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Network Working Group                                    David Meyer (Editor)
INTERNET DRAFT                                           Bill Fenner (Editor)
Category                                                 Standards Track
                                                         November, 2001


               Multicast Source Discovery Protocol (MSDP)
                     <draft-ietf-msdp-spec-13.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 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

   The Multicast Source Discovery Protocol, MSDP, describes a mechanism
   to connect multiple PIM-SM domains together. Each PIM-SM domain uses
   its own independent RP(s) and does not have to depend on RPs in other
   domains.












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3. Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


4.  Introduction

   The Multicast Source Discovery Protocol, MSDP, describes a mechanism
   to connect multiple PIM-SM 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 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 keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
   SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
   in RFC 2119 [RFC2119].


5. Overview

   MSDP-speaking routers in a PIM-SM [RFC2362] 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.

   We envision this virtual topology will essentially be congruent to
   the existing BGP topology used in the unicast-based Internet today.
   That is, the TCP connections between MSDP peers are likely to be
   congruent to the connections in the BGP routing system.



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6. 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. 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 14 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 (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



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   message. Otherwise, they join a distribution tree.


7. 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.


8. Timers

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


8.1. 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.


8.2. 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



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   implemenation SHOULD send all cached SA messages when a connection is
   established. Finally, the timer is deleted when the MSDP process is
   deconfigured.


8.3. 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 a 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.  The timer is reset whether or not the (S,G) entry's SG-
   Rate-Limit timer is running.  [SG-State-Period] MUST NOT be less than
   [SA-Advertisement-Period] + [SA-Hold-Down-Period].


8.4. SG-Rate-Limit Timer

   The SG-Rate-Limit Timer is a per-(S,G) timer which is used to limit
   possible SA storms.  After an SA message passes the peer-RPF check,
   the SG-Rate-Limit timer for each (S,G) in the message is checked.  If
   the SG-Rate-Limit timer is running for a given (S,G), it is removed
   from the message before forwarding.  If this process causes the
   message to become empty, the empty message is discarded.

   When an SA message is forwarded, the SG-Rate-Limit timer for each
   (S,G) mentioned in the message is set to [SG-Rate-Limit-Period]
   seconds.  Note that this sequence means that the SG-Rate-Limit timer
   will never be reset if it is running, since any (S,G) whose timer was
   running was removed from the forwarded message; it acts as a "one-
   shot" timer.

   [SG-Rate-Limit-Period] SHOULD be set to 30 seconds, and MUST NOT be
   greater than [SA-Advertisement-Period].


8.5. Peer Hold Timer

   If a system has not received any MSDP message within the period
   specified by the Hold Timer, then a Notification message with Hold
   Timer Expired Error Code MUST be sent and the MSDP connection MUST be
   closed. [HoldTime-Period] MUST be at least three seconds. The
   recommended value for [HoldTime-Period] is 90 seconds.

   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.



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8.6. 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
   75 seconds.


8.7. 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 deconfigured.



9. 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.









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10. 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. Only the RP co-located in the same domain as the
   source can restrict SA messages. Note, however, that 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 can cause undesired
   lack of connectivity). In general, policy should be expressed using
   MBGP [RFC2283]. 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.


11. SA Requests

   A MSDP speaker MAY accept SA-Requests from other MSDP peers. When an
   MSDP speaker receives an SA-Request for a group range, it will
   respond to the peer with a set of SA entries, in an SA-Response
   message, for all active sources in its SA cache sending to the group
   requested in the SA-Request message. The peer that sends the request
   will not flood the responding SA-Response message to other peers. See
   section 17 for discussion of error handling relating to SA requests
   and responses.


12. 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.






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13. 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. As long as all RPs have a
   interconnected MSDP topology, each can learn about active sources as
   well as RPs in other domains.


14. 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.


14.1. Definitions

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


14.1.1. Multicast RPF Routing Information Base (MRIB)

   The 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 [RFC2283].


14.1.2. 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.













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14.2. 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 P is chosen deterministically, using the
           first of the following rules that matches. In particular,
           P is the RPF neighbor of X with respect to R if

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

           (ii).   P is the BGP 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 P is the neighbor that
                   advertised the Peer-RPF route for R if the
                   route was learned via a distance-vector or
                   path-vector protocol, or P is the IGP next hop
                   for R if learned via a link-state protocol.

