Network Working Group
Request for Comments: 4611
BCP: 121
Category: Best Current Practice

M. McBride
J. Meylor
D. Meyer
August 2006

Multicast Source Discovery Protocol (MSDP) Deployment Scenarios

Status of This Memo

This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited.

Copyright Notice

Abstract

This document describes best current practices for intra-domain and inter-domain deployment of the Multicast Source Discovery Protocol (MSDP) in conjunction with Protocol Independent Multicast Sparse Mode (PIM-SM).

   1. Introduction ....................................................2
      1.1. BCP, Experimental Protocols, and Normative References ......3
   2. Inter-domain MSDP Peering Scenarios .............................4
      2.1. Peering between PIM Border Routers .........................4
      2.2. Peering between Non-Border Routers .........................5
      2.3. MSDP Peering without BGP ...................................7
      2.4. MSDP Peering at a Multicast Exchange .......................7
   3. Intra-domain MSDP Peering Scenarios .............................7
      3.1. Peering between MSDP- and MBGP-Configured Routers ..........8
      3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer) .................8
      3.3. Hierarchical Mesh Groups ...................................9
      3.4. MSDP and Route Reflectors .................................10
      3.5. MSDP and Anycast RPs ......................................11
   4. Security Considerations ........................................11
      4.1. Filtering SA Messages .....................................11
      4.2. SA Message State Limits ...................................12
   5. Acknowledgements ...............................................12
   6. References .....................................................12
      6.1. Normative References ......................................12
      6.2. Informative References ....................................13

1. Introduction

MSDP [RFC3618] is used primarily in two deployment scenarios:

Between PIM Domains

MSDP can be used between Protocol Independent Multicast Sparse Mode (PIM-SM) [RFC4601] domains to convey information about active sources available in other domains. MSDP peering used in such cases is generally one-to-one peering, and utilizes the deterministic peer-RPF (Reverse Path Forwarding) rules described in the MSDP specification (i.e., it does not use mesh-groups). Peerings can be aggregated on a single MSDP peer. Such a peer can typically have from one to hundreds of peerings, which is similar in scale to BGP peerings.

Within a PIM Domain

MSDP is often used between Anycast Rendezvous Points (Anycast-RPs) [RFC3446] within a PIM domain to synchronize information about the active sources being served by each Anycast-RP peer (by virtue of IGP reachability). MSDP peering used in this scenario is typically based on MSDP mesh groups, where anywhere from two to tens of peers can comprise a given mesh group, although more than ten is not typical. One or more of these mesh-group peers may also have additional one-to-one peerings with MSDP peers outside that PIM domain for discovery of external sources. MSDP for anycast-RP without external MSDP peering is a valid deployment option and common.

Current best practice for MSDP deployment utilizes PIM-SM and the Border Gateway Protocol with multi-protocol extensions (MBGP) [RFC4271, RFC2858]. This document outlines how these protocols work together to provide an intra-domain and inter-domain Any Source Multicast (ASM) service.

The PIM-SM specification assumes that SM operates only in one PIM domain. MSDP is used to enable the use of multiple PIM domains by distributing the required information about active multicast sources to other PIM domains. Due to breaking the Internet multicast infrastructure down to multiple PIM domains, MSDP also enables the possibility of setting policy on the visibility of the groups and sources.

Transit IP providers typically deploy MSDP to be part of the global multicast infrastructure by connecting to their upstream and peer multicast networks using MSDP.

Edge multicast networks typically have two choices: to use their Internet providers' RP, or to have their own RP and connect it to their ISP using MSDP. By deploying their own RP and MSDP, they can use internal multicast groups that are not visible to the provider's RP. This helps internal multicast be able to continue to work in the event that there is a problem with connectivity to the provider or that the provider's RP/MSDP is experiencing difficulties. In the simplest cases, where no internal multicast groups are necessary, there is often no need to deploy MSDP.

1.1. BCP, Experimental Protocols, and Normative References

This document describes the best current practice for a widely deployed Experimental protocol, MSDP. There is no plan to advance the MSDP's status (for example, to Proposed Standard). The reasons for this include:

MSDP was originally envisioned as a temporary protocol to be supplanted by whatever the IDMR working group produced as an inter-domain protocol. However, the IDMR WG (or subsequently, the BGMP WG) never produced a protocol that could be deployed to replace MSDP.

One of the primary reasons given for MSDP to be classified as Experimental was that the MSDP Working Group came up with modifications to the protocol that the WG thought made it better but that implementors didn't see any reasons to deploy. Without these modifications (e.g., UDP or GRE encapsulation), MSDP can have negative consequences to initial packets in datagram streams.

