Request for Comments: 3446
Anycast Rendevous Point (RP) mechanism using
Protocol Independent Multicast (PIM)
and Multicast Source Discovery Protocol (MSDP)
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright © The Internet Society (2003). All Rights Reserved.
This document describes a mechanism to allow for an arbitrary number of Rendevous Points (RPs) per group in a single shared-tree Protocol Independent Multicast-Sparse Mode (PIM-SM) domain.
PIM-SM, as defined in RFC 2362, allows for only a single active RP per group, and as such the decision of optimal RP placement can become problematic for a multi-regional network deploying PIM-SM.
Anycast RP relaxes an important constraint in PIM-SM, namely, that there can be only one group to RP mapping can be active at any time. The single mapping property has several implications, including traffic concentration, lack of scalable register decapsulation (when using the shared tree), slow convergence when an active RP fails, possible sub-optimal forwarding of multicast packets, and distant RP dependencies. These properties of PIM-SM have been demonstrated in native continental or inter-continental scale multicast deployments. As a result, it is clear that ISP backbones require a mechanism that allows definition of multiple active RPs per group in a single PIM-SM domain. Further, any such mechanism should also address the issues addressed above.
The mechanism described here is intended to address the need for better fail-over (convergence time) and sharing of the register decapsulation load (again, when using the shared-tree) among RPs in a domain. It is primarily intended for applications within those networks using MBGP, Multicast Source Discovery Protocol [MSDP] and PIM-SM protocols, for native multicast deployment, although it is not limited to those protocols. In particular, Anycast RP is applicable in any PIM-SM network that also supports MSDP (MSDP is required so that the various RPs in the domain maintain a consistent view of the sources that are active). Note however, a domain deploying Anycast RP is not required to run MBGP. Finally, a general requirement of the Anycast RP scheme is that the anycast address MUST NOT be used as the RP address in the RP's SA messages.
2. Problem Definition
The anycast RP solution provides a solution for both fast fail-over and shared-tree load balancing among any number of active RPs in a domain.
2.1. Traffic Concentration and Distributing Decapsulation Load Among RPs
While PIM-SM allows for multiple RPs to be defined for a given group, only one group to RP mapping can be active at a given time. A traditional deployment mechanism for balancing register decapsulation load between multiple RPs covering the multicast group space is to split up the 126.96.36.199/4 space between multiple defined RPs. This is an acceptable solution as long as multicast traffic remains low, but has problems as multicast traffic increases, especially because the network operator defining group space split between RPs does not always have a priori knowledge of traffic distribution between groups. This can be overcome via periodic reconfigurations, but operational considerations cause this type of solution to scale poorly.
2.2. Sub-optimal Forwarding of Multicast Packets
When a single RP serves a given multicast group, all joins to that group will be sent to that RP regardless of the topological distance between the RP and the sources and receivers. Initial data will be sent towards the RP also until configured the shortest path tree switch threshold is reached, or the data will always be sent towards the RP if the network is configured to always use the RP rooted shared tree. This holds true even if all the sources and the receivers are in any given single region, and RP is topologically distant from the sources and the receivers. This is an artifact of the dynamic nature of multicast group members, and of the fact that operators may not always have a priori knowledge of the topological placement of the group members.
Taken together, these effects can mean that (for example) although all the sources and receivers of a given group are in Europe, they are joining towards the RP in the USA and the data will be traversing a relatively expensive pipe(s) twice, once to get to RP, and back down the RP rooted tree again, creating inefficient use of expensive resources.
2.3. Distant RP Dependencies
As outlined above, a single active RP per group may cause local sources and receivers to become dependent on a topologically distant RP. In addition, when multiple RPs are configured, there can be considerable convergence delay involved in switching to the backup RP. This delay may exist independent of the toplogical location of the primary and backup RPs.
Given the problem set outlined above, a good solution would allow an operator to configure multiple RPs per group, and distribute those RPs in a topologically significant manner to the sources and receivers.
