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RFC 4601 - Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)
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RFC4601 - Protocol Independent Multicast - Sparse Mode (PIM-SM)
Network Working Group B. Fenner
Request for Comments: 4601 AT&T Labs - Research
Obsoletes: 2362 M. Handley
Category: Standards Track UCL
H. Holbrook
Arastra
I. Kouvelas
Cisco
August 2006
Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document specifies Protocol Independent Multicast - Sparse Mode
(PIM-SM). PIM-SM is a multicast routing protocol that can use the
underlying unicast routing information base or a separate multicast-
capable routing information base. It builds unidirectional shared
trees rooted at a Rendezvous Point (RP) per group, and optionally
creates shortest-path trees per source.
This document obsoletes RFC 2362, an Experimental version of PIM-SM.
Table of Contents
1. Introduction ....................................................5
2. Terminology .....................................................5
2.1. Definitions ................................................5
2.2. Pseudocode Notation ........................................7
3. PIM-SM Protocol Overview ........................................7
3.1. Phase One: RP Tree .........................................8
3.2. Phase Two: Register-Stop ...................................8
3.3. Phase Three: Shortest-Path Tree ............................9
3.4. Source-Specific Joins .....................................10
3.5. Source-Specific Prunes ....................................11
3.6. Multi-Access Transit LANs .................................11
3.7. RP Discovery ..............................................12
4. Protocol Specification .........................................12
4.1. PIM Protocol State ........................................13
4.1.1. General Purpose State ..............................14
4.1.2. (*,*,RP) State .....................................15
4.1.3. (*,G) State ........................................16
4.1.4. (S,G) State ........................................17
4.1.5. (S,G,rpt) State ....................................20
4.1.6. State Summarization Macros .........................21
4.2. Data Packet Forwarding Rules ..............................26
4.2.1. Last-Hop Switchover to the SPT .....................28
4.2.2. Setting and Clearing the (S,G) SPTbit ..............29
4.3. Designated Routers (DR) and Hello Messages ................30
4.3.1. Sending Hello Messages .............................30
4.3.2. DR Election ........................................32
4.3.3. Reducing Prune Propagation Delay on LANs ...........34
4.3.4. Maintaining Secondary Address Lists ................37
4.4. PIM Register Messages .....................................38
4.4.1. Sending Register Messages from the DR ..............38
4.4.2. Receiving Register Messages at the RP ..............43
4.5. PIM Join/Prune Messages ...................................45
4.5.1. Receiving (*,*,RP) Join/Prune Messages .............45
4.5.2. Receiving (*,G) Join/Prune Messages ................49
4.5.3. Receiving (S,G) Join/Prune Messages ................53
4.5.4. Receiving (S,G,rpt) Join/Prune Messages ............56
4.5.5. Sending (*,*,RP) Join/Prune Messages ...............62
4.5.6. Sending (*,G) Join/Prune Messages ..................66
4.5.7. Sending (S,G) Join/Prune Messages ..................71
4.5.8. (S,G,rpt) Periodic Messages ........................76
4.5.9. State Machine for (S,G,rpt) Triggered Messages .....77
4.5.10. Background: (*,*,RP) and (S,G,rpt) Interaction ....82
4.6. PIM Assert Messages .......................................83
4.6.1. (S,G) Assert Message State Machine .................83
4.6.2. (*,G) Assert Message State Machine .................91
4.6.3. Assert Metrics .....................................98
4.6.4. AssertCancel Messages ..............................99
4.6.5. Assert State Macros ...............................100
4.7. PIM Bootstrap and RP Discovery ...........................103
4.7.1. Group-to-RP Mapping ...............................104
4.7.2. Hash Function .....................................105
4.8. Source-Specific Multicast ................................106
4.8.1. Protocol Modifications for SSM Destination
Addresses .........................................106
4.8.2. PIM-SSM-Only Routers ..............................107
4.9. PIM Packet Formats .......................................108
4.9.1. Encoded Source and Group Address Formats ..........110
4.9.2. Hello Message Format ..............................113
4.9.3. Register Message Format ...........................116
4.9.4. Register-Stop Message Format ......................119
4.9.5. Join/Prune Message Format .........................119
4.9.5.1. Group Set Source List Rules ..............122
4.9.5.2. Group Set Fragmentation ..................126
4.9.6. Assert Message Format .............................126
4.10. PIM Timers ..............................................128
4.11. Timer Values ............................................129
5. IANA Considerations ...........................................135
5.1. PIM Address Family .......................................135
5.2. PIM Hello Options ........................................136
6. Security Considerations .......................................136
6.1. Attacks Based on Forged Messages .........................136
6.1.1. Forged Link-Local Messages ........................136
6.1.2. Forged Unicast Messages ...........................137
6.2. Non-Cryptographic Authentication Mechanisms ..............137
6.3. Authentication Using IPsec ...............................138
6.3.1. Protecting Link-Local Multicast Messages ..........138
6.3.2. Protecting Unicast Messages .......................139
6.3.2.1. Register Messages ........................139
6.3.2.2. Register-Stop Messages ...................139
6.4. Denial-of-Service Attacks ................................140
7. Acknowledgements ..............................................140
8. Normative References ..........................................141
9. Informative References ........................................141
Appendix A. PIM Multicast Border Router Behavior .................143
A.1. Sources External to the PIM-SM Domain ....................143
A.2. Sources Internal to the PIM-SM Domain ...................144
Appendix B. Index ................................................146
List of Figures
Figure 1. Per-(S,G) register state machine at a DR ................38
Figure 2. Downstream per-interface (*,*,RP) state machine .........46
Figure 3. Downstream per-interface (*,G) state machine ............50
Figure 4. Downstream per-interface (S,G) state machine ............53
Figure 5. Downstream per-interface (S,G,rpt) state machine ........57
Figure 6. Upstream (*,*,RP) state machine .........................62
Figure 7. Upstream (*,G) state machine ............................67
Figure 8. Upstream (S,G) state machine ............................71
Figure 9. Upstream (S,G,rpt) state machine for triggered
messages ................................................77
Figure 10. Per-interface (S,G) Assert State machine ...............84
Figure 11. Per-interface (*,G) Assert State machine ...............92
1. Introduction
This document specifies a protocol for efficiently routing multicast
groups that may span wide-area (and inter-domain) internets. This
protocol is called Protocol Independent Multicast - Sparse Mode
(PIM-SM) because, although it may use the underlying unicast routing
to provide reverse-path information for multicast tree building, it
is not dependent on any particular unicast routing protocol.
PIM-SM version 2 was originally specified in RFC 2117 and was revised
in RFC 2362, both Experimental RFCs. This document is intended to
obsolete RFC 2362, to correct a number of deficiencies that have been
identified with the way PIM-SM was previously specified, and to bring
PIM-SM onto the IETF Standards Track. As far as possible, this
document specifies the same protocol as RFC 2362 and only diverges
from the behavior intended by RFC 2362 when the previously specified
behavior was clearly incorrect. Routers implemented according to the
specification in this document will be able to interoperate
successfully with routers implemented according to RFC 2362.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant PIM-SM implementations.
2.1. Definitions
The following terms have special significance for PIM-SM:
Rendezvous Point (RP):
An RP is a router that has been configured to be used as the
root of the non-source-specific distribution tree for a
multicast group. Join messages from receivers for a group are
sent towards the RP, and data from senders is sent to the RP so
that receivers can discover who the senders are and start to
receive traffic destined for the group.
Designated Router (DR):
A shared-media LAN like Ethernet may have multiple PIM-SM
routers connected to it. A single one of these routers, the
DR, will act on behalf of directly connected hosts with respect
to the PIM-SM protocol. A single DR is elected per interface
(LAN or otherwise) using a simple election process.
MRIB Multicast Routing Information Base. This is the multicast
topology table, which is typically derived from the unicast
routing table, or routing protocols such as Multiprotocol BGP
(MBGP) that carry multicast-specific topology information. In
PIM-SM, the MRIB is used to decide where to send Join/Prune
messages. A secondary function of the MRIB is to provide
routing metrics for destination addresses; these metrics are
used when sending and processing Assert messages.
RPF Neighbor
RPF stands for "Reverse Path Forwarding". The RPF Neighbor of
a router with respect to an address is the neighbor that the
MRIB indicates should be used to forward packets to that
address. In the case of a PIM-SM multicast group, the RPF
neighbor is the router that a Join message for that group would
be directed to, in the absence of modifying Assert state.
TIB Tree Information Base. This is the collection of state at a
PIM router that has been created by receiving PIM Join/Prune
messages, PIM Assert messages, and Internet Group Management
Protocol (IGMP) or Multicast Listener Discovery (MLD)
information from local hosts. It essentially stores the state
of all multicast distribution trees at that router.
MFIB Multicast Forwarding Information Base. The TIB holds all the
state that is necessary to forward multicast packets at a
router. However, although this specification defines
forwarding in terms of the TIB, to actually forward packets
using the TIB is very inefficient. Instead, a real router
implementation will normally build an efficient MFIB from the
TIB state to perform forwarding. How this is done is
implementation-specific and is not discussed in this document.
Upstream
Towards the root of the tree. The root of tree may be either
the source or the RP, depending on the context.
Downstream
Away from the root of the tree.
GenID Generation Identifier, used to detect reboots.
PMBR PIM Multicast Border Router, joining a PIM domain with another
multicast domain.
2.2. Pseudocode Notation
We use set notation in several places in this specification.
A (+) B is the union of two sets, A and B.
A (-) B is the elements of set A that are not in set B.
NULL is the empty set or list.
In addition, we use C-like syntax:
= denotes assignment of a variable.
== denotes a comparison for equality.
!= denotes a comparison for inequality.
Braces { and } are used for grouping.
3. PIM-SM Protocol Overview
This section provides an overview of PIM-SM behavior. It is intended
as an introduction to how PIM-SM works, and it is NOT definitive.
For the definitive specification, see Section 4.
PIM relies on an underlying topology-gathering protocol to populate a
routing table with routes. This routing table is called the
Multicast Routing Information Base (MRIB). The routes in this table
may be taken directly from the unicast routing table, or they may be
different and provided by a separate routing protocol such as MBGP
[10]. Regardless of how it is created, the primary role of the MRIB
in the PIM protocol is to provide the next-hop router along a
multicast-capable path to each destination subnet. The MRIB is used
to determine the next-hop neighbor to which any PIM Join/Prune
message is sent. Data flows along the reverse path of the Join
messages. Thus, in contrast to the unicast RIB, which specifies the
next hop that a data packet would take to get to some subnet, the
MRIB gives reverse-path information and indicates the path that a
multicast data packet would take from its origin subnet to the router
that has the MRIB.
Like all multicast routing protocols that implement the service model
from RFC 1112 [3], PIM-SM must be able to route data packets from
sources to receivers without either the sources or receivers knowing
a priori of the existence of the others. This is essentially done in
three phases, although as senders and receivers may come and go at
any time, all three phases may occur simultaneously.
3.1. Phase One: RP Tree
In phase one, a multicast receiver expresses its interest in
receiving traffic destined for a multicast group. Typically, it does
this using IGMP [2] or MLD [4], but other mechanisms might also serve
this purpose. One of the receiver's local routers is elected as the
Designated Router (DR) for that subnet. On receiving the receiver's
expression of interest, the DR then sends a PIM Join message towards
the RP for that multicast group. This Join message is known as a
(*,G) Join because it joins group G for all sources to that group.
The (*,G) Join travels hop-by-hop towards the RP for the group, and
in each router it passes through, multicast tree state for group G is
instantiated. Eventually, the (*,G) Join either reaches the RP or
reaches a router that already has (*,G) Join state for that group.
When many receivers join the group, their Join messages converge on
the RP and form a distribution tree for group G that is rooted at the
RP. This is known as the RP Tree (RPT), and is also known as the
shared tree because it is shared by all sources sending to that
group. Join messages are resent periodically so long as the receiver
remains in the group. When all receivers on a leaf-network leave the
group, the DR will send a PIM (*,G) Prune message towards the RP for
that multicast group. However, if the Prune message is not sent for
any reason, the state will eventually time out.
