path: root/doc/rfc/rfc3913.txt

Network Working Group                                          D. Thaler
Request for Comments: 3913                                     Microsoft
Category: Informational                                   September 2004


               Border Gateway Multicast Protocol (BGMP):
                         Protocol Specification

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes the Border Gateway Multicast Protocol (BGMP),
   a protocol for inter-domain multicast routing.  BGMP builds shared
   trees for active multicast groups, and optionally allows receiver
   domains to build source-specific, inter-domain, distribution branches
   where needed.  BGMP natively supports "source-specific multicast"
   (SSM).  To also support "any-source multicast" (ASM), BGMP requires
   that each multicast group be associated with a single root (in BGMP
   it is referred to as the root domain).  It requires that different
   ranges of the multicast address space are associated (e.g., with
   Unicast-Prefix-Based Multicast addressing) with different domains.
   Each of these domains then becomes the root of the shared domain-
   trees for all groups in its range.  Multicast participants will
   generally receive better multicast service if the session initiator's
   address allocator selects addresses from its own domain's part of the
   space, thereby causing the root domain to be local to at least one of
   the session participants.















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Table of Contents

   1.  Purpose. . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology. . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol Overview. . . . . . . . . . . . . . . . . . . . . . .  5
       3.1.  Design Rationale . . . . . . . . . . . . . . . . . . . .  7
   4.  Protocol Details . . . . . . . . . . . . . . . . . . . . . . .  8
       4.1.  Interaction with the EGP . . . . . . . . . . . . . . . .  8
       4.2.  Multicast Data Packet Processing . . . . . . . . . . . .  9
       4.3.  BGMP processing of Join and Prune messages and
             notifications. . . . . . . . . . . . . . . . . . . . . . 10
             4.3.1.  Receiving Joins. . . . . . . . . . . . . . . . . 10
             4.3.2.  Receiving Prune Notifications. . . . . . . . . . 11
             4.3.3.  Receiving Route Change Notifications . . . . . . 12
             4.3.4.  Receiving (S,G) Poison-Reverse messages. . . . . 12
       4.4.  Interaction with M-IGP components. . . . . . . . . . . . 13
             4.4.1.  Interaction with DVMRP and PIM-DM. . . . . . . . 14
             4.4.2.  Interaction with PIM-SM. . . . . . . . . . . . . 15
             4.4.3.  Interaction with CBT . . . . . . . . . . . . . . 16
             4.4.4.  Interaction with MOSPF . . . . . . . . . . . . . 17
       4.5.  Operation over Multi-access Networks . . . . . . . . . . 17
       4.6.  Interaction between (S,G) state and G-routes . . . . . . 18
   5.  Message Formats. . . . . . . . . . . . . . . . . . . . . . . . 18
       5.1.  Message Header Format. . . . . . . . . . . . . . . . . . 19
       5.2.  OPEN Message Format. . . . . . . . . . . . . . . . . . . 19
       5.3.  UPDATE Message Format. . . . . . . . . . . . . . . . . . 23
       5.4.  Encoding examples. . . . . . . . . . . . . . . . . . . . 27
       5.5.  KEEPALIVE Message Format . . . . . . . . . . . . . . . . 27
       5.6.  NOTIFICATION Message Format. . . . . . . . . . . . . . . 28
   6.  BGMP Error Handling. . . . . . . . . . . . . . . . . . . . . . 30
       6.1.  Message Header error handling. . . . . . . . . . . . . . 30
       6.2.  OPEN message error handling. . . . . . . . . . . . . . . 30
       6.3.  UPDATE message error handling. . . . . . . . . . . . . . 31
       6.4.  NOTIFICATION message error handling. . . . . . . . . . . 32
       6.5.  Hold Timer Expired error handling. . . . . . . . . . . . 32
       6.6.  Finite State Machine error handling. . . . . . . . . . . 32
       6.7.  Cease. . . . . . . . . . . . . . . . . . . . . . . . . . 32
       6.8.  Connection collision detection . . . . . . . . . . . . . 32
   7.  BGMP Version Negotiation . . . . . . . . . . . . . . . . . . . 33
       7.1.  BGMP Capability Negotiation. . . . . . . . . . . . . . . 34
   8.  BGMP Finite State machine. . . . . . . . . . . . . . . . . . . 34
   9.  Security Considerations. . . . . . . . . . . . . . . . . . . . 38
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 39
       11.1. Normative References . . . . . . . . . . . . . . . . . . 39
       11.2. Informative References . . . . . . . . . . . . . . . . . 40
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 41



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1.  Purpose

   It has been suggested that inter-domain "any-source" multicast is
   better supported with a rendezvous mechanism whereby members receive
   sources' data packets without any sort of global broadcast (e.g.,
   MSDP broadcasts source information, PIM-DM [PIMDM] and DVMRP [DVMRP]
   broadcast initial data packets, and MOSPF [MOSPF] broadcasts
   membership information).  PIM-SM [PIMSM] and CBT [CBT] use a shared
   group-tree, to which all members join and thereby hear from all
   sources (and to which non-members do not join and thereby hear from
   no sources).

   This document describes BGMP, a protocol for inter-domain multicast
   routing.  BGMP natively supports "source-specific multicast" (SSM).
   To also support "any-source multicast" (ASM), BGMP builds shared
   trees for active multicast groups, and allows domains to build
   source-specific, inter-domain, distribution branches where needed.
   Building upon concepts from PIM-SM and CBT, BGMP requires that each
   global multicast group be associated with a single root.  However, in
   BGMP, the root is an entire exchange or domain, rather than a single
   router.

   For non-source-specific groups, BGMP assumes that ranges of the
   multicast address space have been associated (e.g., with Unicast-
   Prefix-Based Multicast [V4PREFIX,V6PREFIX] addressing) with selected
   domains.  Each such domain then becomes the root of the shared
   domain-trees for all groups in its range.  An address allocator will
   generally achieve better distribution trees if it takes its multicast
   addresses from its own domain's part of the space, thereby causing
   the root domain to be local.

   BGMP uses TCP as its transport protocol.  This eliminates the need to
   implement message fragmentation, retransmission, acknowledgement, and
   sequencing.  BGMP uses TCP port 264 for establishing its connections.
   This port is distinct from BGP's port to provide protocol
   independence, and to facilitate distinguishing between protocol
   packets (e.g., by packet classifiers, diagnostic utilities, etc.)

   Two BGMP peers form a TCP connection between one another, and
   exchange messages to open and confirm the connection parameters.
   They then send incremental Join/Prune Updates as group memberships
   change.  BGMP does not require periodic refresh of individual
   entries.  KeepAlive messages are sent periodically to ensure the
   liveness of the connection.  Notification messages are sent in
   response to errors or special conditions.  If a connection encounters
   an error condition, a notification message is sent and the connection
   is closed if the error is a fatal one.




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2.  Terminology

   This document uses the following technical terms:

   Domain:
      A set of one or more contiguous links and zero or more routers
      surrounded by one or more multicast border routers.  Note that
      this loose definition of domain also applies to an external link
      between two domains, as well as an exchange.

   Root Domain:
      When constructing a shared tree of domains for some group, one
      domain will be the "root" of the tree.  The root domain receives
      data from each sender to the group, and functions as a rendezvous
      domain toward which member domains can send inter-domain joins,
      and to which sender domains can send data.

   Multicast RIB:
      The Routing Information Base, or routing table, used to calculate
      the "next-hop" towards a particular address for multicast traffic.

   Multicast IGP (M-IGP):
      A generic term for any multicast routing protocol used for tree
      construction within a domain.  Typical examples of M-IGPs are:
      PIM-SM, PIM-DM, DVMRP, MOSPF, and CBT.

   EGP: A generic term for the interdomain unicast routing protocol in
      use.
      Typically, this will be some version of BGP which can support a
      Multicast RIB, such as MBGP [MBGP], containing both unicast and
      multicast address prefixes.

   Component:
      The portion of a border router associated with (and logically
      inside) a particular domain that runs the multicast IGP (M-IGP)
      for that domain, if any.  Each border router thus has zero or more
      components inside routing domains.  In addition, each border
      router with external links that do not fall inside any routing
      domain will have an inter-domain component that runs BGMP.

   External peer:
      A border router in another multicast AS (autonomous system, as
      used in BGP), to which a BGMP TCP-connection is open.  If BGP is
      being used as the EGP, a separate "eBGP" TCP-connection will also
      be open to the same peer.






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   Internal peer:
      Another border router of the same multicast AS.  If BGP is being
      used as the EGP, the border router either speaks iBGP ("internal"
      BGP) directly to internal peers in a full mesh, or indirectly
      through a route reflector [REFLECT].

