Network Working Group                                     Dino Farinacci
INTERNET-DRAFT                                          Procket Networks
Expiration Date: July 2003                                     Yiqun Cai
                                                           cisco Systems
                                                        January 15, 2003

                          Anycast-RP using PIM

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This proposal allows Anycast-RP to be used inside a domain which runs
   PIM only. There are no other multicast protocols required to support
   Anycast-RP, such as MSDP, which has been used traditionally to solve
   this problem.

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1. Introduction

   Anycast-RP as described in [2] is a mechanism ISP-based backbones
   have used to get fast convergence when a PIM Rendezvous Point (RP)
   router fails. To allow receivers and sources to Rendezvous to the
   closest RP, the packets from a source needs to get to all RPs to find
   joined receivers.

   This notion of receivers finding sources is the fundamental problem
   of source discovery which MSDP was intended to solve. However, if one
   would like to retain the Anycast-RP benefits from [2] with less
   protocol machinery, removing MSDP from the solution space is an

   This draft extends the Register mechanism in PIM so Anycast-RP
   functionality can be retained without using MSDP.

2. Requirements

   o Each router acting as an RP MUST be configured with a loopback
     interface using the same (shared) IP address. This address is used to
     tell other routers in the PIM domain, what IP address to use for the
     RP address.

   o The RP address or a prefix that covers the RP address is injected
     into the unicast routing system inside of the domain.

   o Each RP configures all other RPs used in the Anycast-RP set. This
     must be consistently configured in all RPs in the set.

3. Mechanism

   The following diagram illustrates a domain using 3 RPs where
   receivers are joining to the closest RP according to where unicast
   routing metrics take them and 2 sources sending packets to their
   respective RPs.

         S1-----RP1              RP2                RP3------S3
                / \               |
               /   \              |
              R1   R1'            R2

   Assume the above scenario is completely connected where R1, R1', and
   R2 are receivers for a group, and S1 and S2 send to that group.
   Assume RP1, RP2 and RP3 are all assigned the same IP address which is
   used as the Anycast-RP address (let's say the IP address is RPA).

Farinacci, Cai                                                  [Page 2]

Internet Draft            Anycast-RP using PIM              January 2003

   The following procedure is used when S1 starts sourcing traffic:

   o S1 sends a multicast packet.

   o The DR directly attached to S1 will form a PIM Register message to
     send to RP1. The IP address to use is the Anycast-RP address.

   o RP1 will receive the PIM Register message, decapsulate it, send the
     packet down the shared-tree to get the packet to receivers R1 and R1'.

   o RP1 is configured with RP2 and RP3's IP address. It will forward the
     Register message from S1's DR to both of them. RP1 will include
     it's own IP address as the source address for the PIM Register

   o RP1 sends a Register-Stop back to the DR.

   o RP1 MAY join back to the source-tree by triggering a (S1,G) Join
     message toward S1. However, RP1 MUST create (S1,G) state.

   o RP2 receives the Register message from RP1, decapsulates it, and
     also sends the packet down the shared-tree to get the packet to
     receiver R2.

   o RP2 sends a Register-Stop back to the RP1.

   o RP2 MAY join back to the source-tree by triggering a (S1,G) Join
     message toward S1. However, RP2 MUST create (S1,G) state.

   o RP3 receives the Register message from RP1, decapsulates it, but
     since there is no receivers joined for the group, it can discard
     the packet.

   o RP3 sends a Register-Stop back to the RP1.

   o RP3 creates (S1,G) state so when a receiver joins after S1 starts
     sending, RP3 can join quickly to the source-tree for S1.

   The procedure for S3 sending follows the same as above but it is RP3
   which forwards the Register originated by S3's DR to RP1 and RP2.
   Therefore, this example shows how sources anywhere in the domain,
   associated with different RPs, can reach all receivers, also
   associated with different RPs, in the same domain.