A multicast data sender just starts sending data destined for a
multicast group. The sender's local router (DR) takes those data
packets, unicast-encapsulates them, and sends them directly to the
RP. The RP receives these encapsulated data packets, decapsulates
them, and forwards them onto the shared tree. The packets then
follow the (*,G) multicast tree state in the routers on the RP Tree,
being replicated wherever the RP Tree branches, and eventually
reaching all the receivers for that multicast group. The process of
encapsulating data packets to the RP is called registering, and the
encapsulation packets are known as PIM Register packets.
At the end of phase one, multicast traffic is flowing encapsulated to
the RP, and then natively over the RP tree to the multicast
receivers.
3.2. Phase Two: Register-Stop
Register-encapsulation of data packets is inefficient for two
reasons:
o Encapsulation and decapsulation may be relatively expensive
operations for a router to perform, depending on whether or not the
router has appropriate hardware for these tasks.
o Traveling all the way to the RP, and then back down the shared tree
may result in the packets traveling a relatively long distance to
reach receivers that are close to the sender. For some
applications, this increased latency or bandwidth consumption is
undesirable.
Although Register-encapsulation may continue indefinitely, for these
reasons, the RP will normally choose to switch to native forwarding.
To do this, when the RP receives a register-encapsulated data packet
from source S on group G, it will normally initiate an (S,G) source-
specific Join towards S. This Join message travels hop-by-hop
towards S, instantiating (S,G) multicast tree state in the routers
along the path. (S,G) multicast tree state is used only to forward
packets for group G if those packets come from source S. Eventually
the Join message reaches S's subnet or a router that already has
(S,G) multicast tree state, and then packets from S start to flow
following the (S,G) tree state towards the RP. These data packets
may also reach routers with (*,G) state along the path towards the
RP; if they do, they can shortcut onto the RP tree at this point.
While the RP is in the process of joining the source-specific tree
for S, the data packets will continue being encapsulated to the RP.
When packets from S also start to arrive natively at the RP, the RP
will be receiving two copies of each of these packets. At this
point, the RP starts to discard the encapsulated copy of these
packets, and it sends a Register-Stop message back to S's DR to
prevent the DR from unnecessarily encapsulating the packets.
At the end of phase 2, traffic will be flowing natively from S along
a source-specific tree to the RP, and from there along the shared
tree to the receivers. Where the two trees intersect, traffic may
transfer from the source-specific tree to the RP tree and thus avoid
taking a long detour via the RP.
Note that a sender may start sending before or after a receiver joins
the group, and thus phase two may happen before the shared tree to
the receiver is built.
3.3. Phase Three: Shortest-Path Tree
Although having the RP join back towards the source removes the
encapsulation overhead, it does not completely optimize the
forwarding paths. For many receivers, the route via the RP may
involve a significant detour when compared with the shortest path
from the source to the receiver.
To obtain lower latencies or more efficient bandwidth utilization, a
router on the receiver's LAN, typically the DR, may optionally
initiate a transfer from the shared tree to a source-specific
shortest-path tree (SPT). To do this, it issues an (S,G) Join
towards S. This instantiates state in the routers along the path to
S. Eventually, this join either reaches S's subnet or reaches a
router that already has (S,G) state. When this happens, data packets
from S start to flow following the (S,G) state until they reach the
receiver.
At this point, the receiver (or a router upstream of the receiver)
will be receiving two copies of the data: one from the SPT and one
from the RPT. When the first traffic starts to arrive from the SPT,
the DR or upstream router starts to drop the packets for G from S
that arrive via the RP tree. In addition, it sends an (S,G) Prune
message towards the RP. This is known as an (S,G,rpt) Prune. The
Prune message travels hop-by-hop, instantiating state along the path
towards the RP indicating that traffic from S for G should NOT be
forwarded in this direction. The prune is propagated until it
reaches the RP or a router that still needs the traffic from S for
other receivers.
By now, the receiver will be receiving traffic from S along the
shortest-path tree between the receiver and S. In addition, the RP
is receiving the traffic from S, but this traffic is no longer
reaching the receiver along the RP tree. As far as the receiver is
concerned, this is the final distribution tree.
3.4. Source-Specific Joins
IGMPv3 permits a receiver to join a group and specify that it only
wants to receive traffic for a group if that traffic comes from a
particular source. If a receiver does this, and no other receiver on
the LAN requires all the traffic for the group, then the DR may omit
performing a (*,G) join to set up the shared tree, and instead issue
a source-specific (S,G) join only.
The range of multicast addresses from 232.0.0.0 to 232.255.255.255 is
currently set aside for source-specific multicast in IPv4. For
groups in this range, receivers should only issue source-specific
IGMPv3 joins. If a PIM router receives a non-source-specific join
for a group in this range, it should ignore it, as described in
Section 4.8.
3.5. Source-Specific Prunes
IGMPv3 also permits a receiver to join a group and to specify that it
only wants to receive traffic for a group if that traffic does not
come from a specific source or sources. In this case, the DR will
perform a (*,G) join as normal, but may combine this with an
(S,G,rpt) prune for each of the sources the receiver does not wish to
receive.
3.6. Multi-Access Transit LANs
The overview so far has concerned itself with point-to-point transit
links. However, using multi-access LANs such as Ethernet for transit
is not uncommon. This can cause complications for three reasons:
o Two or more routers on the LAN may issue (*,G) Joins to different
upstream routers on the LAN because they have inconsistent MRIB
entries regarding how to reach the RP. Both paths on the RP tree
will be set up, causing two copies of all the shared tree traffic
to appear on the LAN.
o Two or more routers on the LAN may issue (S,G) Joins to different
upstream routers on the LAN because they have inconsistent MRIB
entries regarding how to reach source S. Both paths on the source-
specific tree will be set up, causing two copies of all the traffic
from S to appear on the LAN.
o A router on the LAN may issue a (*,G) Join to one upstream router
on the LAN, and another router on the LAN may issue an (S,G) Join
to a different upstream router on the same LAN. Traffic from S may
reach the LAN over both the RPT and the SPT. If the receiver
behind the downstream (*,G) router doesn't issue an (S,G,rpt)
prune, then this condition would persist.
All of these problems are caused by there being more than one
upstream router with join state for the group or source-group pair.
PIM does not prevent such duplicate joins from occurring; instead,
when duplicate data packets appear on the LAN from different routers,
these routers notice this and then elect a single forwarder. This
election is performed using PIM Assert messages, which resolve the
problem in favor of the upstream router that has (S,G) state; or, if
neither or both router has (S,G) state, then the problem is resolved
in favor of the router with the best metric to the RP for RP trees,
or the best metric to the source to source-specific trees.
These Assert messages are also received by the downstream routers on
the LAN, and these cause subsequent Join messages to be sent to the
upstream router that won the Assert.
3.7. RP Discovery
PIM-SM routers need to know the address of the RP for each group for
which they have (*,G) state. This address is obtained automatically
(e.g., embedded-RP), through a bootstrap mechanism, or through static
configuration.
One dynamic way to do this is to use the Bootstrap Router (BSR)
mechanism [11]. One router in each PIM domain is elected the
Bootstrap Router through a simple election process. All the routers
in the domain that are configured to be candidates to be RPs
periodically unicast their candidacy to the BSR. From the
candidates, the BSR picks an RP-set, and periodically announces this
set in a Bootstrap message. Bootstrap messages are flooded hop-by-
hop throughout the domain until all routers in the domain know the
RP-Set.
To map a group to an RP, a router hashes the group address into the
RP-set using an order-preserving hash function (one that minimizes
changes if the RP-Set changes). The resulting RP is the one that it
uses as the RP for that group.
4. Protocol Specification
The specification of PIM-SM is broken into several parts:
o Section 4.1 details the protocol state stored.
o Section 4.2 specifies the data packet forwarding rules.
o Section 4.3 specifies Designated Router (DR) election and the rules
for sending and processing Hello messages.
o Section 4.4 specifies the PIM Register generation and processing
rules.
o Section 4.5 specifies the PIM Join/Prune generation and processing
rules.
o Section 4.6 specifies the PIM Assert generation and processing
rules.
o Section 4.7 specifies the RP discovery mechanisms.
o The subset of PIM required to support Source-Specific Multicast,
PIM-SSM, is described in Section 4.8.
o PIM packet formats are specified in Section 4.9.
o A summary of PIM-SM timers and their default values is given in
Section 4.10.
o Appendix A specifies the PIM Multicast Border Router behavior.
4.1. PIM Protocol State
This section specifies all the protocol state that a PIM
implementation should maintain in order to function correctly. We
term this state the Tree Information Base (TIB), as it holds the
state of all the multicast distribution trees at this router. In
this specification, we define PIM mechanisms in terms of the TIB.
However, only a very simple implementation would actually implement
packet forwarding operations in terms of this state. Most
implementations will use this state to build a multicast forwarding
table, which would then be updated when the relevant state in the TIB
changes.
Although we specify precisely the state to be kept, this does not
mean that an implementation of PIM-SM needs to hold the state in this
form. This is actually an abstract state definition, which is needed
in order to specify the router's behavior. A PIM-SM implementation
is free to hold whatever internal state it requires and will still be
conformant with this specification so long as it results in the same
externally visible protocol behavior as an abstract router that holds
the following state.
We divide TIB state into four sections:
(*,*,RP) state
State that maintains per-RP trees, for all groups served by a
given RP.
(*,G) state
State that maintains the RP tree for G.
(S,G) state
State that maintains a source-specific tree for source S and
group G.
(S,G,rpt) state
State that maintains source-specific information about source S
on the RP tree for G. For example, if a source is being
received on the source-specific tree, it will normally have been
pruned off the RP tree. This prune state is (S,G,rpt) state.
The state that should be kept is described below. Of course,
implementations will only maintain state when it is relevant to
forwarding operations; for example, the "NoInfo" state might be
assumed from the lack of other state information rather than being
held explicitly.
4.1.1. General Purpose State
A router holds the following non-group-specific state:
For each interface:
o Effective Override Interval
o Effective Propagation Delay
o Suppression state: One of {"Enable", "Disable"}
Neighbor State:
For each neighbor:
o Information from neighbor's Hello
o Neighbor's GenID.
o Neighbor Liveness Timer (NLT)
Designated Router (DR) State:
o Designated Router's IP Address
o DR's DR Priority
The Effective Override Interval, the Effective Propagation Delay and
the Interface suppression state are described in Section 4.3.3.
Designated Router state is described in Section 4.3.
4.1.2. (*,*,RP) State
For every RP, a router keeps the following state:
(*,*,RP) state:
For each interface:
PIM (*,*,RP) Join/Prune State:
o State: One of {"NoInfo" (NI), "Join" (J), "Prune-
Pending" (PP)}
o Prune-Pending Timer (PPT)
o Join/Prune Expiry Timer (ET)
Not interface specific:
Upstream (*,*,RP) Join/Prune State:
o State: One of {"NotJoined(*,*,RP)",
"Joined(*,*,RP)"}
o Upstream Join/Prune Timer (JT)
o Last RPF Neighbor towards RP that was used
PIM (*,*,RP) Join/Prune state is the result of receiving PIM (*,*,RP)
Join/Prune messages on this interface and is specified in Section
4.5.1.
The upstream (*,*,RP) Join/Prune State reflects the state of the
upstream (*,*,RP) state machine described in Section 4.5.5.
The upstream (*,*,RP) Join/Prune Timer is used to send out periodic
Join(*,*,RP) messages, and to override Prune(*,*,RP) messages from
peers on an upstream LAN interface.
The last RPF neighbor towards the RP is stored because if the MRIB
changes, then the RPF neighbor towards the RP may change. If it does
so, then we need to trigger a new Join(*,*,RP) to the new upstream
neighbor and a Prune(*,*,RP) to the old upstream neighbor.
Similarly, if a router detects through a changed GenID in a Hello
message that the upstream neighbor towards the RP has rebooted, then
it should re-instantiate state by sending a Join(*,*,RP). These
mechanisms are specified in Section 4.5.5.
