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draft-ietf-idmr-PIM-SM-spec-02.txt May 7, 1996

Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
Specification

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 its Working Groups. (Note that other groups may also distribute
working documents as Internet Drafts).

Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
``working'' draft'' or ``work in progress.''

Please check the I-D abstract listing contained in each Internet
Draft directory to learn the current status of this or any other
Internet Draft.

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Internet Draft PIM-SM Specification May 1996

1 Introduction

This document describes a protocol for efficiently routing to
multicast groups that may span wide-area (and inter-domain)
internets. We refer to the approach as Protocol Independent
Multicast--Sparse Mode (PIM-SM) because it is not dependent on any
particular unicast routing protocol, and because it is designed to
support sparse groups as defined in [1][2]. This document describes
the protocol details. For the motivation behind the design and a
description of the architecture, see [1][2]. Section 2 summarizes
PIM-SM operation. It describes the protocol from a network
perspective, in particular, how the participating routers interact to
create and maintain the multicast distribution tree. Section 3
describes PIM-SM operations from the perspective of a single router
implementing the protocol; this section constitutes the main body of
the protocol specification. It is organized according to PIM-SM
message type; for each message type we describe its contents, its
generation, and its processing. Interoperability with other protocols
will be further discussed in an appendix to this document.

Section 4 provides packet format details.

The most significant functional changes since the January '95
version, are the Rendezvous Point-related mechanisms and the removal
of the PIM-DM protocol details to a separate [3] (for clarity).

2 PIM-SM Protocol Overview

In this section we provide an overview of the architectural
components of PIM-SM.

A router [*]

receives explicit Join/Prune messages from those neighboring routers
that have downstream group members. The router then forwards data
packets addressed to a multicast group, G, only onto those interfaces
on which explicit joins have been received.

A Designated Router (DR) sends periodic Join/Prune messages toward a
group-specific Rendezvous Point (RP) for each group for which it has
active members. Each router along the path toward the RP builds
wildcard (any-source) forwarding state for the group and sends
messages on toward the RP. The wildcard forwarding entry's incoming
_________________________
[*] All routers mentioned in this document are assumed
to be PIM-SM capable, unless otherwise specified.

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interface points toward the RP; the outgoing interfaces point to the
neighboring downstream routers that have sent Join/Prune messages
toward the RP. This forwarding state creates a shared, RP-centered,
distribution tree that reaches all group members. When a data source
first sends to a group, its DR unicasts Register messages to the RP
with the source's data packets encapsulated within. If the data rate
is high, the RP can send source-specific Join/Prune messages back
towards the source and the source's data packets will follow the
resulting forwarding state and travel unencapsulated to the RP.
Whether they arrive encapsulated or natively, the RP forwards the
source's decapsulated data packets down the RP-centered distribution
tree toward group members. If the data rate warrants it, routers with
local receivers can join a source-specific, shortest path,
distribution tree, and prune these source's packets off of the shared
RP-centered tree. Even if all receivers switch to the shortest path
tree, state for that source will be kept at the RP, so that new
members that join the RP-centered tree will receive data packets from
the source. For low data rate sources, neither the RP, nor last hop
routers need join a source-specific shortest path tree and data
packets can be delivered via the shared, RP-tree.

The following subsections describe SM operation in more detail, in
particular, the control messages, and the actions they trigger.
Section 3 describes protocol operation from an implementors
perspective, i.e., the actions performed by a single router.

2.1 Local hosts joining a group

In order to join a multicast group, G, a host sends an IGMP Host-
Membership-Report message identifying the particular group. As
specified in [4], IGMP Host-Membership-Report messages are sent in
response to a directly-connected router's IGMP Host-Membership-Query
message (see figure 1). [*]

From this point on we refer to such a host as a receiver, R, (or
member) of the group G.

_________________________
[*] All figures used in this section are for illustra-
tion and are not intended to be complete. For complete
and detailed protocol action see Section 3.

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Fig. 1 Example: how a receiver joins, and sets up shared tree

When a DR receives an IGMP Host-Membership-Report for a new group, G,
the DR looks up the associated RP. The DR (e.g., router A in figure
1) creates a wildcard multicast forwarding entry for the group,
referred to here as a (*,G) entry; if there is no more specific match
for a particular source, the packet will be forwarded according to
this entry.

The RP address is included in a special field in the forwarding entry
and is included in periodic upstream Join/Prune messages. The
outgoing interface is set to that over which the IGMP Host-
Membership-Report was received from the new member. The incoming
interface is set to the interface used to send unicast packets to the
RP. An RP-bit associated with this entry is also set, indicating that
this entry, (*,G), represents state on the shared RP-tree. Each DR on
the RP-tree with directly connected members sets a timer for this
entry. If the timer expires and the DR has neither local members nor
downstream receivers, the (*,G) state is deleted. If the DR does have
local members, it refreshes the (*,G) entry timer each time it gets
an IGMP Host-Membership-Report.

2.2 Establishing the RP-rooted shared tree

Triggered by the (*,G) state, the DR creates a Join/Prune message
with the RP address in its join list and the WC-bit and RP-bit set;
nothing is listed in its prune list. The RP-bit flags the join as
being associated with the shared tree and therefore the Join/Prune
message is propagated along the RP-tree. The WC-bit indicates that
the address is an RP and the receiver expects to receive packets from
all sources via this (shared tree) path.

Each upstream router creates or updates its multicast forwarding
entry for (*,G) when it receives a Join/Prune with the RP-bit and
WC-bit set. The interface on which the Join/Prune message arrived is
added to the list of outgoing interfaces (oifs) for (*,G). Based on
this entry each upstream router between the receiver and the RP sends
a Join/Prune message in which the join list includes the RP. The
packet payload contains Multicast-Address=G, Join=RP,WCbit,RPbit,
Prune=NULL.

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2.3 Hosts sending to a group

When a host first sends a multicast data packet to a group, its DR
must deliver the packet to the RP for distribution down the RP-tree
(see figure 2). This is done by the sender's DR unicasting a Register
packet to the RP for the group. The data packet is encapsulated in
the Register packet so that the RP can decapsulate it and deliver it
to downstream members.

Fig. 2 Example: a host sending to a group

If the data rate of the source warrants [*]

the use of a source-specific shortest path tree (SPT), the RP may
construct a new multicast forwarding entry that is specific to the
source, hereafter referred to as (S,G) state, and send periodic
Join/Prune messages toward the source. The routers between the source
and the RP build and maintain (S,G) state in response to these
messages and send (S,G) messages upstream toward the source.

The source's DR must stop encapsulating data packets in Registers
when (and so long as) it receives Register-Stop messages from the RP.
The RP triggers Register-Stop messages in response to Registers, if
the RP has no downstream receivers for the group (or for that
particular source), or if the RP has already joined the (S,G) tree
and is receiving the data packets natively.

2.4 Switching from shared tree (RP-tree) to shortest path tree (SP-
tree)}

When a router has directly-connected members, it first joins the
shared RP-tree. The router can switch to a source's shortest path
tree (SP-tree) after receiving packets from that source over the
shared RP-tree. The recommended policy is to initiate the switch to
the SP-tree after receiving a significant number of data packets
_________________________
[*] This decision is a local policy established at the
RP. For example, when the Register rate exceeds a con-
figured threshold at the RP, this may warrant the use
of the SPT.

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during a specified time interval from a particular source. To realize
this policy the router can monitor data packets from sources for
which it has no source-specific multicast forwarding entry and
initiate such an entry when the data rate exceeds the configured
threshold. As shown in figure 3, router `A' initiates a (S,G) state.

Fig. 3 Example: Switching from shared tree to shortest path tree

When a (S,G) entry is activated (and periodically so long as the
state exists), a Join/Prune message is sent upstream towards the
source, S, with S in the join list. The payload contains Multicast-
Address=G, Join=S, Prune=NULL. When the (S,G) entry is created, the
outgoing interface list is copied from (*,G), i.e., all local shared
tree branches are replicated in the new shortest path tree [*] In
this way when a data packet from S arrives and matches on this entry,
all receivers will continue to receive the source's packets along
this path. Note that (S,G) state must be maintained in all last-hop
routers where an SP-tree is maintained. Even when (*,G) and (S,G)
overlap, both states are needed to trigger the source-specific
Join/Prune messages. (S,G) state is kept alive by data packets
arriving from that source. A timer, S-timer, is set for the (S,G)
entry and this timer is restarted whenever a data packet for (S,G) is
forwarded out at least one oif. When the S-timer expires the state is
deleted.

Only the RP and routers with local members can initiate switching to
the SP-tree; intermediate routers do not. Consequently, last hop
routers create (S,G) state in response to data packets from the
source, S; whereas intermediate routers only create (S,G) state in
response to Join/Prune messages from downstream that have S in the
Join list [*]
_________________________
[*] In more complicated scenarios, other entries in the
router have to be considered. For details see Section 3.
[*] For example, to implement the policy that source-
specific trees are only setup for high-data rate
source, a last-hop router might not create a (S,G) en-
try until it has received m data packets from the
source within some interval of n seconds.

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The (S,G) entry is initialized with the SPT-bit cleared, indicating
that the shortest path tree branch from S has not yet been setup
completely, and the router can still accept packets from S that
arrive on the (*,G) entry's iif.

