<- RFC Index (7701..7800)
(also STD 83)
Obsoletes RFC 4601
Internet Engineering Task Force (IETF) B. Fenner
Request for Comments: 7761 Arista Networks
STD: 83 M. Handley
Obsoletes: 4601 UCL
Category: Standards Track H. Holbrook
ISSN: 2070-1721 I. Kouvelas
Arista Networks
R. Parekh
Cisco Systems, Inc.
Z. Zhang
Juniper Networks
L. Zheng
Huawei Technologies
March 2016
Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)
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 it optionally
creates shortest-path trees per source.
This document obsoletes RFC 4601 by replacing it, addresses the
errata filed against it, removes the optional (*,*,RP), PIM Multicast
Border Router features and authentication using IPsec that lack
sufficient deployment experience (see Appendix A), and moves the PIM
specification to Internet Standard.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7761.
Fenner, et al. Standards Track [Page 1]
RFC 7761 PIM-SM Specification (Revised) March 2016
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 ...................................9
3.3. Phase Three: Shortest-Path Tree ...........................10
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. (*,G) State ........................................15
4.1.3. (S,G) State ........................................17
4.1.4. (S,G,rpt) State ....................................19
4.1.5. State Summarization Macros .........................20
4.2. Data Packet Forwarding Rules ..............................24
4.2.1. Last-Hop Switchover to the SPT .....................27
4.2.2. Setting and Clearing the (S,G) SPTbit ..............27
4.3. Designated Routers (DRs) and Hello Messages ...............29
4.3.1. Sending Hello Messages .............................29
4.3.2. DR Election ........................................31
4.3.3. Reducing Prune Propagation Delay on LANs ...........33
4.3.4. Maintaining Secondary Address Lists ................36
4.4. PIM Register Messages .....................................37
4.4.1. Sending Register Messages from the DR ..............38
4.4.2. Receiving Register Messages at the RP ..............43
Fenner, et al. Standards Track [Page 2]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5. PIM Join/Prune Messages ...................................44
4.5.1. Receiving (*,G) Join/Prune Messages ................45
4.5.2. Receiving (S,G) Join/Prune Messages ................50
4.5.3. Receiving (S,G,rpt) Join/Prune Messages ............54
4.5.4. Sending (*,G) Join/Prune Messages ..................61
4.5.5. Sending (S,G) Join/Prune Messages ..................65
4.5.6. (S,G,rpt) Periodic Messages ........................71
4.5.7. State Machine for (S,G,rpt) Triggered Messages .....72
4.6. PIM Assert Messages .......................................76
4.6.1. (S,G) Assert Message State Machine .................77
4.6.2. (*,G) Assert Message State Machine .................85
4.6.3. Assert Metrics .....................................93
4.6.4. AssertCancel Messages ..............................94
4.6.5. Assert State Macros ................................95
4.7. PIM Bootstrap and RP Discovery ............................98
4.7.1. Group-to-RP Mapping ................................99
4.7.2. Hash Function .....................................100
4.8. Source-Specific Multicast ................................101
4.8.1. Protocol Modifications for SSM Destination
Addresses .........................................102
4.8.2. PIM-SSM-Only Routers ..............................102
4.9. PIM Packet Formats .......................................104
4.9.1. Encoded Source and Group Address Formats ..........105
4.9.2. Hello Message Format ..............................108
4.9.3. Register Message Format ...........................111
4.9.4. Register-Stop Message Format ......................113
4.9.5. Join/Prune Message Format .........................114
4.9.5.1. Group Set Source List Rules ..............117
4.9.5.2. Group Set Fragmentation ..................120
4.9.6. Assert Message Format .............................121
4.10. PIM Timers ..............................................122
4.11. Timer Values ............................................124
5. IANA Considerations ...........................................130
5.1. PIM Address Family .......................................130
5.2. PIM Hello Options ........................................130
6. Security Considerations .......................................131
6.1. Attacks Based on Forged Messages .........................131
6.1.1. Forged Link-Local Messages ........................131
6.1.2. Forged Unicast Messages ...........................132
6.2. Non-cryptographic Authentication Mechanisms ..............132
6.3. Authentication ...........................................133
6.4. Denial-of-Service Attacks ................................133
7. References ....................................................133
7.1. Normative References .....................................133
7.2. Informative References ...................................134
Appendix A. Functionality Removed from RFC 4601 ..................136
Acknowledgements .................................................136
Authors' Addresses ...............................................136
Fenner, et al. Standards Track [Page 3]
RFC 7761 PIM-SM Specification (Revised) March 2016
List of Figures (Shown in Tabular Form)
Figure 1. Per-(S,G) Register State Machine at a DR ................39
Figure 2. Downstream Per-Interface (*,G) State Machine ............47
Figure 3. Downstream Per-Interface (S,G) State Machine ............51
Figure 4. Downstream Per-Interface (S,G,rpt) State Machine ........56
Figure 5. Upstream (*,G) State Machine ............................62
Figure 6. Upstream (S,G) State Machine ............................66
Figure 7. Upstream (S,G,rpt) State Machine for Triggered
Messages ................................................72
Figure 8. Per-Interface (S,G) Assert State Machine ................78
Figure 9. Per-interface (*,G) Assert State Machine ................87
Fenner, et al. Standards Track [Page 4]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 specified in RFC 4601 as a Proposed Standard.
This document is intended to address the reported errata and to
remove the optional (*,*,RP), PIM Multicast Border Router features
and authentication using IPsec that lacks sufficient deployment
experience, to advance PIM-SM to Internet Standard.
This document specifies the same protocol as RFC 4601, and
implementations per the specification in this document will be able
to interoperate successfully with implementations per RFC 4601.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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.
Fenner, et al. Standards Track [Page 5]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 the 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.
Fenner, et al. Standards Track [Page 6]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Unless otherwise noted, operations specified by statements having
multiple (+) and (-) operators should be evaluated from left to
right, i.e., A (+) B (-) C is the set resulting from union of sets A
and B minus elements in set C.
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.
Fenner, et al. Standards Track [Page 7]
RFC 7761 PIM-SM Specification (Revised) March 2016 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. Fenner, et al. Standards Track [Page 8]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 two, 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.
Fenner, et al. Standards Track [Page 9]
RFC 7761 PIM-SM Specification (Revised) March 2016 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. Fenner, et al. Standards Track [Page 10]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
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
Fenner, et al. Standards Track [Page 11]
RFC 7761 PIM-SM Specification (Revised) March 2016
neither router or both routers have (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 for 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 BSR
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.
Fenner, et al. Standards Track [Page 12]
RFC 7761 PIM-SM Specification (Revised) March 2016
o Section 4.7 specifies the RP discovery mechanisms.
o Section 4.8 describes PIM-SSM, the subset of PIM required to
support Source-Specific Multicast.
o Section 4.9 specifies the PIM packet formats.
o Section 4.10 provides a summary of PIM-SM timers, and Section 4.11
provides their default values.
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 three sections:
(*,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.
Fenner, et al. Standards Track [Page 13]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 14]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.1.2. (*,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. It is RECOMMENDED to store this information
Fenner, et al. Standards Track [Page 15]
RFC 7761 PIM-SM Specification (Revised) March 2016 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 4.1.5. PIM (*,G) Join/Prune state is the result of receiving PIM (*,G) Join/Prune messages on this interface and is specified in Section 4.5.1. The state is used by the macros that calculate the outgoing interface list in Section 4.1.5, and in the JoinDesired(*,G) macro (defined in Section 4.5.4) 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 4.5.4. 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 changes, 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 4.5.4. Fenner, et al. Standards Track [Page 16]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.1.3. (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)
Fenner, et al. Standards Track [Page 17]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 (RST)
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
an (S,G) assert on this interface for this group. However, it is
RECOMMENDED to store 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 4.1.5.
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 4.1.5, and in the JoinDesired(S,G)
macro (defined in Section 4.5.5) 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 4.5.5.
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 4.5.5.
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
Fenner, et al. Standards Track [Page 18]
RFC 7761 PIM-SM Specification (Revised) March 2016
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).
4.1.4. (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)
Fenner, et al. Standards Track [Page 19]
RFC 7761 PIM-SM Specification (Revised) March 2016 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 4.1.5. 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 4.5.3. The state is used by the macros that calculate the outgoing interface list in Section 4.1.5, and in the rules for adding Prune(S,G,rpt) messages to Join(*,G) messages specified in Section 4.5.6. 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 4.5.7. 4.1.5. 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(*,G). Fenner, et al. Standards Track [Page 20]
RFC 7761 PIM-SM Specification (Revised) March 2016
Generally speaking, the inherited_olists are used for forwarding, and
the immediate_olists are used to make decisions about state
maintenance.
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(*,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
Fenner, et al. Standards Track [Page 21]
RFC 7761 PIM-SM Specification (Revised) March 2016
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(*,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 4.5.1.
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 4.5.2.
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 4.5.3.
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.
Fenner, et al. Standards Track [Page 22]
RFC 7761 PIM-SM Specification (Revised) March 2016
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
Fenner, et al. Standards Track [Page 23]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 4.5.6).
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 the RP, except at the RP when it is
the decapsulation interface (the "virtual" interface on which
Register packets are received).
Fenner, et al. Standards Track [Page 24]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 (*,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
Fenner, et al. Standards Track [Page 25]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 finite state machine 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 (*,G) state,
asserts, etc.
inherited_olist(S,G,rpt) is the outgoing interface list for packets
forwarded on (*,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 4.5.5.
Keepalive_Period is defined in Section 4.11.
Fenner, et al. Standards Track [Page 26]
RFC 7761 PIM-SM Specification (Revised) March 2016
Data-triggered PIM-Assert messages sent from the above forwarding
code SHOULD be rate-limited in an 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 (*,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 (*,G) state while upstream (S,G) state is being established,
during which time a router should continue to forward only on (*,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.
Fenner, et al. Standards Track [Page 27]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 4.5.5 and indicates whether we
have the appropriate (S,G) Join state to wish to send a Join(S,G)
upstream.
Basically, Update_SPTbit(S,G,iif) 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 apply:
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. No one 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. The RPF'(S,G) != NULL check ensures that the
SPTbit is set only if the RPF neighbor towards S is valid.
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.
Fenner, et al. Standards Track [Page 28]
RFC 7761 PIM-SM Specification (Revised) March 2016
The SPTbit is cleared in the (S,G) upstream state machine (see
Section 4.5.5) when JoinDesired(S,G) becomes FALSE.
4.3. Designated Routers (DRs) 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 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.
Fenner, et al. Standards Track [Page 29]
RFC 7761 PIM-SM Specification (Revised) March 2016 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 Fenner, et al. Standards Track [Page 30]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 31]
RFC 7761 PIM-SM Specification (Revised) March 2016
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
}
Fenner, et al. Standards Track [Page 32]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 33]
RFC 7761 PIM-SM Specification (Revised) March 2016
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
}
Fenner, et al. Standards Track [Page 34]
RFC 7761 PIM-SM Specification (Revised) March 2016
To avoid synchronization of override messages when multiple
downstream routers share a multi-access link, the 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
}
Fenner, et al. Standards Track [Page 35]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 4.11).
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.
Fenner, et al. Standards Track [Page 36]
RFC 7761 PIM-SM Specification (Revised) March 2016 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. Fenner, et al. Standards Track [Page 37]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 38]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 1: Per-(S,G) Register State Machine at a DR
+----------++----------------------------------------------------------+
| || 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 | | | | |
+----------++----------+-----------+-----------+-----------+-----------+
Fenner, et al. Standards Track [Page 39]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 specification 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.
Fenner, et al. Standards Track [Page 40]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 Explicit Congestion Notification (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.
Fenner, et al. Standards Track [Page 41]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 42]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 ( 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
Fenner, et al. Standards Track [Page 43]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 SHOULD NOT make this a special case, but
SHOULD 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 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 (S,G), and (S,G,rpt) Joins and Prunes can only
affect their respective downstream state machines. When considering
Fenner, et al. Standards Track [Page 44]
RFC 7761 PIM-SM Specification (Revised) March 2016 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 (see Section 6.3), then all Join/Prune messages from that neighbor MUST also be authenticated. 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 (*,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). Fenner, et al. Standards Track [Page 45]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 4.5.3) 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.
Fenner, et al. Standards Track [Page 46]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 2: Downstream Per-Interface (*,G) State Machine
+------------++--------------------------------------------------------+
| || 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 it is RECOMMENDED that for backwards compatibility PIM
Join/Prune messages with an upstream neighbor address field of all
zeros also be accepted.
Fenner, et al. Standards Track [Page 47]
RFC 7761 PIM-SM Specification (Revised) March 2016
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. The ET
is set to the 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.
Fenner, et al. Standards Track [Page 48]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 (ET) is restarted and is then set to the
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.
Fenner, et al. Standards Track [Page 49]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5.2. 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.
Fenner, et al. Standards Track [Page 50]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 3: Downstream Per-Interface (S,G) State Machine
+------------++--------------------------------------------------------+
| || 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 it is RECOMMENDED that for backwards compatibility PIM
Join/Prune messages with an upstream neighbor address field of all
zeros also be accepted.
Fenner, et al. Standards Track [Page 51]
RFC 7761 PIM-SM Specification (Revised) March 2016
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. The Expiry Timer (ET) is restarted and is then
set to the 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.
Fenner, et al. Standards Track [Page 52]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 (ET) is restarted and is then set to the
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.
Fenner, et al. Standards Track [Page 53]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5.3. 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 an
(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 the PP state.
Fenner, et al. Standards Track [Page 54]
RFC 7761 PIM-SM Specification (Revised) March 2016
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.
Fenner, et al. Standards Track [Page 55]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 4: Downstream Per-Interface (S,G,rpt) State Machine
+----------++----------------------------------------------------------+
| || 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,
Fenner, et al. Standards Track [Page 56]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 it is RECOMMENDED that PIM Join/Prune messages with an
upstream neighbor address field of all zeros also be 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.
Fenner, et al. Standards Track [Page 57]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 the 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 the NoInfo state. The 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.
Fenner, et al. Standards Track [Page 58]
RFC 7761 PIM-SM Specification (Revised) March 2016
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 the 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 the NoInfo state. The 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
and is then set to the 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.
Fenner, et al. Standards Track [Page 59]
RFC 7761 PIM-SM Specification (Revised) March 2016
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)
that 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 back to the Prune-Pending state. The
Expiry Timer (ET) is restarted and is then set to the
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. The 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 and is then set to the 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.
Note: Receiving a Prune(*,G) does not affect the (S,G,rpt) downstream
state machine.
Fenner, et al. Standards Track [Page 60]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5.4. Sending (*,G) Join/Prune Messages
The per-interface state machines for (*,G) hold join state from
downstream PIM routers. This state then determines whether a router
needs to propagate a Join(*,G) upstream towards the RP.
If a router wishes to propagate a Join(*,G) 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(*,G) to the correct upstream neighbor, it should suppress its
own Join(*,G). If it sees a Prune(*,G) to the correct upstream
neighbor, it should be prepared to override that prune by sending a
Join(*,G) almost immediately. Finally, if it sees the Generation ID
(see Section 4.3) of the correct upstream neighbor change, it knows
that the upstream neighbor has lost state, and it should be prepared
to refresh the state by sending a Join(*,G) almost immediately.
If a (*,G) Assert occurs on the upstream interface, and this changes
this router's idea of the upstream neighbor, it should be prepared to
ensure that the Assert winner is aware of downstream routers by
sending a Join(*,G) almost immediately.
In addition, if the MRIB changes to indicate that the next hop
towards the RP has changed, and either the upstream interface changes
or there is no Assert winner on the upstream interface, the router
should prune off from the old next hop and join towards the new
next hop.
The upstream (*,G) state machine only contains two states:
Not Joined
The downstream state machines indicate that the router does
not need to join the RP tree for this group.
Joined
The downstream state machines indicate that the router
should join the RP tree for this group.
In addition, one timer JT(*,G) is kept that is used to trigger the
sending of a Join(*,G) to the upstream next hop towards the RP,
RPF'(*,G).
