One document matched: draft-ietf-idmr-pim-sm-spec-03.txt
Differences from draft-ietf-idmr-pim-sm-spec-02.txt
Network Working Group Steven Deering (XEROX)
Internet Draft Deborah Estrin (USC)
Dino Farinacci (CISCO)
Mark Handley (UCL)
Ahmed Helmy (USC)
Van Jacobson (LBL)
Chinggung Liu (USC)
Puneet Sharma (USC)
David Thaler (UMICH)
Liming Wei (CISCO)
draft-ietf-idmr-pim-sm-spec-03.txt June 6, 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 June 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.
Section 4 provides packet format details. Sections 3.8 and 3.9
summarize the timers and flags referred to throughout this document.
The most significant functional changes since the January '95 version
involve the Rendezvous Point-related mechanisms, several resulting
simplifications to the protocol, and 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 a
wildcard (any-source) forwarding state. for the group and sends
_________________________
[*] All routers mentioned in this document are assumed
to be PIM-SM capable, unless otherwise specified.
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messages on toward the RP. We use the term entry to refer to the
forwarding state maintained in a router to represent the distribution
tree. Each entry includes such things as the incoming interface from
which packets are accepted, the list of outgoing interfaces to which
packets are sent, timers, flag bits, etc. The wildcard forwarding
entry's incoming 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. The RPT-bit flag associated with this entry is also set to 1,
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 the WC-bit and RPT-bit
set to 1. The prune list is left empty. When the RPT-bit is set to 1
it indicates that the join is associated with the shared RP-tree and
therefore the Join/Prune message is propagated along the RP-tree.
When the WC-bit is set to 1 it indicates that the address is an RP
and the downstream receivers expect to receive packets from all
sources via this (shared tree) path; WC stands for wildcard
[*]
Each upstream router creates or updates its multicast forwarding
_________________________
[*] Note that the term RPT-bit is used to refer to both
the RPT-bit flags associated with forwarding entries,
and the RPT-bit included in each encoded address in a
Join/Prune message.
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entry for (*,G) when it receives a Join/Prune with the RPT-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,WC-bit,RPT-bit,
Prune=NULL.
2.3 Hosts sending to a group
When a host starts sending multicast data packets to a group,
initially its DR must deliver each packet to the RP for distribution
down the RP-tree (see figure 2). The sender's DR initially
encapsulates each data packet in a Register message and unicasts it
to the RP for that group. The RP decapsulates each Register message
and forwards the enclosed data packet natively to downstream members
on the shared RP-tree.
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
_________________________
[*] 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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and is receiving the data packets natively. Each source's DR
maintains, per (S,G), a Register-bit and a Register-bit timer. The
Register-bit timer is started by the Register-Stop message; upon
expiration, the Register-bit is set to 1 and the source's DR resumes
sending data packets encapsulated in Register messages.
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
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 each last-hop
router that is responsible for initiating and maintaining an SP-tree.
[*]
_________________________
[*] In more complicated scenarios, other entries in the
router have to be considered. For details see Section 3.
[*] By last-hop router we mean the router that delivers
the packets to their ultimate end-system destination.
This is the router that monitors if there is group
membership and joins or prunes the appropriate distri-
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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 [*]
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 indicated incoming interface (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 RPT-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 forwarding entry's RPT-bit flag set), it deletes the
arriving interface from the (S,G) oif list. If the router has only
(*,G) state, it creates an entry with the RPT-bit flag set to 1. For
brevity we refer to an (S,G) entry that has the RPT-bit flag set to 1
_________________________
bution trees in response. In general the last-hop
router is the Desgnated Router (DR) for the LAN. Howev-
er, under various conditions described later, a paral-
lel router connected to the same LAN may take over as
the last-hop router in place of the DR.
[*] 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.
[*] As in DVMRP, each PIM multicast forwarding entry
has an associated incoming interface on which packets
are expected to arrive.
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as an (S,G)RPT-bit entry. This notational distinction is useful to
point out the different actions taken for (S,G) entries depending on
the setting of the RPT-bit flag. Note that a router can have no more
than one (S,G) entry for any particular S and G, at any particular
time; whether the RPT-bit flag is set or not. In other words, a
router never has both an (S,G) and an (S,G)RPT-bit entry for the same
S and G at the same time. The Join/Prune message payload contains
Multicast-Address=G, Join=NULL, Prune=S,RPT-bit.