4.1.3. (*,G) State
For every group G, a router keeps the following state:
(*,G) state:
For each interface:
Local Membership:
State: One of {"NoInfo", "Include"}
PIM (*,G) Join/Prune State:
o State: One of {"NoInfo" (NI), "Join" (J), "Prune-
Pending" (PP)}
o Prune-Pending Timer (PPT)
o Join/Prune Expiry Timer (ET)
(*,G) Assert Winner State
o State: One of {"NoInfo" (NI), "I lost Assert" (L),
"I won Assert" (W)}
o Assert Timer (AT)
o Assert winner's IP Address (AssertWinner)
o Assert winner's Assert Metric (AssertWinnerMetric)
Not interface specific:
Upstream (*,G) Join/Prune State:
o State: One of {"NotJoined(*,G)", "Joined(*,G)"}
o Upstream Join/Prune Timer (JT)
o Last RP Used
o Last RPF Neighbor towards RP that was used
Local membership is the result of the local membership mechanism
(such as IGMP or MLD) running on that interface. It need not be kept
if this router is not the DR on that interface unless this router won
a (*,G) assert on this interface for this group, although
implementations may optionally keep this state in case they become
the DR or assert winner. We recommend storing this information if
possible, as it reduces latency converging to stable operating
conditions after a failure causing a change of DR. This information
is used by the pim_include(*,G) macro described in Section 4.1.6.
PIM (*,G) Join/Prune state is the result of receiving PIM (*,G)
Join/Prune messages on this interface and is specified in Section
4.5.2. The state is used by the macros that calculate the outgoing
interface list in Section 4.1.6, and in the JoinDesired(*,G) macro
(defined in Section 4.5.6) that is used in deciding whether a
Join(*,G) should be sent upstream.
(*,G) Assert Winner state is the result of sending or receiving (*,G)
Assert messages on this interface. It is specified in Section 4.6.2.
The upstream (*,G) Join/Prune State reflects the state of the
upstream (*,G) state machine described in Section 4.5.6.
The upstream (*,G) Join/Prune Timer is used to send out periodic
Join(*,G) messages, and to override Prune(*,G) messages from peers on
an upstream LAN interface.
The last RP used must be stored because if the RP-Set changes
(Section 4.7), then state must be torn down and rebuilt for groups
whose RP changes.
The last RPF neighbor towards the RP is stored because if the MRIB
changes, then the RPF neighbor towards the RP may change. If it does
so, then we need to trigger a new Join(*,G) to the new upstream
neighbor and a Prune(*,G) to the old upstream neighbor. Similarly,
if a router detects through a changed GenID in a Hello message that
the upstream neighbor towards the RP has rebooted, then it should
re-instantiate state by sending a Join(*,G). These mechanisms are
specified in Section 4.5.6.
4.1.4. (S,G) State
For every source/group pair (S,G), a router keeps the following
state:
(S,G) state:
For each interface:
Local Membership:
State: One of {"NoInfo", "Include"}
PIM (S,G) Join/Prune State:
o State: One of {"NoInfo" (NI), "Join" (J), "Prune-
Pending" (PP)}
o Prune-Pending Timer (PPT)
o Join/Prune Expiry Timer (ET)
(S,G) Assert Winner State
o State: One of {"NoInfo" (NI), "I lost Assert" (L),
"I won Assert" (W)}
o Assert Timer (AT)
o Assert winner's IP Address (AssertWinner)
o Assert winner's Assert Metric (AssertWinnerMetric)
Not interface specific:
Upstream (S,G) Join/Prune State:
o State: One of {"NotJoined(S,G)", "Joined(S,G)"}
o Upstream (S,G) Join/Prune Timer (JT)
o Last RPF Neighbor towards S that was used
o SPTbit (indicates (S,G) state is active)
o (S,G) Keepalive Timer (KAT)
Additional (S,G) state at the DR:
o Register state: One of {"Join" (J), "Prune" (P),
"Join-Pending" (JP), "NoInfo" (NI)}
o Register-Stop timer
Additional (S,G) state at the RP:
o PMBR: the first PMBR to send a Register for this
source with the Border bit set.
Local membership is the result of the local source-specific
membership mechanism (such as IGMP version 3) running on that
interface and specifying that this particular source should be
included. As stored here, this state is the resulting state after
any IGMPv3 inconsistencies have been resolved. It need not be kept
if this router is not the DR on that interface unless this router won
a (S,G) assert on this interface for this group. However, we
recommend storing this information if possible, as it reduces latency
converging to stable operating conditions after a failure causing a
change of DR. This information is used by the pim_include(S,G) macro
described in Section 4.1.6.
PIM (S,G) Join/Prune state is the result of receiving PIM (S,G)
Join/Prune messages on this interface and is specified in Section
4.5.2. The state is used by the macros that calculate the outgoing
interface list in Section 4.1.6, and in the JoinDesired(S,G) macro
(defined in Section 4.5.7) that is used in deciding whether a
Join(S,G) should be sent upstream.
(S,G) Assert Winner state is the result of sending or receiving (S,G)
Assert messages on this interface. It is specified in Section 4.6.1.
The upstream (S,G) Join/Prune State reflects the state of the
upstream (S,G) state machine described in Section 4.5.7.
The upstream (S,G) Join/Prune Timer is used to send out periodic
Join(S,G) messages, and to override Prune(S,G) messages from peers on
an upstream LAN interface.
The last RPF neighbor towards S is stored because if the MRIB
changes, then the RPF neighbor towards S may change. If it does so,
then we need to trigger a new Join(S,G) to the new upstream neighbor
and a Prune(S,G) to the old upstream neighbor. Similarly, if the
router detects through a changed GenID in a Hello message that the
upstream neighbor towards S has rebooted, then it should re-
instantiate state by sending a Join(S,G). These mechanisms are
specified in Section 4.5.7.
The SPTbit is used to indicate whether forwarding is taking place on
the (S,G) Shortest Path Tree (SPT) or on the (*,G) tree. A router
can have (S,G) state and still be forwarding on (*,G) state during
the interval when the source-specific tree is being constructed.
When SPTbit is FALSE, only (*,G) forwarding state is used to forward
packets from S to G. When SPTbit is TRUE, both (*,G) and (S,G)
forwarding state are used.
The (S,G) Keepalive Timer is updated by data being forwarded using
this (S,G) forwarding state. It is used to keep (S,G) state alive in
the absence of explicit (S,G) Joins. Amongst other things, this is
necessary for the so-called "turnaround rules" -- when the RP uses
(S,G) joins to stop encapsulation, and then (S,G) prunes to prevent
traffic from unnecessarily reaching the RP.
On a DR, the (S,G) Register State is used to keep track of whether to
encapsulate data to the RP on the Register Tunnel; the (S,G)
Register-Stop timer tracks how long before encapsulation begins again
for a given (S,G).
On an RP, the PMBR value must be cleared when the Keepalive Timer
expires.
4.1.5. (S,G,rpt) State
For every source/group pair (S,G) for which a router also has (*,G)
state, it also keeps the following state:
(S,G,rpt) state:
For each interface:
Local Membership:
State: One of {"NoInfo", "Exclude"}
PIM (S,G,rpt) Join/Prune State:
o State: One of {"NoInfo", "Pruned", "Prune-
Pending"}
o Prune-Pending Timer (PPT)
o Join/Prune Expiry Timer (ET)
Not interface specific:
Upstream (S,G,rpt) Join/Prune State:
o State: One of {"RPTNotJoined(G)",
"NotPruned(S,G,rpt)", "Pruned(S,G,rpt)"}
o Override Timer (OT)
Local membership is the result of the local source-specific
membership mechanism (such as IGMPv3) running on that interface and
specifying that although there is (*,G) Include state, this
particular source should be excluded. As stored here, this state is
the resulting state after any IGMPv3 inconsistencies between LAN
members have been resolved. It need not be kept if this router is
not the DR on that interface unless this router won a (*,G) assert on
this interface for this group. However, we recommend storing this
information if possible, as it reduces latency converging to stable
operating conditions after a failure causing a change of DR. This
information is used by the pim_exclude(S,G) macro described in
Section 4.1.6.
PIM (S,G,rpt) Join/Prune state is the result of receiving PIM
(S,G,rpt) Join/Prune messages on this interface and is specified in
Section 4.5.4. The state is used by the macros that calculate the
outgoing interface list in Section 4.1.6, and in the rules for adding
Prune(S,G,rpt) messages to Join(*,G) messages specified in Section
4.5.8.
The upstream (S,G,rpt) Join/Prune state is used along with the
Override Timer to send the correct override messages in response to
Join/Prune messages sent by upstream peers on a LAN. This state and
behavior are specified in Section 4.5.9.
4.1.6. State Summarization Macros
Using this state, we define the following "macro" definitions, which
we will use in the descriptions of the state machines and pseudocode
in the following sections.
The most important macros are those that define the outgoing
interface list (or "olist") for the relevant state. An olist can be
"immediate" if it is built directly from the state of the relevant
type. For example, the immediate_olist(S,G) is the olist that would
be built if the router only had (S,G) state and no (*,G) or (S,G,rpt)
state. In contrast, the "inherited" olist inherits state from other
types. For example, the inherited_olist(S,G) is the olist that is
relevant for forwarding a packet from S to G using both source-
specific and group-specific state.
There is no immediate_olist(S,G,rpt) as (S,G,rpt) state is negative
state; it removes interfaces in the (*,G) olist from the olist that
is actually used to forward traffic. The inherited_olist(S,G,rpt) is
therefore the olist that would be used for a packet from S to G
forwarding on the RP tree. It is a strict subset of
(immediate_olist(*,*,RP) (+) immediate_olist(*,G)).
Generally speaking, the inherited olists are used for forwarding, and
the immediate_olists are used to make decisions about state
maintenance.
immediate_olist(*,*,RP) =
joins(*,*,RP)
immediate_olist(*,G) =
joins(*,G) (+) pim_include(*,G) (-) lost_assert(*,G)
immediate_olist(S,G) =
joins(S,G) (+) pim_include(S,G) (-) lost_assert(S,G)
inherited_olist(S,G,rpt) =
( joins(*,*,RP(G)) (+) joins(*,G) (-) prunes(S,G,rpt) )
(+) ( pim_include(*,G) (-) pim_exclude(S,G))
(-) ( lost_assert(*,G) (+) lost_assert(S,G,rpt) )
inherited_olist(S,G) =
inherited_olist(S,G,rpt) (+)
joins(S,G) (+) pim_include(S,G) (-) lost_assert(S,G)
The macros pim_include(*,G) and pim_include(S,G) indicate the
interfaces to which traffic might be forwarded because of hosts that
are local members on that interface. Note that normally only the DR
cares about local membership, but when an assert happens, the assert
winner takes over responsibility for forwarding traffic to local
members that have requested traffic on a group or source/group pair.
pim_include(*,G) =
{ all interfaces I such that:
( ( I_am_DR( I ) AND lost_assert(*,G,I) == FALSE )
OR AssertWinner(*,G,I) == me )
AND local_receiver_include(*,G,I) }
pim_include(S,G) =
{ all interfaces I such that:
( (I_am_DR( I ) AND lost_assert(S,G,I) == FALSE )
OR AssertWinner(S,G,I) == me )
AND local_receiver_include(S,G,I) }
pim_exclude(S,G) =
{ all interfaces I such that:
( (I_am_DR( I ) AND lost_assert(*,G,I) == FALSE )
OR AssertWinner(*,G,I) == me )
AND local_receiver_exclude(S,G,I) }
The clause "local_receiver_include(S,G,I)" is true if the IGMP/MLD
module or other local membership mechanism has determined that local
members on interface I desire to receive traffic sent specifically by
S to G. "local_receiver_include(*,G,I)" is true if the IGMP/MLD
module or other local membership mechanism has determined that local
members on interface I desire to receive all traffic sent to G
(possibly excluding traffic from a specific set of sources).
"local_receiver_exclude(S,G,I) is true if
"local_receiver_include(*,G,I)" is true but none of the local members
desire to receive traffic from S.
The set "joins(*,*,RP)" is the set of all interfaces on which the
router has received (*,*,RP) Joins:
joins(*,*,RP) =
{ all interfaces I such that
DownstreamJPState(*,*,RP,I) is either Join or
Prune-Pending }
DownstreamJPState(*,*,RP,I) is the state of the finite state machine
in Section 4.5.1.
The set "joins(*,G)" is the set of all interfaces on which the router
has received (*,G) Joins:
joins(*,G) =
{ all interfaces I such that
DownstreamJPState(*,G,I) is either Join or Prune-Pending }
DownstreamJPState(*,G,I) is the state of the finite state machine in
Section 4.5.2.
The set "joins(S,G)" is the set of all interfaces on which the router
has received (S,G) Joins:
joins(S,G) =
{ all interfaces I such that
DownstreamJPState(S,G,I) is either Join or Prune-Pending }
DownstreamJPState(S,G,I) is the state of the finite state machine in
Section 4.5.3.
