





Network Working Group                                           D. Thaler
Request for Comments: 2715                                      Microsoft
Category: Informational                                      October 1999


         Interoperability Rules for Multicast Routing Protocols

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 (1999).  All Rights Reserved.

Abstract

   The rules described in this document will allow efficient
   interoperation among multiple independent multicast routing domains.
   Specific instantiations of these rules are given for the DVMRP,
   MOSPF, PIM-DM, PIM-SM, and CBT multicast routing protocols, as well
   as for IGMP-only links.  Future versions of these protocols, and any
   other multicast routing protocols, may describe their
   interoperability procedure by stating how the rules described herein
   apply to them.

1.  Introduction

   To allow sources and receivers inside multiple autonomous multicast
   routing domains (or "regions") to communicate, the domains must be
   connected by multicast border routers (MBRs).  To prevent black holes
   or routing loops among domains, we assume that these domains are
   organized into one of the following topologies:

   o  A tree (or star) topology (figure 1) with a backbone domain at the
      root, stub domains at the leaves, and possibly "transit" domains
      as branches between the root and the leaves.  Each pair of
      adjacent domains is connected by one or more MBRs.  The root of
      each subtree of domains receives all globally-scoped traffic
      originated anywhere within the subtree, and forwards traffic to
      its parent and children where needed.  Each parent domain's MBR
      injects a default route into its child domains, while child
      domains' MBRs inject actual (but potentially aggregated) routes
      into parent domains.  Thus, the arrows in the figure indicate both
      the direction in which the default route points, as well as the
      direction in which all globally-scoped traffic is sent.



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                                 +--------+
                          +----|        |----+
          +---+    +---+  |  ===>      <===  |
          |   |    |   |  +----|   #    |----+
          |   |    | # |     +-----#------+
          | # |  +---#-------|     v      |-----------+
         +--#----|   v       |            |           |-----+
         |  v  ===>        ===> Backbone <===        <===   |
         +-------|   ^       |            |     ^     |-----+
                 +---#-------|     ^      |-----#-----+
                   | # |     +-----#------+ |   #    |-----+
                   |   |       |   #    |   |       <===   |
                   +---+   +---|        |   |        |-----+
                           | ===>       |   +--------+
                           +---+--------+

                 Figure 1: Tree Topology of Domains

   o  An arbitrary topology, in which a higher level (inter-domain)
      routing protocol, such as HDVMRP [1] or BGMP [9], is used to
      calculate paths among domains.  Each pair of adjacent domains is
      connected by one or more MBRs.

   Section 2 describes rules allowing interoperability between existing
   multicast routing protocols [2,3,4,5,6], and reduces the
   interoperability problem from O(N^2) potential protocol interactions,
   to just N (1 per protocol) instantiations of the same set of
   invariant rules.  This document specifically applies to Multicast
   Border Routers (MBRs) which meet the following assumptions:

   o  The MBR consists of two or more active multicast routing
      components, each running an instance of some multicast routing
      protocol.  No assumption is made about the type of multicast
      routing protocol (e.g., broadcast-and-prune vs. explicit-join) any
      component runs, or the nature of a "component".  Multiple
      components running the same protocol are allowed.

   o  The router is configured to forward packets between two or more
      independent domains.  The router has one or more active interfaces
      in each domain, and one component per domain.  The router also has
      an inter-component "alert dispatcher", which we cover in Section
      3.









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   o  Only one multicast routing protocol is active per interface (we do
      not consider mixed multicast protocol LANs).  Each interface on
      which multicast is enabled is thus "owned" by exactly one of the
      components.

   o  All components share a common forwarding cache of (S,G) entries,
      which are created when data packets are received, and can be
      deleted at any time.  Only the component owning an interface may
      change information about that interface in the forwarding cache.
      Each forwarding cache entry has a single incoming interface (iif)
      and a list of outgoing interfaces (oiflist).  Each component
      typically keeps a separate multicast routing table with any type
      of entries.

   Note that the guidelines in this document are implementation-
   independent.  The same rules given in Section 2 apply in some form,
   regardless of the implementation.  For example, they apply to each of
   the following architectural models:

   o  Single process (e.g., GateD): Several routing components in the
      same user-space process, running on top of a multicast-capable
      kernel.

   o  Multiple peer processes: Several routing components, each as a
      separate user-space process, all sitting on top of a multicast-
      capable kernel, with N*(N-1) interaction channels.

   o  Multiple processes with arbiter: Multiple independent peer routing
      component processes which interact with each other and with the
      kernel solely through an independent arbitration daemon.

   o  Monolith: Several routing components which are part of the
      "kernel" itself.

   We describe all interactions between components in terms of "alerts".
   The nature of an alert is implementation-dependent (e.g., it may
   consist of a simple function call, writing to shared memory, use of
   IPC, or some other method) but alerts of some form exist in every
   model. Similarly, the originator of an alert is also implementation-
   dependent; for example, alerts may be originated by a component
   effecting a change, by an independent arbiter, or by the kernel.

