Multicast Source Discovery Protocol (MSDP)

Network Working Group                                     B. Fenner, Ed.
Request for Comments: 3618                                 D. Meyer, Ed.
Category: Experimental                                      October 2003


               Multicast Source Discovery Protocol (MSDP)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The Multicast Source Discovery Protocol (MSDP) describes a mechanism
   to connect multiple IP Version 4 Protocol Independent Multicast
   Sparse-Mode (PIM-SM) domains together.  Each PIM-SM domain uses its
   own independent Rendezvous Point (RP) and does not have to depend on
   RPs in other domains.  This document reflects existing MSDP
   implementations.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Caching . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Timers. . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
       5.1. SA-Advertisement-Timer . . . . . . . . . . . . . . . . .   5
       5.2. SA-Advertisement-Timer Processing. . . . . . . . . . . .   5
       5.3. SA Cache Timeout (SA-State Timer). . . . . . . . . . . .   5
       5.4. Peer Hold Timer. . . . . . . . . . . . . . . . . . . . .   5
       5.5. KeepAlive Timer. . . . . . . . . . . . . . . . . . . . .   6
       5.6. ConnectRetry Timer . . . . . . . . . . . . . . . . . . .   6
   6.  Intermediate MSDP Peers . . . . . . . . . . . . . . . . . . .   6
   7.  SA Filtering and Policy . . . . . . . . . . . . . . . . . . .   6
   8.  Encapsulated Data Packets . . . . . . . . . . . . . . . . . .   7
   9.  Other Scenarios . . . . . . . . . . . . . . . . . . . . . . .   7
   10. MSDP Peer-RPF Forwarding. . . . . . . . . . . . . . . . . . .   7
       10.1. Definitions . . . . . . . . . . . . . . . . . . . . . .   7
             10.1.1. Multicast RPF Routing Information Base. . . . .   8
             10.1.2. Peer-RPF Route. . . . . . . . . . . . . . . . .   8
             10.1.3. Peer-RPF Forwarding Rules . . . . . . . . . . .   8
       10.2. MSDP mesh-group semantics . . . . . . . . . . . . . . .   9
   11. MSDP Connection State Machine . . . . . . . . . . . . . . . .   9
       11.1. Events. . . . . . . . . . . . . . . . . . . . . . . . .  10
       11.2. Actions . . . . . . . . . . . . . . . . . . . . . . . .  10
       11.3. Peer-specific Events. . . . . . . . . . . . . . . . . .  11
       11.4. Peer-independent Events . . . . . . . . . . . . . . . .  11
   12. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . .  12
       12.1. MSDP TLV format . . . . . . . . . . . . . . . . . . . .  12
       12.2. Defined TLVs. . . . . . . . . . . . . . . . . . . . . .  12
             12.2.1. IPv4 Source-Active TLV. . . . . . . . . . . . .  13
             12.2.2. KeepAlive TLV . . . . . . . . . . . . . . . . .  14
   13. MSDP Error Handling . . . . . . . . . . . . . . . . . . . . .  15
   14. SA Data Encapsulation . . . . . . . . . . . . . . . . . . . .  15
   15. Applicability Statement . . . . . . . . . . . . . . . . . . .  15
       15.1. Between PIM Domains . . . . . . . . . . . . . . . . . .  15
       15.2. Between Anycast-RPs . . . . . . . . . . . . . . . . . .  15
   16. Intellectual Property . . . . . . . . . . . . . . . . . . . .  15
   17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
       19.1. Allocated TLV Range . . . . . . . . . . . . . . . . . .  17
       19.2. Experimental TLV Range. . . . . . . . . . . . . . . . .  17
   20. References. . . . . . . . . . . . . . . . . . . . . . . . . .  17
       20.1. Normative References. . . . . . . . . . . . . . . . . .  17
       20.2. Informative References. . . . . . . . . . . . . . . . .  18
   21. Editors' Addresses. . . . . . . . . . . . . . . . . . . . . .  18
   22. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The Multicast Source Discovery Protocol (MSDP) describes a mechanism
   to connect multiple PIM Sparse-Mode (PIM-SM) [RFC2362] domains
   together.  Each PIM-SM domain uses its own independent RP(s) and does
   not have to depend on RPs in other domains.  Advantages of this
   approach include:

   o  No Third-party resource dependencies on a domain's RP

      PIM-SM domains can rely on their own RPs only.

   o  Receiver only Domains

      Domains with only receivers get data without globally advertising
      group membership.

