GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3973

Network Working Group A. Adams Request for Comments: 3973 NextHop Technologies Category: Experimental J. Nicholas

                                                              ITT A/CD
                                                             W. Siadak
                                                  NextHop Technologies
                                                          January 2005
       Protocol Independent Multicast - Dense Mode (PIM-DM):
                  Protocol Specification (Revised)

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 This document specifies Protocol Independent Multicast - Dense Mode
 (PIM-DM).  PIM-DM is a multicast routing protocol that uses the
 underlying unicast routing information base to flood multicast
 datagrams to all multicast routers.  Prune messages are used to
 prevent future messages from propagating to routers without group
 membership information.

Adams, et al. Experimental [Page 1] RFC 3973 PIM - Dense Mode January 2005

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Pseudocode Notation  . . . . . . . . . . . . . . . . . .  5
 3.  PIM-DM Protocol Overview . . . . . . . . . . . . . . . . . . .  5
 4.  Protocol Specification . . . . . . . . . . . . . . . . . . . .  6
     4.1.  PIM Protocol State . . . . . . . . . . . . . . . . . . .  7
           4.1.1.  General Purpose State  . . . . . . . . . . . . .  7
           4.1.2.  (S,G) State  . . . . . . . . . . . . . . . . . .  8
           4.1.3.  State Summarization Macros . . . . . . . . . . .  8
     4.2.  Data Packet Forwarding Rules . . . . . . . . . . . . . . 10
     4.3.  Hello Messages . . . . . . . . . . . . . . . . . . . . . 11
           4.3.1.  Sending Hello Messages . . . . . . . . . . . . . 11
           4.3.2.  Receiving Hello Messages . . . . . . . . . . . . 11
           4.3.3.  Hello Message Hold Time  . . . . . . . . . . . . 12
           4.3.4.  Handling Router Failures . . . . . . . . . . . . 12
           4.3.5.  Reducing Prune Propagation Delay on LANs . . . . 13
     4.4.  PIM-DM Prune, Join, and Graft Messages . . . . . . . . . 13
           4.4.1.  Upstream Prune, Join, and Graft Messages . . . . 14
                   4.4.1.1.  Transitions from the Forwarding
                             (F) State  . . . . . . . . . . . . . . 17
                   4.4.1.2.  Transitions from the Pruned
                             (P) State  . . . . . . . . . . . . . . 18
                   4.4.1.3.  Transitions from the AckPending
                             (AP) State . . . . . . . . . . . . . . 19
           4.4.2.  Downstream Prune, Join, and Graft Messages . . . 21
                   4.4.2.1.  Transitions from the NoInfo State  . . 23
                   4.4.2.2.  Transitions from the PrunePending
                             (PP) State . . . . . . . . . . . . . . 24
                   4.4.2.3.  Transitions from the Prune
                             (P) State  . . . . . . . . . . . . . . 25
     4.5.  State Refresh  . . . . . . . . . . . . . . . . . . . . . 26
           4.5.1.  Forwarding of State Refresh Messages . . . . . . 26
           4.5.2.  State Refresh Message Origination  . . . . . . . 28
                   4.5.2.1.  Transitions from the NotOriginator
                             (NO) State . . . . . . . . . . . . . . 29
                   4.5.2.2.  Transitions from the Originator
                             (O) State  . . . . . . . . . . . . . . 29