1.1.  Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.




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2.  Requirements

   To insure that a MBR fitting the above assumptions exhibits correct
   interdomain routing behavior, each MBR component MUST adhere to the
   following rules:

   Rule 1: All components must agree on which component owns the
           incoming interface (iif) for a forwarding cache entry. This
           component, which we call the "iif owner" is determined by the
           dispatcher (see Section 3).  The incoming component may
           select ANY interface it owns as the iif according to its own
           rules.

   When a routing change occurs which causes the iif to change to an
   interface owned by a different component, both the component
   previously owning the entry's iif and the component afterwards owning
   the entry's iif MUST notice the change (so the first can prune
   upstream and the second can join/graft upstream, for example).
   Typically, noticing such changes will happen as a result of normal
   protocol behavior.

   Rule 2: The component owning an interface specifies the criteria for
           which packets received on that interface are to be accepted
           or dropped (e.g., whether to perform an RPF check, and what
           scoped boundaries exist on that interface).  Once a packet is
           accepted, however, it is processed according to the
           forwarding rules of all components.

   Furthermore, some multicast routing protocols (e.g. PIM) also require
   the ability to react to packets received on the "wrong" interface. To
   support these protocols, an MBR must allow a component to place any
   of its interfaces in "WrongIf Alert Mode".  If a packet arrives on
   such an interface, and is not accepted according to Rule 2, then the
   component owning the interface MUST be alerted [(S,G) WrongIf alert].
   Typically, WrongIf alerts must be rate-limited.

   Rule 3: Whenever a new (S,G) forwarding cache entry is to be created
           (e.g., upon accepting a packet destined to a non-local
           group), all components MUST be alerted [(S,G) Creation alert]
           so that they can set the forwarding state on their own
           outgoing interfaces (oifs) before the packet is forwarded.

   Note that (S,G) Creation alerts are not necessarily generated by one
   of the protocol components themselves.







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   Rule 4: When a component removes the last oif from an (S,G)
           forwarding cache entry whose iif is owned by another
           component, or when such an (S,G) forwarding cache entry is
           created with an empty oif list, the component owning the iif
           MUST be alerted [(S,G) Prune alert] (so it can send a prune,
           for example).

   Rule 5: When the first oif is added to an (S,G) forwarding cache
           entry whose iif is owned by another component, the component
           owning the iif MUST be alerted [(S,G) Join alert] (so it can
           send a join or graft, for example).

   The oif list in rules 4 and 5 must also logically include any virtual
   encapsulation interfaces such as those used for tunneling or for
   sending encapsulated packets to an RP/core.

   Rule 6: Unless a component reports the aggregate group membership in
           the direction of its interfaces, it MUST be a "wildcard
           receiver" for all sources whose RPF interface is owned by
           another component ("externally-reached" sources).  In
           addition, a component MUST be a "wildcard receiver" for all
           sources whose RPF interface is owned by that component
           ("internally-reached" sources) if any other component of the
           MBR is a wildcard receiver for externally-reached sources;
           this will happen naturally as a result of Rule 5 when it
           receives a (*,*) Join alert.

   For example, if the backbone does not keep global membership
   information, all MBR components in the backbone in a tree topology of
   domains, as well as all components owning the RPF interface towards
   the backbone are wildcard receivers for externally-reached sources.

   MBRs need not be wildcard receivers (for internally- or externally-
   reached sources) if a higher-level routing protocol, such as BGMP, is
   used for routing between domains.

2.1.  Deleting Forwarding Cache Entries

   Special care must be taken to follow Rules 4 and 5 when forwarding
   cache entries can be deleted at will.  Specifically, a component must
   be able to determine when the combined oiflist for (S,G) goes from
   null to non-null, and vice versa.

   This can be done in any implementation-specific manner, including,
   but not limited to, the following possibilities:






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   o  Whenever a component would modify the oiflist of a single
      forwarding cache entry if one existed, one is first created.  The
      oiflist is then modified and Rules 4 and 5 applied after an (S,G)
      Creation alert is sent to all components and all components have
      updated the oiflist.  OR,

   o  When a forwarding cache entry is to be deleted, a new alert [(S,G)
      Deletion alert] is sent to all components, and the entry is only
      deleted if all components then grant permission.  Each component
      could then grant permission only if it had no (S,G) route table
      entry.

2.2.  Additional Recommendation

   Using (*,G) Join alerts and (*,G) Prune alerts can reduce bandwidth
   usage by avoiding broadcast-and-prune behavior among domains when it
   is unnecessary.  This optimization requires that each component be
   able to determine which other components are interested in any given
   group.