When a router with a (S,G) entry and a cleared SPT-bit starts to
receive packets from the new source S on the iif for the (S,G) entry,
and that iif differs from the (*,G) entry's iif, the router sets the
SPT-bit, and sends a Join/Prune message towards the RP, indicating
that the router no longer wants to receive packets from S via the
shared RP-tree. The Join/Prune message sent towards the RP includes S
in the prune list, with the RP-bit set indicating that S's packets
should not be forwarded down this branch of the shared tree. If the
router receiving the Join/Prune message has (S,G) state (with or
without the RPbit set), it deletes the arriving interface from the
(S,G) oif list. If the router has only (*,G) state, it creates an
(S,G)RP-bit entry. The Join/Prune message payload contains
Multicast-Address=G, Join=NULL, Prune=S,RPbit.

If at a later time a new receiver joins the RP-tree, the negative
cache state on the RP-tree must be eradicated to bring all sources'
data packets down to the new receiver. Therefore, when a (*,G) Join
arrives with a null prune list at a router that has any (S,G)RP-bit
entries (which is causing it to send source-specific prunes toward
the RP), all RP-bit state for that group has to be updated upstream
of the router; so as to bring all sources' packets down to the new
member. To accomplish this the router updates all existing (S,G)RP-
bit entries; it adds to each (S,G)RP-bit entry's oif list the
interface on which the (*,G) join arrived. The router also triggers a
(*,G) join upstream to cause the same updating of RP-bit settings
upstream and pull down all active sources' packets. If the arriving
(*,G) join has some sources included in its prune list, then the
corresponding (S,G)RP-bit entries are left unchanged (i.e., the RPbit
remains set and no oif is added).

2.5 Steady state maintenance of distribution tree (i.e., router state)}

In the steady state each router sends periodic Join/Prune messages
for each active (S,G), (*,G) or (*,*,RP) [*]

entry; the Join/Prune messages are sent to the RPF neighbor on the
iif of the corresponding entry. These messages are sent periodically
to capture state, topology, and membership changes. A Join/Prune
_________________________
[*] (*,*,RP) entry is introduced for interoperability,
see Sections 2.10 and 6.

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message is also sent on an event-triggered basis each time a new
forwarding entry is established for some new source (note that some
damping function may be applied, e.g., a merge time). Join/Prune
messages do not elicit any form of explicit acknowledgment; routers
recover from lost packets using the periodic refresh mechanism.

2.6 Obtaining RP information

To obtain the RP information, all routers collect RP-Set messages.
RP-Set messages are sent hop-by-hop within the domain; originating at
the domain's bootstrap router (BSR). The BSR is elected dynamically
within each domain.
[*]

Routers then use the set of RPs to get the proper Group to RP
mapping. Details are as follows:

2.6.1 Bootstrap Router

A (small) set of routers, within a domain, are configured as
candidate bootstrap routers. Initially, each of these candidates
includes its address in `RP-set' messages. Through a simple election
mechanism, a single bootstrap router (BSR) is elected for that domain
(see Section 3.6).
2.6.2 Candidate RPs

A set of routers within a domain are configured as candidate RPs (C-
RPs); typically these will be the same routers that are configured as
C-BSRs. Candidate RPs periodically unicast Candidate-RP-Advertisement
messages (C-RP-Advs) to the BSR of that domain. C-RP-Advs include the
address of the advertising C-RP, as well as an optional group address
and a mask length field, indicating the group prefix(es) for which
the candidacy is advertised. The BSR then includes a set of these
Candidate-RPs in the RP-Set messages, along with the corresponding
group prefixes (see Section
3.6.2). RP-Set messages are periodically sent hop-by-hop throughout
the domain.

_________________________
[*] A domain in this context is a multicast region in
which routers implement PIM-SM. PIM-SM border routers
are assumed to connect a domain to the rest of the in-
ternet.

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2.6.3 Group to RP mapping

Routers receive and store RP-Set messages originated by the BSR. When
a DR receives IGMP Host-Membership-Report (or a data packet) from a
directly connected host, for a group for which it has no entry, the
DR uses a hash function to map the pertinent group to one of the C-
RPs whose Group-prefix includes the group (see Section 3.7). The DR
then sends a Join/Prune message towards (or unicasts Registers to)
that RP.
[*]

2.6.4 Providing RP liveness

The RP-Set message indicates liveness of the RPs included therein; if
an RP is included in the message, then it is tagged as `up' at the
routers, while RPs not included in the message are tagged as `down'
and removed from the list of RPs over which the hash algorithm acts.
Each router continues to use the contents of the most recently
received RP-set message until it receives a new RP-set message.

2.7 Multicast data packet processing

Data packets are processed in a manner similar to existing multicast
schemes. A router first performs a longest match on the source and
group address in the data packet. A (S,G) entry is matched first if
one exists; a (*,G) entry is matched otherwise. If neither state
exists, then a (*,*,RP) entry match is attempted as follows: the
router hashes on G to identify the RP for group G, and looks for a
(*,*,RP) entry that has this RP address associated with it. If none
of the above exists, then the packet is dropped. If a state is
matched, an incoming interface check (RPF check) is performed on the
matching state and if it fails the packet is dropped, otherwise the
packet is forwarded to all interfaces listed in the outgoing
interface list.

Some special actions are needed to deliver packets continuously while
switching from the shared to shortest-path tree. In particular, when
a (S,G) entry is matched, incoming packets are forwarded as follows:

1 If the SPT-bit is set, then:
_________________________
[*] Each intermediate router also uses this same hash
function to determine the (*,*,RP) match for incoming
data packets.

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1 if the incoming interface is the same as a matching
(S,G) iif, the packet is forwarded to the oif-list of
(S,G).

2 if the incoming interface is different than a matching
(S,G) iif , the packet is discarded.

2 If the SPT-bit is cleared, then:

1 if the incoming interface is the same as a matching
(S,G) iif, the packet is forwarded to the oif-list of
(S,G). In addition, the SPT bit is set for that entry
if the incoming interface differs from the incoming
interface of the (*,G) or (*,*,RP) entry.

2 if the incoming interface is different than a matching
(S,G) iif, the incoming interface is tested against a
matching (*,G) or (*,*,RP) entry. IF the iif is the
same as one of those, the packet is forwarded to the
oif-list of the matching entry.

3 Otherwise the iif does not match any entry for G and
the packet is discarded.

Data packets never trigger prunes. However, data packets may
trigger actions that in turn trigger prunes. For example, when
router B in figure 3 decides to switch to SP-tree at step 3, it
creates a (S,G) entry with SPT-bit set to 0. When data packets
from S arrive at interface 2 of B, B sets the SPT-bit to 1
since the iif for (*,G) is different than that for (S,G). This
triggers the sending of prunes towards the RP.

2.8 Operation over Multi-access Networks

This section describes a few additional protocol mechanisms
needed to operate PIM over multi-access networks: Designated
Router election, Assert messages to resolve parallel paths, and
the Joiner bit to suppress redundant Joins on multi-access
networks.

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2.8.1 Designated router election

When there are multiple routers connected to a multi-access
network, one of them should be chosen to operate as the
designated router (DR) at any point in time. The DR is
responsible for sending triggered Join/Prune and Register
messages toward the RP [*]

A simple designated router (DR) election mechanism is used for
both SM and traditional IP multicast routing.

Neighboring routers send Query messages to each other. The
sender with the largest IP address assumes the role of DR. Each
router connected to the multi-access LAN sends the Queries
periodically in order to adapt to changes in router status.

2.8.2 Parallel paths to a source or the RP

If a router receives a multicast datagram on a multi-access LAN
from a source whose corresponding (S,G) outgoing interface list
includes the interface to that LAN, the packet must be a
duplicate. In this case a single forwarder must be elected.
Using Assert messages addressed to `224.0.0.13' (ALL-PIM-ROUTERS
group) on the LAN, upstream routers can resolve which one will
act as the forwarder. Downstream routers listen to the Asserts
so they know which one was elected, and therefore where to send
subsequent Joins. Typically this is the same as the downstream
router's RPF neighbor but there are circumstances where this
might not be the case, e.g., when using different unicast
protocols.

The upstream router elected is the one that has the shortest
distance to the source. Therefore, when a packet is received on
an outgoing interface a router sends an Assert message on the
multi-access LAN indicating what metric it uses to reach the
source of the data packet. The router with the smallest
numerical metric (with ties broken by highest address) will
become the forwarder. All other upstream routers will delete the
interface from their outgoing interface list. The downstream
routers also do the comparison in case the forwarder is
different than the RPF neighbor.
_________________________
[*] IGMP Queries are sent by a PIMv2 DR if it supports
IGMPv1. If a PIMv2 router is using IGMPv2 then Host
queries are not sent by the PIMv2 DR but by the IGMP
querier.

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Associated with the metric is a metric preference value. This is
provided to deal with the case where the upstream routers may
run different unicast routing protocols. The numerically smaller
metric preference is always preferred. The metric preference
should be treated as the high-order part of an assert metric
comparison. Therefore, a metric value can be compared with
another metric value provided both metric preferences are the
same. A metric preference can be assigned per unicast routing
protocol and needs to be consistent for all routers on the
multi-access network.

Asserts are also needed for (*,G) entries since there may be
parallel paths from the RP and sources to a multi-access
network. When an assert is sent for a (*,G) entry, the first bit
in the metric preference (RP-bit) is always set to 1 to indicate
that this path corresponds to the RP tree, and that the match
should be done on (*,G) if exits. Furthermore, the RP-bit is
always cleared for SP-tree entries' metric preference; this
causes an SP-tree path to always look better than an RP-tree
path. When the SP-tree and RPtree cross the same LAN, this
mechanism eliminates the duplicates that would otherwise be
carried over the LAN.