   Note that MSDP may be used with protocols other than PIM-SM, but such
   usage is not specified in this memo.

   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 [RFC2119].

2.  Overview

   MSDP-speaking routers in a PIM-SM domain have a MSDP peering
   relationship with MSDP peers in another domain.  The peering
   relationship is made up of a TCP connection in which control
   information is exchanged.  Each domain has one or more connections to
   this virtual topology.

   The purpose of this topology is to allow domains to discover
   multicast sources from other domains.  If the multicast sources are
   of interest to a domain which has receivers, the normal source-tree
   building mechanism in PIM-SM will be used to deliver multicast data
   over an inter-domain distribution tree.

3.  Procedure

   When an RP in a PIM-SM domain first learns of a new sender, e.g., via
   PIM register messages, it constructs a "Source-Active" (SA) message
   and sends it to its MSDP peers.  All RPs, which intend to originate
   or receive SA messages, must establish MSDP peering with other RPs,
   either directly or via an intermediate MSDP peer.  The SA message
   contains the following fields:

   o  Source address of the data source.

   o  Group address the data source sends to.

   o  IP address of the RP.

   Note that an RP that isn't a DR on a shared network SHOULD NOT
   originate SA's for directly connected sources on that shared network;
   it should only originate in response to receiving Register messages
   from the DR.

   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.  The Multicast
   RPF Routing Information Base (MRIB) is examined to determine which
   peer towards the originating RP of the SA message is selected.  Such
   a peer is called an "RPF peer".  See section 13 for the details of
   peer-RPF forwarding.

   If the MSDP peer receives the SA from a non-RPF peer towards the
   originating RP, it will drop the message.  Otherwise, it forwards the
   message to all its MSDP peers (except the one from which it received
   the SA message).

   When an MSDP peer which is also an RP for its own domain receives a
   new SA message, it determines if there are any group members within
   the domain interested in any group described by an (Source, Group),
   or (S,G) entry within the SA message.  That is, the RP checks for a
   (*,G) entry with a non-empty outgoing interface list; this implies
   that some system in the domain is interested in the group.  In this
   case, the RP triggers a (S,G) join event towards the data source as
   if a Join/Prune message was received addressed to the RP itself.
   This sets up a branch of the source-tree to this domain.  Subsequent
   data packets arrive at the RP via this tree branch, and are forwarded
   down the shared-tree inside the domain.  If leaf routers choose to
   join the source-tree they have the option to do so according to
   existing PIM-SM conventions.  Finally, if an RP in a domain receives
   a PIM Join message for a new group G, the RP SHOULD trigger a (S,G)
   join event for each active (S,G) for that group in its SA cache.

   This procedure has been affectionately named flood-and-join because
   if any RP is not interested in the group, they can ignore the SA
   message.  Otherwise, they join a distribution tree.

4.  Caching

   A MSDP speaker MUST cache SA messages.  Caching allows pacing of MSDP
   messages as well as reducing join latency for new receivers of a
   group G at an originating RP which has existing MSDP (S,G) state.  In
   addition, caching greatly aids in diagnosis and debugging of various
   problems.

   An MSDP speaker must provide a mechanism to reduce the forwarding of
   new SA's.  The SA-cache is used to reduce storms and performs this by
   not forwarding SA's unless they are in the cache or are new SA
   packets that the MSDP speaker will cache for the first time.  The
   SA-cache also reduces storms by advertising from the cache at a
   period of no more than twice per SA-Advertisement-Timer interval and
   not less than 1 time per SA Advertisement period.