The set "prunes(S,G,rpt)" is the set of all interfaces on which the
router has received (*,G) joins and (S,G,rpt) prunes.
prunes(S,G,rpt) =
{ all interfaces I such that
DownstreamJPState(S,G,rpt,I) is Prune or PruneTmp }
DownstreamJPState(S,G,rpt,I) is the state of the finite state machine
in Section 4.5.4.
The set "lost_assert(*,G)" is the set of all interfaces on which the
router has received (*,G) joins but has lost a (*,G) assert. The
macro lost_assert(*,G,I) is defined in Section 4.6.5.
lost_assert(*,G) =
{ all interfaces I such that
lost_assert(*,G,I) == TRUE }
The set "lost_assert(S,G,rpt)" is the set of all interfaces on which
the router has received (*,G) joins but has lost an (S,G) assert.
The macro lost_assert(S,G,rpt,I) is defined in Section 4.6.5.
lost_assert(S,G,rpt) =
{ all interfaces I such that
lost_assert(S,G,rpt,I) == TRUE }
The set "lost_assert(S,G)" is the set of all interfaces on which the
router has received (S,G) joins but has lost an (S,G) assert. The
macro lost_assert(S,G,I) is defined in Section 4.6.5.
lost_assert(S,G) =
{ all interfaces I such that
lost_assert(S,G,I) == TRUE }
The following pseudocode macro definitions are also used in many
places in the specification. Basically, RPF' is the RPF neighbor
towards an RP or source unless a PIM-Assert has overridden the normal
choice of neighbor.
neighbor RPF'(*,G) {
if ( I_Am_Assert_Loser(*, G, RPF_interface(RP(G))) ) {
return AssertWinner(*, G, RPF_interface(RP(G)) )
} else {
return NBR( RPF_interface(RP(G)), MRIB.next_hop( RP(G) ) )
}
}
neighbor RPF'(S,G,rpt) {
if( I_Am_Assert_Loser(S, G, RPF_interface(RP(G)) ) ) {
return AssertWinner(S, G, RPF_interface(RP(G)) )
} else {
return RPF'(*,G)
}
}
neighbor RPF'(S,G) {
if ( I_Am_Assert_Loser(S, G, RPF_interface(S) )) {
return AssertWinner(S, G, RPF_interface(S) )
} else {
return NBR( RPF_interface(S), MRIB.next_hop( S ) )
}
}
RPF'(*,G) and RPF'(S,G) indicate the neighbor from which data packets
should be coming and to which joins should be sent on the RP tree and
SPT, respectively.
RPF'(S,G,rpt) is basically RPF'(*,G) modified by the result of an
Assert(S,G) on RPF_interface(RP(G)). In such a case, packets from S
will be originating from a different router than RPF'(*,G). If we
only have active (*,G) Join state, we need to accept packets from
RPF'(S,G,rpt) and add a Prune(S,G,rpt) to the periodic Join(*,G)
messages that we send to RPF'(*,G) (see Section 4.5.8).
The function MRIB.next_hop( S ) returns an address of the next-hop
PIM neighbor toward the host S, as indicated by the current MRIB. If
S is directly adjacent, then MRIB.next_hop( S ) returns NULL. At the
RP for G, MRIB.next_hop( RP(G)) returns NULL.
The function NBR( I, A ) uses information gathered through PIM Hello
messages to map the IP address A of a directly connected PIM neighbor
router on interface I to the primary IP address of the same router
(Section 4.3.4). The primary IP address of a neighbor is the address
that it uses as the source of its PIM Hello messages. Note that a
neighbor's IP address may be non-unique within the PIM neighbor
database due to scope issues. The address must, however, be unique
amongst the addresses of all the PIM neighbors on a specific
interface.
I_Am_Assert_Loser(S, G, I) is true if the Assert state machine (in
Section 4.6.1) for (S,G) on Interface I is in "I am Assert Loser"
state.
I_Am_Assert_Loser(*, G, I) is true if the Assert state machine (in
Section 4.6.2) for (*,G) on Interface I is in "I am Assert Loser"
state.
4.2. Data Packet Forwarding Rules
The PIM-SM packet forwarding rules are defined below in pseudocode.
iif is the incoming interface of the packet.
S is the source address of the packet.
G is the destination address of the packet (group address).
RP is the address of the Rendezvous Point for this group.
RPF_interface(S) is the interface the MRIB indicates would be used
to route packets to S.
RPF_interface(RP) is the interface the MRIB indicates would be
used to route packets to RP, except at the RP when it is the
decapsulation interface (the "virtual" interface on which register
packets are received).
First, we restart (or start) the Keepalive Timer if the source is on
a directly connected subnet.
Second, we check to see if the SPTbit should be set because we've now
switched from the RP tree to the SPT.
Next, we check to see whether the packet should be accepted based on
TIB state and the interface that the packet arrived on.
If the packet should be forwarded using (S,G) state, we then build an
outgoing interface list for the packet. If this list is not empty,
then we restart the (S,G) state Keepalive Timer.
If the packet should be forwarded using (*,*,RP) or (*,G) state, then
we just build an outgoing interface list for the packet. We also
check if we should initiate a switch to start receiving this source
on a shortest path tree.
Finally we remove the incoming interface from the outgoing interface
list we've created, and if the resulting outgoing interface list is
not empty, we forward the packet out of those interfaces.
On receipt of data from S to G on interface iif:
if( DirectlyConnected(S) == TRUE AND iif == RPF_interface(S) ) {
set KeepaliveTimer(S,G) to Keepalive_Period
# Note: a register state transition or UpstreamJPState(S,G)
# transition may happen as a result of restarting
# KeepaliveTimer, and must be dealt with here.
}
if( iif == RPF_interface(S) AND UpstreamJPState(S,G) == Joined AND
inherited_olist(S,G) != NULL ) {
set KeepaliveTimer(S,G) to Keepalive_Period
}
Update_SPTbit(S,G,iif)
oiflist = NULL
if( iif == RPF_interface(S) AND SPTbit(S,G) == TRUE ) {
oiflist = inherited_olist(S,G)
} else if( iif == RPF_interface(RP(G)) AND SPTbit(S,G) == FALSE) {
oiflist = inherited_olist(S,G,rpt)
CheckSwitchToSpt(S,G)
} else {
# Note: RPF check failed
# A transition in an Assert FSM may cause an Assert(S,G)
# or Assert(*,G) message to be sent out interface iif.
# See section 4.6 for details.
if ( SPTbit(S,G) == TRUE AND iif is in inherited_olist(S,G) ) {
send Assert(S,G) on iif
} else if ( SPTbit(S,G) == FALSE AND
iif is in inherited_olist(S,G,rpt) {
send Assert(*,G) on iif
}
}
oiflist = oiflist (-) iif
forward packet on all interfaces in oiflist
This pseudocode employs several "macro" definitions:
DirectlyConnected(S) is TRUE if the source S is on any subnet that is
directly connected to this router (or for packets originating on this
router).
inherited_olist(S,G) and inherited_olist(S,G,rpt) are defined in
Section 4.1.
Basically, inherited_olist(S,G) is the outgoing interface list for
packets forwarded on (S,G) state, taking into account (*,*,RP) state,
(*,G) state, asserts, etc.
inherited_olist(S,G,rpt) is the outgoing interface list for packets
forwarded on (*,*,RP) or (*,G) state, taking into account (S,G,rpt)
prune state, asserts, etc.
Update_SPTbit(S,G,iif) is defined in Section 4.2.2.
CheckSwitchToSpt(S,G) is defined in Section 4.2.1.
UpstreamJPState(S,G) is the state of the finite state machine in
Section 4.5.7.
Keepalive_Period is defined in Section 4.10.
Data-triggered PIM-Assert messages sent from the above forwarding
code should be rate-limited in a implementation-dependent manner.
4.2.1. Last-Hop Switchover to the SPT
In Sparse-Mode PIM, last-hop routers join the shared tree towards the
RP. Once traffic from sources to joined groups arrives at a last-hop
router, it has the option of switching to receive the traffic on a
shortest path tree (SPT).
The decision for a router to switch to the SPT is controlled as
follows:
void
CheckSwitchToSpt(S,G) {
if ( ( pim_include(*,G) (-) pim_exclude(S,G)
(+) pim_include(S,G) != NULL )
AND SwitchToSptDesired(S,G) ) {
# Note: Restarting the KAT will result in the SPT switch
set KeepaliveTimer(S,G) to Keepalive_Period
}
}
SwitchToSptDesired(S,G) is a policy function that is implementation
defined. An "infinite threshold" policy can be implemented by making
SwitchToSptDesired(S,G) return false all the time. A "switch on
first packet" policy can be implemented by making
SwitchToSptDesired(S,G) return true once a single packet has been
received for the source and group.
4.2.2. Setting and Clearing the (S,G) SPTbit
The (S,G) SPTbit is used to distinguish whether to forward on
(*,*,RP)/(*,G) or on (S,G) state. When switching from the RP tree to
the source tree, there is a transition period when data is arriving
due to upstream (*,*,RP)/(*,G) state while upstream (S,G) state is
being established, during which time a router should continue to
forward only on (*,*,RP)/(*,G) state. This prevents temporary
black-holes that would be caused by sending a Prune(S,G,rpt) before
the upstream (S,G) state has finished being established.
Thus, when a packet arrives, the (S,G) SPTbit is updated as follows:
void
Update_SPTbit(S,G,iif) {
if ( iif == RPF_interface(S)
AND JoinDesired(S,G) == TRUE
AND ( DirectlyConnected(S) == TRUE
OR RPF_interface(S) != RPF_interface(RP(G))
OR inherited_olist(S,G,rpt) == NULL
OR ( ( RPF'(S,G) == RPF'(*,G) ) AND
( RPF'(S,G) != NULL ) )
OR ( I_Am_Assert_Loser(S,G,iif) ) {
Set SPTbit(S,G) to TRUE
}
}
Additionally, a router can set SPTbit(S,G) to TRUE in other cases,
such as when it receives an Assert(S,G) on RPF_interface(S) (see
Section 4.6.1).
JoinDesired(S,G) is defined in Section 4.5.7 and indicates whether we
have the appropriate (S,G) Join state to wish to send a Join(S,G)
upstream.
Basically, Update_SPTbit will set the SPTbit if we have the
appropriate (S,G) join state, and if the packet arrived on the
correct upstream interface for S, and if one or more of the following
conditions applies:
1. The source is directly connected, in which case the switch to the
SPT is a no-op.
2. The RPF interface to S is different from the RPF interface to the
RP. The packet arrived on RPF_interface(S), and so the SPT must
have been completed.
3. Noone wants the packet on the RP tree.
4. RPF'(S,G) == RPF'(*,G). In this case, the router will never be
able to tell if the SPT has been completed, so it should just
switch immediately.
In the case where the RPF interface is the same for the RP and for S,
but RPF'(S,G) and RPF'(*,G) differ, we wait for an Assert(S,G), which
indicates that the upstream router with (S,G) state believes the SPT
has been completed. However, item (3) above is needed because there
may not be any (*,G) state to trigger an Assert(S,G) to happen.
The SPTbit is cleared in the (S,G) upstream state machine (see
Section 4.5.7) when JoinDesired(S,G) becomes FALSE.
4.3. Designated Routers (DR) and Hello Messages
A shared-media LAN like Ethernet may have multiple PIM-SM routers
connected to it. A single one of these routers, the DR, will act on
behalf of directly connected hosts with respect to the PIM-SM
protocol. Because the distinction between LANs and point-to-point
interfaces can sometimes be blurred, and because routers may also
have multicast host functionality, the PIM-SM specification makes no
distinction between the two. Thus, DR election will happen on all
interfaces, LAN or otherwise.
DR election is performed using Hello messages. Hello messages are
also the way that option negotiation takes place in PIM, so that
additional functionality can be enabled, or parameters tuned.
4.3.1. Sending Hello Messages
PIM Hello messages are sent periodically on each PIM-enabled
interface. They allow a router to learn about the neighboring PIM
routers on each interface. Hello messages are also the mechanism
used to elect a Designated Router (DR), and to negotiate additional
capabilities. A router must record the Hello information received
from each PIM neighbor.