In case the packet, or the Assert message, matches on oif for
(*,*,RP) entry, a (*,G) entry is created, and asserts take place
as if the matching state were (*,G).

The DR may lose to another router on the LAN by the Assert
process if there are multiple paths to the RP through the LAN.
From then on, the DR is no longer the last-hop router for local
receivers. The winning router becomes the last-hop router and is
responsible for sending (*,G) join messages to the RP. Asserts
are rate limited.

2.8.3 Join/Prune suppression

If a Join/Prune message arrives on the incoming interface for an
existing (S,G) entry, and the sender of the Join/Prune has a
higher IP address than the recipient of the message, a Joiner-
bit in the multicast routing table entry is cleared to suppress
further Join/Prune messages. A timer is set for the Joiner-bit;
after it expires the Joiner-bit is set indicating further
periodic Join/Prunes should be sent for this entry. The Joiner-
bit timer is restarted each time a Join/Prune message is
received from a higher-IP-addressed PIM neighbor.

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2.9 Unicast Routing Changes

When unicast routing changes, an RPF check is done on all active
(S,G), (*,G) and (*,*,RP) entries, and all affected expected
incoming interfaces are updated. In particular, if the new
incoming interface appears in the outgoing interface list, it is
deleted from the outgoing interface list. The previous incoming
interface may be added to the outgoing interface list by a
subsequent Join/Prune from downstream. Join/Prune messages
received on the current incoming interface are ignored.
Join/Prune messages received on new interfaces or existing
outgoing interfaces are not ignored. Other outgoing interfaces
are left as is until they are explicitly pruned by downstream
routers or are timed out due to lack of appropriate Join/Prune
messages. If the router has a (S,G) entry with the SPT-bit set,
and the updated iif(S,G) does not differ from iif(*,G) or
iif(*,*,RP), then the router resets the SPT-bit.

The router must send a Join/Prune message with S in the Join
list out its new incoming interface to inform upstream routers
that it expects multicast datagrams over the interface. It may
also send a Join/Prune message with S in the Prune list out the
old incoming interface, if the link is operational, to inform
upstream routers that this part of the distribution tree is
going away.

2.10 Interaction with dense mode protocols such as DVMRP

The essential problem in connecting to dense mode protocols is
to pull all packets generated within the PIM-SM region down to
the dense mode routers. To do this, a special entry type,
referred to as (*,*,RP), is introduced. Every (*,*,RP) entry is
associated with a particular RP in the domain; that RP is used
to conduct RPF checks. Border routers initiate the building of
(*,*,RP) towards all internal Candidate RPs. (*,*,RP) entries
represent an aggregation of all the groups supported by the RP.

Most of the mechanisms needed to support interoperability with
dense mode protocols such as DVMRP are implemented in BRs, i.e.,
special routers that sit at the boundary between a PIM-SM
regions and the DVMRP regions and which speak both protocols.
However, all PIM-SM routers must be capable of supporting
(*,*,RP) state and interpreting associated Join messages.
Interaction with non-PIM-SM networks will be discussed in a
separate interoperability appendix.

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2.11 PIM-SM for Inter-Domain Multicast

Future documents will address the use of PIM-SM as a backbone
inter-domain multicast routing protocol. Design choices center
primarily around the distribution and usage of RP information
for wide area, inter-domain groups.

2.12 Security

{ Editors Note: This section requires further work.}

All PIM control messages may use [5] to address security
concerns.

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3 Detailed Protocol Description

This section describes the protocol operations from the
perspective of an individual router implementation. In
particular, for each message type we describe how it is
generated and processed.

3.1 Query

Query messages are sent so neighboring routers can discover each
other.

3.1.1 Sending Queries

Query messages are sent periodically between PIM neighbors. By
default they are transmitted every 30 seconds. This informs
routers what interfaces have PIM neighbors. Query messages are
multicast using address 224.0.0.13 (ALL-PIM-ROUTERS group). The
packet includes the holdtime for neighbors to keep the
information valid. The recommended holdtime is 3 times the query
transmission interval. By default the holdtime is 90 seconds.
Queries are sent on all types of communication links.

3.1.2 Receiving queries

When a router receives a Query packet, it stores the IP address
for that neighbor, sets the PIM neighbor timer based on the
Query holdtime, and determines the Designated Router (DR) for
that interface. The highest IP addressed system is elected DR.
Each query received causes the DR's address to be updated.

When a router that is the active DR receives a query from a new
neighbor (i.e., from an IP address that is not yet in the DRs
neighbor table), the DR unicasts its most recent RP-set
information to the new neighbor.

3.1.3 Timing out neighbor entries

A periodic process is run to time out PIM neighbors that have
not sent queries. If the DR has gone down, a new DR is chosen by
scanning all neighbors on the interface and selecting the new DR
to be the one with the highest IP address. If an interface has
gone down, the router may optionally time out all PIM neighbors
associated with the interface.

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3.2 Join/Prune

Join/Prune messages are sent to join or prune a branch off of
the multicast distribution tree. A single message contains both
a join and prune list, either one of which may be null. Each
list contains a set of source addresses, indicating the source-
specific trees or shared tree that the router wants to join or
prune.

3.2.1 Sending Join/Prune Messages

Join/Prune messages are merged such that a message sent to a
particular upstream neighbor, N, includes all of the current
joined and pruned sources that are reached via N; according to
unicast routing Join/Prune messages are multicast to all routers
on multi-access networks with the target address set to the next
hop router towards S or RP. Join/Prune messages are sent
periodically. Currently the period is set to 60 seconds. [*]

A router sends a periodic Join/Prune message to each
distinct RPF neighbor associated with each (S,G), (*,G) and
(*,*,RP) entry. Join/Prune messages are only sent if the RPF
neighbor is a PIM neighbor. A periodic Join/Prune message sent
towards a particular RPF neighbor is constructed as follows:

1 Each router determines the RP for a (*,G) entry by using
the hash function described. The RP address (with RP and WC
bits set) is included in the join list of a periodic
Join/Prune message under the following conditions:

1 The Join/Prune message is being sent to the RPF
neighbor to the RP for an active (*,G) or (*,*,RP)
entry, and

2 The outgoing interface list in the (*,G) or (*,*,RP)
entry is non-NULL, or the router is the DR on the same
interface as the RPF neighbor.

_________________________
[*] In the future we will introduce mechanisms to
rate-limit this control traffic on a hop by hop basis,
in order to avoid excessive overhead on small links.

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2 A particular source address, S, is included in the join
list with the RP and WC bits cleared under the following
conditions:

1 The Join/Prune message is being sent to the RPF
neighbor to S, and

2 There exists an active (S,G) entry with the RPbit
cleared, and

3 The oif list in the (S,G) entry is not null.

3 A particular source address, S, is included in the prune
list with the RP and WC bits cleared under the following
conditions:

1 The Join/Prune message is being sent to the RPF
neighbor to S, and

2 There exists an active (S,G) entry with the RPbit
cleared, and

3 The oif list in the (S,G) entry is null.

4 A particular source address, S, is included in the prune
list with the RP bit set and the WC bit cleared under the
following conditions:

1 The Join/Prune message is being sent to the RPF
neighbor toward the RP and there exists a (S,G) entry
with the RPbit set and null oif list, or

2 The Join/Prune message is being sent to the RPF
neighbor toward the RP, there exists a (S,G) entry
with the RPbit cleared and SPT-bit set, and the
incoming interface toward S is different than the
incoming interface toward the RP, or

3 The Join/Prune message is being sent to the RPF
neighbor toward the RP, and there exists a (*,G) entry
and (S,G) entry for a directly connected source.

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5 The RP address (with RP and WC bits set) is included in the
prune list if:

1 The Join/Prune message is being sent to the RPF
neighbor toward the RP and there exists a (*,G) entry
with a null oif list (see Section 3.5.2).

In addition to these periodic messages, the following events
will trigger Join/Prune messages (the contents of triggered
messages are the same as the periodic, described above)

1 Receipt of an IGMP Host-Membership-Report message for a
group G will cause building or modifying corresponding
(*,G) state, and subsequent triggering of upstream
Join/Prune messages as follows:

1 If the receiving router does not have a forwarding
entry for G the router creates a (*,G) entry, with the
interface upon which the IGMP Host-Membership-Report
was received included in the oif list. The router
sends a Join/Prune message towards the RP with the RP
address and RP-bit and WC-bits set in the join list. A
timer is initiated for each interface in the oif list.
Or,

2 If the (*,G) already exists, the interface upon which
the IGMP Host-Membership-Report was received is added
to the oif list (if it was not included already) and
the timer for that interface is restarted.

2 Receipt of a Join/Prune message for (S,G), (*,G) or
(*,*,RP) will cause building or modifying corresponding
state, and subsequent triggering of upstream Join/Prune
messages, in the following cases:

1 When there is no current forwarding entry, the RP
address included in the Join/Prune message is checked
against the local RP-Set information. If it matches,
an entry will be created. If the router has no RP-Set

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information it may discard the message, or optionally
use the RP address included in the message.

The new entry will in turn trigger an upstream
Join/Prune message.

2 When the outgoing interface list of (S,G) RPbit entry
is null, the triggered Join/Prune message will contain
S in the prune list.