5.  Timers

   The main timers for MSDP are: SA-Advertisement-Timer, SA Cache Entry
   timer, Peer Hold Timer, KeepAlive timer, and ConnectRetry timer.
   Each is considered below.

5.1.  SA-Advertisement-Timer

   RPs which originate SA messages do so periodically as long as there
   is data being sent by the source.  There is one SA-Advertisement-
   Timer covering the sources that an RP may advertise.  [SA-
   Advertisement-Period] MUST be 60 seconds.  An RP MUST not send more
   than one periodic SA message for a given (S,G) within an SA
   Advertisement interval.  Originating periodic SA messages is required
   to keep announcements alive in caches.  Finally, an originating RP
   SHOULD trigger the transmission of an SA message as soon as it
   receives data from an internal source for the first time.  This
   initial SA message may be in addition to the periodic sa-message
   forwarded in that first 60 seconds for that (S,G).

5.2.  SA-Advertisement-Timer Processing

   An RP MUST spread the generation of periodic SA messages (i.e.,
   messages advertising the active sources for which it is the RP) over
   its reporting interval (i.e., SA-Advertisement-Period).  An RP starts
   the SA-Advertisement-Timer when the MSDP process is configured.  When
   the timer expires, an RP resets the timer to [SA-Advertisement-
   Period] seconds, and begins the advertisement of its active sources.
   Active sources are advertised in the following manner: An RP packs
   its active sources into an SA message until the largest MSDP packet
   that can be sent is built or there are no more sources, and then
   sends the message.  This process is repeated periodically within the
   SA-Advertisement-Period in such a way that all of the RP's sources
   are advertised.  Note that since MSDP is a periodic protocol, an
   implementation SHOULD send all cached SA messages when a connection
   is established.  Finally, the timer is deleted when the MSDP process
   is de-configured.

5.3.  SA Cache Timeout (SA-State Timer)

   Each entry in an SA Cache has an associated SA-State Timer.  A
   (S,G)-SA-State-Timer is started when an (S,G)-SA message is initially
   received by an MSDP peer.  The timer is reset to [SG-State-Period] if
   another (S,G)-SA message is received before the (S,G)-SA-State Timer
   expires.  [SG-State-Period] MUST NOT be less than [SA-Advertisement-
   Period] + [SA-Hold-Down-Period].

5.4.  Peer Hold Timer

   The Hold Timer is initialized to [HoldTime-Period] when the peer's
   transport connection is established, and is reset to [HoldTime-
   Period] when any MSDP message is received.  Finally, the timer is
   deleted when the peer's transport connection is closed.  [HoldTime-
   Period] MUST be at least three seconds.  The recommended value for
   [HoldTime-Period] is 75 seconds.

5.5.  KeepAlive Timer

   Once an MSDP transport connection is established, each side of the
   connection sends a KeepAlive message and sets a KeepAlive timer.  If
   the KeepAlive timer expires, the local system sends a KeepAlive
   message and restarts its KeepAlive timer.

   The KeepAlive timer is set to [KeepAlive-Period] when the peer comes
   up.  The timer is reset to [KeepAlive-Period] each time an MSDP
   message is sent to the peer, and reset when the timer expires.

   Finally, the KeepAlive timer is deleted when the peer's transport
   connection is closed.

   [KeepAlive-Period] MUST be less than [HoldTime-Period], and MUST be
   at least one second.  The recommended value for [KeepAlive-Period] is
   60 seconds.

