Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods. A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred.

HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1".

Table of Contents

   1 Introduction.............................................7
    1.1 Purpose ..............................................7
    1.2 Requirements .........................................7
    1.3 Terminology ..........................................8
    1.4 Overall Operation ...................................11
   2 Notational Conventions and Generic Grammar..............13
    2.1 Augmented BNF .......................................13
    2.2 Basic Rules .........................................15
   3 Protocol Parameters.....................................17
    3.1 HTTP Version ........................................17
   
    3.2 Uniform Resource Identifiers ........................18
     3.2.1 General Syntax ...................................18
     3.2.2 http URL .........................................19
     3.2.3 URI Comparison ...................................20
    3.3 Date/Time Formats ...................................21
     3.3.1 Full Date ........................................21
     3.3.2 Delta Seconds ....................................22
    3.4 Character Sets ......................................22
    3.5 Content Codings .....................................23
    3.6 Transfer Codings ....................................24
    3.7 Media Types .........................................25
     3.7.1 Canonicalization and Text Defaults ...............26
     3.7.2 Multipart Types ..................................27
    3.8 Product Tokens ......................................28
    3.9 Quality Values ......................................28
    3.10 Language Tags ......................................28
    3.11 Entity Tags ........................................29
    3.12 Range Units ........................................30
   4 HTTP Message............................................30
    4.1 Message Types .......................................30
    4.2 Message Headers .....................................31
    4.3 Message Body ........................................32
    4.4 Message Length ......................................32
    4.5 General Header Fields ...............................34
   5 Request.................................................34
    5.1 Request-Line ........................................34
     5.1.1 Method ...........................................35
     5.1.2 Request-URI ......................................35
    5.2 The Resource Identified by a Request ................37
    5.3 Request Header Fields ...............................37
   6 Response................................................38
    6.1 Status-Line .........................................38
     6.1.1 Status Code and Reason Phrase ....................39
    6.2 Response Header Fields ..............................41
   7 Entity..................................................41
    7.1 Entity Header Fields ................................41
    7.2 Entity Body .........................................42
     7.2.1 Type .............................................42
     7.2.2 Length ...........................................43
   8 Connections.............................................43
    8.1 Persistent Connections ..............................43
     8.1.1 Purpose ..........................................43
     8.1.2 Overall Operation ................................44
     8.1.3 Proxy Servers ....................................45
     8.1.4 Practical Considerations .........................45
    8.2 Message Transmission Requirements ...................46
   9 Method Definitions......................................48
    9.1 Safe and Idempotent Methods .........................48
     9.1.1 Safe Methods .....................................48
     9.1.2 Idempotent Methods ...............................49
    9.2 OPTIONS .............................................49
    9.3 GET .................................................50
    9.4 HEAD ................................................50
    9.5 POST ................................................51
    9.6 PUT .................................................52
    9.7 DELETE ..............................................53
    9.8 TRACE ...............................................53
   10 Status Code Definitions................................53
    10.1 Informational 1xx ..................................54
     10.1.1 100 Continue ....................................54
     10.1.2 101 Switching Protocols .........................54
    10.2 Successful 2xx .....................................54
     10.2.1 200 OK ..........................................54
     10.2.2 201 Created .....................................55
     10.2.3 202 Accepted ....................................55
     10.2.4 203 Non-Authoritative Information ...............55
     10.2.5 204 No Content ..................................55
     10.2.6 205 Reset Content ...............................56
     10.2.7 206 Partial Content .............................56
    10.3 Redirection 3xx ....................................56
     10.3.1 300 Multiple Choices ............................57
     10.3.2 301 Moved Permanently ...........................57
     10.3.3 302 Moved Temporarily ...........................58
     10.3.4 303 See Other ...................................58
     10.3.5 304 Not Modified ................................58
     10.3.6 305 Use Proxy ...................................59
    10.4 Client Error 4xx ...................................59
     10.4.1 400 Bad Request .................................60
     10.4.2 401 Unauthorized ................................60
     10.4.3 402 Payment Required ............................60
     10.4.4 403 Forbidden ...................................60
     10.4.5 404 Not Found ...................................60
     10.4.6 405 Method Not Allowed ..........................61
     10.4.7 406 Not Acceptable ..............................61
     10.4.8 407 Proxy Authentication Required ...............61
     10.4.9 408 Request Timeout .............................62
     10.4.10 409 Conflict ...................................62
     10.4.11 410 Gone .......................................62
     10.4.12 411 Length Required ............................63
     10.4.13 412 Precondition Failed ........................63
     10.4.14 413 Request Entity Too Large ...................63
     10.4.15 414 Request-URI Too Long .......................63
     10.4.16 415 Unsupported Media Type .....................63
    10.5 Server Error 5xx ...................................64
     10.5.1 500 Internal Server Error .......................64
     10.5.2 501 Not Implemented .............................64
     10.5.3 502 Bad Gateway .................................64
     10.5.4 503 Service Unavailable .........................64
     10.5.5 504 Gateway Timeout .............................64
     10.5.6 505 HTTP Version Not Supported ..................65
   11 Access Authentication..................................65
    11.1 Basic Authentication Scheme ........................66
    11.2 Digest Authentication Scheme .......................67
   12 Content Negotiation....................................67
    12.1 Server-driven Negotiation ..........................68
    12.2 Agent-driven Negotiation ...........................69
    12.3 Transparent Negotiation ............................70
   13 Caching in HTTP........................................70
     13.1.1 Cache Correctness ...............................72
     13.1.2 Warnings ........................................73
     13.1.3 Cache-control Mechanisms ........................74
     13.1.4 Explicit User Agent Warnings ....................74
     13.1.5 Exceptions to the Rules and Warnings ............75
     13.1.6 Client-controlled Behavior ......................75
    13.2 Expiration Model ...................................75
     13.2.1 Server-Specified Expiration .....................75
     13.2.2 Heuristic Expiration ............................76
     13.2.3 Age Calculations ................................77
     13.2.4 Expiration Calculations .........................79
     13.2.5 Disambiguating Expiration Values ................80
     13.2.6 Disambiguating Multiple Responses ...............80
    13.3 Validation Model ...................................81
     13.3.1 Last-modified Dates .............................82
     13.3.2 Entity Tag Cache Validators .....................82
     13.3.3 Weak and Strong Validators ......................82
     13.3.4 Rules for When to Use Entity Tags and Last-
     modified Dates..........................................85
     13.3.5 Non-validating Conditionals .....................86
    13.4 Response Cachability ...............................86
    13.5 Constructing Responses From Caches .................87
     13.5.1 End-to-end and Hop-by-hop Headers ...............88
     13.5.2 Non-modifiable Headers ..........................88
     13.5.3 Combining Headers ...............................89
     13.5.4 Combining Byte Ranges ...........................90
    13.6 Caching Negotiated Responses .......................90
    13.7 Shared and Non-Shared Caches .......................91
    13.8 Errors or Incomplete Response Cache Behavior .......91
    13.9 Side Effects of GET and HEAD .......................92
    13.10 Invalidation After Updates or Deletions ...........92
    13.11 Write-Through Mandatory ...........................93
    13.12 Cache Replacement .................................93
    13.13 History Lists .....................................93
   14 Header Field Definitions...............................94
    14.1 Accept .............................................95
    14.2 Accept-Charset .....................................97
    14.3 Accept-Encoding ....................................97
    14.4 Accept-Language ....................................98
    14.5 Accept-Ranges ......................................99
    14.6 Age ................................................99
    14.7 Allow .............................................100
    14.8 Authorization .....................................100
    14.9 Cache-Control .....................................101
     14.9.1 What is Cachable ...............................103
     14.9.2 What May be Stored by Caches ...................103
     14.9.3 Modifications of the Basic Expiration Mechanism 104
     14.9.4 Cache Revalidation and Reload Controls .........105
     14.9.5 No-Transform Directive .........................107
     14.9.6 Cache Control Extensions .......................108
    14.10 Connection .......................................109
    14.11 Content-Base .....................................109
    14.12 Content-Encoding .................................110
    14.13 Content-Language .................................110
    14.14 Content-Length ...................................111
    14.15 Content-Location .................................112
    14.16 Content-MD5 ......................................113
    14.17 Content-Range ....................................114
    14.18 Content-Type .....................................116
    14.19 Date .............................................116
    14.20 ETag .............................................117
    14.21 Expires ..........................................117
    14.22 From .............................................118
    14.23 Host .............................................119
    14.24 If-Modified-Since ................................119
    14.25 If-Match .........................................121
    14.26 If-None-Match ....................................122
    14.27 If-Range .........................................123
    14.28 If-Unmodified-Since ..............................124
    14.29 Last-Modified ....................................124
    14.30 Location .........................................125
    14.31 Max-Forwards .....................................125
    14.32 Pragma ...........................................126
    14.33 Proxy-Authenticate ...............................127
    14.34 Proxy-Authorization ..............................127
    14.35 Public ...........................................127
    14.36 Range ............................................128
     14.36.1 Byte Ranges ...................................128
     14.36.2 Range Retrieval Requests ......................130
    14.37 Referer ..........................................131
    14.38 Retry-After ......................................131
    14.39 Server ...........................................132
    14.40 Transfer-Encoding ................................132
    14.41 Upgrade ..........................................132
   
    14.42 User-Agent .......................................134
    14.43 Vary .............................................134
    14.44 Via ..............................................135
    14.45 Warning ..........................................137
    14.46 WWW-Authenticate .................................139
   15 Security Considerations...............................139
    15.1 Authentication of Clients .........................139
    15.2 Offering a Choice of Authentication Schemes .......140
    15.3 Abuse of Server Log Information ...................141
    15.4 Transfer of Sensitive Information .................141
    15.5 Attacks Based On File and Path Names ..............142
    15.6 Personal Information ..............................143
    15.7 Privacy Issues Connected to Accept Headers ........143
    15.8 DNS Spoofing ......................................144
    15.9 Location Headers and Spoofing .....................144
   16 Acknowledgments.......................................144
   17 References............................................146
   18 Authors' Addresses....................................149
   19 Appendices............................................150
    19.1 Internet Media Type message/http ..................150
    19.2 Internet Media Type multipart/byteranges ..........150
    19.3 Tolerant Applications .............................151
    19.4 Differences Between HTTP Entities and
    MIME Entities...........................................152
     19.4.1 Conversion to Canonical Form ...................152
     19.4.2 Conversion of Date Formats .....................153
     19.4.3 Introduction of Content-Encoding ...............153
     19.4.4 No Content-Transfer-Encoding ...................153
     19.4.5 HTTP Header Fields in Multipart Body-Parts .....153
     19.4.6 Introduction of Transfer-Encoding ..............154
     19.4.7 MIME-Version ...................................154
    19.5 Changes from HTTP/1.0 .............................154
     19.5.1 Changes to Simplify Multi-homed Web Servers and
     Conserve IP Addresses .................................155
    19.6 Additional Features ...............................156
     19.6.1 Additional Request Methods .....................156
     19.6.2 Additional Header Field Definitions ............156
    19.7 Compatibility with Previous Versions ..............160
     19.7.1 Compatibility with HTTP/1.0 Persistent
     Connections............................................161

1 Introduction

1.1 Purpose

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved the protocol by allowing messages to be in the format of MIME-like messages, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effects of hierarchical proxies, caching, the need for persistent connections, and virtual hosts. In addition, the proliferation of incompletely-implemented applications calling themselves "HTTP/1.0" has necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities.

This specification defines the protocol referred to as "HTTP/1.1". This protocol includes more stringent requirements than HTTP/1.0 in order to ensure reliable implementation of its features.

Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods that indicate the purpose of a request. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or name (URN) , for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by Internet mail as defined by the Multipurpose Internet Mail Extensions (MIME).

HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems, including those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10] protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.

1.2 Requirements

This specification uses the same words as RFC 1123 [8] for defining the significance of each particular requirement. These words are:

MUST

This word or the adjective "required" means that the item is an absolute requirement of the specification.

SHOULD

This word or the adjective "recommended" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.

MAY

This word or the adjective "optional" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.

An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be "conditionally compliant."

1.3 Terminology

This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.

connection

A transport layer virtual circuit established between two programs for the purpose of communication.

message

The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in section 4 and transmitted via the connection.

request

An HTTP request message, as defined in section 5.

response

An HTTP response message, as defined in section 6.

resource

A network data object or service that can be identified by a URI, as defined in section 3.2. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, resolutions) or vary in other ways.

entity

The information transferred as the payload of a request or response. An entity consists of metainformation in the form of entity-header fields and content in the form of an entity-body, as described in section 7.

representation

An entity included with a response that is subject to content negotiation, as described in section 12. There may exist multiple representations associated with a particular response status.

content negotiation

The mechanism for selecting the appropriate representation when servicing a request, as described in section 12. The representation of entities in any response can be negotiated (including error responses).

variant

A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `variant.' Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.

client

A program that establishes connections for the purpose of sending requests.

user agent

The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools.

server

An application program that accepts connections in order to service requests by sending back responses. Any given program may be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server may act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.

origin server

The server on which a given resource resides or is to be created.

proxy

An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy must implement both the client and server requirements of this specification.

gateway

A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway.

tunnel

An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed.

cache

A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cachable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server may include a cache, though a cache cannot be used by a server that is acting as a tunnel.

cachable

A response is cachable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cachability of HTTP responses are defined in section 13. Even if a resource is cachable, there may be additional constraints on whether a cache can use the cached copy for a particular request.

first-hand

A response is first-hand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also first-hand if its validity has just been checked directly with the origin server.

explicit expiration time

The time at which the origin server intends that an entity should no longer be returned by a cache without further validation.

heuristic expiration time

An expiration time assigned by a cache when no explicit expiration time is available.

age

The age of a response is the time since it was sent by, or successfully validated with, the origin server.

freshness lifetime

The length of time between the generation of a response and its expiration time.

fresh

A response is fresh if its age has not yet exceeded its freshness lifetime.

stale

A response is stale if its age has passed its freshness lifetime.

semantically transparent

A cache behaves in a "semantically transparent" manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response (except for hop-by-hop headers) that it would have received had its request been handled directly by the origin server.

validator

A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find out whether a cache entry is an equivalent copy of an entity.

1.4 Overall Operation

The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible entity-body content. The relationship between HTTP and MIME is described in appendix 19.4.

Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O).

             request chain ------------------------>
          UA -------------------v------------------- O
             <----------------------- response chain

A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages; tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary cannot understand the contents of the messages.

request chain -------------------------------------->

UA -----v----- A -----v----- B -----v----- C -----v----- O

<------------------------------------- response chain

The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain will pass through four separate connections. This distinction is important because some HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple, simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.

Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request which has not been cached by UA or A.

             request chain ---------->
          UA -----v----- A -----v----- B - - - - - - C - - - - - - O
             <--------- response chain

Not all responses are usefully cachable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cachable responses are defined in section 13.

In fact, there are a wide variety of architectures and configurations of caches and proxies currently being experimented with or deployed across the World Wide Web; these systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via CD-ROM, and so on. HTTP systems are used in corporate intranets over high-bandwidth links, and for access via PDAs with low-power radio links and intermittent connectivity. The goal of HTTP/1.1 is to support the wide diversity of configurations already deployed while introducing protocol constructs that meet the needs of those who build web applications that require high reliability and, failing that, at least reliable indications of failure.

HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80, but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.

In HTTP/1.0, most implementations used a new connection for each request/response exchange. In HTTP/1.1, a connection may be used for one or more request/response exchanges, although connections may be closed for a variety of reasons (see section 8.1).

2 Notational Conventions and Generic Grammar

2.1 Augmented BNF

All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by RFC 822 [9]. Implementers will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs:

name = definition

The name of a rule is simply the name itself (without any enclosing "<" and ">") and is separated from its definition by the equal "=" character. Whitespace is only significant in that indentation of continuation lines is used to indicate a rule definition that spans more than one line. Certain basic rules are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names.

"literal"

Quotation marks surround literal text. Unless stated otherwise, the

text is case-insensitive.

rule1 | rule2

Elements separated by a bar ("|") are alternatives, e.g., "yes | no" will accept yes or no.

(rule1 rule2)

Elements enclosed in parentheses are treated as a single element. Thus, "(elem (foo | bar) elem)" allows the token sequences "elem foo elem" and "elem bar elem".

*rule

The character "*" preceding an element indicates repetition. The full form is "<n>*<m>element" indicating at least <n> and at most <m> occurrences of element. Default values are 0 and infinity so that "*(element)" allows any number, including zero; "1*element" requires at least one; and "1*2element" allows one or two.

[rule]

Square brackets enclose optional elements; "[foo bar]" is equivalent to "*1(foo bar)".

N rule

Specific repetition: "<n>(element)" is equivalent to "<n>*<n>(element)"; that is, exactly <n> occurrences of (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters.

#rule

A construct "#" is defined, similar to "*", for defining lists of elements. The full form is "<n>#<m>element " indicating at least <n> and at most <m> elements, each separated by one or more commas (",") and optional linear whitespace (LWS). This makes the usual form of lists very easy; a rule such as "( *LWS element *( *LWS "," *LWS element )) " can be shown as "1#element". Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is, "(element), , (element) " is permitted, but counts as only two elements. Therefore, where at least one element is required, at least one non-null element must be present. Default values are 0 and infinity so that "#element" allows any number, including zero; "1#element" requires at least one; and "1#2element" allows one or two.

; comment

A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel with the specifications.

implied *LWS

The grammar described by this specification is word-based. Except where noted otherwise, linear whitespace (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent tokens and delimiters (tspecials), without changing the interpretation of a field. At least one delimiter (tspecials) must exist between any two tokens, since they would otherwise be interpreted as a single token.

2.2 Basic Rules

The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character set is defined by ANSI X3.4-1986 [21].

          OCTET          = <any 8-bit sequence of data>
          CHAR           = <any US-ASCII character (octets 0 - 127)>
          UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
          LOALPHA        = <any US-ASCII lowercase letter "a".."z">
          ALPHA          = UPALPHA | LOALPHA
          DIGIT          = <any US-ASCII digit "0".."9">
          CTL            = <any US-ASCII control character
                           (octets 0 - 31) and DEL (127)>
          CR             = <US-ASCII CR, carriage return (13)>
          LF             = <US-ASCII LF, linefeed (10)>
          SP             = <US-ASCII SP, space (32)>
          HT             = <US-ASCII HT, horizontal-tab (9)>
          <">            = <US-ASCII double-quote mark (34)>

HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all protocol elements except the entity-body (see appendix 19.3 for tolerant applications). The end-of-line marker within an entity-body is defined by its associated media type, as described in section 3.7.

          CRLF           = CR LF

HTTP/1.1 headers can be folded onto multiple lines if the continuation line begins with a space or horizontal tab. All linear white space, including folding, has the same semantics as SP.

          LWS            = [CRLF] 1*( SP | HT )

The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message parser. Words of *TEXT may contain characters from character sets other than ISO 8859-1 [22] only when encoded according to the rules of RFC 1522 [14].

          TEXT           = <any OCTET except CTLs,
                           but including LWS>

Hexadecimal numeric characters are used in several protocol elements.

          HEX            = "A" | "B" | "C" | "D" | "E" | "F"
          
                         | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters MUST be in a quoted string to be used within a parameter value.

token = 1*<any CHAR except CTLs or tspecials>

          tspecials      = "(" | ")" | "<" | ">" | "@"
                         | "," | ";" | ":" | "\" | <">
                         | "/" | "[" | "]" | "?" | "="
                         | "{" | "}" | SP | HT

Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition. In all other fields, parentheses are considered part of the field value.

          comment        = "(" *( ctext | comment ) ")"
          ctext          = <any TEXT excluding "(" and ")">

A string of text is parsed as a single word if it is quoted using double-quote marks.

          quoted-string  = ( <"> *(qdtext) <"> )
          
          qdtext         = <any TEXT except <">>

The backslash character ("\") may be used as a single-character quoting mechanism only within quoted-string and comment constructs.

          quoted-pair    = "\" CHAR

3 Protocol Parameters

3.1 HTTP Version

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The <minor> number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The <major> number is incremented when the format of a message within the protocol is changed.

The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message.

          HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

Note that the major and minor numbers MUST be treated as separate integers and that each may be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and MUST NOT be sent.

Applications sending Request or Response messages, as defined by this specification, MUST include an HTTP-Version of "HTTP/1.1". Use of this version number indicates that the sending application is at least conditionally compliant with this specification.

The HTTP version of an application is the highest HTTP version for which the application is at least conditionally compliant.

Proxy and gateway applications must be careful when forwarding messages in protocol versions different from that of the application. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway MUST never send a message with a version indicator which is greater than its actual version; if a higher version request is received, the proxy/gateway MUST either downgrade the request version, respond with an error, or switch to tunnel behavior. Requests with a version lower than that of the proxy/gateway's version MAY be upgraded before being forwarded; the proxy/gateway's response to that request MUST be in the same major version as the request.