Adams, et al. Experimental [Page 2] RFC 3973 PIM - Dense Mode January 2005

     4.6.  PIM Assert Messages  . . . . . . . . . . . . . . . . . . 30
           4.6.1.  Assert Metrics . . . . . . . . . . . . . . . . . 30
           4.6.2.  AssertCancel Messages  . . . . . . . . . . . . . 31
           4.6.3.  Assert State Macros  . . . . . . . . . . . . . . 32
           4.6.4.  (S,G) Assert Message State Machine . . . . . . . 32
                   4.6.4.1.  Transitions from NoInfo State  . . . . 34
                   4.6.4.2.  Transitions from Winner State  . . . . 35
                   4.6.4.3.  Transitions from Loser State . . . . . 36
           4.6.5.  Rationale for Assert Rules . . . . . . . . . . . 38
     4.7.  PIM Packet Formats . . . . . . . . . . . . . . . . . . . 38
           4.7.1.  PIM Header . . . . . . . . . . . . . . . . . . . 38
           4.7.2.  Encoded Unicast Address  . . . . . . . . . . . . 39
           4.7.3.  Encoded Group Address  . . . . . . . . . . . . . 40
           4.7.4.  Encoded Source Address . . . . . . . . . . . . . 41
           4.7.5.  Hello Message Format . . . . . . . . . . . . . . 42
                   4.7.5.1.  Hello Hold Time Option . . . . . . . . 43
                   4.7.5.2.  LAN Prune Delay Option . . . . . . . . 43
                   4.7.5.3.  Generation ID Option . . . . . . . . . 44
                   4.7.5.4.  State Refresh Capable Option . . . . . 44
           4.7.6.  Join/Prune Message Format  . . . . . . . . . . . 45
           4.7.7.  Assert Message Format  . . . . . . . . . . . . . 47
           4.7.8.  Graft Message Format . . . . . . . . . . . . . . 48
           4.7.9.  Graft Ack Message Format . . . . . . . . . . . . 48
           4.7.10. State Refresh Message Format . . . . . . . . . . 48
     4.8.  PIM-DM Timers  . . . . . . . . . . . . . . . . . . . . . 50
 5.  Protocol Interaction Considerations  . . . . . . . . . . . . . 53
     5.1.  PIM-SM Interactions  . . . . . . . . . . . . . . . . . . 53
     5.2.  IGMP Interactions  . . . . . . . . . . . . . . . . . . . 54
     5.3.  Source Specific Multicast (SSM) Interactions . . . . . . 54
     5.4.  Multicast Group Scope Boundary Interactions  . . . . . . 54
 6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 54
     6.1.  PIM Address Family . . . . . . . . . . . . . . . . . . . 54
     6.2.  PIM Hello Options  . . . . . . . . . . . . . . . . . . . 55
 7.  Security Considerations. . . . . . . . . . . . . . . . . . . . 55
     7.1.  Attacks Based on Forged Messages . . . . . . . . . . . . 55
     7.2.  Non-cryptographic Authentication Mechanisms  . . . . . . 56
     7.3.  Authentication Using IPsec . . . . . . . . . . . . . . . 56
     7.4.  Denial of Service Attacks  . . . . . . . . . . . . . . . 58
 8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 58
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
     9.1.  Normative References . . . . . . . . . . . . . . . . . . 58
     9.2.  Informative References . . . . . . . . . . . . . . . . . 59
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61

Adams, et al. Experimental [Page 3] RFC 3973 PIM - Dense Mode January 2005

1. Introduction

 This specification defines a multicast routing algorithm for
 multicast groups that are densely distributed across a network.  This
 protocol does not have a topology discovery mechanism often used by a
 unicast routing protocol.  It employs the same packet formats sparse
 mode PIM (PIM-SM) uses.  This protocol is called PIM - Dense Mode.
 The foundation of this design was largely built on Deering's early
 work on IP multicast routing [12].

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
 be interpreted as described in RFC 2119 [11] and indicate requirement
 levels for compliant PIM-DM implementations.

2.1. Definitions

 Multicast Routing Information Base (MRIB)
   This is the multicast topology table, which is typically derived
   from the unicast routing table, or from routing protocols such as
   MBGP that carry multicast-specific topology information.  PIM-DM
   uses the MRIB to make decisions regarding RPF interfaces.
 Tree Information Base (TIB)
   This is the collection of state maintained by a PIM router and
   created by receiving PIM messages and IGMP information from local
   hosts.  It essentially stores the state of all multicast
   distribution trees at that router.
 Reverse Path Forwarding (RPF)
   RPF is a multicast forwarding mode in which a data packet is
   accepted for forwarding only if it is received on an interface used
   to reach the source in unicast.
 Upstream Interface
   Interface toward the source of the datagram.  Also known as the RPF
   Interface.
 Downstream Interface
   All interfaces that are not the upstream interface, including the
   router itself.
 (S,G) Pair
   Source S and destination group G associated with an IP packet.