Fenner, et al. Standards Track [Page 61]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 5: Upstream (*,G) State Machine
+-------------------++-------------------------------------------------+
| || Event |
| Prev State ++------------------------+------------------------+
| || JoinDesired(*,G) | JoinDesired(*,G) |
| || ->True | ->False |
+-------------------++------------------------+------------------------+
| || -> J state | - |
| NotJoined (NJ) || Send Join(*,G); | |
| || set Join Timer to | |
| || t_periodic | |
+-------------------++------------------------+------------------------+
| Joined (J) || - | -> NJ state |
| || | Send Prune(*,G); |
| || | cancel Join Timer |
+-------------------++------------------------+------------------------+
In addition, we have the following transitions, which occur within
the Joined state:
+----------------------------------------------------------------------+
| In Joined (J) State |
+----------------+-----------------+-----------------+-----------------+
|Timer Expires | See Join(*,G) | See Prune(*,G) | RPF'(*,G) |
| | to RPF'(*,G) | to RPF'(*,G) | changes due to |
| | | | an Assert |
+----------------+-----------------+-----------------+-----------------+
|Send | Increase Join | Decrease Join | Decrease Join |
|Join(*,G); set | Timer to | Timer to | Timer to |
|Join Timer to | t_joinsuppress | t_override | t_override |
|t_periodic | | | |
+----------------+-----------------+-----------------+-----------------+
+----------------------------------------------------------------------+
| In Joined (J) State |
+----------------------------------+-----------------------------------+
| RPF'(*,G) changes not | RPF'(*,G) GenID changes |
| due to an Assert | |
+----------------------------------+-----------------------------------+
| Send Join(*,G) to new | Decrease Join Timer to |
| next hop; send | t_override |
| Prune(*,G) to old next | |
| hop; set Join Timer to | |
| t_periodic | |
+----------------------------------+-----------------------------------+
Fenner, et al. Standards Track [Page 62]
RFC 7761 PIM-SM Specification (Revised) March 2016
This state machine uses the following macro:
bool JoinDesired(*,G) {
if (immediate_olist(*,G) != NULL)
return TRUE
else
return FALSE
}
JoinDesired(*,G) is true when the router has forwarding state that
would cause it to forward traffic for G using shared tree state.
Note that although JoinDesired is true, the router's sending of a
Join(*,G) message may be suppressed by another router sending a
Join(*,G) onto the upstream interface.
Transitions from NotJoined State
When the upstream (*,G) state machine is in NotJoined state, the
following event may trigger a state transition:
JoinDesired(*,G) becomes True
The macro JoinDesired(*,G) becomes True, e.g., because the
downstream state for (*,G) has changed so that at least one
interface is in immediate_olist(*,G).
The upstream (*,G) state machine transitions to the Joined
state. Send Join(*,G) to the appropriate upstream neighbor,
which is RPF'(*,G). Set the Join Timer (JT) to expire after
t_periodic seconds.
Transitions from Joined State
When the upstream (*,G) state machine is in Joined state, the
following events may trigger state transitions:
JoinDesired(*,G) becomes False
The macro JoinDesired(*,G) becomes False, e.g., because the
downstream state for (*,G) has changed so no interface is in
immediate_olist(*,G).
The upstream (*,G) state machine transitions to the
NotJoined state. Send Prune(*,G) to the appropriate
upstream neighbor, which is RPF'(*,G). Cancel the
Join Timer (JT).
Fenner, et al. Standards Track [Page 63]
RFC 7761 PIM-SM Specification (Revised) March 2016
Join Timer Expires
The Join Timer (JT) expires, indicating time to send a
Join(*,G).
Send Join(*,G) to the appropriate upstream neighbor, which
is RPF'(*,G). Restart the Join Timer (JT) to expire after
t_periodic seconds.
See Join(*,G) to RPF'(*,G)
This event is only relevant if RPF_interface(RP(G)) is a
shared medium. This router sees another router on
RPF_interface(RP(G)) send a Join(*,G) to RPF'(*,G). This
causes this router to suppress its own Join.
The upstream (*,G) state machine remains in Joined state.
Let t_joinsuppress be the minimum of t_suppressed and the
HoldTime from the Join/Prune message triggering this event.
If the Join Timer is set to expire in less than
t_joinsuppress seconds, reset it so that it expires after
t_joinsuppress seconds. If the Join Timer is set to expire
in more than t_joinsuppress seconds, leave it unchanged.
See Prune(*,G) to RPF'(*,G)
This event is only relevant if RPF_interface(RP(G)) is a
shared medium. This router sees another router on
RPF_interface(RP(G)) send a Prune(*,G) to RPF'(*,G). As
this router is in Joined state, it must override the Prune
after a short random interval.
The upstream (*,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds. If the Join Timer is set to expire in
less than t_override seconds, leave it unchanged.
RPF'(*,G) changes due to an Assert
The current next hop towards the RP changes due to an
Assert(*,G) on the RPF_interface(RP(G)).
The upstream (*,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds. If the Join Timer is set to expire in
less than t_override seconds, leave it unchanged.
Fenner, et al. Standards Track [Page 64]
RFC 7761 PIM-SM Specification (Revised) March 2016
RPF'(*,G) changes not due to an Assert
An event occurred that caused the next hop towards the RP
for G to change. This may be caused by a change in the MRIB
routing database or the group-to-RP mapping. Note that this
transition does not occur if an Assert is active and the
upstream interface does not change.
The upstream (*,G) state machine remains in Joined state.
Send Join(*,G) to the new upstream neighbor, which is the
new value of RPF'(*,G). Send Prune(*,G) to the old upstream
neighbor, which is the old value of RPF'(*,G). Use the new
value of RP(G) in the Prune(*,G) message or all zeros if
RP(G) becomes unknown (old value of RP(G) may be used
instead to improve behavior in routers implementing older
versions of this specification). Set the Join Timer (JT) to
expire after t_periodic seconds.
RPF'(*,G) GenID changes
The Generation ID of the router that is RPF'(*,G) changes.
This normally means that this neighbor has lost state, and
so the state must be refreshed.
The upstream (*,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds.
4.5.5. Sending (S,G) Join/Prune Messages
The per-interface state machines for (S,G) hold join state from
downstream PIM routers. This state then determines whether a router
needs to propagate a Join(S,G) upstream towards the source.
If a router wishes to propagate a Join(S,G) 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(S,G) to the correct upstream neighbor, it should suppress its
own Join(S,G). If it sees a Prune(S,G), Prune(S,G,rpt), or
Prune(*,G) to the correct upstream neighbor towards S, it should be
prepared to override that prune by scheduling a Join(S,G) to be sent
almost immediately. Finally, if it sees the Generation ID of its
upstream neighbor change, it knows that the upstream neighbor has
lost state, and it should refresh the state by scheduling a Join(S,G)
to be sent almost immediately.
Fenner, et al. Standards Track [Page 65]
RFC 7761 PIM-SM Specification (Revised) March 2016
If an (S,G) Assert occurs on the upstream interface, and this changes
this router's idea of the upstream neighbor, it should be prepared to
ensure that the Assert winner is aware of downstream routers by
scheduling a Join(S,G) to be sent almost immediately.
In addition, if MRIB changes cause the next hop towards the source to
change, and either the upstream interface changes or there is no
Assert winner on the upstream interface, the router should send a
prune to the old next hop and a join to the new next hop.
The upstream (S,G) state machine only contains two states:
Not Joined
The downstream state machines and local membership
information do not indicate that the router needs to join
the shortest-path tree for this (S,G).
Joined
The downstream state machines and local membership
information indicate that the router should join the
shortest-path tree for this (S,G).
In addition, one timer JT(S,G) is kept that is used to trigger the
sending of a Join(S,G) to the upstream next hop towards S, RPF'(S,G).
Figure 6: Upstream (S,G) State Machine
+-------------------+--------------------------------------------------+
| | Event |
| Prev State +-------------------------+------------------------+
| | JoinDesired(S,G) | JoinDesired(S,G) |
| | ->True | ->False |
+-------------------+-------------------------+------------------------+
| NotJoined (NJ) | -> J state | - |
| | Send Join(S,G); | |
| | set Join Timer to | |
| | t_periodic | |
+-------------------+-------------------------+------------------------+
| Joined (J) | - | -> NJ state |
| | | Send Prune(S,G); |
| | | set SPTbit(S,G) to |
| | | FALSE; cancel Join |
| | | Timer |
+-------------------+-------------------------+------------------------+
Fenner, et al. Standards Track [Page 66]
RFC 7761 PIM-SM Specification (Revised) March 2016
In addition, we have the following transitions, which occur within
the Joined state:
+----------------------------------------------------------------------+
| In Joined (J) State |
+-----------------+-----------------+-----------------+----------------+
| Timer Expires | See Join(S,G) | See Prune(S,G) | See Prune |
| | to RPF'(S,G) | to RPF'(S,G) | (S,G,rpt) to |
| | | | RPF'(S,G) |
+-----------------+-----------------+-----------------+----------------+
| Send | Increase Join | Decrease Join | Decrease Join |
| Join(S,G); set | Timer to | Timer to | Timer to |
| Join Timer to | t_joinsuppress | t_override | t_override |
| t_periodic | | | |
+-----------------+-----------------+-----------------+----------------+
+----------------------------------------------------------------------+
| In Joined (J) State |
+-----------------+-----------------+----------------+-----------------+
| See Prune(*,G) | RPF'(S,G) | RPF'(S,G) | RPF'(S,G) |
| to RPF'(S,G) | changes not | GenID changes | changes due to |
| | due to an | | an Assert |
| | Assert | | |
+-----------------+-----------------+----------------+-----------------+
| Decrease Join | Send Join(S,G) | Decrease Join | Decrease Join |
| Timer to | to new next | Timer to | Timer to |
| t_override | hop; send | t_override | t_override |
| | Prune(S,G) to | | |
| | old next hop; | | |
| | set Join Timer | | |
| | to t_periodic | | |
+-----------------+-----------------+----------------+-----------------+
This state machine uses the following macro:
bool JoinDesired(S,G) {
return( immediate_olist(S,G) != NULL
OR ( KeepaliveTimer(S,G) is running
AND inherited_olist(S,G) != NULL ) )
}
JoinDesired(S,G) is true when the router has forwarding state that
would cause it to forward traffic for G using source tree state. The
source tree state can be as a result of either active source-specific
join state, or the (S,G) Keepalive Timer and active non-source-
specific state. Note that although JoinDesired is true, the router's
sending of a Join(S,G) message may be suppressed by another router
sending a Join(S,G) onto the upstream interface.
Fenner, et al. Standards Track [Page 67]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from NotJoined State
When the upstream (S,G) state machine is in NotJoined state, the
following event may trigger a state transition:
JoinDesired(S,G) becomes True
The macro JoinDesired(S,G) becomes True, e.g., because the
downstream state for (S,G) has changed so that at least one
interface is in inherited_olist(S,G).
The upstream (S,G) state machine transitions to the Joined
state. Send Join(S,G) to the appropriate upstream neighbor,
which is RPF'(S,G). Set the Join Timer (JT) to expire after
t_periodic seconds.
Transitions from Joined State
When the upstream (S,G) state machine is in Joined state, the
following events may trigger state transitions:
JoinDesired(S,G) becomes False
The macro JoinDesired(S,G) becomes False, e.g., because the
downstream state for (S,G) has changed so no interface is in
inherited_olist(S,G).
The upstream (S,G) state machine transitions to the
NotJoined state. Send Prune(S,G) to the appropriate
upstream neighbor, which is RPF'(S,G). Cancel the
Join Timer (JT), and set SPTbit(S,G) to FALSE.
Join Timer Expires
The Join Timer (JT) expires, indicating time to send a
Join(S,G).
Send Join(S,G) to the appropriate upstream neighbor, which
is RPF'(S,G). Restart the Join Timer (JT) to expire after
t_periodic seconds.
Fenner, et al. Standards Track [Page 68]
RFC 7761 PIM-SM Specification (Revised) March 2016
See Join(S,G) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared
medium. This router sees another router on RPF_interface(S)
send a Join(S,G) to RPF'(S,G). This causes this router to
suppress its own Join.
The upstream (S,G) state machine remains in Joined state.
Let t_joinsuppress be the minimum of t_suppressed and the
HoldTime from the Join/Prune message triggering this event.
If the Join Timer is set to expire in less than
t_joinsuppress seconds, reset it so that it expires after
t_joinsuppress seconds. If the Join Timer is set to expire
in more than t_joinsuppress seconds, leave it unchanged.
See Prune(S,G) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared
medium. This router sees another router on RPF_interface(S)
send a Prune(S,G) to RPF'(S,G). As this router is in Joined
state, it must override the Prune after a short random
interval.
The upstream (S,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds.
See Prune(S,G,rpt) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared
medium. This router sees another router on RPF_interface(S)
send a Prune(S,G,rpt) to RPF'(S,G). If the upstream router
is an RFC-2362-compliant PIM router, then the Prune(S,G,rpt)
will cause it to stop forwarding. For backwards
compatibility, this router should override the prune so that
forwarding continues.
The upstream (S,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds.
Fenner, et al. Standards Track [Page 69]
RFC 7761 PIM-SM Specification (Revised) March 2016
See Prune(*,G) to RPF'(S,G)
This event is only relevant if RPF_interface(S) is a shared
medium. This router sees another router on RPF_interface(S)
send a Prune(*,G) to RPF'(S,G). If the upstream router is
an RFC-2362-compliant PIM router, then the Prune(*,G) will
cause it to stop forwarding. For backwards compatibility,
this router should override the prune so that forwarding
continues.
The upstream (S,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds.
RPF'(S,G) changes due to an Assert
The current next hop towards S changes due to an Assert(S,G)
on the RPF_interface(S).
The upstream (S,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds. If the Join Timer is set to expire in
less than t_override seconds, leave it unchanged.
RPF'(S,G) changes not due to an Assert
An event occurred that caused the next hop towards S to
change. Note that this transition does not occur if an
Assert is active and the upstream interface does not change.
The upstream (S,G) state machine remains in Joined state.
Send Join(S,G) to the new upstream neighbor, which is the
new value of RPF'(S,G). Send Prune(S,G) to the old upstream
neighbor, which is the old value of RPF'(S,G). Set the
Join Timer (JT) to expire after t_periodic seconds.
RPF'(S,G) GenID changes
The Generation ID of the router that is RPF'(S,G) changes.
This normally means that this neighbor has lost state, and
so the state must be refreshed.
The upstream (S,G) state machine remains in Joined state.
If the Join Timer is set to expire in more than
t_override seconds, reset it so that it expires after
t_override seconds.
Fenner, et al. Standards Track [Page 70]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5.6. (S,G,rpt) Periodic Messages
(S,G,rpt) Joins and Prunes are (S,G) Joins or Prunes sent on the RP
tree with the RPT bit set, either to modify the results of (*,G)
Joins, or to override the behavior of other upstream LAN peers. The
next section describes the rules for sending triggered messages.
This section describes the rules for including a Prune(S,G,rpt)
message with a Join(*,G).
When a router is going to send a Join(*,G), it should use the
following pseudocode, for each (S,G) for which it has state, to
decide whether to include a Prune(S,G,rpt) in the compound Join/Prune
message:
if( SPTbit(S,G) == TRUE ) {
# Note: If receiving (S,G) on the SPT, we only prune off the
# shared tree if the RPF neighbors differ.
if( RPF'(*,G) != RPF'(S,G) ) {
add Prune(S,G,rpt) to compound message
}
} else if ( inherited_olist(S,G,rpt) == NULL ) {
# Note: All (*,G) olist interfaces received RPT prunes for (S,G).
add Prune(S,G,rpt) to compound message
} else if ( RPF'(*,G) != RPF'(S,G,rpt) {
# Note: We joined the shared tree, but there was an (S,G) assert
# and the source tree RPF neighbor is different.
add Prune(S,G,rpt) to compound message
}
Note that Join(S,G,rpt) is normally sent not as a periodic message,
but only as a triggered message.
Fenner, et al. Standards Track [Page 71]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.5.7. State Machine for (S,G,rpt) Triggered Messages
The state machine for (S,G,rpt) triggered messages is required
per-(S,G) when there is (*,G) join state at a router, and the router
or any of its upstream LAN peers wishes to prune S off the RP tree.
There are three states in the state machine. One of the states is
when there is no (*,G) join state at this router. If there is (*,G)
join state at the router, then the state machine must be at one of
the other two states. The three states are:
Pruned(S,G,rpt)
(*,G) Joined, but (S,G,rpt) pruned.
NotPruned(S,G,rpt)
(*,G) Joined, and (S,G,rpt) not pruned.
RPTNotJoined(G)
(*,G) has not been joined.
In addition, there is an (S,G,rpt) Override Timer, OT(S,G,rpt), which
is used to delay triggered Join(S,G,rpt) messages to prevent
implosions of triggered messages.