Hello messages MUST be sent on all active interfaces, including
physical point-to-point links, and are multicast to the 'ALL-PIM-
ROUTERS' group address ('224.0.0.13' for IPv4 and 'ff02::d' for
IPv6).
We note that some implementations do not send Hello messages on
point-to-point interfaces. This is non-compliant behavior. A
compliant PIM router MUST send Hello messages, even on point-to-
point interfaces.
A per-interface Hello Timer (HT(I)) is used to trigger sending Hello
messages on each active interface. When PIM is enabled on an
interface or a router first starts, the Hello Timer of that interface
is set to a random value between 0 and Triggered_Hello_Delay. This
prevents synchronization of Hello messages if multiple routers are
powered on simultaneously. After the initial randomized interval,
Hello messages must be sent every Hello_Period seconds. The Hello
Timer should not be reset except when it expires.
Note that neighbors will not accept Join/Prune or Assert messages
from a router unless they have first heard a Hello message from that
router. Thus, if a router needs to send a Join/Prune or Assert
message on an interface on which it has not yet sent a Hello message
with the currently configured IP address, then it MUST immediately
send the relevant Hello message without waiting for the Hello Timer
to expire, followed by the Join/Prune or Assert message.
The DR_Priority Option allows a network administrator to give
preference to a particular router in the DR election process by
giving it a numerically larger DR Priority. The DR_Priority Option
SHOULD be included in every Hello message, even if no DR Priority is
explicitly configured on that interface. This is necessary because
priority-based DR election is only enabled when all neighbors on an
interface advertise that they are capable of using the DR_Priority
Option. The default priority is 1.
The Generation_Identifier (GenID) Option SHOULD be included in all
Hello messages. The GenID option contains a randomly generated
32-bit value that is regenerated each time PIM forwarding is started
or restarted on the interface, including when the router itself
restarts. When a Hello message with a new GenID is received from a
neighbor, any old Hello information about that neighbor SHOULD be
discarded and superseded by the information from the new Hello
message. This may cause a new DR to be chosen on that interface.
The LAN Prune Delay Option SHOULD be included in all Hello messages
sent on multi-access LANs. This option advertises a router's
capability to use values other than the defaults for the
Propagation_Delay and Override_Interval, which affect the setting of
the Prune-Pending, Upstream Join, and Override Timers (defined in
Section 4.10).
The Address List Option advertises all the secondary addresses
associated with the source interface of the router originating the
message. The option MUST be included in all Hello messages if there
are secondary addresses associated with the source interface and MAY
be omitted if no secondary addresses exist.
To allow new or rebooting routers to learn of PIM neighbors quickly,
when a Hello message is received from a new neighbor, or a Hello
message with a new GenID is received from an existing neighbor, a new
Hello message should be sent on this interface after a randomized
delay between 0 and Triggered_Hello_Delay. This triggered message
need not change the timing of the scheduled periodic message. If a
router needs to send a Join/Prune to the new neighbor or send an
Assert message in response to an Assert message from the new neighbor
before this randomized delay has expired, then it MUST immediately
send the relevant Hello message without waiting for the Hello Timer
to expire, followed by the Join/Prune or Assert message. If it does
not do this, then the new neighbor will discard the Join/Prune or
Assert message.
Before an interface goes down or changes primary IP address, a Hello
message with a zero HoldTime should be sent immediately (with the old
IP address if the IP address changed). This will cause PIM neighbors
to remove this neighbor (or its old IP address) immediately. After
an interface has changed its IP address, it MUST send a Hello message
with its new IP address. If an interface changes one of its
secondary IP addresses, a Hello message with an updated Address_List
option and a non-zero HoldTime should be sent immediately. This will
cause PIM neighbors to update this neighbor's list of secondary
addresses immediately.
4.3.2. DR Election
When a PIM Hello message is received on interface I, the following
information about the sending neighbor is recorded:
neighbor.interface
The interface on which the Hello message arrived.
neighbor.primary_ip_address
The IP address that the PIM neighbor used as the source
address of the Hello message.
neighbor.genid
The Generation ID of the PIM neighbor.
neighbor.dr_priority
The DR Priority field of the PIM neighbor, if it is present in
the Hello message.
neighbor.dr_priority_present
A flag indicating if the DR Priority field was present in the
Hello message.
neighbor.timeout
A timer value to time out the neighbor state when it becomes
stale, also known as the Neighbor Liveness Timer.
The Neighbor Liveness Timer (NLT(N,I)) is reset to
Hello_Holdtime (from the Hello Holdtime option) whenever a
Hello message is received containing a Holdtime option, or to
Default_Hello_Holdtime if the Hello message does not contain
the Holdtime option.
Neighbor state is deleted when the neighbor timeout expires.
The function for computing the DR on interface I is:
host
DR(I) {
dr = me
for each neighbor on interface I {
if ( dr_is_better( neighbor, dr, I ) == TRUE ) {
dr = neighbor
}
}
return dr
}
The function used for comparing DR "metrics" on interface I is:
bool
dr_is_better(a,b,I) {
if( there is a neighbor n on I for which n.dr_priority_present
is false ) {
return a.primary_ip_address > b.primary_ip_address
} else {
return ( a.dr_priority > b.dr_priority ) OR
( a.dr_priority == b.dr_priority AND
a.primary_ip_address > b.primary_ip_address )
}
}
The trivial function I_am_DR(I) is defined to aid readability:
bool
I_am_DR(I) {
return DR(I) == me
}
The DR Priority is a 32-bit unsigned number, and the numerically
larger priority is always preferred. A router's idea of the current
DR on an interface can change when a PIM Hello message is received,
when a neighbor times out, or when a router's own DR Priority
changes. If the router becomes the DR or ceases to be the DR, this
will normally cause the DR Register state machine to change state.
Subsequent actions are determined by that state machine.
We note that some PIM implementations do not send Hello messages on
point-to-point interfaces and thus cannot perform DR election on
such interfaces. This is non-compliant behavior. DR election MUST
be performed on ALL active PIM-SM interfaces.
4.3.3. Reducing Prune Propagation Delay on LANs
In addition to the information recorded for the DR Election, the
following per neighbor information is obtained from the LAN Prune
Delay Hello option:
neighbor.lan_prune_delay_present
A flag indicating if the LAN Prune Delay option was present in
the Hello message.
neighbor.tracking_support
A flag storing the value of the T bit in the LAN Prune Delay
option if it is present in the Hello message. This indicates
the neighbor's capability to disable Join message suppression.
neighbor.propagation_delay
The Propagation Delay field of the LAN Prune Delay option (if
present) in the Hello message.
neighbor.override_interval
The Override_Interval field of the LAN Prune Delay option (if
present) in the Hello message.
The additional state described above is deleted along with the DR
neighbor state when the neighbor timeout expires.
Just like the DR_Priority option, the information provided in the LAN
Prune Delay option is not used unless all neighbors on a link
advertise the option. The function below computes this state:
bool
lan_delay_enabled(I) {
for each neighbor on interface I {
if ( neighbor.lan_prune_delay_present == false ) {
return false
}
}
return true
}
The Propagation Delay inserted by a router in the LAN Prune Delay
option expresses the expected message propagation delay on the link
and should be configurable by the system administrator. It is used
by upstream routers to figure out how long they should wait for a
Join override message before pruning an interface.
PIM implementers should enforce a lower bound on the permitted values
for this delay to allow for scheduling and processing delays within
their router. Such delays may cause received messages to be
processed later as well as triggered messages to be sent later than
intended. Setting this Propagation Delay to too low a value may
result in temporary forwarding outages because a downstream router
will not be able to override a neighbor's Prune message before the
upstream neighbor stops forwarding.
When all routers on a link are in a position to negotiate a
Propagation Delay different from the default, the largest value from
those advertised by each neighbor is chosen. The function for
computing the Effective_Propagation_Delay of interface I is:
time_interval
Effective_Propagation_Delay(I) {
if ( lan_delay_enabled(I) == false ) {
return Propagation_delay_default
}
delay = Propagation_Delay(I)
for each neighbor on interface I {
if ( neighbor.propagation_delay > delay ) {
delay = neighbor.propagation_delay
}
}
return delay
}
To avoid synchronization of override messages when multiple
downstream routers share a multi-access link, sending of such
messages is delayed by a small random amount of time. The period of
randomization should represent the size of the PIM router population
on the link. Each router expresses its view of the amount of
randomization necessary in the Override Interval field of the LAN
Prune Delay option.
When all routers on a link are in a position to negotiate an Override
Interval different from the default, the largest value from those
advertised by each neighbor is chosen. The function for computing
the Effective Override Interval of interface I is:
time_interval
Effective_Override_Interval(I) {
if ( lan_delay_enabled(I) == false ) {
return t_override_default
}
delay = Override_Interval(I)
for each neighbor on interface I {
if ( neighbor.override_interval > delay ) {
delay = neighbor.override_interval
}
}
return delay
}
Although the mechanisms are not specified in this document, it is
possible for upstream routers to explicitly track the join membership
of individual downstream routers if Join suppression is disabled. A
router can advertise its willingness to disable Join suppression by
using the T bit in the LAN Prune Delay Hello option. Unless all PIM
routers on a link negotiate this capability, explicit tracking and
the disabling of the Join suppression mechanism are not possible.
The function for computing the state of Suppression on interface I
is:
bool
Suppression_Enabled(I) {
if ( lan_delay_enabled(I) == false ) {
return true
}
for each neighbor on interface I {
if ( neighbor.tracking_support == false ) {
return true
}
}
return false
}
Note that the setting of Suppression_Enabled(I) affects the value of
t_suppressed (see Section 4.10).
4.3.4. Maintaining Secondary Address Lists
Communication of a router's interface secondary addresses to its PIM
neighbors is necessary to provide the neighbors with a mechanism for
mapping next_hop information obtained through their MRIB to a primary
address that can be used as a destination for Join/Prune messages.
The mapping is performed through the NBR macro. The primary address
of a PIM neighbor is obtained from the source IP address used in its
PIM Hello messages. Secondary addresses are carried within the Hello
message in an Address List Hello option. The primary address of the
source interface of the router MUST NOT be listed within the Address
List Hello option.
In addition to the information recorded for the DR Election, the
following per neighbor information is obtained from the Address List
Hello option:
neighbor.secondary_address_list
The list of secondary addresses used by the PIM neighbor on
the interface through which the Hello message was transmitted.
When processing a received PIM Hello message containing an Address
List Hello option, the list of secondary addresses in the message
completely replaces any previously associated secondary addresses for
that neighbor. If a received PIM Hello message does not contain an
Address List Hello option, then all secondary addresses associated
with the neighbor must be deleted. If a received PIM Hello message
contains an Address List Hello option that includes the primary
address of the sending router in the list of secondary addresses
(although this is not expected), then the addresses listed in the
message, excluding the primary address, are used to update the
associated secondary addresses for that neighbor.
All the advertised secondary addresses in received Hello messages
must be checked against those previously advertised by all other PIM
neighbors on that interface. If there is a conflict and the same
secondary address was previously advertised by another neighbor, then
only the most recently received mapping MUST be maintained, and an
error message SHOULD be logged to the administrator in a rate-limited
manner.
Within one Address List Hello option, all the addresses MUST be of
the same address family. It is not permitted to mix IPv4 and IPv6
addresses within the same message. In addition, the address family
of the fields in the message SHOULD be the same as the IP source and
destination addresses of the packet header.
4.4. PIM Register Messages
The Designated Router (DR) on a LAN or point-to-point link
encapsulates multicast packets from local sources to the RP for the
relevant group unless it recently received a Register-Stop message
for that (S,G) or (*,G) from the RP. When the DR receives a
Register-Stop message from the RP, it starts a Register-Stop Timer to
maintain this state. Just before the Register-Stop Timer expires,
the DR sends a Null-Register Message to the RP to allow the RP to
refresh the Register-Stop information at the DR. If the Register-
Stop Timer actually expires, the DR will resume encapsulating packets
from the source to the RP.
4.4.1. Sending Register Messages from the DR
Every PIM-SM router has the capability to be a DR. The state machine
below is used to implement Register functionality. For the purposes
of specification, we represent the mechanism to encapsulate packets
to the RP as a Register-Tunnel interface, which is added to or
removed from the (S,G) olist. The tunnel interface then takes part
in the normal packet forwarding rules as specified in Section 4.2.