Adams, et al. Experimental [Page 4] RFC 3973 PIM - Dense Mode January 2005

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
   are the elements of set A that are not in set B.
 NULL
   is the empty set or list.
 Note that operations MUST be conducted in the order specified.  This
 is due to the fact that (-) is not a true difference operator,
 because B is not necessarily a subset of A.  That is, A (+) B (-) C =
 A (-) C (+) B is not a true statement unless C is a subset of both A
 and B.
 In addition, we use C-like syntax:
   =   denotes assignment of a variable.
   ==  denotes a comparison for equality.
   !=  denotes a comparison for inequality.
 Braces { and } are used for grouping.

3. PIM-DM Protocol Overview

 This section provides an overview of PIM-DM behavior.  It is intended
 as an introduction to how PIM-DM works and is NOT definitive.  For
 the definitive specification, see Section 4, Protocol Specification.
 PIM-DM assumes that when a source starts sending, all downstream
 systems want to receive multicast datagrams.  Initially, multicast
 datagrams are flooded to all areas of the network.  PIM-DM uses RPF
 to prevent looping of multicast datagrams while flooding.  If some
 areas of the network do not have group members, PIM-DM will prune off
 the forwarding branch by instantiating prune state.
 Prune state has a finite lifetime.  When that lifetime expires, data
 will again be forwarded down the previously pruned branch.
 Prune state is associated with an (S,G) pair.  When a new member for
 a group G appears in a pruned area, a router can "graft" toward the
 source S for the group, thereby turning the pruned branch back into a
 forwarding branch.

Adams, et al. Experimental [Page 5] RFC 3973 PIM - Dense Mode January 2005

 The broadcast of datagrams followed by pruning of unwanted branches
 is often referred to as a flood and prune cycle and is typical of
 dense mode protocols.
 To minimize repeated flooding of datagrams and subsequent pruning
 associated with a particular (S,G) pair, PIM-DM uses a state refresh
 message.  This message is sent by the router(s) directly connected to
 the source and is propagated throughout the network.  When received
 by a router on its RPF interface, the state refresh message causes an
 existing prune state to be refreshed.
 Compared with multicast routing protocols with built-in topology
 discovery mechanisms (e.g., DVMRP [13]), PIM-DM has a simplified
 design and is not hard-wired into a specific topology discovery
 protocol.  However, this simplification does incur more overhead by
 causing flooding and pruning to occur on some links that could be
 avoided if sufficient topology information were available; i.e., to
 decide whether an interface leads to any downstream members of a
 particular group.  Additional overhead is chosen in favor of the
 simplification and flexibility gained by not depending on a specific
 topology discovery protocol.
 PIM-DM differs from PIM-SM in two essential ways: 1) There are no
 periodic joins transmitted, only explicitly triggered prunes and
 grafts.  2) There is no Rendezvous Point (RP).  This is particularly
 important in networks that cannot tolerate a single point of failure.
 (An RP is the root of a shared multicast distribution tree.  For more
 details, see [4]).

4. Protocol Specification

 The specification of PIM-DM is broken into several parts:
  • Section 4.1 details the protocol state stored.
  • Section 4.2 specifies the data packet forwarding rules.
  • Section 4.3 specifies generation and processing of Hello messages.
  • Section 4.4 specifies the Join, Prune, and Graft generation and

processing rules.

  • Section 4.5 specifies the State Refresh generation and forwarding

rules.

  • Section 4.6 specifies the Assert generation and processing rules.
  • Section 4.7 gives details on PIM-DM Packet Formats.
  • Section 4.8 summarizes PIM-DM timers and their defaults.

Adams, et al. Experimental [Page 6] RFC 3973 PIM - Dense Mode January 2005

4.1. PIM Protocol State

 This section specifies all the protocol states that a PIM-DM
 implementation should maintain to function correctly.  We term this
 state the Tree Information Base or TIB, as it holds the state of all
 the multicast distribution trees at this router.  In this
 specification, we define PIM-DM 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.
 Unlike PIM-SM, PIM-DM does not maintain a keepalive timer associated
 with each (S,G) route.  Within PIM-DM, route and state information
 associated with an (S,G) entry MUST be maintained as long as any
 timer associated with that (S,G) entry is active.  When no timer
 associated with an (S,G) entry is active, all information concerning
 that (S,G) route may be discarded.
 Although we precisely specify the state to be kept, this does not
 mean that an implementation of PIM-DM has 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-DM implementation
 is free to hold whatever internal state it requires and will still be
 conformant with this specification as long as it results in the same
 externally visible protocol behavior as an abstract router that holds
 the following state.