Note: Converting between versions of HTTP may involve modification of header fields required or forbidden by the versions involved.

3.2 Uniform Resource Identifiers

URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers , and finally the combination of Uniform Resource Locators (URL) and Names (URN). As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location, or any other characteristic--a resource.

3.2.1 General Syntax

URIs in HTTP can be represented in absolute form or relative to some known base URI, depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with a scheme name followed by a colon.

          URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]

absoluteURI = scheme ":" *( uchar | reserved )

          relativeURI    = net_path | abs_path | rel_path
          
          net_path       = "//" net_loc [ abs_path ]
          abs_path       = "/" rel_path
          rel_path       = [ path ] [ ";" params ] [ "?" query ]
          
          path           = fsegment *( "/" segment )
          fsegment       = 1*pchar
          segment        = *pchar
          
          params         = param *( ";" param )
          param          = *( pchar | "/" )
          scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
          net_loc        = *( pchar | ";" | "?" )
          
          query          = *( uchar | reserved )
          fragment       = *( uchar | reserved )

pchar = uchar | ":" | "@" | "&" | "=" | "+"

uchar = unreserved | escape

unreserved = ALPHA | DIGIT | safe | extra | national

          escape         = "%" HEX HEX
          reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
          extra          = "!" | "*" | "'" | "(" | ")" | ","
          safe           = "$" | "-" | "_" | "."
          unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
          national       = <any OCTET excluding ALPHA, DIGIT,
                           reserved, extra, safe, and unsafe>

For definitive information on URL syntax and semantics, see RFC 1738 [4] and RFC 1808 [11]. The BNF above includes national characters not allowed in valid URLs as specified by RFC 1738, since HTTP servers are not restricted in the set of unreserved characters allowed to represent the rel_path part of addresses, and HTTP proxies may receive requests for URIs not defined by RFC 1738.

The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return 414 (Request-URI Too Long) status if a URI is longer than the server can handle (see section 10.4.15).

Note: Servers should be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations may not properly support these lengths.

3.2.2 http URL

The "http" scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax and semantics for http URLs.

          http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]
          
          host           = <A legal Internet host domain name
                            or IP address (in dotted-decimal form),
                            as defined by Section 2.1 of RFC 1123>
          
          port           = *DIGIT

If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path. The use of IP addresses in URL's SHOULD be avoided whenever possible (see RFC 1900 [24]). If the abs_path is not present in the URL, it MUST be given as "/" when used as a Request-URI for a resource (section 5.1.2).

3.2.3 URI Comparison

When comparing two URIs to decide if they match or not, a client SHOULD use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions:

• A port that is empty or not given is equivalent to the default port for that URI;

• Comparisons of host names MUST be case-insensitive;

• Comparisons of scheme names MUST be case-insensitive;

• An empty abs_path is equivalent to an abs_path of "/".

Characters other than those in the "reserved" and "unsafe" sets (see section 3.2) are equivalent to their ""%" HEX HEX" encodings.

For example, the following three URIs are equivalent:

         http://abc.com:80/~smith/home.html
         http://ABC.com/%7Esmith/home.html
         http://ABC.com:/%7esmith/home.html

3.3 Date/Time Formats

3.3.1 Full Date

HTTP applications have historically allowed three different formats for the representation of date/time stamps:

          Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
          Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
          Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
   
   The first format is preferred as an Internet standard and represents
   a fixed-length subset of that defined by RFC 1123  (an update to RFC
   822).  The second format is in common use, but is based on the
   obsolete RFC 850 [12] date format and lacks a four-digit year.
   HTTP/1.1 clients and servers that parse the date value MUST accept
   all three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields.

Note: Recipients of date values are encouraged to be robust in accepting date values that may have been sent by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and MUST be assumed when reading the asctime format.

HTTP-date = rfc1123-date | rfc850-date | asctime-date

rfc1123-date = wkday "," SP date1 SP time SP "GMT"

rfc850-date = weekday "," SP date2 SP time SP "GMT"

asctime-date = wkday SP date3 SP time SP 4DIGIT

          date1        = 2DIGIT SP month SP 4DIGIT
                         ; day month year (e.g., 02 Jun 1982)
          date2        = 2DIGIT "-" month "-" 2DIGIT
                         ; day-month-year (e.g., 02-Jun-82)
          date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                         ; month day (e.g., Jun  2)
          
          time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                         ; 00:00:00 - 23:59:59
          
          wkday        = "Mon" | "Tue" | "Wed"
                       | "Thu" | "Fri" | "Sat" | "Sun"
          
          weekday      = "Monday" | "Tuesday" | "Wednesday"
                       | "Thursday" | "Friday" | "Saturday" | "Sunday"

month = "Jan" | "Feb" | "Mar" | "Apr"

| "May" | "Jun" | "Jul" | "Aug"

| "Sep" | "Oct" | "Nov" | "Dec"

Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.

3.3.2 Delta Seconds

Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after the time that the message was received.

          delta-seconds  = 1*DIGIT

3.4 Character Sets

HTTP uses the same definition of the term "character set" as that described for MIME:

The term "character set" is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encodings, from simple single-table mappings such as US- ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, the definition associated with a MIME character set name MUST fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted.

Note: This use of the term "character set" is more commonly referred to as a "character encoding." However, since HTTP and MIME share the same registry, it is important that the terminology also be shared.

HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry [19].

          charset = token

Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry.

3.5 Content Codings

Content coding values indicate an encoding transformation that has been or can be applied to an entity. Content codings are primarily used to allow a document to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the entity is stored in coded form, transmitted directly, and only decoded by the recipient.

          content-coding   = token

All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (section 14.3) and Content-Encoding (section 14.12) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.

The Internet Assigned Numbers Authority (IANA) acts as a registry for content-coding value tokens. Initially, the registry contains the following tokens:

gzip An encoding format produced by the file compression program "gzip"

(GNU zip) as described in RFC 1952 [25]. This format is a Lempel- Ziv coding (LZ77) with a 32 bit CRC.

compress

The encoding format produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).

Note: Use of program names for the identification of encoding formats is not desirable and should be discouraged for future encodings. Their use here is representative of historical practice, not good design. For compatibility with previous implementations of HTTP, applications should consider "x-gzip" and "x-compress" to be equivalent to "gzip" and "compress" respectively.

deflate The "zlib" format defined in RFC 1950[31] in combination with

the "deflate" compression mechanism described in RFC 1951[29].

New content-coding value tokens should be registered; to allow interoperability between clients and servers, specifications of the content coding algorithms needed to implement a new value should be publicly available and adequate for independent implementation, and conform to the purpose of content coding defined in this section.

3.6 Transfer Codings

Transfer coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to an entity-body in order to ensure "safe transport" through the network. This differs from a content coding in that the transfer coding is a property of the message, not of the original entity.

transfer-coding = "chunked" | transfer-extension

          transfer-extension      = token

All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer coding values in the Transfer-Encoding header field (section 14.40).

Transfer codings are analogous to the Content-Transfer-Encoding values of MIME , which were designed to enable safe transport of binary data over a 7-bit transport service. However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message-bodies is the difficulty in determining the exact body length (section 7.2.2), or the desire to encrypt data over a shared transport.

The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an optional footer containing entity-header fields. This allows dynamically-produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.

       Chunked-Body   = *chunk
                        "0" CRLF
                        footer
                        CRLF
       
       chunk          = chunk-size [ chunk-ext ] CRLF
                        chunk-data CRLF
       
       hex-no-zero    = <HEX excluding "0">
       
       chunk-size     = hex-no-zero *HEX
       chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)
       
       footer         = *entity-header

The chunked encoding is ended by a zero-sized chunk followed by the footer, which is terminated by an empty line. The purpose of the footer is to provide an efficient way to supply information about an entity that is generated dynamically; applications MUST NOT send header fields in the footer which are not explicitly defined as being appropriate for the footer, such as Content-MD5 or future extensions to HTTP for digital signatures or other facilities.

An example process for decoding a Chunked-Body is presented in appendix 19.4.6.

All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer coding, and MUST ignore transfer coding extensions they do not understand. A server which receives an entity-body with a transfer-coding it does not understand SHOULD return 501 (Unimplemented), and close the connection. A server MUST NOT send transfer-codings to an HTTP/1.0 client.

3.7 Media Types

   HTTP uses Internet Media Types  in the Content-Type (section 14.18)
   and Accept (section 14.1) header fields in order to provide open and
   extensible data typing and type negotiation.
   
          media-type     = type "/" subtype *( ";" parameter )
          type           = token
          subtype        = token

Parameters may follow the type/subtype in the form of attribute/value pairs.

          parameter      = attribute "=" value
          attribute      = token
          value          = token | quoted-string

The type, subtype, and parameter attribute names are case- insensitive. Parameter values may or may not be case-sensitive, depending on the semantics of the parameter name. Linear white space (LWS) MUST NOT be used between the type and subtype, nor between an attribute and its value. User agents that recognize the media-type MUST process (or arrange to be processed by any external applications used to process that type/subtype by the user agent) the parameters for that MIME type as described by that type/subtype definition to the and inform the user of any problems discovered.

Note: some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations should only use media type parameters when they are required by that type/subtype definition.

Media-type values are registered with the Internet Assigned Number Authority (IANA). The media type registration process is outlined in RFC 2048 [17]. Use of non-registered media types is discouraged.

3.7.1 Canonicalization and Text Defaults

Internet media types are registered with a canonical form. In general, an entity-body transferred via HTTP messages MUST be represented in the appropriate canonical form prior to its transmission; the exception is "text" types, as defined in the next paragraph.

When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire entity-body. HTTP applications MUST accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP. In addition, if the text is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the entity-body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).

If an entity-body is encoded with a Content-Encoding, the underlying data MUST be in a form defined above prior to being encoded.

The "charset" parameter is used with some media types to define the character set (section 3.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value.

Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should guess." Senders wishing to defeat this behavior MAY include a charset parameter even when the charset is ISO-8859-1 and SHOULD do so when it is known that it will not confuse the recipient.

Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients MUST respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset MUST use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially displaying a document.

3.7.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message-body. All multipart
   types share a common syntax, as defined in  MIME [7], and MUST
   include a boundary parameter as part of the media type value. The
   message body is itself a protocol element and MUST therefore use only
   CRLF to represent line breaks between body-parts. Unlike in MIME, the
   epilogue of any multipart message MUST be empty; HTTP applications
   MUST NOT transmit the epilogue (even if the original multipart
   contains an epilogue).

In HTTP, multipart body-parts MAY contain header fields which are significant to the meaning of that part. A Content-Location header field (section 14.15) SHOULD be included in the body-part of each enclosed entity that can be identified by a URL.

In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".

Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 [15].

3.8 Product Tokens

Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application.

          product         = token ["/" product-version]
          product-version = token

Examples:

          User-Agent: CERN-LineMode/2.15 libwww/2.17b3
          Server: Apache/0.8.4

Product tokens should be short and to the point -- use of them for advertising or other non-essential information is explicitly forbidden. Although any token character may appear in a product- version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value).

3.9 Quality Values

HTTP content negotiation (section 12) uses short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. HTTP/1.1 applications MUST NOT generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion.

          qvalue         = ( "0" [ "." 0*3DIGIT ] )
                         | ( "1" [ "." 0*3("0") ] )

"Quality values" is a misnomer, since these values merely represent relative degradation in desired quality.

3.10 Language Tags

A language tag identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language and Content- Language fields.

The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags:

           language-tag  = primary-tag *( "-" subtag )
           
           primary-tag   = 1*8ALPHA
           subtag        = 1*8ALPHA

Whitespace is not allowed within the tag and all tags are case- insensitive. The name space of language tags is administered by the IANA. Example tags include:

          en, en-US, en-cockney, i-cherokee, x-pig-latin

where any two-letter primary-tag is an ISO 639 language abbreviation and any two-letter initial subtag is an ISO 3166 country code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered in future.)

3.11 Entity Tags

Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the ETag (section 14.20), If-Match (section 14.25), If-None-Match (section 14.26), and If-Range (section 14.27) header fields. The definition of how they are used and compared as cache validators is in section 13.3.3. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.

         entity-tag = [ weak ] opaque-tag
         
         weak       = "W/"
         opaque-tag = quoted-string

A "strong entity tag" may be shared by two entities of a resource only if they are equivalent by octet equality.

A "weak entity tag," indicated by the "W/" prefix, may be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no significant change in semantics. A weak entity tag can only be used for weak comparison.

An entity tag MUST be unique across all versions of all entities associated with a particular resource. A given entity tag value may be used for entities obtained by requests on different URIs without implying anything about the equivalence of those entities.

3.12 Range Units

HTTP/1.1 allows a client to request that only part (a range of) the response entity be included within the response. HTTP/1.1 uses range units in the Range (section 14.36) and Content-Range (section 14.17) header fields. An entity may be broken down into subranges according to various structural units.

         range-unit       = bytes-unit | other-range-unit
         
         bytes-unit       = "bytes"
         other-range-unit = token

The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1

implementations may ignore ranges specified using other units. HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.

4 HTTP Message

4.1 Message Types

HTTP messages consist of requests from client to server and responses from server to client.

HTTP-message = Request | Response ; HTTP/1.1 messages

Request (section 5) and Response (section 6) messages use the generic message format of RFC 822 [9] for transferring entities (the payload of the message). Both types of message consist of a start-line, one or more header fields (also known as "headers"), an empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header fields, and an optional message-body.

generic-message = start-line

*message-header

CRLF

[ message-body ]

           start-line      = Request-Line | Status-Line

In the interest of robustness, servers SHOULD ignore any empty line(s) received where a Request-Line is expected. In other words, if the server is reading the protocol stream at the beginning of a message and receives a CRLF first, it should ignore the CRLF.

Note: certain buggy HTTP/1.0 client implementations generate an extra CRLF's after a POST request. To restate what is explicitly forbidden by the BNF, an HTTP/1.1 client must not preface or follow a request with an extra CRLF.

4.2 Message Headers

HTTP header fields, which include general-header (section 4.5), request-header (section 5.3), response-header (section 6.2), and entity-header (section 7.1) fields, follow the same generic format as that given in Section 3.1 of RFC 822 [9]. Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. The field value may be preceded by any amount of LWS, though a single SP is preferred. Header fields can be extended over multiple lines by preceding each extra line with at least one SP or HT. Applications SHOULD follow "common form" when generating HTTP constructs, since there might exist some implementations that fail to accept anything beyond the common forms.

message-header = field-name ":" [ field-value ] CRLF

          field-name     = token
          field-value    = *( field-content | LWS )
          
          field-content  = <the OCTETs making up the field-value
                           and consisting of either *TEXT or combinations
                           of token, tspecials, and quoted-string>

The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send general-header fields first, followed by request-header or response- header fields, and ending with the entity-header fields.

Multiple message-header fields with the same field-name may be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

4.3 Message Body

The message-body (if any) of an HTTP message is used to carry the entity-body associated with the request or response. The message-body differs from the entity-body only when a transfer coding has been applied, as indicated by the Transfer-Encoding header field (section 14.40).

message-body = entity-body

                       | <entity-body encoded as per Transfer-Encoding>

Transfer-Encoding MUST be used to indicate any transfer codings applied by an application to ensure safe and proper transfer of the message. Transfer-Encoding is a property of the message, not of the entity, and thus can be added or removed by any application along the request/response chain.

The rules for when a message-body is allowed in a message differ for requests and responses.

The presence of a message-body in a request is signaled by the inclusion of a Content-Length or Transfer-Encoding header field in the request's message-headers. A message-body MAY be included in a request only when the request method (section 5.1.1) allows an entity-body.

For response messages, whether or not a message-body is included with a message is dependent on both the request method and the response status code (section 6.1.1). All responses to the HEAD request method MUST NOT include a message-body, even though the presence of entity- header fields might lead one to believe they do. All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a message-body. All other responses do include a message-body, although it may be of zero length.

4.4 Message Length

When a message-body is included with a message, the length of that body is determined by one of the following (in order of precedence):

1. Any response message which MUST NOT include a message-body

(such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity-header fields present in the message.

2. If a Transfer-Encoding header field (section 14.40) is present and

indicates that the "chunked" transfer coding has been applied, then the length is defined by the chunked encoding (section 3.6).

3. If a Content-Length header field (section 14.14) is present, its

value in bytes represents the length of the message-body.

4. If the message uses the media type "multipart/byteranges", which is

self-delimiting, then that defines the length. This media type MUST NOT be used unless the sender knows that the recipient can parse it; the presence in a request of a Range header with multiple byte-range specifiers implies that the client can parse multipart/byteranges responses.

5. By the server closing the connection. (Closing the connection

cannot be used to indicate the end of a request body, since that would leave no possibility for the server to send back a response.)

For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing a message-body MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. If a request contains a message-body and a Content-Length is not given, the server SHOULD respond with 400 (bad request) if it cannot determine the length of the message, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length.

All HTTP/1.1 applications that receive entities MUST accept the "chunked" transfer coding (section 3.6), thus allowing this mechanism to be used for messages when the message length cannot be determined in advance.

Messages MUST NOT include both a Content-Length header field and the "chunked" transfer coding. If both are received, the Content-Length MUST be ignored.

When a Content-Length is given in a message where a message-body is allowed, its field value MUST exactly match the number of OCTETs in the message-body. HTTP/1.1 user agents MUST notify the user when an invalid length is received and detected.

4.5 General Header Fields

There are a few header fields which have general applicability for both request and response messages, but which do not apply to the entity being transferred. These header fields apply only to the message being transmitted.

          general-header = Cache-Control            ; Section 14.9
          
                         | Connection               ; Section 14.10
                         | Date                     ; Section 14.19
                         | Pragma                   ; Section 14.32
                         | Transfer-Encoding        ; Section 14.40
                         | Upgrade                  ; Section 14.41
                         | Via                      ; Section 14.44

General-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize them to be general-header fields. Unrecognized header fields are treated as entity-header fields.

5 Request

A request message from a client to a server includes, within the first line of that message, the method to be applied to the resource, the identifier of the resource, and the protocol version in use.

           Request       = Request-Line              ; Section 5.1
                           *( general-header         ; Section 4.5
                            | request-header         ; Section 5.3
                            | entity-header )        ; Section 7.1
                           CRLF
                           [ message-body ]          ; Section 7.2

5.1 Request-Line

The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The elements are separated by SP characters. No CR or LF are allowed except in the final CRLF sequence.

Request-Line = Method SP Request-URI SP HTTP-Version CRLF

5.1.1 Method

The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive.

          Method         = "OPTIONS"                ; Section 9.2
          
                         | "GET"                    ; Section 9.3
                         | "HEAD"                   ; Section 9.4
                         | "POST"                   ; Section 9.5
                         | "PUT"                    ; Section 9.6
                         | "DELETE"                 ; Section 9.7
                         | "TRACE"                  ; Section 9.8
                         | extension-method
          
          extension-method = token

The list of methods allowed by a resource can be specified in an Allow header field (section 14.7). The return code of the response always notifies the client whether a method is currently allowed on a resource, since the set of allowed methods can change dynamically. Servers SHOULD return the status code 405 (Method Not Allowed) if the method is known by the server but not allowed for the requested resource, and 501 (Not Implemented) if the method is unrecognized or not implemented by the server. The list of methods known by a server can be listed in a Public response-header field (section 14.35).

The methods GET and HEAD MUST be supported by all general-purpose servers. All other methods are optional; however, if the above methods are implemented, they MUST be implemented with the same semantics as those specified in section 9.

5.1.2 Request-URI

The Request-URI is a Uniform Resource Identifier (section 3.2) and identifies the resource upon which to apply the request.

          Request-URI    = "*" | absoluteURI | abs_path

The three options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource. One example would be

          OPTIONS * HTTP/1.1

The absoluteURI form is required when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache, and return the response. Note that the proxy MAY forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request loops, a proxy MUST be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example Request-Line would be:

GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.

The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute path of the URI MUST be transmitted (see section 3.2.1, abs_path) as the Request-URI, and the network location of the URI (net_loc) MUST be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin server would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:

          GET /pub/WWW/TheProject.html HTTP/1.1
          Host: www.w3.org

followed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as "/" (the server root).

If a proxy receives a request without any path in the Request-URI and the method specified is capable of supporting the asterisk form of request, then the last proxy on the request chain MUST forward the request with "*" as the final Request-URI. For example, the request

          OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

would be forwarded by the proxy as

          OPTIONS * HTTP/1.1
          Host: www.ics.uci.edu:8001

after connecting to port 8001 of host "www.ics.uci.edu".

The Request-URI is transmitted in the format specified in section 3.2.1. The origin server MUST decode the Request-URI in order to properly interpret the request. Servers SHOULD respond to invalid Request-URIs with an appropriate status code.