3 Receipt of a packet on a (S,G) entry whose SPT-bit is
cleared triggers the following if the packet arrived on the
correct incoming interface and there is a (*,G) or (*,*,RP)
entry with a different incoming RPF neighbor: a) setting of
the SPT-bit on (S,G) entry, and b) sending a Join/Prune
message towards the RP with S,RP-bit in the prune list if
the iif of (S,G) is different from the iif of (*,G) or
(*,*,RP).

4 When a Join/Prune message is received for a group G, the
prune list is checked. If it contains a source for which
the receiving router has a corresponding active (S,G),
(*,G) or (*,*,RP) entry, and whose iif is that on which
the Join/Prune was received, then a join for (S,G), (*,G)
or (*,*,RP) is triggered to override the prune,
respectively. (This is necessary in the case of parallel
downstream routers connected to a multi-access network.)

5 When the RP fails, the RP will not be included in the RP-
Set messages sent to the receivers' last-hop routers. This
triggers the last-hop routers to send (*,G) joins towards
the new RP for the group, as determined by the RP-Set and
the hash function [*]

_________________________
[*] PIM Multicast Border Routers (PMBRs), handling in-
teroperability functionality, trigger (*,*,RP) joins
towards each RP in the RP-Set.

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We do not trigger prunes onto interfaces for SM groups based on
data packets. Data packets that arrive on the wrong incoming
interface for an SM group are silently dropped.

It is possible that a Join/Prune message constructed according
to the preceeding rules could exceed the MTU of a network. In
this case, the message can undergo semantic fragmentation
whereby information corresponding to different groups can be
sent in different messages. However, if a Join/Prune message
must be fragmented the following rule must be followed:

1 The complete prune list corresponding to a group G must be
included in the same Join/Prune message as the associated
RP-tree Join for G.

3.2.2 Receiving Join/Prune Messages When a router receives a
Join/Prune message, it processes it as follows:

1 The receiver of the Join/Prune notes the interface on which
the PIM message arrived, call it I. The router accepts this
Join/Prune message if this Join/Prune message is addressed
to the router itself. If the Join/Prune is for this router
the following actions are taken:

1 If an address Sj in the join list has RP-bit and WC-
bit set, then Sj is the RP address used by the
downstream router and the following actions are taken:

1 If Sj is not the same as the receiving router's
RP mapping for G, the receiving router may ignore
that group entry in the Join/Prune message. If
the router does not have any RP-Set information,
it may use the address Sj included in the
Join/Prune message as the RP for the group.

2 If Sj is the same as the receiving router's RP
mapping for G, it adds I to the outgoing
interface list of the (*,G) forwarding entry and

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sets the timer for that interface (if there is no
(*,G) entry, the router creates one first). If a
(*,*,RP) exists, for the RP associated with G,
then the oif list of the newly created (*,G) is
copied from that (*,*,RP) state, excluding
iif(*,G),

3 For each (Si,G) entry associated with group G, if
Si is not included in the prune list, and if I is
not the iif then interface I is added to the
oif list and the timers are restarted for that
interface in each affected entry. If the G in the
join message is `*' [*] , then every (*,G) and
(S,G) entry, whose group address hashes to the RP
indicated in the (*,*,RP) join message, is
updated accordingly,

4 If the (Si,G) entry is an RP-bit entry and its
oif list is the same as (*,G) oif list,
then the (Si,G,RPbit) entry is deleted,

5 The incoming interface is set to the interface
used to send unicast packets to the RP in the
(*,G) forwarding entry, i.e., RPF interface to
the RP.

2 For each address Si in the join list whose RP-bit and
WC-bit are not set, and for which there is no
existing (Si,G) forwarding entry, the router initiates
one.
[*]
_________________________
[*] A `*' in the group field of the Join/Prune is
represented by a group address 224.0.0.0 and a group
mask length of 4, indicating a (*,*,RP) Join.
[*] The router creates a (S,G) entry and copies all
outgoing interfaces, excluding iif(S,G), from the
(S,G)RP-bit, (*,G), or (*,*,RP), entry, if it exists.
If a router does not copy all outgoing interfaces from
the (*,G), or (*,*,RP) entry, all receivers on RP-tree,
downstream from outgoing interfaces other than the one

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1 The outgoing interface for (Si,G) is set to I.
The incoming interface for (Si,G) is set to the
interface used to send unicast packets to Si
(i.e., the RPF neighbor).

2 If the interface used to reach Si is the same as
the outgoing interface being built, I, this
represents an error and the Join/Prune should not
be processed.

3 For any Si included in the join list of the Join/Prune
message, for which there is an existing (Si,G)
forwarding entry,

1 If the RP-bit is not set for Si listed in the
Join/Prune message, but the RP-bit is set on the
existing (Si,G) entry, the router clears the RP-
bit on (Si,G) entry, sets the incoming interface
to point towards Si for that (Si,G) entry, and
sends a Join/Prune to the new incoming interface;
and

2 The router adds I to the list of outgoing
interfaces if I is not the same as the existing
incoming interface; the timer for I is restarted.

3 The (Si,G) SPT bit is initialized to be cleared
until data comes down the shortest path tree.

4 For each address Si in the prune list, with the RP-bit
is either set or cleared, and the WC-bit cleared:
_________________________
newly added to (S,G), will not receive packets from
source S. Data packets of S arriving from the RP will
match the (S,G) entry instead of (*,G), or (*,*,RP),
entry, and will be dropped because the incoming inter-
face is incorrect.

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1 If there is an existing (Si,G) forwarding entry,
the router schedules a deletion of I from the
list of outgoing interfaces by lowering that oif
timer to 5 seconds (unless it is already lower).
The deletion is not executed until this timer
expires, allowing for other downstream routers on
a multi-access LAN to override the prune.

2 If the router has a current (*,G), or (*,*,RP),
forwarding entry, and if a (Si,G)RP-bit entry
also exists then the (Si,G)RP-bit entry is
maintained even if its outgoing interface list is
null.

5 For any Si in the prune list that has the RP-bit set,
and the WC-bit cleared:

1 If (*,G), or corresponding (*,*,RP), state
exists, but there is no (Si,G) entry, an
(Si,G)RP-bit entry is created . The outgoing
interface list is copied from the (*,G), or
(*,*,RP), entry, with the interface, I, on which
the prune was received deleted. Packets from the
pruned source, Si, match on this state and are
not forwarded toward the pruned receivers.

2 If there exists a (Si,G) entry, with or without
the RPbit set, the iif on which the prune was
received, I, is deleted from the oif list,
and the entry timer is restarted.

6 For each address Si in the prune list, with the RP-bit
and the WC-bit set:

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1 If there is an existing (*,G) entry, with Si as
the RP for G, the router schedules a deletion of
I from the list of outgoing interfaces by
lowering that oif timer to 5 seconds (unless it
is already lower). The deletion is not executed
until this timer expires, allowing for other
downstream routers on a multi-access LAN to
override the prune.

2 If the corresponding (*,*,RP) state exists, but
there is no (*,G) entry, a (*,G) entry is
created. The outgoing interface list is copied
from (*,*,RP) entry, with the interface, I, on
which the prune was received, deleted.

3 If there exists a (*,G) entry, the interface on
which the prune was received, I, is deleted from
the oif list, and the entry timer is
restarted.

2 If the received Join/Prune does not indicate the router as
its target, then if the Join/Prune is for a (S,G) pair for
which the router has an active (S,G) entry, and if the
Join/Prune arrived on the iif for that entry, then the
router compares the IP address of the generator of the
Join/Prune, to its own IP address.

1 If its own IP address is higher, the Joiner-bit in the
(S,G) entry is set.

2 If its own IP address is lower, the Joiner-bit in the
(S,G) entry is cleared, and the Joiner-bit timer is
activated.

After the timer expires the Joiner-bit is set indicating
further periodic Join/Prunes should be sent for this entry.
The Joiner-bit timer is restarted each time a Join/Prune
message is received from a higher-IP-addressed PIM

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neighbor.

For any new (S,G), (*,G) or (*,*,RP) entry created by an
incoming Join/Prune message, the Joiner-bit is set and the
SPT-bit is cleared.

3.3 Register and Register-Stop

When a source first starts sending to a group its packets are
encapsulated in Register messages and sent to the RP. If the
data rate warrants source-specific paths, the RP sets up source
specific state and starts sending (S,G) Join/Prune messages
toward the source.

3.3.1 Sending Registers and Receiving Register-Stops

1 When a DR receives a packet from a directly connected
source, S [*] :

1 If there is no corresponding (S,G) entry, and the
router has RP-Set information, the DR creates one with
the Register-bit set to 1 and the RP address set
according to the hash function mapping for the
corresponding group. The Register-bit-timer is
initialized to zero; the Register-bit-timer is non-
zero only when the Register-bit is set to 0.

2 If there is a (S,G) entry in existence, the DR simply
restarts the corresponding S-timer (entry timer).
_________________________
[*] When a border router (e.g., a router that connects
the PIM-SM region to a dense mode region running DVMRP
or PIM-DM) receives a packet from a source in the dense
mode region, the router treats the packet as if it were
from a directly connected source. See the Appendix on
Interoperability for more details.

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2 If the new or previously-existing (S,G) entry has the
Register-bit set, the data packet is encapsulated in a
Register message and unicast to the RP for that group. The
data packet is also forwarded according to (S,G) state in
the DR if the oif list is not null; since a receiver may
join the SP-tree while the DR is still registering to the
RP.