   Next-hop peer:
      The next-hop peer towards a given IP address is the next EGP
      router on the path to the given address, according to multicast
      RIB routes in the EGP's routing table (e.g., in MBGP, routes whose
      Subsequent Address Family Identifier field indicates that the
      route is valid for multicast traffic).

   target:
      Either an EGP peer, or an M-IGP component.

   Tree State Table:
      This is a table of (S-prefix,G) and (*,G-prefix) entries that have
      been explicitly joined by a set of targets.  Each entry has, in
      addition to the source and group addresses and masks, a list of
      targets that have explicitly requested data (on behalf of directly
      connected hosts or downstream routers).  (S,G) entries also have
      an "SPT" bit.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as described in [RFC2119].

3.  Protocol Overview

   BGMP maintains group-prefix state in response to messages from BGMP
   peers and notifications from M-IGP components.  Group-shared trees
   are rooted at the domain advertising the group prefix covering those
   groups.  When a receiver joins a specific group address, the border
   router towards the root domain generates a group-specific Join
   message, which is then forwarded Border-Router-by-Border-Router
   towards the root domain (see Figure 1).  BGMP Join and Prune messages
   are sent over TCP connections between BGMP peers, and BGMP protocol
   state is refreshed by KEEPALIVE messages periodically sent over TCP.

   BGMP routers build group-specific bidirectional forwarding state as
   they process the BGMP Join messages.  Bidirectional forwarding state
   means that packets received from any target are forwarded to all
   other targets in the target list without any RPF checks.  No group-
   specific state or traffic exists in parts of the network where there
   are no members of that group.






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   BGMP routers optionally build source-specific unidirectional
   forwarding state, only where needed, to be compatible with source-
   specific trees (SPTs) used by some M-IGPs (e.g., DVMRP, PIM-DM, or
   PIM-SM), or to construct trees for source-specific groups.  A domain
   that uses an SPT-based M-IGP may need to inject multicast packets
   from external sources via different border routers (to be compatible
   with the M-IGP RPF checks) which thus act as "surrogates".  For
   example, in the Transit_1 domain, data from Src_A arrives at BR12,
   but must be injected by BR11.  A surrogate router may create a
   source-specific BGMP branch if no shared tree state exists.  Note:
   stub domains with a single border router, such as Rcvr_Stub_7 in
   Figure 1, receive all multicast data packets through that router, to
   which all RPF checks point.  Therefore, stub domains never build
   source-specific state.

             Root_Domain
              [BR91]--------------------------\
                 |                            |
              [BR32]                         [BR41]
             Transit_3                     Transit_4
              [BR31]                      [BR42] [BR43]
                 |                          |      |
              [BR22]                      [BR52] [BR53]
             Transit_2                     Transit_5
              [BR21]                         [BR51]
                 |                            |
              [BR12]                         [BR61]
             Transit_1[BR11]----------[BR62]Stub_6
              [BR13]                        (Src_A)
                 |                          (Rcvr_D)
       -------------------
       |                 |
    [BR71]              [BR81]
   Rcvr_Stub_7       Src_only_Stub_8
   (Rcvr_C)             (Src_B)

   Figure 1: Example inter-domain topology.  [BRxy] represents a BGMP
   border router.  Transit_X is a transit domain network.  *_Stub_X is a
   stub domain network.

   Data packets are forwarded based on a combination of BGMP and M-IGP
   rules.  The router forwards to a set of targets according to a
   matching (S,G) BGMP tree state entry if it exists.  If not found, the
   router checks for a matching (*,G) BGMP tree state entry.  If neither
   is found, then the packet is sent natively to the next-hop EGP peer
   for G, according to the Multicast RIB (for example, in the case of a
   non-member sender such as Src_B in Figure 1).  If a matching entry
   was found, the packet is forwarded to all other targets in the target



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   list.  In this way BGMP trees forward data in a bidirectional manner.
   If a target is an M-IGP component then forwarding is subject to the
   rules of that M-IGP protocol.

3.1.  Design Rationale

   Several other protocols, or protocol proposals, build shared trees
   within domains [PIMSM, CBT].  The design choices made for BGMP result
   from our focus on Inter-Domain multicast in particular.  The design
   choices made by PIM-SM and CBT are better suited to the wide-area
   intra-domain case.  There are three major differences between BGMP
   and other shared-tree protocols:

   (1) Unidirectional vs. Bidirectional trees

   Bidirectional trees (using bidirectional forwarding state as
   described above) minimize third party dependence which is essential
   in the inter-domain context.  For example, in Figure 1, stub domains
   7 and 8 would like to exchange multicast packets without being
   dependent on the quality of connectivity of the root domain.
   However, unidirectional shared trees (i.e., those using RPF checks)
   have more aggressive loop prevention and share the same processing
   rules as source-specific entries which are inherently unidirectional.

   The lack of third party dependence concerns in the INTRA domain case
   reduces the incentive to employ bidirectional trees.  BGMP supports
   bidirectional trees because it has to, and because it can without
   excessive cost.

   (2) Source-specific distribution trees/branches

   In a departure from other shared tree protocols, source-specific BGMP
   state is built ONLY where (a) it is needed to pull the multicast
   traffic down to a BGMP router that has source-specific (S,G) state,
   and (b) that router is NOT already on the shared tree (i.e., has no
   (*,G) state), and (c) that router does not want to receive packets
   via encapsulation from a router which is on the shared tree.  BGMP
   provides source-specific branches because most M-IGP protocols in use
   today build source-specific trees.  BGMP's source-specific branches
   eliminate the unnecessary overhead of encapsulations for high data
   rate sources from the shared tree's ingress router to the surrogate
   injector (e.g., from BR12 to BR11 in Figure 1).  Moreover, cases in
   which shared paths are significantly longer than SPT paths will also
   benefit.

   However, except for source-specific group distribution trees, we do
   not build source-specific inter-domain trees in general because (a)
   inter-domain connectivity is generally less rich than intra-domain



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   connectivity, so shared distribution trees should have more
   acceptable path length and traffic concentration properties in the
   inter-domain context, than in the intra-domain case, and (b) by
   having the shared tree state always take precedence over source-
   specific tree state, we avoid ambiguities that can otherwise arise.

   In summary, BGMP trees are, in a sense, a hybrid between PIM-SM and
   CBT trees.

   (3) Method of choosing root of group shared tree

   The choice of a group's shared-tree-root has implications for
   performance and policy.  In the intra-domain case it is sometimes
   assumed that all potential shared-tree roots (RPs/Cores) within the
   domain are equally suited to be the root for a group that is
   initiated within that domain.  In the INTER-domain case, there is far
   more opportunity for unacceptably poor locality, and administrative
   control of a group's shared-tree root.  Therefore in the intra-domain
   case, other protocols sometimes treat all candidate roots (RPs or
   Cores) as equivalent and emphasize load sharing and stability to
   maximize performance.  In the Inter-Domain case, all roots are not
   equivalent, and we adopt an approach whereby a group's root domain is
   not random but is subject to administrative control.

4.  Protocol Details

   In this section, we describe the detailed protocol that border
   routers perform.  We assume that each border router conforms to the
   component-based model described in [INTEROP], modulo one correction
   to section 3.2 ("BGMP" Dispatcher), as follows:

   The iif owner of a (*,G) entry is the component owning the next-hop
   interface towards the nominal root of G, in the multicast RIB.

4.1.  Interaction with the EGP

   The fundamental requirements imposed by BGMP are that:

   (1)   For a given source-specific group and source, BGMP must be able
         to look up the next-hop towards the source in the Multicast
         RIB, and

   (2)   For a given non-source-specific group, BGMP will map the group
         address to a nominal "root" address, and must be able to look
         up the next-hop towards that address in the Multicast RIB.






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   BGMP determines the nominal "root" address as follows.  If the
   multicast address is a Unicast-Prefix-based Multicast address, then
   the nominal root address is the embedded unicast prefix, padded with
   a suffix of 0 bits to form a full address.

   For example, if the IPv6 group address is
   ff2e:0100:1234:5678:9abc:def0::123, then the unicast prefix is
   1234:5678:9abc:def0/64, and the nominal root address would be
   1234:5678:9abc:def0::.  (This address is in fact the subnet router
   anycast address [IPv6AA].)

   Support for any-source-multicast using any address other than a
   Unicast-prefix-based Multicast Address is outside the scope of this
   document.

