Network Working Group                                     Dino Farinacci
INTERNET DRAFT                                          Procket Networks
                                                           Yakov Rekhter
                                                             David Meyer
                                                           Cisco Systems
                                                          Peter Lothberg
                                                                  Sprint
                                                             Hank Kilmer
                                                             Jeremy Hall
                                                                   UUnet
Category                                                 Standards Track
                                                         February, 2000


               Multicast Source Discovery Protocol (MSDP)
                     <draft-ietf-msdp-spec-05.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.













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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.


3. Copyright Notice

   Copyright (C) The Internet Society (2000).  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.

   o Global Source State

     Global source state is not required, since a router need not
     cache Source  Active (SA) messages (see below). MSDP is a
     periodic protocol.

   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].













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5. Overview

   MSDP-speaking routers in a PIM-SM [RFC2362] domain will have a MSDP
   peering relationship with MSDP peers in another domain. The peering
   relationship will be made up of a TCP connection in which control
   information is exchanged. Each domain will have one or more
   connections to this virtual topology.

   The purpose of this topology is to allow domains 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.



6. Procedure

   A source in a PIM-SM domain originates traffic to a multicast group.
   The PIM DR which is directly connected to the source sends the data
   encapsulated in a PIM Register message to the RP in the domain.

   The RP will construct a "Source-Active" (SA) message and send 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.

   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 BGP routing
   table is examined to determine which peer is the NEXT_HOP towards the
   originating RP of the SA message.  Such a peer is called an "RPF
   peer". See section 14 below 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).

   The flooding can be further constrained to children of the peer by
   interrogating BGP reachability information. That is, if a BGP peer



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   advertises a route (back to you) and you are the next to last AS in
   the AS_PATH, the peer is using you as the NEXT_HOP. This is known in
   other circles as Split-Horizon with Poison Reverse. An implementation
   SHOULD NOT forward SA messages (which were originated from the RP
   address covered by a route) to peers which have not Poison Reversed
   that route.

   When an MSDP peer which is also an RP for its own domain receives a
   new SA message, it determines if it has any group members interested
   in the group which the SA message describes. That is, the RP checks
   for a (*,G) entry with a non-empty outgoing interface list; this
   implies that 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 which 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, and it is caching SAs, then the RP should
   trigger a (S,G) join event for each SA for that group in its 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.


7. Controlling State

   While RPs which receive SA messages are not required to keep MSDP
   (S,G) state, an RP SHOULD cache SA messages by default. One of the
   main advantages of caching is that since the RP has MSDP (S,G) state,
   join latency is greatly reduced for new receivers of G. In addition,
   caching greatly aids in diagnosis and debugging of various problems.


8. Timers

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










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8.1. SA-Advertisement-Timer

   RPs which originate SA messages do it 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 important so that new
   receivers who join after a source has been active can get data
   quickly via the receiver's own RP when it is not caching SA state.
   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 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 the largest MSDP packet that can be sent has
   size that is the minimum of MTU of outgoing link minus size of TCP
   and IP headers, and 1400 (largest MSDP packet). Finally, the timer is
   deleted when the MSDP process is deconfigured. Note that a caching
   implementation may also wish to check the SA-Cache on this timer
   event.




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 caching MSDP peer. The timer is reset to [SA-State-
   Period] if another (S,G)-SA message is received before the (S,G)-SA-
   State-Timer expires. [SA-State-Period] MUST NOT be less than 90
   seconds.





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8.4. SA-Hold-Down-Timer

   A caching MSDP peer SHOULD NOT forward an SA message it has received
   in during the previous [SA-Hold-Down-Period] seconds. [SA-Hold-Down-
   Period] SHOULD be set to 30 seconds. The per-SA message timer is set
   to [SA-Hold-Down-Period] when forwarding an (S,G)-SA message, and a
   (S,G)-SA message MUST only be forwarded when it's associated timer is
   not running. Finally, the timer is deleted when the (S,G)-SA cache
   entry is deleted.


8.5. KeepAlive Timer

   The KeepAlive timer is used by the  active connect side of the MSDP
   connection to track the state of the passive-connect side of the
   connection. In particular, the KeepAlive timer is used to reset the
   TCP connection when the passive-connect side of the connection goes
   down. The KeepAlive timer is set to [KeepAlive-Period] when the
   passive-connect peer comes up. [KeepAlive-Period] SHOULD NOT be less
   that 75 seconds. The timer is reset to [KeepAlive-Period] upon
   receipt of an MSDP message from peer, and deleted when the timer
   expires or the passive-connect peer closes the connection.