3 If the (S,G) entry has the Register-bit cleared, the data
packet is not sent in a Register message, it is just
forwarded according to the (S,G) oif list.

The DR processes Register-Stop messages as follows:

1 The DR clears the Register-bit and restarts the Register-
bit-timer in the corresponding (S,G) entry(ies).

When a Register-bit-timer expires, the corresponding entry(ies)
Register-bit is set to 1 to reinstigate encapsulation of data
packets in Register messages.

3.3.2 Receiving Register Messages and Sending Register-Stops

When a router (i.e., the RP) receives a Register message, the
router does the following:

1 Decapsulates the data packet, and checks for a
corresponding (S,G) entry.

1 If a (S,G) entry exists, the packet is forwarded but
the SPT bit is left cleared (0). If the SPT bit is 1,
the packet is dropped, and Register-Stop messages are
triggered. Register-Stops are rate limited. [*]
_________________________
[*] Register-Stops should be rate limited so that no
more than a few are sent per round trip time. This

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2 If there is no (S,G) entry, but there is a (*,G)
entry, or a (*,*,RP) entry with the RP corresponding
to G, the packet is forwarded according to that entry.

3 If there is a (*,*,RP) entry but no (*,G) entry, a
(*,G) or (S,G) entry is created and the oif is copied
from the (*,*,RP) entry to the new entry.

4 If there is no G or (*,*,RP) entry corresponding to G,
the packet is dropped, and a Register-Stop is
triggered.

5 A ``Border bit'' bit is added to the Register message,
to facilitate interoperability mechanisms. PIM MBRs
set this bit when registering for external sources
(see Sections 2.10 and 6). If the ``Border bit'' is
set in the Register, the RP does the following:

1 If there is no matching (S,G) state, the RP
creates one, with a `PMBR' field. This field
holds the source of the Register (i.e. the outer
IP address of the register packet). The RP
triggers a (S,G) join towards the source of the
data packet, and clears the SPT bit for the (S,G)
entry, else

2 If the `PMBR' field for the corresponding (S,G)
entry matches the source of the Register packet,
the decapsulated packet is forwarded to the oif
list of that entry, else

3 The packet is dropped, and a Register-stop is
triggered towards the source of the Register.

_________________________
prevents a high datarate stream of packets from
triggering a large number of Register-stop messages
between the time that the first packet is received and
the time when the source receives the first Register-
Stop.

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The (S,G) state timer is restarted by Registers arriving
from that source to that group.

2 If the matching (S,G) or (*,G) state contains a null oif
list, the RP unicasts a Register-Stop message to the source
of the Register message; in the latter case, the source-
address field, within the Register-Stop message, is set to
the wildcard value (all 0's). This message is not processed
by intermediate routers, hence no (S,G) state is
constructed between the RP and the source.

3 If the Register message arrival rate warrants it and there
is no existing (S,G) entry, the RP sets up a (S,G)
forwarding entry with the outgoing interface list,
excluding iif(S,G), copied from the (*,G) outgoing
interface list, its SPT-bit is initialized to 0. If a (*,G)
entry does not exist, but there exists a (*,*,RP) entry
with the RP corresponding to G , the oif list for (S,G) is
copied -excluding the iif- from that (*,*,RP) entry.

A timer is set for the (S,G) entry and this timer is
restarted by receipt of data packets for (S,G). The (S,G)
entry causes the RP to send a Join/Prune message for the
indicated group towards the source of the register message.

If the (S,G) oif list becomes null, Join/Prune messages
will not be sent towards the source, S.

3.4 Multicast Data Packet Forwarding

Processing a multicast data packet involves the following steps:

1 Lookup forwarding state based on a longest match
[*]

of the source address, and an exact match of the
destination address in the data packet and compare the RPF
check on the source address in the packet header with the
_________________________
[*] The longest match is performed in the following
order: (1) (S,G), (2) (*,G). If neither is matched,
then a lookup is performed on (*,*,RP) entries.

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iif specified in the forwarding entry.

2 If the packet arrived on the interface found in the
matching-entry's iif field, and the oif list is not
null:

1 Forward the packet to the oif list for that entry
and restarted the entry's timer if the matching entry
is (S,G) [*]

2 If the entry is a (S,G) entry with a cleared SPT-bit,
and a (*,G) or associated (*,*,RP) also exists whose
incoming interface is different than that for (S,G),
set the SPT-bit for the (S,G) entry and trigger an
(S,G) RP-bit prune towards the RP.

3 If the source of the packet is a directly-connected
host and the router is the DR on a multi-access
network, check the Register-bit associated with the
(S,G) entry. If it is set, then the router
encapsulates the data packet in a register message and
sends it to the RP.

This covers the common case of a packet arriving on the RPF
interface to the source or RP and being forwarded to all
joined branches. It also detects when packets arrive on the
SP-tree, and triggers their pruning from the RP-tree. If it
is the DR for the source, it sends data packets
encapsulated in Registers to the RPs.

3 If the packet matches to an entry but did not arrive on the
interface found in the entry's iif field, check the
SPT-bit of the entry. If the SPT-bit is set, drop the
packet. If the SPT-bit is cleared, then lookup the (*,G),
or (*,*,RP), entry for G. If the packet arrived on the
_________________________
[*] Optionally, the (S,G) timer may be restarted by
periodic checking of the matching packet count.

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iif found in (*,G), or the corresponding (*,*,RP),
forward the packet to the oif list of the matching
entry. This covers the case when a data packet matches on a
(S,G) entry for which the SP-tree has not yet been
completely established upstream.

4 If the packet does not match to any entry, but the source
of the data packet is a local, directly-connected host, and
the router is the DR on a multi-access LAN and has RP-Set
information, the DR uses the hash function to determine the
RP associated with the destination group, G. The DR then
checks the Register-bit associated with the local sender
(if there is no such a Register-bit, a new register flag,
associated with the local sender, is created and set), and
encapsulates the data packet in a Register message and
unicasts it to the RP.

5 If the packet does not match to any entry, and it is not a
local host or the router is not the DR, drop the packet.

3.4.1 Data triggered switch to shortest path tree (SP-tree)

Different criteria can be applied to trigger switching over from
the RP-based shared tree to source-specific, shortest path
trees.

One proposed example is to do so based on data rate. For
example, when a (*,G), or corresponding (*,*,RP), entry is
created, a data rate counter may be initiated at the last-hop
routers. The counter is incremented with every data packet
received for directly connected members of an SM group, if the
longest match is (*,G) or (*,*,RP). If and when the data rate
for the group exceeds a certain configured threshold (t1), the
router initiates `source-specific' data rate counters for the
following data packets. Then, each counter for a source, is
incremented when packets matching on (*,G), or (*,*,RP), are
received from that source. If the data rate from the particular
source exceeds a configured threshold (t2), a (S,G) entry is
created and a Join/Prune message is sent towards the source. If
the RPF interface for (S,G) is
not the same as that for (*,G) -or (*,*,RP), then the SPT-bit
is cleared in the (S,G) entry.

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Other configured rules may be enforced to cause or prevent
establishment of (S,G) state.

3.5 Assert

Asserts are used to resolve which of the parallel routers
connected to a multi-access LAN is responsible for forwarding
packets onto the LAN.

3.5.1 Sending Asserts

The following Assert rules are provided when a multicast packet
is received on an outgoing multi-access interface of an existing
(S,G) entry:

1 Do unicast routing table lookup on source IP address from
data packet, and send assert on interface for source IP
address in data packet; include metric preference of
routing protocol and metric from routing table lookup.

2 If route is not found, use metric preference of 0x7fffffff
and metric 0xffffffff.

3 When an assert is sent for a (*,G) entry, the first bit in
the metric preference (the RP-bit) is set to 1, indicating
the data packet is routed down the RP-tree.

Asserts are rate-limited by the router.

3.5.2 Receiving Asserts

When an assert is received the router performs a longest match
on the source and group address in the assert message. The
router checks the first bit of the metric preference (RP-bit).
If the RP-bit is set, the router does a match on (*,G), or
(*,*,RP), entries, otherwise, the router matches (S,G) entries.
If the matching entry is (*,*,RP), the router creates a (*,G)
entry.

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If the interface that received the Assert message is in the
oif list of the matched entry, then this assert should be
processed by this router as follows:

1 Compare the metric received in the Assert with the one the
router would have advertised in an assert. The metric
preference should be treated as the high-order part of an
assert metric comparison. If the value in the assert is
less than the router's value, delete the interface from the
entry. If the value is the same, compare IP addresses, if
the routers address is less than the assert sender, delete
the interface.

2 If the router has won the election and there are directly
connected members on the multi-access LAN, the router keeps
the interface in its outgoing interface list. It acts as
the forwarder for the LAN.

3 If the router won the election but there are no directly
connected members on the multi-access LAN, the router
schedules to delete the interface. The LAN might be a stub
LAN with no members (and no downstream routers). If no
subsequent Join/Prunes are received, the router deletes the
interface from the outgoing interface list; otherwise it
keeps the interface in its outgoing interface and acts as
the forwarder for the LAN.

The winning router should send out an assert message including
its own metric to that outgoing interface. This will cause other
routers on the LAN to prune that interface from their forwarding
entries.

