RFC2715 - Interoperability Rules for Multicast Routing Protocols

　　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.

+--------+
+---- ----+
+---+ +---+ ===> <===
+---- # ----+
# +-----#------+
# +---#------- 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.

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 RFC2119.

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.

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:

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.

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.

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 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.

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.

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.

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

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
(*,*,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 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, 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.

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:

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.

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.

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", RFC1584, 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", RFC2362, June 1998.

[5] Ballardie, A., "Core Based Trees (CBT version 2) Multicast
Routing", RFC2189, 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",
RFC2236, November 1997.

[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: [email protected]

8. Full Copyright Statement

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