Scalability: Although we don't know what the hard limits might be, readvertising everything you know every 60 seconds clearly limits the amount of state you can advertise.

MSDP reached nearly ubiquitous deployment as the de facto standard inter-domain multicast protocol in the IPv4 Internet.

No consensus could be reached regarding the reworking of MSDP to address the many concerns of various constituencies within the IETF. As a result, a decision was taken to document what is (ubiquitously) deployed and to move that document to Experimental. While advancement of MSDP to Proposed Standard was considered, for the reasons mentioned above, it was immediately discarded.

The advent of protocols such as source-specific multicast and bi- directional PIM, as well as embedded RP techniques for IPv6, have further reduced consensus that a replacement protocol for MSDP for the IPv4 Internet is required.

The RFC Editor's policy regarding references is that they be split into two categories known as "normative" and "informative". Normative references specify those documents that must be read for one to understand or implement the technology in an RFC (or whose technology must be present for the technology in the new RFC to work) [RFCED]. In order to understand this document, one must also understand both the PIM and MSDP documents. As a result, references to these documents are normative.

The IETF has adopted the policy that BCPs must not have normative references to Experimental protocols. However, this document is a special case in that the underlying Experimental document (MSDP) is not planned to be advanced to Proposed Standard.

The MBONED Working Group has requested approval under the Variance Procedure as documented in RFC 2026 [RFC2026]. The IESG followed the Variance Procedure and, after an additional 4 week IETF Last Call, evaluated the comments and status, and has approved this document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

2. Inter-domain MSDP Peering Scenarios

The following sections describe the most common inter-domain MSDP peering possibilities and their deployment options.

2.1. Peering between PIM Border Routers

In this case, the MSDP peers within the domain have their own RP located within a bounded PIM domain. In addition, the domain will typically have its own Autonomous System (AS) number and one or more MBGP speakers. The domain may also have multiple MSDP speakers. Each border router has an MSDP and MBGP peering with its peer routers. These external MSDP peering deployments typically configure the MBGP peering and MSDP peering using the same directly connected next hop peer IP address or other IP address from the same router. Typical deployments of this type are providers who have a direct peering with other providers, providers peering at an exchange, or providers who use their edge router to MSDP/MBGP peer with customers.

For a direct peering inter-domain environment to be successful, the first AS in the MBGP best path to the originating RP should be the same as the AS of the MSDP peer. As an example, consider the following topology:

         AS1----AS2----AS4
         |    /
         |   /
         |  /
         AS3

In this case, AS4 receives a Source Active (SA) message, originated by AS1, from AS2. AS2 also has an MBGP peering with AS4. The MBGP first hop AS from AS4, in the best path to the originating RP, is AS2. The AS of the sending MSDP peer is also AS2. In this case, the peer-Reverse Path Forwarding check (peer-RPF check) passes, and the SA message is forwarded.

A peer-RPF failure would occur in this topology when the MBGP first hop AS, in the best path to the originating RP, is AS2 and the origin AS of the sending MSDP peer is AS3. This reliance upon BGP AS PATH information prevents endless looping of SA packets.

Router code, which has adopted the latest rules in the MSDP document, will relax the rules between AS's a bit. In the following topology, we have an MSDP peering between AS1<->AS3 and AS3<->AS4:

                               RP
         AS1----AS2----AS3----AS4

If the first AS in best path to the RP does not equal the MSDP peer, MSDP peer-RPF fails. So AS1 cannot MSDP peer with AS3, since AS2 is the first AS in the MBGP best path to AS4 RP. With the latest MSDP document compliant code, AS1 will choose the peer in the closest AS along best AS path to the RP. AS1 will then accept SA's coming from AS3. If there are multiple MSDP peers to routers within the same AS, the peer with the highest IP address is chosen as the RPF peer.

2.2. Peering between Non-Border Routers

For MSDP peering between border routers, intra-domain MSDP scalability is restricted because it is necessary to also maintain MBGP and MSDP peerings internally towards their border routers. Within the intra-domain, the border router becomes the announcer of the next hop towards the originating RP. This requires that all intra-domain MSDP peerings mirror the MBGP path back towards the border router. External MSDP (eMSDP) peerings rely upon AS path for peer RPF checking, while internal MSDP (iMSDP) peerings rely upon the announcer of the next hop.