8.6. ConnectRetry Timer

   The ConnectRetry timer is used by an MSDP peer 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 peer's active
   connect attempt fails. 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.



8.7. Peer Hold Timer

   If a system does not receive successive KeepAlive messages (or any SA
   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. [Hold-Time-Period] MUST be at
   least three seconds. A suggested value for [Hold-Time-Period] is 90
   seconds.

   The Hold Timer is initialized to [Hold-Time-Period] when the peer's
   transport connection is established, and is reset to [Hold-Time-



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   Period] when any MSDP message is received.


9. Intermediate MSDP Peers

   Intermediate RPs 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.


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 will 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

   If an MSDP peer decides to cache SA state, it MAY accept SA-Requests
   from other MSDP peers. When an MSDP peer 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 sending to the group
   range 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.













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12. Encapsulated Data Packets

   For bursty sources, 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.



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, the Peer-RPF check is against the
   RP address carried in the SA message.


14.1. Peer-RPF Forwarding Rules

   An SA message originated by an MSDP originator R and received by a
   MSDP router from MSDP peer N is accepted if N is the appropriate RPF
   neighbor for originator R (the RP in the SA message), and discarded
   otherwise.

   The RPF neighbor is chosen using the first of the following rules
   that matches:

   (i).   R is the RPF neighbor if we have an MSDP peering with R.

   (ii).  The external MBGP neighbor towards which we are



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          poison-reversing the MBGP route towards R is the RPF neighbor
          if we have an MSDP peering with it.

   (iii). If we have any MSDP peerings with neighbors in the first
          AS along the AS_PATH (the AS from which we learned this
          route), but no external MBGP peerings with them,
          the neighbor with the highest IP address is the RPF neighbor.

   (vi).  The internal MBGP advertiser of the router towards R is
          the RPF neighbor if we have an MSDP peering with it.

   (v).   If none of the above match, and we have an MSDP
          default-peer configured, the MSDP default-peer is
          the RPF neighbor.



14.2. MSDP default-peer semantics

   An MSDP default-peer is much like a default route. It is intended to
   be used in those cases where a stub network isn't running BGP or
   MBGP. An MSDP peer configured with a default-peer accepts all SA
   messages from the default-peer. Note that a router running BGP or
   MBGP SHOULD NOT allow configuration of default peers, since this
   allows the possibility for SA looping or black-holes to occur.


15. MSDP Connection Establishment

   MSDP messages will be encapsulated in a TCP connection. An MSDP peer
   listens for new TCP connections on port 639. One side of the MSDP
   peering relationship will listen on the well-known port and the other
   side will do an active connect to the well-known port. The side with
   the higher peer IP address will do the 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 it may result in
   longer startup times at the passive end.

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










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                De-configured or
                  disabled
              +-------------------------------------------+
              |                                           |
              |                                           |
          Enable                                          |
        +-----|--------->+----------+ Connect Retry Timer |
        |     |       +->| INACTIVE |----------------+    |
        |     |       |  +----------+                |    |
   Deconf'ed  |       |   |  /|\ /|\                 |    |     Lower Address
       or     |       |   |   |   |                  |    |
    disabled  |       |   |   |   |                 \|/   |
        |     |       |   |   |   |               +-------------+
        |     |       |   |   |   +---------------|  CONNECTING |
        |     |       |   |   |     Timeout or    +-------------+
        |     |       |   |   |     Local Address Change      |
       \|/   \|/      |   |   |                               |
     +----------+     |   |   |                               |
     | DISABLED |     |   |   +---------------------+         | TCP Established
     +----------+     |   |                         |         |
     /|\ /|\          |   |   Connection Timeout,   |         |
      |   |           |   |   Local Address change, |         |
      |   |           |   |   Authorization Failure |         |
      |   |           |   |                         |         |
      |   |           |   |                         |        \|/
      |   |           |   |                       +-------------+
      |   |     Local |   |                       | ESTABLISHED |
      |   |   Address |   |   Higher Address      +-------------+
      |   |    Change |  \|/                          /|\    |
      |   |           |  +--------+                    |     |
      |   |           +--| LISTEN |--------------------+     |
      |   |              +--------+     TCP Accept           |
      |   |               |                                  |
      |   |               |                                  |
      |   +---------------+                                  |
      |    De-configured or                                  |
      |      disabled                                        |
      |                                                      |
      +------------------------------------------------------+
          De-configured or
           disabled










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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 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.