5.6.  ConnectRetry Timer

   The ConnectRetry timer is used by the MSDP peer with the lower IP
   address to transition from INACTIVE to CONNECTING states.  There is
   one timer per peer, and the [ConnectRetry-Period] SHOULD be set to 30
   seconds.  The timer is initialized to [ConnectRetry-Period] when an
   MSDP speaker attempts to actively open a TCP connection to its peer
   (see section 15, event E2, action A2 ).  When the timer expires, the
   peer retries the connection and the timer is reset to [ConnectRetry-
   Period].  It is deleted if either the connection transitions into
   ESTABLISHED state or the peer is de-configured.

6.  Intermediate MSDP Peers

   Intermediate MSDP speakers do not originate periodic SA messages on
   behalf of sources in other domains.  In general, an RP MUST only
   originate an SA for a source which would register to it, and ONLY RPs
   may originate SA messages.  Intermediate MSDP speakers MAY forward SA
   messages received from other domains.

7.  SA Filtering and Policy

   As the number of (S,G) pairs increases in the Internet, an RP may
   want to filter which sources it describes in SA messages.  Also,
   filtering may be used as a matter of policy which at the same time
   can reduce state.  MSDP peers in transit domains should not filter SA
   messages or the flood-and-join model can not guarantee that sources
   will be known throughout the Internet (i.e., SA filtering by transit
   domains may cause undesired lack of connectivity).  In general,
   policy should be expressed using MBGP [RFC2858].  This will cause
   MSDP messages to flow in the desired direction and peer-RPF fail
   otherwise.  An exception occurs at an administrative scope [RFC2365]
   boundary.  In particular, a SA message for a (S,G) MUST NOT be sent
   to peers which are on the other side of an administrative scope
   boundary for G.

8.  Encapsulated Data Packets

   The RP MAY encapsulate multicast data from the source.  An interested
   RP may decapsulate the packet, which SHOULD be forwarded as if a PIM
   register encapsulated packet was received.  That is, if packets are
   already arriving over the interface toward the source, then the
   packet is dropped.  Otherwise, if the outgoing interface list is
   non-null, the packet is forwarded appropriately.  Note that when
   doing data encapsulation, an implementation MUST bound the time
   during which packets are encapsulated.

   This allows for small bursts to be received before the multicast tree
   is built back toward the source's domain.  For example, an
   implementation SHOULD encapsulate at least the first packet to
   provide service to bursty sources.

9.  Other Scenarios

   MSDP is not limited to deployment across different routing domains.
   It can be used within a routing domain when it is desired to deploy
   multiple RPs for the same group ranges such as with Anycast RP's.  As
   long as all RPs have a interconnected MSDP topology, each can learn
   about active sources as well as RPs in other domains.

10.  MSDP Peer-RPF Forwarding

   The MSDP Peer-RPF Forwarding rules are used for forwarding SA
   messages throughout an MSDP enabled internet.  Unlike the RPF check
   used when forwarding data packets, which generally compares the
   packet's source address against the interface upon which the packet
   was received, the Peer-RPF check compares the RP address carried in
   the SA message against the MSDP peer from which the message was
   received.

10.1.  Definitions

   The following definitions are used in the description of the Peer-RPF
   Forwarding Rules:

10.1.1.  Multicast RPF Routing Information Base

   The Multicast RPF Routing Information Base (MRIB) is the multicast
   topology table.  It is typically derived from the unicast routing
   table or from other routing protocols such as multi-protocol BGP
   [RFC2858].

10.1.2.  Peer-RPF Route

   The Peer-RPF route is the route that the MRIB chooses for a given
   address.  The Peer-RPF route for a SA's originating RP is used to
   select the peer from which the SA is accepted.

10.1.3.  Peer-RPF Forwarding Rules

   An SA message originated by R and received by X from N is accepted if
   N is the peer-RPF neighbor for X, and is discarded otherwise.

              MPP(R,N)                 MP(N,X)
      R ---------....-------> N ------------------> X
              SA(S,G,R)                SA(S,G,R)

   MP(N,X) is an MSDP peering between N and X.  MPP(R,N) is an MSDP
   peering path (zero or more MSDP peers) between R and N, e.g.,
   MPP(R,N) = MP(R, A) + MP(A, B) + MP(B, N).  SA(S,G,R) is an SA
   message for source S on group G originated by an RP R.