4.2.  Multicast Data Packet Processing

   For BGMP rules to be applied, an incoming packet must first be
   "accepted":

   o  If the packet arrived on an interface owned by an M-IGP, the M-IGP
      component determines whether the packet should be accepted or
      dropped according to its rules.  If the packet is accepted, the
      packet is forwarded (or not forwarded) out any other interfaces
      owned by the same component, as specified by the M-IGP.

   o  If the packet was received over a point-to-point interface owned
      by BGMP, the packet is accepted.

   o  If the packet arrived on a multiaccess network interface owned by
      BGMP, the packet is accepted if it is receiving data on a source-
      specific branch, if it is the designated forwarder for the longest
      matching route for S, or for the longest matching route for the
      nominal root of G.

   If the packet is accepted, then the router checks the tree state
   table for a matching (S,G) entry.  If one is found, but the packet
   was not received from the next hop target towards S (if the entry's
   SPT bit is True), or was not received from the next hop target
   towards G (if the entry's SPT bit is False) then the packet is
   dropped and no further actions are taken.  If no (S,G) entry was
   found, the router then checks for a matching (*,G) entry.

   If neither is found, then the packet is forwarded towards the next-
   hop peer for the nominal root of G, according to the Multicast RIB.
   If a matching entry was found, the packet is forwarded to all other
   targets in the target list.




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   Forwarding to a target which is an M-IGP component means that the
   packet is forwarded out any interfaces owned by that component
   according to that component's multicast forwarding rules.

4.3.  BGMP processing of Join and Prune messages and notifications

4.3.1.  Receiving Joins

   When the BGMP component receives a (*,G) or (S,G) Join alert from
   another component, or a BGMP (S,G) or (*,G) Join message from an
   external peer, it searches the tree state table for a matching entry.
   If an entry is found, and that peer is already listed in the target
   list, then no further actions are taken.

   Otherwise, if no (*,G) or (S,G) entry was found, one is created.  In
   the case of a (*,G), the target list is initialized to contain the
   next-hop peer towards the nominal root of G, if it is an external
   peer.  If the peer is internal, the target list is initialized to
   contain the M-IGP component owning the next-hop interface.  If there
   is no next-hop peer (because the nominal root of G is inside the
   domain), then the target  list is initialized to contain the next-hop
   component.  If an (S,G) entry exists for the same G for which the
   (*,G) Join is being processed, and the next-hop peers toward S and
   the nominal root of G are different, the BGMP router must first send
   a (S,G) Prune message toward the source and clear the SPT bit on the
   (S,G) entry, before activating the (*,G) entry.

   When creating (S,G) state, if the source is internal to the BGMP
   speaker's domain, a "Poison-Reverse" bit (PR-bit) is set.  This bit
   indicates that the router may receive packets matching (S,G) anyway
   due to the BGMP speaker being a member of a domain on the path
   between S and the root domain.  (Depending on the M-IGP protocol, it
   may in fact receive such packets anyway only if it is the best exit
   for the nominal root of G.)

   The target from which the Join was received is then added to the
   target list.  The router then looks up S or the nominal root of G in
   the Multicast RIB to find the next-hop EGP peer.  If the target list,
   not including the next-hop target towards G for a (*,G) entry,
   becomes non-null as a result, the next-hop EGP peer must be notified
   as follows:

   a) If the next-hop peer towards the nominal root of G (for a (*,G)
      entry) is an external peer, a BGMP (*,G) Join message is unicast
      to the external peer.  If the next-hop peer towards S (for an
      (S,G) entry) is an external peer, and the router does NOT have any
      active (*,G) state for that group address G, a BGMP (S,G) Join
      message is unicast to the external peer.  A BGMP (S,G) Join



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      message is never sent to an external peer by a router that also
      contains active (*,G) state for the same group.  If the next-hop
      peer towards S (for an (S,G entry) is an external peer and the
      router DOES have active (*,G) state for that group G, the SPT bit
      is always set to False.

   b) If the next-hop peer is an internal peer, a (*,G) or (S,G) Join
      alert is sent to the M-IGP component owning the next-hop
      interface.

   c) If there is no next-hop peer, a (*,G) or (S,G) Join alert is sent
      to the M-IGP component owning the next-hop interface.

   Finally, if an (S,G) Join is received from an internal peer, the peer
   should be stored with the M-IGP component target.  If (S,G) state
   exists with the PR-bit set, and the next-hop towards the nominal root
   for G is through the M-IGP component, an (S,G) Poison-Reverse message
   is immediately sent to the internal peer.

   If an (S,G) Join is received from an external peer, and (S,G) state
   exists with the PR-bit set, and the local BGMP speaker is the best
   exit for the nominal root of G, and the next-hop towards the nominal
   root for G is through the interface towards the external peer, an
   (S,G) Poison-Reverse message is immediately sent to the external
   peer.

4.3.2.  Receiving Prune Notifications

   When the BGMP component receives a (*,G) or (S,G) Prune alert from
   another component, or a BGMP (*,G) or (S,G) Prune message from an
   external peer, it searches the tree state table for a matching entry.
   If no (S,G) entry was found for an (S,G) Prune, but (*,G) state
   exists, an (S,G) entry is created, with the target list copied from
   the (*,G) entry.  If no matching entry exists, or if the component or
   peer is not listed in the target list, no further actions are taken.

   Otherwise, the component or peer is removed from the target list.  If
   the target list becomes null as a result, the next-hop peer towards
   the nominal root of G (for a (*,G) entry), or towards S (for an (S,G)
   entry if and only if the BGMP router does NOT have any corresponding
   (*,G) entry), must be notified as follows.

   a) If the peer is an external peer, a BGMP (*,G) or (S,G) Prune
      message is unicast to it.

   b) If the next-hop peer is an internal peer, a (*,G) or (S,G) Prune
      alert is sent to the M-IGP component owning the next-hop
      interface.



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   c) If there is no next-hop peer, a (*,G) or (S,G) Prune alert is sent
      to the M-IGP component owning the next-hop interface.

4.3.3.  Receiving Route Change Notifications

   When a border router receives a route for a new prefix in the
   multicast RIB, or a existing route for a prefix is withdrawn, a route
   change notification for that prefix must be sent to the BGMP
   component.  In addition, when the next hop peer (according to the
   multicast RIB) changes, a route change notification for that prefix
   must be sent to the BGMP component.

   In addition, in IPv4 (only), an internal route for each class-D
   prefix associated with the domain (if any) MUST be injected into the
   multicast RIB in the EGP by the domain's border routers.

   When a route for a new group prefix is learned, or an existing route
   for a group prefix is withdrawn, or the next-hop peer for a group
   prefix changes, a BGMP router updates all affected (*,G) target
   lists.  The router sends a (*,G) Join to the new next-hop target, and
   a (*,G) Prune to the old next-hop target, as appropriate.  In
   addition, if any (S,G) state exists with the PR-bit set:

   o  If the BGMP speaker has just become the best exit for the nominal
      root of G, an (S,G) Poison Reverse message with the PR-bit set is
      sent as noted below.

   o  If the BGMP speaker was the best exit for the nominal root of G
      and is no longer, an (S,G) Poison Reverse message with the PR-bit
      clear is sent as noted below.

   The (S,G) Poison-Reverse messages are sent to all external peers on
   the next-hop interface towards the nominal root of G from which (S,G)
   Joins have been received.

   When an existing route for a source prefix is withdrawn, or the
   next-hop peer for a source prefix changes, a BGMP router updates all
   affected (S,G) target lists.  The router sends a (S,G) Join to the
   new next-hop target, and a (S,G) Prune to the old next-hop target, as
   appropriate.

4.3.4.  Receiving (S,G) Poison-Reverse messages

   When a BGMP speaker receives an (S,G) Poison-Reverse message from a
   peer, it sets the PR-bit on the (S,G) state to match the PR-bit in
   the message, and looks up the next-hop towards the nominal root of G.
   If the next-hop target is an M-IGP component, it forwards the (S,G)
   Poison Reverse message to all internal peers of that component from



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   which it has received (S,G) Joins.  If the next-hop target is an
   external peer on a given interface, it forwards the (S,G) Poison
   Reverse message to all external peers on that interface.

   When a BGMP speaker receives an (S,G) Poison-Reverse message from an
   external peer, with the PR-bit set, and the speaker has received no
   (S,G) Joins from any other peers (e.g., only from the M-IGP, or has
   (S,G) state due to encapsulation as described in 5.4.1), it knows
   that its own (S,G) Join is unnecessary, and should send an (S,G)
   Prune.

   When a BGMP speaker receives an (S,G) Poison-Reverse message from an
   internal peer, with the PR-bit set, and the speaker is the best exit
   for the nominal root of G, and has (S,G) prune state, an (S,G) Join
   message is sent to cancel the prune state and the state is deleted.