Figure 7: Upstream (S,G,rpt) State Machine for Triggered Messages
+------------++--------------------------------------------------------+
| || Event |
| ++--------------+--------------+-------------+------------+
|Prev State || PruneDesired | PruneDesired | RPTJoin | inherited_ |
| || (S,G,rpt) | (S,G,rpt) | Desired(G) | olist |
| || ->True | ->False | ->False | (S,G,rpt) |
| || | | | ->non-NULL |
+------------++--------------+--------------+-------------+------------+
|RPTNotJoined|| -> P state | - | - | -> NP state|
|(G) (NJ) || | | | |
+------------++--------------+--------------+-------------+------------+
|Pruned || - | -> NP state | -> NJ state | - |
|(S,G,rpt) || | Send Join | | |
|(P) || | (S,G,rpt) | | |
+------------++--------------+--------------+-------------+------------+
|NotPruned || -> P state | - | -> NJ state | - |
|(S,G,rpt) || Send Prune | | Cancel OT | |
|(NP) || (S,G,rpt); | | | |
| || cancel OT | | | |
+------------++--------------+--------------+-------------+------------+
Fenner, et al. Standards Track [Page 72]
RFC 7761 PIM-SM Specification (Revised) March 2016
Additionally, we have the following transitions within the
NotPruned(S,G,rpt) state, which are all used for prune override
behavior.
+----------------------------------------------------------------------+
| In NotPruned(S,G,rpt) State |
+----------+--------------+--------------+--------------+--------------+
|Override | See Prune | See Join | See Prune | RPF' |
|Timer | (S,G,rpt) to | (S,G,rpt) to | (S,G) to | (S,G,rpt) -> |
|expires | RPF' | RPF' | RPF' | RPF' (*,G) |
| | (S,G,rpt) | (S,G,rpt) | (S,G,rpt) | |
+----------+--------------+--------------+--------------+--------------+
|Send Join | OT = min(OT, | Cancel OT | OT = min(OT, | OT = min(OT, |
|(S,G,rpt);| t_override) | | t_override) | t_override) |
|leave OT | | | | |
|unset | | | | |
+----------+--------------+--------------+--------------+--------------+
Note that the min function in the above state machine considers a
non-running timer to have an infinite value (e.g., min(not-running,
t_override) = t_override).
This state machine uses the following macros:
bool RPTJoinDesired(G) {
return (JoinDesired(*,G))
}
RPTJoinDesired(G) is true when the router has forwarding state that
would cause it to forward traffic for G using (*,G) shared tree
state.
bool PruneDesired(S,G,rpt) {
return ( RPTJoinDesired(G) AND
( inherited_olist(S,G,rpt) == NULL
OR (SPTbit(S,G)==TRUE
AND (RPF'(*,G) != RPF'(S,G)) )))
}
PruneDesired(S,G,rpt) can only be true if RPTJoinDesired(G) is true.
If RPTJoinDesired(G) is true, then PruneDesired(S,G,rpt) is true
either if there are no outgoing interfaces that S would be forwarded
on, or if the router has active (S,G) forwarding state but RPF'(*,G)
!= RPF'(S,G).
Fenner, et al. Standards Track [Page 73]
RFC 7761 PIM-SM Specification (Revised) March 2016
The state machine contains the following transition events:
See Join(S,G,rpt) to RPF'(S,G,rpt)
This event is only relevant in the "Not Pruned" state.
The router sees a Join(S,G,rpt) from someone else to
RPF'(S,G,rpt), which is the correct upstream neighbor. If
we're in "NotPruned" state and the (S,G,rpt) Override Timer
is running, then this is because we have been triggered to
send our own Join(S,G,rpt) to RPF'(S,G,rpt). Someone else
beat us to it, so there's no need to send our own Join.
The action is to cancel the Override Timer.
See Prune(S,G,rpt) to RPF'(S,G,rpt)
This event is only relevant in the "NotPruned" state.
The router sees a Prune(S,G,rpt) from someone else to
RPF'(S,G,rpt), which is the correct upstream neighbor. If
we're in the "NotPruned" state, then we want to continue to
receive traffic from S destined for G, and that traffic is
being supplied by RPF'(S,G,rpt). Thus, we need to override
the Prune.
The action is to set the (S,G,rpt) Override Timer to the
randomized prune-override interval, t_override. However, if
the Override Timer is already running, we only set the timer
if doing so would set it to a lower value. At the end of
this interval, if no one else has sent a Join, then we will
do so.
See Prune(S,G) to RPF'(S,G,rpt)
This event is only relevant in the "NotPruned" state.
This transition and action are the same as the above
transition and action, except that the Prune does not have
the RPT bit set. This transition is necessary to be
compatible with routers implemented from RFC 2362 that don't
maintain separate (S,G) and (S,G,rpt) state.
Fenner, et al. Standards Track [Page 74]
RFC 7761 PIM-SM Specification (Revised) March 2016
The (S,G,rpt) prune Override Timer expires
This event is only relevant in the "NotPruned" state.
When the Override Timer expires, we must send a
Join(S,G,rpt) to RPF'(S,G,rpt) to override the Prune message
that caused the timer to be running. We only send this if
RPF'(S,G,rpt) equals RPF'(*,G); if this were not the case,
then the Join might be sent to a router that does not have
(*,G) Join state, and so the behavior would not be well
defined. If RPF'(S,G,rpt) is not the same as RPF'(*,G),
then it may stop forwarding S. However, if this happens,
then the router will send an AssertCancel(S,G), which would
then cause RPF'(S,G,rpt) to become equal to RPF'(*,G) (see
below).
RPF'(S,G,rpt) changes to become equal to RPF'(*,G)
This event is only relevant in the "NotPruned" state.
RPF'(S,G,rpt) can only be different from RPF'(*,G) if an
(S,G) Assert has happened, which means that traffic from S
is arriving on the SPT, and so Prune(S,G,rpt) will have been
sent to RPF'(*,G). When RPF'(S,G,rpt) changes to become
equal to RPF'(*,G), we need to trigger a Join(S,G,rpt) to
RPF'(*,G) to cause that router to start forwarding S again.
The action is to set the (S,G,rpt) Override Timer to the
randomized prune-override interval t_override. However, if
the timer is already running, we only set the timer if doing
so would set it to a lower value. At the end of this
interval, if no one else has sent a Join, then we will
do so.
PruneDesired(S,G,rpt)->TRUE
See macro above. This event is relevant in the "NotPruned"
and "RPTNotJoined(G)" states.
The router wishes to receive traffic for G but does not wish
to receive traffic from S destined for G. This causes the
router to transition into the Pruned state.
If the router was previously in NotPruned state, then the
action is to send a Prune(S,G,rpt) to RPF'(S,G,rpt), and to
cancel the Override Timer. If the router was previously in
RPTNotJoined(G) state, then there is no need to trigger an
action in this state machine because sending a
Prune(S,G,rpt) is handled by the rules for sending the
Join(*,G).
Fenner, et al. Standards Track [Page 75]
RFC 7761 PIM-SM Specification (Revised) March 2016
PruneDesired(S,G,rpt)->FALSE
See macro above. This transition is only relevant in the
"Pruned" state.
If the router is in the Pruned(S,G,rpt) state, and
PruneDesired(S,G,rpt) changes to FALSE, this could be
because the router no longer has RPTJoinDesired(G) true, or
it now wishes to receive traffic from S again. If it is the
former, then this transition should not happen, but instead
the "RPTJoinDesired(G)->FALSE" transition should happen.
Thus, this transition should be interpreted as
"PruneDesired(S,G,rpt)->FALSE AND RPTJoinDesired(G)==TRUE".
The action is to send a Join(S,G,rpt) to RPF'(S,G,rpt).
RPTJoinDesired(G)->FALSE
This event is relevant in the "Pruned" and "NotPruned"
states.
The router no longer wishes to receive any traffic destined
for G on the RP Tree. This causes a transition to the
RPTNotJoined(G) state, and the Override Timer is canceled if
it was running. Any further actions are handled by the
appropriate upstream state machine for (*,G).
inherited_olist(S,G,rpt) becomes non-NULL
This transition is only relevant in the RPTNotJoined(G)
state.
The router has joined the RP tree (handled by the (*,G)
upstream state machine as appropriate) and wants to receive
traffic from S. This does not trigger any events in this
state machine, but causes a transition to the
NotPruned(S,G,rpt) state.
4.6. PIM Assert Messages
Where multiple PIM routers peer over a shared LAN, it is possible for
more than one upstream router to have valid forwarding state for a
packet, which can lead to packet duplication (see Section 3.6). PIM
does not attempt to prevent this from occurring. Instead, it detects
when this has happened and elects a single forwarder amongst the
upstream routers to prevent further duplication. This election is
performed using PIM Assert messages. Assert messages are also
received by downstream routers on the LAN, and these cause subsequent
Join/Prune messages to be sent to the upstream router that won the
Assert.
Fenner, et al. Standards Track [Page 76]
RFC 7761 PIM-SM Specification (Revised) March 2016
In general, a PIM Assert 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
an Assert message from a particular IP source address and it has not
seen a PIM Hello message from that source address, then the Assert
message SHOULD be discarded without further processing. In addition,
if the Hello message from a neighbor was authenticated (see
Section 6.3), then all Assert messages from that neighbor MUST also
be authenticated.
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.
4.6.1. (S,G) Assert Message State Machine
The (S,G) Assert state machine for interface I is shown in Figure 8.
There are three states:
NoInfo (NI)
This router has no (S,G) assert state on interface I.
I am Assert Winner (W)
This router has won an (S,G) assert on interface I. It is
now responsible for forwarding traffic from S destined for G
out of interface I. Irrespective of whether it is the DR
for I, while a router is the assert winner, it is also
responsible for forwarding traffic onto I on behalf of local
hosts on I that have made membership requests that
specifically refer to S (and G).
I am Assert Loser (L)
This router has lost an (S,G) assert on interface I. It
must not forward packets from S destined for G onto
interface I. If it is the DR on I, it is no longer
responsible for forwarding traffic onto I to satisfy local
hosts with membership requests that specifically refer to S
and G.
In addition, there is also an Assert Timer (AT) that is used to
time out asserts on the assert losers and to resend asserts on the
assert winner.
Fenner, et al. Standards Track [Page 77]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 8: Per-Interface (S,G) Assert State Machine
+----------------------------------------------------------------------+
| In NoInfo (NI) State |
+---------------+-------------------+------------------+---------------+
| Receive | Receive Assert | Data arrives | Receive |
| Inferior | with RPTbit | from S to G on | Acceptable |
| Assert with | set and | I and | Assert with |
| RPTbit clear | CouldAssert | CouldAssert | RPTbit clear |
| | (S,G,I) | (S,G,I) | and AssTrDes |
| | | | (S,G,I) |
+---------------+-------------------+------------------+---------------+
| -> W state | -> W state | -> W state | -> L state |
| [Actions A1] | [Actions A1] | [Actions A1] | [Actions A6] |
+---------------+-------------------+------------------+---------------+
+----------------------------------------------------------------------+
| In I Am Assert Winner (W) State |
+----------------+------------------+-----------------+----------------+
| Assert Timer | Receive | Receive | CouldAssert |
| Expires | Inferior | Preferred | (S,G,I) -> |
| | Assert | Assert | FALSE |
+----------------+------------------+-----------------+----------------+
| -> W state | -> W state | -> L state | -> NI state |
| [Actions A3] | [Actions A3] | [Actions A2] | [Actions A4] |
+----------------+------------------+-----------------+----------------+
+---------------------------------------------------------------------+
| In I Am Assert Loser (L) State |
+-------------+-------------+-------------+-------------+-------------+
|Receive |Receive |Receive |Assert Timer |Current |
|Preferred |Acceptable |Inferior |Expires |Winner's |
|Assert |Assert with |Assert or | |GenID |
| |RPTbit clear |Assert | |Changes or |
| |from Current |Cancel from | |NLT Expires |
| |Winner |Current | | |
| | |Winner | | |
+-------------+-------------+-------------+-------------+-------------+
|-> L state |-> L state |-> NI state |-> NI state |-> NI state |
|[Actions A2] |[Actions A2] |[Actions A5] |[Actions A5] |[Actions A5] |
+-------------+-------------+-------------+-------------+-------------+
Fenner, et al. Standards Track [Page 78]
RFC 7761 PIM-SM Specification (Revised) March 2016
+----------------------------------------------------------------------+
| In I Am Assert Loser (L) State |
+----------------+-----------------+------------------+----------------+
| AssTrDes | my_metric -> | RPF_interface | Receive |
| (S,G,I) -> | better than | (S) stops | Join(S,G) on |
| FALSE | winner's | being I | interface I |
| | metric | | |
+----------------+-----------------+------------------+----------------+
| -> NI state | -> NI state | -> NI state | -> NI State |
| [Actions A5] | [Actions A5] | [Actions A5] | [Actions A5] |
+----------------+-----------------+------------------+----------------+
Note that for reasons of compactness, "AssTrDes(S,G,I)" is used in
the state machine table to refer to AssertTrackingDesired(S,G,I).
Terminology:
A "preferred assert" is one with a better metric than the current
winner.
An "acceptable assert" is one that has a better metric than
my_assert_metric(S,G,I). An assert is never considered acceptable
if its metric is infinite.
An "inferior assert" is one with a worse metric than
my_assert_metric(S,G,I). An assert is never considered inferior
if my_assert_metric(S,G,I) is infinite.
The state machine uses the following macros:
CouldAssert(S,G,I) =
SPTbit(S,G)==TRUE
AND (RPF_interface(S) != I)
AND (I in ( ( joins(*,G) (-) prunes(S,G,rpt) )
(+) ( pim_include(*,G) (-) pim_exclude(S,G) )
(-) lost_assert(*,G)
(+) joins(S,G) (+) pim_include(S,G) ) )
CouldAssert(S,G,I) is true for downstream interfaces that would be in
the inherited_olist(S,G) if (S,G) assert information was not taken
into account.
Fenner, et al. Standards Track [Page 79]
RFC 7761 PIM-SM Specification (Revised) March 2016
AssertTrackingDesired(S,G,I) =
(I in ( joins(*,G) (-) prunes(S,G,rpt)
(+) ( pim_include(*,G) (-) pim_exclude(S,G) )
(-) lost_assert(*,G)
(+) joins(S,G) ) )
OR (local_receiver_include(S,G,I) == TRUE
AND (I_am_DR(I) OR (AssertWinner(S,G,I) == me)))
OR ((RPF_interface(S) == I) AND (JoinDesired(S,G) == TRUE))
OR ((RPF_interface(RP(G)) == I) AND (JoinDesired(*,G) == TRUE)
AND (SPTbit(S,G) == FALSE))
AssertTrackingDesired(S,G,I) is true on any interface in which an
(S,G) assert might affect the router's behavior on that interface.
The first three lines of AssertTrackingDesired account for (*,G) join
and local membership information received on I that might cause the
router to be interested in asserts on I.
The 4th line accounts for (S,G) join information received on I that
might cause the router to be interested in asserts on I.
The 5th and 6th lines account for (S,G) local membership information
on I. Note that we can't use the pim_include(S,G) macro, since it
uses lost_assert(S,G,I) and would result in the router forgetting
that it lost an assert if the only reason it was interested was local
membership. The AssertWinner(S,G,I) check forces an assert winner to
keep on being responsible for forwarding as long as local receivers
are present. Removing this check would make the assert winner
give up forwarding as soon as the information that originally caused
it to forward went away, and the task of forwarding for local
receivers would revert back to the DR.
The last three lines account for the fact that a router must keep
track of assert information on upstream interfaces in order to send
joins and prunes to the proper neighbor.
Fenner, et al. Standards Track [Page 80]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from NoInfo State
When in NoInfo state, the following events may trigger transitions:
Receive Inferior Assert with RPTbit cleared
An assert is received for (S,G) with the RPT bit cleared
that is inferior to our own assert metric. The RPT bit
cleared indicates that the sender of the assert had (S,G)
forwarding state on this interface. If the assert is
inferior to our metric, then we must also have (S,G)
forwarding state (i.e., CouldAssert(S,G,I)==TRUE) as (S,G)
asserts have priority over (*,G) asserts, and so we should
be the assert winner. We transition to the "I am Assert
Winner" state and perform Actions A1 (below).
Receive Assert with RPTbit set AND CouldAssert(S,G,I)==TRUE
An assert is received for (S,G) on I with the RPT bit set
(it is a (*,G) assert). CouldAssert(S,G,I) is TRUE only if
we have (S,G) forwarding state on this interface, so we
should be the assert winner. We transition to the "I am
Assert Winner" state and perform Actions A1 (below).
An (S,G) data packet arrives on interface I, AND
CouldAssert(S,G,I)==TRUE
An (S,G) data packet arrived on a downstream interface that
is in our (S,G) outgoing interface list. We optimistically
assume that we will be the assert winner for this (S,G), and
so we transition to the "I am Assert Winner" state and
perform Actions A1 (below), which will initiate the assert
negotiation for (S,G).