In requests that they forward, proxies MUST NOT rewrite the "abs_path" part of a Request-URI in any way except as noted above to replace a null abs_path with "*", no matter what the proxy does in its internal implementation.

Note: The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using a non-reserved URL character for a reserved purpose. Implementers should be aware that some pre-HTTP/1.1 proxies have been known to rewrite the Request-URI.

5.2 The Resource Identified by a Request

HTTP/1.1 origin servers SHOULD be aware that the exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field.

An origin server that does not allow resources to differ by the requested host MAY ignore the Host header field value. (But see section 19.5.1 for other requirements on Host support in HTTP/1.1.)

An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or vanity hostnames) MUST use the following rules for determining the requested resource on an HTTP/1.1 request:

1. If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request MUST be ignored.

2. If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field value.

     3. If the host as determined by rule 1 or 2 is not a valid host on
     
        the server, the response MUST be a 400 (Bad Request) error
        message.

Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested.

5.3 Request Header Fields

The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.

          request-header = Accept                   ; Section 14.1
                         | Accept-Charset           ; Section 14.2
                         | Accept-Encoding          ; Section 14.3
                         | Accept-Language          ; Section 14.4
                         | Authorization            ; Section 14.8
                         | From                     ; Section 14.22
                         | Host                     ; Section 14.23
                         | If-Modified-Since        ; Section 14.24
                         | If-Match                 ; Section 14.25
                         | If-None-Match            ; Section 14.26
                         | If-Range                 ; Section 14.27
                         | If-Unmodified-Since      ; Section 14.28
                         | Max-Forwards             ; Section 14.31
                         | Proxy-Authorization      ; Section 14.34
                         | Range                    ; Section 14.36
                         | Referer                  ; Section 14.37
                         | User-Agent               ; Section 14.42

Request-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of request- header fields if all parties in the communication recognize them to be request-header fields. Unrecognized header fields are treated as entity-header fields.

6 Response

After receiving and interpreting a request message, a server responds with an HTTP response message.

       Response      = Status-Line               ; Section 6.1
       
                       *( general-header         ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header )        ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2

6.1 Status-Line

The first line of a Response message is the Status-Line, consisting of the protocol version followed by a numeric status code and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.

       Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

6.1.1 Status Code and Reason Phrase

The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes are fully defined in section 10. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason- Phrase.

The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:

• 1xx: Informational - Request received, continuing process

• 2xx: Success - The action was successfully received, understood, and accepted

• 3xx: Redirection - Further action must be taken in order to complete the request

• 4xx: Client Error - The request contains bad syntax or cannot be fulfilled

• 5xx: Server Error - The server failed to fulfill an apparently valid request

The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's, are presented below. The reason phrases listed here are only recommended -- they may be replaced by local equivalents without affecting the protocol.

          Status-Code    = "100"   ; Continue
                         | "101"   ; Switching Protocols
                         | "200"   ; OK
                         | "201"   ; Created
                         | "202"   ; Accepted
                         | "203"   ; Non-Authoritative Information
                         | "204"   ; No Content
                         | "205"   ; Reset Content
                         | "206"   ; Partial Content
                         | "300"   ; Multiple Choices
                         | "301"   ; Moved Permanently
                         | "302"   ; Moved Temporarily
                         | "303"   ; See Other
                         | "304"   ; Not Modified
                         | "305"   ; Use Proxy
                         | "400"   ; Bad Request
                         | "401"   ; Unauthorized
                         | "402"   ; Payment Required
                         | "403"   ; Forbidden
                         | "404"   ; Not Found
                         | "405"   ; Method Not Allowed
                         | "406"   ; Not Acceptable
                         | "407"   ; Proxy Authentication Required
                         | "408"   ; Request Time-out
                         | "409"   ; Conflict
                         | "410"   ; Gone
                         | "411"   ; Length Required
                         | "412"   ; Precondition Failed
                         | "413"   ; Request Entity Too Large
                         | "414"   ; Request-URI Too Large
                         | "415"   ; Unsupported Media Type
                         | "500"   ; Internal Server Error
                         | "501"   ; Not Implemented
                         | "502"   ; Bad Gateway
                         | "503"   ; Service Unavailable
                         | "504"   ; Gateway Time-out
                         | "505"   ; HTTP Version not supported
                         | extension-code
          
          extension-code = 3DIGIT
          
          Reason-Phrase  = *<TEXT, excluding CR, LF>

HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST NOT be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the entity returned with the response, since that entity is likely to include human- readable information which will explain the unusual status.

6.2 Response Header Fields

The response-header fields allow the server to pass additional information about the response which cannot be placed in the Status- Line. These header fields give information about the server and about further access to the resource identified by the Request-URI.

          response-header = Age                     ; Section 14.6
                          | Location                ; Section 14.30
                          | Proxy-Authenticate      ; Section 14.33
                          | Public                  ; Section 14.35
                          | Retry-After             ; Section 14.38
                          | Server                  ; Section 14.39
                          | Vary                    ; Section 14.43
                          | Warning                 ; Section 14.45
                          | WWW-Authenticate        ; Section 14.46

Response-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of response- header fields if all parties in the communication recognize them to be response-header fields. Unrecognized header fields are treated as entity-header fields.

7 Entity

Request and Response messages MAY transfer an entity if not otherwise restricted by the request method or response status code. An entity consists of entity-header fields and an entity-body, although some responses will only include the entity-headers.

In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.

7.1 Entity Header Fields

Entity-header fields define optional metainformation about the entity-body or, if no body is present, about the resource identified by the request.

          entity-header  = Allow                    ; Section 14.7
          
                         | Content-Base             ; Section 14.11
                         | Content-Encoding         ; Section 14.12
                         | Content-Language         ; Section 14.13
                         | Content-Length           ; Section 14.14
                         | Content-Location         ; Section 14.15
                         | Content-MD5              ; Section 14.16
                         | Content-Range            ; Section 14.17
                         | Content-Type             ; Section 14.18
                         | ETag                     ; Section 14.20
                         | Expires                  ; Section 14.21
                         | Last-Modified            ; Section 14.29
                         | extension-header
          
          extension-header = message-header

The extension-header mechanism allows additional entity-header fields to be defined without changing the protocol, but these fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields SHOULD be ignored by the recipient and forwarded by proxies.

7.2 Entity Body

The entity-body (if any) sent with an HTTP request or response is in a format and encoding defined by the entity-header fields.

          entity-body    = *OCTET

An entity-body is only present in a message when a message-body is present, as described in section 4.3. The entity-body is obtained from the message-body by decoding any Transfer-Encoding that may have been applied to ensure safe and proper transfer of the message.

7.2.1 Type

When an entity-body is included with a message, the data type of that body is determined via the header fields Content-Type and Content- Encoding. These define a two-layer, ordered encoding model:

entity-body := Content-Encoding( Content-Type( data ) )

Content-Type specifies the media type of the underlying data. Content-Encoding may be used to indicate any additional content codings applied to the data, usually for the purpose of data compression, that are a property of the requested resource. There is no default encoding.

Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URL used to identify the resource. If the media type remains unknown, the recipient SHOULD treat it as type "application/octet-stream".

7.2.2 Length

The length of an entity-body is the length of the message-body after any transfer codings have been removed. Section 4.4 defines how the length of a message-body is determined.

8 Connections

8.1 Persistent Connections

8.1.1 Purpose

Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make multiple requests of the same server in a short amount of time. Analyses of these performance problems are available [30][27]; analysis and results from a prototype implementation are in [26].

Persistent HTTP connections have a number of advantages:

     o  By opening and closing fewer TCP connections, CPU time is saved,
        and memory used for TCP protocol control blocks is also saved.
     o  HTTP requests and responses can be pipelined on a connection.
        Pipelining allows a client to make multiple requests without
        waiting for each response, allowing a single TCP connection to be
        used much more efficiently, with much lower elapsed time.
     o  Network congestion is reduced by reducing the number of packets
        caused by TCP opens, and by allowing TCP sufficient time to
        determine the congestion state of the network.
     o  HTTP can evolve more gracefully; since errors can be reported
        without the penalty of closing the TCP connection. Clients using
        future versions of HTTP might optimistically try a new feature, but
        if communicating with an older server, retry with old semantics
        after an error is reported.

HTTP implementations SHOULD implement persistent connections.

8.1.2 Overall Operation

A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior of any HTTP connection. That is, unless otherwise indicated, the client may assume that the server will maintain a persistent connection.

Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling takes place using the Connection header field. Once a close has been signaled, the client MUST not send any more requests on that connection.

8.1.2.1 Negotiation

An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to maintain a persistent connection unless a Connection header including the connection-token "close" was sent in the request. If the server chooses to close the connection immediately after sending the response, it SHOULD send a Connection header including the connection-token close.

An HTTP/1.1 client MAY expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains a Connection header with the connection-token close. In case the client does not want to maintain a connection for more than that request, it SHOULD send a Connection header including the connection-token close.

If either the client or the server sends the close token in the Connection header, that request becomes the last one for the connection.

Clients and servers SHOULD NOT assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See section 19.7.1 for more information on backwards compatibility with HTTP/1.0 clients.

In order to remain persistent, all messages on the connection must have a self-defined message length (i.e., one not defined by closure of the connection), as described in section 4.4.

8.1.2.2 Pipelining

A client that supports persistent connections MAY "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server MUST send its responses to those requests in the same order that the requests were received.

Clients which assume persistent connections and pipeline immediately after connection establishment SHOULD be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it MUST NOT pipeline before it knows the connection is persistent. Clients MUST also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.

8.1.3 Proxy Servers

It is especially important that proxies correctly implement the properties of the Connection header field as specified in 14.2.1.

The proxy server MUST signal persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects to. Each persistent connection applies to only one transport link.

A proxy server MUST NOT establish a persistent connection with an HTTP/1.0 client.

8.1.4 Practical Considerations

Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length of this time-out for either the client or the server.

When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side's close promptly it could cause unnecessary resource drain on the network.

A client, server, or proxy MAY close the transport connection at any time. For example, a client MAY have started to send a new request at the same time that the server has decided to close the "idle" connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress.

This means that clients, servers, and proxies MUST be able to recover from asynchronous close events. Client software SHOULD reopen the transport connection and retransmit the aborted request without user interaction so long as the request method is idempotent (see section

9.1.2); other methods MUST NOT be automatically retried, although user agents MAY offer a human operator the choice of retrying the request.

However, this automatic retry SHOULD NOT be repeated if the second request fails.

Servers SHOULD always respond to at least one request per connection, if at all possible. Servers SHOULD NOT close a connection in the middle of transmitting a response, unless a network or client failure is suspected.

Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD maintain AT MOST 2 connections with any server or proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion of the Internet or other networks.

8.2 Message Transmission Requirements

General requirements:

• HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.

• An HTTP/1.1 (or later) client sending a message-body SHOULD monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it SHOULD immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (section 3.6), a zero length chunk and empty footer MAY be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header, the client MUST close the connection.

o An HTTP/1.1 (or later) client MUST be prepared to accept a 100

(Continue) status followed by a regular response.

• An HTTP/1.1 (or later) server that receives a request from a HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue) response; it SHOULD either wait for the request to be completed normally (thus avoiding an interrupted request) or close the connection prematurely.

Upon receiving a method subject to these requirements from an HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either respond with 100 (Continue) status and continue to read from the input stream, or respond with an error status. If it responds with an error status, it MAY close the transport (TCP) connection or it MAY continue to read and discard the rest of the request. It MUST NOT perform the requested method if it returns an error status.

Clients SHOULD remember the version number of at least the most recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or later response from the server, and it sees the connection close before receiving any status from the server, the client SHOULD retry the request without user interaction so long as the request method is idempotent (see section 9.1.2); other methods MUST NOT be automatically retried, although user agents MAY offer a human operator the choice of retrying the request.. If the client does retry the request, the client

• MUST first send the request header fields, and then

• MUST wait for the server to respond with either a 100 (Continue) response, in which case the client should continue, or with an error status.

If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from the server, it should assume that the server implements HTTP/1.0 or older and will not use the 100 (Continue) response. If in this case the client sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry the request to this HTTP/1.0 server, it should use the following "binary exponential backoff" algorithm to be assured of obtaining a reliable response:

1. Initiate a new connection to the server

2. Transmit the request-headers

3. Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection), or to a constant value of 5 seconds if the round-trip time is not available.

4. Compute T = R * (2**N), where N is the number of previous retries of this request.

5. Wait either for an error response from the server, or for T seconds (whichever comes first)

6. If no error response is received, after T seconds transmit the body of the request.

7. If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response is received, or the user becomes impatient and terminates the retry process.

No matter what the server version, if an error status is received, the client

• MUST NOT continue and

• MUST close the connection if it has not completed sending the message.

An HTTP/1.1 (or later) client that sees the connection close after receiving a 100 (Continue) but before receiving any other status SHOULD retry the request, and need not wait for 100 (Continue) response (but MAY do so if this simplifies the implementation).

9 Method Definitions

The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers.

The Host request-header field (section 14.23) MUST accompany all HTTP/1.1 requests.

9.1 Safe and Idempotent Methods

9.1.1 Safe Methods

Implementers should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they may take which may have an unexpected significance to themselves or others.

In particular, the convention has been established that the GET and HEAD methods should never have the significance of taking an action other than retrieval. These methods should be considered "safe." This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.

Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.

9.1.2 Idempotent Methods

   Methods may also have the property of "idempotence" in that (aside
   from error or expiration issues) the side-effects of  N > 0 identical
   requests is the same as for a single request. The methods GET, HEAD,
   PUT and DELETE share this property.

9.2 OPTIONS

The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.

Unless the server's response is an error, the response MUST NOT include entity information other than what can be considered as communication options (e.g., Allow is appropriate, but Content-Type is not). Responses to this method are not cachable.

If the Request-URI is an asterisk ("*"), the OPTIONS request is intended to apply to the server as a whole. A 200 response SHOULD include any header fields which indicate optional features implemented by the server (e.g., Public), including any extensions not defined by this specification, in addition to any applicable general or response-header fields. As described in section 5.1.2, an "OPTIONS *" request can be applied through a proxy by specifying the destination server in the Request-URI without any path information.

If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource. A 200 response SHOULD include any header fields which indicate optional features implemented by the server and applicable to that resource (e.g., Allow), including any extensions not defined by this specification, in addition to any applicable general or response-header fields. If the OPTIONS request passes through a proxy, the proxy MUST edit the response to exclude those options which apply to a proxy's capabilities and which are known to be unavailable through that proxy.

9.3 GET

The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field. A conditional GET method requests that the entity be transferred only under the circumstances described by the conditional header field(s). The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred, as described in section 14.36. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client.

The response to a GET request is cachable if and only if it meets the requirements for HTTP caching described in section 13.

9.4 HEAD

The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the entity implied by the request without transferring the entity-body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification.

The response to a HEAD request may be cachable in the sense that the information contained in the response may be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current entity (as would be indicated by a change in Content-Length, Content-MD5, ETag or Last-Modified), then the cache MUST treat the cache entry as stale.

9.5 POST

The POST method is used to request that the destination server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions:

• Annotation of existing resources;

• Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles;

• Providing a block of data, such as the result of submitting a form, to a data-handling process;

• Extending a database through an append operation.

The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database.

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result.

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity which describes the status of the request and refers to the new resource, and a Location header (see section 14.30).

Responses to this method are not cachable, unless the response includes appropriate Cache-Control or Expires header fields. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cachable resource.

POST requests must obey the message transmission requirements set out in section 8.2.

9.6 PUT

The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server MUST inform the user agent via the 201 (Created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. The recipient of the entity MUST NOT ignore any Content-* (e.g. Content-Range) headers that it does not understand or implement and MUST return a 501 (Not Implemented) response in such cases.

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries should be treated as stale. Responses to this method are not cachable.

The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity. That resource may be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI, it MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request.

A single resource MAY be identified by many different URIs. For example, an article may have a URI for identifying "the current version" which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI may result in several other URIs being defined by the origin server.

HTTP/1.1 does not define how a PUT method affects the state of an origin server.

PUT requests must obey the message transmission requirements set out in section 8.2.

9.7 DELETE

The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD not indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the response is OK but does not include an entity.

If the request passes through a cache and the Request-URI identifies one or more currently cached entities, those entries should be treated as stale. Responses to this method are not cachable.

9.8 TRACE

The TRACE method is used to invoke a remote, application-layer loop- back of the request message. The final recipient of the request SHOULD reflect the message received back to the client as the entity-body of a 200 (OK) response. The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of zero (0) in the request (see section 14.31). A TRACE request MUST NOT include an entity.

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (section 14.44) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.

If successful, the response SHOULD contain the entire request message in the entity-body, with a Content-Type of "message/http". Responses to this method MUST NOT be cached.

10 Status Code Definitions

Each Status-Code is described below, including a description of which method(s) it can follow and any metainformation required in the response.

10.1 Informational 1xx

This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.

10.1.1 100 Continue

The client may continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed.

10.1.2 101 Switching Protocols

The server understands and is willing to comply with the client's request, via the Upgrade message header field (section 14.41), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response.

The protocol should only be switched when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol may be advantageous when delivering resources that use such features.

10.2 Successful 2xx

This class of status code indicates that the client's request was successfully received, understood, and accepted.

10.2.1 200 OK

The request has succeeded. The information returned with the response is dependent on the method used in the request, for example:

   GET  an entity corresponding to the requested resource is sent in the
   
        response;

HEAD the entity-header fields corresponding to the requested resource

are sent in the response without any message-body;

POST an entity describing or containing the result of the action;

TRACE an entity containing the request message as received by the end

server.

10.2.2 201 Created

The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced by the URI(s) returned in the entity of the response, with the most specific URL for the resource given by a Location header field. The origin server MUST create the resource before returning the 201 status code. If the action cannot be carried out immediately, the server should respond with 202 (Accepted) response instead.

10.2.3 202 Accepted

The request has been accepted for processing, but the processing has not been completed. The request MAY or MAY NOT eventually be acted upon, as it MAY be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this.

The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.

10.2.4 203 Non-Authoritative Information

The returned metainformation in the entity-header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented MAY be a subset or superset of the original version. For example, including local annotation information about the resource MAY result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be 200 (OK).

10.2.5 204 No Content

The server has fulfilled the request but there is no new information to send back. If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be sent. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view. The response MAY include new metainformation in the form of entity-headers, which SHOULD apply to the document currently in the user agent's active view.

The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response MUST NOT include an entity.

10.2.7 206 Partial Content

The server has fulfilled the partial GET request for the resource. The request must have included a Range header field (section 14.36) indicating the desired range. The response MUST include either a Content-Range header field (section 14.17) indicating the range included with this response, or a multipart/byteranges Content-Type including Content-Range fields for each part. If multipart/byteranges is not used, the Content-Length header field in the response MUST match the actual number of OCTETs transmitted in the message-body.

A cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent SHOULD NOT automatically redirect a request more than 5 times, since such redirections usually indicate an infinite loop.

10.3.1 300 Multiple Choices

The requested resource corresponds to any one of a set of representations, each with its own specific location, and agent- driven negotiation information (section 12) is being provided so that the user (or user agent) can select a preferred representation and redirect its request to that location.

Unless it was a HEAD request, the response SHOULD include an entity containing a list of resource characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content- Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. However, this specification does not define any standard for such automatic selection.

If the server has a preferred choice of representation, it SHOULD include the specific URL for that representation in the Location field; user agents MAY use the Location field value for automatic redirection. This response is cachable unless indicated otherwise.

10.3.2 301 Moved Permanently

The requested resource has been assigned a new permanent URI and any future references to this resource SHOULD be done using one of the returned URIs. Clients with link editing capabilities SHOULD automatically re-link references to the Request-URI to one or more of the new references returned by the server, where possible. This response is cachable unless indicated otherwise.

If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).

If the 301 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.

Note: When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.

10.3.3 302 Moved Temporarily

The requested resource resides temporarily under a different URI. Since the redirection may be altered on occasion, the client SHOULD continue to use the Request-URI for future requests. This response is only cachable if indicated by a Cache-Control or Expires header field.

If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).

If the 302 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued.

Note: When automatically redirecting a POST request after receiving a 302 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request.

10.3.4 303 See Other

The response to the request can be found under a different URI and SHOULD be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new URI is not a substitute reference for the originally requested resource. The 303 response is not cachable, but the response to the second (redirected) request MAY be cachable.

If the new URI is a location, its URL SHOULD be given by the Location field in the response. Unless the request method was HEAD, the entity of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s).

10.3.5 304 Not Modified

If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The response MUST NOT contain a message-body.