   The peer-RPF neighbor N is chosen deterministically, using the first
   of the following rules that matches.  In particular, N is the RPF
   neighbor of X with respect to R if

   (i).    N == R (X has an MSDP peering with R).

   (ii).   N is the eBGP NEXT_HOP of the Peer-RPF route for R.

   (iii).  The Peer-RPF route for R is learned through a distance-vector
           or path-vector routing protocol (e.g., BGP, RIP, DVMRP) and N
           is the neighbor that advertised the Peer-RPF route for R
           (e.g., N is the iBGP advertiser of the route for R), or N is
           the IGP next hop for R if the route for R is learned via a
           link-state protocol (e.g., OSPF [RFC2328] or IS-IS
           [RFC1142]).

   (iv).   N resides in the closest AS in the best path towards R.  If
           multiple MSDP peers reside in the closest AS, the peer with
           the highest IP address is the rpf-peer.

   (v).    N is configured as the static RPF-peer for R.

   MSDP peers, which are NOT in state ESTABLISHED (i.e., down peers),
   are not eligible for peer RPF consideration.

10.2.  MSDP mesh-group semantics

   An MSDP mesh-group is a operational mechanism for reducing SA
   flooding, typically in an intra-domain setting.  In particular, when
   some subset of a domain's MSDP speakers are fully meshed, they can be
   configured into a mesh-group.

   Note that mesh-groups assume that a member doesn't have to forward an
   SA to other members of the mesh-group because the originator will
   forward to all members.  To be able for the originator to forward to
   all members (and to have each member also be a potential originator),
   the mesh-group must be a full mesh of MSDP peering among all members.

   The semantics of the mesh-group are as follows:

   (i).    If a member R of a mesh-group M receives a SA message from an
           MSDP peer that is also a member of mesh-group M, R accepts
           the SA message and forwards it to all of its peers that are
           not part of mesh-group M.  R MUST NOT forward the SA message
           to other members of mesh-group M.

   (ii).   If a member R of a mesh-group M receives an SA message from
           an MSDP peer that is not a member of mesh-group M, and the SA
           message passes the peer-RPF check, then R forwards the SA
           message to all members of mesh-group M and to any other msdp
           peers.

11.  MSDP Connection State Machine

   MSDP uses TCP as its transport protocol.  In a peering relationship,
   one MSDP peer listens for new TCP connections on the well-known port
   639.  The other side makes an active connect to this port.  The peer
   with the higher IP address will listen.  This connection
   establishment algorithm avoids call collision.  Therefore, there is
   no need for a call collision procedure.  It should be noted, however,
   that the disadvantage of this approach is that the startup time
   depends completely upon the active side and its connect retry timer;
   the passive side cannot cause the connection to be established.

   An MSDP peer starts in the DISABLED state.  MSDP peers establish
   peering sessions according to the following state machine:

              --------------->+----------+
             /                | DISABLED |<----------
            |          ------>+----------+           \
            |         /            |E1->A1            |
            |        |             |                  |
            |        |             V                  |E7->A7
            |        |        +----------+ E3->A3 +--------+
            |        |        | INACTIVE |------->| LISTEN |
            |        |        +----------+        +--------+
            |        |     E2->A2|    ^               |E5->A5
            |        |           |    |               |
            |        |E7->A6     V    |E6             |
            |         \      +------------+           |
            |          ------| CONNECTING |           |
            |                +------------+           |
   E7->A8   |                      |E4->A4            |
   E8->A8   |                      |                  |
   E9->A8   |                      V                  |
            \               +-------------+          /
              --------------| ESTABLISHED |<---------
                            +-------------+
                               |       ^
                               |       |
                       E10->A9 \______/