Receive Acceptable Assert with RPT bit clear AND
AssertTrackingDesired(S,G,I)==TRUE
We're interested in (S,G) Asserts, either because I is a
downstream interface for which we have (S,G) or (*,G)
forwarding state, or because I is the upstream interface for
S and we have (S,G) forwarding state. The received assert
has a better metric than our own, so we do not win the
Assert. We transition to "I am Assert Loser" and perform
Actions A6 (below).
Fenner, et al. Standards Track [Page 81]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from "I am Assert Winner" State
When in "I am Assert Winner" state, the following events trigger
transitions:
Assert Timer Expires
The (S,G) Assert Timer expires. As we're in the Winner
state, we must still have (S,G) forwarding state that is
actively being kept alive. We resend the (S,G) Assert and
restart the Assert Timer (Actions A3 below). Note that the
assert winner's Assert Timer is engineered to expire shortly
before timers on assert losers; this prevents unnecessary
thrashing of the forwarder and periodic flooding of
duplicate packets.
Receive Inferior Assert
We receive an (S,G) assert or (*,G) assert mentioning S that
has a worse metric than our own. Whoever sent the assert is
in error, and so we resend an (S,G) Assert and restart the
Assert Timer (Actions A3 below).
Receive Preferred Assert
We receive an (S,G) assert that has a better metric than our
own. We transition to "I am Assert Loser" state and perform
Actions A2 (below). Note that this may affect the value of
JoinDesired(S,G) and PruneDesired(S,G,rpt), which could
cause transitions in the upstream (S,G) or (S,G,rpt) state
machines.
CouldAssert(S,G,I) -> FALSE
Our (S,G) forwarding state or RPF interface changed so as to
make CouldAssert(S,G,I) become false. We can no longer
perform the actions of the assert winner, and so we
transition to NoInfo state and perform Actions A4 (below).
This includes sending a "canceling assert" with an infinite
metric.
Fenner, et al. Standards Track [Page 82]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from "I am Assert Loser" State
When in "I am Assert Loser" state, the following transitions can
occur:
Receive Preferred Assert
We receive an assert that is better than that of the current
assert winner. We stay in Loser state and perform
Actions A2 below.
Receive Acceptable Assert with RPTbit clear from Current Winner
We receive an assert from the current assert winner that is
better than our own metric for this (S,G) (although the
metric may be worse than the winner's previous metric). We
stay in Loser state and perform Actions A2 below.
Receive Inferior Assert or Assert Cancel from Current Winner
We receive an assert from the current assert winner that is
worse than our own metric for this group (typically, because
the winner's metric became worse or because it is an assert
cancel). We transition to NoInfo state, deleting the (S,G)
assert information and allowing the normal PIM Join/Prune
mechanisms to operate. Usually, we will eventually
re-assert and win when data packets from S have started
flowing again.
Assert Timer Expires
The (S,G) Assert Timer expires. We transition to NoInfo
state, deleting the (S,G) assert information (Actions A5
below).
Current Winner's GenID Changes or NLT Expires
The Neighbor Liveness Timer associated with the current
winner expires or we receive a Hello message from the
current winner reporting a different GenID from the one it
previously reported. This indicates that the current
winner's interface or router has gone down (and may have
come back up), and so we must assume that it no longer knows
it was the winner. We transition to the NoInfo state,
deleting this (S,G) assert information (Actions A5 below).
AssertTrackingDesired(S,G,I)->FALSE
AssertTrackingDesired(S,G,I) becomes FALSE. Our forwarding
state has changed so that (S,G) Asserts on interface I are
no longer of interest to us. We transition to the NoInfo
state, deleting the (S,G) assert information.
Fenner, et al. Standards Track [Page 83]
RFC 7761 PIM-SM Specification (Revised) March 2016
My metric becomes better than the assert winner's metric
my_assert_metric(S,G,I) has changed so that now my assert
metric for (S,G) is better than the metric we have stored
for the current assert winner. This might happen when the
underlying routing metric changes, or when
CouldAssert(S,G,I) becomes true, for example, when
SPTbit(S,G) becomes true. We transition to NoInfo state,
delete this (S,G) assert state (Actions A5 below), and allow
the normal PIM Join/Prune mechanisms to operate. Usually,
we will eventually re-assert and win when data packets from
S have started flowing again.
RPF_interface(S) stops being interface I
Interface I used to be the RPF interface for S, and now it
is not. We transition to NoInfo state, deleting this (S,G)
assert state (Actions A5 below).
Receive Join(S,G) on Interface I
We receive a Join(S,G) that has the Upstream Neighbor
Address field set to my primary IP address on interface I.
The action is to transition to NoInfo state, delete this
(S,G) assert state (Actions A5 below), and allow the normal
PIM Join/Prune mechanisms to operate. If whoever sent the
Join was in error, then the normal assert mechanism will
eventually re-apply, and we will lose the assert again.
However, whoever sent the assert may know that the previous
assert winner has died, and so we may end up being the new
forwarder.
(S,G) Assert State Machine Actions
A1: Send Assert(S,G).
Set Assert Timer to (Assert_Time - Assert_Override_Interval).
Store self as AssertWinner(S,G,I).
Store spt_assert_metric(S,I) as AssertWinnerMetric(S,G,I).
A2: Store new assert winner as AssertWinner(S,G,I) and assert
winner metric as AssertWinnerMetric(S,G,I).
Set Assert Timer to Assert_Time.
A3: Send Assert(S,G).
Set Assert Timer to (Assert_Time - Assert_Override_Interval).
A4: Send AssertCancel(S,G).
Delete assert information (AssertWinner(S,G,I) and
AssertWinnerMetric(S,G,I) will then return to their default
values).
Fenner, et al. Standards Track [Page 84]
RFC 7761 PIM-SM Specification (Revised) March 2016
A5: Delete assert information (AssertWinner(S,G,I) and
AssertWinnerMetric(S,G,I) will then return to their default
values).
A6: Store new assert winner as AssertWinner(S,G,I) and assert
winner metric as AssertWinnerMetric(S,G,I).
Set Assert Timer to Assert_Time.
If (I is RPF_interface(S)) AND (UpstreamJPState(S,G) ==
Joined) set SPTbit(S,G) to TRUE.
Note that some of these actions may cause the value of
JoinDesired(S,G), PruneDesired(S,G,rpt), or RPF'(S,G) to change,
which could cause further transitions in other state machines.
4.6.2. (*,G) Assert Message State Machine
The (*,G) Assert state machine for interface I is shown in Figure 9.
There are three states:
NoInfo (NI)
This router has no (*,G) assert state on interface I.
I am Assert Winner (W)
This router has won a (*,G) assert on interface I. It is
now responsible for forwarding traffic destined for G onto
interface I with the exception of traffic for which it has
(S,G) "I am Assert Loser" state. Irrespective of whether it
is the DR for I, it is also responsible for handling the
membership requests for G from local hosts on I.
I am Assert Loser (L)
This router has lost a (*,G) assert on interface I. It must
not forward packets for G onto interface I with the
exception of traffic from sources for which it has (S,G) "I
am Assert Winner" state. If it is the DR, it is no longer
responsible for handling the membership requests for group G
from local hosts on I.
In addition, there is also an Assert Timer (AT) that is used to time
out asserts on the assert losers and to resend asserts on the assert
winner.
When an Assert message is received with a source address other than
zero, a PIM implementation must first match it against the possible
events in the (S,G) assert state machine and process any transitions
and actions, before considering whether the Assert message matches
against the (*,G) assert state machine.
Fenner, et al. Standards Track [Page 85]
RFC 7761 PIM-SM Specification (Revised) March 2016 It is important to note that NO TRANSITION CAN OCCUR in the (*,G) state machine as a result of receiving an Assert message unless the (S,G) assert state machine for the relevant S and G is in the "NoInfo" state after the (S,G) state machine has processed the message. Also, NO TRANSITION CAN OCCUR in the (*,G) state machine as a result of receiving an assert message if that message triggers any change of state in the (S,G) state machine. Obviously, when the source address in the received message is set to zero, an (S,G) state machine for the S and G does not exist and can be assumed to be in the "NoInfo" state. For example, if both the (S,G) and (*,G) assert state machines are in the NoInfo state when an Assert message arrives, and the message causes the (S,G) state machine to transition to either "W" or "L" state, then the assert will not be processed by the (*,G) assert state machine. Another example: if the (S,G) assert state machine is in "L" state when an assert message is received, and the assert metric in the message is worse than my_assert_metric(S,G,I), then the (S,G) assert state machine will transition to NoInfo state. In such a case, if the (*,G) assert state machine were in NoInfo state, it might appear that it would transition to "W" state, but this is not the case because this message already triggered a transition in the (S,G) assert state machine. Fenner, et al. Standards Track [Page 86]
RFC 7761 PIM-SM Specification (Revised) March 2016
Figure 9: Per-Interface (*,G) Assert State Machine
+----------------------------------------------------------------------+
| In NoInfo (NI) State |
+-----------------------+-----------------------+----------------------+
| Receive Inferior | Data arrives for G | Receive Acceptable |
| Assert with RPTbit | on I and | Assert with RPTbit |
| set and | CouldAssert | set and AssTrDes |
| CouldAssert(*,G,I) | (*,G,I) | (*,G,I) |
+-----------------------+-----------------------+----------------------+
| -> W state | -> W state | -> L state |
| [Actions A1] | [Actions A1] | [Actions A2] |
+-----------------------+-----------------------+----------------------+
+---------------------------------------------------------------------+
| In I Am Assert Winner (W) State |
+----------------+-----------------+-----------------+----------------+
| Assert Timer | Receive | Receive | CouldAssert |
| Expires | Inferior | Preferred | (*,G,I) -> |
| | Assert | Assert | FALSE |
+----------------+-----------------+-----------------+----------------+
| -> W state | -> W state | -> L state | -> NI state |
| [Actions A3] | [Actions A3] | [Actions A2] | [Actions A4] |
+----------------+-----------------+-----------------+----------------+
+---------------------------------------------------------------------+
| In I Am Assert Loser (L) State |
+-------------+-------------+-------------+-------------+-------------+
|Receive |Receive |Receive |Assert Timer |Current |
|Preferred |Acceptable |Inferior |Expires |Winner's |
|Assert with |Assert from |Assert or | |GenID |
|RPTbit set |Current |Assert | |Changes or |
| |Winner with |Cancel from | |NLT Expires |
| |RPTbit set |Current | | |
| | |Winner | | |
+-------------+-------------+-------------+-------------+-------------+
|-> L state |-> L state |-> NI state |-> NI state |-> NI state |
|[Actions A2] |[Actions A2] |[Actions A5] |[Actions A5] |[Actions A5] |
+-------------+-------------+-------------+-------------+-------------+
Fenner, et al. Standards Track [Page 87]
RFC 7761 PIM-SM Specification (Revised) March 2016
+----------------------------------------------------------------------+
| In I Am Assert Loser (L) State |
+----------------+----------------+-----------------+------------------+
| AssTrDes | my_metric -> | RPF_interface | Receive |
| (*,G,I) -> | better than | (RP(G)) stops | Join(*,G) on |
| FALSE | Winner's | being I | Interface I |
| | metric | | |
+----------------+----------------+-----------------+------------------+
| -> NI state | -> NI state | -> NI state | -> NI State |
| [Actions A5] | [Actions A5] | [Actions A5] | [Actions A5] |
+----------------+----------------+-----------------+------------------+
The state machine uses the following macros:
CouldAssert(*,G,I) =
( I in ( joins(*,G) (+) pim_include(*,G)) )
AND (RPF_interface(RP(G)) != I)
CouldAssert(*,G,I) is true on downstream interfaces for which we have
(*,G) join state, or local members that requested any traffic
destined for G.
AssertTrackingDesired(*,G,I) =
CouldAssert(*,G,I)
OR (local_receiver_include(*,G,I)==TRUE
AND (I_am_DR(I) OR AssertWinner(*,G,I) == me))
OR (RPF_interface(RP(G)) == I AND RPTJoinDesired(G))
AssertTrackingDesired(*,G,I) is true on any interface on which a
(*,G) assert might affect the router's behavior on that interface.
Note that for reasons of compactness, "AssTrDes(*,G,I)" is used in
the state machine table to refer to AssertTrackingDesired(*,G,I).
Terminology:
A "preferred assert" is one with a better metric than the current
winner.
An "acceptable assert" is one that has a better metric than
my_assert_metric(*,G,I). An assert is never considered acceptable
if its metric is infinite.
An "inferior assert" is one with a worse metric than
my_assert_metric(*,G,I). An assert is never considered inferior
if my_assert_metric(*,G,I) is infinite.
Fenner, et al. Standards Track [Page 88]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from NoInfo State
When in NoInfo state, the following events trigger transitions, but
only if the (S,G) assert state machine is in NoInfo state before and
after consideration of the received message:
Receive Inferior Assert with RPTbit set AND
CouldAssert(*,G,I)==TRUE
An Inferior (*,G) assert is received for G on Interface I.
If CouldAssert(*,G,I) is TRUE, then I is our downstream
interface, and we have (*,G) forwarding state on this
interface, so we should be the assert winner. We transition
to the "I am Assert Winner" state and perform Actions A1
(below).
A data packet destined for G arrives on interface I, AND
CouldAssert(*,G,I)==TRUE
A data packet destined for G arrived on a downstream
interface that is in our (*,G) outgoing interface list. We
therefore believe we should be the forwarder for this (*,G),
and so we transition to the "I am Assert Winner" state and
perform Actions A1 (below).
Receive Acceptable Assert with RPT bit set AND
AssertTrackingDesired(*,G,I)==TRUE
We're interested in (*,G) Asserts, either because I is a
downstream interface for which we have (*,G) forwarding
state, or because I is the upstream interface for RP(G) and
we have (*,G) forwarding state. We get a (*,G) Assert that
has a better metric than our own, so we do not win the
Assert. We transition to "I am Assert Loser" and perform
Actions A2 (below).
Fenner, et al. Standards Track [Page 89]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from "I am Assert Winner" State
When in "I am Assert Winner" state, the following events trigger
transitions, but only if the (S,G) assert state machine is in NoInfo
state before and after consideration of the received message:
Receive Inferior Assert
We receive a (*,G) assert that has a worse metric than our
own. Whoever sent the assert has lost, and so we resend a
(*,G) Assert and restart the Assert Timer (Actions A3
below).
Receive Preferred Assert
We receive a (*,G) assert that has a better metric than our
own. We transition to "I am Assert Loser" state and perform
Actions A2 (below).
When in "I am Assert Winner" state, the following events trigger
transitions:
Assert Timer Expires
The (*,G) Assert Timer expires. As we're in the Winner
state, then we must still have (*,G) forwarding state that
is actively being kept alive. To prevent unnecessary
thrashing of the forwarder and periodic flooding of
duplicate packets, we resend the (*,G) Assert and restart
the Assert Timer (Actions A3 below).
CouldAssert(*,G,I) -> FALSE
Our (*,G) forwarding state or RPF interface changed so as to
make CouldAssert(*,G,I) become false. We can no longer
perform the actions of the assert winner, and so we
transition to NoInfo state and perform Actions A4 (below).
Fenner, et al. Standards Track [Page 90]
RFC 7761 PIM-SM Specification (Revised) March 2016
Transitions from "I am Assert Loser" State
When in "I am Assert Loser" state, the following events trigger
transitions, but only if the (S,G) assert state machine is in NoInfo
state before and after consideration of the received message:
Receive Preferred Assert with RPTbit set
We receive a (*,G) assert that is better than that of the
current assert winner. We stay in Loser state and perform
Actions A2 below.
Receive Acceptable Assert from Current Winner with RPTbit set
We receive a (*,G) assert from the current assert winner
that is better than our own metric for this group (although
the metric may be worse than the winner's previous metric).
We stay in Loser state and perform Actions A2 below.
Receive Inferior Assert or Assert Cancel from Current Winner
We receive an assert from the current assert winner that is
worse than our own metric for this group (typically because
the winner's metric became worse or is now an assert
cancel). We transition to NoInfo state, delete this (*,G)
assert state (Actions A5), and allow the normal PIM
Join/Prune mechanisms to operate. Usually, we will
eventually re-assert and win when data packets for G have
started flowing again.
When in "I am Assert Loser" state, the following events trigger
transitions:
Assert Timer Expires
The (*,G) Assert Timer expires. We transition to NoInfo
state and delete this (*,G) assert information (Actions A5).