A new receiver may join an existing RP-tree on which source-specific
prune state has been established (e.g., because downstream receivers
have switched to SP-trees). In this case the prune state must be
eradicated upstream of the new receiver to bring all sources' data
packets down to the new receiver. Therefore, when a (*,G) Join
arrives at a router that has any (Si,G)RPT-bit entries (i.e., entries
that cause the router to send source-specific prunes toward the RP),
these entries must be updated upstream of the router so as to bring
all sources' packets down to the new member. To accomplish this, each
router that receives a (*,G) Join/Prune message updates any existing
(S,G)RPT-bit entries. The router may also trigger a (*,G) join
upstream to cause the same updating of RPT-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)RPT-bit entries are left unchanged (i.e., the RPT-bit 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 PIM forwarding entry; the Join/Prune messages are
sent to the neighbor indicated in the iif field of the corresponding
entry. These messages are sent periodically to capture state,
topology, and membership changes. A Join/Prune 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 within a PIM domain collect
RP-Set messages. RP-Set messages are sent hop-by-hop within the
domain; the domain's bootstrap router (BSR) is responsible for
originating the RP-set messages. The BSR is elected dynamically
within each domain.
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[*]
Routers then use the set of RPs to get the proper Group to RP
mapping. Details are as follows:
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).
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.
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.
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.
_________________________
[*] A domain in this context is a contiguous set of
routers that all implement PIM and are configured to
operate within a common boundary defined by PIM Multi-
cast Border Routers (PMBRs). PMBRs connect each PIM
domain to the rest of the internet.
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2.7 Interoperation with dense mode protocols such as DVMRP
In order to interoperate with networks that run dense-mode,
broadcast and prune, protocols, such as DVMRP, all packets generated
within a PIM-SM region must be pulled down to that region's PIM
Multicast Border Routers (PMBRs) and injected (i.e., broadcast) into
the DVMRP network. [*]
To achieve this capability, a special entry type, referred to as
(*,*,RP), must be supported by all PIM routers. For this reason we
include details about (*,*,RP) entry handling in this general PIM
specification.
A data packet will match on a (*,*,RP) entry if there is no more
specific entry (such as (S,G) or (*,G)) and the destination group
address in the packet maps to the RP listed in the (*,*,RP) entry. In
this sense, a (*,*,RP) entry represents an aggregation of all the
groups supported by that RP. PMBRs initialize (*,*,RP) state for each
RP in the domain's RPset. The (*,*,RP) state causes the PMBRs to send
Join/Prune messages toward each of the active RPs in the domain. As a
result distribution trees are built that carry all data packets
originated within the PIM domain (and sent to the RPs) down to the
PMBRs.
All PIM routers must be capable of supporting (*,*,RP) state and
interpreting associated Join/Prune messages. We describe the handling
of (*,*,RP) entries and messages throughout this document. However,
detailed PIM Multicast Border Router functions will be specified in a
separate interoperability document.
2.8 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
_________________________
[*] A PMBR is a router that sits at the boundary of a
PIM-SM domain and interoperates with other types of
multicast routers such as those that run DVMRP. Gen-
erally a PMBR would speak both protocols and implement
interoperability functions not required by regular PIM
routers.
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(*,*,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, the router compares the interface on which the packet
arrived to the incoming interface field in the matched forwarding
entry. If the iif check 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:
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
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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.
2.9 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.
2.9.1 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.
2.9.2 Parallel paths to a source or the RP--Assert process
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
_________________________
[*] 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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subsequent Joins. Typically this is the same as the downstream
router's RPF (Reverse Path Forwarding) 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.
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 (RPT-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 it exists. Furthermore, the
RPT-bit is always cleared for metric preferences that refer to
SP-tree entries; 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
_________________________
[*] The RPF neighbor for a particular source (or RP) is
the next-hop router to which packets are forwarded en
route to that source (or RP); and therefore is con-
sidered a good path via which to accept packets from
that source.
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Internet Draft PIM-SM Specification June 1996
(*,*,RP) entry, a (*,G) entry is created, and asserts take place
as if the matching state were (*,G).