           (iv).   P resides in an AS that is in the AS_PATH of the
                   Peer-RPF route for R, and P has the highest IP address among
                   the MSDP peers that reside in ASs in that AS_PATH.

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













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   When an SA message with RP R is received from neighbor N, it is
   discarded unless N == P as determined above.



14.3. MSDP static RPF-peer semantics

   If none of the rules (i) - (iv) are able to determine an RPF peer for
   R, a longest-match lookup is performed in the static RPF peer table.
   This table MUST be able to contain a default entry, and SHOULD be
   able to contain prefix or per-host (RP) entries.  This table
   statically maps RP addresses to peers, and allows configuration of
   topology that is e.g. unknown to the multicast topology gathering
   protocol.

   The result of the longest-match lookup of an RP address R in the
   static RPF peer table is an MSDP peer, which is the RPF neighbor for
   R.


14.4. MSDP mesh-group semantics

   A 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, then 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 any mesh-group. 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 a 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.

   (iii).  Cross mesh-group forwarding




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           If a member R of a mesh-groups M and N receives an SA
           message from an MSDP peer in mesh-group M, R forwards the SA
           to its MSDP peers in mesh-group N if it receives that SA
           message from a peer that is in the same mesh-group as its
           peer-RPF neighbor for that SA.

           For example, consider the case in which three routers (R1, R2,
           and R3) and three mesh-groups (A, B, and C) are arranged in a
           triangle, e.g.,


                                  [R2] {A,B}
                                  /  \
                                 /    \
                                /      \
                               /        \
                      {A,C} [R1]--------[R3] {B,C}


           Now, when R1 receives an SA message from R2 and R1's
           peer-RPF neighbor for this SA lies in mesh-group A, R1
           forwards the SA message its peers in other mesh-groups
           (in particular, R3 in mesh-group C). Similarly, if R3's
           peer-RPF neighbor lies in mesh-group B, R3 will forward an
           SA message from R2. In this case, both R1 and R3 will send
           SA messages to each other (because they share common mesh-group
           C), but neither of them will forward any further the SA messages
           received from each other (as their peer-RPF neighbors do
           not lie in mesh-group C).



   Note that since mesh-groups suspend peer-RPF checking of SAs received
   from a mesh-group member ((i). above), they allow for mis-
   configuration to cause SA looping.
















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15.  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             |
                |         \      +------------+           |
         E7->A8 |          ------| CONNECTING |           |
         E8->A9 |                +------------+           |
         E9->A10|                      |E4->A4            |
        E10->A11|                      |                  |
        E11->A12|                      V                  |
                 \              +-------------+          /
                  --------------| ESTABLISHED |<---------
                                +-------------+










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15.1. 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
           An example of when to do this is when one's own address is
           changed)
       E8) Hold Timer expired
       E9) Authorization failure
      E10) Notification TLV received
      E11) Error detected


15.2. 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

       In action sets 8)-12), the action "Close peering session" includes
       the following steps:
          Close TCP connection
          Delete KeepAlive timer
          Delete Hold Timer
          Release resources allocated for peering with P

       A8) Send Notification TLV with Error Code "Cease"
           Close peering session
       A9) Send Notification TLV with Error Code "Hold Timer Expired"
           Close peering session



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      A10) Notify management system unless this has already been done by
           the security mechanism
           Close peering session
      A11) Notify management system
           If the received Notification TLV's O-bit was cleared, close
           peering session. Otherwise, remain in ESTABLISHED state.
      A12) Send Notification TLV with appropriate Error Code
           Notify management system
           If the sent Notification TLV's O-bit was cleared, close peering
           session. Otherwise, remain in ESTABLISHED state.


15.3. 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 (consider SG-Rate-Limit
            Timer and SA-State Timer)
         -> 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
      *) Source-Active Request TLV received:
         -> Set Hold Timer to [HoldTime-Period]
         -> If SA-Requests are accepted, send Source-Active Response
            TLV and set KeepAlive timer to [KeepAlive-Period]
      *) Source-Active Response TLV received:
         -> Set Hold Timer to [HoldTime-Period]
         -> If a corresponding SA-Request were previously sent, send
            information to PIM-SM. If not, an error has occured
            (event 11 above)
         -> Store information in cache









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15.4. 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]
      *) Source-Active Request triggered (event not specified here):
         -> Send Source-Active Request 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































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16. Packet Formats

   MSDP messages will be encoded in TLV format. If an implementation
   receives a TLV that has length that is longer than expected, the TLV
   SHOULD be accepted. Any additional data SHOULD be ignored.


