MBONED Working Group                               Dorian Kim
Internet Draft                                     Verio
                                                   David Meyer
                                                   Cisco Systems
                                                   Henry Kilmer
                                                   Dino Farinacci

Category                                           Informational
                                                   October,
                                                   November, 1999

                Anycast RP mechanism using PIM and MSDP
                 <draft-ietf-mboned-anycast-rp-00.txt>
                 <draft-ietf-mboned-anycast-rp-01.txt>

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.

   Internet-Drafts RFC2026.

   2026 are working documents of the Internet Engineering Task Force
   (IETF), its areas, and its working groups.  Note that other groups
   may also distribute working documents as Internet- Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

2. Abstract

   This document describes a mechanism to allow for an arbitrary number
   of RPs per group in a single share-tree PIM-SM domain.

   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 <mboned@ns.uoregon.edu> or the
   authors.

3. Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

4. Introduction

   PIM-SM as currently defined 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.

   The single active RP, or flat RP space design of PIM-SM has several
   implications, including traffic concentration, lack of scalable load
   balancing and redundancy between RPs, sub-optimal forwarding of
   multicast packets, and distant RP dependencies. These properties of
   PIM-SM have been demonstrated in recent native continental or inter-
   continental scale multicast deployments. As a result, it became clear
   that ISP backbones require a mechanism that allows definition of
   multiple active RPs per group in single PIM-SM domain. Further, any
   such mechanism should also addresses the issues addressed above.

   The mechanism described here is intended to address the need for
   redundancy and load sharing among RPs in a domain. It is primarily
   intended for application within those networks which are using MBGP,
   MSDP and PIM-SM protocols for native multicast deployment, although
   it 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.

5. Problem Definition

   The anycast RP solution provides a solution for both redundancy and
   load balancing among any number of active RPs in a domain.

5.1. Traffic Concentration and Load Balancing Between RPs

   While PIM-SM allows for multiple RPs to be defined for a given group,
   only one group to RP mapping can active at a given time. A
   traditional deployment mechanism for load balancing between multiple
   RPs covering the multicast group space is to split up the 224.0.0.0/4
   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 alway 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. The other alternative to periodic
   reconfiguration is to split 224.0.0.0/4 space more finely between
   more RPs, but this solution can have the disadvantage of creating
   more complex RP configurations, along with the attendant operational
   problems when RPs are configured [CLUSTERS].

5.2. 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 shortest path tree switch
   threshold is is reached, or the data will always be sent towards the
   RP if the network is configured to always use 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 USA and the data will be traversing
   relatively expensive pipe(s) twice, once to get to RP, and back down
   the RP rooted tree again, creating inefficient use of expensive
   resources.

5.3. Distant RP Dependencies

   As outlined above, single active RP per group may cause local sources
   and receivers to become dependent on a topologically distant RP. In
   case of a scenario where there are backup RPs configured, distant RP
   dependence can be created due to the failure of the primary RP, which
   is topologically closer, and may become exacerbated by switching to
   the backup RP, which may be even more distant topologically, which
   may lead to inferior performance, if not outright loss of
   connectivity to an RP serving the group, depending on the network
   condition at the given moment.

6. Solution

   Given the problem set outlined above, a good solution would allow an
   operator to define multiple RPs per group, and distribute those RPs
   in a topologically significant manner to the sources and receivers.

6.1. Mechanisms

   All the RPs serving a given group or set of groups are configured
   with identical unicast 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 the
   unique shared addresses. Note that if the router implementation
   chooses the shared address for the BGP router ID, then BGP peerings
   will not be established. As a result, care should be taken to avoid
   the ambiguity of the BGP router ID with the RP address (for example,
   if the logical address chosen is the highest IP address configured on
   the router, and the router implementation that automatically chooses
   a router ID based upon highest IP address assigned to interfaces).
   Finally, the solution described here can be implemented without any
   modification to existing protocols or their implementations.

6.2. 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. messages [MSDP]. The BGP or
   MBGP 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".  There are a few simple rules that govern how MSDP
   Peer-RPF checks. These rules should be kept in mind when configuring
   Anycast RP:

6.2.1. Singly Homed MSDP Speaker

   A singly homed MSDP speaker always accepts SA messages from its peer.

6.2.2. RP in SA is a MSDP Peer

   A MSDP speaker always accepts SAs for which the RP in the SA message
   is a peer.

6.2.3. Router is itself RP in SA message

   A MSDP speaker always rejects an SA from any peer if it the RP in the
   SA message.

6.2.4. Router has default peers

   If a MSDP speaker has one or more default peers configured, then it
   will accept an SA message if comes from the default peer for the RP
   in the SA message.

6.2.5. Complex MSDP Scenario

   Consider routers R1, R2, and R3 form an Anycast RP mesh  See [MSDP] for an AS. C1
   and C2 are customer routers (and the BGP session not multi-hop); RP
   is C1's RP. P1 is a peer router. The picture is as follows:

              MSDP/MBGP        Anycast RPs
         C1 ============= R1----------------R2
          |  SA(S,G,RP)                    /
          |                               /
          |  MBGP/MSDP        MSDP  MSDP /
          |                             /
          |                            /
         C2 SA(S',G',RP')             /
                                     /
                                   R3 ============== P1
                                        SA(S,G,RP)
                                         ------->

6.2.6. Internal MSDP Peering

   When R1 sees SA(S,G,RP) from C1, it sees the next-hop toward prefix
   covering RP is through C1, so R1 accepts the SA. R1 will forward
   SA(S,G,RP) to R2 and R3, which will only accept SA(S,G,RP) if R1 is
   announcing  (and is) the next-hop towards the originating RP each SA
   message. Note that operationally, this means next-hop needs to be the
   same as the MSDP connect-source.