   Although this may be done in any implementation-dependent method, one
   example would be to maintain a common table (which we call the
   Component-Group Table) indexed by group-prefix, listing which
   components are interested in each group(prefix).  Thus, any
   components which are wildcard receivers for externally-reached
   sources (i.e., those whose RPF interface is owned by another
   component) would be listed in all entries of this table, including a
   default entry.  This table is thus loosely analogous to a forwarding
   cache of (*,G) entries, except that no distinction is made between
   incoming and outgoing interfaces.

3.  Alert Dispatchers

   We assume that each MBR has an "alert dispatcher".  The dispatcher is
   responsible for selecting, for each (S,G) entry in the shared
   forwarding cache, the component owning the iif.  It is also
   responsible for selecting to which component(s) a given alert should
   be sent.

3.1.  The "Interop" Dispatcher

   We describe here rules that may be used in the absence of any inter-
   domain multicast routing protocol, to enable interoperability in a
   tree topology of domains.  If an inter-domain multicast routing
   protocol is in use, another dispatcher should be used instead.  The
   Interop dispatcher does not own any interfaces.





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   To select the iif of an (S,G) entry, the iif owner is the component
   owning the next-hop interface towards S in the multicast RIB.

   The "iif owner" of (*,G) and (*,*) entries is the Interop dispatcher
   itself.  This allows the Interop dispatcher to receive relevant
   alerts without owning any interfaces.

3.1.1.  Processing Alerts

   If the Interop dispatcher receives an (S,G) Creation alert, it adds
   no interfaces to the entry's oif list, since it owns none.

   When the Interop dispatcher receives a (*,G) Prune alert, the
   following actions are taken, depending on the number of components N
   which want to receive data for G.  If N has just changed from 2 to 1,
   a (*,G) Prune alert is sent to the remaining component. If N has just
   changed from 1 to 0, a (*,G) Prune alert is sent to ALL components
   other than the 1.

   When the Interop dispatcher receives a (*,G) Join alert, the
   following actions are taken, depending on the number of components N
   which want to receive data for G.  If N has just changed from 0 to 1,
   a (*,G) Join alert is sent to ALL components other than the 1.  If N
   has just changed from 1 to 2, a (*,G) Join alert is sent to the
   original (1) component.

3.2.  "BGMP" Dispatcher

   This dispatcher can be used with an inter-domain multicast routing
   protocol (such as BGMP) which allows global (S,G) and (*,G) trees.

   The iif owner of an (S,G) entry is the component owning the next-hop
   interface towards S in the multicast RIB.

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

3.2.1.  Processing Alerts

   This dispatcher simply forwards all (S,G) and (*,G) alerts to the iif
   owner of the associated entry.

4.  Multicast Routing Protocol Components

   In this section, we describe how the rules in section 2 apply to
   current versions of various protocols.  Future versions, and
   additional protocols, should describe how these rules apply in a
   separate document.



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4.1.  DVMRP

   In this section we describe how the rules in section 2 apply to
   DVMRP.  We assume that the reader is familiar with normal DVMRP
   behavior as specified in [2].

   As with all broadcast-and-prune protocols, DVMRP components are
   automatically wildcard receivers for internally-reached sources.
   Unless some form of Domain-Wide-Reports (DWRs) [10] (synonymous with
   Regional-Membership-Reports as described in [1]) are added to DVMRP
   in the future, all DVMRP components also act as wildcard receivers
   for externally-reached sources.  If DWRs are available for the
   domain, then a DVMRP component acts as a wildcard receiver for
   externally-reached sources only if internally-reached domains exist
   which do not support some form of DWRs.

   One simple heuristic to approximate DWRs is to assume that if there
   are any internally-reached members, then at least one of them is a
   sender.  With this heuristic, the presense of any (S,G) state for
   internally-reached sources can be used instead.  Sending a data
   packet to a group is then equivalent to sending a DWR for the group.

4.1.1.  Generating Alerts

   A (*,*) Join alert is sent to the iif owner of the (*,*) entry (e.g.,
   the Interop dispatcher) when the first component becomes a wildcard
   receiver for external sources.  This may occur when a DVMRP component
   starts up which does not support some form of DWRs.

   A (*,*) Prune alert is sent to the iif owner of the (*,*) entry
   (e.g., the Interop dispatcher) when all components are no longer
   wildcard receivers for external sources.  This may occur when a DVMRP
   component which does not support some form of DWRs shuts down.

   An (S,G) Prune alert is sent to the component owning the iif for a
   forwarding cache entry whenever the last oif is removed from the
   entry, and the iif is owned by another component.  In DVMRP, this may
   happen when:

      o  A DVMRP (S,G) Prune message is received on the logical
         interface.