Note that when an Assert is received, the router performs an
exact match based on the source address, group address and the
RP-bit of the metric preference in the assert message. This is
not a longest match; only exact state will be matched. If there
is no such state, then the router drops the Assert message.
Otherwise, If the interface that received the Assert matches the
incoming interface of the exactly matched entry, then the Assert
message is processed as follows:

1 Downstream routers will select the upstream router with the

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smallest metric as their RPF neighbor. If two metrics are
the same, the highest IP address is chosen to break the
tie. [*]

2 If the downstream routers have downstream members, they
must schedule a join to inform the upstream router that
packets should be forwarded on the multi-access network.
This will cause the upstream forwarder to cancel its
scheduled deletion of the interface.

_________________________
[*] This is important so that downstream routers send
subsequent Joins/Prunes (in SM) to the correct neigh-
bor. An Assert timer is initiated when changing the RPF
neighbor to the Assert winner. When the timer expires
the router resets its RPF neighbor according to its un-
icast routing tables to capture failures of the Assert
winner.

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3.6 Candidate-RP-Advertisements and RP-Set messages

Candidate-RP-Advertisements (C-RP-Advs) are periodic PIM
messages unicast by those routers that are configured as
Candidate-RPs (C-RPs).

RP-Set messages are periodic PIM messages originated by the
Bootstrap router (BSR) within a domain, and forwarded hop-by-hop
to distribute the current RP-set to all routers in that domain.
The RP-Set messages also support a simple mechanism by which the
Candidate BSR (C-BSR) with the highest BSR-priority and IP
address (referred to as the preferred BSR) is elected as the BSR
for the domain [*]

3.6.1 Sending Candidate-RP-Advertisements

C-RPs periodically unicast C-RP-Advs to the BSR for that domain.
The interval for sending these messages is subject to local
configuration at the C-RP. A recommended default value is 60
seconds.

Candidate-RP-Advertisements carry group address and group mask
fields. This enables the advertising router to limit the
advertisement to certain prefixes or scopes of groups. The
advertising router may enforce this scope acceptance when
receiving Registers or Join/Prune messages.

3.6.2 Receiving C-RP-Advs and Originating RP-Set

Upon receiving a C-RP-Adv, a router does the following:

1 If the router is not the elected BSR, it ignores the
message, else

2 The BSR adds the RP address to its local pool of candidate
RPs, according to the associated group prefix(es) in the
C-RP-Adv message [*] The BSR may override the prefix
indicated in a C-RP-Adv.

_________________________
[*] We recommend that each router configured as a C-RP
also be configured as a C-BSR.
[*] The BSR may apply a local policy to limit the
number of Candidate RPs included in the RP-Set message.

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The BSR keeps an RP-timer per RP in its local RP-set. The RP-
timer is initialized to three times the holdtime in the RP's C-
RP-Adv. When the timer expires, the corresponding RP is removed
from the RP-set. The RP-timer is restarted by the C-RP-Advs from
the corresponding RP.

The BSR also keeps an RP-Set timer to send RP-Set messages
periodically. In particular, when the RP-Set timer expires, the
BSR originates an RP-Set message on each of its interfaces. The
message is sent with a TTL of 1 to the `ALL-PIM-ROUTERS' group.
In steady state, the BSR originates RP-Set messages every 60
seconds. At startup, the RP-Set timer is initialized to 180
seconds, causing the first RP-Set message to be originated after
180 seconds, when/if the timer expires. For timer details see
Section 3.6.3. A DR unicasts an RP-Set message to new PIM
neighbors starting up, after receiving their Query messages.
(since after DR election the new neighbor may become the new
DR.)

The RP-Set message is subdivided into sets of group-prefix,RP-
Count,RP-addresses. The format of the RP-Set message allows
`semantic fragmentation', if the length of the original RP-Set
message exceeds the packet maximum boundaries (see Section 4).
However, we recommend against configuring a large number of
routers as C-RPs, to reduce the semantic fragmentation required.

3.6.3 Receiving and Forwarding RP-Set

Each router keeps an RP-Set timer, initialized to 180 seconds at
startup.

When a router receives RP-Set message sent to `ALL-PIM-ROUTERS'
group, it performs the following:

1 If the message was not sent by the RPF neighbor towards the
BSR address included, the message is dropped. Else

2 If the included BSR is not preferred over, and not equal
to, the currently active BSR:

1 If the RP-Set timer is not yet expired, or if the
receiving router is a C-BSR, then the RP-Set message
is dropped. Else

2 The RP-Set timer is expired and the receiving router

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not a C-BSR, so the receiving router stores the RP-Set
and BSR address found in the message. The RP-Set
message is then forwarded out all PIM interfaces,
excluding the one over which the message arrived, to
`ALL-PIM-ROUTERS' group, with a TTL of 1.

3 If the RP-Set message includes a BSR address that is
preferred over, or equal to, the currently active BSR, the
router resets its RP-Set timer to 180 seconds, and stores
the BSR address and RP-Set information. The RP-Set message
is then forwarded out all PIM interfaces, excluding the one
over which the message arrived, to `ALL-PIM-ROUTERS' group,
with a TTL of 1.

If the receiving router has no current RP set information and
the RP-set was unicast to it from a directly connected neighbor,
the router stores the information as its new RP-set. This covers
the startup condition when a newly booted router obtains the
RP-Set and BSR address from its DR.

When a router receives a new RP-Set it checks if each of the RPs
referred to by existing state (i.e., by (*,G), (*,*,RP), or
(S,G)RPbit entries) is in the new RP-Set. If an RP is not in the
new RP-set, that RP is considered unreachable and the hash
algorithm (see below) is re-performed for each group with
locally active state that previously hashed to that RP. This
will cause those groups to be distributed among the remaining
RPs. When the new RP-Set contains a new RP, the value of the new
RP is calculated for each group covered by that C-RP's Group-
prefix. Any group for which the new RP's value is greater than
the previously active RP's value is switched over to the new RP.

3.7 Hash Function

The hash function is used by all routers within a domain, to map
a group to one of the C-RPs from the RP-Set. For a particular
group, G, the hash function uses only those C-RPs whose Group-
prefix covers G. The algorithm takes as input the group address,
and the addresses of the Candidate RPs, and gives as output one
RP address to be used.

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The protocol requires that all routers hash to the same RP
within a domain (except for transients). The following hash
function must be used in each router:

1 For each candidate RP address Ci in the Candidate-RP-
Set, whose Group-prefix covers G, compute a value:
Value(G,M,Ci) =
1103515245 ((1103515245 (G&M)+12345) XOR Ci)+ 12345 mod 2^31
where M is a hash-mask included in RP-Set messages.
This hash-mask allows a small number of
consecutive groups (e.g., 4) to always hash to the same RP.
For instance, hierarchically-encoded data can be sent on
consecutive group addresses to get the same delay and
fate-sharing characteristics.

In standard C, this corresponds to:

srand(G & M);
srand(rand() ^ Ci);
value = rand();

2 The candidate with the highest resulting value is then
chosen as the RP for that group, and its identity and hash
value are stored with the entry created.

Ties between C-RPs having the same hash value, are broken
in advantage of the highest address.

The hash function algorithm is invoked by a DR, upon reception
of a packet, or IGMP Host-Membership-Report, for a group, for
which the DR has no entry. It is invoked by any router that has
(*,*,RP) state when a packet is received for which there is no
corresponding (S,G) or (*,G) entry. Furthermore, the hash
function is invoked by all routers upon receiving a Join/Prune
message with WC-bit set.

3.8 Processing Timer Events

{ Editors Note: Timers are also discussed individually in the
sections that pertain to the protocol messages that they

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trigger/affect. Until we finalize this section, if discrepencies
exist, then assume that the individual sections are
authoritative over this table.}

In this subsection, we enumerate all timers that have been
discussed or implied. Since some critical timer events are not
associated with the receipt or sending of messages, they are not
fully covered by earlier subsections.

In many cases, the values for timers come from Holdtime fields
in PIM control messages, in which case the default values used
in these Holdtime fields are shown in the tables below.
Otherwise, the default value used when setting the timer is
shown. In general, the default timeout value for state
information is three times the refresh period. For example,
Queries refresh Neighbor state and the default Query-timer
period is 30 seconds, so a default Neighbor-timer duration of 90
seconds is included in the Holdtime field of the Queries.

In this version of the spec we suggest particular numerical
timer settings. A future version of the specification will
specify a mechanism for timers to be set as a function of the
outgoing link bandwidth.

bsubsection*Timers related to tree maintenance

Each (S,G), (*,G), and (*,*,RP) entry has multiple timers
associated with it: one for each interface in the outgoing
interface list, one for the multicast routing entry itself, and
one for the Joiner-bit. Each (S,G) and (*,G) entry also has an
Assert timer and an Assert-rate-limit timer. In addition, DR's
have a Register-bit-timer for each (S,G) entry and every router
has a single Join/Prune timer.

Because some of the outgoing interfaces in an (S,G) entry are
copied from the (*,G) outgoing interface list, they may not have
explicit (S,G) join messages from some of the downstream routers
(i.e., where members are joining to the (*,G) tree only). Thus,
when a timer is reset for an outgoing interface listed in a
(*,G) entry, the timers are reset for that interface in each
existing (S,G) entry whose oif list contains that interface [*]
_________________________
[*] If there are sources in the prune list of the (*,G)
join, then the timers for the arriving interface will
first be reset for those sources, and then this inter-
face will be deleted from these same entries; producing
a correct result, even though the updating of the ti-
mers was unnecessary. An implementation could optimize

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The same rule applies to (*,G) and (S,G) entries when resetting
an oif timer on a (*,*,RP) entry.