4.4.  Interaction with M-IGP components

   When an M-IGP component on a border router first learns that there
   are internally-reached members for a group G (whose scope is larger
   than that domain), a (*,G) Join alert is sent to the BGMP component.
   Similarly, when an M-IGP component on a border router learns that
   there are no longer internally-reached members for a group G (whose
   scope is larger than a single domain), a (*,G) Prune alert is sent to
   the BGMP component.

   At any time, any M-IGP domain MAY decide to join a source-specific
   branch for some external source S and group G.  When the M-IGP
   component in the border router that is the next-hop router for a
   particular source S learns that a receiver wishes to receive data
   from S on a source-specific path, an (S,G) Join alert is sent to the
   BGMP component.  When it is learned that such receivers no longer
   exist, an (S,G) Prune alert is sent to the BGMP component.  Recall
   that the BGMP component will generate external source-specific Joins
   only where the source-specific branch does not coincide with the
   shared tree distribution tree for that group.

   Finally, we will require that the border router that is the next-hop
   internal peer for a particular address S or the nominal root of G be
   able to forward data for a matching tree state table entry to all
   members within the domain.  This requirement has implications on
   specific M-IGPs as follows.









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4.4.1.  Interaction with DVMRP and PIM-DM

   DVMRP and PIM-DM are both "broadcast and prune" protocols in which
   every data packet must pass an RPF check against the packet's source
   address, or be dropped.  If the border router receiving packets from
   an external source is the only BR to inject the route for the source
   into the domain, then there are no problems.  For example, this will
   always be true for stub domains with a single border router (see
   Figure 1).  Otherwise, the border router receiving packets externally
   is responsible for encapsulating the data to any other border routers
   that must inject the data into the domain for RPF checks to succeed.

   When an intended border router injector for a source receives
   encapsulated packets from another border router in its domain, it
   should create source-specific (S,G) BGMP state.  Note that the border
   router may be configured to do this on a data-rate triggered basis so
   that the state is not created for very low data-rate/intermittent
   sources.  If source-specific state is created, then its incoming
   interface points to the virtual encapsulation interface from the
   border router that forwarded the packet, and it has an SPT flag that
   is initialized to be False.

   When the (S,G) BGMP state is created, the BGMP component will in turn
   send a BGMP (S,G) Join message to the next-hop external peer towards
   S if there is no (*,G) state for that same group, G.  The (S,G) BGMP
   state will have the SPT bit set to False if (*,G) BGMP state is
   present.

   When the first data packet from S arrives from the external peer and
   matches on the BGMP (S,G) state, and IF there is no (*,G) state, the
   router sets the SPT flag to True, resets the incoming interface to
   point to the external peer, and sends a BGMP (S,G) Prune message to
   the border router that was encapsulating the packets (e.g., in Figure
   1, BR11 sends the (Src_A,G) Prune to BR12).  When the border router
   with (*,G) state receives the prune for (S,G), it then deletes that
   border router from its list of targets.

   If the decapsulator receives a (S,G) Poison Reverse message with the
   PR-bit set, it will forward it to the encapsulator (which may again
   forward it up the shared tree according to normal BGMP rules), and
   both will delete their BGMP (S,G) state.

   PIM-DM and DVMRP present an additional problem, i.e., no protocol
   mechanism exists for joining and pruning entire groups; only joins
   and prunes for individual sources are available.  As a result, BGMP
   does not currently support such protocols being used in a transit
   domain.




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4.4.2.  Interaction with PIM-SM

   Protocols such as PIM-SM build unidirectional shared and source-
   specific trees.  As with DVMRP and PIM-DM, every data packet must
   pass an RPF check against some group-specific or source-specific
   address.

   The fewest encapsulations/decapsulations will be done when the
   intra-domain tree is rooted at the next-hop internal peer (which
   becomes the RP) towards the nominal root of G, since in general that
   router will receive the most packets from external sources.  To
   achieve this, each BGMP border router to a PIM-SM domain should send
   Candidate-RP-Advertisements within the domain for those groups for
   which it is the shared-domain tree ingress router.  When the border
   router that is the RP for a group G receives an external data packet,
   it forwards the packet according to the M-IGP (i.e., PIM-SM) shared-
   tree outgoing interface list.

   Other border routers will receive data packets from external sources
   that are farther down the bidirectional tree of domains.  When a
   border router that is not the RP receives an external packet for
   which it does not have a source-specific entry, the border router
   treats it like a local source by creating (S,G) state with a Register
   flag set, based on normal PIM-SM rules; the Border router then
   encapsulates the data packets in PIM-SM Registers and unicasts them
   to the RP for the group.  As explained above, the RP for the inter-
   domain group will be one of the other border routers of the domain.

   If a source's data rate is high enough, DRs within the PIM-SM domain
   may switch to the shortest path tree.  If the shortest path to an
   external source is via the group's ingress router for the shared
   tree, the new (S,G) state in the BGMP border router will not cause
   BGMP (S,G) Joins because that border router will already have (*,G)
   state.  If however, the shortest path to an external source is via
   some other border router, that border router will create (S,G) BGMP
   state in response to the M-IGP (S,G) Join alert.  In this case,
   because there is no local (*,G) state to suppress it, the border
   router will send a BGMP (S,G) Join to the next-hop external peer
   towards S, in order to pull the data down directly.  (See BR11 in
   Figure 1).  As in normal PIM-SM operation, those PIM-SM routers that
   have (*,G) and (S,G) state pointing to different incoming interfaces
   will prune that source off the shared tree.  Therefore, all internal
   interfaces may be eventually pruned off the internal shared tree.








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   After the border router sends a BGMP (S,G) Join, if its (S,G) state
   has the PR-bit clear, a (S,G) Poison-Reverse message (with the PR-bit
   clear) is sent to the ingress router for G.  The ingress router then
   creates (S,G) if it does not already exist, and removes the next hop
   towards the nominal root of G from the target list.

   If the border router later receives an (S,G) Poison-Reverse message
   with the PR-bit set, the Poison-Reverse message is forwarded to the
   ingress router for G.  The best-exit router then creates (S,G) state
   if it does not already exist, and puts the next hop towards the
   nominal root of G in the target list if not already present.

4.4.3.  Interaction with CBT

   CBT builds bidirectional shared trees but must address two points of
   compatibility with BGMP.  First, CBT can not accommodate more than
   one border router injecting a packet.  Therefore, if a CBT domain
   does have multiple external connections, the M-IGP components of the
   border routers are responsible for insuring that only one of them
   will inject data from any given source.

   Second, CBT cannot process source-specific Joins or Prunes.  Two
   options thus exist for each CBT domain:

   Option A:
      The CBT component interprets a (S,G) Join alert as if it were an
      (*,G) Join alert, as described in [INTEROP].  That is, if it is
      not already on the core-tree for G, then it sends a CBT (*,G)
      JOIN-REQUEST message towards the core for G.  Similarly, when the
      CBT component receives an (S,G) Prune alert, and the child
      interface list for a group is NULL, then it sends a (*,G)
      QUIT_NOTIFICATION towards the core for G.  This option has the
      disadvantage of pulling all data for the group G down to the CBT
      domain when no members exist.

   Option B:
      The CBT domain does not propagate any routes to their external
      peers for the Multicast RIB unless it is known that no other path
      exists to that prefix (e.g., routes for prefixes internal to the
      domain or in a singly-homed customer's domain may be propagated).
      This insures that source-specific joins are never received unless
      the source's data already passes through the domain on the shared
      tree, in which case the (S,G) Join need not be propagated anyway.
      BGMP border routers will only send source-specific Joins or Prunes
      to an external peer if that external peer advertises source-
      prefixes in the EGP.  If a BGMP-CBT border router does receive an
      (S,G) Join or Prune, that border router should ignore the message.




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   To minimize en/de-capsulations, CBTv2 BR's may follow the same scheme
   as described under PIM-SM above, in which Candidate-Core
   advertisements are sent for those groups for which it is the shared-
   tree ingress router.

4.4.4.  Interaction with MOSPF

   As with CBTv2, MOSPF cannot process source-specific Joins or Prunes,
   and the same two options are available.  Therefore, an MOSPF domain
   may either:

   Option A:
      send a Group-Membership-LSA for all of G in response to a (S,G)
      Join alert, and "prematurely age" it out (when no other downstream
      members exist) in response to an (S,G) Prune alert, OR

   Option B:
      not propagate any routes to their external peers for the Multicast
      RIB unless it is known that no other path exists to that prefix
      (e.g., routes for prefixes internal to the domain or in a singly-
      homed customer's domain may be propagated)

4.5.  Operation over Multi-access Networks

   Multiaccess links require special handling to prevent duplicates.
   The following mechanism enables BGMP to operate over multiaccess
   links which do not run an M-IGP.  This avoids broadcast-and-prune
   behavior and does not require (S,G) state.