   This implies that if you want to pass on an SA internally, you have
   to be announcing the next-hop towards the AS that originates the
   prefix covering the originating RP. Note that the MSDP connect-source
   has to be the interface that is configured with the address of the
   next-hop.

   Now, if C2 tries MSDP peer with R1 directly (cutting out transit
   provider C1), then C2's SA RPF fails at R1, because R1 expects SA
   message to come from a MSDP peer in the next AS in the AS-PATH
   towards the originating RP, which in this case would C1.

6.2.6.1. RULE

   An internal MSDP peer will accept an SA message from another internal
   peer iff that peer is the advertiser details of towards the prefix covering the RP which originated the SA.

6.2.7. External MSDP Peering

   External peer P1 will accept an SA from R3 iff R3 comes from the next
   AS in the path. This breaks, for example, if P1 peers with C1.

6.2.7.1. RULE

   An external MSDP peer will accept an SA message from another peer iff
   the peer is in the next AS in the path towards the AS originating the
   prefix covering the RP in the SA message. Peer-RPF check.

6.3. Further Applications of Anycast RP mechanism

   The solution described above can also be applied to external MSDP
   peers that are used to join two PIM-SM domains together.  This can
   provide redundancy to the MSDP peering session, ease operational
   complexity as well as simplify configuration management.  A side
   effect to be aware of with this design is that which of the
   configured MSDP sessions comes up will be determined via the unicast
   topology between two providers, and can be some what unpredictable.
   If any of the backup peering sessions resets, the active session will
   also reset.

7. Multicast State Scaling

       Let  k = m + r, where

       r = registering to an RP
       m = number internal sources learned through MSDP
       p = number of anycast (internal) MSDP peers

       For p = 1, m = 0

        0 receivers              ==> 1 (*,G) + 0 SAs
        Greater than 1 receiver  ==> k (S,G) + 0 SAs

       For p > 1, m != 0

        0 receivers              ==> 1 (*,G) + m SAs
        Greater than 1 receiver  ==> k (S,G) + m SAs

   Importantly, the multicast state growth is O(k), where k is not a
   function of p, the number of anycast RP peers.

8. 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.

8.1. Unicast Routing

   Both internal and external unicast routing can be weakly protected
   with keyed MD5 [RFC1828], as implemented in an internal protocol such
   as OSPF [RFC2382] or in BGP [RFC2385]. More generally,  IPSEC
   [RFC1825] could be used to provide protocol integrity for the unicast
   routing system.

8.2. Multicast Protocol Integrity

   The mechanisms described in [PIMAUTH] should be used to provide
   protocol message integrity protection and group-wise message origin
   authentication.

8.3. MSDP Peer Integrity

   As is the the case for BGP, MSDP peers can be protected using keyed
   MD5 [RFC1828].

9. Acknowledgments

   John Meylor, Dave Thaler and Tom Pusateri provided insightful
   comments on earlier versions for this idea.

10. References

    [CLUSTERS] D. Farinacci, et. al., "Use of Anycast Clusters for
               Inter-Domain Multicast Routing",
               draft-ietf-farinacci-anycast-clusters-01.txt, March,
               1998. ftp://ftpeng.cisco.com/ipmulticast/internet-drafts

    [MSDP]     D. Farinacci, et. al., "Multicast Source Discovery
               Protocol (MSDP)", draft-farinacci-msdp-00.txt,
               June, 1998. draft-ietf-msdp-spec-02.txt,
               November, 1999.

    [PIMAUTH]  L. Wei, et al., "Authenticating PIM version 2 messages",
               draft-ietf-pim-v2-auth-00.txt, November, 1998.

    [RFC1825]  Atkinson, R., "IP Security Architecture", August 1995.

    [RFC1828]  P. Metzger and W. Simpson, "IP Authentication using Keyed
               MD5", RFC 1828, August, 1995.

    [RFC2362]  D. Estrin, et. al., "Protocol Independent Multicast-
               Sparse Mode (PIM-SM): Protocol Specification", RFC
               2362, June, 1998.

    [RFC2382]  Moy, J., "OSPF Version 2", RFC 2382, April 1998.

    [RFC2385]  Herrernan, A., "Protection of BGP Sessions via the TCP
               MD5 Signature Option", RFC 2385, August, 1998.

    [RFC2403]  C. Madson and R. Glenn, "The Use of HMAC-MD5-96 within
               ESP and AH", RFC 2403, November, 1998.

11. Author's Address

   Dorian Kim
   Verio, Inc.
   2361 Lancashire Dr. #2A
   Ann Arbor, MI 48015
   Email: dorian@blackrose.org

   Hank Kilmer
   Email: hank@rem.com

   Dino Farinacci
   Email: dino@dinof.net

   David Meyer
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134
   Email: dmm@cisco.com