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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
                                                         December, 1999


               Multicast Source Discovery Protocol (MSDP)
                     <draft-ietf-msdp-spec-01.txt>



1. Status of this Memo

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

   Internet Drafts 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
   it's own independent RP(s) and does not have to depend on RPs in
   other domains.


3. Copyright Notice

   Copyright (C) The Internet Society (1999).  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

   An RP (or other MSDP SA originator) in a PIM-SM [RFC2362] domain will
   have a MSDP peering relationship with a MSDP speaker in another
   domain. The peering relationship will be made up of a TCP connection
   in which control information exchanged. Each domain will have one or
   more connections to this virtual topology.

   The purpose of this topology is to have 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 speakers can be realized by
   the underlying 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 10 below for the details of peer-RPF fowarding.

   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 it's MSDP peers.

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



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   implementation SHOULD forward an SA message (which was originated
   from  the RP address covered by that route) to the peer. This is
   known in other circles as Split-Horizon with Poison Reverse.

   When an MSDP peer which is also an RP for its own domain receives an
   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 (See
   [RFC2362] Section 3.2.2). 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. The advantage
   of caching is that newly formed MSDP peers can get MSDP (S,G) state
   sooner and therefore reduce join latency for new joiners. In
   addition, caching greatly aids in diagnosis and debugging of various
   problems.


7.1. Timers

   The main timers for MSDP are: SA Advertisement period, SA Hold-down
   period, the SA Cache timeout period, KeepAlive, HoldTimer, and
   ConnectRetry. Each is described below.


7.1.1. SA Advertisement Period

   RPs which originate SA messages do it periodically as long as there
   is data being sent by the source. The SA Advertisement Period MUST be
   60 seconds. An RP will not send more than one SA message for a given
   (S,G) within an SA Advertisement period. Originating periodic SA



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   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, 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 for each SA for that group in its
   cache.


7.1.2. SA Hold-down Period

   A caching MSDP speaker SHOULD NOT forward an SA message it has
   received in the last SA-Hold-down period. The SA-Hold-down period
   SHOULD be set to 30 seconds.


7.1.3. SA Cache Timeout

   A caching MSDP speaker times out it's SA cache at SA-State-Timer.
   The SA-State-Timer MUST NOT be less than 90 seconds.


7.1.4. KeepAlive, HoldTimer, and ConnectRetry

   The KeepAlive, HoldTimer, and ConnectRetry timers are defined in RFC
   1771 [RFC1771].


7.2. Intermediate MSDP Speakers

   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 its own sources.


7.3. 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, hoever, 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 black
   holes). 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)



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   MUST NOT be sent to peers which are on the other side of an
   administrative scope boundary for G.


7.4. 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.

   If an implementation receives an SA-Request message and is not
   caching SA messages, it sends a notification with Error code 7
   subcode 1, as defined in section 12.2.7.


8. 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 number of packets from the source which 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.

   Finally, if an implementation supports an encapsulation of SA data
   other than default TCP encapsulation, then it MUST support GRE
   encapsulation. In addition, an implementation MUST learn about not
   TCP encapsulations via capability advertisement (see section 11.2.5).












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9. Other Scenarios

   MSDP is not limited to deployment across different routing domains.
   It can be used within a routing domain when it is desired to deploy
   multiple RPs for different 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. Another example is the Anycast RP
   mechanism [ANYCASTRP].


10. MSDP Peer-RPF Forwarding

   The MSDP Peer-RPF Forwarding rules are used for forwarding SA
   messages throughout an MSDP enabled internet. Unlike the RPF check
   used when forwarding data packets, the Peer-RPF check is against the
   RP address carried in the SA message.


10.1. 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 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
          poison-reversing the MBGP route towards R is the RPF neighbor
          if we have an MSDP peering with it.

   (iii). If we have an MSDP peering with a neighbor in the first
          AS along the AS_PATH (the AS from which we learned this
          route), but no exeternal MBGP peering with that neighbor,
          pick a neighbor via a deterministic rule if you have have
          several, and that 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.

   Once an RPF neighbor is chosen (as defined above), an SA message is
   accepted if it was received from the RPF neighbor, and discarded
   otherwise.




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10.2. MSDP default-peer semantics

   A 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. A MSDP speaker 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 to occur.