The response MUST include the following header fields:

• Date

• ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request

  o  Expires, Cache-Control, and/or Vary, if the field-value might
  
     differ from that sent in any previous response for the same variant

If the conditional GET used a strong cache validator (see section 13.3.3), the response SHOULD NOT include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response MUST NOT include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers.

If a 304 response indicates an entity not currently cached, then the cache MUST disregard the response and repeat the request without the conditional.

If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.

The 304 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.

10.3.6 305 Use Proxy

The requested resource MUST be accessed through the proxy given by the Location field. The Location field gives the URL of the proxy. The recipient is expected to repeat the request via the proxy.

10.4 Client Error 4xx

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included entity to the user.

Note: If the client is sending data, a server implementation using TCP should be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.

10.4.1 400 Bad Request

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

10.4.2 401 Unauthorized

The request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.46) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity MAY include relevant diagnostic information. HTTP access authentication is explained in section 11.

10.4.3 402 Payment Required

This code is reserved for future use.

10.4.4 403 Forbidden

The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.

If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

10.4.6 405 Method Not Allowed

The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response MUST include an Allow header containing a list of valid methods for the requested resource.

10.4.7 406 Not Acceptable

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

Unless it was a HEAD request, the response SHOULD include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice may be performed automatically. However, this specification does not define any standard for such automatic selection.

Note: HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable. If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.

10.4.8 407 Proxy Authentication Required

This code is similar to 401 (Unauthorized), but indicates that the client MUST first authenticate itself with the proxy. The proxy MUST return a Proxy-Authenticate header field (section 14.33) containing a challenge applicable to the proxy for the requested resource. The client MAY repeat the request with a suitable Proxy-Authorization header field (section 14.34). HTTP access authentication is explained in section 11.

10.4.9 408 Request Timeout

The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time.

10.4.10 409 Conflict

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user agent to fix the problem; however, that may not be possible and is not required.

Conflicts are most likely to occur in response to a PUT request. If versioning is being used and the entity being PUT includes changes to a resource which conflict with those made by an earlier (third-party) request, the server MAY use the 409 response to indicate that it can't complete the request. In this case, the response entity SHOULD contain a list of the differences between the two versions in a format defined by the response Content-Type.

10.4.11 410 Gone

The requested resource is no longer available at the server and no forwarding address is known. This condition SHOULD be considered permanent. Clients with link editing capabilities SHOULD delete references to the Request-URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) SHOULD be used instead. This response is cachable unless indicated otherwise.

The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time -- that is left to the discretion of the server owner.

10.4.12 411 Length Required

The server refuses to accept the request without a defined Content- Length. The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the message-body in the request message.

10.4.13 412 Precondition Failed

The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metainformation (header field data) and thus prevent the requested method from being applied to a resource other than the one intended.

10.4.14 413 Request Entity Too Large

The server is refusing to process a request because the request entity is larger than the server is willing or able to process. The server may close the connection to prevent the client from continuing the request.

If the condition is temporary, the server SHOULD include a Retry- After header field to indicate that it is temporary and after what time the client may try again.

10.4.15 414 Request-URI Too Long

The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URL "black hole" of redirection (e.g., a redirected URL prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URI.

10.4.16 415 Unsupported Media Type

The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.

10.5 Server Error 5xx

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. User agents SHOULD display any included entity to the user. These response codes are applicable to any request method.

10.5.1 500 Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

10.5.2 501 Not Implemented

The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.

10.5.3 502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.

10.5.4 503 Service Unavailable

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay may be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.

Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.

10.5.5 504 Gateway Timeout

The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server it accessed in attempting to complete the request.

10.5.6 505 HTTP Version Not Supported

The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in section 3.1, other than with this error message. The response SHOULD contain an entity describing why that version is not supported and what other protocols are supported by that server.

11 Access Authentication

HTTP provides a simple challenge-response authentication mechanism which MAY be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible, case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs which carry the parameters necessary for achieving authentication via that scheme.

          auth-scheme    = token
          
          auth-param     = token "=" quoted-string

The 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response MUST include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource.

challenge = auth-scheme 1*SP realm *( "," auth-param )

          realm          = "realm" "=" realm-value
          realm-value    = quoted-string

The realm attribute (case-insensitive) is required for all authentication schemes which issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL (see section 5.1.2) of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme.

A user agent that wishes to authenticate itself with a server-- usually, but not necessarily, after receiving a 401 or 411 response- -MAY do so by including an Authorization header field with the request. The Authorization field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested.

          credentials    = basic-credentials
                         | auth-scheme #auth-param

The domain over which credentials can be automatically applied by a user agent is determined by the protection space. If a prior request has been authorized, the same credentials MAY be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server.

If the server does not wish to accept the credentials sent with a request, it SHOULD return a 401 (Unauthorized) response. The response MUST include a WWW-Authenticate header field containing the (possibly new) challenge applicable to the requested resource and an entity explaining the refusal.

The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms MAY be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification.

Proxies MUST be completely transparent regarding user agent authentication. That is, they MUST forward the WWW-Authenticate and Authorization headers untouched, and follow the rules found in section 14.8.

HTTP/1.1 allows a client to pass authentication information to and from a proxy via the Proxy-Authenticate and Proxy-Authorization headers.

11.1 Basic Authentication Scheme

The "basic" authentication scheme is based on the model that the user agent must authenticate itself with a user-ID and a password for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms on that server. The server will service the request only if it can validate the user-ID and password for the protection space of the Request-URI. There are no optional authentication parameters.

Upon receipt of an unauthorized request for a URI within the protection space, the server MAY respond with a challenge like the following:

WWW-Authenticate:

                            Basic realm="WallyWorld"

where "WallyWorld" is the string assigned by the server to identify the protection space of the Request-URI.

   To receive authorization, the client sends the userid and password,
   separated by a single colon (":") character, within a base64  encoded
   string in the credentials.
   
          basic-credentials = "Basic" SP basic-cookie
   
          basic-cookie   = <base64 [7] encoding of user-pass,
                           except not limited to 76 char/line>
   
          user-pass   = userid ":" password
   
          userid      = *<TEXT excluding ":">
   
          password    = *TEXT

Userids might be case sensitive.

If the user agent wishes to send the userid "Aladdin" and password "open sesame", it would use the following header field:

Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

See section 15 for security considerations associated with Basic authentication.

11.2 Digest Authentication Scheme

A digest authentication for HTTP is specified in RFC 2069 [32].

12 Content Negotiation

Most HTTP responses include an entity which contains information for interpretation by a human user. Naturally, it is desirable to supply the user with the "best available" entity corresponding to the request. Unfortunately for servers and caches, not all users have the same preferences for what is "best," and not all user agents are equally capable of rendering all entity types. For that reason, HTTP has provisions for several mechanisms for "content negotiation" -- the process of selecting the best representation for a given response when there are multiple representations available.

Note: This is not called "format negotiation" because the alternate representations may be of the same media type, but use different capabilities of that type, be in different languages, etc.

Any response containing an entity-body MAY be subject to negotiation, including error responses.

There are two kinds of content negotiation which are possible in HTTP: server-driven and agent-driven negotiation. These two kinds of negotiation are orthogonal and thus may be used separately or in combination. One method of combination, referred to as transparent negotiation, occurs when a cache uses the agent-driven negotiation information provided by the origin server in order to provide server-driven negotiation for subsequent requests.

12.1 Server-driven Negotiation

If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g. language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).

Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.

Server-driven negotiation has disadvantages:

1. It is impossible for the server to accurately determine what might be

"best" for any given user, since that would require complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want to view it on screen or print it on paper?).

2. Having the user agent describe its capabilities in every request can

be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of the user's privacy.

3. It complicates the implementation of an origin server and the

algorithms for generating responses to a request.

4. It may limit a public cache's ability to use the same response for

multiple user's requests.

HTTP/1.1 includes the following request-header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (section 14.1), Accept- Charset (section 14.2), Accept-Encoding (section 14.3), Accept- Language (section 14.4), and User-Agent (section 14.42). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including information outside the request-header fields or within extension header fields not defined by this specification.

HTTP/1.1 origin servers MUST include an appropriate Vary header field (section 14.43) in any cachable response based on server-driven negotiation. The Vary header field describes the dimensions over which the response might vary (i.e. the dimensions over which the origin server picks its "best guess" response from multiple representations).

HTTP/1.1 public caches MUST recognize the Vary header field when it is included in a response and obey the requirements described in section 13.6 that describes the interactions between caching and content negotiation.

12.2 Agent-driven Negotiation

With agent-driven negotiation, selection of the best representation for a response is performed by the user agent after receiving an initial response from the origin server. Selection is based on a list of the available representations of the response included within the header fields (this specification reserves the field-name Alternates, as described in appendix 19.6.2.1) or entity-body of the initial response, with each representation identified by its own URI. Selection from among the representations may be performed automatically (if the user agent is capable of doing so) or manually by the user selecting from a generated (possibly hypertext) menu.

Agent-driven negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.

Agent-driven negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.

HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) status codes for enabling agent-driven negotiation when the server is unwilling or unable to provide a varying response using server-driven negotiation.

12.3 Transparent Negotiation

Transparent negotiation is a combination of both server-driven and agent-driven negotiation. When a cache is supplied with a form of the list of available representations of the response (as in agent-driven negotiation) and the dimensions of variance are completely understood by the cache, then the cache becomes capable of performing server- driven negotiation on behalf of the origin server for subsequent requests on that resource.

Transparent negotiation has the advantage of distributing the negotiation work that would otherwise be required of the origin server and also removing the second request delay of agent-driven negotiation when the cache is able to correctly guess the right response.

This specification does not define any mechanism for transparent negotiation, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1. An HTTP/1.1 cache performing transparent negotiation MUST include a Vary header field in the response (defining the dimensions of its variance) if it is cachable to ensure correct interoperation with all HTTP/1.1 clients. The agent-driven negotiation information supplied by the origin server SHOULD be included with the transparently negotiated response.

13 Caching in HTTP

HTTP is typically used for distributed information systems, where performance can be improved by the use of response caches. The HTTP/1.1 protocol includes a number of elements intended to make caching work as well as possible. Because these elements are inextricable from other aspects of the protocol, and because they interact with each other, it is useful to describe the basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc.

Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former reduces the number of network round-trips required for many operations; we use an "expiration" mechanism for this purpose (see section 13.2). The latter reduces network bandwidth requirements; we use a "validation" mechanism for this purpose (see section 13.3).

Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and may be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency be relaxed

• only by an explicit protocol-level request when relaxed by client or origin server

• only with an explicit warning to the end user when relaxed by cache or client

Therefore, the HTTP/1.1 protocol provides these important elements:

1. Protocol features that provide full semantic transparency when this is required by all parties.

2. Protocol features that allow an origin server or user agent to explicitly request and control non-transparent operation.

3. Protocol features that allow a cache to attach warnings to responses that do not preserve the requested approximation of semantic transparency.

A basic principle is that it must be possible for the clients to detect any potential relaxation of semantic transparency.

Note: The server, cache, or client implementer may be faced with design decisions not explicitly discussed in this specification. If a decision may affect semantic transparency, the implementer ought to err on the side of maintaining transparency unless a careful and complete analysis shows significant benefits in breaking transparency.

13.1.1 Cache Correctness

A correct cache MUST respond to a request with the most up-to-date response held by the cache that is appropriate to the request (see sections 13.2.5, 13.2.6, and 13.12) which meets one of the following conditions:

1. It has been checked for equivalence with what the origin server would have returned by revalidating the response with the origin server (section 13.3);

2. It is "fresh enough" (see section 13.2). In the default case, this means it meets the least restrictive freshness requirement of the client, server, and cache (see section 14.9); if the origin server so specifies, it is the freshness requirement of the origin server alone.

3. It includes a warning if the freshness demand of the client or the origin server is violated (see section 13.1.5 and 14.45).

4. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or error (4xx or 5xx) response message.

If the cache can not communicate with the origin server, then a correct cache SHOULD respond as above if the response can be correctly served from the cache; if not it MUST return an error or warning indicating that there was a communication failure.

If a cache receives a response (either an entire response, or a 304 (Not Modified) response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache SHOULD forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache SHOULD NOT attempt to revalidate a response simply because that response became stale in transit; this might lead to an infinite loop. An user agent that receives a stale response without a Warning MAY display a warning indication to the user.

13.1.2 Warnings

Whenever a cache returns a response that is neither first-hand nor "fresh enough" (in the sense of condition 2 in section 13.1.1), it must attach a warning to that effect, using a Warning response- header. This warning allows clients to take appropriate action.

Warnings may be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish these responses from true failures.

Warnings are always cachable, because they never weaken the transparency of a response. This means that warnings can be passed to HTTP/1.0 caches without danger; such caches will simply pass the warning along as an entity-header in the response.

Warnings are assigned numbers between 0 and 99. This specification defines the code numbers and meanings of each currently assigned warnings, allowing a client or cache to take automated action in some (but not all) cases.

Warnings also carry a warning text. The text may be in any appropriate natural language (perhaps based on the client's Accept headers), and include an optional indication of what character set is used.

Multiple warnings may be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number. For example, a server may provide the same warning with texts in both English and Basque.

When multiple warnings are attached to a response, it may not be practical or reasonable to display all of them to the user. This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but does suggest some heuristics.

The Warning header and the currently defined warnings are described in section 14.45.

13.1.3 Cache-control Mechanisms

The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches. In some cases, a server or client may need to provide explicit directives to the HTTP caches. We use the Cache-Control header for this purpose.

The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between header values, the most restrictive interpretation should be applied (that is, the one that is most likely to preserve semantic transparency). However, in some cases, Cache-Control directives are explicitly specified as weakening the approximation of semantic transparency (for example, "max-stale" or "public").

The Cache-Control directives are described in detail in section 14.9.

13.1.4 Explicit User Agent Warnings

Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent may allow the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually add "Cache-Control: max-stale=3600" to every request. The user should have to explicitly request either non-transparent behavior, or behavior that results in abnormally ineffective caching.

If the user has overridden the basic caching mechanisms, the user agent should explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other visual indicator.

If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user agent should continually display an indication (for example, a picture of currency in flames) so that the user does not inadvertently consume excess resources or suffer from excessive latency.

13.1.5 Exceptions to the Rules and Warnings

In some cases, the operator of a cache may choose to configure it to return stale responses even when not requested by clients. This decision should not be made lightly, but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin server. Whenever a cache returns a stale response, it MUST mark it as such (using a Warning header). This allows the client software to alert the user that there may be a potential problem.

It also allows the user agent to take steps to obtain a first-hand or fresh response. For this reason, a cache SHOULD NOT return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply for technical or policy reasons.

13.1.6 Client-controlled Behavior

While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are the primary source of expiration information, in some cases the client may need to control a cache's decision about whether to return a cached response without validating it. Clients do this using several directives of the Cache-Control header.

A client's request may specify the maximum age it is willing to accept of an unvalidated response; specifying a value of zero forces the cache(s) to revalidate all responses. A client may also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior of caches, and so cannot further relax the cache's approximation of semantic transparency.

A client may also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on the caches, and so may violate the origin server's specified constraints on semantic transparency, but may be necessary to support disconnected operation, or high availability in the face of poor connectivity.

13.2 Expiration Model

13.2.1 Server-Specified Expiration

HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future, indicating that a response may be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server.

Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic transparency, as long as the server's expiration times are carefully chosen.

The expiration mechanism applies only to responses taken from a cache and not to first-hand responses forwarded immediately to the requesting client.

If an origin server wishes to force a semantically transparent cache to validate every request, it may assign an explicit expiration time in the past. This means that the response is always stale, and so the cache SHOULD validate it before using it for subsequent requests. See section 14.9.4 for a more restrictive way to force revalidation.

If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it should use the "must-revalidate" Cache-Control directive (see section 14.9).

Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header.

An expiration time cannot be used to force a user agent to refresh its display or reload a resource; its semantics apply only to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource is initiated. See section 13.13 for explanation of the difference between caches and history mechanisms.

13.2.2 Heuristic Expiration

Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results. Since heuristic expiration times may compromise semantic transparency, they should be used cautiously, and we encourage origin servers to provide explicit expiration times as much as possible.

13.2.3 Age Calculations

In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how to calculate the latter in section 13.2.4; this section describes how to calculate the age of a response or cache entry.

In this discussion, we use the term "now" to mean "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially hosts running origin servers and caches, should use NTP [28] or some similar protocol to synchronize their clocks to a globally accurate time standard.

Also note that HTTP/1.1 requires origin servers to send a Date header with every response, giving the time at which the response was generated. We use the term "date_value" to denote the value of the Date header, in a form appropriate for arithmetic operations.

HTTP/1.1 uses the Age response-header to help convey age information between caches. The Age header value is the sender's estimate of the amount of time since the response was generated at the origin server. In the case of a cached response that has been revalidated with the origin server, the Age value is based on the time of revalidation, not of the original response.

In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths.

We use the term "age_value" to denote the value of the Age header, in a form appropriate for arithmetic operations.

A response's age can be calculated in two entirely independent ways:

1. now minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero.

2. age_value, if all of the caches along the response path implement HTTP/1.1.

Given that we have two independent ways to compute the age of a response when it is received, we can combine these as

corrected_received_age = max(now - date_value, age_value)

and as long as we have either nearly synchronized clocks or all-

HTTP/1.1 paths, one gets a reliable (conservative) result.

Note that this correction is applied at each HTTP/1.1 cache along the path, so that if there is an HTTP/1.0 cache in the path, the correct received age is computed as long as the receiving cache's clock is nearly in sync. We don't need end-to-end clock synchronization (although it is good to have), and there is no explicit clock synchronization step.

Because of network-imposed delays, some significant interval may pass from the time that a server generates a response and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low ages.

Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation, we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age value is received, it MUST be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm results in conservative behavior no matter how much delay is experienced. So, we compute:

corrected_initial_age = corrected_received_age

• (now - request_time)

where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.

Summary of age calculation algorithm, when a cache receives a response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */
      apparent_age = max(0, response_time - date_value);
      
      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

When a cache sends a response, it must add to the corrected_initial_age the amount of time that the response was resident locally. It must then transmit this total age, using the Age header, to the next recipient cache.

Note that a client cannot reliably tell that a response is first- hand, but the presence of an Age header indicates that a response is definitely not first-hand. Also, if the Date in a response is earlier than the client's local request time, the response is probably not first-hand (in the absence of serious clock skew).

13.2.4 Expiration Calculations

In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is calculated as described in section 13.2.3; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. In the discussion below, the values can be represented in any form appropriate for arithmetic operations.

We use the term "expires_value" to denote the value of the Expires header. We use the term "max_age_value" to denote an appropriate value of the number of seconds carried by the max-age directive of the Cache-Control header in a response (see section 14.10.

The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:

         freshness_lifetime = max_age_value

Otherwise, if Expires is present in the response, the calculation is:

freshness_lifetime = expires_value - date_value

Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server.

If neither Expires nor Cache-Control: max-age appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. If the value is greater than 24 hours, the cache must attach Warning 13 to any response whose age is more than 24 hours if such warning has not already been added.

Also, if the response does have a Last-Modified time, the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.

The calculation to determine if a response has expired is quite simple:

response_is_fresh = (freshness_lifetime > current_age)

13.2.5 Disambiguating Expiration Values

Because expiration values are assigned optimistically, it is possible for two caches to contain fresh values for the same resource that are different.

If a client performing a retrieval receives a non-first-hand response for a request that was already fresh in its own cache, and the Date header in its existing cache entry is newer than the Date on the new response, then the client MAY ignore the response. If so, it MAY retry the request with a "Cache-Control: max-age=0" directive (see section 14.9), to force a check with the origin server.

If a cache has two fresh responses for the same representation with different validators, it MUST use the one with the more recent Date header. This situation may arise because the cache is pooling responses from other caches, or because a client has asked for a reload or a revalidation of an apparently fresh cache entry.

13.2.6 Disambiguating Multiple Responses

Because a client may be receiving responses via multiple paths, so that some responses flow through one set of caches and other responses flow through a different set of caches, a client may receive responses in an order different from that in which the origin server sent them. We would like the client to use the most recently generated response, even if older responses are still apparently fresh.

Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response intentionally carries an earlier expiration time. However, the HTTP/1.1 specification requires the transmission of Date headers on every response, and the Date values are ordered to a granularity of one second.

When a client tries to revalidate a cache entry, and the response it receives contains a Date header that appears to be older than the one for the existing entry, then the client SHOULD repeat the request unconditionally, and include

Cache-Control:

                         max-age=0

to force any intermediate caches to validate their copies directly with the origin server, or

Cache-Control:

                         no-cache

to force any intermediate caches to obtain a new copy from the origin server.

If the Date values are equal, then the client may use either response (or may, if it is being extremely prudent, request a new response). Servers MUST NOT depend on clients being able to choose deterministically between responses generated during the same second, if their expiration times overlap.