4. Observations and Guidelines about this Proposal

   o An RP will forward a Register only if the Register is received

Farinacci, Cai                                                  [Page 3]

Internet Draft            Anycast-RP using PIM              January 2003

     from an IP address not in the Anycast-RP list (i.e. the Register
     came from a DR and not another RP).

   o Each DR that PIM registers for a source will send the message to it's
     closest RP address. Therefore there are no changes to the DR logic.

   o Packets flow to all receivers no matter what RP they have joined to.

   o The source gets Registered to a single RP by the DR, it's the
     responsibility of the RP, the DR selects, to get the packet to all
     other RPs in the Anycast-RP set.

   o Logic is changed only in the RPs. The logic change is for forwarding
     Register messages. Register-Stop processing is unchanged. However, an
     implementation MAY suppress sending Register-Stop messages in response
     to a Register received from an RP.

   o The rate-limiting of Register and Register-Stop messages are done
     end-to-end. That is from DR -> RP1 -> {RP2 and RP3}. There is no need for
     specific rate-limiting logic between the RPs.

   o When topology changes occur, the existing source-tree adjusts as it
     does today according to [1]. The existing shared-trees, as well,
     adjust as it does today according to [1].

   o Physical RP changes are as fast as unicast route convergence.
     Retaining the benefit of [2].

   o An RP that doesn't support this draft can be mixed with RPs that do
     support this draft. However, the non-supporter RPs should not have
     sources registering to it but may have receivers joined to it.

   o If Null Registers are sent (Registers with an IP header and no IP
     payload), they MUST be replicated to all of the RPs in the Anycast-RP
     set so that source state remains alive for active sources.

   o The number of RPs in the Anycast-RP set should remain small so the
     amount of non-native replication is kept to a minimum.

5. Possible Configuration Language

A possible set of commands to be used could be:

    ip pim anycast-rp <anycast-rp-addr> <rp-addr>


Farinacci, Cai                                                  [Page 4]

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    <anycast-rp-addr> describes the Anycast-RP set for the RP which
    is assigned to the group range. This IP address is the address
    that first-hop and last-hop PIM routers use to register and join

    <rp-addr> describes the IP address where Register messages are
    forwarded to. This IP address is any address assigned to the RP
    router not including the <anycast-rp-addr>.


    From the illustration above, the configuration commands would be:

    ip pim anycast-rp RPA RP1
    ip pim anycast-rp RPA RP2
    ip pim anycast-rp RPA RP3


    It may be useful to include the local router's IP address in the
    command set so the above lines can be cut-and-pasted or scripted
    into all the RPs in the Anycast-RP set.

    But the implementation would have to be aware of it's own address
    and not inadvertently send a Register to itself.

6. Interaction with MSDP running in an Anycast-PIM Router

   The objective of this Anycast-PIM proposal is to remove the
   dependence on using MSDP. This can be achieved by removing MSDP
   peering between the Anycast RPs. However, to advertise internal
   sources to routers outside of a PIM routing domain, MSDP may still be

   In this capacity, an Anycast-RP that receives a Register message
   should process it as described in this specification as well as how
   to process a Register message as described in [2].

   A source should be considered internal to the domain, when it is
   discovered by an Anycast-RP through a received Register message.
   Regardless, if the Register message was sent by a DR, another
   Anycast-RP member, or the router itself.

   It is recommended that an Anycast-RP set defined in this
   specification should not be mixed with MSDP peering amoung the
   members. In some cases, the source discovery will work but it may not

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   be obvious to the implementations what sources are local to the
   domain and which are not. This may affect external MSDP advertisement
   of internal sources.

Farinacci, Cai                                                  [Page 6]

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7. Acknowledgments

   The authors would like to thank Yiqun Cai and Dino Farinacci for
   prototyping this draft in the cisco IOS implementation and Procket
   implementation, respectively.

   The authors would like to thank John Zwiebel for doing
   interoperability testing of the two prototype implementations.

   And finally, the authors would like to thank Greg Shepard for
   comments on the draft.

8. Author Information

   Dino Farinacci
   Procket Networks

   Yiqun Cai
   cisco Systems

9. References

   [1] Estrin, et al., "Protocol Independent Multicast-Sparse Mode (PIM-
       SM): Protocol Specification", RFC 2362, June 1998.

   [2] Kim, Meyer, Kilmer, Farinacci, "Anycast RP mechanism using PIM
       and MSDP", Internet Draft draft-ietf-mboned-anycast-rp-08.txt,
       May 2001.

Farinacci, Cai                                                  [Page 7]