   An (S,G) Join alert is sent to the component owning the iif for a
   forwarding cache entry whenever the first logical oif is added to an
   entry, and the iif is owned by another component.  In DVMRP, this may
   happen when any of the following occur:





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      o  The oif's prune timer expires, or
      o  A DVMRP (S,G) Graft message is received on the logical
         interface, or
      o  IGMP [7] notifies DVMRP that directly-connected members of G
         now exist on the interface.

   When it is known, for a group G, that there are no longer any members
   in the DVMRP domain which receive data for externally-reached sources
   from the local router, a (*,G) Prune alert is sent to the "iif owner"
   for (*,G) according to the dispatcher.  In DVMRP, this may happen
   when:

      o  The DWR for G times out, or
      o  The members-are-senders approximation is being used and the
         last (S,G) entry for G is timed out.

   When it is first known that there are members of a group G in the
   DVMRP domain, a (*,G) Join alert is sent to the "iif owner" of (*,G).
   In DVMRP, this may happen when either of the following occurs:

      o  A DWR is received for G, or
      o  The members-are-senders approximation is being used and a data
         packet for G is received on one of the component's interfaces.

4.1.2.  Processing Alerts

   When a DVMRP component receives an (S,G) Creation alert, it adds all
   the component's interfaces to the entry's oif list (according to
   normal DVMRP behavior) EXCEPT:

      o  the iif,
      o  interfaces without local members of the entry's group, and for
         which DVMRP (S,G) Prune messages have been received from all
         downstream dependent neighbors.
      o  interfaces for which the router is not the designated forwarder
         for S,
      o  and interfaces with scoped boundaries covering the group.

   When a DVMRP component receives an (S,G) Prune alert, and the
   forwarding cache entry's oiflist is empty, it sends a DVMRP (S,G)
   Prune message to the upstream neighbor according to normal DVMRP
   behavior.

   When a DVMRP component receives a (*,G) or (*,*) Prune alert, it is
   treated as if an (S,G) Prune alert were received for every existing
   DVMRP (S,G) entry covered.  In addition, if DWRs are being used, a
   DWR Leave message is sent within its domain.




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   When a DVMRP component receives an (S,G) Join alert, and a prune was
   previously sent upstream, it sends a DVMRP (S,G) Graft message to the
   upstream neighbor according to normal DVMRP behavior.

   When a DVMRP component receives a (*,G) or (*,*) Join alert, it is
   treated as if an (S,G) Join alert were received for every existing
   DVMRP (S,G) entry covered.  In addition, if DWRs are being used, the
   component sends a DWR Join message within its domain.

4.2.  MOSPF

   In this section we describe how the rules in section 2 apply to
   MOSPF.  We assume that the reader is familiar with normal MOSPF
   behavior as specified in [3].  We note that MOSPF allows joining and
   pruning entire groups, but not individual sources within groups.

   Although interoperability between MOSPF and dense-mode protocols
   (such as DVMRP) is specified in [3], we describe here how an MOSPF
   implementation may interoperate with all other multicast routing
   protocols.

   An MOSPF component acts as a wildcard receiver for internally-reached
   sources if and only if any other component is a wildcard receiver for
   externally-reached sources.  An MOSPF component acts as a wildcard
   receiver for externally-reached sources only if internally-reached
   domains exist which do not support some form of Domain-Wide-Reports
   (DWRs) [10].  Since MOSPF floods membership information throughout
   the domain, MOSPF itself is considered to support a form of DWRs
   natively.

4.2.1.  Generating Alerts

   A (*,*) Join alert is sent to the iif owner of the (*,*) entry (e.g.,
   the Interop dispatcher) when the first component becomes a wildcard
   receiver for external sources.  This may occur when an MOSPF
   component starts up and decides to act in this role.

   A (*,*) Prune alert is sent to the iif owner of the (*,*) entry
   (e.g., the Interop dispatcher) when all components are no longer
   wildcard receivers for external sources.  This may occur when an
   MOSPF component which was acting in this role shuts down.

   When it is known that there are no longer any members of a group G in
   the MOSPF domain, a (*,G) Prune alert is sent to the "iif owner" for
   (*,G) according to the dispatcher.  In MOSPF, this may happen when
   either:





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      o  IGMP notifies MOSPF that there are no longer any directly-
         connected group members on an interface, or
      o  Any router's group-membership-LSA for G is aged out.

      When it is first known that there are members of a group G in the
      MOSPF domain, a (*,G) Join alert is sent to the "iif owner" of
      (*,G), according to the dispatcher.  In MOSPF, this may happen
      when any of the following occur:

      o  IGMP notifies MOSPF that directly-connected group members now
         exist on the interface, or
      o  A group-membership-LSA is received for G.

4.2.2.  Processing Alerts

   When an MOSPF component receives an (S,G) Creation alert, it
   calculates the shortest path tree for the MOSPF domain, and adds the
   downstream interfaces to the entry's oif list according to normal
   MOSPF behavior.

   When an MOSPF component receives an (S,G) Prune alert, the alert is
   ignored, since MOSPF can only prune entire groups at a time.