If register state is maintained, it is maintained only for directly
connected sources and is per-(S,G). There are four states in the
DR's per-(S,G) Register state machine:
Join (J)
The register tunnel is "joined" (the join is actually implicit,
but the DR acts as if the RP has joined the DR on the tunnel
interface).
Prune (P)
The register tunnel is "pruned" (this occurs when a Register-
Stop is received).
Join-Pending (JP)
The register tunnel is pruned but the DR is contemplating adding
it back.
NoInfo (NI)
No information. This is the initial state, and the state when
the router is not the DR.
In addition, a Register-Stop Timer (RST) is kept if the state machine
is not in the NoInfo state.
Figure 1: Per-(S,G) register state machine at a DR in tabular form
+----------++----------------------------------------------------------+
| || Event |
| ++----------+-----------+-----------+-----------+-----------+
|Prev State||Register- | Could | Could | Register- | RP changed|
| ||Stop Timer| Register | Register | Stop | |
| ||expires | ->True | ->False | received | |
+----------++----------+-----------+-----------+-----------+-----------+
|NoInfo ||- | -> J state| - | - | - |
|(NI) || | add reg | | | |
| || | tunnel | | | |
+----------++----------+-----------+-----------+-----------+-----------+
| ||- | - | -> NI | -> P state| -> J state|
| || | | state | | |
| || | | remove reg| remove reg| update reg|
|Join (J) || | | tunnel | tunnel; | tunnel |
| || | | | set | |
| || | | | Register- | |
| || | | | Stop | |
| || | | | Timer(*) | |
+----------++----------+-----------+-----------+-----------+-----------+
| ||-> J state| - | -> NI | -> P state| -> J state|
| || | | state | | |
|Join- ||add reg | | | set | add reg |
|Pending ||tunnel | | | Register- | tunnel; |
|(JP) || | | | Stop | cancel |
| || | | | Timer(*) | Register- |
| || | | | | Stop Timer|
+----------++----------+-----------+-----------+-----------+-----------+
| ||-> JP | - | -> NI | - | -> J state|
| ||state | | state | | |
| ||set | | | | add reg |
|Prune (P) ||Register- | | | | tunnel; |
| ||Stop | | | | cancel |
| ||Timer(**);| | | | Register- |
| ||send Null-| | | | Stop Timer|
| ||Register | | | | |
+----------++----------+-----------+-----------+-----------+-----------+
Notes:
(*) The Register-Stop Timer is set to a random value chosen
uniformly from the interval ( 0.5 * Register_Suppression_Time,
1.5 * Register_Suppression_Time) minus Register_Probe_Time.
Subtracting off Register_Probe_Time is a bit unnecessary because
it is really small compared to Register_Suppression_Time, but
this was in the old spec and is kept for compatibility.
(**) The Register-Stop Timer is set to Register_Probe_Time.
The following three actions are defined:
Add Register Tunnel
A Register-Tunnel virtual interface, VI, is created (if it doesn't
already exist) with its encapsulation target being RP(G).
DownstreamJPState(S,G,VI) is set to Join state, causing the tunnel
interface to be added to immediate_olist(S,G) and
inherited_olist(S,G).
Remove Register Tunnel
VI is the Register-Tunnel virtual interface with encapsulation
target of RP(G). DownstreamJPState(S,G,VI) is set to NoInfo
state, causing the tunnel interface to be removed from
immediate_olist(S,G) and inherited_olist(S,G). If
DownstreamJPState(S,G,VI) is NoInfo for all (S,G), then VI can be
deleted.
Update Register Tunnel
This action occurs when RP(G) changes.
VI_old is the Register-Tunnel virtual interface with encapsulation
target old_RP(G). A Register-Tunnel virtual interface, VI_new, is
created (if it doesn't already exist) with its encapsulation
target being new_RP(G). DownstreamJPState(S,G,VI_old) is set to
NoInfo state and DownstreamJPState(S,G,VI_new) is set to Join
state. If DownstreamJPState(S,G,VI_old) is NoInfo for all (S,G),
then VI_old can be deleted.
Note that we cannot simply change the encapsulation target of
VI_old because not all groups using that encapsulation tunnel will
have moved to the same new RP.
CouldRegister(S,G)
The macro "CouldRegister" in the state machine is defined as:
bool CouldRegister(S,G) {
return ( I_am_DR( RPF_interface(S) ) AND
KeepaliveTimer(S,G) is running AND
DirectlyConnected(S) == TRUE )
}
Note that on reception of a packet at the DR from a directly
connected source, KeepaliveTimer(S,G) needs to be set by the
packet forwarding rules before computing CouldRegister(S,G) in the
register state machine, or the first packet from a source won't be
registered.
Encapsulating Data Packets in the Register Tunnel
Conceptually, the Register Tunnel is an interface with a smaller
MTU than the underlying IP interface towards the RP. IP
fragmentation on packets forwarded on the Register Tunnel is
performed based upon this smaller MTU. The encapsulating DR may
perform Path MTU Discovery to the RP to determine the effective
MTU of the tunnel. Fragmentation for the smaller MTU should take
both the outer IP header and the PIM register header overhead into
account. If a multicast packet is fragmented on the way into the
Register Tunnel, each fragment is encapsulated individually so it
contains IP, PIM, and inner IP headers.
In IPv6, the DR MUST perform Path MTU discovery, and an ICMP
Packet Too Big message MUST be sent by the encapsulating DR if it
receives a packet that will not fit in the effective MTU of the
tunnel. If the MTU between the DR and the RP results in the
effective tunnel MTU being smaller than 1280 (the IPv6 minimum
MTU), the DR MUST send Fragmentation Required messages with an MTU
value of 1280 and MUST fragment its PIM register messages as
required, using an IPv6 fragmentation header between the outer
IPv6 header and the PIM Register header.
The TTL of a forwarded data packet is decremented before it is
encapsulated in the Register Tunnel. The encapsulating packet
uses the normal TTL that the router would use for any locally-
generated IP packet.
The IP ECN bits should be copied from the original packet to the
IP header of the encapsulating packet. They SHOULD NOT be set
independently by the encapsulating router.
The Diffserv Code Point (DSCP) should be copied from the original
packet to the IP header of the encapsulating packet. It MAY be
set independently by the encapsulating router, based upon static
configuration or traffic classification. See [12] for more
discussion on setting the DSCP on tunnels.
Handling Register-Stop(*,G) Messages at the DR
An old RP might send a Register-Stop message with the source
address set to all zeros. This was the normal course of action in
RFC 2362 when the Register message matched against (*,G) state at
the RP, and it was defined as meaning "stop encapsulating all
sources for this group". However, the behavior of such a
Register-Stop(*,G) is ambiguous or incorrect in some
circumstances.
We specify that an RP should not send Register-Stop(*,G) messages,
but for compatibility, a DR should be able to accept one if it is
received.
A Register-Stop(*,G) should be treated as a Register-Stop(S,G) for
all (S,G) Register state machines that are not in the NoInfo
state. A router should not apply a Register-Stop(*,G) to sources
that become active after the Register-Stop(*,G) was received.
4.4.2. Receiving Register Messages at the RP
When an RP receives a Register message, the course of action is
decided according to the following pseudocode:
packet_arrives_on_rp_tunnel( pkt ) {
if( outer.dst is not one of my addresses ) {
drop the packet silently.
# Note: this may be a spoofing attempt
}
if( I_am_RP(G) AND outer.dst == RP(G) ) {
sentRegisterStop = FALSE;
if ( register.borderbit == TRUE ) {
if ( PMBR(S,G) == unknown ) {
PMBR(S,G) = outer.src
} else if ( outer.src != PMBR(S,G) ) {
send Register-Stop(S,G) to outer.src
drop the packet silently.
}
}
if ( SPTbit(S,G) OR
( SwitchToSptDesired(S,G) AND
( inherited_olist(S,G) == NULL ))) {
send Register-Stop(S,G) to outer.src
sentRegisterStop = TRUE;
}
if ( SPTbit(S,G) OR SwitchToSptDesired(S,G) ) {
if ( sentRegisterStop == TRUE ) {
set KeepaliveTimer(S,G) to RP_Keepalive_Period;
} else {
set KeepaliveTimer(S,G) to Keepalive_Period;
}
}
if( !SPTbit(S,G) AND ! pkt.NullRegisterBit ) {
decapsulate and forward the inner packet to
inherited_olist(S,G,rpt) # Note (+)
}
} else {
send Register-Stop(S,G) to outer.src
# Note (*)
}
}
outer.dst is the IP destination address of the encapsulating header.
outer.src is the IP source address of the encapsulating header, i.e.,
the DR's address.
I_am_RP(G) is true if the group-to-RP mapping indicates that this
router is the RP for the group.
Note (*): This may block traffic from S for Register_Suppression_Time
if the DR learned about a new group-to-RP mapping before the RP
did. However, this doesn't matter unless we figure out some way
for the RP also to accept (*,G) joins when it doesn't yet realize
that it is about to become the RP for G. This will all get sorted
out once the RP learns the new group-to-rp mapping. We decided to
do nothing about this and just accept the fact that PIM may suffer
interrupted (*,G) connectivity following an RP change.
Note (+): Implementations are advised not to make this a special
case, but to arrange that this path rejoin the normal packet
forwarding path. All of the appropriate actions from the "On
receipt of data from S to G on interface iif" pseudocode in
Section 4.2 should be performed.
KeepaliveTimer(S,G) is restarted at the RP when packets arrive on the
proper tunnel interface and the RP desires to switch to the SPT or
the SPTbit is already set. This may cause the upstream (S,G) state
machine to trigger a join if the inherited_olist(S,G) is not NULL.
An RP should preserve (S,G) state that was created in response to a
Register message for at least ( 3 * Register_Suppression_Time );
otherwise, the RP may stop joining (S,G) before the DR for S has
restarted sending registers. Traffic would then be interrupted until
the Register-Stop Timer expires at the DR.
Thus, at the RP, KeepaliveTimer(S,G) should be restarted to ( 3 *
Register_Suppression_Time + Register_Probe_Time ).
When forwarding a packet from the Register Tunnel, the TTL of the
original data packet is decremented after it is decapsulated.
The IP ECN bits should be copied from the IP header of the Register
packet to the decapsulated packet.
The Diffserv Code Point (DSCP) should be copied from the IP header of
the Register packet to the decapsulated packet. The RP MAY retain
the DSCP of the inner packet or re-classify the packet and apply a
different DSCP. Scenarios where each of these might be useful are
discussed in [12].
4.5. PIM Join/Prune Messages
A PIM Join/Prune message consists of a list of groups and a list of
Joined and Pruned sources for each group. When processing a received
Join/Prune message, each Joined or Pruned source for a Group is
effectively considered individually, and applies to one or more of
the following state machines. When considering a Join/Prune message
whose Upstream Neighbor Address field addresses this router, (*,G)
Joins and Prunes can affect both the (*,G) and (S,G,rpt) downstream
state machines, while (*,*,RP), (S,G), and (S,G,rpt) Joins and Prunes
can only affect their respective downstream state machines. When
considering a Join/Prune message whose Upstream Neighbor Address
field addresses another router, most Join or Prune messages could
affect each upstream state machine.
In general, a PIM Join/Prune message should only be accepted for
processing if it comes from a known PIM neighbor. A PIM router hears
about PIM neighbors through PIM Hello messages. If a router receives
a Join/Prune message from a particular IP source address and it has
not seen a PIM Hello message from that source address, then the
Join/Prune message SHOULD be discarded without further processing.
In addition, if the Hello message from a neighbor was authenticated
using IPsec AH (see Section 6.3), then all Join/Prune messages from
that neighbor MUST also be authenticated using IPsec AH.
We note that some older PIM implementations incorrectly fail to send
Hello messages on point-to-point interfaces, so we also RECOMMEND
that a configuration option be provided to allow interoperation with
such older routers, but that this configuration option SHOULD NOT be
enabled by default.
4.5.1. Receiving (*,*,RP) Join/Prune Messages
The per-interface state machine for receiving (*,*,RP) Join/Prune
Messages is given below. There are three states:
NoInfo (NI)
The interface has no (*,*,RP) Join state and no timers
running.
Join (J)
The interface has (*,*,RP) Join state, which will cause the
router to forward packets destined for any group handled by RP
from this interface except if there is also (S,G,rpt) prune
information (see Section 4.5.4) or the router lost an assert
on this interface.