4.1.1. General Purpose State

 A router stores the following non-group-specific state:
 For each interface:
   Hello Timer (HT)
   State Refresh Capable
   LAN Delay Enabled
   Propagation Delay (PD)
   Override Interval (OI)
   Neighbor State:
     For each neighbor:
       Information from neighbor's Hello
       Neighbor's Gen ID.
       Neighbor's LAN Prune Delay
       Neighbor's Override Interval
       Neighbor's State Refresh Capability
       Neighbor Liveness Timer (NLT)

Adams, et al. Experimental [Page 7] RFC 3973 PIM - Dense Mode January 2005

4.1.2. (S,G) State

 For every source/group pair (S,G), a router stores the following
 state:
 (S,G) state:
   For each interface:
     Local Membership:
       State: One of {"NoInfo", "Include"}
     PIM (S,G) Prune State:
       State: One of {"NoInfo" (NI), "Pruned" (P), "PrunePending"
                      (PP)}
                      Prune Pending Timer (PPT)
                      Prune Timer (PT)
     (S,G) Assert Winner State:
       State: One of {"NoInfo" (NI), "I lost Assert" (L), "I won
                      Assert" (W)}
       Assert Timer (AT)
       Assert winner's IP Address
       Assert winner's Assert Metric
   Upstream interface-specific:
     Graft/Prune State:
       State: One of {"NoInfo" (NI), "Pruned" (P), "Forwarding" (F),
                      "AckPending" (AP) }
       GraftRetry Timer (GRT)
       Override Timer (OT)
       Prune Limit Timer (PLT)
     Originator State:
       Source Active Timer (SAT)
       State Refresh Timer (SRT)

4.1.3. State Summarization Macros

 Using the state defined above, the following "macros" are defined and
 will be used in the descriptions of the state machines and pseudocode
 in the following sections.
 The most important macros are those defining the outgoing interface
 list (or "olist") for the relevant state.
 immediate_olist(S,G) = pim_nbrs (-) prunes(S,G) (+)
                        (pim_include(*,G) (-) pim_exclude(S,G) ) (+)
                        pim_include(S,G) (-) lost_assert(S,G) (-)
                        boundary(G)

Adams, et al. Experimental [Page 8] RFC 3973 PIM - Dense Mode January 2005

 olist(S,G) = immediate_olist(S,G) (-) RPF_interface(S)
 The macros pim_include(*,G) and pim_include(S,G) indicate the
 interfaces to which traffic might or might not be forwarded because
 of hosts that are local members on those interfaces.
 pim_include(*,G) = {all interfaces I such that:
                     local_receiver_include(*,G,I)}
 pim_include(S,G) = {all interfaces I such that:
                     local_receiver_include(S,G,I)}
 pim_exclude(S,G) = {all interfaces I such that:
                     local_receiver_exclude(S,G,I)}
 The macro RPF_interface(S) returns the RPF interface for source S.
 That is to say, it returns the interface used to reach S as indicated
 by the MRIB.
 The macro local_receiver_include(S,G,I) is true if the IGMP module or
 other local membership mechanism ([1], [2], [3], [6]) has determined
 that there are local members on interface I that seek to receive
 traffic sent specifically by S to G.
 The macro local_receiver_include(*,G,I) is true if the IGMP module or
 other local membership mechanism has determined that there are local
 members on interface I that seek to receive all traffic sent to G.
 Note that this determination is expected to account for membership
 joins initiated on or by the router.
 The macro local_receiver_exclude(S,G,I) is true if
 local_receiver_include(*,G,I) is true but none of the local members
 seek to receive traffic from S.
 The set pim_nbrs is the set of all interfaces on which the router has
 at least one active PIM neighbor.
 The set prunes(S,G) is the set of all interfaces on which the router
 has received Prune(S,G) messages:
 prunes(S,G) = {all interfaces I such that
                DownstreamPState(S,G,I) is in Pruned state}