   When an MOSPF component receives a (*,G) Prune alert, and there are
   no directly-connected members on any MOSPF interface, the router
   "prematurely ages" out its group-membership-LSA for G in the MOSPF
   domain according to normal MOSPF behavior.

   When an MOSPF component receives either an (S,G) Join alert or a
   (*,G) Join alert, and G was not previously included in the router's
   group-membership-LSA (and the component is not a wildcard multicast
   receiver), it originates a group-membership-LSA in the MOSPF domain
   according to normal MOSPF behavior.

   When an MOSPF component receives a (*,*) Prune alert, it ceases to be
   a wildcard multicast receiver in its domain.

   When an MOSPF component receives a (*,*) Join alert, it becomes a
   wildcard multicast receiver in its domain.

4.3.  PIM-DM

   In this section we describe how the rules in section 2 apply to
   Dense-mode PIM.  We assume that the reader is familiar with normal
   PIM-DM behavior as specified in [6].






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   As with all broadcast-and-prune protocols, PIM-DM components are
   automatically wildcard receivers for internally-reached sources.
   Unless some form of Domain-Wide-Reports (DWRs) [10] are added to
   PIM-DM in the future, all PIM-DM components also act as wildcard
   receivers for externally-reached sources.  If DWRs are available for
   the domain, then a PIM-DM component acts as a wildcard receiver for
   externally-reached sources only if internally-reached domains exist
   which do not support some form of DWRs.

   One simple heuristic to approximate DWRs is to assume that if there
   are any internally-reached members, then at least one of them is a
   sender.  With this heuristic, the presense of any (S,G) state for
   internally-reached sources can be used instead.  Sending a data
   packet to a group is then equivalent to sending a DWR for the group.

4.3.1.  Generating Alerts

   A (*,*) Join alert is sent to the iif owner of the (*,*) entry (e.g.,
   the Interop dispatcher) when the first component becomes a wildcard
   receiver for external sources.  This may occur when a PIM-DM
   component starts up which does not support some form of DWRs.

   A (*,*) Prune alert is sent to the iif owner of the (*,*) entry
   (e.g., the Interop dispatcher) when all components are no longer
   wildcard receivers for external sources.  This may occur when a PIM-
   DM component which does not support some form of DWRs shuts down.

   A (S,G) Prune alert is sent to the component owning the iif for a
   forwarding cache entry whenever the last oif is removed from the
   forwarding cache entry, and the iif is owned by another component. In
   PIM-DM, this may happen when:

      o  A PIM (S,G) Join/Prune message with S in the prune list is
         received on a point-to-point interface.
      o  The Oif-Timer in an (S,G) route table entry expires.
      o  A PIM (S,G) Assert message from a preferred neighbor is
         received on the interface.

   A (S,G) Join alert is sent to the component owning the iif for a
   forwarding cache entry whenever the first oif is added to an entry,
   and the iif is owned by another component.  In PIM-DM, this may
   happen when any of the following occur:

      o  The oif's prune timer expires, or
      o  A PIM-DM (S,G) Graft message is received on the interface, or
      o  IGMP notifies PIM-DM that directly-connected group members now
         exist on the interface.




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   When it is known that there are no longer any members of a group G in
   the PIM-DM domain which receive data for externally-reached sources
   from the local router, a (*,G) Prune alert is sent to the "iif owner"
   for (*,G) according to the dispatcher.  In PIM-DM, this may happen
   when:

      o  The DWR for G times out.
      o  The members-are-senders approximation is being used and PIM-
         DM's last (S,G) entry for G is timed out.

   When it is first known that there are members of a group G in the
   PIM-DM domain, a (*,G) Join alert is sent to the "iif owner" of
   (*,G), according to the dispatcher.  In PIM-DM, this may happen when
   either of the following occurs:

      o  A DWR is received for G.
      o  The members-are-senders approximation is being used and a data
         packet for G is received on one of the component's interfaces.

4.3.2.  Processing Alerts

   When a PIM-DM component receives an (S,G) Creation alert, it adds the
   component's interfaces to the entry's oif list (according to normal
   PIM-DM behavior) EXCEPT:

      o  the iif,
      o  leaf networks without local members of the entry's group,
      o  and interfaces with scoped boundaries covering the group.

   When a PIM-DM component receives an (S,G) Prune alert, and the
   forwarding cache entry's oiflist is empty, it sends a PIM-DM (S,G)
   Prune message to the upstream neighbor according to normal PIM-DM
   behavior.

   When a PIM-DM component receives a (*,G) or (*,*) Prune alert, it is
   treated as if an (S,G) Prune alert were received for every matching
   (S,G) entry.

   When a PIM-DM component receives an (S,G) Join alert, and an (S,G)
   prune was previously sent upstream, it sends a PIM-DM (S,G) Graft
   message to the upstream neighbor according to normal PIM-DM behavior.