11. MSDP Connection Establishment

   MSDP speakers establish peering sessions according to the following
   state machine:





































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                De-configured or
                  disabled
              +-------------------------------------------+
              |                                           |
        +-----|--------->+----------+                     |
        |     |       +->| INACTIVE |----------------+    |
        |     |       |  +----------+                |    |
   Deconf'ed  |       |   |  /|\ /|\                 | Timer + Higher Address
       or     |       |   |   |   |                  |    |
    disabled  |       |   |   |   |                 \|/   |
        |     |       |   |   |   |               +-------------+
        |     |       |   |   |   +---------------|  CONNECTING |
        |     |       |   |   |     Timeout or    +-------------+
        |     |       |   |   |     Router ID Change          |
       \|/   \|/      |   |   |                               |
     +----------+     |   |   |                               |
     | DISABLED |     |   |   +---------------------+         | TCP Established
     +----------+     |   |                         |         |
     /|\ /|\          |   |   Connection Timeout or |         |
      |   |           |   |   Router ID change   or |         |
      |   |           |   |   Authorization Failure |         |
      |   |           |   |                         |         |
      |   |           |   |                         |        \|/
      |   |           |   |                       +-------------+
      |   | Router ID |   |   Timer +             | ESTABLISHED |
      |   | Change    |   |   Low Address        +-------------+
      |   |           |  \|/                          /|\    |
      |   |           |  +--------+                    |     |
      |   |           +--| LISTEN |--------------------+     |
      |   |              +--------+     TCP Accept           |
      |   |               |                                  |
      |   |               |                                  |
      |   +---------------+                                  |
      |    De-configured or                                  |
      |      disabled                                        |
      |                                                      |
      +------------------------------------------------------+
          De-configured or
           disabled












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12. Packet Formats

   MSDP messages will be encapsulated in a TCP connection using well-
   known 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
   on 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.

   Finally, 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.


12.1. MSDP messages will be encoded in 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. The
    minimum length required is 3 octets.

   Value (variable length)
    Format is based on the Type value. See below. The length of
    the value field is Length field minus 3.


12.2. 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                  Encapsulation Capability Advertisement
    6                  Encapsulation Capability Request
    7                  Notification



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    8                  GRE Encapsulation

   Each TLV is described below.


12.2.1. IPv4 Source-Active TLV

   The maximum size SA message that can be sent is 1400 bytes. If an
   MSDP peer needs to originate a message with information greater than
   1400 bytes, it sends successive 1400-byte messages. The 1400 byte
   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           |  Gprefix Len  |  Sprefix Len  | \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  \
   |                      Group Address Prefix                     |   ) z
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  /
   |                      Source Address Prefix                    | /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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



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    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
    by a receiver.

   Gprefix Len and Sprefix Len
    The route prefix length associated with the group address
    prefix and source address prefix, respectively.

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

   Source Address Prefix
    The route prefix associated with 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.


12.2.2. 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 Prefix
    The group address prefix the MSDP peer is requesting.






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12.2.3. 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.


12.2.4. 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 bytes which encompasses the 1-byte
   Type field and the 2-byte Length field.


12.2.5. Encapsulation Capability Advertisement TLV

   This TLV is sent by an MSDP speaker to advertise its ability to
   receive data packets encapsulated as described by the TLV (in
   addition to the default TCP encapsulation).

   A MSDP speaker receiving this TLV can choose to either default TCP



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   encapsulation, or may send a IPv4 Encapsulation Request to change to
   the advertised encapsulation type.

   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       |             8                 |   ENCAP_TYPE  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port        |           Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    IPv4 Encapsulation Advertisement TLV is type 5.

   Length
    Length is a two byte field with value 8.

   ENCAP_TYPE
    The following data encapsulation types are defined for MSDP:

    Value             Meaning
    ---------------------------------------
     0           TCP Encapsulation
     1           UDP Encapsulation [RFC768]
     2           GRE Encapsulation [GRE]

   Source Port
    Port for use by the requester.

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

   Note that since the TLV does not carry endpoint addresses for the GRE
   or UDP tunnels, an implementation using these encapsulations MUST use
   the endpoints that are used for the MSDP peering.


12.2.6. Encapsulation Capability Request TLV

   The Encapsulation Capability Request is sent to notify a peer that
   has advertised an encapsulation capability that it will encapsulate
   SA data according to the advertised ENCAP_TYPE.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       6       |             4                 |   ENCAP_TYPE  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Type
    IPv4 Encapsulation Request TLV is type 6.

   Length
    Length is a two byte field with value 4.

   ENCAP_TYPE
    ENCAP_TYPE is described above.

   A requester MAY also provide a source port, in which case
   the TLV has 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       6       |             8                 |   ENCAP_TYPE  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port        |           Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


12.2.7. NOTIFICATION TLV

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       |          x + 5                |   Error Code  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Error subcode |          ...                                  |
   +-+-+-+-+-+-+-+-+                                               |
   |                         Data                                  |
   |                          ...                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    The Notification TLV is type 7.

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

   Error code
    See [RFC1771]. In addition, Error code 7 indicates an
    an SA-Request Error.

   Error subcode
    See [RFC1771]. In addition, Error code 7 subcode 1 indicates
    the receipt of an SA-Request message by a non-caching
    MSDP speaker.




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   Data
    See [RFC1771]. In addition, for Error code 7 subcode 1 (receipt
    of an SA-Request message by a non-caching MSDP speaker), the
    TLV has 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       |           20                  |       7       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       |   Reserved    |  Gprefix Len  |  Sprefix Len  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising RP Address                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Group Address Prefix                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Source Address Prefix                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   See [RFC1771] for NOTIFICATION error handling.