Adams, et al. Experimental [Page 9] RFC 3973 PIM - Dense Mode January 2005

 The set lost_assert(S,G) is the set of all interfaces on which the
 router has lost an (S,G) Assert.
 lost_assert(S,G) = {all interfaces I such that
                     lost_assert(S,G,I) == TRUE}
 boundary(G) = {all interfaces I with an administratively scoped
                boundary for group G}
 The following pseudocode macro definitions are also used in many
 places in the specification.  Basically RPF' is the RPF neighbor
 toward a source unless a PIM-DM Assert has overridden the normal
 choice of neighbor.
 neighbor RPF'(S,G) {
   if ( I_Am_Assert_loser(S, G, RPF_interface(S) )) {
     return AssertWinner(S, G, RPF_interface(S) )
   } else {
     return MRIB.next_hop( S )
   }
 }
 The macro I_Am_Assert_loser(S, G, I) is true if the Assert state
 machine (in Section 4.6) for (S,G) on interface I is in the "I am
 Assert Loser" state.

4.2. Data Packet Forwarding Rules

 The PIM-DM 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).  RPF_interface(S) is the interface the MRIB indicates would
 be used to route packets to S.
 First, an RPF check MUST be performed to determine whether the packet
 should be accepted based on TIB state and the interface on which that
 the packet arrived.  Packets that fail the RPF check MUST NOT be
 forwarded, and the router will conduct an assert process for the
 (S,G) pair specified in the packet.  Packets for which a route to the
 source cannot be found MUST be discarded.
 If the RPF check has been passed, an outgoing interface list is
 constructed for the packet.  If this list is not empty, then the
 packet MUST be forwarded to all listed interfaces.  If the list is
 empty, then the router will conduct a prune process for the (S,G)
 pair specified in the packet.

Adams, et al. Experimental [Page 10] RFC 3973 PIM - Dense Mode January 2005

 Upon receipt of a data packet from S addressed to G on interface iif:
 if (iif == RPF_interface(S) AND UpstreamPState(S,G) != Pruned) {
     oiflist = olist(S,G)
 } else {
     oiflist = NULL
 }
 forward packet on all interfaces in oiflist
 This pseudocode employs the following "macro" definition:
 UpstreamPState(S,G) is the state of the Upstream(S,G) state machine
 in Section 4.4.1.

4.3. Hello Messages

 This section describes the generation and processing of Hello
 messages.

4.3.1. Sending Hello Messages

 PIM-DM uses Hello messages to detect other PIM routers.  Hello
 messages are sent periodically on each PIM enabled interface.  Hello
 messages are multicast to the ALL-PIM-ROUTERS group.  When PIM is
 enabled on an interface or when a router first starts, the Hello
 Timer (HT) MUST be set to random value between 0 and
 Triggered_Hello_Delay.  This prevents synchronization of Hello
 messages if multiple routers are powered on simultaneously.
 After the initial Hello message, a Hello message MUST be sent every
 Hello_Period.  A single Hello timer MAY be used to trigger sending
 Hello messages on all active interfaces.  The Hello Timer SHOULD NOT
 be reset except when it expires.

4.3.2. Receiving Hello Messages

 When a Hello message is received, the receiving router SHALL record
 the receiving interface, the sender, and any information contained in
 recognized options.  This information is retained for a number of
 seconds in the Hold Time field of the Hello Message.  If a new Hello
 message is received from a particular neighbor N, the Neighbor
 Liveness Timer (NLT(N,I)) MUST be reset to the newly received Hello
 Holdtime.  If a Hello message is received from a new neighbor, the
 receiving router SHOULD send its own Hello message after a random
 delay between 0 and Triggered_Hello_Delay.

Adams, et al. Experimental [Page 11] RFC 3973 PIM - Dense Mode January 2005

4.3.3. Hello Message Hold Time

 The Hold Time in the Hello Message should be set to a value that can
 reasonably be expected to keep the Hello active until a new Hello
 message is received.  On most links, this will be 3.5 times the value
 of Hello_Period.
 If the Hold Time is set to '0xffff', the receiving router MUST NOT
 time out that Hello message.  This feature might be used for on-
 demand links to avoid keeping the link up with periodic Hello
 messages.
 If a Hold Time of '0' is received, the corresponding neighbor state
 expires immediately.  When a PIM router takes an interface down or
 changes IP address, a Hello message with a zero Hold Time SHOULD be
 sent immediately (with the old IP address if the IP address is
 changed) to cause any PIM neighbors to remove the old information
 immediately.