Prune-Pending (PP)
The router has received a Prune(*,*,RP) on this interface from
a downstream neighbor and is waiting to see whether the prune
will be overridden by another downstream router. For
forwarding purposes, the Prune-Pending state functions exactly
like the Join state.
In addition, the state machine uses two timers:
ExpiryTimer (ET)
This timer is restarted when a valid Join(*,*,RP) is received.
Expiry of the ExpiryTimer causes the interface state to revert
to NoInfo for this RP.
Prune-Pending Timer (PPT)
This timer is set when a valid Prune(*,*,RP) is received.
Expiry of the Prune-Pending Timer causes the interface state
to revert to NoInfo for this RP.
Figure 2: Downstream per-interface (*,*,RP) state machine
in tabular form
+------------++--------------------------------------------------------+
| || Event |
| ++-------------+-------------+--------------+-------------+
|Prev State ||Receive | Receive | Prune- | Expiry Timer|
| ||Join(*,*,RP) | Prune | Pending | Expires |
| || | (*,*,RP) | Timer | |
| || | | Expires | |
+------------++-------------+-------------+--------------+-------------+
| ||-> J state | -> NI state | - | - |
|NoInfo (NI) ||start Expiry | | | |
| ||Timer | | | |
+------------++-------------+-------------+--------------+-------------+
| ||-> J state | -> PP state | - | -> NI state |
|Join (J) ||restart | start Prune-| | |
| ||Expiry Timer | Pending | | |
| || | Timer | | |
+------------++-------------+-------------+--------------+-------------+
|Prune- ||-> J state | -> PP state | -> NI state | -> NI state |
|Pending (PP)||restart | | Send Prune- | |
| ||Expiry Timer | | Echo(*,*,RP) | |
+------------++-------------+-------------+--------------+-------------+
The transition events "Receive Join(*,*,RP)" and "Receive
Prune(*,*,RP)" imply receiving a Join or Prune targeted to this
router's primary IP address on the received interface. If the
upstream neighbor address field is not correct, these state
transitions in this state machine must not occur, although seeing
such a packet may cause state transitions in other state machines.
On unnumbered interfaces on point-to-point links, the router's
address should be the same as the source address it chose for the
Hello message it sent over that interface. However, on point-to-
point links we also recommend that for backwards compatibility PIM
Join/Prune messages with an upstream neighbor address field of all
zeros are also accepted.
Transitions from NoInfo State
When in NoInfo state, the following event may trigger a transition:
Receive Join(*,*,RP)
A Join(*,*,RP) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,*,RP) downstream state machine on interface I
transitions to the Join state. The Expiry Timer (ET) is
started and set to the HoldTime from the triggering Join/Prune
message.
Note that it is possible to receive a Join(*,*,RP) message for
an RP for which we do not have information telling us that it
is an RP. In the case of (*,*,RP) state, so long as we have a
route to the RP, this will not cause a problem, and the
transition should still take place.
Transitions from Join State
When in Join state, the following events may trigger a transition:
Receive Join(*,*,RP)
A Join(*,*,RP) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,*,RP) downstream state machine on interface I remains
in Join state, and the Expiry Timer (ET) is restarted, set to
maximum of its current value and the HoldTime from the
triggering Join/Prune message.
Receive Prune(*,*,RP)
A Prune(*,*,RP) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,*,RP) downstream state machine on interface I
transitions to the Prune-Pending state. The Prune-Pending
Timer is started. It is set to the J/P_Override_Interval(I)
if the router has more than one neighbor on that interface;
otherwise, it is set to zero, causing it to expire
immediately.
Expiry Timer Expires
The Expiry Timer for the (*,*,RP) downstream state machine on
interface I expires.
The (*,*,RP) downstream state machine on interface I
transitions to the NoInfo state.
Transitions from Prune-Pending State
When in Prune-Pending state, the following events may trigger a
transition:
Receive Join(*,*,RP)
A Join(*,*,RP) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,*,RP) downstream state machine on interface I
transitions to the Join state. The Prune-Pending Timer is
canceled (without triggering an expiry event). The Expiry
Timer is restarted, set to maximum of its current value and
the HoldTime from the triggering Join/Prune message.
Expiry Timer Expires
The Expiry Timer for the (*,*,RP) downstream state machine on
interface I expires.
The (*,*,RP) downstream state machine on interface I
transitions to the NoInfo state.
Prune-Pending Timer Expires
The Prune-Pending Timer for the (*,*,RP) downstream state
machine on interface I expires.
The (*,*,RP) downstream state machine on interface I
transitions to the NoInfo state. A PruneEcho(*,*,RP) is sent
onto the subnet connected to interface I.
The action "Send PruneEcho(*,*,RP)" is triggered when the
router stops forwarding on an interface as a result of a
prune. A PruneEcho(*,*,RP) is simply a Prune(*,*,RP) message
sent by the upstream router on a LAN with its own address in
the Upstream Neighbor Address field. Its purpose is to add
additional reliability so that if a Prune that should have
been overridden by another router is lost locally on the LAN,
then the PruneEcho may be received and cause the override to
happen. A PruneEcho(*,*,RP) need not be sent on an interface
that contains only a single PIM neighbor during the time this
state machine was in Prune-Pending state.
4.5.2. Receiving (*,G) Join/Prune Messages
When a router receives a Join(*,G), it must first check to see
whether the RP in the message matches RP(G) (the router's idea of who
the RP is). If the RP in the message does not match RP(G), the
Join(*,G) should be silently dropped. (Note that other source list
entries, such as (S,G,rpt) or (S,G), in the same Group-Specific Set
should still be processed.) If a router has no RP information (e.g.,
has not recently received a BSR message), then it may choose to
accept Join(*,G) and treat the RP in the message as RP(G). Received
Prune(*,G) messages are processed even if the RP in the message does
not match RP(G).
The per-interface state machine for receiving (*,G) Join/Prune
Messages is given below. There are three states:
NoInfo (NI)
The interface has no (*,G) Join state and no timers running.
Join (J)
The interface has (*,G) Join state, which will cause the
router to forward packets destined for G from this interface
except if there is also (S,G,rpt) prune information (see
Section 4.5.4) or the router lost an assert on this interface.
Prune-Pending (PP)
The router has received a Prune(*,G) on this interface from a
downstream neighbor and is waiting to see whether the prune
will be overridden by another downstream router. For
forwarding purposes, the Prune-Pending state functions exactly
like the Join state.
In addition, the state machine uses two timers:
Expiry Timer (ET)
This timer is restarted when a valid Join(*,G) is received.
Expiry of the Expiry Timer causes the interface state to
revert to NoInfo for this group.
Prune-Pending Timer (PPT)
This timer is set when a valid Prune(*,G) is received. Expiry
of the Prune-Pending Timer causes the interface state to
revert to NoInfo for this group.
Figure 3: Downstream per-interface (*,G) state machine in tabular form
+------------++--------------------------------------------------------+
| || Event |
| ++-------------+--------------+-------------+-------------+
|Prev State ||Receive | Receive | Prune- | Expiry Timer|
| ||Join(*,G) | Prune(*,G) | Pending | Expires |
| || | | Timer | |
| || | | Expires | |
+------------++-------------+--------------+-------------+-------------+
| ||-> J state | -> NI state | - | - |
|NoInfo (NI) ||start Expiry | | | |
| ||Timer | | | |
+------------++-------------+--------------+-------------+-------------+
| ||-> J state | -> PP state | - | -> NI state |
|Join (J) ||restart | start Prune- | | |
| ||Expiry Timer | Pending | | |
| || | Timer | | |
+------------++-------------+--------------+-------------+-------------+
|Prune- ||-> J state | -> PP state | -> NI state | -> NI state |
|Pending (PP)||restart | | Send Prune- | |
| ||Expiry Timer | | Echo(*,G) | |
+------------++-------------+--------------+-------------+-------------+
The transition events "Receive Join(*,G)" and "Receive Prune(*,G)"
imply receiving a Join or Prune targeted to this router's primary IP
address on the received interface. If the upstream neighbor address
field is not correct, these state transitions in this state machine
must not occur, although seeing such a packet may cause state
transitions in other state machines.
On unnumbered interfaces on point-to-point links, the router's
address should be the same as the source address it chose for the
Hello message it sent over that interface. However, on point-to-
point links we also recommend that for backwards compatibility PIM
Join/Prune messages with an upstream neighbor address field of all
zeros are also accepted.
Transitions from NoInfo State
When in NoInfo state, the following event may trigger a transition:
Receive Join(*,G)
A Join(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,G) downstream state machine on interface I transitions
to the Join state. The Expiry Timer (ET) is started and set
to the HoldTime from the triggering Join/Prune message.
Transitions from Join State
When in Join state, the following events may trigger a transition:
Receive Join(*,G)
A Join(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,G) downstream state machine on interface I remains in
Join state, and the Expiry Timer (ET) is restarted, set to
maximum of its current value and the HoldTime from the
triggering Join/Prune message.
Receive Prune(*,G)
A Prune(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,G) downstream state machine on interface I transitions
to the Prune-Pending state. The Prune-Pending Timer is
started. It is set to the J/P_Override_Interval(I) if the
router has more than one neighbor on that interface;
otherwise, it is set to zero, causing it to expire
immediately.
Expiry Timer Expires
The Expiry Timer for the (*,G) downstream state machine on
interface I expires.
The (*,G) downstream state machine on interface I transitions
to the NoInfo state.
Transitions from Prune-Pending State
When in Prune-Pending state, the following events may trigger a
transition:
Receive Join(*,G)
A Join(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (*,G) downstream state machine on interface I transitions
to the Join state. The Prune-Pending Timer is canceled
(without triggering an expiry event). The Expiry Timer is
restarted, set to maximum of its current value and the
HoldTime from the triggering Join/Prune message.
Expiry Timer Expires
The Expiry Timer for the (*,G) downstream state machine on
interface I expires.
The (*,G) downstream state machine on interface I transitions
to the NoInfo state.
Prune-Pending Timer Expires
The Prune-Pending Timer for the (*,G) downstream state machine
on interface I expires.
The (*,G) downstream state machine on interface I transitions
to the NoInfo state. A PruneEcho(*,G) is sent onto the subnet
connected to interface I.
The action "Send PruneEcho(*,G)" is triggered when the router
stops forwarding on an interface as a result of a prune. A
PruneEcho(*,G) is simply a Prune(*,G) message sent by the
upstream router on a LAN with its own address in the Upstream
Neighbor Address field. Its purpose is to add additional
reliability so that if a Prune that should have been
overridden by another router is lost locally on the LAN, then
the PruneEcho may be received and cause the override to
happen. A PruneEcho(*,G) need not be sent on an interface
that contains only a single PIM neighbor during the time this
state machine was in Prune-Pending state.
4.5.3. Receiving (S,G) Join/Prune Messages
The per-interface state machine for receiving (S,G) Join/Prune
messages is given below and is almost identical to that for (*,G)
messages. There are three states:
NoInfo (NI)
The interface has no (S,G) Join state and no (S,G) timers
running.
Join (J)
The interface has (S,G) Join state, which will cause the
router to forward packets from S destined for G from this
interface if the (S,G) state is active (the SPTbit is set)
except if the router lost an assert on this interface.
Prune-Pending (PP)
The router has received a Prune(S,G) on this interface from a
downstream neighbor and is waiting to see whether the prune
will be overridden by another downstream router. For
forwarding purposes, the Prune-Pending state functions exactly
like the Join state.
In addition, there are two timers:
Expiry Timer (ET)
This timer is set when a valid Join(S,G) is received. Expiry
of the Expiry Timer causes this state machine to revert to
NoInfo state.
Prune-Pending Timer (PPT)
This timer is set when a valid Prune(S,G) is received. Expiry
of the Prune-Pending Timer causes this state machine to revert
to NoInfo state.