12.2.8. Encapsulation Capability State Machine

   The active connect side of an MSDP peering SHALL begin in ADVERTISING
   state, and the passive side of the TCP connection begins in DEFAULT
   state. This will cause the state machine to behave deterministically.

























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                                  +-------+
                                  |       | Receive TLV which isn't
                                  |       |   understood or
                                  |       | Receive Request (TLV 6) or
                                  |       | Receive Advertisement (TLV 5)
                                 \|/      |  that isn't understood
                           +---------+----+
                           | DEFAULT |----------------+
                           +---------+                |
                                                      |
                         +-------------+              |
                         | ADVERTISING |              |
                         +-------------+              |
                              |                       |
     Timeout  +--------+      |                       |
    +-------->| FAILED |      | Send Advertisement    | Receive Advertisement
    |         +--------+      |   (TLV 5)             |    (TLV 5)
    |                         |                       |
    |                         |                       |
    |                         |                       |
    |                         |                       |
    |  Receive non-matching   |                       |
    |  Request (TLV 6)        |                       |
    | +----+                  |                       |
    | |    |                  |                       |
    | |    |                  |                       |
    | |   \|/                 |                      \|/
    | | +------+              |                   +----------+
    | +-| SENT |<-------------+                   | RECEIVED |
    +---+------+                                  +----------+
           |                                         \|/
           |                                          |
           | Receive matching                         |  Send matching
           | Request (TLV 6)                          |  Request (TLV 6)
           |             +--------+                   |
           +------------>| AGREED |<------------------+
                         +--------+

   Note that if an advertiser transitions into the FAILED state, it
   SHOULD assume that it has an old-style peer which can only support
   TCP encapsulation. If an implementation wishes to be backwardly
   compatible, it SHOULD support TCP encapsulation. In addition, a
   requester in any state other than AGREED MUST only encapsulate data
   in the TCP stream.







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12.2.9. UDP Data Encapsulation

   When using UDP encapsulation, the UDP psuedo-header has 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port        |            Dest Port            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Length             |             Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Origin RP Address                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Source Port
    When using UDP encapsulation, a capability requester
    uses the advertiser's Source Port as its destination
    port. The advertiser MUST provide a Source Port.

   Destination Port
    When using UDP encapsulation, a capability advertiser
    uses the well known port 639 as the destination port.
    A capability requester MUST listen on this well-known
    port. The requester MAY provide a Source Port in it's
    reply to the advertiser.

   Length
    Length is the length in octets of this user datagram
    including  this header and the data. The minimum value
    of the length is twelve.

   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.














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12.2.10. GRE Encapsulation TLV

   A TLV is defined to describe GRE encapsulated data packets. The TLV
   has 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       8       |             8 + x             |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Originating RP IPv4 Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  (S,G) Data Packet ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type
    GRE encapsulated data packet TLV is type 8.

   Length
    Length is a two byte field with value 8 + x, where
    x is the length of the (S,G) Data packet.

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

   The entire GRE header, then, will 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Delivery Headers .....                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|       Reserved0       | Ver |         Protocol Type         |\
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ GRE Header
   |      Checksum (optional)      |          Reserved1            |/
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+\
   |       8       |             8 + x             |   Reserved    | \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Payload
   |                  Originating RP IPv4 Address                  |  /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
   |                    (S,G) Data Packet ....                       .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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12.2.10.1. Problems with MTU Exceeded by Encapsulation

   Black holes can arise when PMTU [RFC1191] is used and the tunnel
   entry point does not relay MTU exceeded errors back to the originator
   of the packet. A black hole can be realized by the following
   behavior: the originator sets the Don't Fragment bit in the Delivery
   Header, the packet gets dropped within the tunnel (MTU is exceeded),
   but since the originator doesn't receive feedback, it retransmits
   with the same PMTU, causing subsequently transmitted packets to be
   dropped. While GRE [GRE] does not require that such errors be relayed
   back to the originator, known implementations of GRE do not set the
   Don't Fragment bit in the Delivery Header.


13. 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.


14. Acknowledgments

   The authors would like to thank Dave Thaler, Bill Nickless, John
   Meylor, Liming Wei, Manoj Leelanivas, Mark Turner, and John Zwiebel
   for their design feedback and comments. Bill Fenner also made many
   contributions, including clarification of the Peer-RPF rules.

















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15. 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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16. REFERENCES


   [ANYCASTRP] Meyer, et. al, "Anycast RP mechanism using PIM and
               MSDP", draft-ietf-mboned-anycast-rp-04.txt, November,
               1999. Work in Progress.

   [GRE]       Farinacci, D., at el., "Generic Routing Encapsulation
               (GRE)", draft-meyer-gre-update-01.txt, December,
               1999. 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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