11.1.  Events

   E1) Enable MSDP peering with P
   E2) Own IP address < P's IP address
   E3) Own IP address > P's IP address
   E4) TCP established (active side)
   E5) TCP established (passive side)
   E6) ConnectRetry timer expired
   E7) Disable MSDP peering with P (e.g., when one's own address is
       changed)
   E8) Hold Timer expired
   E9) MSDP TLV format error detected
   E10) Any other error detected

11.2.  Actions

   A1) Allocate resources for peering with P Compare one's own and
       peer's IP addresses
   A2) TCP active OPEN Set ConnectRetry timer to
       [ConnectRetry-Period]
   A3) TCP passive OPEN (listen)
   A4) Delete ConnectRetry timer Send KeepAlive TLV
       Set KeepAlive timer to [KeepAlive-Period]
       Set Hold Timer to [HoldTime-Period]
   A5) Send KeepAlive TLV
       Set KeepAlive timer to [KeepAlive-Period]
       Set Hold Timer to [HoldTime-Period]
   A6) Abort TCP active OPEN attempt
       Release resources allocated for peering with P
   A7) Abort TCP passive OPEN attempt
       Release resources allocated for peering with P
   A8) Close the TCP connection
       Release resources allocated for peering with P
   A9) Drop the packet

11.3.  Peer-specific Events

   The following peer-specific events can occur in the ESTABLISHED
   state, they do not cause a state transition.  Appropriate actions are
   listed for each event.

   *) KeepAlive timer expired:
      -> Send KeepAlive TLV
      -> Set KeepAlive timer to [KeepAlive-Period]
   *) KeepAlive TLV received:
      -> Set Hold Timer to [HoldTime-Period]
   *) Source-Active TLV received:
      -> Set Hold Timer to [HoldTime-Period]
      -> Run Peer-RPF Forwarding algorithm
      -> Set KeepAlive timer to [KeepAlive-Period] for those peers
         the Source-Active TLV is forwarded to
      -> Send information to PIM-SM
      -> Store information in cache

11.4.  Peer-independent Events

   There are also a number of events that affect more than one peering
   session, but still require actions to be performed on a per-peer
   basis.

   *) SA-Advertisement-Timer expired:
      -> Start periodic transmission of Source-Active TLV(s)
       -> Set KeepAlive timer to [KeepAlive-Period] each time a
          Source-Active TLV is sent
   *) MSDP learns of a new active internal source (e.g., PIM-SM
      register received for a new source):
      -> Send Source-Active TLV
      -> Set KeepAlive timer to [KeepAlive-Period]
   *) SG-State-Timer expired (one timer per cache entry):

      -> Implementation specific, typically mark the cache entry
         for deletion

12.  Packet Formats

   MSDP messages are encoded in TLV format.  If an implementation
   receives a TLV whose length exceeds the maximum TLV length specified
   below, the TLV SHOULD be accepted.  Any additional data, including
   possible next TLV's in the same message, SHOULD be ignored, and the
   MSDP session should not be reset.

12.1.  MSDP TLV format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Type       |           Length              |  Value ....   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type (8 bits)
    Describes the format of the Value field.

   Length (16 bits)
    Length of Type, Length, and Value fields in octets.  Minimum length
    required is 4 octets, except for Keepalive messages.  The maximum
    TLV length is 9192.

   Value (variable length)
    Format is based on the Type value.  See below.  The length of the
    value field is Length field minus 3.  All reserved fields in the
    Value field MUST be transmitted as zeros and ignored on receipt.

12.2.  Defined TLVs

   The following TLV Types are defined:

   Code                        Type
   ===================================================
     1                  IPv4 Source-Active
     2                  IPv4 Source-Active Request
     3                  IPv4 Source-Active Response
     4                  KeepAlive
     5                  Reserved (Previously: Notification)
   Each TLV is described below.