   When a PIM-DM component receives a (*,G) or (*,*) Join alert, then
   for each matching (S,G) entry in the PIM-DM routing table for which a
   prune was previously sent upstream, it sends a PIM-DM (S,G) Graft
   message to the upstream neighbor according to normal PIM-DM behavior.
   In addition, if DWR's are being used, the component sends a DWR Join
   message within its domain.



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4.4.  PIM-SM

   In this section we describe how the rules in section 2 apply to
   Sparse-mode PIM.  We assume that the reader is familiar with normal
   PIM-SM behavior, as specified in [4].

   To achieve correct PIM-SM behavior within the domain, the PIM-SM
   domain MUST be convex so that Bootstrap messages reach all routers in
   the domain.  That is, the shortest-path route from any internal
   router to any other internal router must lie entirely within the PIM
   domain.

   Unless some form of Domain-Wide-Reports (DWRs) [10] are added to
   PIM-SM in the future, all PIM-SM components act as wildcard receivers
   for externally-reached sources.  If DWRs are available for the
   domain, then a PIM-SM component acts as a wildcard receiver for
   externally-reached sources only if internally-reached domains exist
   which do not support some form of DWRs.

   A PIM-SM component acts as a wildcard receiver for internally-reached
   sources if and only if any other component is a wildcard receiver for
   externally-reached sources.  It does this by periodically sending
   (*,*,RP) Joins to all RPs for non-local groups (for example,
   239.x.x.x is considered locally-scoped, and PIM-SM components do not
   send (*,*,RP) Joins to RPs supporting only that portion of the
   address space).  The period is set according to standard PIM-SM rules
   for periodic Join/Prune messages.

   To properly instantiate Rule 1, whenever PIM creates a PIM (S,G)
   entry for an externally-reached source, and the next hop towards S is
   reached via an interface owned by another component, the iif should
   always point towards S and not towards the RP for G.  In addition,
   the Border-bit is set in all PIM Register messages for this entry.

   Finally, the PIM-SM component acts as a DR for externally-reached
   receivers in terms of being able to switch to the shortest-path tree
   for internally-reached sources.

4.4.1.  Generating Alerts

   A (*,*) Join alert is sent to the iif owner of the (*,*) entry (e.g.,
   the Interop dispatcher) when the first component becomes a wildcard
   receiver for external sources.  This may occur when a PIM-SM
   component starts up and decides to act in this role.







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   A (*,*) Prune alert is sent to the iif owner of the (*,*) entry
   (e.g., the Interop dispatcher) when all components are no longer
   wildcard receivers for external sources.  This may occur when a PIM-
   SM component which was acting in this role shuts down.

   A (S,G) Prune alert is sent to the component owning the iif for a
   forwarding cache entry whenever the last oif is removed from the
   entry and the iif is owned by another component.  In PIM-SM, this may
   happen when:

      o  A PIM (S,G) Join/Prune message with S in the prune list is
         received on a point-to-point interface, or
      o  A PIM (S,G) Assert from a preferred neighbor was received on
         the interface, or
      o  A PIM Register-Stop message is received for (S,G), or
      o  The interface's Oif-Timer for PIM's (S,G) route table entry
         expires.
      o  The Entry-Timer for PIM's (S,G) route table entry expires.

   When it is known that there are no longer any members of a group G in
   the PIM-SM domain which receive data for externally-reached sources
   from the local router, a (*,G) Prune alert is sent to the "iif owner"
   for (*,G) according to the dispatcher.  In PIM-SM, this may happen
   when:

      o  A PIM (*,G) Join/Prune message with G in the prune list is
         received on a point-to-point interface, or
      o  A PIM (*,G) Assert from a preferred neighbor was received on
         the interface, or
      o  IGMP notifies PIM-SM that directly-connected members no longer
         exist on the interface.
      o  The Entry-Timer for PIM's (*,G) route table entry expires.

   A (S,G) Join alert is sent to the component owning the iif for a
   forwarding cache entry whenever the first logical oif is added to an
   entry and the iif is owned by another component.  In PIM-SM, this may
   happen when any of the following occur:

      o  A PIM (S,G) Join/Prune message is received on the interface, or
      o  The Register-Suppression-Timer for (S,G) expires, or
      o  The Entry-Timer for an (S,G) negative-cache state route table
         entry expires.

   When it is first known that there are members of a group G in the
   PIM-SM domain, a (*,G) Join alert is sent to the "iif owner" of
   (*,G), according to the dispatcher.  In PIM-SM, this may happen when
   any of the following occur:




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      o  A PIM (*,G) Join/Prune message is received on the interface, or
      o  A PIM (*,*,RP) Join/Prune message is received on the interface,
         or
      o  (*,G) negative cache state expires, or
      o  IGMP notifies PIM that directly-connected group members now
         exist on the interface.