The DR may lose the (*,G) Assert process to another router on
the LAN 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 and removes the LAN from its (*,G) oif list. The
winning router becomes the last-hop router and is responsible
for sending (*,G) join messages to the RP. Asserts are rate
limited.
2.9.3 Join/Prune suppression
If a Join/Prune message arrives and matches on the incoming
interface for an existing (S,G), (*,G), or (*,*,RP) entry, and
the sender of the Join/Prune has a higher IP address than the
recipient of the message, the Joiner-bit in the recipient's
multicast routing table entry is cleared to suppress further
Join/Prune messages. A timer is set for the Joiner-bit; after it
expires the recipient sets the Joiner-bit to resume further
periodic Join/Prunes for this entry. The Joiner-bit timer is
restarted each time a Join/Prune message is received from a
higher-IP-addressed PIM neighbor.
2.10 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
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upstream routers that this part of the distribution tree is
going away.
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
All PIM control messages may use [5] to address security
concerns. Security mechanisms are likely to be enhanced in the
near future.
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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. [*]
In addition, certain events cause triggered Join/Prune messages
to be sent.
3.2.1.1 Periodic Join/Prune Messages
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 to 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 toward the RP for an active (*,G) or (*,*,RP)
entry, and
_________________________
[*] 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 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.
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 toward S, and
2 There exists an active (S,G) entry with the RPT-bit
flag 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 toward S, and
2 There exists an active (S,G) entry with the RPT-bit
flag 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 RPT-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 RPT-bit flag set and null oif list, or
2 The Join/Prune message is being sent to the RPF
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neighbor toward the RP, there exists a (S,G) entry
with the RPT-bit flag 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.
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).
3.2.1.2 Triggered Join/Prune Messages
In addition to 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 RPT-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.
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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
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)RPT-bit entry
is null, the triggered Join/Prune message will contain
S in the prune list.
3 Receipt of a packet that matches 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) the router sets the SPT-bit on the (S,G)
entry, and b) if the iif of the (S,G) entry is different
from the iif of the local (*,G) or (*,*,RP) entries, the
router sends a Join/Prune message towards the RP with S and
a set RPT-bit in the prune list.
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-
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Set messages sent to all routers in that domain. This
triggers the DRs to send (*,G) Join/Prune messages towards
the new RP for the group, as determined by the RP-Set and
the hash function [*]
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.
3.2.1.3 Fragmentation
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 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.
The receiver of the Join/Prune notes the interface on which the
PIM message arrived, call it I. The receiver then checks to see
if the Join/Prune message was addressed to the receiving router
itself (i.e., the router's address appears in the Unicast
Upstream Neighbor Router field of the the Join/Prune message)
[*] If the Join/Prune is for this router the following actions
are taken.
For each Sj in the join list of the Join/Prune message:
1 If an address, Sj, in the join list of the Join/Prune
messagehas the RPT-bit and WC-bit set, then Sj is the RP
address used by the downstream router(s) 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 the
_________________________
[*] As described earlier, PMBRs trigger (*,*,RP) joins
towards each RP in the RP-Set.
[*] If the router is connected to a multiaccess LAN,
the message could be intended for a different router.
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Join/Prune message with respect to that group entry.
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, the receiving router adds I to the outgoing
interface list of the (*,G) forwarding entry and sets
the timer for that interface (if there is no (*,G)
entry, the router creates one first). If a (*,*,RP)
entry exists, for the RP associated with G, then the
oif list of the newly created (*,G) entry is copied
from that (*,*,RP) entry.
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 group address in the Join/Prune
message is `*' then every (*,G) and (S,G) entry, whose
group address hashes to the RP indicated in the
(*,*,RP) Join/Prune message, is updated accordingly
[*]
4 If the (Si,G) entry has its RPT-bit flag set to 1, and
its oif list is the same as the (*,G) oif
list, then the (Si,G)RPT-bit 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 toward the RP.
2 For each address, Sj, in the join list whose RPT-bit and
WC-bit are not set, and for which there is no existing
(Sj,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
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1 The outgoing interface for (Sj,G) is set to I. The
incoming interface for (Sj,G) is set to the interface
used to send unicast packets to Sj (i.e., the RPF
neighbor).