   In addition, the following TLV Types are assigned but not described
   in this memo:

   Code                        Type
   ====================================================
     6                  MSDP traceroute in progress
     7                  MSDP traceroute reply

12.2.1.  IPv4 Source-Active TLV

   The maximum size SA message that can be sent is 9192 octets.  The
   9192 octet size does not include the TCP, IP, layer-2 headers.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       1       |           x + y               |  Entry Count  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          RP Address                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Reserved            |  Sprefix Len  | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  \
|                         Group Address                         |   ) z
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  /
|                         Source Address                        | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    IPv4 Source-Active TLV is type 1.

   Length x
    Is the length of the control information in the message.  x is 8
    octets (for the first two 32-bit quantities) plus 12 times Entry
    Count octets.

   Length y
    If 0, then there is no data encapsulated.  Otherwise an IPv4 packet
    follows and y is the value of the total length field in the header
    of the encapsulated IP packet.  If there are multiple (S,G) entries
    in an SA message, only the last entry may have encapsulated data and
    it must reflect the source and destination addresses in the header
    of the encapsulated IP packet.

   Entry Count
    Is the count of z entries (note above) which follow the RP address
    field.  This is so multiple (S,G)s from the same domain can be
    encoded efficiently for the same RP address.  An SA message
    containing encapsulated data typically has an entry count of 1
    (i.e., only contains a single entry, for the (S,G) representing the
    encapsulated packet).

   RP Address
    The address of the RP in the domain the source has become active in.

   Reserved
    The Reserved field MUST be transmitted as zeros and MUST be ignored
    by a receiver.

   Sprefix Len
    The route prefix length associated with source address.  This field
    MUST be transmitted as 32 (/32).

   Group Address
    The group address the active source has sent data to.

   Source Address
    The IP address of the active source.

   Multiple (S,G) entries MAY appear in the same SA and can be batched
   for efficiency at the expense of data latency.  This would typically
   occur on intermediate forwarding of SA messages.

12.2.2.  KeepAlive TLV

   A KeepAlive TLV is sent to an MSDP peer if and only if there were no
   MSDP messages sent to the peer within [KeepAlive-Period] seconds.
   This message is necessary to keep the MSDP connection alive.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4       |             3                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The length of the message is 3 octets which encompasses the one octet
   Type field and the two octet Length field.

13.  MSDP Error Handling

   If an MSDP message is received with a TLV format error, the session
   SHOULD be reset with that peer.  MSDP messages with other errors,
   such as unrecognized type code, received from MSDP peers, SHOULD be
   silently discarded and the session SHOULD not be reset.

14.  SA Data Encapsulation

   As discussed earlier, TCP encapsulation of data in SA messages MAY be
   supported for backwards compatibility with legacy MSDP peers.

15.  Applicability Statement

   MSDP is used primarily in two deployment scenarios:

15.1.  Between PIM Domains

   MSDP can be used between PIM 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 rules described in this spec (i.e., does not use mesh-
   groups).  Peerings can be aggregated on a single MSDP peer, typically
   from one to hundreds of peerings, similar in scale, although not
   necessarily consistent, with BGP peerings.

15.2.  Between Anycast-RPs

   MSDP is also used between 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 then also have additional one-to-one peering with
   msdp peers outside that PIM domain as described in scenario A, for
   discovery of external sources.  MSDP for anycast-RP without external
   MSDP peering is a valid deployment option and common.

16.  Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

17.  Acknowledgments

   The editors would like to thank the original authors, Dino Farinacci,
   Yakov Rehkter, Peter Lothberg, Hank Kilmer, and Jermey Hall for their
   original contribution to the MSDP specification.  In addition, Bill
   Nickless, John Meylor, Liming Wei, Manoj Leelanivas, Mark Turner,
   John Zwiebel, Cristina Radulescu-Banu, Brian Edwards, Selina
   Priestley, IJsbrand Wijnands, Tom Pusateri, Kristofer Warell, Henning
   Eriksson, Thomas Eriksson, Dave Thaler, and Ravi Shekhar provided
   useful and productive design feedback and comments.  Toerless Eckert,
   Leonard Giuliano, Mike McBride, David Meyer, John Meylor, Pekka
   Savola, Ishan Wu, and Swapna Yelamanchi contributed to the final
   version of the document.