While the eMBGP peer is typically directly connected between border routers, it is common for the eMSDP peer to be located deeper into the transit provider's AS. Providers, which desire more flexibility in MSDP peering placement, commonly choose a few dedicated routers within their core networks for the inter-domain MSDP peerings to their customers. These core MSDP routers will also typically be in the provider's intra-domain MSDP mesh group and be configured for Anycast RP. All multicast routers in the provider's AS should statically point to the Anycast RP address. Static RP assignment is the most commonly used method for group-to-RP mapping due to its deterministic nature. Auto-RP [RFC4601] and/or the Bootstrap Router (BSR) [BSR] dynamic RP mapping mechanisms could also be used to disseminate RP information within the provider's network

For an SA message to be accepted in this (multi-hop peering) environment, we rely upon the next (or closest, with latest MSDP spec) AS in the best path towards the originating RP for the RPF check. The MSDP peer address should be in the same AS as the AS of the border router's MBGP peer. The MSDP peer address should be advertised via MBGP.

For example, in the diagram below, if customer R1 router is MBGP peering with the R2 router and if R1 is MSDP peering with the R3 router, then R2 and R3 must be in the same AS (or must appear, to AS1, to be from the same AS in the event that private AS numbers are deployed). The MSDP peer with the highest IP address will be chosen as the MSDP RPF peer. R1 must also have the MSDP peer address of R3 in its MBGP table.

         +--+    +--+    +--+
         |R1|----|R2|----|R3|
         +--+    +--+    +--+
         AS1     AS2     AS2

From R3's perspective, AS1 (R1) is the MBGP next AS in the best path towards the originating RP. As long as AS1 is the next AS (or closest) in the best path towards the originating RP, RPF will succeed on SAs arriving from R1.

In contrast, with the single hop scenario, with R2 (instead of R3) border MSDP peering with R1 border, R2's MBGP address becomes the announcer of the next hop for R3, towards the originating RP, and R3 must peer with that R2 address. Moreover, all AS2 intra-domain MSDP peers need to follow iMBGP (or other IGP) peerings towards R2 since iMSDP has a dependence upon peering with the address of the MBGP (or other IGP) announcer of the next hop.

2.3. MSDP Peering without BGP

In this case, an enterprise maintains its own RP and has an MSDP peering with its service provider but does not BGP peer with them. MSDP relies upon BGP path information to learn the MSDP topology for the SA peer-RPF check. MSDP can be deployed without BGP, however, and as a result, there are some special cases where the requirement to perform a peer-RPF check on the BGP path information is suspended. These cases are:

There is only a single MSDP peer connection.

A default peer (default MSDP route) is configured.

The originating RP is directly connected.

A mesh group is used.

An implementation is used that allows for an MSDP peer-RPF check using an IGP.

An enterprise will typically configure a unicast default route from its border router to the provider's border router and then MSDP peer with the provider's MSDP router. If internal MSDP peerings are also used within the enterprise, then an MSDP default peer will need to be configured on the border router that points to the provider. In this way, all external multicast sources will be learned, and internal sources can be advertised. If only a single MSDP peering was used (no internal MSDP peerings) towards the provider, then this stub site will MSDP default peer towards the provider and skip the peer-RPF check.

2.4. MSDP Peering at a Multicast Exchange

Multicast exchanges allow multicast providers to peer at a common IP subnet (or by using point-to-point virtual LANs) and share MSDP SA updates. Each provider will MSDP and MBGP peer with each others directly connected exchange IP address. Each exchange router will send/receive SAs to/from their MSDP peers. They will then be able to forward SAs throughout their domain to their customers and any direct provider peerings.

3. Intra-domain MSDP Peering Scenarios

The following sections describe the different intra-domain MSDP peering possibilities and their deployment options.

3.1. Peering between MSDP- and MBGP-Configured Routers

The next hop IP address of the iBGP peer is typically used for the MSDP peer-RPF check (IGP can also be used). This is different from the inter-domain BGP/MSDP case, where AS path information is used for the peer-RPF check. For this reason, it is necessary for the IP address of the MSDP peer connection to be the same as the internal MBGP peer connection whether or not the MSDP/MBGP peers are directly connected. A successful deployment would be similar to the following:

                                 +----+
                                 | Rb | 3.3.3.3
                               / +----+
          AS1          AS2    /     |
         +---+         +--+  /      |
         |RP1|---------|Ra|         |
         +---+         +--+         |
         1.1.1.1     2.2.2.2        |
                             \      |
                              \     |
                               \ +-----+
                                 | RP2 |
                                 +-----+

where RP2 MSDP and MBGP peers with Ra (using 2.2.2.2) and with Rb (using 3.3.3.3). When the MSDP SA update arrives on RP2 from Ra, the MSDP RPF check for 1.1.1.1 passes because RP2 receives the SA update from MSDP peer 2.2.2.2, which is also the correct MBGP next hop for 1.1.1.1.