Current Winner's GenID Changes or NLT Expires
The Neighbor Liveness Timer associated with the current
winner expires or we receive a Hello message from the
current winner reporting a different GenID from the one it
previously reported. This indicates that the current
winner's interface or router has gone down (and may have
come back up), and so we must assume that it no longer knows
it was the winner. We transition to the NoInfo state,
deleting the (*,G) assert information (Actions A5).
Fenner, et al. Standards Track [Page 91]
RFC 7761 PIM-SM Specification (Revised) March 2016
AssertTrackingDesired(*,G,I)->FALSE
AssertTrackingDesired(*,G,I) becomes FALSE. Our forwarding
state has changed so that (*,G) Asserts on interface I are
no longer of interest to us. We transition to NoInfo state
and delete this (*,G) assert information (Actions A5).
My metric becomes better than the assert winner's metric
My routing metric, rpt_assert_metric(G,I), has changed so
that now my assert metric for (*,G) is better than the
metric we have stored for the current assert winner. We
transition to NoInfo state, delete this (*,G) assert state
(Actions A5), and allow the normal PIM Join/Prune mechanisms
to operate. Usually, we will eventually re-assert and win
when data packets for G have started flowing again.
RPF_interface(RP(G)) stops being interface I
Interface I used to be the RPF interface for RP(G), and now
it is not. We transition to NoInfo state and delete this
(*,G) assert state (Actions A5).
Receive Join(*,G) on interface I
We receive a Join(*,G) that has the Upstream Neighbor
Address field set to my primary IP address on interface I.
The action is to transition to NoInfo state, delete this
(*,G) assert state (Actions A5), and allow the normal PIM
Join/Prune mechanisms to operate. If whoever sent the Join
was in error, then the normal assert mechanism will
eventually re-apply, and we will lose the assert again.
However, whoever sent the assert may know that the previous
assert winner has died, so we may end up being the new
forwarder.
(*,G) Assert State Machine Actions
A1: Send Assert(*,G).
Set Assert Timer to (Assert_Time - Assert_Override_Interval).
Store self as AssertWinner(*,G,I).
Store rpt_assert_metric(G,I) as AssertWinnerMetric(*,G,I).
A2: Store new assert winner as AssertWinner(*,G,I) and assert
winner metric as AssertWinnerMetric(*,G,I).
Set Assert Timer to Assert_Time.
A3: Send Assert(*,G).
Set Assert Timer to (Assert_Time - Assert_Override_Interval).
Fenner, et al. Standards Track [Page 92]
RFC 7761 PIM-SM Specification (Revised) March 2016
A4: Send AssertCancel(*,G).
Delete assert information (AssertWinner(*,G,I) and
AssertWinnerMetric(*,G,I) will then return to their default
values).
A5: Delete assert information (AssertWinner(*,G,I) and
AssertWinnerMetric(*,G,I) will then return to their default
values).
Note that some of these actions may cause the value of
JoinDesired(*,G) or RPF'(*,G) to change, which could cause further
transitions in other state machines.
4.6.3. Assert Metrics
Assert metrics are defined as:
struct assert_metric {
rpt_bit_flag;
metric_preference;
route_metric;
ip_address;
};
When comparing assert_metrics, the rpt_bit_flag, metric_preference,
and route_metric fields are compared in order, where the first lower
value wins. If all fields are equal, the primary IP address of the
router that sourced the Assert message is used as a tie-breaker, with
the highest IP address winning.
An assert metric for (S,G) to include in (or compare against) an
Assert message sent on interface I should be computed using the
following pseudocode:
assert_metric
my_assert_metric(S,G,I) {
if( CouldAssert(S,G,I) == TRUE ) {
return spt_assert_metric(S,I)
} else if( CouldAssert(*,G,I) == TRUE ) {
return rpt_assert_metric(G,I)
} else {
return infinite_assert_metric()
}
}
Fenner, et al. Standards Track [Page 93]
RFC 7761 PIM-SM Specification (Revised) March 2016
spt_assert_metric(S,I) gives the assert metric we use if we're
sending an assert based on active (S,G) forwarding state:
assert_metric
spt_assert_metric(S,I) {
return {0,MRIB.pref(S),MRIB.metric(S),my_ip_address(I)}
}
rpt_assert_metric(G,I) gives the assert metric we use if we're
sending an assert based only on (*,G) forwarding state:
assert_metric
rpt_assert_metric(G,I) {
return {1,MRIB.pref(RP(G)),MRIB.metric(RP(G)),my_ip_address(I)}
}
MRIB.pref(X) and MRIB.metric(X) are the routing preference and
routing metrics associated with the route to a particular (unicast)
destination X, as determined by the MRIB. my_ip_address(I) is simply
the router's primary IP address that is associated with the local
interface I.
infinite_assert_metric() is an assert metric that the router uses for
an Assert that does not match either (S,G) or (*,G) forwarding state:
assert_metric
infinite_assert_metric() {
return {1,infinity,infinity,0}
}
4.6.4. AssertCancel Messages
An AssertCancel message is simply an RPT Assert message but with an
infinite metric. It is sent by the assert winner when it deletes the
forwarding state that had caused the assert to occur. Other routers
will see this metric, and it will cause any other router that has
forwarding state to send its own assert, and to take over forwarding.
An AssertCancel(S,G) is an infinite metric assert with the RPT bit
set that names S as the source.
An AssertCancel(*,G) is an infinite metric assert with the RPT bit
set and the source set to zero.
Fenner, et al. Standards Track [Page 94]
RFC 7761 PIM-SM Specification (Revised) March 2016
AssertCancel messages are simply an optimization. The original
Assert timeout mechanism will allow a subnet to eventually become
consistent; the AssertCancel mechanism simply causes faster
convergence. No special processing is required for an AssertCancel
message, since it is simply an Assert message from the current
winner.
4.6.5. Assert State Macros
The macros lost_assert(S,G,rpt,I), lost_assert(S,G,I), and
lost_assert(*,G,I) are used in the olist computations of Section 4.1
and are defined as:
bool lost_assert(S,G,rpt,I) {
if ( RPF_interface(RP(G)) == I OR
( RPF_interface(S) == I AND SPTbit(S,G) == TRUE ) ) {
return FALSE
} else {
return ( AssertWinner(S,G,I) != NULL AND
AssertWinner(S,G,I) != me )
}
}
bool lost_assert(S,G,I) {
if ( RPF_interface(S) == I ) {
return FALSE
} else {
return ( AssertWinner(S,G,I) != NULL AND
AssertWinner(S,G,I) != me AND
(AssertWinnerMetric(S,G,I) is better
than spt_assert_metric(S,I) )
}
}
Note: The term "AssertWinnerMetric(S,G,I) is better than
spt_assert_metric(S,I)" is required to correctly handle the
transition phase when a router has (S,G) join state but has not yet
set the SPTbit. In this case, it needs to ignore the assert state if
it will win the assert once the SPTbit is set.
bool lost_assert(*,G,I) {
if ( RPF_interface(RP(G)) == I ) {
return FALSE
} else {
return ( AssertWinner(*,G,I) != NULL AND
AssertWinner(*,G,I) != me )
}
}
Fenner, et al. Standards Track [Page 95]
RFC 7761 PIM-SM Specification (Revised) March 2016
AssertWinner(S,G,I) is the IP source address of the Assert(S,G)
packet that won an Assert.
AssertWinner(*,G,I) is the IP source address of the Assert(*,G)
packet that won an Assert.
AssertWinnerMetric(S,G,I) is the Assert metric of the Assert(S,G)
packet that won an Assert.
AssertWinnerMetric(*,G,I) is the Assert metric of the Assert(*,G)
packet that won an Assert.
AssertWinner(S,G,I) defaults to NULL and AssertWinnerMetric(S,G,I)
defaults to Infinity when in the NoInfo state.
Summary of Assert Rules and Rationale
This section summarizes the key rules for sending and reacting to
asserts and the rationale for these rules. This section is not
intended to be and should not be treated as a definitive
specification of protocol behavior. The state machines and
pseudocode should be consulted for that purpose. Rather, this
section is intended to document important aspects of the Assert
protocol behavior and to provide information that may prove helpful
to the reader in understanding and implementing this part of the
protocol.
1. Behavior: Downstream neighbors send Join(*,G) and Join(S,G)
periodic messages to the appropriate RPF' neighbor, i.e., the RPF
neighbor as modified by the assert process. They are not always
sent to the RPF neighbor as indicated by the MRIB. Normal
suppression and override rules apply.
Rationale: By sending the periodic and triggered Join messages to
the RPF' neighbor instead of the RPF neighbor, the downstream
router avoids re-triggering the Assert process with every Join.
A side effect of sending Joins to the Assert winner is that
traffic will not switch back to the "normal" RPF neighbor until
the Assert times out. This will not happen until data stops
flowing, if item 8, below, is implemented.
2. Behavior: The assert winner for (*,G) acts as the local DR for
(*,G) on behalf of IGMP/MLD members.
Rationale: This is required to allow a single router to merge
PIM and IGMP/MLD joins and leaves. Without this, overrides
don't work.
Fenner, et al. Standards Track [Page 96]
RFC 7761 PIM-SM Specification (Revised) March 2016
3. Behavior: The assert winner for (S,G) acts as the local DR for
(S,G) on behalf of IGMPv3 members.
Rationale: Same rationale as for item 2.
4. Behavior: (S,G) and (*,G) prune overrides are sent to the RPF'
neighbor and not to the regular RPF neighbor.
Rationale: Same rationale as for item 1.
5. Behavior: An (S,G,rpt) prune override is not sent (at all) if
RPF'(S,G,rpt) != RPF'(*,G).
Rationale: This avoids keeping state alive on the (S,G) tree when
only (*,G) downstream members are left. Also, it avoids sending
(S,G,rpt) joins to a router that is not on the (*,G) tree. This
behavior might be confusing, although this specification does
indicate that such a join SHOULD be dropped.
6. Behavior: An assert loser that receives a Join(S,G) with an
Upstream Neighbor Address that is its primary IP address on that
interface expires the (S,G) Assert Timer.
Rationale: This is necessary in order to have rapid convergence
in the event that the downstream router that initially sent a
join to the prior Assert winner has undergone a topology change.
7. Behavior: An assert loser that receives a Join(*,G) with an
Upstream Neighbor Address that is its primary IP address on that
interface expires the (*,G) Assert Timer and all (S,G) assert
timers that do not have corresponding Prune(S,G,rpt) messages in
the compound Join/Prune message.
Rationale: Same rationale as for item 6.
8. Behavior: An assert winner for (*,G) or (S,G) sends a canceling
assert when it is about to stop forwarding on a (*,G) or an (S,G)
entry. This behavior does not apply to (S,G,rpt).
Rationale: This allows switching back to the shared tree after
the last SPT router on the LAN leaves. Doing this prevents
downstream routers on the shared tree from keeping SPT state
alive.
Fenner, et al. Standards Track [Page 97]
RFC 7761 PIM-SM Specification (Revised) March 2016
9. Behavior: Resend the assert messages before timing out an assert.
(This behavior is optional.)
Rationale: This prevents the periodic duplicates that would
otherwise occur each time that an assert times out and is then
re-established.
10. Behavior: When RPF'(S,G,rpt) changes to be the same as RPF'(*,G),
we need to trigger a Join(S,G,rpt) to RPF'(*,G).
Rationale: This allows switching back to the RPT after the last
SPT member leaves.
4.7. PIM Bootstrap and RP Discovery
For correct operation, every PIM router within a PIM domain must be
able to map a particular multicast group address to the same RP. If
this is not the case, then black holes may appear, where some
receivers in the domain cannot receive some groups. A domain in this
context is a contiguous set of routers that all implement PIM and are
configured to operate within a common boundary.
A notable exception to this is where a PIM domain is broken up into
multiple administrative scope regions; these are regions where a
border has been configured so that a range of multicast groups will
not be forwarded across that border. For more information on
Administratively Scoped IP Multicast, see RFC 2365. The modified
criteria for admin-scoped regions are that the region is convex with
respect to forwarding based on the MRIB, and that all PIM routers
within the scope region map scoped groups to the same RP within that
region.
This specification does not mandate the use of a single mechanism to
provide routers with the information to perform the group-to-RP
mapping. Currently, four mechanisms are possible, and all four have
associated problems:
Static Configuration
A PIM router MUST support the static configuration of group-to-
RP mappings. Such a mechanism is not robust to failures but
does at least provide a basic interoperability mechanism.
Embedded-RP
Embedded-RP defines an address allocation policy in which the
address of the Rendezvous Point (RP) is encoded in an IPv6
multicast group address [16].
Fenner, et al. Standards Track [Page 98]
RFC 7761 PIM-SM Specification (Revised) March 2016
Cisco's Auto-RP
Auto-RP uses a PIM Dense-Mode (PIM-DM) multicast group to
announce group-to-RP mappings from a central location. This
mechanism is not useful if PIM Dense Mode is not being run in
parallel with PIM Sparse Mode; it was only intended for use
with PIM Sparse Mode Version 1. No standard specification
currently exists.
Bootstrap Router (BSR)
RFC 2362 specifies a bootstrap mechanism based on the automatic
election of a BSR. Any router in the domain that is configured
to be a possible RP reports its candidacy to the BSR, and then
a domain-wide flooding mechanism distributes the BSR's chosen
set of RPs throughout the domain. As specified in RFC 2362,
the BSR mechanism is flawed in its handling of admin-scoped
regions that are smaller than a PIM domain, but the mechanism
does work for global-scoped groups.
As far as PIM-SM is concerned, the only important requirement is that
all routers in the domain (or admin scope zone for scoped regions)
receive the same set of group-range-to-RP mappings. This may be
achieved through the use of any of these mechanisms, or through
alternative mechanisms not currently specified.
It must be operationally ensured that any RP address configured,
learned, or advertised is reachable from all routers in the PIM
domain.
4.7.1. Group-to-RP Mapping
Using one of the mechanisms described above, a PIM router receives
one or more possible group-range-to-RP mappings. Each mapping
specifies a range of multicast groups (expressed as a group and mask)
and the RP to which such groups should be mapped. Each mapping may
also have an associated priority. It is possible to receive multiple
mappings, all of which might match the same multicast group; this is
the common case with the BSR mechanism. The algorithm for performing
the group-to-RP mapping is as follows:
1. Perform longest match on group range to obtain a list of RPs.
2. From this list of matching RPs, find the ones with highest
priority.
Eliminate any RPs from the list that have lower priorities.
Fenner, et al. Standards Track [Page 99]
RFC 7761 PIM-SM Specification (Revised) March 2016
3. If only one RP remains in the list, use that RP.
4. If multiple RPs are in the list, use the PIM hash function to
choose one.
Thus, if two or more group-range-to-RP mappings cover a particular
group, the one with the longest mask is the mapping to use. If the
mappings have the same mask length, then the one with the highest
priority is chosen. If there is more than one matching entry with
the same longest mask and the priorities are identical, then a hash
function (see Section 4.7.2) is applied to choose the RP.
This algorithm is invoked by a DR when it needs to determine an RP
for a given group, e.g., upon reception of a packet or IGMP/MLD
membership indication for a group for which the DR does not know
the RP.
Furthermore, the mapping function is invoked by all routers upon
receiving a (*,G) Join/Prune message.
Note that if the set of possible group-range-to-RP mappings changes,
each router will need to check whether any existing groups are
affected. This may, for example, cause a DR or acting DR to re-join
a group, or cause it to restart register encapsulation to the new RP.
Implementation note: The bootstrap mechanism described in RFC 2362
omitted step 1 above. However, of the implementations we are
aware of, approximately half performed step 1 anyway. Note that
implementations of BSR that omit step 1 will not correctly
interoperate with implementations of this specification when used
with the BSR mechanism described in [11].
4.7.2. Hash Function
The hash function is used by all routers within a domain, to map a
group to one of the RPs from the matching set of group-range-to-RP
mappings (this set of mappings all have the same longest mask length
and same highest priority). The algorithm takes as input the group
address, and the addresses of the candidate RPs from the mappings,
and gives as output one RP address to be used.
Fenner, et al. Standards Track [Page 100]
RFC 7761 PIM-SM Specification (Revised) March 2016
The protocol requires that all routers hash to the same RP within a
domain (except for transients). The following hash function must be
used in each router:
1. For RP addresses in the matching group-range-to-RP mappings,
compute a value:
Value(G,M,C(i))=
(1103515245 * ((1103515245 * (G&M)+12345) XOR C(i)) + 12345) mod 2^31
where C(i) is the RP address and M is a hash-mask. If BSR is
being used, the hash-mask is given in the Bootstrap messages. If
BSR is not being used, the alternative mechanism that supplies
the group-range-to-RP mappings may supply the value, or else it
defaults to a mask with the most significant 30 bits being one
for IPv4 and the most significant 126 bits being one for IPv6.