   To elect a designated forwarder per prefix, BGMP uses a FWDR_PREF
   message to exchange "forwarder preference" values for each prefix.
   The peer with the highest forwarder preference becomes the designated
   forwarder, with ties broken by lowest BGMP Identifier.  The
   designated forwarder is the router responsible for forwarding packets
   up the tree, and is the peer to which joins will be sent.

   When BGMP first learns that a route exists in the multicast RIB whose
   next-hop interface is NOT the multiaccess link, the BGMP router sends
   a BGMP FWDR_PREF message for the prefix, to all BGMP peers on the
   LAN.  The FWDR_PREF message contains a "forwarder preference value"
   for the local router, and the same value MUST be sent to all peers on
   the LAN.  Likewise, when the prefix is no longer reachable, a
   FWDR_PREF of 0 is sent to all peers on the LAN.

   Whenever a BGMP router calculates the next-hop peer towards a
   particular address, and that peer is reached over a BGMP-owned
   multiaccess LAN, the designated forwarder is used instead.




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   When a BGMP router receives a FWDR_PREF message from a peer, it looks
   up the matching route in its multicast RIB, and calculates the new
   designated forwarder.  If the router has tree state entries whose
   parent target was the old forwarder, it sends Joins to the new
   forwarder and Prunes to the old forwarder.

   When a BGMP router which is NOT the designated forwarder receives a
   packet on the multiaccess link, it is silently dropped.

   Finally, this mechanism prevents duplicates where full peering exists
   on a "logical" link.  Where full peering does not exist, steps must
   be taken (outside of BGMP) to present separate logical interfaces to
   BGMP, each of which is a link with full peering.  This might entail,
   for example, using different link-layer address mappings, doing
   encapsulation, or changing the physical media.

4.6.  Interaction between (S,G) state and G-routes

   As discussed earlier, routers with (*,G) state will not propagate
   (S,G) joins.  However, a special case occurs when (S,G) state
   coincides with the G-route (or route towards the nominal root of G).
   When this occurs, care must be taken so that the data will reach the
   root domain without causing duplicates or black holes.  For this
   reason, (S,G) state on the path between the source and the root
   domain is annotated as being "poison-reversed".  A PR-bit is kept for
   this purpose, which is updated by (UN)POISON_REVERSE messages.

   The PR-bit indicates to BGMP nodes whether they need to forward
   packets up towards the root domain.  For example, in a case where an
   (S,G) branch exists, a transit domain may get packets along the (S,G)
   branch, and needs to know whether to (also) forward them up towards
   the root domain.  If the domain in question is on the path between S
   and the root domain, then the answer is yes (and the PR bit will be
   set on the S,G state).  If the domain in question is not on the path
   between S and the root domain, then the answer is no (and the PR bit
   will be clear on the S,G state).

5.  Message Formats

   This section describes message formats used by BGMP.

   Messages are sent over a reliable transport protocol connection.  A
   message is processed only after it is entirely received.  The maximum
   message size is 4096 octets.  All implementations are required to
   support this maximum message size.

   All fields labelled "Reserved" below must be transmitted as 0, and
   ignored upon receipt.



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5.1.  Message Header Format

   Each message has a fixed-size (4-byte) header.  There may or may not
   be a data portion following the header, depending on the message
   type.  The layout of these fields is shown below:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Length               |      Type     |    Reserved   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length:
      This 2-octet unsigned integer indicates the total length of the
      message, including the header, in octets.  Thus, e.g., it allows
      one to locate in the transport-level stream the start of the next
      message.  The value of the Length field must always be at least 4
      and no greater than 4096, and may be further constrained,
      depending on the message type.  No "padding" of extra data after
      the message is allowed, so the Length field must have the smallest
      value required given the rest of the message.

   Type:
      This 1-octet unsigned integer indicates the type code of the
      message.  The following type codes are defined:

         1 - OPEN
         2 - UPDATE
         3 - NOTIFICATION
         4 - KEEPALIVE

5.2.  OPEN Message Format

   After a transport protocol connection is established, the first
   message sent by each side is an OPEN message.  If the OPEN message is
   acceptable, a KEEPALIVE message confirming the OPEN is sent back.
   Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
   messages may be exchanged.

   In addition to the fixed-size BGMP header, the OPEN message contains
   the following fields:










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Version     | Rsvd| AddrFam |           Hold Time           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                BGMP Identifier (variable length)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                      (Optional Parameters)                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version:
      This 1-octet unsigned integer indicates the protocol version
      number of the message.  The current BGMP version number is 1.

   AddrFam:
      The IANA-assigned address family number of the BGMP Identifier.
      These include (among others):

      Number    Description
      ------    -----------
         1      IP (IP version 4)
         2      IPv6 (IP version 6)

   Hold Time:
      This 2-octet unsigned integer indicates the number of seconds that
      the sender proposes for the value of the Hold Timer.  Upon receipt
      of an OPEN message, a BGMP speaker MUST calculate the value of the
      Hold Timer by using the smaller of its configured Hold Time and
      the Hold Time received in the OPEN message.  The Hold Time MUST be
      either zero or at least three seconds.  An implementation may
      reject connections on the basis of the Hold Time.  The calculated
      value indicates the maximum number of seconds that may elapse
      between the receipt of successive KEEPALIVE, and/or UPDATE
      messages by the sender.

   BGMP Identifier:
      This 4-octet (for IPv4) or 16-octet (IPv6) unsigned integer
      indicates the BGMP Identifier of the sender.  A given BGMP speaker
      sets the value of its BGMP Identifier to a globally-unique value
      assigned to that BGMP speaker (e.g., an IPv4 address).  The value
      of the BGMP Identifier is determined on startup and is the same
      for every BGMP session opened.







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   Optional Parameters:
      This field may contain a list of optional parameters, where each
      parameter is encoded as a <Parameter Length, Parameter Type,
      Parameter Value> triplet.  The combined length of all optional
      parameters can be derived from the Length field in the message
      header.

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
      |  Parm. Type   | Parm. Length  |  Parameter Value (variable)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...

      Parameter Type is a one octet field that unambiguously identifies
      individual parameters.  Parameter Length is a one octet field that
      contains the length of the Parameter Value field in octets.
      Parameter Value is a variable length field that is interpreted
      according to the value of the Parameter Type field.

      This document defines the following Optional Parameters:

   a) Authentication Information (Parameter Type 1):  This optional
      parameter may be used to authenticate a BGMP peer.  The Parameter
      Value field contains a 1-octet Authentication Code followed by a
      variable length Authentication Data.

       0 1 2 3 4 5 6 7 8
      +-+-+-+-+-+-+-+-+
      |  Auth. Code   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                     |
      |              Authentication Data                    |
      |                                                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Authentication Code:

      This 1-octet unsigned integer indicates the authentication
      mechanism being used.  Whenever an authentication mechanism is
      specified for use within BGMP, three things must be included in
      the specification:

      - the value of the Authentication Code which indicates use of the
      mechanism, and - the form and meaning of the Authentication Data.

      Note that a separate authentication mechanism may be used in
      establishing the transport level connection.




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   Authentication Data:

      The form and meaning of this field is a variable-length field
      depend on the Authentication Code.

   The minimum length of the OPEN message is 12 octets (including
   message header).

   b) Capability Information (Parameter Type 2):  This is an Optional
      Parameter that is used by a BGMP-speaker to convey to its peer the
      list of capabilities supported by the speaker.  The parameter
      contains one or more triples <Capability Code, Capability Length,
      Capability Value>, where each triple is encoded as shown below:

      +------------------------------+
      | Capability Code (1 octet)    |
      +------------------------------+
      | Capability Length (1 octet)  |
      +------------------------------+
      | Capability Value (variable)  |
      +------------------------------+

   Capability Code:

      Capability Code is a one octet field that unambiguously identifies
      individual capabilities.

   Capability Length:

      Capability Length is a one octet field that contains the length of
      the Capability Value field in octets.

   Capability Value:

      Capability Value is a variable length field that is interpreted
      according to the value of the Capability Code field.

   A particular capability, as identified by its Capability Code, may
   occur more than once within the Optional Parameter.

   This document reserves Capability Codes 128-255 for vendor-specific
   applications.

   This document reserves value 0.

   Capability Codes (other than those reserved for vendor specific use)
   are assigned only by the IETF consensus process and IESG approval.




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5.3.  UPDATE Message Format

   UPDATE messages are used to transfer Join/Prune/FwdrPref information
   between BGMP peers.  The UPDATE message always includes the fixed-
   size BGMP header, and one or more attributes as described below.