4.4.2.  Processing Alerts

   When a PIM-SM component receives an (S,G) Creation alert, it does a
   longest match search ((S,G), then (*,G), then (*,*,RP)) in its
   multicast routing table.  All outgoing interfaces of that entry are
   then added to the forwarding cache entry.  Unless the PIM-SM
   component owns the iif, the oiflist is also modified to support
   sending PIM Registers with the Border-bit set to the corresponding
   RP.

   When a PIM-SM component receives an (S,G) Prune alert, and the
   forwarding cache entry's oiflist is empty, then for each PIM (S,G)
   state entry covered, it sends an (S,G) Join/Prune message with S in
   the prune list to the upstream neighbor according to normal PIM-SM
   behavior.

   When a PIM-SM component receives a (*,G) Prune alert, it sends a
   (*,G) Join/Prune message with G in the prune list to the upstream
   neighbor towards the RP for G, according to normal PIM-SM behavior.

   When a PIM-SM component receives an (S,G) Join alert, it sends an
   (S,G) Join/Prune message to the next-hop neighbor towards S, and
   resets the (S,G) Entry-timer, according to normal PIM-SM behavior.

   When a PIM-SM component receives a (*,G) Join alert, then it sends a
   (*,G) Join/Prune message to the next-hop neighbor towards the RP for
   G, and resets the (*,G) Entry-timer, according to normal PIM-SM
   behavior.

   When a PIM-SM component receives a (*,*) Join alert, then it sends
   (*,*,RP) Join/Prune messages towards each RP.

   When a PIM-SM component receives a (*,*) Prune alert, then it sends a
   (*,*,RP) Prune towards each RP.










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4.5.  CBTv2

   In this section we describe how the rules in section 2 apply to
   CBTv2.  We assume that the reader is familiar with normal CBTv2
   behavior as specified in [5]. We note that, like MOSPF, CBTv2 allows
   joining and pruning entire groups, but not individual sources within
   groups.

   Interoperability between a single CBTv2 stub domain and a DVMRP
   backbone is outlined in [8].  Briefly, CBTv2 MBR components are
   statically configured such that, whenever an external route exists
   between two or more MBRs, one is designated as the primary, and the
   others act as non-forwarding (to prevent duplicate packets) backups.
   Thus, a CBTv2 domain must not serve as transit between two domains if
   another route between them exists.

   We now describe how a CBTv2 implementation may extend this to
   interoperate with all other multicast routing protocols.  A CBTv2
   component acts as a wildcard receiver for internally-reached sources
   if and only if any other component is a wildcard receiver for
   externally-reached sources.  It does this by sending JOIN-REQUESTs
   for all non-local group ranges to all known cores, as described in
   [8].

   Unless some form of Domain-Wide-Reports (DWRs) [10] are added to
   CBTv2 in the future, all CBTv2 components act as wildcard receivers
   for externally-reached sources.  If DWRs are available for the
   domain, then a CBTv2 component acts as a wildcard receiver for
   externally-reached sources only if internally-reached domains exist
   which do not support some form of DWRs.

4.5.1.  Generating Alerts

   A (*,*) Join alert is sent to the iif owner of the (*,*) entry (e.g.,
   the Interop dispatcher) when the first component becomes a wildcard
   receiver for external sources.  This may occur when a PIM-SM
   component starts up and decides to act in this role.

   A (*,*) Prune alert is sent to the iif owner of the (*,*) entry
   (e.g., the Interop dispatcher) when all components are no longer
   wildcard receivers for external sources.  This may occur when a PIM-
   SM component which was acting in this role shuts down.

   When the last oif is removed from the core tree for G, a (*,G) Prune
   alert is sent to the "iif owner" for (*,G) according to the
   dispatcher.  Since CBTv2 always sends all data to the core, the only
   time this can occur after the entry is created is when the MBR is the
   core.  In this case, the last oif is removed from the entry when:



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      o  A QUIT-REQUEST is received on the logical interface, and there
         are no directly-connected members present on the interface, or
      o  IGMP notifies CBT that there are no longer directly-connected
         members present on the interface, and the interface is not a
         CBT child interface for group G.

   When the first CBT outgoing interface is added to an existing core
   tree, a (*,G) Join alert is sent to the "iif owner" of (*,G)
   according to the dispatcher.  Since CBTv2 always sends all data to
   the core, the only time these can occur, other than when the entry is
   created, is when the MBR is the core.  In this case, the first
   logical oif is added to an entry when:

      o  A JOIN-REQUEST for G is received on the interface, or
      o  IGMP notifies CBT that directly-connected group members now
         exist on the interface.

4.5.2.  Processing Alerts

   When a CBTv2 component receives an (S,G) Creation alert, and the
   router is functioning as the designated BR, any CBT interfaces which
   are on the tree for G are added to the forwarding cache entry's oif
   list (according to normal CBTv2 behavior).