_________________________
this by checking the prune list before processing the
join list.

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Timer DefVal Notes

Joiner-bit 90 Started : When Joiner bit is cleared
per route entry Reset by: Receiving Join from higher-IP neighbor on iif
Action : Set Joiner bit

Join/Prune 60 Started : When booting
Reset by: Nothing
Action : Send Join/Prune to each RPF neighbor, restart timer

oif 180 Started : When adding oif to oiflist
per (*,*,RP) oif Restarted by: Receiving (*,*,RP) Join on that iface
Action : Remove oif from oiflist

oif 180 Started : When adding oif to oiflist
per (*,G) oif Restarted by: Receiving (*,G) Join or IGMP
Host-Membership-Report for G on that iface, or
restartedting oif timer in (*,*,RP).
Action : Remove oif from oiflist

oif 180 Started : When adding oif to oiflist
per (S,G) oif Restarted by: Receiving (S,G) Join on that
iface, or restartedting oif timer in (*,G) or
(*,*,RP).
Action : Remove oif from oiflist

(*,*,RP) entry 180 Started : When entry is created
per (*,*,RP) Restarted by: Restartedting timer on any oif
Action : Delete entry

(*,G) entry 180 Started : When entry is created
per (*,G) Restarted by: Receiving (*,G) prune,
restarting timer on any oif, or receiving an
Assert with RP-bit set.
Action : Delete entry and any associated
(S,G)RP-bit entries

(S,G) entry 180 Started : When entry is created
aka S-timer Restarted by: Forwarding data packet,
per (S,G) receiving Register, receiving (S,G) RP-bit
prune, restarting timer on any oif,
or receiving an Assert without RP-bit set.
Action : Delete entry

Register-bit 60 Started : When Register bit is cleared by
per (S,G) receiving a Register-Stop
Restarted by: Receiving Register-Stop

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Action : Set Register bit

Assert 180 Started : Receiving an Assert where the
per (S,G) upstream RPF neighbor is not your unicast RPF
and (*,G) neighbor.
Restarted by: Receiving an Assert where the
upstream RPF neighbor is not your unicast
RPF neighbor.
Action : Change RPF neighbor to unicast RPF neighbor

Assert-Rate-limit 5 Started : When an Assert is sent
per (S,G) Restarted by: Nothing
and (*,G) Action : Allow asserts to be triggered by
data packets

*Timers relating to neighbor discovery

Timer DefVal Notes

Query 30 Started : When booting
Restarted by: Nothing
Action : Send Query on all ifaces, restart timer

Neighbor 90 Started : When receive first Query from neighbor
per neighbor Restarted by: When receive subsequent Queries
Action : Delete neighbor entry

*Timers relating to RP information

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Timer DefVal Notes

C-RP-Adv 60 Started : When booting if you're a Cand-RP
Restarted by: Nothing
Action : Send C-RP-Adv, restart C-RP-Adv timer

RP 180 Started : When adding an RP to the RP-Set if
per RP you are BSR
Restarted by: Receiving C-RP-Adv
Action : Remove RP from RP-Set

RP-Set 180/60 Started : Set to 180 when booting if
you're a C-BSR
Restarted by: Restarted to 180 when receive
RP-Set from preferred router if you're a C-BSR
Action : Send RP-Set and restart timer to 60 secs

3.9 Summary of flags used

Following is a summary of all the flags used in our scheme.

Bit Used in Definition

Border Register Register is coming from a PIM border router.
Joiner Route entry Periodic Join/Prunes should be sent for this entry.
Register (S,G) entry Encapsulate packets from directly connected
sources in Register messages unicast to the RP
for that group.
RP Route entry Entry represents state on the RP-tree.
RP Join/Prune Join is associated with the shared tree and therefore
the Join/Prune message is propagated along the RP-tree.
RP Assert The data packet was routed down the shared tree; thus,
the path indicated corresponds to the RP tree.
SPT (S,G) entry Packets have arrived on the iif towards S,
and the iif is different from the (*,G) iif.
WC Join Included address is an RP and the receiver expects to
receive packets from all sources via this (shared tree)
path. Thus, the Join/Prune applies to a (*,G) entry.
WC Route entry Wildcard entry; if there is no more specific match for
a particular source, packets will be forwarded according
to this entry.

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3.10 Security

{ Editors Note: this section is to be completed.}

All PIM control messages may use [5] to address security
concerns.

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4 Packet Formats

This section describes the details of the packet formats for PIM
control messages.

All PIM control messages have protocol number 103.

Basically, PIM messages are either unicast (e.g. Registers and
Register-Stop), or multicast hop-by-hop to `ALL-PIM-ROUTERS'
group `224.0.0.13' (e.g. Join/Prune, Asserts, etc.).

PIM Ver
PIM Version number is 2.

Type Types for specific PIM messages. PIM Types are:

0 = Query
1 = Register
2 = Register-Stop
3 = Join/Prune
4 = RP-Set
5 = Assert
6 = Graft (used in PIM-DM only)
7 = Graft-Ack (used in PIM-DM only)
8 = Candidate-RP-Advertisement

Addr length
Address length in bytes. Throughout this section this
would indicate the number of bytes in the Address field of
an address, including unicast and group addresses.

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Checksum
The checksum is the 16-bit one's complement of the one's
complement sum of the entire PIM message, (excluding the
data portion in the Register message). For computing the
checksum, the checksum field is zeroed.

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4.1 Encoded Source and Group Address formats

1 Unicast address: Only the address is included. The length
of the unicast address in bytes is specified in the `Addr
length' field in the header.

2 Encoded-Group-Address: Takes the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Mask Len | Group multicast Address ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...Group multicast Address ...|
+-+-+-+-+-+-+-+-+-+-+~+~+~+~+~+~+

Reserved
Transmitted as zero. Ignored upon receipt.

Mask Len
The Mask length is 8 bits. The value is the number of
contiguous bits left justified used as a mask which
describes the address. It is less than or equal to
Addr length * 8. If the message is sent for a single
group then the Mask length should equal Addr length *
8 (i.e. 32 for IPv4 and 128 for IPv6).

Group multicast Address
contains the group address, and has number of bytes
equal to that specified in the Addr length field.

3 Encoded-Source-Address: Takes the following format:

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rsrvd |S|W|R| Mask Len | Source Address ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+~+~+-+

Reserved
Transmitted as zero, ignored on receipt.

S,W,R See Section7 ef{Join_format} for details.

Mask Length
Mask length is 8 bits. The value is the number of
contiguous bits left justified used as a mask which
describes the address. The mask length must be less
than or equal to Addr Length * 8. If the message is
sent for a single source then the Mask length should
equal Addr length * 8. In version 2 of PIM, it is
strongly recommended that this field be set to 32 for
IPv4.

Source Address
The address length is indicated from the Addr length
field at the beginning of the header. For IPv4, the
address length is 4 octets.

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4.2 Query Message

It is sent periodically by routers on all interfaces.

PIM Version, Type, Addr length, Checksum
Described above.

Reserved
Transmitted as zero, ignored on receipt.

Holdtime
The amount of time a receiver should keep the neighbor
reachable, in seconds.

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4.3 Register Message

It is sent by the Designated Router (DR) to the RP when a
multicast packet needs to be transmitted on the RP-tree. Source
IP address is set to the address of the DR, destination IP
address is to the RP's address.

PIM Version, Type, Addr length, Checksum
Described above. { Note that the checksum for Registers
is done only on the PIM header, excluding the data packet
portion.}

B The Border bit. Set to zero by all DRs. Set to `1' by the
PIM Multicast Border Routers, when registering for external
sources.

Multicast data packet
The original packet sent by the source.

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4.4 Register-Stop Message

A Register-Stop is unicast from the RP to the sender of the
Register message. Source IP address is the address to which the
register was addressed. Destination IP address is the source
address of the register message.

PIM Version, Type, Addr length, Checksum
Described above.

Encoded-Group Address
Format described above. Note that for Register-Stops the
Mask Len field should contain Addr length * 8 (32 for
IPv4), if the message is sent for a single group.

Unicast-Source Address
IP host address of source from multicast data packet in
register. The length of this field in bytes is specified in
the Addr length field. A special wild card value (0.0.0.0),
can be used to indicate any source.

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4.5 Join/Prune Message

It is sent by routers towards upstream sources and RPs. A join
creates forwarding state and a prune destroys forwarding state.
Joins are sent to build shared trees (RP trees) or source trees
(SPT). Prunes are sent to prune source trees when members leave
groups as well as sources that do not use the shared tree.

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Addr length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-Upstream Neighbor Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Num groups | Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Multicast Group Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Joined Source Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Joined Source Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Pruned Source Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Pruned Source Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Multicast Group Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Joined Source Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Joined Source Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Pruned Source Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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| Encoded-Pruned Source Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PIM Version, Type, Addr length, Checksum
Described above.

Upstream Neighbor Address
The IP address of the RPF or upstream neighbor.

Reserved
Transmitted as zero, ignored on receipt.

Holdtime
The amount of time a receiver should keep the Join/Prune
state alive, in seconds.

Number of Groups
The number of multicast group sets contained in the
message.

Encoded-Multicast group address
For format description see Section
4.1. A wild card group in the (*,*,RP) join is represented
by a 224.0.0.0 in the group address field and `4' in the
mask length field. A (*,*,RP) join also has the WC-bit and
the RP-bit set.