16.1. 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.



16.2. 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                  Notification

   Each TLV is described below.

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



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


16.2.1. 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 length of the total length field
    of the IPv4 header encapsulated. If there are multiple SA TLVs
    in a message, and data is also included, y must be 0 in all SA
    TLVs except the last one and the last SA TLV must reflect the
    source and destination addresses in the IP header of the
    encapsulated data.

   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



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    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). An Invalid
    Sprefix Len Notification SHOULD be sent upon receipt
    of any other value.

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

   Source Address
    The IP address of the active source.

   Multiple SA TLVs MAY appear in the same message and can be batched
   for efficiency at the expense of data latency. This would typically
   occur on intermediate forwarding of SA messages.


16.2.2. IPv4 Source-Active Request TLV

   The Source-Active Request is used to request SA-state from a MSDP
   peer. If an RP in a domain receives a PIM Join message for a group,
   creates (*,G) state and wants to know all active sources for group G,
   it may send an SA-Request message for the group.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |             8                 |    Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Group Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    IPv4 Source-Active Request TLV is type 2.

   Reserved
    Must be transmitted as zero and ignored on receipt.




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   Group Address
    The group address the MSDP peer is requesting.


16.2.3. IPv4 Source-Active Response TLV

   The Source-Active Response is sent in response to a Source-Active
   Request message. The Source-Active Response message has the same
   format as a Source-Active message but does not allow encapsulation of
   multicast data.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       3       |             x                 |     ....      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    IPv4 Source-Active Response TLV is type 3.

   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.


16.2.4. 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.










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16.2.5. Notification TLV

   A Notification message is sent when an error condition is detected,
   and has the following form:


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |          x + 5                |O| Error Code  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Error subcode |          ...                                  |
   +-+-+-+-+-+-+-+-+                                               |
   |                         Data                                  |
   |                          ...                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    The Notification TLV is type 5.

   Length
    Length is a two octet field with value x + 5, where x is
    the length of the notification data field.

   O-bit
    Open-bit. If clear, the connection will be closed.

   Error code
    This 7-bit unsigned integer indicates the type of Notification.
    The following Error Codes have been defined:

    Error Code       Symbolic Name                  Reference

        1           Message Header Error           Section 17.1
        2           SA-Request Error               Section 17.2
        3           SA-Message/SA-Response Error   Section 17.3
        4           Hold Timer Expired             Section 17.4
        5           Finite State Machine Error     Section 17.5
        6           Notification                   Section 17.6
        7           Cease                          Section 17.7

   Error subcode:
    This one-octet unsigned integer provides more specific information
    about the reported error.  Each Error Code may have one or more Error
    Subcodes associated with it.  If no appropriate Error Subcode is
    defined, then a zero (Unspecific) value is used for the Error Subcode
    field, and the O-bit must be cleared (i.e. the connection will be
    closed).  The used notation in the error description below is: MC =



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    Must Close connection = O-bit clear; CC = Can Close connection =
    O-bit MAY be cleared.

   Message Header Error subcodes:

            0 - Unspecific                              (MC)
            2 - Bad Message Length                      (MC)
            3 - Bad Message Type                        (CC)

   SA-Request Error subcodes (the O-bit is always clear):

            0 - Unspecific                              (MC)
            1 - Invalid Group                           (MC)

   SA-Message/SA-Response Error subcodes

            0 - Unspecific                              (MC)
            1 - Invalid Entry Count                     (CC)
            2 - Invalid RP Address                      (MC)
            3 - Invalid Group Address                   (MC)
            4 - Invalid Source Address                  (MC)
            5 - Invalid Sprefix Length                  (MC)
            6 - Looping SA (Self is RP)                 (MC)
            7 - Unknown Encapsulation                   (MC)
            8 - Administrative Scope Boundary Violated  (MC)

   Hold Timer Expired subcodes (the O-bit is always clear):

           0 - Unspecific                               (MC)

   Finite State Machine Error subcodes (the O-bit is always clear):

            0 - Unspecific                              (MC)
            1 - Unexpected Message Type FSM Error       (MC)

   Notification subcodes (the O-bit is always clear):

            0 - Unspecific                              (MC)

   Cease subcodes (the O-bit is always clear):

            0 - Unspecific                              (MC)









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17. MSDP Error Handling

   This section describes actions to be taken when errors are detected
   while processing MSDP messages. MSDP Error Handling is similar to
   that of BGP [RFC1771].