When RP2 receives the same SA update from MSDP peer 3.3.3.3, the MBGP lookup for 1.1.1.1 shows a next hop of 2.2.2.2, so RPF correctly fails, preventing a loop.

This deployment could also fail on an update from Ra to RP2 if RP2 was MBGP peering to an address other than 2.2.2.2 on Ra. Intra- domain deployments must have MSDP and MBGP (or other IGP) peering addresses that match, unless a method to skip the peer-RPF check is deployed.

3.2. MSDP Peer Is Not BGP Peer (or No BGP Peer)

This is a common MSDP intra-domain deployment in environments where few routers are running MBGP or where the domain is not running MBGP. The problem here is that the MSDP peer address needs to be the same as the MBGP peer address. To get around this requirement, the intra- domain MSDP RPF rules have been relaxed in the following topologies:

By configuring the MSDP peer as a mesh group peer.

By having the MSDP peer be the only MSDP peer.

By configuring a default MSDP peer.

By peering with the originating RP.

By relying upon an IGP for MSDP peer-RPF.

The common choice around the intra-domain BGP peering requirement, when more than one MSDP peer is configured, is to deploy MSDP mesh groups. When an MSDP mesh group is deployed, there is no RPF check on arriving SA messages when they are received from a mesh group peer. Subsequently, SA messages are always accepted from mesh group peers. MSDP mesh groups were developed to reduce the amount of SA traffic in the network since SAs, which arrive from a mesh group peer, are not flooded to peers within that same mesh group. Mesh groups must be fully meshed.

If recent (but not currently widely deployed) router code is running that is fully compliant with the latest MSDP document, another option, to work around not having BGP to MSDP RPF peer, is to RPF using an IGP like OSPF, IS-IS, RIP, etc. This new capability will allow for enterprise customers, who are not running BGP and who don't want to run mesh groups, to use their existing IGP to satisfy the MSDP peer-RPF rules.

3.3. Hierarchical Mesh Groups

Hierarchical mesh groups are occasionally deployed in intra-domain environments where there are a large number of MSDP peers. Allowing multiple mesh groups to forward to one another can reduce the number of MSDP peerings per router (due to the full mesh requirement) and hence reduce router load. A good hierarchical mesh group implementation (one that prevents looping) contains a core mesh group in the backbone, and these core routers serve as mesh group aggregation routers:

                      [R2]{A,2}
                      /  \
                     /    \
                    /      \
                   /        \
                  /          \
                 /            \
                /              \
         {A,1}[R1]-------------[R3]{A,3}

In this example, R1, R2, and R3 are in MSDP mesh group A (the core mesh group), and each serves as MSDP aggregation routers for their leaf (or second tier) mesh groups 1, 2, and 3. Since SA messages received from a mesh group peer are not forwarded to peers within that same mesh group, SA messages will not loop. Do not create topologies that connect mesh groups in a loop. In the above example, for instance, second-tier mesh groups 1, 2, and 3 must not directly exchange SA messages with each other or an endless SA loop will occur.

Redundancy between mesh groups will also cause a loop and is subsequently not available with hierarchical mesh groups. For instance, assume that R3 had two routers connecting its leaf mesh group 3 with the core mesh group A. A loop would be created between mesh group 3 and mesh group A because each mesh group must be fully meshed between peers.

3.4. MSDP and Route Reflectors

BGP requires all iBGP speakers that are not route-reflector clients or confederation members be fully meshed to prevent loops. In the route reflector environment, MSDP requires that the route reflector clients peer with the route reflector since the router reflector (RR) is the BGP announcer of the next hop towards the originating RP. The RR is not the BGP next hop, but is the announcer of the BGP next hop. The announcer of the next hop is the address typically used for MSDP peer-RPF checks. For example, consider the following case:

               Ra--------RR
                         /|\
                        / | \
                       A  B  C

Ra is forwarding MSDP SAs to the route reflector RR. Routers A, B, and C also MSDP peer with RR. When RR forwards the SA to A, B, and C, these RR clients will accept the SA because RR is the announcer of the next hop to the originating RP address.

An SA will peer-RPF fail if Ra MSDP peers directly with Routers A, B, or C because the announcer of the next hop is RR but the SA update came from Ra. Proper deployment is to have RR clients MSDP peer with the RR. MSDP mesh groups may be used to work around this requirement. External MSDP peerings will also prevent this requirement since the next AS is compared between MBGP and MSDP peerings, rather than the IP address of the announcer of the next hop.