The hash-mask allows a small number of consecutive groups
(e.g., 4) to always hash to the same RP. For instance,
hierarchically encoded data can be sent on consecutive group
addresses to get the same delay and fate-sharing characteristics.
For address families other than IPv4, a 32-bit digest to be used
as C(i) and G must first be derived from the actual RP or group
address. Such a digest method must be used consistently
throughout the PIM domain. For IPv6 addresses, it is RECOMMENDED
to use the equivalent IPv4 address for an IPv4-compatible
address, and the exclusive-or of each 32-bit segment of the
address for all other IPv6 addresses. For example, the digest of
the IPv6 address 3ffe:b00:c18:1::10 would be computed as
0x3ffe0b00 ^ 0x0c180001 ^ 0x00000000 ^ 0x00000010,
where the '^' symbol represents the exclusive-or operation.
2. The candidate RP with the highest resulting hash value is then
the RP chosen by this hash function. If more than one RP has the
same highest hash value, the RP with the highest IP address is
chosen.
4.8. Source-Specific Multicast
The Source-Specific Multicast (SSM) service model [6] can be
implemented with a strict subset of the PIM-SM protocol mechanisms.
Both regular IP Multicast and SSM semantics can coexist on a single
router, and both can be implemented using the PIM-SM protocol. A
range of multicast addresses, currently 232.0.0.0/8 in IPv4 and
ff3x::/32 for IPv6, is reserved for SSM, and the choice of semantics
is determined by the multicast group address in both data packets and
PIM messages.
Fenner, et al. Standards Track [Page 101]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.8.1. Protocol Modifications for SSM Destination Addresses
The following rules override the normal PIM-SM behavior for a
multicast address G in the SSM range:
o A router MUST NOT send a (*,G) Join/Prune message for any reason.
o A router MUST NOT send an (S,G,rpt) Join/Prune message for any
reason.
o A router MUST NOT send a Register message for any packet that is
destined to an SSM address.
o A router MUST NOT forward packets based on (*,G) or (S,G,rpt)
state. The (*,G)- and (S,G,rpt)-related state summarization
macros are NULL for any SSM address, for the purposes of packet
forwarding.
o A router acting as an RP MUST NOT forward any Register-
encapsulated packet that has an SSM destination address and SHOULD
respond with a Register-Stop message to such a Register message.
o A router MAY optimize out the creation and maintenance of
(S,G,rpt) and (*,G) state for SSM destination addresses -- this
state is not needed for SSM packets.
The last three rules are present to deal with SSM-unaware "legacy"
routers that may be sending (*,G) and (S,G,rpt) Join/Prunes, or
Register messages for SSM destination addresses. Note that this
specification does not attempt to aid an SSM-unaware "legacy" router
with SSM operations.
4.8.2. PIM-SSM-Only Routers
An implementer may choose to implement only the subset of PIM
Sparse Mode that provides SSM forwarding semantics.
A PIM-SSM-only router MUST implement the following portions of this
specification:
o Upstream (S,G) state machine (Section 4.5.5)
o Downstream (S,G) state machine (Section 4.5.2)
o (S,G) Assert state machine (Section 4.6.1)
Fenner, et al. Standards Track [Page 102]
RFC 7761 PIM-SM Specification (Revised) March 2016
o Hello messages, neighbor discovery, and DR election (Section 4.3)
o Packet forwarding rules (Section 4.2)
A PIM-SSM-only router does not need to implement the following
protocol elements:
o Register state machine (Section 4.4)
o (*,G) and (S,G,rpt) downstream state machines (Sections 4.5.1 and
4.5.3)
o (*,G) and (S,G,rpt) upstream state machines (Sections 4.5.4,
4.5.6, and 4.5.7)
o (*,G) Assert state machine (Section 4.6.2)
o Bootstrap RP election (Section 4.7)
o Keepalive Timer
o SPTbit (Section 4.2.2)
The Keepalive Timer should be treated as always running, and the
SPTbit should be treated as always being set for an SSM address.
Additionally, the packet forwarding rules of Section 4.2 can be
simplified in a PIM-SSM-only router:
oiflist = NULL
if( iif == RPF_interface(S) AND UpstreamJPState(S,G) == Joined ) {
oiflist = inherited_olist(S,G)
} else if( iif is in inherited_olist(S,G) ) {
send Assert(S,G) on iif
}
oiflist = oiflist (-) iif
forward packet on all interfaces in oiflist
This is nothing more than the reduction of the normal PIM-SM
forwarding rule, with all (S,G,rpt) and (*,G) clauses replaced
with NULL.
Fenner, et al. Standards Track [Page 103]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.9. PIM Packet Formats
This section describes the details of the packet formats for PIM
control messages.
All PIM control messages have IP protocol number 103.
PIM messages are either unicast (e.g., Registers and Register-Stop)
or multicast with TTL 1 to the 'ALL-PIM-ROUTERS' group (e.g.,
Join/Prune, Asserts). The source address used for unicast messages
is a domain-wide reachable address; the source address used for
multicast messages is the link-local address of the interface on
which the message is being sent.
The IPv4 'ALL-PIM-ROUTERS' group is '224.0.0.13'. The IPv6
'ALL-PIM-ROUTERS' group is 'ff02::d'.
The PIM header common to all PIM messages is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Ver
PIM Version number is 2.
Type
Types for specific PIM messages. PIM Types are:
Message Type Destination
---------------------------------------------------------------------
0 = Hello Multicast to ALL-PIM-ROUTERS
1 = Register Unicast to RP
2 = Register-Stop Unicast to source of Register
packet
3 = Join/Prune Multicast to ALL-PIM-ROUTERS
4 = Bootstrap Multicast to ALL-PIM-ROUTERS
5 = Assert Multicast to ALL-PIM-ROUTERS
6 = Graft (used in PIM-DM only) Unicast to RPF'(S)
7 = Graft-Ack (used in PIM-DM only) Unicast to source of Graft
packet
8 = Candidate-RP-Advertisement Unicast to Domain's BSR
Fenner, et al. Standards Track [Page 104]
RFC 7761 PIM-SM Specification (Revised) March 2016
Reserved
Set to zero on transmission. Ignored upon receipt.
Checksum
The checksum is a standard IP checksum, i.e., the 16-bit one's
complement of the one's complement sum of the entire PIM
message, excluding the "Multicast data packet" section of the
Register message. For computing the checksum, the checksum
field is zeroed. If the packet's length is not an integral
number of 16-bit words, the packet is padded with a trailing
byte of zero before performing the checksum.
For IPv6, the checksum also includes the IPv6 "pseudo-header",
as specified in RFC 2460, Section 8.1 [5]. This
"pseudo-header" is prepended to the PIM header for the purposes
of calculating the checksum. The "Upper-Layer Packet Length"
in the pseudo-header is set to the length of the PIM message,
except in Register messages where it is set to the length of
the PIM register header (8). The Next Header value used in the
pseudo-header is 103.
If a message is received with an unrecognized PIM Ver or Type field,
or if a message's destination does not correspond to the table above,
the message MUST be discarded, and an error message SHOULD be logged
to the administrator in a rate-limited manner.
4.9.1. Encoded Source and Group Address Formats
Encoded Unicast Address
An encoded unicast address takes the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
The PIM address family of the 'Unicast Address' field of this
address.
Values 0-127 are as assigned by the IANA for Internet Address
Families in [7]. Values 128-250 are reserved to be assigned by
the IANA for PIM-specific Address Families. Values 251 through
255 are designated for Private Use. As there is no assignment
authority for this space, collisions should be expected.
Fenner, et al. Standards Track [Page 105]
RFC 7761 PIM-SM Specification (Revised) March 2016
Encoding Type
The type of encoding used within a specific Address Family.
The value '0' is reserved for this field and represents the
native encoding of the Address Family.
Unicast Address
The unicast address as represented by the given Address Family
and Encoding Type.
Encoded Group Address
Encoded group addresses take the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type |B| Reserved |Z| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
Described above.
Encoding Type
Described above.
[B]idirectional PIM
Indicates that the group range uses Bidirectional PIM [13].
For PIM-SM as defined in this specification, this bit MUST be
zero.
Reserved
Transmitted as zero. Ignored upon receipt.
Admin Scope [Z]one
Indicates that the group range is an admin scope zone. This is
used in the Bootstrap Router mechanism [11] only. For all
other purposes, this bit is set to zero and ignored on receipt.
Fenner, et al. Standards Track [Page 106]
RFC 7761 PIM-SM Specification (Revised) March 2016
Mask Len
The Mask length field is 8 bits. The value is the number of
contiguous one bits that are left-justified and used as a mask;
when combined with the group address, it describes a range of
groups. It is less than or equal to the address length in bits
for the given Address Family and Encoding Type. If the message
is sent for a single group, then the Mask length must equal the
address length in bits for the given Address Family and
Encoding Type (e.g., 32 for IPv4 native encoding, 128 for IPv6
native encoding).
Group multicast Address
Contains the group address.
Encoded Source Address
An encoded source address takes the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Rsrvd |S|W|R| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Addr Family
Described above.
Encoding Type
Described above.
Reserved
Transmitted as zero, ignored on receipt.
S The Sparse bit is a 1-bit value, set to 1 for PIM-SM. It is
used for PIM Version 1 compatibility.
W The WC (or WildCard) bit is a 1-bit value for use with PIM
Join/Prune messages (see Section 4.9.5.1).
R The RPT (or Rendezvous Point Tree) bit is a 1-bit value for use
with PIM Join/Prune messages (see Section 4.9.5.1). If the
WC bit is 1, the RPT bit MUST be 1.
Fenner, et al. Standards Track [Page 107]
RFC 7761 PIM-SM Specification (Revised) March 2016
Mask Len
The mask length field is 8 bits. The value is the number of
contiguous one bits that are left-justified and used as a mask;
when combined with the source address, it describes a source
subnet. The mask length MUST be equal to the mask length in
bits for the given Address Family and Encoding Type (32 for
IPv4 native and 128 for IPv6 native). A router SHOULD ignore
any messages received with any other mask length.
Source Address
The source address.
4.9.2. Hello Message Format
A Hello message is sent periodically by routers on all interfaces.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType | OptionLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionValue |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType | OptionLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionValue |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version, Type, Reserved, Checksum
Described in Section 4.9.
OptionType
The type of the option given in the following OptionValue
field.
OptionLength
The length of the OptionValue field in bytes.
OptionValue
A variable-length field, carrying the value of the option.
Fenner, et al. Standards Track [Page 108]
RFC 7761 PIM-SM Specification (Revised) March 2016
The Option fields may contain the following values:
o OptionType 1: Holdtime
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Holdtime is the amount of time a receiver must keep the neighbor
reachable, in seconds. If the Holdtime is set to '0xffff', the
receiver of this message never times out the neighbor. This may
be used with dial-on-demand links, to avoid keeping the link up
with periodic Hello messages.
An implementation MAY provide a configuration mechanism to reject
a Hello message with holdtime 0xffff, and/or provide a mechanism
to remove a neighbor.
Hello messages with a Holdtime value set to '0' are also sent by a
router on an interface about to go down or changing IP address
(see Section 4.3.1). These are effectively goodbye messages, and
the receiving routers SHOULD immediately time out the neighbor
information for the sender.
o OptionType 2: LAN Prune Delay
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T| Propagation_Delay | Override_Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LAN Prune Delay option is used to tune the prune propagation
delay on multi-access LANs. The T bit specifies the ability of
the sending router to disable Join suppression. Propagation_Delay
and Override_Interval are time intervals in units of milliseconds.
A router originating a LAN Prune Delay option on interface I sets
the Propagation_Delay field to the configured value of
Propagation_Delay(I) and the value of the Override_Interval field
to the value of Override_Interval(I). On a receiving router, the
values of the fields are used to tune the value of the
Effective_Override_Interval(I) and its derived timer values.
Fenner, et al. Standards Track [Page 109]
RFC 7761 PIM-SM Specification (Revised) March 2016
Section 4.3.3 describes how these values affect the behavior of a
router.
o OptionTypes 3 through 16: Reserved; to be defined in future
versions of this document.
o OptionType 18: Deprecated and should not be used.
o OptionType 19: DR Priority
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 19 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DR Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DR Priority is a 32-bit unsigned number and should be considered
in the DR election as described in Section 4.3.2.
o OptionType 20: Generation ID
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 20 | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generation ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Generation ID is a random 32-bit value for the interface on which
the Hello message is sent. The Generation ID is regenerated
whenever PIM forwarding is started or restarted on the interface.
Fenner, et al. Standards Track [Page 110]
RFC 7761 PIM-SM Specification (Revised) March 2016
o OptionType 24: Address List
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 24 | Length = <Variable> |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secondary Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secondary Address N (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The contents of the Address List Hello option are described in
Section 4.3.4. All addresses within a single Address List must
belong to the same address family.
OptionTypes 17 through 65000 are assigned by the IANA.
OptionTypes 65001 through 65535 are reserved for Private Use,
as defined in [9].
Unknown options MUST be ignored and MUST NOT prevent a neighbor
relationship from being formed. The Holdtime option MUST be
implemented; the DR Priority and Generation ID options SHOULD be
implemented. The Address List option MUST be implemented for IPv6.
4.9.3. Register Message Format
A Register message is sent by the DR to the RP when a multicast
packet needs to be transmitted on the RP-tree. The IP source address
is set to the address of the DR, the destination address to the RP's
address. The IP TTL of the PIM packet is the system's normal
unicast TTL.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B|N| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Multicast data packet .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fenner, et al. Standards Track [Page 111]
RFC 7761 PIM-SM Specification (Revised) March 2016
PIM Version, Type, Reserved, Checksum
Described in Section 4.9. Note that in order to reduce
encapsulation overhead, the checksum for Registers is done only
on the first 8 bytes of the packet, including the PIM header
and the next 4 bytes, excluding the data packet portion. For
interoperability reasons, a message carrying a checksum
calculated over the entire PIM Register message should also be
accepted. When calculating the checksum, the IPv6
pseudo-header "Upper-Layer Packet Length" is set to 8.
B The Border bit. This specification deprecates the Border bit.
A router MUST set the B bit to 0 on transmission and MUST
ignore this bit on reception.
N The Null-Register bit. Set to 1 by a DR that is probing the RP
before expiring its local Register-Suppression Timer. Set to 0
otherwise.
Reserved2
Transmitted as zero, ignored on receipt.
Multicast data packet
The original packet sent by the source. This packet must be of
the same address family as the encapsulating PIM packet, e.g.,
an IPv6 data packet must be encapsulated in an IPv6 PIM packet.
Note that the TTL of the original packet is decremented before
encapsulation, just like any other packet that is forwarded.
In addition, the RP decrements the TTL after decapsulating,
before forwarding the packet down the shared tree.
For (S,G) Null-Registers, the Multicast data packet portion
contains a dummy IP header with S as the source address
and G as the destination address. When generating an IPv4
Null-Register message, the fields in the dummy IPv4 header
SHOULD be filled in according to the following table. Other
IPv4 header fields may contain any value that is valid for
that field.
Field Value
---------------------------------------
IP Version 4
Header Length 5
Checksum Header checksum
Fragmentation offset 0
More Fragments 0
Total Length 20
IP Protocol 103 (PIM)
Fenner, et al. Standards Track [Page 112]
RFC 7761 PIM-SM Specification (Revised) March 2016
On receipt of an (S,G) Null-Register, if the Header Checksum
field is non-zero, the recipient SHOULD check the checksum and
discard Null-Registers that have a bad checksum. The recipient
SHOULD NOT check the value of any individual fields; a correct
IP header checksum is sufficient. If the Header Checksum field
is zero, the recipient MUST NOT check the checksum.
With IPv6, an implementation generates a dummy IP header
followed by a dummy PIM header with values according to the
following table in addition to the source and group. Other
IPv6 header fields may contain any value that is valid for that
field.
Header Field Value
--------------------------------------
IP Version 6
Next Header 103 (PIM)
Length 4
PIM Version 0
PIM Type 0
PIM Reserved 0
PIM Checksum PIM checksum, including
IPv6 "pseudo-header";
see Section 4.9
On receipt of an IPv6 (S,G) Null-Register, if the dummy PIM
header is present, the recipient SHOULD check the checksum and
discard Null-Registers that have a bad checksum.
4.9.4. Register-Stop Message Format
A Register-Stop is unicast from the RP to the sender of the Register
message. The IP source address is the address to which the register
was addressed. The IP destination address is the source address of
the register message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fenner, et al. Standards Track [Page 113]
RFC 7761 PIM-SM Specification (Revised) March 2016
PIM Version, Type, Reserved, Checksum
Described in Section 4.9.