   When any of the conditions described here are detected, a
   Notification message with the indicated Error Code, Error Subcode,
   and Data fields is sent.  In addition, the MSDP connection MAY be
   closed.  If no Error Subcode is specified, then a zero (Unspecific)
   must be used.

   The phrase "the MSDP connection is closed" means that the transport
   protocol connection has been closed and that all resources for that
   MSDP connection have been deallocated.


17.1.  Message Header Error Handling

   All errors detected while processing the Message Header are indicated
   by sending the Notification message with Error Code Message Header
   Error. The Error Subcode describes the specific nature of the error.
   The Data field contains the erroneous Message (including the message
   header).

   If the Length field of the message header is less than 4 or greater
   than 9192, or the length of a KeepAlive message is not equal to 3,
   then the Error Subcode is set to Bad Message Length.

   If the Type field of the message header is not recognized, then the
   Error Subcode is set to Bad Message Type.


17.2. SA-Request Error Handling

   The SA-Request Error code is used to signal the receipt of a SA
   request at a MSDP peer when an invalid group address requested.

   When a MSDP peer receives a request for an invalid group, it returns
   the following notification:











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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |           12                  |O|     2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |                   Reserved                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Group Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3. SA-Message/SA-Response Error Handling

   The SA-Message/SA-Response Error code is used to signal the receipt
   of a erroneous SA Message at an MSDP peer, or the receipt of an SA-
   Response Message by a peer that did not issue a SA-Request. It has
   the following form:


17.3.1. Invalid Entry Count (IEC)

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |            6                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |  Entry Count  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.2. Invalid RP Address

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |           12                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         RP Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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17.3.3. Invalid Group Address

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |           12                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       3       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Group Address                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.4. Invalid Source Address

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |           12                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4       |             Reserved                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Source Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.5.  Invalid Sprefix Length (ISL)

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |            6                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |  Sprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.6.  Looping SAs (Self is RP in received SA)

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |          x + 5                |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       6       |         SA Message   ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length x



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     x is the length of the looping SA message contained in the data
     field of the Notification message.

















































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17.3.7. Unknown Encapsulation

   This notification is sent on receipt of SA data that is encapsulated
   in an unknown encapsulation type. See section 18 for known
   encapsulations.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |          x + 5                |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       |   SA Message ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length x
     x is the length of the SA message (which contained data which
     was encapsulated in some unknown way) that is contained in the
     data field of the Notification message.


17.3.8. Administrative Scope Boundary Violated

   This notification is used when an SA message is received for a group
   G from a peer which is across an administrative scope boundary for G.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |           12                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Group Address                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




17.4. Hold Time Expired

   If a system has not received any MSDP message within the period
   specified in the Hold Timer, the notification message with Hold Timer
   Expired Error Code and no additional data MUST be sent and the MSDP
   connection closed.







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17.5. Finite State Machine Error Handling

   Any error detected by the MSDP Finite State Machine (e.g., receipt of
   an unexpected event) is indicated by sending the Notification message
   with Error Code Finite State Machine Error.



17.6. Notification Message Error Handling

   If a node sends a Notification message, and there is an error in that
   message, and the O-bit of that message is not clear, a Notification
   with O-bit clear, Error Code of Notification Error, and subcode
   Unspecific must be sent.  In addition, the Data field must include
   the Notification message that triggered the error.  However, if the
   erroneous Notification message had the O-bit clear, then any error,
   such as an unrecognized Error Code or Error Subcode, should be
   noticed, logged locally, and brought to the attention of the
   administrator of the remote node.


17.7. Cease

   In absence of any fatal errors (that are indicated in this section),
   an MSDP node may choose at any given time to close its MSDP
   connection by sending the Notification message with Error Code Cease.
   However, the Cease Notification message MUST NOT be used when a fatal
   error indicated by this section does exist.


18.  SA Data Encapsulation

   This section describes UDP, GRE, and TCP encapsulation of data
   packets to be included with SA messages. Encapsulation type is a
   configuration option.


18.1. UDP Data Encapsulation

   Data packets  MAY be encapsulated in UDP. In this case, the UDP
   pseudo-header has the following form:










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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port        |         Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Length             |             Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Origin RP Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The Source port, Destination Port, Length, and Checksum are used
   according to RFC 768.  Source and Destination ports are known via
   an implementation-specific method (e.g. per-peer configuration).