Figure 4: Downstream per-interface (S,G) state machine in tabular form
+------------++--------------------------------------------------------+
| || Event |
| ++-------------+--------------+-------------+-------------+
|Prev State ||Receive | Receive | Prune- | Expiry Timer|
| ||Join(S,G) | Prune(S,G) | Pending | Expires |
| || | | Timer | |
| || | | Expires | |
+------------++-------------+--------------+-------------+-------------+
| ||-> J state | -> NI state | - | - |
|NoInfo (NI) ||start Expiry | | | |
| ||Timer | | | |
+------------++-------------+--------------+-------------+-------------+
| ||-> J state | -> PP state | - | -> NI state |
|Join (J) ||restart | start Prune- | | |
| ||Expiry Timer | Pending | | |
| || | Timer | | |
+------------++-------------+--------------+-------------+-------------+
|Prune- ||-> J state | -> PP state | -> NI state | -> NI state |
|Pending (PP)||restart | | Send Prune- | |
| ||Expiry Timer | | Echo(S,G) | |
+------------++-------------+--------------+-------------+-------------+
The transition events "Receive Join(S,G)" and "Receive Prune(S,G)"
imply receiving a Join or Prune targeted to this router's primary IP
address on the received interface. If the upstream neighbor address
field is not correct, these state transitions in this state machine
must not occur, although seeing such a packet may cause state
transitions in other state machines.
On unnumbered interfaces on point-to-point links, the router's
address should be the same as the source address it chose for the
Hello message it sent over that interface. However, on point-to-
point links we also recommend that for backwards compatibility PIM
Join/Prune messages with an upstream neighbor address field of all
zeros are also accepted.
Transitions from NoInfo State
When in NoInfo state, the following event may trigger a transition:
Receive Join(S,G)
A Join(S,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G) downstream state machine on interface I transitions
to the Join state. The Expiry Timer (ET) is started and set
to the HoldTime from the triggering Join/Prune message.
Transitions from Join State
When in Join state, the following events may trigger a transition:
Receive Join(S,G)
A Join(S,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G) downstream state machine on interface I remains in
Join state, and the Expiry Timer (ET) is restarted, set to
maximum of its current value and the HoldTime from the
triggering Join/Prune message.
Receive Prune(S,G)
A Prune(S,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G) downstream state machine on interface I transitions
to the Prune-Pending state. The Prune-Pending Timer is
started. It is set to the J/P_Override_Interval(I) if the
router has more than one neighbor on that interface;
otherwise, it is set to zero, causing it to expire
immediately.
Expiry Timer Expires
The Expiry Timer for the (S,G) downstream state machine on
interface I expires.
The (S,G) downstream state machine on interface I transitions
to the NoInfo state.
Transitions from Prune-Pending State
When in Prune-Pending state, the following events may trigger a
transition:
Receive Join(S,G)
A Join(S,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G) downstream state machine on interface I transitions
to the Join state. The Prune-Pending Timer is canceled
(without triggering an expiry event). The Expiry Timer is
restarted, set to maximum of its current value and the
HoldTime from the triggering Join/Prune message.
Expiry Timer Expires
The Expiry Timer for the (S,G) downstream state machine on
interface I expires.
The (S,G) downstream state machine on interface I transitions
to the NoInfo state.
Prune-Pending Timer Expires
The Prune-Pending Timer for the (S,G) downstream state machine
on interface I expires.
The (S,G) downstream state machine on interface I transitions
to the NoInfo state. A PruneEcho(S,G) is sent onto the subnet
connected to interface I.
The action "Send PruneEcho(S,G)" is triggered when the router
stops forwarding on an interface as a result of a prune. A
PruneEcho(S,G) is simply a Prune(S,G) message sent by the
upstream router on a LAN with its own address in the Upstream
Neighbor Address field. Its purpose is to add additional
reliability so that if a Prune that should have been
overridden by another router is lost locally on the LAN, then
the PruneEcho may be received and cause the override to
happen. A PruneEcho(S,G) need not be sent on an interface
that contains only a single PIM neighbor during the time this
state machine was in Prune-Pending state.
4.5.4. Receiving (S,G,rpt) Join/Prune Messages
The per-interface state machine for receiving (S,G,rpt) Join/Prune
messages is given below. There are five states:
NoInfo (NI)
The interface has no (S,G,rpt) Prune state and no (S,G,rpt)
timers running.
Prune (P)
The interface has (S,G,rpt) Prune state, which will cause the
router not to forward packets from S destined for G from this
interface even though the interface has active (*,G) Join
state.
Prune-Pending (PP)
The router has received a Prune(S,G,rpt) on this interface
from a downstream neighbor and is waiting to see whether the
prune will be overridden by another downstream router. For
forwarding purposes, the Prune-Pending state functions exactly
like the NoInfo state.
PruneTmp (P')
This state is a transient state that for forwarding purposes
behaves exactly like the Prune state. A (*,G) Join has been
received (which may cancel the (S,G,rpt) Prune). As we parse
the Join/Prune message from top to bottom, we first enter this
state if the message contains a (*,G) Join. Later in the
message, we will normally encounter an (S,G,rpt) prune to
reinstate the Prune state. However, if we reach the end of
the message without encountering such a (S,G,rpt) prune, then
we will revert to NoInfo state in this state machine.
As no time is spent in this state, no timers can expire.
Prune-Pending-Tmp (PP')
This state is a transient state that is identical to P' except
that it is associated with the PP state rather than the P
state. For forwarding purposes, PP' behaves exactly like PP
state.
In addition, there are two timers:
Expiry Timer (ET)
This timer is set when a valid Prune(S,G,rpt) is received.
Expiry of the Expiry Timer causes this state machine to revert
to NoInfo state.
Prune-Pending Timer (PPT)
This timer is set when a valid Prune(S,G,rpt) is received.
Expiry of the Prune-Pending Timer causes this state machine to
move on to Prune state.
Figure 5: Downstream per-interface (S,G,rpt) state machine
in tabular form
+----------++----------------------------------------------------------+
| || Event |
| ++---------+----------+----------+--------+--------+--------+
|Prev ||Receive | Receive | Receive | End of | Prune- | Expiry |
|State ||Join(*,G)| Join | Prune | Message| Pending| Timer |
| || | (S,G,rpt)| (S,G,rpt)| | Timer | Expires|
| || | | | | Expires| |
+----------++---------+----------+----------+--------+--------+--------+
| ||- | - | -> PP | - | - | - |
| || | | state | | | |
| || | | start | | | |
|NoInfo || | | Prune- | | | |
|(NI) || | | Pending | | | |
| || | | Timer; | | | |
| || | | start | | | |
| || | | Expiry | | | |
| || | | Timer | | | |
+----------++---------+----------+----------+--------+--------+--------+
| ||-> P' | -> NI | -> P | - | - | -> NI |
| ||state | state | state | | | state |
|Prune (P) || | | restart | | | |
| || | | Expiry | | | |
| || | | Timer | | | |
+----------++---------+----------+----------+--------+--------+--------+
|Prune- ||-> PP' | -> NI | - | - | -> P | - |
|Pending ||state | state | | | state | |
|(PP) || | | | | | |
+----------++---------+----------+----------+--------+--------+--------+
| ||- | - | -> P | -> NI | - | - |
|PruneTmp || | | state | state | | |
|(P') || | | restart | | | |
| || | | Expiry | | | |
| || | | Timer | | | |
+----------++---------+----------+----------+--------+--------+--------+
| ||- | - | -> PP | -> NI | - | - |
|Prune- || | | state | state | | |
|Pending- || | | restart | | | |
|Tmp (PP') || | | Expiry | | | |
| || | | Timer | | | |
+----------++---------+----------+----------+--------+--------+--------+
The transition events "Receive Join(S,G,rpt)", "Receive
Prune(S,G,rpt)", and "Receive Join(*,G)" imply receiving a Join or
Prune targeted to this router's primary IP address on the received
interface. If the upstream neighbor address field is not correct,
these state transitions in this state machine must not occur,
although seeing such a packet may cause state transitions in other
state machines.
On unnumbered interfaces on point-to-point links, the router's
address should be the same as the source address it chose for the
Hello message it sent over that interface. However, on point-to-
point links we also recommend that PIM Join/Prune messages with an
upstream neighbor address field of all zeros are also accepted.
Transitions from NoInfo State
When in NoInfo (NI) state, the following event may trigger a
transition:
Receive Prune(S,G,rpt)
A Prune(S,G,rpt) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I
transitions to the Prune-Pending state. The Expiry Timer (ET)
is started and set to the HoldTime from the triggering
Join/Prune message. The Prune-Pending Timer is started. It
is set to the J/P_Override_Interval(I) if the router has more
than one neighbor on that interface; otherwise, it is set to
zero, causing it to expire immediately.
Transitions from Prune-Pending State
When in Prune-Pending (PP) state, the following events may trigger a
transition:
Receive Join(*,G)
A Join(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I
transitions to Prune-Pending-Tmp state whilst the remainder of
the compound Join/Prune message containing the Join(*,G) is
processed.
Receive Join(S,G,rpt)
A Join(S,G,rpt) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I
transitions to NoInfo state. ET and PPT are canceled.
Prune-Pending Timer Expires
The Prune-Pending Timer for the (S,G,rpt) downstream state
machine on interface I expires.
The (S,G,rpt) downstream state machine on interface I
transitions to the Prune state.
Transitions from Prune State
When in Prune (P) state, the following events may trigger a
transition:
Receive Join(*,G)
A Join(*,G) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I
transitions to PruneTmp state whilst the remainder of the
compound Join/Prune message containing the Join(*,G) is
processed.
Receive Join(S,G,rpt)
A Join(S,G,rpt) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I
transitions to NoInfo state. ET and PPT are canceled.
Receive Prune(S,G,rpt)
A Prune(S,G,rpt) is received on interface I with its Upstream
Neighbor Address set to the router's primary IP address on I.
The (S,G,rpt) downstream state machine on interface I remains
in Prune state. The Expiry Timer (ET) is restarted, set to
maximum of its current value and the HoldTime from the
triggering Join/Prune message.
Expiry Timer Expires
The Expiry Timer for the (S,G,rpt) downstream state machine on
interface I expires.
The (S,G,rpt) downstream state machine on interface I
transitions to the NoInfo state.
Transitions from Prune-Pending-Tmp State
When in Prune-Pending-Tmp (PP') state and processing a compound
Join/Prune message, the following events may trigger a transition:
Receive Prune(S,G,rpt)
The compound Join/Prune message contains a Prune(S,G,rpt).
The (S,G,rpt) downstream state machine on interface I
transitions back to the Prune-Pending state. The Expiry Timer
(ET) is restarted, set to maximum of its current value and the
HoldTime from the triggering Join/Prune message.
End of Message
The end of the compound Join/Prune message is reached.
The (S,G,rpt) downstream state machine on interface I
transitions to the NoInfo state. ET and PPT are canceled.
Transitions from PruneTmp State
When in PruneTmp (P') state and processing a compound Join/Prune
message, the following events may trigger a transition:
Receive Prune(S,G,rpt)
The compound Join/Prune message contains a Prune(S,G,rpt).
The (S,G,rpt) downstream state machine on interface I
transitions back to the Prune state. The Expiry Timer (ET) is
restarted, set to maximum of its current value and the
HoldTime from the triggering Join/Prune message.
End of Message
The end of the compound Join/Prune message is reached.
The (S,G,rpt) downstream state machine on interface I
transitions to the NoInfo state. ET is canceled.
Notes:
Receiving a Prune(*,G) does not affect the (S,G,rpt) downstream state
machine.
Receiving a Join(*,*,RP) does not affect the (S,G,rpt) downstream
state machine. If a router has originated Join(*,*,RP) and pruned a
source off it using Prune(S,G,rpt), then to receive that source again
it should explicitly re-join using Join(S,G,rpt) or Join(*,G). In
some LAN topologies it is possible for a router sending a new
Join(*,*,RP) to have to wait as much as a Join/Prune Interval before
noticing that it needs to override a neighbor's preexisting
Prune(S,G,rpt). This is considered acceptable, as (*,*,RP) state is
intended to be used only in long-lived and persistent scenarios.
4.5.5. Sending (*,*,RP) Join/Prune Messages
The per-interface state machines for (*,*,RP) hold join state from
downstream PIM routers. This state then determines whether a router
needs to propagate a Join(*,*,RP) upstream towards the RP.
If a router wishes to propagate a Join(*,*,RP) upstream, it must also
watch for messages on its upstream interface from other routers on
that subnet, and these may modify its behavior. If it sees a
Join(*,*,RP) to the correct upstream neighbor, it should suppress its
own Join(*,*,RP). If it sees a Prune(*,*,RP) to the correct upstream
neighbor, it should be prepared to override that prune by sending a
Join(*,*,RP) almost immediately. Finally, if it sees the Generation
ID (see Section