13.3 Validation Model

When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call this "validating" the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if the cached entry is good, and we do not want to pay the overhead of an extra round trip if the cached entry is invalid, the HTTP/1.1 protocol supports the use of conditional methods.

The key protocol features for supporting conditional methods are those concerned with "cache validators." When an origin server generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (user agent or proxy cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated validator in the request.

The server then checks that validator against the current validator for the entity, and, if they match, it responds with a special status code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it returns a full response (including entity-body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round trip if it does not match.

Note: the comparison functions used to decide if validators match are defined in section 13.3.3.

In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional.

The protocol includes both positive and negative senses of cache- validating conditions. That is, it is possible to request either that a method be performed if and only if a validator matches or if and only if no validators match.

Note: a response that lacks a validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited by a Cache-Control directive. However, a cache cannot do a conditional retrieval if it does not have a validator for the entity, which means it will not be refreshable after it expires.

13.3.1 Last-modified Dates

The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered to be valid if the entity has not been modified since the Last-Modified value.

13.3.2 Entity Tag Cache Validators

The ETag entity-header field value, an entity tag, provides for an "opaque" cache validator. This may allow more reliable validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date values is not sufficient, or where the origin server wishes to avoid certain paradoxes that may arise from the use of modification dates.

Entity Tags are described in section 3.11. The headers used with entity tags are described in sections 14.20, 14.25, 14.26 and 14.43.

13.3.3 Weak and Strong Validators

Since both origin servers and caches will compare two validators to decide if they represent the same or different entities, one normally would expect that if the entity (the entity-body or any entity- headers) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a "strong validator."

However, there may be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the entity change. A validator that does not always change when the resource changes is a "weak validator."

Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities.

Note: One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed.

An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible that the resource may be modified twice during a single second.

Support for weak validators is optional; however, weak validators allow for more efficient caching of equivalent objects; for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during that period is likely "good enough" to be equivalent.

A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators.

Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client may end up with an internally inconsistent entity.

The only function that the HTTP/1.1 protocol defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not:

  o  The strong comparison function: in order to be considered equal,
     both validators must be identical in every way, and neither may be
     weak.
  o  The weak comparison function: in order to be considered equal, both
     validators must be identical in every way, but either or both of
     them may be tagged as "weak" without affecting the result.

The weak comparison function MAY be used for simple (non-subrange)

GET requests. The strong comparison function MUST be used in all other cases.

An entity tag is strong unless it is explicitly tagged as weak. Section 3.11 gives the syntax for entity tags.

A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules:

  o  The validator is being compared by an origin server to the actual
     current validator for the entity and,
  o  That origin server reliably knows that the associated entity did
     not change twice during the second covered by the presented
     validator.
or

  o  The validator is about to be used by a client in an If-Modified-
     Since or If-Unmodified-Since header, because the client has a cache
     entry for the associated entity, and
  o  That cache entry includes a Date value, which gives the time when
     the origin server sent the original response, and
  o  The presented Last-Modified time is at least 60 seconds before the
     Date value.
or

  o  The validator is being compared by an intermediate cache to the
     validator stored in its cache entry for the entity, and
  o  That cache entry includes a Date value, which gives the time when
     the origin server sent the original response, and
  o  The presented Last-Modified time is at least 60 seconds before the
     Date value.

This method relies on the fact that if two different responses were sent by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60- second limit guards against the possibility that the Date and Last- Modified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation may use a value larger than 60 seconds, if it is believed that 60 seconds is too short.

If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it may do this only if the Last-Modified time is strong in the sense described here.

A cache or origin server receiving a cache-conditional request, other than a full-body GET request, MUST use the strong comparison function to evaluate the condition.

These rules allow HTTP/1.1 caches and clients to safely perform sub- range retrievals on values that have been obtained from HTTP/1.0 servers.

13.3.4 Rules for When to Use Entity Tags and Last-modified Dates

We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types should be used, and for what purposes.

HTTP/1.1 origin servers:

  o  SHOULD send an entity tag validator unless it is not feasible to
     generate one.
  o  MAY send a weak entity tag instead of a strong entity tag, if
     performance considerations support the use of weak entity tags, or
     if it is unfeasible to send a strong entity tag.
  o  SHOULD send a Last-Modified value if it is feasible to send one,
     unless the risk of a breakdown in semantic transparency that could
     result from using this date in an If-Modified-Since header would
     lead to serious problems.

In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value.

In order to be legal, a strong entity tag MUST change whenever the associated entity value changes in any way. A weak entity tag SHOULD change whenever the associated entity changes in a semantically significant way.

Note: in order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity tag value for two different entities, or reusing a specific weak entity tag value for two semantically different entities. Cache entries may persist for arbitrarily long periods, regardless of expiration times, so it may be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past.

HTTP/1.1 clients:

• If an entity tag has been provided by the origin server, MUST use that entity tag in any cache-conditional request (using If-Match or If-None-Match).

     o  If only a Last-Modified value has been provided by the origin
        server, SHOULD use that value in non-subrange cache-conditional
        requests (using If-Modified-Since).
     o  If only a Last-Modified value has been provided by an HTTP/1.0
        origin server, MAY use that value in subrange cache-conditional
        requests (using If-Unmodified-Since:). The user agent should
        provide a way to disable this, in case of difficulty.
     o  If both an entity tag and a Last-Modified value have been
        provided by the origin server, SHOULD use both validators in
        cache-conditional requests. This allows both HTTP/1.0 and
        HTTP/1.1 caches to respond appropriately.

An HTTP/1.1 cache, upon receiving a request, MUST use the most restrictive validator when deciding whether the client's cache entry matches the cache's own cache entry. This is only an issue when the request contains both an entity tag and a last-modified-date validator (If-Modified-Since or If-Unmodified-Since).

A note on rationale: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non- redundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive.

HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers should not provide one.

13.3.5 Non-validating Conditionals

The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry.

13.4 Response Cachability

Unless specifically constrained by a Cache-Control (section 14.9) directive, a caching system may always store a successful response (see section 13.8) as a cache entry, may return it without validation if it is fresh, and may return it after successful validation. If there is neither a cache validator nor an explicit expiration time associated with a response, we do not expect it to be cached, but certain caches may violate this expectation (for example, when little or no network connectivity is available). A client can usually detect that such a response was taken from a cache by comparing the Date header to the current time.

Note that some HTTP/1.0 caches are known to violate this expectation without providing any Warning.

However, in some cases it may be inappropriate for a cache to retain an entity, or to return it in response to a subsequent request. This may be because absolute semantic transparency is deemed necessary by the service author, or because of security or privacy considerations. Certain Cache-Control directives are therefore provided so that the server can indicate that certain resource entities, or portions thereof, may not be cached regardless of other considerations.

Note that section 14.8 normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization header.

A response received with a status code of 200, 203, 206, 300, 301 or 410 may be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a Cache-Control directive prohibits caching. However, a cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial Content) responses.

A response received with any other status code MUST NOT be returned in a reply to a subsequent request unless there are Cache-Control directives or another header(s) that explicitly allow it. For example, these include the following: an Expires header (section 14.21); a "max-age", "must-revalidate", "proxy-revalidate", "public" or "private" Cache-Control directive (section 14.9).

13.5 Constructing Responses From Caches

The purpose of an HTTP cache is to store information received in response to requests, for use in responding to future requests. In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a cache entry based on a previous response, it may have to combine parts of a new response with what is held in the cache entry.

13.5.1 End-to-end and Hop-by-hop Headers

For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories:

  o  End-to-end headers, which must be transmitted to the
     ultimate recipient of a request or response. End-to-end
     headers in responses must be stored as part of a cache entry
     and transmitted in any response formed from a cache entry.
  o  Hop-by-hop headers, which are meaningful only for a single
     transport-level connection, and are not stored by caches or
     forwarded by proxies.

The following HTTP/1.1 headers are hop-by-hop headers:

o Connection
o Keep-Alive
o Public
o Proxy-Authenticate
o Transfer-Encoding
o Upgrade

All other headers defined by HTTP/1.1 are end-to-end headers.

Hop-by-hop headers introduced in future versions of HTTP MUST be listed in a Connection header, as described in section 14.10.

13.5.2 Non-modifiable Headers

Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers. A cache or non-caching proxy SHOULD NOT modify an end-to-end header unless the definition of that header requires or specifically allows that.

A cache or non-caching proxy MUST NOT modify any of the following fields in a request or response, nor may it add any of these fields if not already present:

o Content-Location
o ETag
o Expires
o Last-Modified

A cache or non-caching proxy MUST NOT modify or add any of the following fields in a response that contains the no-transform Cache- Control directive, or in any request:

     o  Content-Encoding
     o  Content-Length
     o  Content-Range
     o  Content-Type

A cache or non-caching proxy MAY modify or add these fields in a response that does not include no-transform, but if it does so, it MUST add a Warning 14 (Transformation applied) if one does not already appear in the response.

Warning: unnecessary modification of end-to-end headers may cause authentication failures if stronger authentication mechanisms are introduced in later versions of HTTP. Such authentication mechanisms may rely on the values of header fields not listed here.

13.5.3 Combining Headers

When a cache makes a validating request to a server, and the server provides a 304 (Not Modified) response, the cache must construct a response to send to the requesting client. The cache uses the entity-body stored in the cache entry as the entity-body of this outgoing response. The end-to-end headers stored in the cache entry are used for the constructed response, except that any end-to-end headers provided in the 304 response MUST replace the corresponding headers from the cache entry. Unless the cache decides to remove the cache entry, it MUST also replace the end-to-end headers stored with the cache entry with corresponding headers received in the incoming response.

In other words, the set of end-to-end headers received in the incoming response overrides all corresponding end-to-end headers stored with the cache entry. The cache may add Warning headers (see section 14.45) to this set.

If a header field-name in the incoming response matches more than one header in the cache entry, all such old headers are replaced.

Note: this rule allows an origin server to use a 304 (Not Modified) response to update any header associated with a previous response for the same entity, although it might not always be meaningful or correct to do so. This rule does not allow an origin server to use a 304 (not Modified) response to entirely delete a header that it had provided with a previous response.

13.5.4 Combining Byte Ranges

A response may transfer only a subrange of the bytes of an entity- body, either because the request included one or more Range specifications, or because a connection was broken prematurely. After several such transfers, a cache may have received several ranges of the same entity-body.

If a cache has a stored non-empty set of subranges for an entity, and an incoming response transfers another subrange, the cache MAY combine the new subrange with the existing set if both the following conditions are met:

     o  Both the incoming response and the cache entry must have a cache
        validator.
     o  The two cache validators must match using the strong comparison
        function (see section 13.3.3).

If either requirement is not meant, the cache must use only the most recent partial response (based on the Date values transmitted with every response, and using the incoming response if these values are equal or missing), and must discard the other partial information.

13.6 Caching Negotiated Responses

Use of server-driven content negotiation (section 12), as indicated by the presence of a Vary header field in a response, alters the conditions and procedure by which a cache can use the response for subsequent requests.

A server MUST use the Vary header field (section 14.43) to inform a cache of what header field dimensions are used to select among multiple representations of a cachable response. A cache may use the selected representation (the entity included with that particular response) for replying to subsequent requests on that resource only when the subsequent requests have the same or equivalent values for all header fields specified in the Vary response-header. Requests with a different value for one or more of those header fields would be forwarded toward the origin server.

If an entity tag was assigned to the representation, the forwarded request SHOULD be conditional and include the entity tags in an If- None-Match header field from all its cache entries for the Request- URI. This conveys to the server the set of entities currently held by the cache, so that if any one of these entities matches the requested entity, the server can use the ETag header in its 304 (Not Modified) response to tell the cache which entry is appropriate. If the entity-tag of the new response matches that of an existing entry, the new response SHOULD be used to update the header fields of the existing entry, and the result MUST be returned to the client.

The Vary header field may also inform the cache that the representation was selected using criteria not limited to the request-headers; in this case, a cache MUST NOT use the response in a reply to a subsequent request unless the cache relays the new request to the origin server in a conditional request and the server responds with 304 (Not Modified), including an entity tag or Content-Location that indicates which entity should be used.

If any of the existing cache entries contains only partial content for the associated entity, its entity-tag SHOULD NOT be included in the If-None-Match header unless the request is for a range that would be fully satisfied by that entry.

If a cache receives a successful response whose Content-Location field matches that of an existing cache entry for the same Request- URI, whose entity-tag differs from that of the existing entry, and whose Date is more recent than that of the existing entry, the existing entry SHOULD NOT be returned in response to future requests, and should be deleted from the cache.

13.7 Shared and Non-Shared Caches

For reasons of security and privacy, it is necessary to make a distinction between "shared" and "non-shared" caches. A non-shared cache is one that is accessible only to a single user. Accessibility in this case SHOULD be enforced by appropriate security mechanisms. All other caches are considered to be "shared." Other sections of this specification place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls.

13.8 Errors or Incomplete Response Cache Behavior

A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) may store the response. However, the cache MUST treat this as a partial response. Partial responses may be combined as described in section 13.5.4; the result might be a full response or might still be partial. A cache MUST NOT return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code. A cache MUST NOT return a partial response using a status code of 200 (OK).

If a cache receives a 5xx response while attempting to revalidate an entry, it may either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously received response unless the cached entry includes the "must-revalidate" Cache-Control directive (see section 14.9).

13.9 Side Effects of GET and HEAD

Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any resources SHOULD NOT have side effects that would lead to erroneous behavior if these responses are taken from a cache. They may still have side effects, but a cache is not required to consider such side effects in its caching decisions. Caches are always expected to observe an origin server's explicit restrictions on caching.

We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those containing a "?" in the rel_path part) to perform operations with significant side effects, caches MUST NOT treat responses to such URLs as fresh unless the server provides an explicit expiration time. This specifically means that responses from HTTP/1.0 servers for such URIs should not be taken from a cache. See section 9.1.1 for related information.

13.10 Invalidation After Updates or Deletions

The effect of certain methods at the origin server may cause one or more existing cache entries to become non-transparently invalid. That is, although they may continue to be "fresh," they do not accurately reflect what the origin server would return for a new request.

There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request that caused the change at the origin server may not have gone through the proxy where a cache entry is stored. However, several rules help reduce the likelihood of erroneous behavior.

In this section, the phrase "invalidate an entity" means that the cache should either remove all instances of that entity from its storage, or should mark these as "invalid" and in need of a mandatory revalidation before they can be returned in response to a subsequent request.

Some HTTP methods may invalidate an entity. This is either the entity referred to by the Request-URI, or by the Location or Content- Location response-headers (if present). These methods are:

o PUT
o DELETE
o POST

In order to prevent denial of service attacks, an invalidation based on the URI in a Location or Content-Location header MUST only be performed if the host part is the same as in the Request-URI.

13.11 Write-Through Mandatory

All methods that may be expected to cause modifications to the origin server's resources MUST be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache MUST NOT reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding response from the inbound server. This does not prevent a cache from sending a 100 (Continue) response before the inbound server has replied.

The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing consistent updates and the problems arising from server, cache, or network failure prior to write-back.

13.12 Cache Replacement

If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) response is received from a resource while any existing responses for the same resource are cached, the cache SHOULD use the new response to reply to the current request. It may insert it into cache storage and may, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response to be returned. If it inserts the new response into cache storage it should follow the rules in section 13.5.3.

Note: a new response that has an older Date header value than existing cached responses is not cachable.

13.13 History Lists

User agents often have history mechanisms, such as "Back" buttons and history lists, which can be used to redisplay an entity retrieved earlier in a session.

History mechanisms and caches are different. In particular history mechanisms SHOULD NOT try to show a semantically transparent view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the user saw at the time when the resource was retrieved.

By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism should display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents.

This should not be construed to prohibit the history mechanism from telling the user that a view may be stale.

Note: if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider it important that users not be presented with error messages or warning messages when they use navigation controls (such as BACK) to view previously fetched resources. Even though sometimes such resources ought not to cached, or ought to expire quickly, user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects of improperly functioning history mechanisms.

14 Header Field Definitions

This section defines the syntax and semantics of all standard HTTP/1.1 header fields. For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.

14.1 Accept

The Accept request-header field can be used to specify certain media types which are acceptable for the response. Accept headers can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image.

          Accept         = "Accept" ":"
                           #( media-range [ accept-params ] )
          
          media-range    = ( "*/*"
                           | ( type "/" "*" )
                           | ( type "/" subtype )
                           ) *( ";" parameter )

accept-params = ";" "q" "=" qvalue *( accept-extension )

          accept-extension = ";" token [ "=" ( token | quoted-string ) ]

The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating all subtypes of that type. The media-range MAY include media type parameters that are applicable to that range.

Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative quality factor. The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params. Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1 (section 3.9). The default value is q=1.

Note: Use of the "q" parameter name to separate media type parameters from Accept extension parameters is due to historical practice. Although this prevents any media type parameter named "q" from being used with a media range, such an event is believed to be unlikely given the lack of any "q" parameters in the IANA media type registry and the rare usage of any media type parameters in Accept. Future media types should be discouraged from registering any parameter named "q".

   The example
   
          Accept: audio/*; q=0.2, audio/basic

SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality."

If no Accept header field is present, then it is assumed that the client accepts all media types. If an Accept header field is present, and if the server cannot send a response which is acceptable according to the combined Accept field value, then the server SHOULD send a 406 (not acceptable) response.

A more elaborate example is

          Accept: text/plain; q=0.5, text/html,
                  text/x-dvi; q=0.8, text/x-c

Verbally, this would be interpreted as "text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity."

Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,

Accept: text/*, text/html, text/html;level=1, */*

have the following precedence:

          1) text/html;level=1
          2) text/html
          3) text/*
          4) */*

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example,

Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,

text/html;level=2;q=0.4, */*;q=0.5

would cause the following values to be associated:

          text/html;level=1         = 1
          text/html                 = 0.7
          text/plain                = 0.3
          image/jpeg                = 0.5
          text/html;level=2         = 0.4
          text/html;level=3         = 0.7

Note: A user agent may be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents, this default set should be configurable by the user.

14.2 Accept-Charset

The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. This field allows clients capable of understanding more comprehensive or special- purpose character sets to signal that capability to a server which is capable of representing documents in those character sets. The ISO- 8859-1 character set can be assumed to be acceptable to all user agents.

Accept-Charset = "Accept-Charset" ":"

1#( charset [ ";" "q" "=" qvalue ] )

Character set values are described in section 3.4. Each charset may be given an associated quality value which represents the user's preference for that charset. The default value is q=1. An example is

          Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

If no Accept-Charset header is present, the default is that any character set is acceptable. If an Accept-Charset header is present, and if the server cannot send a response which is acceptable according to the Accept-Charset header, then the server SHOULD send an error response with the 406 (not acceptable) status code, though the sending of an unacceptable response is also allowed.

14.3 Accept-Encoding

The Accept-Encoding request-header field is similar to Accept, but restricts the content-coding values (section 14.12) which are acceptable in the response.

          Accept-Encoding  = "Accept-Encoding" ":"
                                    #( content-coding )

An example of its use is

Accept-Encoding:

                           compress, gzip

If no Accept-Encoding header is present in a request, the server MAY assume that the client will accept any content coding. If an Accept- Encoding header is present, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 (Not Acceptable) status code.

An empty Accept-Encoding value indicates none are acceptable.

14.4 Accept-Language

The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request.

Accept-Language = "Accept-Language" ":"

                            1#( language-range [ ";" "q" "=" qvalue ] )

language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

Each language-range MAY be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The quality value defaults to "q=1". For example,

          Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and other types of English." A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefix of the tag such that the first tag character following the prefix is "-". The special range "*", if present in the Accept-Language field, matches every tag not matched by any other range present in the Accept-Language field.

Note: This use of a prefix matching rule does not imply that language tags are assigned to languages in such a way that it is always true that if a user understands a language with a certain tag, then this user will also understand all languages with tags for which this tag is a prefix. The prefix rule simply allows the use of prefix tags if this is the case.

The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest language- range in the field that matches the language-tag. If no language- range in the field matches the tag, the language quality factor assigned is 0. If no Accept-Language header is present in the request, the server SHOULD assume that all languages are equally acceptable. If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable.

It may be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request. For a discussion of this issue, see section 15.7.

Note: As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field must not be given in the request.

14.5 Accept-Ranges

The Accept-Ranges response-header field allows the server to indicate its acceptance of range requests for a resource:

Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges

          acceptable-ranges = 1#range-unit | "none"

Origin servers that accept byte-range requests MAY send

Accept-Ranges:

                         bytes

but are not required to do so. Clients MAY generate byte-range requests without having received this header for the resource involved.

   Servers that do not accept any kind of range request for a  resource
   MAY send

Accept-Ranges:

                         none

to advise the client not to attempt a range request.