All the RPs serving a given group or set of groups are configured with an identical anycast address, using a numbered interface on the RPs (frequently a logical interface such as a loopback is used). RPs then advertise group to RP mappings using this interface address. This will cause group members (senders) to join (register) towards the topologically closest RP. RPs MSDP peer with each other using an address unique to each RP. Since the anycast address is not a unique address (by definition), a router MUST NOT choose the anycast unicast address as the router ID, as this can prevent peerings and/or adjacencies from being established.
In summary then, the following steps are required:
3.1.1. Create the set of group-to-anycast-RP-address mappings
The first step is to create the set of group-to-anycast-RP-address mappings to be used in the domain. Each RP participating in an anycast RP set must be configured with a consistent set of group to RP address mappings. This mapping will be used by the non-RP routers in the domain.
3.1.2. Configure each RP for the group range with the anycast RP address
The next step is to configure each RP for the group range with the anycast RP address. If a dynamic mechanism, such as auto-RP or the PIMv2 bootstrap mechanism, is being used to advertise group to RP mappings, the anycast IP address should be used for the RP address.
3.1.3. Configure MSDP peerings between each of the anycast RPs in the
Unlike the group to RP mapping advertisements, MSDP peerings must use an IP address that is unique to the endpoints; that is, the MSDP peering endpoints MUST use a unicast rather than anycast address. A general guideline is to follow the addressing of the BGP peerings, e.g., loopbacks for iBGP peering, physical interface addresses for eBGP peering. Note that the anycast address MUST NOT be used as the RP address in SA messages (as this would case the peer-RPF check to fail).
3.1.4. Configure the non-RP's with the group-to-anycast-RP-address
Finally, each non-RP router must learn the set of group to RP mappings. This could be done via static configuration, auto-RP, or by PIMv2 bootstrap mechanism.
3.1.5. Ensure that the anycast IP address is reachable by all routers in
This is typically accomplished by causing each RP to inject the /32 into the domain's IGP.
3.2. Interaction with MSDP Peer-RPF check
Each MSDP peer receives and forwards the message away from the RP address in a "peer-RPF flooding" fashion. The notion of peer-RPF flooding is with respect to forwarding SA messages [MSDP]. The BGP routing tables are examined to determine which peer is the next hop towards the originating RP of the SA message. Such a peer is called an "RPF peer". See [MSDP] for details of the Peer-RPF check.
3.3. State Implications
It should be noted that using MSDP in this way forces the creation of (S,G) state along the path from the receiver to the source. This state may not be present if a single RP was used and receivers were forced to stay on the shared tree.
4. Security considerations
Since the solution described here makes heavy use of anycast addressing, care must be taken to avoid spoofing. In particular unicast routing and PIM RPs must be protected.
4.1. Unicast Routing
4.1.1. Effects of Unicast Routing Instability
While not a security issue, it is worth noting that if unicast routing is unstable, then the actual RP that source or receiver is using will be subject to the same instability.
4.2. Multicast Protocol Integrity
The mechanisms described in [RFC2362] should be used to provide protocol message integrity protection and group-wise message origin authentication.
4.3. MSDP Peer Integrity
As is the the case for BGP, MSDP peers can be protected using keyed MD5 [RFC1828].
John Meylor, Bill Fenner, Dave Thaler and Tom Pusateri provided insightful comments on earlier versions for this idea.
This memo is a product of the MBONE Deployment Working Group (MBONED) in the Operations and Management Area of the Internet Engineering Task Force. Submit comments to <firstname.lastname@example.org> or the authors.
[MSDP] D. Meyer and B. Fenner, Editors, "Multicast Source Discovery Protocol (MSDP)", Work in Progress. [RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, August 1995. [RFC1828] Metzger, P. and W. Simpson, "IP Authentication using Keyed MD5", RFC 1828, August 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M., Jacobson, V., Liu, C., Sharma, P. and L. Wei, "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC 2362, June 1998. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998. [RFC2403] Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within ESP and AH", RFC 2403, November 1998.
7. Author's Address
Dorian Kim Verio, Inc. EMail: email@example.com
Dino Farinacci Procket Networks EMail: firstname.lastname@example.org
8. Full Copyright Statement
Copyright © The Internet Society (2003). All Rights Reserved.
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