2 If the interface, I, used to reach Sj, is the same as
the outgoing interface being initialized, this
represents an error (or a unicast routing change) and
the Join/Prune should not be processed.
3 For each address, Sj, in the join list of the Join/Prune
message, for which there is an existing (Sj,G) forwarding
entry,
1 If the RPT-bit is not set for Sj listed in the
Join/Prune message, but the RPT-bit flag is set on the
existing (Sj,G) entry, the router clears the RPT-bit
flag on the (Sj,G) entry, sets the incoming interface
to point towards Sj for that (Sj,G) entry, and sends a
Join/Prune message corresponding to that entry through
the new incoming interface; and
2 If I is not the same as the existing incoming
interface, the router adds I to the list of outgoing
interfaces.
3 The timer for I is restarted.
4 The (Sj,G) entry's SPT bit is cleared until data comes
down the shortest path tree.
_________________________
outgoing interfaces from the (S,G)RPT-bit entry, if it
exists. If there is no (S,G) entry, the oif list is
copied from the (*,G) entry; and if there is no (*,G)
entry, the oif list is copied from the (*,*,RP) entry,
if it exists. In all cases, the iif of the (S,G) entry
is always excluded from the oif list.
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For each Sp in the prune list of the Join/Prune message:
1 For each address, Sp, in the prune list whose RPT-bit and
WC-bit are cleared:
1 If there is an existing (Sp,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 the existing (Sp,G) entry has
its RPT-bit flag set to 1, then this (Sp,G)RPT-bit
entry is maintained (not deleted) even if its outgoing
interface list is null.
2 For each address, Sp, in the prune list whose RPT-bit is
set and whose WC-bit cleared:
1 If there is an existing (Sp,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 the existing (Sp,G) entry has
its RPT-bit flag set to 1, then this (Sp,G)RPT-bit
entry is maintained (not deleted) even if its outgoing
interface list is null.
3 If (*,G), or corresponding (*,*,RP), state exists, but
there is no (Sp,G) entry, an (Sp,G)RPT-bit entry is
created . The outgoing interface list is copied from
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the (*,G), or (*,*,RP), entry, with the interface, I,
on which the prune was received, is deleted. Packets
from the pruned source, Sp, match on this state and
are not forwarded toward the pruned receivers.
4 If there exists a (Sp,G) entry, with or without the
RPT-bit set, the iif on which the prune was received,
I, is deleted from the oif list, and the entry
timer is restarted.
3 For each address, Sp, in the prune list whose RPT-bit and
WC-bit are both set:
1 If there is an existing (*,G) entry, with Sp 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.
For any new (S,G), (*,G) or (*,*,RP) entry created by an
incoming Join/Prune message, the Joiner-bit is initialized
to 1 and the SPT-bit is cleared.
If the received Join/Prune does not indicate the router as its
target, then if the Join/Prune matches an existing (S,G), (*,G),
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or (*,*,RP) entry and 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 and sets the
Joiner-bit as follows.
1 If its own IP address is higher, the Joiner-bit in the
entry is set.
2 If its own IP address is lower, the Joiner-bit in the 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 neighbor.
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, with S in the join list.
3.3.1 Sending Registers and Receiving Register-Stops
Register messages are sent as follows:
1 When a DR receives a packet from a directly connected
source, S [*] :
1 If there is no corresponding (S,G) entry, and the
_________________________
[*] When a PMBR (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. A separate document will
describe the details of interoperabiity.
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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).
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.
When the DR receives a Register-Stop message it 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.
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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. [*]
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. PMBRs set
this bit when registering for external sources (see
Section 2.7). 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
_________________________
[*] Register-Stops should be rate limited so that no
more than a few are sent per round trip time. This
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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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.
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:
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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. If neither S, nor G, find a
longest match entry, and the RP for the packet's
destination group address has a corresponding (*,*,RP)
entry, then the longest match does not require an exact
match on the destination group address. In summary, 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.
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) RPT-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
_________________________
[*] Optionally, the (S,G) timer may be restarted by
periodic checking of the matching packet count.
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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
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
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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.
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.
When an assert is sent for a (*,G) entry, the first bit in the
metric preference (the RPT-bit) is set to 1, indicating the data
packet is routed down the RP-tree.
Asserts are rate-limited by the router.
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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 (RPT-bit).