4.3.4. Handling Router Failures

 If a Hello message is received from an active neighbor with a
 different Generation ID (GenID), the neighbor has restarted and may
 not contain the correct (S,G) state.  A Hello message SHOULD be sent
 after a random delay between 0 and Triggered_Hello_Delay (see 4.8)
 before any other messages are sent.  If the neighbor is downstream,
 the router MAY replay the last State Refresh message for any (S,G)
 pairs for which it is the Assert Winner indicating Prune and Assert
 status to the downstream router.  These State Refresh messages SHOULD
 be sent out immediately after the Hello message.  If the neighbor is
 the upstream neighbor for an (S,G) entry, the router MAY cancel its
 Prune Limit Timer to permit sending a prune and reestablishing a
 Pruned state in the upstream router.
 Upon startup, a router MAY use any State Refresh messages received
 within Hello_Period of its first Hello message on an interface to
 establish state information.  The State Refresh source will be the
 RPF'(S), and Prune status for all interfaces will be set according to
 the Prune Indicator bit in the State Refresh message.  If the Prune
 Indicator is set, the router SHOULD set the PruneLimitTimer to
 Prune_Holdtime and set the PruneTimer on all downstream interfaces to
 the State Refresh's Interval times two.  The router SHOULD then
 propagate the State Refresh as described in Section 4.5.1.

Adams, et al. Experimental [Page 12] RFC 3973 PIM - Dense Mode January 2005

4.3.5. Reducing Prune Propagation Delay on LANs

 If all routers on a LAN support the LAN Prune Delay option, then the
 PIM routers on that LAN will use the values received to adjust their
 J/P_Override_Interval on that interface and the interface is LAN
 Delay Enabled.  Briefly, to avoid synchronization of Prune Override
 (Join) messages when multiple downstream routers share a multi-access
 link, sending of these messages is delayed by a small random amount
 of time.  The period of randomization is configurable and has a
 default value of 3 seconds.
 Each router on the LAN 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 LAN use the LAN Prune
 Delay Option, all routers on the LAN MUST set their Override_Interval
 to the largest Override value on the LAN.
 The LAN 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.  When all routers
 on a link use the LAN Prune Delay Option, all routers on the LAN MUST
 set Propagation Delay to the largest LAN Delay on the LAN.
 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 and triggered messages to be sent later than
 intended.  Setting this LAN Prune 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.

4.4. PIM-DM Prune, Join, and Graft Messages

 This section describes the generation and processing of PIM-DM Join,
 Prune, and Graft messages.  Prune messages are sent toward the
 upstream neighbor for S to indicate that traffic from S addressed to
 group G is not desired.  In the case of downstream routers A and B,
 where A wishes to continue receiving data and B does not, A will send
 a Join in response to B's Prune to override the Prune.  This is the
 only situation in PIM-DM in which a Join message is used.  Finally, a
 Graft message is used to re-join a previously pruned branch to the
 delivery tree.