14.6 Age

The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation) was generated at the origin server. A cached response is "fresh" if its age does not exceed its freshness lifetime. Age values are calculated as specified in section 13.2.3.

Age = "Age" ":" age-value

           age-value = delta-seconds

Age values are non-negative decimal integers, representing time in seconds.

If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows, it MUST transmit an Age header with a value of 2147483648 (2^31). HTTP/1.1 caches MUST send an Age header in every response. Caches SHOULD use an arithmetic type of at least 31 bits of range.

14.7 Allow

The Allow entity-header field lists the set of methods supported by the resource identified by the Request-URI. The purpose of this field is strictly to inform the recipient of valid methods associated with the resource. An Allow header field MUST be present in a 405 (Method Not Allowed) response.

          Allow          = "Allow" ":" 1#method

Example of use:

Allow:

                 GET, HEAD, PUT

This field cannot prevent a client from trying other methods. However, the indications given by the Allow header field value SHOULD be followed. The actual set of allowed methods is defined by the origin server at the time of each request.

The Allow header field MAY be provided with a PUT request to recommend the methods to be supported by the new or modified resource. The server is not required to support these methods and SHOULD include an Allow header in the response giving the actual supported methods.

A proxy MUST NOT modify the Allow header field even if it does not understand all the methods specified, since the user agent MAY have other means of communicating with the origin server.

The Allow header field does not indicate what methods are implemented at the server level. Servers MAY use the Public response-header field (section 14.35) to describe what methods are implemented on the server as a whole.

14.8 Authorization

A user agent that wishes to authenticate itself with a server-- usually, but not necessarily, after receiving a 401 response--MAY do so by including an Authorization request-header field with the request. The Authorization field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested.

          Authorization  = "Authorization" ":" credentials

HTTP access authentication is described in section 11. If a request is authenticated and a realm specified, the same credentials SHOULD be valid for all other requests within this realm.

When a shared cache (see section 13.7) receives a request containing an Authorization field, it MUST NOT return the corresponding response as a reply to any other request, unless one of the following specific exceptions holds:

     1. If the response includes the "proxy-revalidate" Cache-Control
        directive, the cache MAY use that response in replying to a
        subsequent request, but a proxy cache MUST first revalidate it with
        the origin server, using the request-headers from the new request
        to allow the origin server to authenticate the new request.
     2. If the response includes the "must-revalidate" Cache-Control
        directive, the cache MAY use that response in replying to a
        subsequent request, but all caches MUST first revalidate it with
        the origin server, using the request-headers from the new request
        to allow the origin server to authenticate the new request.
     3. If the response includes the "public" Cache-Control directive, it
        may be returned in reply to any subsequent request.

14.9 Cache-Control

The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain. The directives specify behavior intended to prevent caches from adversely interfering with the request or response. These directives typically override the default caching algorithms. Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive should be given in the response.

Note that HTTP/1.0 caches may not implement Cache-Control and may only implement Pragma: no-cache (see section 14.32).

Cache directives must be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a cache- directive for a specific cache.

Cache-Control = "Cache-Control" ":" 1#cache-directive

cache-directive = cache-request-directive

| cache-response-directive

cache-request-directive =

                            "no-cache" [ "=" <"> 1#field-name <"> ]
                          | "no-store"
                          | "max-age" "=" delta-seconds
                          | "max-stale" [ "=" delta-seconds ]
                          | "min-fresh" "=" delta-seconds
                          | "only-if-cached"
                          | cache-extension

cache-response-directive =

                            "public"
                          | "private" [ "=" <"> 1#field-name <"> ]
                          | "no-cache" [ "=" <"> 1#field-name <"> ]
                          | "no-store"
                          | "no-transform"
                          | "must-revalidate"
                          | "proxy-revalidate"
                          | "max-age" "=" delta-seconds
                          | cache-extension

cache-extension = token [ "=" ( token | quoted-string ) ]

When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol may apply these directives to header fields not defined in HTTP/1.1.

The cache-control directives can be broken down into these general categories:

     o  Restrictions on what is cachable; these may only be imposed by the
        origin server.
     o  Restrictions on what may be stored by a cache; these may be imposed
        by either the origin server or the user agent.
     o  Modifications of the basic expiration mechanism; these may be
        imposed by either the origin server or the user agent.
     o  Controls over cache revalidation and reload; these may only be
        imposed by a user agent.
     o  Control over transformation of entities.
     o  Extensions to the caching system.

14.9.1 What is Cachable

By default, a response is cachable if the requirements of the request method, request header fields, and the response status indicate that it is cachable. Section 13.4 summarizes these defaults for cachability. The following Cache-Control response directives allow an origin server to override the default cachability of a response:

public

Indicates that the response is cachable by any cache, even if it would normally be non-cachable or cachable only within a non-shared cache. (See also Authorization, section 14.8, for additional details.)

private

Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache. This allows an origin server to state that the specified parts of the response are intended for only one user and are not a valid response for requests by other users. A private (non-shared) cache may cache the response.

Note: This usage of the word private only controls where the response may be cached, and cannot ensure the privacy of the message content.

no-cache

Indicates that all or part of the response message MUST NOT be cached anywhere. This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests.

Note: Most HTTP/1.0 caches will not recognize or obey this directive.

14.9.2 What May be Stored by Caches

The purpose of the no-store directive is to prevent the inadvertent release or retention of sensitive information (for example, on backup tapes). The no-store directive applies to the entire message, and may be sent either in a response or in a request. If sent in a request, a cache MUST NOT store any part of either this request or any response to it. If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited it. This directive applies to both non-shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.

Even when this directive is associated with a response, users may explicitly store such a response outside of the caching system (e.g., with a "Save As" dialog). History buffers may store such responses as part of their normal operation.

The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive may improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches may not recognize or obey this directive; and communications networks may be vulnerable to eavesdropping.

14.9.3 Modifications of the Basic Expiration Mechanism

The expiration time of an entity may be specified by the origin server using the Expires header (see section 14.21). Alternatively, it may be specified using the max-age directive in a response.

If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be useful if certain HTTP/1.0 caches improperly calculate ages or expiration times, perhaps due to desynchronized clocks.

Note: most older caches, not compliant with this specification, do not implement any Cache-Control directives. An origin server wishing to use a Cache-Control directive that restricts, but does not prevent, caching by an HTTP/1.1-compliant cache may exploit the requirement that the max-age directive overrides the Expires header, and the fact that non-HTTP/1.1-compliant caches do not observe the max-age directive.

Other directives allow an user agent to modify the basic expiration mechanism. These directives may be specified on a request:

max-age

Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.

min-fresh

Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number of seconds.

max-stale

Indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age.

If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning 10 (Response is stale).

14.9.4 Cache Revalidation and Reload Controls

Sometimes an user agent may want or need to insist that a cache revalidate its cache entry with the origin server (and not just with the next cache along the path to the origin server), or to reload its cache entry from the origin server. End-to-end revalidation may be necessary if either the cache or the origin server has overestimated the expiration time of the cached response. End-to-end reload may be necessary if the cache entry has become corrupted for some reason.

End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it "unspecified end-to-end revalidation", or when the client does have a local cached copy, in which case we call it "specific end-to-end revalidation."

The client can specify these three kinds of action using Cache- Control request directives:

End-to-end reload

The request includes a "no-cache" Cache-Control directive or, for compatibility with HTTP/1.0 clients, "Pragma: no-cache". No field names may be included with the no-cache directive in a request. The server MUST NOT use a cached copy when responding to such a request.

Specific end-to-end revalidation

The request includes a "max-age=0" Cache-Control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional with the client's current validator.

Unspecified end-to-end revalidation

The request includes "max-age=0" Cache-Control directive, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator.

When an intermediate cache is forced, by means of a max-age=0 directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator may differ from the validator currently stored with the cache entry. In this case, the cache may use either validator in making its own request without affecting semantic transparency.

However, the choice of validator may affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache should return its now validated copy to the client with a 200 (OK) response. If the server replies with a new entity and cache validator, however, the intermediate cache should compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new entity with a 200 (OK) response.

If a request includes the no-cache directive, it should not include min-fresh, max-stale, or max-age.

In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the only-if-cached directive in a request. If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches.

Because a cache may be configured to ignore a server's specified expiration time, and because a client request may include a max-stale directive (which has a similar effect), the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use. When the must-revalidate directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. (I.e., the cache must do an end-to-end revalidation every time, if, based solely on the origin server's Expires or max-age value, the cached response is stale.)

The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response.

Servers should send the must-revalidate directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction. Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an unvalidated copy of the entity if revalidation fails.

Although this is not recommended, user agents operating under severe connectivity constraints may violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided. The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation.

The proxy-revalidate directive has the same meaning as the must- revalidate directive, except that it does not apply to non-shared user agent caches. It can be used on a response to an authenticated request to permit the user's cache to store and later return the response without needing to revalidate it (since it has already been authenticated once by that user), while still requiring proxies that service many users to revalidate each time (in order to make sure that each user has been authenticated). Note that such authenticated responses also need the public cache control directive in order to allow them to be cached at all.

14.9.5 No-Transform Directive

Implementers of intermediate caches (proxies) have found it useful to convert the media type of certain entity bodies. A proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. HTTP has to date been silent on these transformations.

Serious operational problems have already occurred, however, when these transformations have been applied to entity bodies intended for certain kinds of applications. For example, applications for medical imaging, scientific data analysis and those using end-to-end authentication, all depend on receiving an entity body that is bit for bit identical to the original entity-body.

Therefore, if a response includes the no-transform directive, an intermediate cache or proxy MUST NOT change those headers that are listed in section 13.5.2 as being subject to the no-transform directive. This implies that the cache or proxy must not change any aspect of the entity-body that is specified by these headers.

14.9.6 Cache Control Extensions

The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional assigned value. Informational extensions (those which do not require a change in cache behavior) may be added without changing the semantics of other directives. Behavioral extensions are designed to work by acting as modifiers to the existing base of cache directives. Both the new directive and the standard directive are supplied, such that applications which do not understand the new directive will default to the behavior specified by the standard directive, and those that understand the new directive will recognize it as modifying the requirements associated with the standard directive. In this way, extensions to the Cache-Control directives can be made without requiring changes to the base protocol.

This extension mechanism depends on a HTTP cache obeying all of the cache-control directives defined for its native HTTP-version, obeying certain extensions, and ignoring all directives that it does not understand.

For example, consider a hypothetical new response directive called "community" which acts as a modifier to the "private" directive. We define this new directive to mean that, in addition to any non-shared cache, any cache which is shared only by members of the community named within its value may cache the response. An origin server wishing to allow the "UCI" community to use an otherwise private response in their shared cache(s) may do so by including

Cache-Control:

private, community="UCI"

A cache seeing this header field will act correctly even if the cache does not understand the "community" cache-extension, since it will also see and understand the "private" directive and thus default to the safe behavior.

Unrecognized cache-directives MUST be ignored; it is assumed that any cache-directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard directives (or the response's default cachability) such that the cache behavior will remain minimally correct even if the cache does not understand the extension(s).

14.10 Connection

The Connection general-header field allows the sender to specify options that are desired for that particular connection and MUST NOT be communicated by proxies over further connections.

The Connection header has the following grammar:

          Connection-header = "Connection" ":" 1#(connection-token)
          connection-token  = token

HTTP/1.1 proxies MUST parse the Connection header field before a message is forwarded and, for each connection-token in this field, remove any header field(s) from the message with the same name as the connection-token. Connection options are signaled by the presence of a connection-token in the Connection header field, not by any corresponding additional header field(s), since the additional header field may not be sent if there are no parameters associated with that connection option. HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion of the response. For example,

Connection:

                      close

in either the request or the response header fields indicates that the connection should not be considered `persistent' (section 8.1) after the current request/response is complete.

HTTP/1.1 applications that do not support persistent connections MUST include the "close" connection option in every message.

14.11 Content-Base

The Content-Base entity-header field may be used to specify the base URI for resolving relative URLs within the entity. This header field is described as Base in RFC 1808, which is expected to be revised.

Content-Base = "Content-Base" ":" absoluteURI

If no Content-Base field is present, the base URI of an entity is defined either by its Content-Location (if that Content-Location URI is an absolute URI) or the URI used to initiate the request, in that order of precedence. Note, however, that the base URI of the contents within the entity-body may be redefined within that entity-body.

14.12 Content-Encoding

The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the entity-body, and thus what decoding mechanisms MUST be applied in order to obtain the media-type referenced by the Content-Type header field. Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type.

Content-Encoding = "Content-Encoding" ":" 1#content-coding

Content codings are defined in section 3.5. An example of its use is

Content-Encoding:

                            gzip

The Content-Encoding is a characteristic of the entity identified by the Request-URI. Typically, the entity-body is stored with this encoding and is only decoded before rendering or analogous usage.

If multiple encodings have been applied to an entity, the content codings MUST be listed in the order in which they were applied.

Additional information about the encoding parameters MAY be provided by other entity-header fields not defined by this specification.

14.13 Content-Language

The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this may not be equivalent to all the languages used within the entity-body.

Content-Language = "Content-Language" ":" 1#language-tag

Language tags are defined in section 3.10. The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is

Content-Language:

                            da

If no Content-Language is specified, the default is that the content is intended for all language audiences. This may mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended.

Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi," presented simultaneously in the original Maori and English versions, would call for

Content-Language:

                            mi, en

However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin," which is clearly intended to be used by an English-literate audience. In this case, the Content-Language should only include "en".

Content-Language may be applied to any media type -- it is not limited to textual documents.

14.14 Content-Length

The Content-Length entity-header field indicates the size of the message-body, in decimal number of octets, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET.

          Content-Length    = "Content-Length" ":" 1*DIGIT
   
   An example is

Content-Length:

Applications SHOULD use this field to indicate the size of the message-body to be transferred, regardless of the media type of the entity. It must be possible for the recipient to reliably determine the end of HTTP/1.1 requests containing an entity-body, e.g., because the request has a valid Content-Length field, uses Transfer-Encoding: chunked or a multipart body.

Any Content-Length greater than or equal to zero is a valid value. Section 4.4 describes how to determine the length of a message-body if a Content-Length is not given.

Note: The meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the "message/external-body" content-type. In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred.

14.15 Content-Location

The Content-Location entity-header field may be used to supply the resource location for the entity enclosed in the message. In the case where a resource has multiple entities associated with it, and those entities actually have separate locations by which they might be individually accessed, the server should provide a Content-Location for the particular variant which is returned. In addition, a server SHOULD provide a Content-Location for the resource corresponding to the response entity.

Content-Location = "Content-Location" ":"

                            ( absoluteURI | relativeURI )

If no Content-Base header field is present, the value of Content- Location also defines the base URL for the entity (see section 14.11).

The Content-Location value is not a replacement for the original requested URI; it is only a statement of the location of the resource corresponding to this particular entity at the time of the request. Future requests MAY use the Content-Location URI if the desire is to identify the source of that particular entity.

A cache cannot assume that an entity with a Content-Location different from the URI used to retrieve it can be used to respond to later requests on that Content-Location URI. However, the Content- Location can be used to differentiate between multiple entities retrieved from a single requested resource, as described in section 13.6.

If the Content-Location is a relative URI, the URI is interpreted relative to any Content-Base URI provided in the response. If no Content-Base is provided, the relative URI is interpreted relative to the Request-URI.

14.16 Content-MD5

The Content-MD5 entity-header field, as defined in RFC 1864 [23], is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.)

           Content-MD5   = "Content-MD5" ":" md5-digest
           
           md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

The Content-MD5 header field may be generated by an origin server to function as an integrity check of the entity-body. Only origin servers may generate the Content-MD5 header field; proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check. Any recipient of the entity-body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received.

The MD5 digest is computed based on the content of the entity-body, including any Content-Encoding that has been applied, but not including any Transfer-Encoding that may have been applied to the message-body. If the message is received with a Transfer-Encoding, that encoding must be removed prior to checking the Content-MD5 value against the received entity.

This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no Transfer-Encoding were being applied.

   HTTP extends RFC 1864 to permit the digest to be computed for MIME
   composite media-types (e.g., multipart/* and message/rfc822), but
   this does not change how the digest is computed as defined in the
   preceding paragraph.

Note: There are several consequences of this. The entity-body for composite types may contain many body-parts, each with its own MIME and HTTP headers (including Content-MD5, Content-Transfer-Encoding, and Content-Encoding headers). If a body-part has a Content- Transfer-Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i.e., after the application. The Transfer-Encoding header field is not allowed within body-parts.

Note: while the definition of Content-MD5 is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways in which the application of Content-MD5 to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does use Transfer-Encoding and Content-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that, in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CRLF. Conversion of all line breaks to CRLF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest.

14.17 Content-Range

The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be inserted. It also indicates the total size of the full entity-body. When a server returns a partial response to a client, it must describe both the extent of the range covered by the response, and the length of the entire entity-body.

Content-Range = "Content-Range" ":" content-range-spec

content-range-spec = byte-content-range-spec

          byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                    last-byte-pos "/" entity-length
          
          entity-length           = 1*DIGIT

Unlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range.

A byte-content-range-spec whose last-byte-pos value is less than its first-byte-pos value, or whose entity-length value is less than or equal to its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec MUST ignore it and any content transferred along with it.

Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:

• The first 500 bytes:

          bytes 0-499/1234

• The second 500 bytes:

          bytes 500-999/1234

• All except for the first 500 bytes:

          bytes 500-1233/1234

• The last 500 bytes:

          bytes 734-1233/1234

When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example,

          HTTP/1.1 206 Partial content
          Date: Wed, 15 Nov 1995 06:25:24 GMT
          Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
          Content-Range: bytes 21010-47021/47022
          Content-Length: 26012
          Content-Type: image/gif

When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be "multipart/byteranges". See appendix 19.2 for its definition.

A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request.

When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.

If the server ignores a byte-range-spec because it is invalid, the server should treat the request as if the invalid Range header field did not exist. (Normally, this means return a 200 response containing the full entity). The reason is that the only time a client will make such an invalid request is when the entity is smaller than the entity retrieved by a prior request.

14.18 Content-Type

The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET.

          Content-Type   = "Content-Type" ":" media-type
   Media types are defined in section 3.7. An example of the field is
   
          Content-Type: text/html; charset=ISO-8859-4

Further discussion of methods for identifying the media type of an entity is provided in section 7.2.1.

14.19 Date

The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822. The field value is an HTTP-date, as described in section 3.3.1.

          Date  = "Date" ":" HTTP-date
   
   An example is
   
          Date: Tue, 15 Nov 1994 08:12:31 GMT

If a message is received via direct connection with the user agent (in the case of requests) or the origin server (in the case of responses), then the date can be assumed to be the current date at the receiving end. However, since the date--as it is believed by the origin--is important for evaluating cached responses, origin servers MUST include a Date header field in all responses. Clients SHOULD only send a Date header field in messages that include an entity- body, as in the case of the PUT and POST requests, and even then it is optional. A received message which does not have a Date header field SHOULD be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date.

In theory, the date SHOULD represent the moment just before the entity is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.

The format of the Date is an absolute date and time as defined by HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.

14.20 ETag

The ETag entity-header field defines the entity tag for the associated entity. The headers used with entity tags are described in sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used for comparison with other entities from the same resource (see section 13.3.2).

ETag = "ETag" ":" entity-tag

Examples:

         ETag: "xyzzy"
         ETag: W/"xyzzy"
         ETag: ""

14.21 Expires

The Expires entity-header field gives the date/time after which the response should be considered stale. A stale cache entry may not normally be returned by a cache (either a proxy cache or an user agent cache) unless it is first validated with the origin server (or with an intermediate cache that has a fresh copy of the entity). See section 13.2 for further discussion of the expiration model.

The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time.

The format is an absolute date and time as defined by HTTP-date in section 3.3; it MUST be in RFC1123-date format:

Expires = "Expires" ":" HTTP-date

An example of its use is

         Expires: Thu, 01 Dec 1994 16:00:00 GMT

Note: if a response includes a Cache-Control field with the max-age directive, that directive overrides the Expires field.

HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").

To mark a response as "already expired," an origin server should use an Expires date that is equal to the Date header value. (See the rules for expiration calculations in section 13.2.4.)

To mark a response as "never expires," an origin server should use an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers should not send Expires dates more than one year in the future.

The presence of an Expires header field with a date value of some time in the future on an response that otherwise would by default be non-cacheable indicates that the response is cachable, unless indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

The From request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent. The address SHOULD be machine-usable, as defined by mailbox in RFC 822 (as updated by RFC 1123 ):

          From   = "From" ":" mailbox

An example is:

From:

                [email protected]

This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end.

The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.

Note: The client SHOULD not send the From header field without the user's approval, as it may conflict with the user's privacy interests or their site's security policy. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request.