1 If the RPT-bit is set, the router first does a match on
(*,G), or (*,*,RP), entries; if no matching entry is found,
the router matches (S,G) entries.
2 If the RPT-bit is not set in the Assert, the router first
does a match on (S,G) entries; if no matching entry is
found, the router matches (*,G) or (*,*,RP) entries.
3.5.2.1 Receiving Asserts on an entry's outgoing interface
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 If the Assert's RPT-bit is set and the matching entry is
(*,*,RP), the router creates a (*,G) entry. If the Assert's
RPT-bit is cleared and the matching entry is (*,G), or
(*,*,RP), the router creates a (S,G)RPT-bit entry.
2 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.
3 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
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the forwarder for the LAN.
4 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.
3.5.2.2 Receiving Asserts on an entry's incoming interface
If the Assert arrived on the incoming interface of an existing
(S,G), (*,G), or (*,*,RP) entry, the Assert is processed as
follows. If the Assert message does not match the entry,
exactly, it is ignored; i.e, longest-match is not used in this
case. If the Assert message does match exactly, then:
1 Downstream routers will select the upstream router with the
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
_________________________
[*] 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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scheduled deletion of the interface.
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 [*] Sections 3.6.2 and 3.6.3 describe the
combined function of RP-Set messages as the vehicle for BSR
election and RP-Set distribution.
_________________________
[*] We recommend that each router configured as a C-RP
also be configured as a C-BSR.
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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.
The BSR keeps an RP-timer per RP in its local RP-set. The RP-
timer is initialized to 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 PIM 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.
_________________________
[*] The BSR may apply a local policy to limit the
number of Candidate RPs included in the RP-Set message.
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(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 has expired and the receiving router
is not a C-BSR, so the receiving router stores the
RP-Set and BSR address and priority found in the
message, and restarts the timer with its maximum
value. 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,
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with a TTL of 1.
4 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)RPT-bit 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.
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.
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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
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.
Timers may either count up or count down. If they count up then
expiration means that the timer has reached its configured
maximum value. If they count down then expiration means that the
timer has reached zero.
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
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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.
3.8.1 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 [*]
The same rule applies to (*,G) and (S,G) entries when resetting
an oif timer on a (*,*,RP) entry.
_________________________
[*] 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
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 RPT-bit set.
Action : Delete entry and any associated
(S,G)RPT-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)RPT-bit
prune, restarting timer on any oif,
or receiving an Assert without RPT-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
3.8.2 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
3.8.3 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 multicast 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.).
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 Section 4.5 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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
Multicast data packet
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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Internet Draft PIM-SM Specification June 1996
| 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 RPT-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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Internet Draft PIM-SM Specification June 1996
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 RPT-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 < WC-bit >< RPT-bit ><
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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Internet Draft PIM-SM Specification June 1996
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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Internet Draft PIM-SM Specification June 1996
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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Internet Draft PIM-SM Specification June 1996
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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Internet Draft PIM-SM Specification June 1996
group prefix. The length of this field in bytes is
specified in Addr length.
[Page 59]
Internet Draft PIM-SM Specification June 1996
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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 RPT-bit is a 1 bit value. If the IP multicast datagram
that triggered the Assert packet is routed down the RP
tree, then the RPT-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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Internet Draft PIM-SM Specification June 1996
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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Internet Draft PIM-SM Specification June 1996
4.10 Candidate-RP-Advertisement
Candidate-RP-Advertisements are periodically unicast from the
C-RPs to the BSR.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Cnt | Reserved | Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unicast-RP-Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address-1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoded-Group Address-n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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Internet Draft PIM-SM Specification June 1996
length.
Encoded-Group Address-1..n
The group prefixes for which the C-RP is advertising.
Format previously described.
[Page 63]
Internet Draft PIM-SM Specification June 1996
5 Appendix I: Major 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.
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
to specify the address length in bytes of the underlying
protocol, see section 4.
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Internet Draft PIM-SM Specification June 1996
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]
RP-Set [Feb. 96]
3 Modified messages:
(a) Join/Prune [Feb. 96]
(b) Register [Feb. 96]
(c) Register-Stop [Feb. 96]
[Page 65]
Internet Draft PIM-SM Specification June 1996
[Page 66]
Internet Draft PIM-SM Specification June 1996
6 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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