   When a CBTv2 component receives an (S,G) Prune alert, the alert is
   ignored, since CBTv2 cannot prune specific sources.  Thus, it will
   continue to receive packets from S since it must receive packets from
   other sources in group G.

   When a CBTv2 component receives a (*,G) Prune alert, and the router
   is not the primary core for G, and the only CBT on-tree interface is
   the interface towards the core, it sends a QUIT-REQUEST to the next-
   hop neighbor towards the core, according to normal CBTv2 behavior.

   When a CBTv2 component receives either an (S,G) Join alert or a (*,G)
   Join alert, and the router is not the primary core for G, and the
   router is not already on the core-tree for G, it sends a CBT (*,G)
   JOIN-REQUEST to the next-hop neighbor towards the core, according to
   normal CBTv2 behavior.












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4.6.  IGMP-only links

   In this section we describe how the rules in section 2 apply to a
   link which is not within any routing domain, and hence no routing
   protocol messages are exchanged and the interface is not owned by any
   multicast routing protocol component.  We assume that the reader is
   familiar with normal IGMP behavior as specified in [7].  We note that
   IGMPv2 allows joining and pruning entire groups, but not individual
   sources within groups.

   An IGMP-only "component" may only own a single interface; hence an
   IGMP-only domain only consists of a single link.  Since an IGMP-only
   component can only act as a wildcard receiver for internally-reached
   sources if all internally-reached sources are directly-connected,
   then either the IGMP-only domain (link) must be a stub domain, or
   else there must be no other components which are wildcard receivers
   for externally-reached sources.

4.6.1.  Generating Alerts

   When it is known that there are no longer any directly-connected
   members of a group G on the IGMP-only interface, a (*,G) Prune alert
   is sent to the "iif owner" for (*,G) according to the dispatcher.  In
   IGMP, this may happen when:

      o  The group membership times out.

   When it is first known that there are directly-connected members of a
   group G on the interface, a (*,G) Join alert is sent to the "iif
   owner" of (*,G), according to the dispatcher.  In IGMP, this may
   happen when any of the following occur:

      o  A Membership Report is received for G.

4.6.2.  Processing Alerts

   When an IGMP-only component receives an (S,G) Creation alert, and
   there are directly-connected members of G present on its interface,
   it adds the interface to the entry's oif list.

   When an IGMP-only component receives an (S,G) Prune alert, the alert
   is ignored, since IGMP can only prune entire groups at a time.

   When an IGMP-only component receives a (*,G) Prune alert, the router
   leaves the group G, sending an IGMP Leave message if it was the last
   reporter, according to normal IGMPv2 behavior.





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   When an IGMP-only component receives a (*,*) Prune alert, it leaves
   promiscuous multicast mode.

   When an IGMP-only component receives either an (S,G) Join alert or a
   (*,G) Join alert, and the component was not previously a member of G
   on the IGMP-only interface (and the component is not a wildcard
   receiver for internally reached sources), it joins the group on the
   interface, causing it to send an unsolicited Membership Report
   according to normal IGMP behavior.

   When an IGMP-only component receives a (*,*) Join alert, it enters
   promiscuous multicast mode.

5.  Security Considerations

   All operations described herein are internal to multicast border
   routers.  The rules described herein do not change the security
   issues underlying individual multicast routing protcols.  Allowing
   different protocols to interact, however, means that security
   weaknesses of any particular protocol may also apply to the other
   protocols as a result.

6. References

   [1]   Ajit S. Thyagarajan and Stephen E. Deering.  Hierarchical
         distance-vector multicast routing for the MBone.  In
         "Proceedings of the ACM SIGCOMM", pages 60--66, October 1995.

   [2]   Pusateri, T., "Distance Vector Multicast Routing Protocol",
         Work in Progress.

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

   [4]   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.

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

   [6]   Estrin, Farinacci, Helmy, Jacobson, and Wei, "Protocol
         Independent Multicast (PIM), Dense Mode Protocol
         Specification", Work in Progress.

   [7]   Fenner, W., "Internet Group Management Protocol, Version 2",
         RFC 2236, November 1997.




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   [8]   Ballardie, A., "Core Based Tree (CBT) Multicast Border Router
         Specification", Work in Progress.

   [9]   Thaler, D., Estrin, D. and D. Meyer, "Border Gateway Multicast
         Protocol (BGMP): Protocol Specification", Work in Progress.

   [10]  Fenner, W., "Domain Wide Multicast Group Membership Reports",
         Work in Progress.

7.  Author's Address

   Dave Thaler
   Microsoft
   One Microsoft Way
   Redmond, WA 98052

   Phone: (425) 703-8835
   EMail: dthaler@microsoft.com

































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8.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

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

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

Acknowledgement

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



















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