Number of Joined Sources
Number of join source addresses listed for a given group.

Join Source Address-1 .. n
This list contains the sources that the sending router
will forward multicast datagrams for if received on the
interface this message is sent on.

See format section 4.1. The fields explanation for the
Encoded-Source-Address format follows:

Reserved
Described above.

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S The Sparse bit is a 1 bit value, set to 1 for PIM-SM.
It is used for PIM v.1 compatability.

W The WC bit is a 1 bit value. If 1, the join or prune
applies to the (*,G) or (*,*,RP) entry. If 0, the join
or prune applies to the (S,G) entry where S is Source
Address. Joins and prunes sent towards the RP should
have this bit set.

R The RP bit is a 1 bit value. If 1, the information
about (S,G) is sent towards the RP. If 0, the
information should be sent about (S,G) toward S, where
S is Source Address.

Mask Length, Source Address
Described above.

Represented in the form of
< WCbit >< RPbit >< Mask length>< Source address>:

A source address could be a host IP address :

< 0 >< 0 >< 32 >< 192.1.1.17 >

A source address could be the RP's IP address :

< 1 >< 1 >< 32 >< 131.108.13.111 >

A source address could be a subnet address to prune from
the RP-tree :

< 0 >< 1 >< 28 >< 192.1.1.16 >

A source address could be a general aggregate :

< 0 >< 0 >< 16 >< 192.1.0.0 >

Number of Pruned Sources
Number of prune source addresses listed for a group.

Prune Source Address-1 .. n
This list contains the sources that the sending router
does not want to forward multicast datagrams for when
received on the interface this message is sent on [*]
_________________________
[*] If the Join/Prune message boundary exceeds the max-

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4.6 RP-Set

The RP-Set messages are multicast to `ALL-PIM-ROUTERS' group,
out all interfaces having PIM neighbors (excluding the one over
which the message was received). RP-Set messages are sent with
TTL value of 1. RP-Set messages originate at the BSR, and are
forwarded by intermediate routers.

RP-Set message is divided up into `semantic fragments', if the
original message exceeds the maximum packet size boundaries.

The semantics of a single `fragment' is given below:

_________________________
imum packet size, then the join and prune lists for the
same group must be included in the same packet.

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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Addr length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Tag | Hash Mask len | BSR-priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-BSR-Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP-Count-1 | Frag RP-Cnt-1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-RP-Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-RP-Address-m |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP-Count-m | Frag RP-Cnt-m | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-RP-Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-RP-Address-m |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PIM Version, Type, Addr length, Checksum
Described above.

Fragment Tag

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A randomly generated number, acts to distinguish the
fragments belonging to different RP-Set messages; fragments
belonging to same RP-Set message carry the same `Fragment
Tag'.

Hash Mask len
The length (in bits) of the mask to use in the hash
function. For IPv4 we recommend a value of 30. For IPv6 we
recommend a value of 126.

BSR-priority
Contains the BSR priority value of the included BSR. This
field is considered as a high order byte when comparing BSR
addresses.

Unicast-BSR-Address
The IP address of the bootstrap router for the domain. The
length of this field in bytes is specified in Addr length.

Encoded-Group Address-1..n
The group prefix (address and mask) with which the
Candidate RPs are associated. Format previously described.

RP-Count-1..n
The number of Candidate RP addresses included in the whole
RP-Set message for the corresponding group prefix [*]

Frag RP-Cnt-1..m
The number of Candidate RP addresses included in this
fragment of the RP-Set message, for the corresponding group
prefix. The `Frag RP-Cnt' field facilitates parsing of the
RP-Set for a given group prefix, when carried over more
than one fragment.

Unicast-RP-address-1..m
The address of the Candidate RPs, for the corresponding
_________________________
[*] A router does not replace its old RP-Set for a
given group prefix until/unless it receives `RP-Count'
addresses for that prefix; the addresses could be car-
ried over several fragments. If only part of the RP-Set
for a given group prefix was received, the router dis-
cards it, without updating that specific group prefix's
RP-Set.

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group prefix. The length of this field in bytes is
specified in Addr length.

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4.7 Assert Message

The Assert message is sent when a multicast data packet is
received on an outgoing interface corresponding to the (S,G) or
(*,G) associated with the source.

PIM Version, Type, Addr length, Checksum
Described above.

Encoded-Group Address
The group address to which the data packet was addressed,
and which triggered the Assert. Format previously
described.

Unicast-Source Address
Source IP address from IP multicast datagram that
triggered the Assert packet to be sent. The length of this
field in bytes is specified in Addr length.

R RP bit is a 1 bit value. If the IP multicast datagram that
triggered the Assert packet is routed down the RP tree,
then the RP bit is 1; if the IP multicast datagram is
routed down the SPT, it is 0.

Metric Preference
Preference value assigned to the unicast routing protocol
that provided the route to Host address.

Metric The unicast routing table metric. The metric is in units
applicable to the unicast routing protocol used.

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4.8 Graft Message

Used in dense-mode. Refer to PIM dense mode specification.

4.9 Graft-Ack Message

Used in dense-mode. Refer to PIM dense mode specification.

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4.10 Candidate-RP-Advertisement

Candidate-RP-Advertisements are periodically unicast from the
C-RPs to the BSR.

PIM Version, Type, Addr length, Checksum
Described above.

Prefix-Cnt
The number of encoded group addresses included in the
message; indicating the group prefixes for which the C-RP
is advertising. A Prefix-Cnt of `0' implies a prefix of
224.0.0.0 with mask length of 4; i.e. all multicast groups.
If the C-RP is not configured with Group-prefix
information, the C-RP puts a default value of `0' in this
field.

Holdtime
The amount of time the advertisement is valid. This field
allows advertisements to be aged out.

Unicast-RP-Address
The address of the interface to advertise as a Candidate
RP. The length of this field in bytes is specified in Addr

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length.

Encoded-Group Address-1..n
The group prefixes for which the C-RP is advertising.
Format previously described.

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5 Appendix I: Changes and Updates to the Spec

This appendix populates the major changes in the specification
document as compared to `draft-ietf-idmr-pim-spec-01.ps,txt'.

5.1 Major Changes

List of changes since March '96 IETF:

1. (*,*,RP) Joins state and data forwarding check; replaces (*,G-
Prefix) Joins state for interoperability. (*,G) negative cache
introduced for the (*,*,RP) state supporting mechanisms.

2. Semantic fragmentation for the RP-Set message.

3. Appendix II on interoperability details with DVMRP in
preparation.

List of changes incurred since version 1 of the spec:

1. Proposal and refinement of bootstrap router (BSR) election
mechanisms

2. Introduction of hash functions for Group to RP mapping

3. New RP-liveness indication mechanisms based upon the the
Bootstrap Router (BSR) and the RP-Set messages.

4. Removal of reachability messages, RP reports and multiple RPs
per group.

5.2 Packet Format Changes

Packet Format incurred updates to accommodate different address
lengths, and address aggregation.

1 The `Addr length' field was added to the PIM fixed header

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to specify the address length in bytes of the underlying
protocol, see section 4.

2 The Encoded source and group address formats were
introduced, with the use of a `Mask length' field to allow
aggregation, section 4.1.

3 Packet formats are no longer IGMP messages; rather PIM
messages.

PIM message types and formats were also modified:

[{ Note: most changes were made to the May 95 version, unless
otherwise specified}].

1 Obsolete messages:

a. Register-Ack [Feb. 96]

b. Poll and Poll Response [Feb. 96]

c. RP-Reachability [Feb. 96]

d. RPlist-Mapping [Feb. 96]

2 New messages:

a. Candidate-RP-Advertisement [change made in October 95]

b. RP-Set [Feb. 96]

3 Modified messages:

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a. Join/Prune [Feb. 96]

b. Register [Feb. 96]

c. Register-Stop [Feb. 96]

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6 Appendix II: Interoperability with Dense Mode Protocols

{ Editors Note: This section is to be completed.}

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7 Acknowledgments

Tony Ballardie, Scott Brim, Jon Crowcroft, Bill Fenner, Paul
Francis, Joel Halpern, Horst Hodel, Polly Huang, Stephen
Ostrowski, and Lixia Zhang provided detailed comments on
previous drafts. The authors of [6] and membership of the IDMR
WG provided many of the motivating ideas for this work and
useful feedback on design details.

This work was supported by the National Science Foundation,
ARPA, cisco Systems and Sun Microsystems.

References

1. S.Deering, D.Estrin, D.Farinacci, V.Jacobson, C.Liu, L.Wei,
P.Sharma, and A.Helmy. Protocol independent multicast (pim) :
Motivation and architecture.
Internet Draft, May 1995.

2. S.Deering, D.Estrin, D.Farinacci, V.Jacobson, C.Liu, and L.Wei.
The pim architecture for wide-area multicast routing.
ACM Transactions on Networks, April 1996.

3. D.Estrin, D.Farinacci, V.Jacobson, C.Liu, L.Wei, P.Sharma, and
A.Helmy. Protocol independent multicast-dense mode (pim-dm) :
Protocol specification. Internet Draft, November 1995.

4. S.Deering. Host extensions for ip multicasting, aug 1989.
RFC1112.

5. R.Atkinson. Security architecture for the internet protocol,
August 1995. RFC-1825.

6. A.J. Ballardie, P.F. Francis, and J.Crowcroft. Core based trees.
In Proceedings of the ACM SIGCOMM, San Francisco, 1993.

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