14.23 Host

The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URL given by the user or referring resource (generally an HTTP URL, as described in section 3.2.2). The Host field value MUST represent the network location of the origin server or gateway given by the original URL. This allows the origin server or gateway to differentiate between internally-ambiguous URLs, such as the root "/" URL of a server for multiple host names on a single IP address.

Host = "Host" ":" host [ ":" port ] ; Section 3.2.2

A "host" without any trailing port information implies the default port for the service requested (e.g., "80" for an HTTP URL). For example, a request on the origin server for <http://www.w3.org/pub/WWW/> MUST include:

          GET /pub/WWW/ HTTP/1.1
          Host: www.w3.org

A client MUST include a Host header field in all HTTP/1.1 request messages on the Internet (i.e., on any message corresponding to a request for a URL which includes an Internet host address for the service being requested). If the Host field is not already present, an HTTP/1.1 proxy MUST add a Host field to the request message prior to forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST respond with a 400 status code to any HTTP/1.1 request message which lacks a Host header field.

See sections 5.2 and 19.5.1 for other requirements relating to Host.

14.24 If-Modified-Since

The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body.

If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

          If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:

a)If the request would normally result in anything other than a 200

(OK) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.

b)If the variant has been modified since the If-Modified-Since date,

the response is exactly the same as for a normal GET.

c)If the variant has not been modified since a valid If-Modified-Since

date, the server MUST return a 304 (Not Modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

Note that the Range request-header field modifies the meaning of If-Modified-Since; see section 14.36 for full details.

Note that If-Modified-Since times are interpreted by the server, whose clock may not be synchronized with the client.

Note that if a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-Modified- Since date of a subsequent request, and the possibility of clock- skew-related problems if the If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency.

14.25 If-Match

The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that one of those entities is current by including a list of their associated entity tags in the If-Match header field. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special case, the value "*" matches any current entity of the resource.

          If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MAY perform the requested method as if the If-Match header field did not exist.

A server MUST use the strong comparison function (see section 3.11) to compare the entity tags in If-Match.

If none of the entity tags match, or if "*" is given and no current entity exists, the server MUST NOT perform the requested method, and MUST return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, such as PUT, from modifying a resource that has changed since the client last retrieved it.

If the request would, without the If-Match header field, result in anything other than a 2xx status, then the If-Match header MUST be ignored.

The meaning of "If-Match: *" is that the method SHOULD be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.43) exists, and MUST NOT be performed if the representation does not exist.

A request intended to update a resource (e.g., a PUT) MAY include an If-Match header field to signal that the request method MUST NOT be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without their knowledge. Examples:

          If-Match: "xyzzy"
          If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-Match: *

14.26 If-None-Match

The If-None-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of a resource which was not known to exist.

As a special case, the value "*" matches any current entity of the resource.

If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource, then the server MUST NOT perform the requested method. Instead, if the request method was GET or HEAD, the server SHOULD respond with a 304 (Not Modified) response, including the cache-related entity-header fields (particularly ETag) of one of the entities that matched. For all other request methods, the server MUST respond with a status of 412 (Precondition Failed).

See section 13.3.3 for rules on how to determine if two entity tags match. The weak comparison function can only be used with GET or HEAD requests.

If none of the entity tags match, or if "*" is given and no current entity exists, then the server MAY perform the requested method as if the If-None-Match header field did not exist.

If the request would, without the If-None-Match header field, result in anything other than a 2xx status, then the If-None-Match header MUST be ignored.

The meaning of "If-None-Match: *" is that the method MUST NOT be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see section 14.43) exists, and SHOULD be performed if the representation does not exist. This feature may be useful in preventing races between PUT operations.

Examples:

          If-None-Match: "xyzzy"
          If-None-Match: W/"xyzzy"
          If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
          If-None-Match: *

14.27 If-Range

If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body.

The If-Range header allows a client to "short-circuit" the second request. Informally, its meaning is `if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.'

If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

If the client has no entity tag for an entity, but does have a Last- Modified date, it may use that date in a If-Range header. (The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.) The If-Range header should only be used together with a Range header, and must be ignored if the request does not include a Range header, or if the server does not support the sub-range operation.

If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server should provide the specified sub-range of the entity using a 206 (Partial content) response. If the entity tag does not match, then the server should return the entire entity using a 200 (OK) response.

14.28 If-Unmodified-Since

The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested resource has not been modified since the time specified in this field, the server should perform the requested operation as if the If-Unmodified-Since header were not present.

If the requested variant has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed).

If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date

An example of the field is:

If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx status, the If- Unmodified-Since header should be ignored.

If the specified date is invalid, the header is ignored.

14.29 Last-Modified

The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified.

          Last-Modified  = "Last-Modified" ":" HTTP-date

An example of its use is

          Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource. For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time stamp of the record. For virtual objects, it may be the last time the internal state changed.

An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date.

An origin server should obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response. This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes near the time that the response is generated.

HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource. For 201 (Created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URL for automatic redirection to the resource. The field value consists of a single absolute URL.

          Location       = "Location" ":" absoluteURI
   
   An example is
   
          Location: http://www.w3.org/pub/WWW/People.html

Note: The Content-Location header field (section 14.15) differs from Location in that the Content-Location identifies the original location of the entity enclosed in the request. It is therefore possible for a response to contain header fields for both Location and Content-Location. Also see section 13.10 for cache requirements of some methods.

14.31 Max-Forwards

The Max-Forwards request-header field may be used with the TRACE method (section 14.31) to limit the number of proxies or gateways that can forward the request to the next inbound server. This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain.

          Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded.

Each proxy or gateway recipient of a TRACE request containing a Max- Forwards header field SHOULD check and update its value prior to forwarding the request. If the received value is zero (0), the recipient SHOULD NOT forward the request; instead, it SHOULD respond as the final recipient with a 200 (OK) response containing the received request message as the response entity-body (as described in section 9.8). If the received Max-Forwards value is greater than zero, then the forwarded message SHOULD contain an updated Max- Forwards field with a value decremented by one (1).

The Max-Forwards header field SHOULD be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition.

14.32 Pragma

The Pragma general-header field is used to include implementation- specific directives that may apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.

Pragma = "Pragma" ":" 1#pragma-directive

pragma-directive = "no-cache" | extension-pragma

extension-pragma = token [ "=" ( token | quoted-string ) ]

When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache cache-directive (see section 14.9) and is defined here for backwards compatibility with HTTP/1.0. Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant.

Pragma directives MUST be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient.

HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". No new Pragma directives will be defined in HTTP.

14.33 Proxy-Authenticate

The Proxy-Authenticate response-header field MUST be included as part of a 407 (Proxy Authentication Required) response. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI.

Proxy-Authenticate = "Proxy-Authenticate" ":" challenge

The HTTP access authentication process is described in section 11. Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and SHOULD NOT be passed on to downstream clients. However, an intermediate proxy may need to obtain its own credentials by requesting them from the downstream client, which in some circumstances will appear as if the proxy is forwarding the Proxy-Authenticate header field.

14.34 Proxy-Authorization

The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested.

       Proxy-Authorization     = "Proxy-Authorization" ":" credentials

The HTTP access authentication process is described in section 11. Unlike Authorization, the Proxy-Authorization header field applies only to the next outbound proxy that demanded authentication using the Proxy-Authenticate field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first outbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.

14.35 Public

The Public response-header field lists the set of methods supported by the server. The purpose of this field is strictly to inform the recipient of the capabilities of the server regarding unusual methods. The methods listed may or may not be applicable to the

Request-URI; the Allow header field (section 14.7) MAY be used to indicate methods allowed for a particular URI.

          Public         = "Public" ":" 1#method

Example of use:

          Public: OPTIONS, MGET, MHEAD, GET, HEAD

This header field applies only to the server directly connected to the client (i.e., the nearest neighbor in a chain of connections). If the response passes through a proxy, the proxy MUST either remove the Public header field or replace it with one applicable to its own capabilities.

14.36 Range

14.36.1 Byte Ranges

Since all HTTP entities are represented in HTTP messages as sequences of bytes, the concept of a byte range is meaningful for any HTTP entity. (However, not all clients and servers need to support byte- range operations.)

Byte range specifications in HTTP apply to the sequence of bytes in the entity-body (not necessarily the same as the message-body).

A byte range operation may specify a single range of bytes, or a set of ranges within a single entity.

       ranges-specifier = byte-ranges-specifier
       
       byte-ranges-specifier = bytes-unit "=" byte-range-set
       
       byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
       
       byte-range-spec = first-byte-pos "-" [last-byte-pos]
       
       first-byte-pos  = 1*DIGIT
       
       last-byte-pos   = 1*DIGIT

The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.

If the last-byte-pos value is present, it must be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte- range-spec is invalid. The recipient of an invalid byte-range-spec must ignore it.

If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-byte-pos is taken to be equal to one less than the current length of the entity- body in bytes.

By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity.

          suffix-byte-range-spec = "-" suffix-length
          
          suffix-length = 1*DIGIT

A suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity-body.) If the entity is shorter than the specified suffix-length, the entire entity-body is used.

Examples of byte-ranges-specifier values (assuming an entity-body of length 10000):

• The first 500 bytes (byte offsets 0-499, inclusive):

          bytes=0-499

• The second 500 bytes (byte offsets 500-999, inclusive):

          bytes=500-999

• The final 500 bytes (byte offsets 9500-9999, inclusive):

          bytes=-500

• Or

          bytes=9500-

• The first and last bytes only (bytes 0 and 9999):

          bytes=0-0,-1

• Several legal but not canonical specifications of the second 500 bytes (byte offsets 500-999, inclusive):

          bytes=500-600,601-999
          
          bytes=500-700,601-999

14.36.2 Range Retrieval Requests

HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request:

Range = "Range" ":" ranges-specifier

A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities.

If the server supports the Range header and the specified range or ranges are appropriate for the entity:

• The presence of a Range header in an unconditional GET modifies what is returned if the GET is otherwise successful. In other words, the response carries a status code of 206 (Partial Content) instead of 200 (OK).

• The presence of a Range header in a conditional GET (a request using one or both of If-Modified-Since and If-None-Match, or one or both of If-Unmodified-Since and If-Match) modifies what is returned if the GET is otherwise successful and the condition is true. It does not affect the 304 (Not Modified) response returned if the conditional is false.

In some cases, it may be more appropriate to use the If-Range header (see section 14.27) in addition to the Range header.

If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client. It SHOULD store the entire received response in its cache, if that is consistent with its cache allocation policies.

14.37 Referer

The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the "referrer", although the header field is misspelled.) The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard.

Referer = "Referer" ":" ( absoluteURI | relativeURI )

Example:

Referer: http://www.w3.org/hypertext/DataSources/Overview.html

If the field value is a partial URI, it SHOULD be interpreted relative to the Request-URI. The URI MUST NOT include a fragment.

Note: Because the source of a link may be private information or may reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information.

14.38 Retry-After

The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.

          Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )

Two examples of its use are

          Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
          Retry-After: 120

In the latter example, the delay is 2 minutes.

14.39 Server

The Server response-header field contains information about the software used by the origin server to handle the request. The field can contain multiple product tokens (section 3.8) and comments identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application.

Server = "Server" ":" 1*( product | comment )

Example:

Server:

                  CERN/3.0 libwww/2.17

If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server response-header. Instead, it SHOULD include a Via field (as described in section 14.44).

Note: Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementers are encouraged to make this field a configurable option.

14.40 Transfer-Encoding

The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This differs from the Content-Encoding in that the transfer coding is a property of the message, not of the entity.

          Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-
   coding

Transfer codings are defined in section 3.6. An example is:

Transfer-Encoding:

                             chunked

Many older HTTP/1.0 applications do not understand the Transfer- Encoding header.

14.41 Upgrade

The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols. The server

MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched.

          Upgrade        = "Upgrade" ":" 1#product

For example,

          Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by the server, possibly according to the nature of the method and/or resource being requested).

The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field.

The Upgrade header field only applies to the immediate connection. Therefore, the upgrade keyword MUST be supplied within a Connection header field (section 14.10) whenever Upgrade is present in an HTTP/1.1 message.

The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response.

This specification only defines the protocol name "HTTP" for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section 3.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol.

14.42 User-Agent

The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. User agents SHOULD include this field with requests. The field can contain multiple product tokens (section 3.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application.

User-Agent = "User-Agent" ":" 1*( product | comment )

Example:

          User-Agent: CERN-LineMode/2.15 libwww/2.17b3

14.43 Vary

The Vary response-header field is used by a server to signal that the response entity was selected from the available representations of the response using server-driven negotiation (section 12). Field- names listed in Vary headers are those of request-headers. The Vary field value indicates either that the given set of header fields encompass the dimensions over which the representation might vary, or that the dimensions of variance are unspecified ("*") and thus may vary over any aspect of future requests.

          Vary  = "Vary" ":" ( "*" | 1#field-name )

An HTTP/1.1 server MUST include an appropriate Vary header field with any cachable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server SHOULD include an appropriate Vary header field with a non-cachable response that is subject to server-driven negotiation, since this might provide the user agent with useful information about the dimensions over which the response might vary.

The set of header fields named by the Vary field value is known as the "selecting" request-headers.

When the cache receives a subsequent request whose Request-URI specifies one or more cache entries including a Vary header, the cache MUST NOT use such a cache entry to construct a response to the new request unless all of the headers named in the cached Vary header are present in the new request, and all of the stored selecting request-headers from the previous request match the corresponding headers in the new request.

The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first request can be transformed to the selecting request-headers in the second request by adding or removing linear whitespace (LWS) at places where this is allowed by the corresponding BNF, and/or combining multiple message- header fields with the same field name following the rules about message headers in section 4.2.

A Vary field value of "*" signals that unspecified parameters, possibly other than the contents of request-header fields (e.g., the network address of the client), play a role in the selection of the response representation. Subsequent requests on that resource can only be properly interpreted by the origin server, and thus a cache MUST forward a (possibly conditional) request even when it has a fresh response cached for the resource. See section 13.6 for use of the Vary header by caches.

A Vary field value consisting of a list of field-names signals that the representation selected for the response is based on a selection algorithm which considers ONLY the listed request-header field values in selecting the most appropriate representation. A cache MAY assume that the same selection will be made for future requests with the same values for the listed field names, for the duration of time in which the response is fresh.

The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names are case-insensitive.

14.44 Via

The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received" field of RFC 822 and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain.

      Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
      
      received-protocol = [ protocol-name "/" ] protocol-version
      protocol-name     = token
      protocol-version  = token
      received-by       = ( host [ ":" port ] ) | pseudonym
      pseudonym         = token

The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients.

The protocol-name is optional if and only if it would be "HTTP". The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol.

Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications.

Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message.

For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field:

          Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)

Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region. This information SHOULD only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.

For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example,

          Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

could be collapsed to

          Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Applications MUST NOT combine entries which have different received-protocol values.

14.45 Warning

The Warning response-header field is used to carry additional information about the status of a response which may not be reflected by the response status code. This information is typically, though not exclusively, used to warn about a possible lack of semantic transparency from caching operations.

Warning headers are sent with responses using:

          Warning    = "Warning" ":" 1#warning-value
          
          warning-value = warn-code SP warn-agent SP warn-text
          warn-code  = 2DIGIT
          warn-agent = ( host [ ":" port ] ) | pseudonym
                          ; the name or pseudonym of the server adding
                          ; the Warning header, for use in debugging
          warn-text  = quoted-string

A response may carry more than one Warning header.

The warn-text should be in a natural language and character set that is most likely to be intelligible to the human user receiving the response. This decision may be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc. The default language is English and the default character set is ISO- 8859-1.

If a character set other than ISO-8859-1 is used, it MUST be encoded in the warn-text using the method described in RFC 1522 [14].

Any server or cache may add Warning headers to a response. New Warning headers should be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a response. However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry except as specified for specific Warning codes. It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response.

When multiple Warning headers are attached to a response, the user agent SHOULD display as many of them as possible, in the order that they appear in the response. If it is not possible to display all of the warnings, the user agent should follow these heuristics:

     o  Warnings that appear early in the response take priority over those
        appearing later in the response.
     o  Warnings in the user's preferred character set take priority over
        warnings in other character sets but with identical warn-codes and
        warn-agents.

Systems that generate multiple Warning headers should order them with this user agent behavior in mind.

This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning.

10 Response is stale

MUST be included whenever the returned response is stale. A cache may add this warning to any response, but may never remove it until the response is known to be fresh.

11 Revalidation failed

MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server. A cache may add this warning to any response, but may never remove it until the response is successfully revalidated.

12 Disconnected operation

SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time.

13 Heuristic expiration

MUST be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.

14 Transformation applied

MUST be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, unless this Warning code already appears in the response. MUST NOT be deleted from a response even after revalidation.

99 Miscellaneous warning

The warning text may include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action.

14.46 WWW-Authenticate

The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI.

WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge

The HTTP access authentication process is described in section 11. User agents MUST take special care in parsing the WWW-Authenticate field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.

15 Security Considerations

This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1 as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does make some suggestions for reducing security risks.

15.1 Authentication of Clients

The Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the entity, which is transmitted in clear text across the physical network used as the carrier. HTTP does not prevent additional authentication schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes to use one-time passwords) to Basic authentication.

The most serious flaw in Basic authentication is that it results in the essentially clear text transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.

Because Basic authentication involves the clear text transmission of passwords it SHOULD never be used (without enhancements) to protect sensitive or valuable information.

A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords.

If a server permits users to select their own passwords, then the threat is not only illicit access to documents on the server but also illicit access to the accounts of all users who have chosen to use their account password. If users are allowed to choose their own password that also means the server must maintain files containing the (presumably encrypted) passwords. Many of these may be the account passwords of users perhaps at distant sites. The owner or administrator of such a system could conceivably incur liability if this information is not maintained in a secure fashion.

Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by basic authentication when in fact he is connecting to a hostile server or gateway then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication [32]. Server implementers SHOULD guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway since that gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client.

15.2 Offering a Choice of Authentication Schemes

An HTTP/1.1 server may return multiple challenges with a 401 (Authenticate) response, and each challenge may use a different scheme. The order of the challenges returned to the user agent is in the order that the server would prefer they be chosen. The server should order its challenges with the "most secure" authentication scheme first. A user agent should choose as the challenge to be made to the user the first one that the user agent understands.

When the server offers choices of authentication schemes using the WWW-Authenticate header, the "security" of the authentication is only as malicious user could capture the set of challenges and try to authenticate him/herself using the weakest of the authentication schemes. Thus, the ordering serves more to protect the user's credentials than the server's information.

A possible man-in-the-middle (MITM) attack would be to add a weak authentication scheme to the set of choices, hoping that the client will use one that exposes the user's credentials (e.g. password). For this reason, the client should always use the strongest scheme that it understands from the choices accepted.

An even better MITM attack would be to remove all offered choices, and to insert a challenge that requests Basic authentication. For this reason, user agents that are concerned about this kind of attack could remember the strongest authentication scheme ever requested by a server and produce a warning message that requires user confirmation before using a weaker one. A particularly insidious way to mount such a MITM attack would be to offer a "free" proxy caching service to gullible users.

15.3 Abuse of Server Log Information

A server is in the position to save personal data about a user's requests which may identify their reading patterns or subjects of interest. This information is clearly confidential in nature and its handling may be constrained by law in certain countries. People using the HTTP protocol to provide data are responsible for ensuring that such material is not distributed without the permission of any individuals that are identifiable by the published results.

15.4 Transfer of Sensitive Information

Like any generic data transfer protocol, HTTP cannot regulate the content of the data that is transferred, nor is there any a priori method of determining the sensitivity of any particular piece of information within the context of any given request. Therefore, applications SHOULD supply as much control over this information as possible to the provider of that information. Four header fields are worth special mention in this context: Server, Via, Referer and From.

Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementers SHOULD make the Server header field a configurable option.

Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall.

The Referer field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer field may indicate a private document's URI whose publication would be inappropriate.

The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration.

We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information.

15.5 Attacks Based On File and Path Names

Implementations of HTTP origin servers SHOULD be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators. If an HTTP server translates HTTP URIs directly into file system calls, the server MUST take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft Windows, and other operating systems use ".." as a path component to indicate a directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the Request-URI if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control files, configuration files, and script code) MUST be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor bugs in such HTTP server implementations have turned into security risks.

15.6 Personal Information

HTTP clients are often privy to large amounts of personal information (e.g. the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information via the HTTP protocol to other sources. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementers be particularly careful in this area. History shows that errors in this area are often both serious security and/or privacy problems, and often generate highly adverse publicity for the implementer's company.