   Checksum
    The checksum is computed according to RFC 768 [RFC768].

   Originating RP Address
    The Originating RP Address is the address of the RP sending
    the encapsulated data.



18.2. GRE Encapsulation

   MSDP SA-data MAY be encapsulated in GRE using protocol type [MSDP-
   GRE-ProtocolType]. The GRE header and payload packet have the
   following form:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|       Reserved0       | Ver |     [MSDP-GRE-ProtocolType]   |\
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ GRE Header
   |      Checksum (optional)      |          Reserved1            |/
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Originating RP IPv4 Address                  |\
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Payload
   |                    (S,G) Data Packet ....                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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18.2.1. Encapsulation and Path MTU Discovery [RFC1191]

   Existing implementations of GRE, when using IPv4 as the Delivery
   Header, do not implement Path MTU discovery and do not set the Don't
   Fragment bit in the Delivery Header.  This can cause large packets to
   become fragmented within the tunnel and reassembled at the tunnel
   exit (independent of whether the payload packet is using PMTU).  If a
   tunnel entry point were to use Path MTU discovery, however, that
   tunnel entry point would also need to relay ICMP unreachable error
   messages (in particular the "fragmentation needed and DF set" code)
   back to the originator of the packet, which is not required by the
   GRE specification [RFC2784]. Failure to properly relay Path MTU
   information to an originator can result in the following behavior:
   the originator sets the don't fragment bit, the packet gets dropped
   within the tunnel, but since the originator doesn't receive proper
   feedback, it retransmits with the same PMTU, causing subsequently
   transmitted packets to be dropped.


18.3. TCP Data Encapsulation

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


19. IANA Considerations

   The IANA should assign 0x0009 from the IANA SNAP Protocol IDs [IANA]
   to MSDP-GRE-ProtocolType.


20. Security Considerations

   An MSDP implementation MUST use IPsec [RFC2401] to secure control
   messages. In particular, the TCP connection between MSDP peers MUST
   be secured using IPsec. When encapsulating data packets in GRE,
   security should be relatively similar to security in a normal IPv4
   network, as routing using GRE follows the same routing that IPv4 uses
   natively. Route filtering will remain unchanged. However packet
   filtering at a firewall requires either that a firewall look inside
   the GRE packet or that the filtering is done on the GRE tunnel
   endpoints. In those environments in which this is considered to be a
   security issue it may be desirable to terminate the tunnel at the
   firewall.







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21. Acknowledgments

   The editors would like to thank the original authors, Dino Farinacci,
   Yakov Rehkter, Peter Lothberg, Hank Kilmer, and Jermey Hall for their
   orginal 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 and IJsbrand Wijnands provided useful and productive design
   feedback and comments. In addition to many other contributions, Tom
   Pusateri, Kristofer Warell, Henning Eriksson, and Thomas Eriksson
   helped to clarify the connection state machine, Dave Thaler helped to
   clarify the Notification message types. Ravi Shekhar helped clarify
   the semantics of mesh-groups, and countless others helped to clarify
   the Peer-RPF rules.





































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22. Editors' Address:

   David Meyer
   Sprint
   12502 Sunrise Valley Drive
   Reston VA, 20191
   Email: dmm@sprint.net

   Bill Fenner
   AT&T Labs -- Research
   75 Willow Road
   Menlo Park, CA 94025
   Email: fenner@research.att.com






































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23. REFERENCES

   [IANA]      http://www.iana.org

   [RFC768]    Postel, J. "User Datagram Protocol", RFC 768, August,
               1980.

   [RFC1191]   Mogul, J., and S. Deering, "Path MTU Discovery",
               RFC 1191, November 1990.

   [RFC1771]   Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
               (BGP-4)", RFC 1771, March 1995.

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

   [RFC2283]   Bates, T., Chandra, R., Katz, D., and Y. Rekhter.,
               "Multiprotocol Extensions for BGP-4", RFC 2283,
               February 1998.

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

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

   [RFC2401]   Kent, S. and  R. Atkinson, "Security Architecture for
               the Internet Protocol", RFC 2401, November 1998.

   [RFC2784]   Farinacci, D., et al., "Generic Routing Encapsulation
               (GRE)", RFC 2784, March 2000.



24. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of



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   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.





































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