   The message format below allows compact encoding of (*,G) Joins and
   Prunes, while allowing the flexibility needed to do other updates
   such as (S,G) Joins and Prunes towards sources as well as on the
   shared tree.  In the discussion below, an Encoded-Address-Prefix is
   of the 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
                                                   +-+-+-+-+-+-+-+-+
                                                   |EnTyp| AddrFam |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Address (variable length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Mask    (variable length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   EnTyp:
     0 - All 1's Mask.  The Mask field is 0 bytes long.
     1 - Mask length included.  The Mask field is 4 bytes long, and
         contains the mask length, in bits.
     2 - Full Mask included.  The Mask field is the same length
         as the Address field, and contains the full bitmask.

   AddrFam:
     The IANA-assigned address family number of the encoded prefix.

   Address:
     The address associated with the given prefix to be encoded.  The
     length is determined based on the Address Family.

   Mask:
     The mask associated with the given prefix.  The format (or absence)
     of this field is determined by the EnTyp field.

   Each attribute is of the 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Length           |     Type      |   Data ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   All attributes are 4-byte aligned.

   Length:
     The Length is the length of the entire attribute, including the
     length, type, and data fields.  If other attributes are nested
     within the data field, the length includes the size of all such
     nested attributes.

   Type:

     Types 128-255 are reserved for "optional" attributes.  If a
     required attribute is unrecognized, a NOTIFICATION will be sent and
     the connection will be closed if the error is a fatal one.
     Unrecognized optional attributes are simply ignored.

        0 - JOIN
        1 - PRUNE
        2 - GROUP
        3 - SOURCE
        4 - FWDR_PREF
        5 - POISON_REVERSE

   a) JOIN (Type Code 0)

   The JOIN attribute indicates that all GROUP or SOURCE options
   nested immediately within the JOIN option should be joined.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Length           |    Type=0     |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Nested Attributes ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   No JOIN, PRUNE, or FWDR_PREF attributes may be immediately nested
   within a JOIN attribute.

   b) PRUNE (Type Code 1)

   The PRUNE attribute indicates that all GROUP or SOURCE attributes
   nested immediately within the PRUNE attribute should be pruned.









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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Length           |    Type=1     |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Nested Attributes ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   No JOIN, PRUNE, or FWDR_PREF attributes may be immediately nested
   within a PRUNE attribute.

   c) GROUP (Type Code 2)

   The GROUP attribute identifies a given group-prefix.  In addition,
   any attributes nested immediately within the GROUP attribute also
   apply to the given group-prefix.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Length           |    Type=2     |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                                                               |
   |                   Encoded-Address-Prefix                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Nested Attributes (optional) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Encoded-Address-Prefix The multicast group prefix to be joined to
                          pruned, in the format described above.
   Nested Attributes      No GROUP, SOURCE, or FWDR_PREF attributes may
                          be immediately nested within a GROUP
                          attribute.

   d) SOURCE (Type Code 3):

   The SOURCE attribute identifies a given source-prefix.  In
   addition, any attributes nested immediately within the SOURCE
   attribute also apply to the given source-prefix.











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   The SOURCE attribute has the following 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Length           |    Type=2     |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                                                               |
   |                   Encoded-Address-Prefix                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Nested Attributes (optional) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Encoded-Address-Prefix  The Source-prefix in the format described
                           above.
   Nested Attributes       No GROUP, SOURCE, or FWDR_PREF attributes may
                           be immediately nested within a SOURCE
                           attribute.

   e) FWDR_PREF (Type Code 4)

   The FWDR_PREF attribute provides a forwarder preference value for
   all GROUP or SOURCE attributes nested immediately within the
   FWDR_PREF attribute.  It is used by a BGMP speaker to inform other
   BGMP speakers of the originating speaker's degree of preference for
   a given group or source prefix.  Usage of this attribute is
   described in 5.5.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Length           |    Type=1     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Preference Value                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Nested Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Preference Value    A 32-bit non-negative integer.
   Nested Attributes   No JOIN, PRUNE, or FWDR_PREF attributes may be
                       immediately nested within a FWDR_PREF attribute.

   e) POISON_REVERSE (Type Code 5)

   The POISON_REVERSE attribute provides a "poison-reverse" (PR-bit)
   value for all SOURCE attributes nested immediately within the
   POISON_REVERSE attribute.  It is used by a BGMP speaker to inform



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   other BGMP speakers from which it has received (S,G) Joins that
   they are on the path of domains between the source and the root
   domain.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Length           |    Type=1     |   Reserved  |P|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Nested Attributes ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   P                   The PR-bit value.
   Nested Attributes   No attributes in the document other than SOURCE
                       may be immediately nested within a POISON_REVERSE
                       attribute.

5.4.  Encoding examples

   Below are enumerated examples of how various updates are built using
   nested attributes, where A ( B ) denotes that attribute B is nested
   within attribute A.

(*,G-prefix) Join: JOIN ( GROUP )
(*,G-prefix) Prune: PRUNE ( GROUP )
(S,G) Join towards S : GROUP ( JOIN ( SOURCE ) )
(S,G) Join cancelling prune towards root of G: GROUP ( JOIN ( SOURCE ) )
(S,G) Prune towards S: GROUP ( PRUNE ( SOURCE ) )
(S,G) Prune towards root of G: GROUP ( PRUNE ( SOURCE ) )
Switch from (*,G) to (S,G): PRUNE ( GROUP ( JOIN ( SOURCE ) ) )
Switch from (S,G) to (*,G): JOIN ( GROUP )
Initial (*,G) Join with S pruned: JOIN ( GROUP ( PRUNE ( SOURCE ) ) )
Forwarder preference announcement for G-prefix: FWDR_PREF ( GROUP )
Forwarder preference announcement for S-prefix: FWDR_PREF ( SOURCE )

5.5.  KEEPALIVE Message Format

   BGMP does not use any transport protocol-based keep-alive mechanism
   to determine if peers are reachable.  Instead, KEEPALIVE messages are
   exchanged between peers often enough as not to cause the Hold Timer
   to expire.  A reasonable maximum time between the last KEEPALIVE or
   UPDATE message sent, and the time at which a KEEPALIVE message is
   sent, would be one third of the Hold Time interval.  KEEPALIVE
   messages MUST NOT be sent more frequently than one per second.  An
   implementation MAY adjust the rate at which it sends KEEPALIVE
   messages as a function of the Hold Time interval.





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   If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
   messages MUST NOT be sent.

   A KEEPALIVE message consists of only a message header, and has a
   length of 4 octets.

5.6.  NOTIFICATION Message Format

   A NOTIFICATION message is sent when an error condition is detected.
   The BGMP connection is closed immediately after sending it if the
   error is a fatal one.

   In addition to the fixed-size BGMP header, the NOTIFICATION message
   contains the following fields:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |O| Error code  | Error subcode |           Data                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   O-bit:
      Open-bit.  If clear, the connection will be closed.  If set,
      indicates the error is not fatal.

   Error Code:
      This 1-octet unsigned integer indicates the type of NOTIFICATION.
      The following Error Codes have been defined:

         Error Code       Symbolic Name               Reference

           1         Message Header Error             Section 9.1

           2         OPEN Message Error               Section 9.2

           3         UPDATE Message Error             Section 9.3

           4         Hold Timer Expired               Section 9.5

           5         Finite State Machine Error       Section 9.6

           6         Cease                            Section 9.7







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   Error subcode:
      This 1-octet unsigned integer provides more specific information
      about the nature of 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.  The notation (MC) below indicates
      the error is a fatal one and the O-bit must be clear.  Non-fatal
      subcodes SHOULD be sent with the O-bit set.

      Message Header Error subcodes:

                            2  - Bad Message Length (MC)
                            3  - Bad Message Type (MC)

      OPEN Message Error subcodes:

                            1  - Unsupported Version (MC)
                            4  - Unsupported Optional Parameter
                            5  - Authentication Failure (MC)
                            6  - Unacceptable Hold Time (MC)
                            7  - Unsupported Capability (MC)

      UPDATE Message Error subcodes:

                            1 - Malformed Attribute List (MC)
                            2 - Unrecognized Attribute Type
                            5 - Attribute Length Error (MC)
                           10 - Invalid Address
                           11 - Invalid Mask
                           13 - Unrecognized Address Family
   Data:
      This variable-length field is used to diagnose the reason for the
      NOTIFICATION.  The contents of the Data field depend upon the
      Error Code and Error Subcode.  See Section 7 below for more
      details.

      Note that the length of the Data field can be determined from the
      message Length field by the formula:

         Message Length = 6 + Data Length

      The minimum length of the NOTIFICATION message is 6 octets
      (including message header).