Some recent MSDP implementations conform to the latest MSDP document, which relaxes the requirement of peering with the Advertiser of the next hop (the Route Reflector). This new rule allows for peering with the next hop, in addition to the Advertiser of the next hop. In the example above, for instance, if Ra is the next hop (perhaps due to using BGP's next hop self attribute), and if routers A, B, and C are peering with Ra, the SA's received from Ra will now succeed.

3.5. MSDP and Anycast RPs

A network with multiple RPs can achieve RP load sharing and redundancy by using the Anycast RP mechanism in conjunction with MSDP mesh groups [RFC3446]. This mechanism is a common deployment technique used within a domain by service providers and enterprises that deploy several RPs within their domains. These RPs will each have the same IP address configured on a Loopback interface (making this the Anycast address). These RPs will MSDP peer with each other using a separate loopback interface and are part of the same fully meshed MSDP mesh group. This loopback interface, used for MSDP peering, will typically also be used for the MBGP peering. All routers within the provider's domain will learn of the Anycast RP address through Auto-RP, BSR, or a static RP assignment. Each designated router in the domain will send source registers and group joins to the Anycast RP address. Unicast routing will direct those registers and joins to the nearest Anycast RP. If a particular Anycast RP router fails, unicast routing will direct subsequent registers and joins to the nearest Anycast RP. That RP will then forward an MSDP update to all peers within the Anycast MSDP mesh group. Each RP will then forward (or receive) the SAs to (from) external customers and providers.

4. Security Considerations

An MSDP service should be secured by explicitly controlling the state that is created by, and passed within, the MSDP service. As with unicast routing state, MSDP state should be controlled locally, at the edge origination points. Selective filtering at the multicast service edge helps ensure that only intended sources result in SA message creation, and this control helps to reduce the likelihood of state-aggregation related problems in the core. There are a variety of points where local policy should be applied to the MSDP service.

4.1. Filtering SA Messages

The process of originating SA messages should be filtered to ensure that only intended local sources are resulting in SA message origination. In addition, MSDP speakers should filter which SA messages get received and forwarded.

Typically, there is a fair amount of (S,G) state in a PIM-SM domain that is local to the domain. However, without proper filtering, SA messages containing these local (S,G) announcements may be advertised to the global MSDP infrastructure. Examples of this include domain- local applications that use global IP multicast addresses and sources that use RFC 1918 addresses [RFC1918]. To improve on the scalability of MSDP and to avoid global visibility of domain local (S,G) information, an external SA filter list is recommended to help prevent unnecessary creation, forwarding, and caching of well-known domain local sources.

4.2. SA Message State Limits

Proper filtering on SA message origination, receipt, and forwarding will significantly reduce the likelihood of unintended and unexpected spikes in MSDP state. However, an SA-cache state limit SHOULD be configured as a final safeguard to state spikes. When an MSDP peering has reached a stable state (i.e., when the peering has been established and the initial SA state has been transferred), it may also be desirable to configure a rate limiter for the creation of new SA state entries.

5. Acknowledgements

The authors would like to thank Pekka Savola, John Zwiebel, Swapna Yelamanchi, Greg Shepherd, and Jay Ford for their feedback on earlier versions of this document.

6. References

6.1. Normative References

   [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
   
             "Protocol Independent Multicast - Sparse Mode (PIM-SM):
             Protocol Specification (Revised)", RFC 4601, August 2006.
   
   [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
             Protocol 4 (BGP-4)", RFC 4271, January 2006.
   
   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
             and E. Lear, "Address Allocation for Private Internets",
             BCP 5, RFC 1918, February 1996.
   
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC2858] Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
             "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.
   
   [RFC3446] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast
             Rendevous Point (RP) mechanism using Protocol Independent
             Multicast (PIM) and Multicast Source Discovery Protocol
             (MSDP)", RFC 3446, January 2003.
   
   [RFC3618] Fenner, B. and D. Meyer, "Multicast Source Discovery
             Protocol (MSDP)", RFC 3618, October 2003.

6.2. Informative References

   [BSR]     Fenner, W., et. al., "Bootstrap Router (BSR) Mechanism for
             PIM Sparse Mode", Work in Progress, February 2003.
   
   [RFCED]   http://www.rfc-editor.org/policy.html

Authors' Addresses

Mike McBride
Cisco Systems

EMail:

          [email protected]

John Meylor
Cisco Systems

EMail:

          [email protected]
   
   David Meyer
   
   EMail: [email protected]

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