Group Address
The group address from the multicast data packet in the
Register. The format for this address is described in
Section 4.9.1. Note that for Register-Stops the Mask Len field
contains the full address length * 8 (e.g., 32 for IPv4 native
encoding), if the message is sent for a single group.
Source Address
The host address of the source from the multicast data packet
in the register. The format for this address is given in the
encoded unicast address in Section 4.9.1. A special wildcard
value consisting of an address field of all zeros can be used
to indicate any source.
4.9.5. Join/Prune Message Format
A Join/Prune message is sent by routers towards upstream sources and
RPs. Joins are sent to build shared trees (RP trees) or source trees
(SPT). Prunes are sent to prune source trees when members leave
groups as well as sources that do not use the shared tree.
Fenner, et al. Standards Track [Page 114]
RFC 7761 PIM-SM Specification (Revised) March 2016
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Upstream Neighbor Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Num groups | Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address 1 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address m (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Joined Sources | Number of Pruned Sources |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address 1 (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Joined Source Address n (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address 1 (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pruned Source Address n (Encoded-Source format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fenner, et al. Standards Track [Page 115]
RFC 7761 PIM-SM Specification (Revised) March 2016
PIM Version, Type, Reserved, Checksum
Described in Section 4.9.
Unicast Upstream Neighbor Address
The primary address of the upstream neighbor that is the target
of the message. The format for this address is given in the
encoded unicast address in Section 4.9.1.
Reserved
Transmitted as zero, ignored on receipt.
Holdtime
The amount of time a receiver MUST keep the Join/Prune state
alive, in seconds. If the Holdtime is set to '0xffff', the
receiver of this message SHOULD hold the state until canceled
by the appropriate canceling Join/Prune message, or timed out
according to local policy. This may be used with dial-on-
demand links, to avoid keeping the link up with periodic
Join/Prune messages.
Note that the HoldTime MUST be larger than the
J/P_Override_Interval(I).
Number of Groups
The number of multicast group sets contained in the message.
Multicast group address
For format description, see Section 4.9.1.
Number of Joined Sources
Number of joined source addresses listed for a given group.
Joined Source Address 1 .. n
This list contains the sources for a given group that the
sending router will forward multicast datagrams from if
received on the interface on which the Join/Prune message
is sent.
See Section 4.9.1 for the format description for the
encoded source address.
Number of Pruned Sources
Number of pruned source addresses listed for a group.
Fenner, et al. Standards Track [Page 116]
RFC 7761 PIM-SM Specification (Revised) March 2016
Pruned Source Address 1 .. n
This list contains the sources for a given group that the
sending router does not want to forward multicast datagrams
from when received on the interface on which the Join/Prune
message is sent.
Within one PIM Join/Prune message, all the Multicast Group addresses,
Joined Source addresses, and Pruned Source 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. This permits maximum
implementation flexibility for dual-stack IPv4/IPv6 routers. If a
router receives a message with mixed family addresses, it SHOULD only
process the addresses that are of the same family as the unicast
upstream neighbor address.
4.9.5.1. Group Set Source List Rules
As described above, Join/Prune messages are composed of one or more
group sets. Each set contains two source lists: the Joined Sources
and the Pruned Sources. This section describes the different types
of group sets and source list entries that can exist in a Join/Prune
message.
There is one valid group set type:
Group-Specific Set
A Group-Specific Set is represented by a valid IP multicast
address in the group address field and the full length of the
IP address in the mask length field of the Multicast Group
Address. Each Join/Prune message SHOULD NOT contain more than
one group-specific set for the same IP multicast address. Each
group-specific set may contain (*,G), (S,G,rpt), and (S,G)
source list entries in the Joined or Pruned lists.
(*,G)
The (*,G) source list entry is used in Join/Prune messages
sent towards the RP for the specified group. It expresses
interest (or lack thereof) in receiving traffic sent to the
group through the RP shared tree. There MUST only be one
such entry in both the Joined and Pruned lists of a group-
specific set.
(*,G) source list entries have the Source-Address set to the
address of the RP for group G, the Source-Address Mask-Len
set to the full length of the IP address, and both the WC
and RPT bits of the encoded-source-address set.
Fenner, et al. Standards Track [Page 117]
RFC 7761 PIM-SM Specification (Revised) March 2016
(S,G,rpt)
The (S,G,rpt) source list entry is used in Join/Prune
messages sent towards the RP for the specified group. It
expresses interest (or lack thereof) in receiving traffic
through the shared tree sent by the specified source to this
group. For each source address, the entry MUST exist in
only one of the Joined and Pruned source lists of a group-
specific set, but not both.
(S,G,rpt) source list entries have the Source-Address set to
the address of the source S, the Source-Address Mask-Len set
to the full length of the IP address, and the WC bit cleared
and the RPT bit set in the encoded source address.
(S,G)
The (S,G) source list entry is used in Join/Prune messages
sent towards the specified source. It expresses interest
(or lack thereof) in receiving traffic through the shortest
path tree sent by the source to the specified group. For
each source address, the entry MUST exist in only one of the
Joined and Pruned source lists of a group-specific set, but
not both.
(S,G) source list entries have the Source-Address set to the
address of the source S, the Source-Address Mask-Len set to
the full length of the IP address, and both the WC and RPT
bits of the encoded source address cleared.
The rules described above are sufficient to prevent invalid
combinations of source list entries in group-specific sets. There
are, however, a number of combinations that have a valid
interpretation but that are not generated by the protocol as
described in this specification:
o Combining a (*,G) Join and an (S,G,rpt) Join entry in the same
message is redundant, as the (*,G) entry covers the information
provided by the (S,G,rpt) entry.
o The same applies for a (*,G) Prune and an (S,G,rpt) Prune.
o The combination of a (*,G) Prune and an (S,G,rpt) Join is also not
generated. (S,G,rpt) Joins are only sent when the router is
receiving all traffic for a group on the shared tree and it wishes
to indicate a change for the particular source. As a (*,G) prune
indicates that the router no longer wishes to receive shared tree
traffic, the (S,G,rpt) Join would be meaningless.
Fenner, et al. Standards Track [Page 118]
RFC 7761 PIM-SM Specification (Revised) March 2016
o As Join/Prune messages are targeted to a single PIM neighbor,
including both an (S,G) Join and an (S,G,rpt) Prune in the same
message is usually redundant. The (S,G) Join informs the neighbor
that the sender wishes to receive the particular source on the
shortest path tree. It is therefore unnecessary for the router to
say that it no longer wishes to receive it on the shared tree.
However, there is a valid interpretation for this combination of
entries. A downstream router may have to instruct its upstream
only to start forwarding a specific source once it has started
receiving the source on the shortest-path tree.
o The combination of an (S,G) Prune and an (S,G,rpt) Join could
possibly be used by a router to switch from receiving a particular
source on the shortest-path tree back to receiving it on the
shared tree (provided that the RPF neighbor for the shortest-path
and shared trees is common). However, Sparse-Mode PIM does not
provide a mechanism for explicitly switching back to the shared
tree.
The rules are summarized in the table below.
+----------++------+-------+-----------+-----------+-------+-------+
| ||Join | Prune | Join | Prune | Join | Prune |
| ||(*,G) | (*,G) | (S,G,rpt) | (S,G,rpt) | (S,G) | (S,G) |
+----------++------+-------+-----------+-----------+-------+-------+
|Join ||- | no | ? | yes | yes | yes |
|(*,G) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
|Prune ||no | - | ? | ? | yes | yes |
|(*,G) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
|Join ||? | ? | - | no | yes | ? |
|(S,G,rpt) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
|Prune ||yes | ? | no | - | yes | ? |
|(S,G,rpt) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
|Join ||yes | yes | yes | yes | - | no |
|(S,G) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
|Prune ||yes | yes | ? | ? | no | - |
|(S,G) || | | | | | |
+----------++------+-------+-----------+-----------+-------+-------+
Fenner, et al. Standards Track [Page 119]
RFC 7761 PIM-SM Specification (Revised) March 2016
yes Allowed and expected.
no Combination is not allowed by the protocol and MUST NOT be
generated by a router. A router MAY accept these messages, but
the result is undefined. An error message MAY be logged to the
administrator in a rate-limited manner.
? Combination not expected by the protocol, but well defined. A
router MAY accept it but SHOULD NOT generate it.
The order of source list entries in a group set source list is not
important, except where limited by the packet format itself.
4.9.5.2. Group Set Fragmentation
When building a Join/Prune for a particular neighbor, a router should
try to include in the message as much of the information it needs to
convey to the neighbor as possible. This implies adding one group
set for each multicast group that has information pending
transmission and within each set including all relevant source list
entries.
On a router with a large amount of multicast state, the number of
entries that must be included may result in packets that are larger
than the maximum IP packet size. In most such cases, the information
may be split into multiple messages.
There is an exception with group sets that contain a (*,G) Joined
source list entry. The group set expresses the router's interest in
receiving all traffic for the specified group on the shared tree, and
it MUST include an (S,G,rpt) Pruned source list entry for every
source that the router does not wish to receive. This list of
(S,G,rpt) Pruned source list entries MUST NOT be split in multiple
messages.
If only N (S,G,rpt) Prune entries fit into a maximum-sized Join/Prune
message, but the router has more than N (S,G,rpt) Prunes to add, then
the router MUST choose to include the first N (numerically smallest
in network byte order) IP addresses, and the rest are ignored (not
included).
Fenner, et al. Standards Track [Page 120]
RFC 7761 PIM-SM Specification (Revised) March 2016
4.9.6. Assert Message Format
The Assert message is used to resolve forwarder conflicts between
routers on a link. It is sent when a router receives a multicast
data packet on an interface on which the router would normally have
forwarded that packet. Assert messages may also be sent in response
to an Assert message from another router.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Metric Preference |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version, Type, Reserved, Checksum
Described in Section 4.9.
Group Address
The group address for which the router wishes to resolve the
forwarding conflict. This is an encoded group address, as
specified in Section 4.9.1.
Source Address
Source address for which the router wishes to resolve the
forwarding conflict. The source address MAY be set to zero for
(*,G) asserts (see below). The format for this address is
given in the encoded unicast address in Section 4.9.1.
R RPTbit is a 1-bit value. The RPTbit is set to 1 for
Assert(*,G) messages and 0 for Assert(S,G) messages.
Metric Preference
Preference value assigned to the unicast routing protocol that
provided the route to the multicast source or Rendezvous Point.
Metric
The unicast routing table metric associated with the route used
to reach the multicast source or Rendezvous Point. The metric
is in units applicable to the unicast routing protocol used.
Fenner, et al. Standards Track [Page 121]
RFC 7761 PIM-SM Specification (Revised) March 2016
Assert messages can be sent to resolve a forwarding conflict for all
traffic to a given group or for a specific source and group.
Assert(S,G)
Source-specific asserts are sent by routers forwarding a
specific source on the shortest-path tree (SPTbit is TRUE).
(S,G) Asserts have the Group-Address field set to the group G
and the Source-Address field set to the source S. The RPTbit
is set to 0, the Metric-Preference is set to MRIB.pref(S), and
the Metric is set to MRIB.metric(S).
Assert(*,G)
Group-specific asserts are sent by routers forwarding data for
the group and source(s) under contention on the shared tree.
(*,G) asserts have the Group-Address field set to the group G.
For data-triggered Asserts, the Source-Address field MAY be set
to the IP source address of the data packet that triggered the
Assert and is set to zero otherwise. The RPTbit is set to 1,
the Metric-Preference is set to MRIB.pref(RP(G)), and the
Metric is set to MRIB.metric(RP(G)).
4.10. PIM Timers
PIM-SM maintains the following timers, as discussed in Section 4.11.
All timers are countdown timers; they are set to a value and count
down to zero, at which point they typically trigger an action. Of
course, they can just as easily be implemented as count-up timers,
where the absolute expiry time is stored and compared against a
real-time clock, but the language in this specification assumes that
they count downwards to zero.