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6.  BGMP Error Handling

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

   When any of the conditions described here are detected, a
   NOTIFICATION message with the indicated Error Code, Error Subcode,
   and Data fields is sent, and the BGMP connection is closed if the
   error is a fatal one.  If no Error Subcode is specified, then a zero
   must be used.

   The phrase "the BGMP connection is closed" means that the transport
   protocol connection has been closed and that all resources for that
   BGMP connection have been deallocated.  The remote peer is removed
   from the target list of all tree state entries.

   Unless specified explicitly, the Data field of the NOTIFICATION
   message that is sent to indicate an error is empty.

6.1.  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 elaborates on the specific nature of the
   error.

   If the Length field of the message header is less than 4 or greater
   than 4096, or if the Length field of an OPEN message is less  than
   the minimum length of the OPEN message, or if the Length field of an
   UPDATE message is less than the minimum length of the UPDATE message,
   or if the Length field of a KEEPALIVE message is not equal to 4, then
   the Error Subcode is set to Bad Message Length.  The Data field
   contains the erroneous Length field.

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

6.2.  OPEN message error handling

   All errors detected while processing the OPEN message are indicated
   by sending the NOTIFICATION message with Error Code OPEN Message
   Error.  The Error Subcode elaborates on the specific nature of the
   error.






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   If the version number contained in the Version field of the received
   OPEN message is not supported, then the Error Subcode is set to
   Unsupported Version Number.  The Data field is a 2-octet unsigned
   integer, which indicates the largest locally supported version number
   less than the version the remote BGMP peer bid (as indicated in the
   received OPEN message).

   If the Hold Time field of the OPEN message is unacceptable, then the
   Error Subcode MUST be set to Unacceptable Hold Time.  An
   implementation MUST reject Hold Time values of one or two seconds.
   An implementation MAY reject any proposed Hold Time.  An
   implementation which accepts a Hold Time MUST use the negotiated
   value for the Hold Time.

   If one of the Optional Parameters in the OPEN message is not
   recognized, then the Error Subcode is set to Unsupported Optional
   Parameters.

   If the OPEN message carries Authentication Information (as an
   Optional Parameter), then the corresponding authentication procedure
   is invoked.  If the authentication procedure (based on Authentication
   Code and Authentication Data) fails, then the Error Subcode is set to
   Authentication Failure.

   If the OPEN message indicates that the peer does not support a
   capability which the receiver requires, the receiver may send a
   NOTIFICATION message to the peer, and terminate peering.  The Error
   Subcode in the message is set to Unsupported Capability.  The Data
   field in the NOTIFICATION message lists the set of capabilities that
   cause the speaker to send the message.  Each such capability is
   encoded the same way as it was encoded in the received OPEN message.

6.3.  UPDATE message error handling

   All errors detected while processing the UPDATE message are indicated
   by sending the NOTIFICATION message with Error Code UPDATE Message
   Error.  The error subcode elaborates on the specific nature of the
   error.

   If any recognized attribute has Attribute Length that conflicts with
   the expected length (based on the attribute type code), then the
   Error Subcode is set to Attribute Length Error.  The Data field
   contains the erroneous attribute (type, length and value).

   If the Encoded-Address-Prefix field in some attribute is
   syntactically incorrect, then the Error Subcode is set to Invalid
   Prefix Field.




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   If any other is encountered when processing attributes (such as
   invalid nestings), then the Error Subcode is set to Malformed
   Attribute List, and the problematic attribute is included in the data
   field.

6.4.  NOTIFICATION message error handling

   If a peer sends a NOTIFICATION message, and there is an error in that
   message, there is unfortunately no means of reporting this error via
   a subsequent NOTIFICATION message.  Any such error, such as an
   unrecognized Error Code or Error Subcode, should be noticed, logged
   locally, and brought to the attention of the administration of the
   peer.  The means to do this, however, lies outside the scope of this
   document.

6.5.  Hold Timer Expired error handling

   If a system does not receive successive KEEPALIVE and/or UPDATE
   and/or NOTIFICATION messages within the period specified in the Hold
   Time field of the OPEN message, then the NOTIFICATION message with
   Hold Timer Expired Error Code must be sent and the BGMP connection
   closed.

6.6.  Finite State Machine error handling

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

6.7.  Cease

   In absence of any fatal errors (that are indicated in this section),
   a BGMP peer may choose at any given time to close its BGMP 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.

6.8.  Connection collision detection

   If a pair of BGMP speakers try simultaneously to establish a TCP
   connection to each other, then two parallel connections between this
   pair of speakers might well be formed.  We refer to this situation as
   connection collision.  Clearly, one of these connections must be
   closed.

   Based on the value of the BGMP Identifier a convention is established
   for detecting which BGMP connection is to be preserved when a
   collision does occur.  The convention is to compare the BGMP



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   Identifiers of the peers involved in the collision and to retain only
   the connection initiated by the BGMP speaker with the higher-valued
   BGMP Identifier.

   Upon receipt of an OPEN message, the local system must examine all of
   its connections that are in the OpenConfirm state.  A BGMP speaker
   may also examine connections in an OpenSent state if it knows the
   BGMP Identifier of the peer by means outside of the protocol.  If
   among these connections there is a connection to a remote BGMP
   speaker whose BGMP Identifier equals the one in the OPEN message,
   then the local system performs the following collision resolution
   procedure:

   1. The BGMP Identifier of the local system is compared to the BGMP
      Identifier of the remote system (as specified in the OPEN
      message).

   2. If the value of the local BGMP Identifier is less than the remote
      one, the local system closes BGMP connection that already exists
      (the one that is already in the OpenConfirm state), and accepts
      BGMP connection initiated by the remote system.

   3. Otherwise, the local system closes newly created BGMP connection
      (the one associated with the newly received OPEN message), and
      continues to use the existing one (the one that is already in the
      OpenConfirm state).

   Comparing BGMP Identifiers is done by treating them as (4-octet long)
   unsigned integers.

   A connection collision with an existing BGMP connection that is in
   Established states causes unconditional closing of the newly created
   connection.  Note that a connection collision cannot be detected with
   connections that are in Idle, or Connect, or Active states.

   Closing the BGMP connection (that results from the collision
   resolution procedure) is accomplished by sending the NOTIFICATION
   message with the Error Code Cease.

7.  BGMP Version Negotiation

   BGMP speakers may negotiate the version of the protocol by making
   multiple attempts to open a BGMP connection, starting with the
   highest version number each supports.  If an open attempt fails with
   an Error Code OPEN Message Error, and an Error Subcode Unsupported
   Version Number, then the BGMP speaker has available the version
   number it tried, the version number its peer tried, the version
   number passed by its peer in the NOTIFICATION message, and the



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   version numbers that it supports.  If the two peers do support one or
   more common versions, then this will allow them to rapidly determine
   the highest common version.  In order to support BGMP version
   negotiation, future versions of BGMP must retain the format of the
   OPEN and NOTIFICATION messages.

7.1.  BGMP Capability Negotiation

   When a BGMP speaker sends an OPEN message to its BGMP peer, the
   message may include an Optional Parameter, called Capabilities.  The
   parameter lists the capabilities supported by the speaker.

   A BGMP speaker may use a particular capability when peering with
   another speaker only if both speakers support that capability.  A
   BGMP speaker determines the capabilities supported by its peer by
   examining the list of capabilities present in the Capabilities
   Optional Parameter carried by the OPEN message that the speaker
   receives from the peer.

8.  BGMP Finite State machine

   This section specifies BGMP operation in terms of a Finite State
   Machine (FSM).  Following is a brief summary and overview of BGMP
   operations by state as determined by this FSM.

   Initially BGMP is in the Idle state.

   Idle state:

      In this state BGMP refuses all incoming BGMP connections.  No
      resources are allocated to the peer.  In response to the Start
      event (initiated by either system or operator) the local system
      initializes all BGMP resources, starts the ConnectRetry timer,
      initiates a transport connection to the other BGMP peer, while
      listening for a connection that may be initiated by the remote
      BGMP peer, and changes its state to Connect.  The exact value of
      the ConnectRetry timer is a local matter, but should be
      sufficiently large to allow TCP initialization.

      If a BGMP speaker detects an error, it shuts down the connection
      and changes its state to Idle.  Getting out of the Idle state
      requires generation of the Start event.  If such an event is
      generated automatically, then persistent BGMP errors may result in
      persistent flapping of the speaker.  To avoid such a condition it
      is recommended that Start events should not be generated
      immediately for a peer that was previously transitioned to Idle
      due to an error.  For a peer that was previously transitioned to
      Idle due to an error, the time between consecutive generation of



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      Start events, if such events are generated automatically, shall
      exponentially increase.  The value of the initial timer shall be
      60 seconds.  The time shall be doubled for each consecutive retry.