18.  Security Considerations

   An MSDP implementation MUST implement Keyed MD5 [RFC2385] to secure
   control messages, and MUST be capable of interoperating with peers
   that do not support it.  However, if one side of the connection is
   configured with Keyed MD5 and the other side is not, the connection
   SHOULD NOT be established.

   In addition, to mitigate state explosion during denial of service and
   other attacks, SA filters and limits SHOULD be used with MSDP to
   limit the sources and  groups that will be passed between RPs
   [DEPLOY].  These filtering and limiting functions may include, for
   example, access lists of source or group addresses which should not
   be propagated to other domains using MSDP, the absolute highest
   acceptable number of SA-state entries or a rate-limit of for the
   creation of new SA-state entries after the connection has been
   established.

   If follow-on work is done in this area, a more robust integrity
   mechanism, such as HMAC-SHA1 [RFC2104, RFC2202] ought to be employed.

19.  IANA Considerations

   This document creates a new namespace called "MSDP TLV Values" that
   the IANA will manage.  The initial seven MSDP TLV values are
   specified in Section 12.2.  The following two sections describe the
   rules for allocating new MSDP TLV values.

19.1.  IANA Allocated TLV Range

   MSDP TLV values in the range [8,200] (inclusive) are to be allocated
   using an IESG Approval or Standards Action process [RFC2434].

19.2.  Experimental TLV Range

   TLV values in the range [201,255] (inclusive) are allocated for
   experimental use.

20.  References

20.1.  Normative References

   [RFC1142]       Oran, D., Ed., "OSI IS-IS Intra-domain Routing
                   Protocol", RFC 1142, February 1990.

   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2328]       Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                   1998.

   [RFC2858]       Bates, T., Rekhter, Y., Chandra, R. and D. Katz,
                   "Multiprotocol Extensions for BGP-4", RFC 2858, June
                   2000.

   [RFC2362]       Estrin, D., Farinacci, D., Helmy, A., Thaler, D.,
                   Deering, S., Handley, M., Jacobson, V., Lin, C.,
                   Sharma, P. and L. Wei, "Protocol Independent
                   Multicast - Sparse Mode (PIM-SM):  Protocol
                   Specification", RFC 2362, June 1998.

   [RFC2365]       Meyer, D., "Administratively Scoped IP Multicast",
                   BCP 23, RFC 2365, July 1998.

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

   [RFC2434]       Narten, T. and H. Alvestrand, "Guidelines for Writing
                   an IANA Considerations Section in RFCs", BCP 26, RFC
                   2434, October 1998.

   [RFC3446]       Kim, D., Meyer, D., Kilmer, H. and D. Farinacci,
                   "Anycast Rendezvous Point (RP) Mechanism using
                   Protocol Independent Multicast (PIM) and Multicast
                   Source Discovery Protocol (MSDP)", RFC 3446, January
                   2003.

20.2.  Informative References

   [DEPLOY]        McBride, M., Meylor, J. and D. Meyer, "Multicast
                   Source Discovery Protocol (MSDP) Deployment
                   Scenarios", Work in Progress, July 2003.

   [RFC2104]       Krawczyk, H., Bellare, M. and R.  Canetti, "HMAC:
                   Keyed-Hashing for Message Authentication", RFC 2104,
                   February 1997.

   [RFC2202]       Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
                   HMAC-SHA-1", RFC 2202, September 1997.

21.  Editors' Addresses

   Bill Fenner
   AT&T Labs -- Research
   75 Willow Road
   Menlo Park, CA 94025

   EMail: fenner@research.att.com


   David Meyer

   EMail: dmm@1-4-5.net

22.  Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.