   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.







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16.2.1. IPv4 Source-Active TLV

   The maximum size SA message that can be sent is 1400 octets. If an
   MSDP peer needs to originate a message with information greater than
   1400 octets, it sends successive 1400 octet or smaller messages. The
   1400 octet size does not include the TCP, IP, layer-2 headers.

    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.

   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 ignored



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    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 caching
   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, and it has been configured to peer with an SA-state caching
   peer, it may send an SA-Request message for the group.

    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                 |  Gprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Group Address Prefix                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    IPv4 Source-Active Request TLV is type 2.

   Gprefix Len
    The route prefix length associated with the group address prefix.

   Group Address
    The group address the MSDP peer is requesting.









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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 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 after a period of time. This message
   is necessary for the active connect side of the MSDP connection. The
   passive connect side of the connection knows that the connection will
   be reestablished when a TCP SYN packet is sent from the active
   connect side. However, the active connect side will not know when the
   passive connect side goes down. Therefore, the KeepAlive timeout will
   be used to reset the TCP connection.

    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.



16.2.5. Notification TLV

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





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

   Message Header Error subcodes:

            0 - Unspecific                              (MC)
            2 - Bad Message Length                      (MC)



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            3 - Bad Message Type                        (CC)

   SA-Request Error subcodes:

            0 - Unspecific                              (MC)
            1 - Does not cache SA                       (MC)
            2 - 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:

            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 might 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 1400, 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 non-caching MSDP peer, or at a caching MSDP peer when an
   invalid group address requested.

   When a non-caching MSDP peer receives an SA-Request, it returns the
   following notification:

    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       |           16                  |O|     2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |           Reserved            |  Gprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   |                          Gprefix                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Group Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   y.fi


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

    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       |           16                  |O|     2       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |           Reserved            |  Gprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Gprefix                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Invalid 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 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       |      IEC      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.2. Invalid RP Address

    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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   |                     Invalid RP Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.3. Invalid Group Address

    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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Invalid Group Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.4. Invalid Source Address

    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                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Invalid Source Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


17.3.5.  Invalid Sprefix Length (ISL)

    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       |      ISL      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

    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       |         Looping 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.


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 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 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       |           16                  |O|     3       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |           Reserved            |  Gprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Gprefix                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Group Address                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




17.4. Hold Time Expired

   If a system does not receive successive KeepAlive or any SA Message
   and/or Notification messages 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 SA data.
   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 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 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 ....                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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



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   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 [GRE]. 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. Security Considerations

   An MSDP implementation MAY use IPsec [RFC1825] or keyed MD5 [RFC1828]
   to secure control messages. When encapsulating SA data 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.


20. Acknowledgments

   The authors would like to thank Bill Nickless, John Meylor, Liming
   Wei, Manoj Leelanivas, Mark Turner, John Zwiebel, and Cristina
   Radulescu-Banu for their design feedback and comments. In addition to
   many other contributions, Tom Pusateri helped to clarify the
   connection state machine, Dave Thaler helped to clarify the
   Notification message types, and Bill Fenner helped to clarify the
   Peer-RPF rules.










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21. Author's Address:

   Dino Farinacci
   Procket Networks
   3850 No. First St., Ste. C
   San Jose, CA 95134
   Email: dino@procket.com

   Yakov Rehkter
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134
   Email: yakov@cisco.com

   Peter Lothberg
   Sprint
   VARESA0104
   12502 Sunrise Valley Drive
   Reston VA, 20196
   Email: roll@sprint.net

   Hank Kilmer
   Email: hank@rem.com

   Jeremy Hall
   UUnet Technologies
   3060 Williams Drive
   Fairfax, VA 22031
   Email: jhall@uu.net

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
















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22. REFERENCES

   [GRE]       Farinacci, D., et al., "Generic Routing Encapsulation
               (GRE)", draft-meyer-gre-update-03.txt, January,
               2000. Work in Progress.

   [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.

   [RFC1825]   Atkinson, R., "Security Architecture for the Internet
               Protocol", RFC 1825, August, 1995.

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

















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