      Any other event received in the Idle state is ignored.

   Connect state:

      In this state BGMP is waiting for the transport protocol
      connection to be completed.

      If the transport protocol connection succeeds, the local system
      clears the ConnectRetry timer, completes initialization, sends an
      OPEN message to its peer, and changes its state to OpenSent.  If
      the transport protocol connect fails (e.g., retransmission
      timeout), the local system restarts the ConnectRetry timer,
      continues to listen for a connection that may be initiated by the
      remote BGMP peer, and changes its state to Active state.

      In response to the ConnectRetry timer expired event, the local
      system restarts the ConnectRetry timer, initiates a transport
      connection to the other BGMP peer, continues to listen for a
      connection that may be initiated by the remote BGMP peer, and
      stays in the Connect state.

      The Start event is ignored in the Connect state.

      In response to any other event (initiated by either system or
      operator), the local system releases all BGMP resources associated
      with this connection and changes its state to Idle.

   Active state:

      In this state BGMP is trying to acquire a peer by listening for an
      incoming transport protocol connection.

      If the transport protocol connection succeeds, the local system
      clears the ConnectRetry timer, completes initialization, sends an
      OPEN message to its peer, sets its Hold Timer to a large value,
      and changes its state to OpenSent.  A Hold Timer value of 4
      minutes is suggested.

      In response to the ConnectRetry timer expired event, the local
      system restarts the ConnectRetry timer, initiates a transport
      connection to other BGMP peer, continues to listen for a
      connection that may be initiated by the remote BGMP peer, and
      changes its state to Connect.




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      If the local system detects that a remote peer is trying to
      establish BGMP connection to it, and the IP address of the remote
      peer is not an expected one, the local system restarts the
      ConnectRetry timer, rejects the attempted connection, continues to
      listen for a connection that may be initiated by the remote BGMP
      peer, and stays in the Active state.

      The Start event is ignored in the Active state.

      In response to any other event (initiated by either system or
      operator), the local system releases all BGMP resources associated
      with this connection and changes its state to Idle.

   OpenSent state:

      In this state BGMP waits for an OPEN message from its peer.  When
      an OPEN message is received, all fields are checked for
      correctness.  If the BGMP message header checking or OPEN message
      checking detects an error (see Section 6.2), or a connection
      collision (see Section 6.8) the local system sends a NOTIFICATION
      message and changes its state to Idle.

      If there are no errors in the OPEN message, BGMP sends a KEEPALIVE
      message and sets a KeepAlive timer.  The Hold Timer, which was
      originally set to a large value (see above), is replaced with the
      negotiated Hold Time value (see section 4.2).  If the negotiated
      Hold Time value is zero, then the Hold Time timer and KeepAlive
      timers are not started.  If the configured remote Autonomous
      System value for this peering is the same as the local Autonomous
      System number, then the connection is an "internal" connection;
      otherwise, it is "external".  Finally, the state is changed to
      OpenConfirm.

      If a disconnect notification is received from the underlying
      transport protocol, the local system closes the BGMP connection,
      restarts the ConnectRetry timer, while continue listening for
      connection that may be initiated by the remote BGMP peer, and goes
      into the Active state.

      If the Hold Timer expires, the local system sends NOTIFICATION
      message with error code Hold Timer Expired and changes its state
      to Idle.

      In response to the Stop event (initiated by either system or
      operator) the local system sends NOTIFICATION message with Error
      Code Cease and changes its state to Idle.

      The Start event is ignored in the OpenSent state.



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      In response to any other event the local system sends NOTIFICATION
      message with Error Code Finite State Machine Error and changes its
      state to Idle.

      Whenever BGMP changes its state from OpenSent to Idle, it closes
      the BGMP (and transport-level) connection and releases all
      resources associated with that connection.

   OpenConfirm state:

      In this state BGMP waits for a KEEPALIVE or NOTIFICATION message.

      If the local system receives a KEEPALIVE message, it changes its
      state to Established.

      If the Hold Timer expires before a KEEPALIVE message is received,
      the local system sends NOTIFICATION message with error code Hold
      Timer Expired and changes its state to Idle.

      If the local system receives a NOTIFICATION message, it changes
      its state to Idle.

      If the KeepAlive timer expires, the local system sends a KEEPALIVE
      message and restarts its KeepAlive timer.

      If a disconnect notification is received from the underlying
      transport protocol, the local system changes its state to Idle.

      In response to the Stop event (initiated by either system or
      operator) the local system sends NOTIFICATION message with Error
      Code Cease and changes its state to Idle.

      The Start event is ignored in the OpenConfirm state.

      In response to any other event the local system sends NOTIFICATION
      message with Error Code Finite State Machine Error and changes its
      state to Idle.

      Whenever BGMP changes its state from OpenConfirm to Idle, it
      closes the BGMP (and transport-level) connection and releases all
      resources associated with that connection.

   Established state:

      In the Established state BGMP can exchange UPDATE, NOTIFICATION,
      and KEEPALIVE messages with its peer.





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      If the local system receives an UPDATE or KEEPALIVE message, it
      restarts its Hold Timer, if the negotiated Hold Time value is
      non-zero.

      If the local system receives a NOTIFICATION message, it changes
      its state to Idle.

      If the local system receives an UPDATE message and the UPDATE
      message error handling procedure (see Section 6.3) detects an
      error, the local system sends a NOTIFICATION message and changes
      its state to Idle.

      If a disconnect notification is received from the underlying
      transport protocol, the local system changes its state to Idle.

      If the Hold Timer expires, the local system sends a NOTIFICATION
      message with Error Code Hold Timer Expired and changes its state
      to Idle.

      If the KeepAlive timer expires, the local system sends a KEEPALIVE
      message and restarts its KeepAlive timer.

      Each time the local system sends a KEEPALIVE or UPDATE message, it
      restarts its KeepAlive timer, unless the negotiated Hold Time
      value is zero.

      In response to the Stop event (initiated by either system or
      operator), the local system sends a NOTIFICATION message with
      Error Code Cease and changes its state to Idle.

      The Start event is ignored in the Established state.

      In response to any other event, the local system sends
      NOTIFICATION message with Error Code Finite State Machine Error
      and changes its state to Idle.

      Whenever BGMP changes its state from Established to Idle, it
      closes the BGMP (and transport-level) connection, releases all
      resources associated with that connection, and deletes all routes
      derived from that connection.

9.  Security Considerations

   If a BGMP speaker accepts unauthorized or altered BGMP messages,
   denial of service due to excess bandwidth consumption or lack of
   multicast connectivity can result.  Authentication of BGMP messages
   can protect against this behavior.




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

   This provides a weak security mechanism, as it is still possible for
   denial of service to occur as a result of messages relayed through a
   trusted peer.  However, this model is the same as the currently
   practiced security mechanism for BGP.  It is anticipated that future
   work will provide different stronger mechanisms for dealing with
   these issues in routing protocols.

10.  Acknowledgements

   In addition to the editor, the following individuals have contributed
   to the design of BGMP: Cengiz Alaettinoglu, Tony Ballardie, Steve
   Casner, Steve Deering, Deborah Estrin, Dino Farinacci, Bill Fenner,
   Mark Handley, Ahmed Helmy, Van Jacobson, Dave Meyer, and Satish
   Kumar.

   This document is the product of the IETF BGMP Working Group with Dave
   Thaler as editor.

   Rusty Eddy, Isidor Kouvelas, and Pavlin Radoslavov also provided
   valuable feedback on this document.

11.  References

11.1.  Normative References

   [INTEROP]  Thaler, D., "Interoperability Rules for Multicast Routing
              Protocols", RFC 2715, October 1999.

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

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

   [V6PREFIX] Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002.









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11.2.  Informative References

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

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

   [CBT]      Ballardie, A., "Core Based Trees (CBT version 2) Multicast
              Routing -- Protocol Specification", RFC 2189, September
              1997.

   [DVMRP]    Pusateri, T., "Distance Vector Multicast Routing
              Protocol", Work in Progress, October 2003.

   [IPv6AA]   Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [MOSPF]    Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
              1994.

   [PIMDM]    Adams, A., Nicholas, J. and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", Work in Progress, September
              2003.

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

   [REFLECT]  Bates, T. and R. Chandra, "BGP Route Reflection: An
              alternative to full mesh IBGP", RFC 1966, June 1996.

   [V4PREFIX] Thaler, D., "Unicast-Prefix-based IPv4 Multicast
              Addresses", Work in Progress, August 2004.

Authors' Address

   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA 98052

   EMail: dthaler@microsoft.com






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







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