15.7 Privacy Issues Connected to Accept Headers

Accept request-headers can reveal information about the user to all servers which are accessed. The Accept-Language header in particular can reveal information the user would consider to be of a private nature, because the understanding of particular languages is often strongly correlated to the membership of a particular ethnic group. User agents which offer the option to configure the contents of an Accept-Language header to be sent in every request are strongly encouraged to let the configuration process include a message which makes the user aware of the loss of privacy involved.

An approach that limits the loss of privacy would be for a user agent to omit the sending of Accept-Language headers by default, and to ask the user whether it should start sending Accept-Language headers to a server if it detects, by looking for any Vary response-header fields generated by the server, that such sending could improve the quality of service.

Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents should be conservative in offering accept header configuration options to end users. As an extreme privacy measure, proxies could filter the accept headers in relayed requests. General purpose user agents which provide a high degree of header configurability should warn users about the loss of privacy which can be involved.

15.8 DNS Spoofing

Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based on the deliberate mis-association of IP addresses and DNS names. Clients need to be cautious in assuming the continuing validity of an IP number/DNS name association.

In particular, HTTP clients SHOULD rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they SHOULD be configured to do so. These lookups should be cached, however, only when the TTL (Time To Live) information reported by the name server makes it likely that the cached information will remain useful.

If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they MUST observe the TTL information reported by DNS.

If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As network renumbering is expected to become increasingly common, the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability.

This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites which use that strategy.

15.9 Location Headers and Spoofing

If a single server supports multiple organizations that do not trust one another, then it must check the values of Location and Content- Location headers in responses that are generated under control of said organizations to make sure that they do not attempt to invalidate resources over which they have no authority.

16 Acknowledgments

This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP and Internet mail message formats.

The HTTP protocol has evolved considerably over the past four years. It has benefited from a large and active developer community--the many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol.

This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already mentioned, the following individuals have contributed to this specification:

          Gary Adams                  Albert Lunde
          Harald Tveit Alvestrand     John C. Mallery
          Keith Ball                  Jean-Philippe Martin-Flatin
          Brian Behlendorf            Larry Masinter
          Paul Burchard               Mitra
          Maurizio Codogno            David Morris
          Mike Cowlishaw              Gavin Nicol
          Roman Czyborra              Bill Perry
          Michael A. Dolan            Jeffrey Perry
          David J. Fiander            Scott Powers
          Alan Freier                 Owen Rees
          Marc Hedlund                Luigi Rizzo
          Greg Herlihy                David Robinson
          Koen Holtman                Marc Salomon
          Alex Hopmann                Rich Salz
          Bob Jernigan                Allan M. Schiffman
          Shel Kaphan                 Jim Seidman
          Rohit Khare                 Chuck Shotton
          John Klensin                Eric W. Sink
          Martijn Koster              Simon E. Spero
          Alexei Kosut                Richard N. Taylor
          David M. Kristol            Robert S. Thau
          Daniel LaLiberte            Bill (BearHeart) Weinman
          Ben Laurie                  Francois Yergeau
          Paul J. Leach               Mary Ellen Zurko
          Daniel DuBois

Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

Most of the specification of ranges is based on work originally done by Ari Luotonen and John Franks, with additional input from Steve Zilles.

Thanks to the "cave men" of Palo Alto. You know who you are.

Jim Gettys (the current editor of this document) wishes particularly to thank Roy Fielding, the previous editor of this document, along with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen Holtman, John Franks, Alex Hopmann, and Larry Masinter for their help.

17 References

   [1] Alvestrand, H., "Tags for the identification of languages", RFC
   1766, UNINETT, March 1995.
   
   [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
   D., and B. Alberti. "The Internet Gopher Protocol: (a distributed
   document search and retrieval protocol)", RFC 1436, University of
   Minnesota, March 1993.

[3] Berners-Lee, T., "Universal Resource Identifiers in WWW", A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web", RFC 1630, CERN, June 1994.

   [4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
   Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
   December 1994.
   
   [5] Berners-Lee, T., and D. Connolly, "HyperText Markup Language
   Specification - 2.0", RFC 1866, MIT/LCS, November 1995.
   
   [6] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
   Transfer Protocol -- HTTP/1.0.", RFC 1945 MIT/LCS, UC Irvine, May
   1996.
   
   [7] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
   Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
   2045, Innosoft, First Virtual, November 1996.
   
   [8] Braden, R., "Requirements for Internet hosts - application and
   support", STD 3,  RFC 1123, IETF, October 1989.
   
   [9] Crocker, D., "Standard for the Format of ARPA Internet Text
   Messages", STD 11, RFC 822, UDEL, August 1982.
   [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
   Sui, J., and M. Grinbaum. "WAIS Interface Protocol Prototype
   Functional Specification", (v1.5), Thinking Machines Corporation,
   April 1990.
   
   [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
   Irvine, June 1995.
   
   [12] Horton, M., and R. Adams. "Standard for interchange of USENET
   messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic
   Studies, December 1987.
   
   [13] Kantor, B., and P. Lapsley. "Network News Transfer Protocol." A
   Proposed Standard for the Stream-Based Transmission of News", RFC
   977, UC San Diego, UC Berkeley, February 1986.
   
   [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
   Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
   University of Tennessee, November 1996.
   
   [15] Nebel, E., and L. Masinter. "Form-based File Upload in HTML",
   RFC 1867, Xerox Corporation, November 1995.
   
   [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
   USC/ISI, August 1982.
   
   [17] Postel, J., "Media Type Registration Procedure", RFC 2048,
   USC/ISI, November 1996.
   
   [18] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD
   9, RFC 959, USC/ISI, October 1985.
   
   [19] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
   1700, USC/ISI, October 1994.
   
   [20] Sollins, K., and L. Masinter, "Functional Requirements for
   Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
   December 1994.
   
   [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
   Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
   
   [22] ISO-8859. International Standard -- Information Processing --
     8-bit Single-Byte Coded Graphic Character Sets --
     Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
     Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
     Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
     Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
     Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
     Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
     Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
     Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
     Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
   
   [23] Meyers, J., and M. Rose "The Content-MD5 Header Field", RFC
   1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
   
   [24] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC
   1900, IAB, February 1996.
   
   [25] Deutsch, P., "GZIP file format specification version 4.3." RFC
   1952, Aladdin Enterprises, May 1996.
   
   [26] Venkata N. Padmanabhan and Jeffrey C. Mogul. Improving HTTP
   Latency. Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec.
   1995.  Slightly revised version of paper in Proc. 2nd International
   WWW Conf. '94: Mosaic and the Web, Oct. 1994, which is available at
   http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/
   HTTPLatency.html.
   
   [27] Joe Touch, John Heidemann, and Katia Obraczka, "Analysis of HTTP
   Performance", <URL: http://www.isi.edu/lsam/ib/http-perf/>,
   USC/Information Sciences Institute, June 1996
   
   [28] Mills, D., "Network Time Protocol, Version 3, Specification,
   Implementation and Analysis", RFC 1305, University of Delaware, March
   1992.
   
   [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
   version 1.3." RFC 1951, Aladdin Enterprises, May 1996.
   
   [30] Spero, S., "Analysis of HTTP Performance Problems"
   <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.
   
   [31] Deutsch, P., and J-L. Gailly, "ZLIB Compressed Data Format
   Specification version 3.3", RFC 1950, Aladdin Enterprises, Info-ZIP,
   May 1996.
   
   [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
   Luotonen, A., Sink, E., and L. Stewart, "An Extension to HTTP :
   Digest Access Authentication", RFC 2069, January 1997.

18 Authors' Addresses

Roy T. Fielding
Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA

   Fax: +1 (714) 824-4056
   EMail: [email protected]

Jim Gettys
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: [email protected]

Jeffrey C. Mogul
Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, USA

   EMail: [email protected]

Henrik Frystyk Nielsen
W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: [email protected]

Tim Berners-Lee
Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: [email protected]

19 Appendices

19.1 Internet Media Type message/http

In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type "message/http". The following is to be registered with IANA.

       Media Type name:         message
       Media subtype name:      http
       Required parameters:     none
       Optional parameters:     version, msgtype

version: The HTTP-Version number of the enclosed message

(e.g., "1.1"). If not present, the version can be

determined from the first line of the body.

msgtype:

The message type -- "request" or "response". If not present, the type can be determined from the first line of the body.

Encoding considerations:

only "7bit", "8bit", or "binary" are permitted

       Security considerations: none

19.2 Internet Media Type multipart/byteranges

When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart media type for this purpose is called "multipart/byteranges".

The multipart/byteranges media type includes two or more parts, each with its own Content-Type and Content-Range fields. The parts are separated using a MIME boundary parameter.

          Media Type name:         multipart
          Media subtype name:      byteranges
          Required parameters:     boundary
          Optional parameters:     none

Encoding considerations:

only "7bit", "8bit", or "binary" are permitted

          Security considerations: none

For example:

   HTTP/1.1 206 Partial content
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
   
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 500-999/8000
   
   ...the first range...
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 7000-7999/8000
   
   ...the second range
   --THIS_STRING_SEPARATES--

19.3 Tolerant Applications

Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations can be interpreted unambiguously.

Clients SHOULD be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they SHOULD accept any amount of SP or HT characters between fields, even though only a single SP is required.

The line terminator for message-header fields is the sequence CRLF. However, we recommend that applications, when parsing such headers, recognize a single LF as a line terminator and ignore the leading CR.

The character set of an entity-body should be labeled as the lowest common denominator of the character codes used within that body, with the exception that no label is preferred over the labels US-ASCII or ISO-8859-1.

Additional rules for requirements on parsing and encoding of dates and other potential problems with date encodings include:

• HTTP/1.1 clients and caches should assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the "year 2000" problem).

• An HTTP/1.1 implementation may internally represent a parsed Expires date as earlier than the proper value, but MUST NOT internally represent a parsed Expires date as later than the proper value.

• All expiration-related calculations must be done in GMT. The local time zone MUST NOT influence the calculation or comparison of an age or expiration time.

• If an HTTP header incorrectly carries a date value with a time zone other than GMT, it must be converted into GMT using the most conservative possible conversion.

19.4 Differences Between HTTP Entities and MIME Entities

HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822) and the Multipurpose Internet Mail Extensions (MIME ) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, MIME [7] discusses mail, and HTTP has a few features that are different from those described in MIME. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients.

This appendix describes specific areas where HTTP differs from MIME. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions may be required.

19.4.1 Conversion to Canonical Form

MIME requires that an Internet mail entity be converted to canonical form prior to being transferred. Section 3.7.1 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. MIME requires that content with a type of "text" represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP.

Where it is possible, a proxy or gateway from HTTP to a strict MIME environment SHOULD translate all line breaks within the text media types described in section 3.7.1 of this document to the MIME canonical form of CRLF. Note, however, that this may be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF, as is the case for some multi-byte character sets.

19.4.2 Conversion of Date Formats

HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to simplify the process of date comparison. Proxies and gateways from other protocols SHOULD ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.

19.4.3 Introduction of Content-Encoding

MIME does not include any concept equivalent to HTTP/1.1's Content- Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols MUST either change the value of the Content-Type header field or decode the entity-body before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<content-coding>" to perform an equivalent function as Content-Encoding. However, this parameter is not part of MIME.)

19.4.4 No Content-Transfer-Encoding

HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE ("quoted-printable" or "base64") encoding prior to delivering the response message to an HTTP client.

Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.

19.4.5 HTTP Header Fields in Multipart Body-Parts

In MIME, most header fields in multipart body-parts are generally ignored unless the field name begins with "Content-". In HTTP/1.1, multipart body-parts may contain any HTTP header fields which are significant to the meaning of that part.

19.4.6 Introduction of Transfer-Encoding

HTTP/1.1 introduces the Transfer-Encoding header field (section 14.40). Proxies/gateways MUST remove any transfer coding prior to forwarding a message via a MIME-compliant protocol.

A process for decoding the "chunked" transfer coding (section 3.6) can be represented in pseudo-code as:

          length := 0
          read chunk-size, chunk-ext (if any) and CRLF
          while (chunk-size > 0) {
             read chunk-data and CRLF
             append chunk-data to entity-body
             length := length + chunk-size
             read chunk-size and CRLF
          }
          read entity-header
          while (entity-header not empty) {
             append entity-header to existing header fields
             read entity-header
          }
          Content-Length := length
          Remove "chunked" from Transfer-Encoding

19.4.7 MIME-Version

HTTP is not a MIME-compliant protocol (see appendix 19.4). However, HTTP/1.1 messages may include a single MIME-Version general-header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full compliance with the MIME protocol. Proxies/gateways are responsible for ensuring full compliance (where possible) when exporting HTTP messages to strict MIME environments.

MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification.

19.5 Changes from HTTP/1.0

This section summarizes major differences between versions HTTP/1.0 and HTTP/1.1.

19.5.1 Changes to Simplify Multi-homed Web Servers and Conserve IP

Addresses

The requirements that clients and servers support the Host request- header, report an error if the Host request-header (section 14.23) is missing from an HTTP/1.1 request, and accept absolute URIs (section 5.1.2) are among the most important changes defined by this specification.

Older HTTP/1.0 clients assumed a one-to-one relationship of IP addresses and servers; there was no other established mechanism for distinguishing the intended server of a request than the IP address to which that request was directed. The changes outlined above will allow the Internet, once older HTTP clients are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been allocated for the sole purpose of allowing special-purpose domain names to be used in root-level HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely important that all implementations of HTTP (including updates to existing HTTP/1.0 applications) correctly implement these requirements:

• Both clients and servers MUST support the Host request-header.

• Host request-headers are required in HTTP/1.1 requests.

• Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 request does not include a Host request-header.

• Servers MUST accept absolute URIs.

19.6 Additional Features

This appendix documents protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementers should be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications. Some of these describe proposed experimental features, and some describe features that experimental deployment found lacking that are now addressed in the base HTTP/1.1 specification.

19.6.1 Additional Request Methods

19.6.1.1 PATCH

The PATCH method is similar to PUT except that the entity contains a list of differences between the original version of the resource identified by the Request-URI and the desired content of the resource after the PATCH action has been applied. The list of differences is in a format defined by the media type of the entity (e.g., "application/diff") and MUST include sufficient information to allow the server to recreate the changes necessary to convert the original version of the resource to the desired version.

If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.

The actual method for determining how the patched resource is placed, and what happens to its predecessor, is defined entirely by the origin server. If the original version of the resource being patched included a Content-Version header field, the request entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts.

PATCH requests must obey the message transmission requirements set out in section 8.2.

Caches that implement PATCH should invalidate cached responses as defined in section 13.10 for PUT.

19.6.1.2 LINK

The LINK method establishes one or more Link relationships between the existing resource identified by the Request-URI and other existing resources. The difference between LINK and other methods allowing links to be established between resources is that the LINK method does not allow any message-body to be sent in the request and does not directly result in the creation of new resources.

If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.

Caches that implement LINK should invalidate cached responses as defined in section 13.10 for PUT.

19.6.1.3 UNLINK

The UNLINK method removes one or more Link relationships from the existing resource identified by the Request-URI. These relationships may have been established using the LINK method or by any other method supporting the Link header. The removal of a link to a resource does not imply that the resource ceases to exist or becomes inaccessible for future references.

If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.

Caches that implement UNLINK should invalidate cached responses as defined in section 13.10 for PUT.

19.6.2 Additional Header Field Definitions

19.6.2.1 Alternates

The Alternates response-header field has been proposed as a means for the origin server to inform the client about other available representations of the requested resource, along with their distinguishing attributes, and thus providing a more reliable means for a user agent to perform subsequent selection of another representation which better fits the desires of its user (described as agent-driven negotiation in section 12).

The Alternates header field is orthogonal to the Vary header field in that both may coexist in a message without affecting the interpretation of the response or the available representations. It is expected that Alternates will provide a significant improvement over the server-driven negotiation provided by the Vary field for those resources that vary over common dimensions like type and language.

The Alternates header field will be defined in a future specification.

19.6.2.2 Content-Version

The Content-Version entity-header field defines the version tag associated with a rendition of an evolving entity. Together with the Derived-From field described in section 19.6.2.3, it allows a group of people to work simultaneously on the creation of a work as an iterative process. The field should be used to allow evolution of a particular work along a single path rather than derived works or renditions in different representations.

Content-Version = "Content-Version" ":" quoted-string

Examples of the Content-Version field include:

          Content-Version: "2.1.2"
          Content-Version: "Fred 19950116-12:26:48"
          Content-Version: "2.5a4-omega7"

19.6.2.3 Derived-From

The Derived-From entity-header field can be used to indicate the version tag of the resource from which the enclosed entity was derived before modifications were made by the sender. This field is used to help manage the process of merging successive changes to a resource, particularly when such changes are being made in parallel and from multiple sources.

Derived-From = "Derived-From" ":" quoted-string

An example use of the field is:

Derived-From:

                        "2.1.1"

The Derived-From field is required for PUT and PATCH requests if the entity being sent was previously retrieved from the same URI and a Content-Version header was included with the entity when it was last retrieved.

19.6.2.4 Link

The Link entity-header field provides a means for describing a relationship between two resources, generally between the requested resource and some other resource. An entity MAY include multiple Link values. Links at the metainformation level typically indicate relationships like hierarchical structure and navigation paths. The Link field is semantically equivalent to the <LINK> element in HTML.[5]

          Link           = "Link" ":" #("<" URI ">" *( ";" link-param )
          
          link-param     = ( ( "rel" "=" relationship )
                             | ( "rev" "=" relationship )
                             | ( "title" "=" quoted-string )
                             | ( "anchor" "=" <"> URI <"> )
                             | ( link-extension ) )

link-extension = token [ "=" ( token | quoted-string ) ]

          relationship   = sgml-name
          
                         | ( <"> sgml-name *( SP sgml-name) <"> )

sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" )

Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource.

Examples of usage include:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
       
       Link: <mailto:[email protected]>; rev="Made"; title="Tim Berners-Lee"

The first example indicates that chapter2 is previous to this resource in a logical navigation path. The second indicates that the person responsible for making the resource available is identified by the given e-mail address.

19.6.2.5 URI

The URI header field has, in past versions of this specification, been used as a combination of the existing Location, Content- Location, and Vary header fields as well as the future Alternates field (above). Its primary purpose has been to include a list of additional URIs for the resource, including names and mirror locations. However, it has become clear that the combination of many different functions within this single field has been a barrier to consistently and correctly implementing any of those functions. Furthermore, we believe that the identification of names and mirror locations would be better performed via the Link header field. The URI header field is therefore deprecated in favor of those other fields.

          URI-header    = "URI" ":" 1#( "<" URI ">" )

19.7 Compatibility with Previous Versions

It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. It is worth noting that at the time of composing this specification, we would expect commercial HTTP/1.1 servers to:

• recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests;

• understand any valid request in the format of HTTP/0.9, 1.0, or 1.1;

• respond appropriately with a message in the same major version used by the client.

And we would expect HTTP/1.1 clients to:

• recognize the format of the Status-Line for HTTP/1.0 and 1.1 responses;

• understand any valid response in the format of HTTP/0.9, 1.0, or 1.1.

For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. A few implementations implement the Keep-Alive version of persistent connections described in section 19.7.1.1.

19.7.1 Compatibility with HTTP/1.0 Persistent Connections

Some clients and servers may wish to be compatible with some previous implementations of persistent connections in HTTP/1.0 clients and servers. Persistent connections in HTTP/1.0 must be explicitly negotiated as they are not the default behavior. HTTP/1.0 experimental implementations of persistent connections are faulty, and the new facilities in HTTP/1.1 are designed to rectify these problems. The problem was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting for the close on the response. The result is that HTTP/1.0 clients must be prevented from using Keep-Alive when talking to proxies.

However, talking to proxies is the most important use of persistent connections, so that prohibition is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. Persistent connections are the default for HTTP/1.1 messages; we introduce a new keyword (Connection: close) for declaring non-persistence.

The following describes the original HTTP/1.0 form of persistent connections.

When it connects to an origin server, an HTTP client MAY send the Keep-Alive connection-token in addition to the Persist connection- token:

Connection:

                      Keep-Alive

An HTTP/1.0 server would then respond with the Keep-Alive connection token and the client may proceed with an HTTP/1.0 (or Keep-Alive) persistent connection.

An HTTP/1.1 server may also establish persistent connections with HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However, a persistent connection with an HTTP/1.0 client cannot make use of the chunked transfer-coding, and therefore MUST use a Content-Length for marking the ending boundary of each message.

A client MUST NOT send the Keep-Alive connection token to a proxy server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing the Connection header field.

19.7.1.1 The Keep-Alive Header

When the Keep-Alive connection-token has been transmitted with a request or a response, a Keep-Alive header field MAY also be included. The Keep-Alive header field takes the following form:

Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param

          keepalive-param = param-name "=" value

The Keep-Alive header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters.

If the Keep-Alive header is sent, the corresponding connection token MUST be transmitted. The Keep-Alive header MUST be ignored if received without the connection token.