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Versions: 01 02 03 RFC 2715

Inter-Domain Multicast Routing (IDMR)                          D. Thaler
Internet Engineering Task Force                                Microsoft
INTERNET-DRAFT                                              31 July 1998
draft-thaler-multicast-interop-03.txt             Expires: January, 1999


         Interoperability Rules for Multicast Routing Protocols
                <draft-thaler-multicast-interop-03.txt>

Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
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(US West Coast).

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.



Copyright Notice Copyright (C) The Internet Society (1998).  All Rights
Reserved.










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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.

                                 +--------+
                            +----|        |----+
            +---+    +---+  |  ===>      <===  |
            |   |    |   |  +----|   #    |----+
            |   |    | # |     +-----#------+
            | # |  +---#-------|     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



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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.

  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.



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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.

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.




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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.

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



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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:

  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



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dispatcher does not own any interfaces.

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:
  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



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     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.

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.




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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:
  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.




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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].

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.




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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.

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



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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.


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



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(*,*,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.

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



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     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:
  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,



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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.


7.  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.

7.1.  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



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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:
  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.

7.2.  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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4.5.  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.

4.5.1.  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.

4.5.2.  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.

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




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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]  T. Pusateri.  Distance vector multicast routing protocol.  Internet
     Draft, March 1998.  Available from ftp://ds.internic.net/internet-
     drafts/draft-ietf-idmr-dvmrp-v3-06.ps.

[3]  J. Moy.  Multicast extensions to OSPF.  RFC 1584, July 1993.

[4]  Estrin, Farinacci, Helmy, Thaler, Deering, Handley, Jacobson, Liu,
     Sharma, and Wei.  Protocol independent multicast-sparse mode (PIM-
     SM): Protocol specification.  RFC 2362, June 1998.

[5]  A. Ballardie.  Core based trees (CBT version 2) multicast routing:
     Protocol specification.  RFC 2189, September 1997.

[6]  Estrin, Farinacci, Helmy, Jacobson, and Wei.  Protocol independent
     multicast (PIM), dense mode protocol specification.  Internet
     Draft, May 1997. Available from ftp://ds.internic.net/internet-
     drafts/draft-ietf-idmr-pim-dm-spec-05.ps.

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

[8]  A. J. Ballardie.  Core Based Tree (CBT) Multicast Border Router
     Specification.  Internet Draft, November 1997. Available from



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     ftp://ds.internic.net/internet-drafts/draft-ietf-idmr-cbt-br-spec-

     01.txt.
[9]  D. Thaler, D. Estrin, and D. Meyer.  Border Gateway Multicast
     Protocol (BGMP): Protocol Specification.  Internet Draft, October
     1997. Available from ftp://ds.internic.net/internet-drafts/draft-
     ietf-idmr-gum-01.txt.

[10]  W. Fenner.  Domain Wide Multicast Group Membership Reports,
     Internet Draft, November 1997. Available from
     ftp://ds.internic.net/internet-drafts/draft-ietf-idmr-membership-
     reports-00.txt.



7.  Address of Author

Dave Thaler
Microsoft
One Microsoft Way
Redmond, WA 98052
Phone: (425) 703-8835
Email: thalerd@eecs.umich.edu




























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8.  Full Copyright Statement
Copyright (C) The Internet Society (1998).  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 implmentation 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."





























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