Fenner, et al. Standards Track [Page 122]
RFC 7761 PIM-SM Specification (Revised) March 2016
Global Timers
Per interface (I):
Hello Timer: HT(I)
Per neighbor (N):
Neighbor Liveness Timer: NLT(N,I)
Per Group (G):
(*,G) Join Expiry Timer: ET(*,G,I)
(*,G) Prune-Pending Timer: PPT(*,G,I)
(*,G) Assert Timer: AT(*,G,I)
Per Source (S):
(S,G) Join Expiry Timer: ET(S,G,I)
(S,G) Prune-Pending Timer: PPT(S,G,I)
(S,G) Assert Timer: AT(S,G,I)
(S,G,rpt) Prune Expiry Timer: ET(S,G,rpt,I)
(S,G,rpt) Prune-Pending Timer: PPT(S,G,rpt,I)
Per Group (G):
(*,G) Upstream Join Timer: JT(*,G)
Per Source (S):
(S,G) Upstream Join Timer: JT(S,G)
(S,G) Keepalive Timer: KAT(S,G)
(S,G,rpt) Upstream Override Timer: OT(S,G,rpt)
At the DRs or relevant Assert Winners only:
Per Source,Group pair (S,G):
Register-Stop Timer: RST(S,G)
Fenner, et al. Standards Track [Page 123]
RFC 7761 PIM-SM Specification (Revised) March 2016 4.11. Timer Values When timers are started or restarted, they are set to default values. This section summarizes those default values. Note that protocol events or configuration may change the default value of a timer on a specific interface. When timers are initialized in this document, the value specific to the interface in context must be used. Some of the timers listed below (Prune-Pending, Upstream Join, Upstream Override) can be set to values that depend on the settings of the Propagation_Delay and Override_Interval of the corresponding interface. The default values for these are given below. Variable Name: Propagation_Delay(I) +-------------------------------+--------------+----------------------+ | Value Name | Value | Explanation | +-------------------------------+--------------+----------------------+ | Propagation_delay_default | 0.5 secs | Expected | | | | propagation delay | | | | over the local | | | | link. | +-------------------------------+--------------+----------------------+ The default value of the Propagation_delay_default is chosen to be relatively large to provide compatibility with older PIM implementations. Variable Name: Override_Interval(I) +--------------------------+-----------------+-------------------------+ | Value Name | Value | Explanation | +--------------------------+-----------------+-------------------------+ | t_override_default | 2.5 secs | Default delay | | | | interval over | | | | which to randomize | | | | when scheduling a | | | | delayed Join | | | | message. | +--------------------------+-----------------+-------------------------+ Fenner, et al. Standards Track [Page 124]
RFC 7761 PIM-SM Specification (Revised) March 2016 Timer Name: Hello Timer (HT(I)) +---------------------+--------+---------------------------------------+ |Value Name | Value | Explanation | +---------------------+--------+---------------------------------------+ |Hello_Period | 30 secs| Periodic interval for Hello messages. | +---------------------+--------+---------------------------------------+ |Triggered_Hello_Delay| 5 secs | Randomized interval for initial Hello | | | | message on bootup or triggered Hello | | | | message to a rebooting neighbor. | +---------------------+--------+---------------------------------------+ At system power-up, the timer is initialized to rand(0, Triggered_Hello_Delay) to prevent synchronization. When a new or rebooting neighbor is detected, a responding Hello is sent within rand(0, Triggered_Hello_Delay). Timer Name: Neighbor Liveness Timer (NLT(N,I)) +--------------------------+----------------------+--------------------+ | Value Name | Value | Explanation | +--------------------------+----------------------+--------------------+ | Default_Hello_Holdtime | 3.5 * Hello_Period | Default holdtime | | | | to keep neighbor | | | | state alive | +--------------------------+----------------------+--------------------+ | Hello_Holdtime | from message | Holdtime from | | | | Hello message | | | | Holdtime option. | +--------------------------+----------------------+--------------------+ The Holdtime in a Hello message should be set to (3.5 * Hello_Period), giving a default value of 105 seconds. Timer Names: Expiry Timer (ET(*,G,I), ET(S,G,I), ET(S,G,rpt,I)) +----------------+----------------+------------------------------------+ | Value Name | Value | Explanation | +----------------+----------------+------------------------------------+ | J/P_HoldTime | from message | Holdtime from Join/Prune message | +----------------+----------------+------------------------------------+ The value of J/P Holdtime that is included in Join/Prune messages is specified below, in the description of "Upstream Join Timer (JT(*,G), JT(S,G))". Fenner, et al. Standards Track [Page 125]
RFC 7761 PIM-SM Specification (Revised) March 2016 Timer Names: Prune-Pending Timer (PPT(*,G,I), PPT(S,G,I), PPT(S,G,rpt,I)) +--------------------------+---------------------+---------------------+ |Value Name | Value | Explanation | +--------------------------+---------------------+---------------------+ |J/P_Override_Interval(I) | Default: | Short period after | | | Effective_ | a join or prune to | | | Propagation_ | allow other | | | Delay(I) + | routers on the LAN | | | Effective_Override_ | to override the | | | Interval(I) | join or prune | +--------------------------+---------------------+---------------------+ Note that both Effective_Propagation_Delay(I) and Effective_Override_Interval(I) are interface-specific values that may change when Hello messages are received (see Section 4.3.3). Timer Names: Assert Timer (AT(*,G,I), AT(S,G,I)) +---------------------------+---------------------+--------------------+ | Value Name | Value | Explanation | +---------------------------+---------------------+--------------------+ | Assert_Override_Interval | Default: 3 secs | Short interval | | | | before an assert | | | | times out where | | | | the assert winner | | | | resends an Assert | | | | message | +---------------------------+---------------------+--------------------+ | Assert_Time | Default: 180 secs | Period after last | | | | assert before | | | | assert state is | | | | timed out | +---------------------------+---------------------+--------------------+ Note that for historical reasons, the Assert message lacks a Holdtime field. Thus, changing the Assert Time from the default value is not recommended. Fenner, et al. Standards Track [Page 126]
RFC 7761 PIM-SM Specification (Revised) March 2016 Timer Names: Upstream Join Timer (JT(*,G), JT(S,G)) +-------------+--------------------+-----------------------------------+ |Value Name | Value | Explanation | +-------------+--------------------+-----------------------------------+ |t_periodic | Default: 60 secs | Period between Join/Prune messages| +-------------+--------------------+-----------------------------------+ |t_suppressed | rand(1.1 * | Suppression period when someone | | | t_periodic, 1.4 * | else sends a J/P message so we | | | t_periodic) when | don't need to do so. | | | Suppression_ | | | | Enabled(I) is | | | | true, 0 otherwise | | +-------------+--------------------+-----------------------------------+ |t_override | rand(0, Effective_ | Randomized delay to prevent | | | Override_ | response implosion when sending a | | | Interval(I)) | Join message to override someone | | | | else's Prune message. | +-------------+--------------------+-----------------------------------+ t_periodic may be set to take into account such things as the configured bandwidth and expected average number of multicast route entries for the attached network or link (e.g., the period would be longer for lower-speed links, or for routers in the center of the network that expect to have a larger number of entries). If the Join/Prune-Period is modified during operation, these changes should be made relatively infrequently, and the router should continue to refresh at its previous Join/Prune-Period for at least Join/Prune-Holdtime, in order to allow the upstream router to adapt. The Holdtime specified in a Join/Prune message should be set to (3.5 * t_periodic). t_override depends on the Effective Override Interval of the upstream interface, which may change when Hello messages are received. t_suppressed depends on the Suppression State of the upstream interface (Section 4.3.3) and becomes zero when suppression is disabled. Fenner, et al. Standards Track [Page 127]
RFC 7761 PIM-SM Specification (Revised) March 2016 Timer Name: Upstream Override Timer (OT(S,G,rpt)) +---------------+--------------------------+---------------------------+ | Value Name | Value | Explanation | +---------------+--------------------------+---------------------------+ | t_override | see Upstream Join Timer | see Upstream Join Timer | +---------------+--------------------------+---------------------------+ The Upstream Override Timer is only ever set to the t_override value; this value is defined earlier in this section, under "Timer Names: Upstream Join Timer (JT(*,G), JT(S,G))". Timer Name: Keepalive Timer (KAT(S,G)) +-----------------------+-----------------------+----------------------+ | Value Name | Value | Explanation | +-----------------------+-----------------------+----------------------+ | Keepalive_Period | Default: 210 secs | Period after last | | | | (S,G) data packet | | | | during which (S,G) | | | | Join state will be | | | | maintained even in | | | | the absence of | | | | (S,G) Join | | | | messages. | +-----------------------+-----------------------+----------------------+ | RP_Keepalive_Period | ( 3 * Register_ | As | | | Suppression_Time ) | Keepalive_Period, | | | + Register_ | but at the RP when | | | Probe_Time | a Register-Stop is | | | | sent. | +-----------------------+-----------------------+----------------------+ The normal keepalive period for the KAT(S,G) defaults to 210 seconds. However, at the RP, the keepalive period must be at least the Register_Suppression_Time, or the RP may time out the (S,G) state before the next Null-Register arrives. Thus, the KAT(S,G) is set to max(Keepalive_Period, RP_Keepalive_Period) when a Register-Stop is sent. Fenner, et al. Standards Track [Page 128]
RFC 7761 PIM-SM Specification (Revised) March 2016 Timer Name: Register-Stop Timer (RST(S,G)) +---------------------------+--------------------+---------------------+ |Value Name | Value | Explanation | +---------------------------+--------------------+---------------------+ |Register_Suppression_Time | Default: 60 secs | Period during | | | | which a DR stops | | | | sending Register- | | | | encapsulated data | | | | to the RP after | | | | receiving a | | | | Register-Stop | | | | message. | +---------------------------+--------------------+---------------------+ |Register_Probe_Time | Default: 5 secs | Time before RST | | | | expires when a DR | | | | may send a Null- | | | | Register to the RP | | | | to cause it to | | | | resend a Register- | | | | Stop message. | +---------------------------+--------------------+---------------------+ If the Register_Suppression_Time or the Register_Probe_Time is configured to values other than the defaults, it MUST be ensured that the value of the Register_Probe_Time is less than half the value of the Register_Suppression_Time to prevent a possible negative value in the setting of the Register-Stop Timer. Fenner, et al. Standards Track [Page 129]
RFC 7761 PIM-SM Specification (Revised) March 2016
5. IANA Considerations
5.1. PIM Address Family
The PIM Address Family field was chosen to be 8 bits as a tradeoff
between packet format and use of the IANA-assigned numbers. Because
when the PIM packet format was designed only 15 values were assigned
for Address Families, and large numbers of new Address Family values
were not envisioned, 8 bits seemed large enough. However, the IANA
assigns Address Families in a 16-bit field. Therefore, the PIM
Address Family is allocated as follows:
Values 0 through 127 are designated to have the same meaning as
IANA-assigned Address Family Numbers [7].
Values 128 through 250 are designated to be assigned for PIM by
the IANA based upon IESG Approval, as defined in [9].
Values 251 through 255 are designated for Private Use, as defined
in [9].
5.2. PIM Hello Options
Values 17 through 65000 are to be assigned by the IANA. Since the
space is large, they may be assigned as First Come First Served, as
defined in [9]. Such assignments are valid for one year and may be
renewed. Permanent assignments require a specification (see
"Specification Required" in [9]).
Fenner, et al. Standards Track [Page 130]
RFC 7761 PIM-SM Specification (Revised) March 2016
6. Security Considerations
This section describes various possible security concerns related to
the PIM-SM protocol. The reader is referred to [8], [14], and [15]
for further discussion of PIM-SM and multicast security.
Note that PIM relies upon an MRIB populated outside of PIM;
therefore, securing the sources of change to the MRIB is RECOMMENDED.
6.1. Attacks Based on Forged Messages
The extent of possible damage depends on the type of counterfeit
messages accepted. We next consider the impact of possible
forgeries, including forged link-local (Join/Prune, Hello, and
Assert) and forged unicast (Register and Register-Stop) messages.
6.1.1. Forged Link-Local Messages
Join/Prune, Hello, and Assert messages are all sent to the link-local
ALL-PIM-ROUTERS multicast address and thus are not forwarded by a
compliant router. A forged message of this type can only reach a LAN
if it was sent by a local host or if it was allowed onto the LAN by a
compromised or non-compliant router.
1. A forged Join/Prune message can cause multicast traffic to be
delivered to links where there are no legitimate requesters,
potentially wasting bandwidth on that link. A forged leave
message on a multi-access LAN is generally not a significant
attack in PIM, because any legitimately joined router on the LAN
would override the leave with a join before the upstream router
stops forwarding data to the LAN.
2. By forging a Hello message, an unauthorized router can cause
itself to be elected as the Designated Router on a LAN. The
Designated Router on a LAN is (in the absence of asserts)
responsible for forwarding traffic to that LAN on behalf of any
local members. The Designated Router is also responsible for
register-encapsulating to the RP any packets that are originated
by hosts on the LAN. Thus, the ability of local hosts to send
and receive multicast traffic may be compromised by a forged
Hello message.
3. By forging an Assert message on a multi-access LAN, an attacker
could cause the legitimate designated forwarder to stop
forwarding traffic to the LAN. Such a forgery would prevent any
hosts downstream of that LAN from receiving traffic.
Fenner, et al. Standards Track [Page 131]
RFC 7761 PIM-SM Specification (Revised) March 2016
6.1.2. Forged Unicast Messages
Register messages and Register-Stop messages are forwarded by
intermediate routers to their destination using normal IP forwarding.
Without data origin authentication, an attacker who is located
anywhere in the network may be able to forge a Register or
Register-Stop message. We next consider the effect of a forgery of
each of these messages.
1. By forging a Register message, an attacker can cause the RP to
inject forged traffic onto the shared multicast tree.
2. By forging a Register-Stop message, an attacker can prevent a
legitimate DR from registering packets to the RP. This can
prevent local hosts on that LAN from sending multicast packets.
The above two PIM messages are not changed by intermediate routers
and need only be examined by the intended receiver. Thus, these
messages can be authenticated end-to-end. Attacks on Register and
Register-Stop messages do not apply to a PIM-SSM-only implementation,
as these messages are not required for PIM-SSM.
6.2. Non-cryptographic Authentication Mechanisms
A PIM router SHOULD provide an option to limit the set of neighbors
from which it will accept Join/Prune, Assert, and Hello messages.
Either static configuration of IP addresses or an IPsec security
association MAY be used. Furthermore, a PIM router SHOULD NOT accept
protocol messages from a router from which it has not yet received a
valid Hello message.
A Designated Router MUST NOT register-encapsulate a packet and send
it to the RP unless the source address of the packet is a legal
address for the subnet on which the packet was received. Similarly,
a Designated Router SHOULD NOT accept a Register-Stop packet whose IP
source address is not a valid RP address for the local domain.
An implementation SHOULD provide a mechanism to allow an RP to
restrict the range of source addresses from which it accepts
Register-encapsulated packets.
All options that restrict the range of addresses from which packets
are accepted MUST default to allowing all packets.
Fenner, et al. Standards Track [Page 132]
RFC 7761 PIM-SM Specification (Revised) March 2016
6.3. Authentication
This document refers to RFC 5796 [8], which specifies mechanisms to
authenticate PIM-SM link-local messages using the IPsec Encapsulating
Security Payload (ESP) or (optionally) the Authentication Header
(AH). It also points out that non-link-local PIM-SM messages (i.e.,
Register and Register-Stop messages) can be secured by a normal
unicast IPsec Security Association (SA) between two communicants.
6.4. Denial-of-Service Attacks
There are a number of possible denial-of-service attacks against PIM
that can be caused by generating false PIM protocol messages or even
by generating false traffic. Authenticating PIM protocol traffic
prevents some, but not all, of these attacks. Two of the possible
attacks include the following:
o Sending packets to many different group addresses quickly can be a
denial-of-service attack in and of itself. This will cause many
register-encapsulated packets, loading the DR, the RP, and the
routers between the DR and the RP.
o Forging Join messages can cause a multicast tree to get set up.
A large number of forged joins can consume router resources and
result in denial of service.
7. References
7.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[2] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, DOI 10.17487/RFC3376, October 2002,
<http://www.rfc-editor.org/info/rfc3376>.
[3] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, DOI 10.17487/RFC1112, August 1989,
<http://www.rfc-editor.org/info/rfc1112>.
[4] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, DOI 10.17487/RFC2710,
October 1999, <http://www.rfc-editor.org/info/rfc2710>.
Fenner, et al. Standards Track [Page 133]
RFC 7761 PIM-SM Specification (Revised) March 2016
[5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998,
<http://www.rfc-editor.org/info/rfc2460>.
[6] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
RFC 4607, DOI 10.17487/RFC4607, August 2006,
<http://www.rfc-editor.org/info/rfc4607>.
[7] IANA, "Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
[8] Atwood, W., Islam, S., and M. Siami, "Authentication and
Confidentiality in Protocol Independent Multicast Sparse Mode
(PIM-SM) Link-Local Messages", RFC 5796, DOI 10.17487/RFC5796,
March 2010, <http://www.rfc-editor.org/info/rfc5796>.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
7.2. Informative References
[10] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 4760, DOI 10.17487/RFC4760,
January 2007, <http://www.rfc-editor.org/info/rfc4760>.
[11] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas, "Bootstrap
Router (BSR) Mechanism for Protocol Independent Multicast
(PIM)", RFC 5059, DOI 10.17487/RFC5059, January 2008,
<http://www.rfc-editor.org/info/rfc5059>.
[12] Black, D., "Differentiated Services and Tunnels", RFC 2983,
DOI 10.17487/RFC2983, October 2000,
<http://www.rfc-editor.org/info/rfc2983>.
[13] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
RFC 5015, DOI 10.17487/RFC5015, October 2007,
<http://www.rfc-editor.org/info/rfc5015>.
[14] Savola, P., Lehtonen, R., and D. Meyer, "Protocol Independent
Multicast - Sparse Mode (PIM-SM) Multicast Routing Security
Issues and Enhancements", RFC 4609, DOI 10.17487/RFC4609,
October 2006, <http://www.rfc-editor.org/info/rfc4609>.
Fenner, et al. Standards Track [Page 134]
RFC 7761 PIM-SM Specification (Revised) March 2016
[15] Savola, P. and J. Lingard, "Host Threats to Protocol Independent
Multicast (PIM)", RFC 5294, DOI 10.17487/RFC5294, August 2008,
<http://www.rfc-editor.org/info/rfc5294>.
[16] Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP)
Address in an IPv6 Multicast Address", RFC 3956,
DOI 10.17487/RFC3956, November 2004,
<http://www.rfc-editor.org/info/rfc3956>.
[17] Zheng, L., Zhang, J., and R. Parekh, "Survey Report on Protocol
Independent Multicast - Sparse Mode (PIM-SM) Implementations and
Deployments", RFC 7063, DOI 10.17487/RFC7063, December 2013,
<http://www.rfc-editor.org/info/rfc7063>.
Fenner, et al. Standards Track [Page 135]
RFC 7761 PIM-SM Specification (Revised) March 2016 Appendix A. Functionality Removed from RFC 4601 Based on a survey of PIM implementations and deployments [17] conducted by the IETF PIM working group, the following functionality of RFC 4601 has been removed due to lack of sufficient implementation and deployment experience: o (*,*,RP) State o PIM Multicast Border Router (PMBR) o Authentication using IPsec Acknowledgements PIM-SM was designed over many years by a large group of people, including ideas, comments, and corrections from Deborah Estrin, Dino Farinacci, Ahmed Helmy, David Thaler, Steve Deering, Van Jacobson, C. Liu, Puneet Sharma, Liming Wei, Tom Pusateri, Tony Ballardie, Scott Brim, Jon Crowcroft, Paul Francis, Joel Halpern, Horst Hodel, Polly Huang, Stephen Ostrowski, Lixia Zhang, Girish Chandranmenon, Brian Haberman, Hal Sandick, Mike Mroz, Garry Kump, Pavlin Radoslavov, Mike Davison, James Huang, Christopher Thomas Brown, and James Lingard. Thanks are due to the American Licorice Company, for its obscure but possibly essential role in the creation of this document. Authors' Addresses Bill Fenner Arista Networks Email: fenner@arista.com Mark Handley Department of Computer Science University College London Gower Street London WC1E 6BT United Kingdom Email: M.Handley@cs.ucl.ac.uk Fenner, et al. Standards Track [Page 136]
RFC 7761 PIM-SM Specification (Revised) March 2016 Hugh Holbrook Arista Networks 5453 Great America Parkway Santa Clara, CA 95054 Email: holbrook@arista.com Isidor Kouvelas Arista Networks 5453 Great America Parkway Santa Clara, CA 95054 Email: kouvelas@arista.com Rishabh Parekh Cisco Systems, Inc. 170 W. Tasman Drive San Jose, CA 95134 Email: riparekh@cisco.com Zhaohui Zhang Juniper Networks 10 Technology Park Drive Westford, MA 01886 Email: zzhang@juniper.net Lianshu Zheng Huawei Technologies Co., Ltd Huawei Campus, 156 Beiqing Road, Hai-dian District Beijing 100089 China Email: vero.zheng@huawei.com Fenner, et al. Standards Track [Page 137]