Adams, et al. Experimental [Page 13] RFC 3973 PIM - Dense Mode January 2005

4.4.1. Upstream Prune, Join, and Graft Messages

 The Upstream(S,G) state machine for sending Prune, Graft, and Join
 messages is given below.  There are three states.
   Forwarding (F)
     This is the starting state of the Upsteam(S,G) state machine.
     The state machine is in this state if it just started or if
     oiflist(S,G) != NULL.
   Pruned (P)
     The set, olist(S,G), is empty.  The router will not forward data
     from S addressed to group G.
   AckPending (AP)
     The router was in the Pruned(P) state, but a transition has
     occurred in the Downstream(S,G) state machine for one of this
     (S,G) entry's outgoing interfaces, indicating that traffic from S
     addressed to G should again be forwarded.  A Graft message has
     been sent to RPF'(S), but a Graft Ack message has not yet been
     received.
 In addition, there are three state-machine-specific timers:
   GraftRetry Timer (GRT(S,G))
     This timer is set when a Graft is sent upstream.  If a
     corresponding GraftAck is not received before the timer expires,
     then another Graft is sent, and the GraftRetry Timer is reset.
     The timer is stopped when a Graft Ack message is received.  This
     timer is normally set to Graft_Retry_Period (see 4.8).
   Override Timer (OT(S,G))
     This timer is set when a Prune(S,G) is received on the upstream
     interface where olist(S,G) != NULL.  When the timer expires, a
     Join(S,G) message is sent on the upstream interface.  This timer
     is normally set to t_override (see 4.8).
   Prune Limit Timer (PLT(S,G))
     This timer is used to rate-limit Prunes on a LAN.  It is only
     used when the Upstream(S,G) state machine is in the Pruned state.
     A Prune cannot be sent if this timer is running.  This timer is
     normally set to t_limit (see 4.8).

Adams, et al. Experimental [Page 14] RFC 3973 PIM - Dense Mode January 2005

        +-------------+                        +-------------+
        |             |     olist == NULL      |             |
        |   Forward   |----------------------->|   Pruned    |
        |             |                        |             |
        +-------------+                        +-------------+
             ^   |                                  ^   |
             |   |                                  |   |
             |   |RPF`(S) Changes      olist == NULL|   |
             |   |                                  |   |
             |   |         +-------------+          |   |
             |   +-------->|             |----------+   |
             |             | AckPending  |              |
             +-------------|             |<-------------+
           Rcv GraftAck OR +-------------+ olist != NULL
         Rcv State Refresh
            With (P==0) OR
        S Directly Connect
              Figure 1: Upstream Interface State Machine
 In tabular form, the state machine is defined as follows:

+——————————-+————————————–+

Previous State
Event Forwarding Pruned AckPending

+——————————-+————+————+————+

Data packet arrives on →P Send →P Send N/A
RPF_Interface(S) AND Prune(S,G) Prune(S,G)
olist(S,G) == NULL AND Set PLT(S,G)Set PLT(S,G)
PLT(S,G) not running

+——————————-+————+————+————+

State Refresh(S,G) received →F Set →P Reset →AP Set
from RPF`(S) AND OT(S,G) PLT(S,G) OT(S,G)
Prune Indicator == 1

+——————————-+————+————+————+

State Refresh(S,G) received →F →P Send →F Cancel
from RPF`(S) AND Prune(S,G) GRT(S,G)
Prune Indicator == 0 AND Set PLT(S,G)
PLT(S,G) not running

+——————————-+————+————+————+

Adams, et al. Experimental [Page 15] RFC 3973 PIM - Dense Mode January 2005

+——————————-+————————————–+

Previous State

+ +————+————+————+

Event Forwarding Pruned AckPending

+——————————-+————+————+————+

See Join(S,G) to RPF'(S) →F Cancel →P →AP Cancel
OT(S,G) OT(S,G)

+——————————-+————+————+————+

See Prune(S,G) →F Set →P →AP Set
OT(S,G) OT(S,G)

+——————————-+————+————+————+

OT(S,G) Expires →F Send N/A →AP Send
Join(S,G) Join(S,G)

+——————————-+————+————+————+

olist(S,G)→NULL →P Send N/A →P Send
Prune(S,G) Prune(S,G)
Set PLT(S,G) Set PLT(S,G)
Cancel
GRT(S,G)

+——————————-+————+————+————+

olist(S,G)→non-NULL N/A →AP Send N/A
Graft(S,G)
Set GRT(S,G)

+——————————-+————+————+————+

RPF'(S) Changes AND →AP Send →AP Send →AP Send
olist(S,G) != NULL Graft(S,G) Graft(S,G) Graft(S,G)
Set GRT(S,G)Set GRT(S,G)Set GRT(S,G)

+——————————-+————+————+————+

RPF'(S) Changes AND →P →P Cancel →P Cancel
olist(S,G) == NULL PLT(S,G) GRT(S,G)

+——————————-+————+————+————+

S becomes directly connected →F →P →F Cancel
GRT(S,G)

+——————————-+————+————+————+

GRT(S,G) Expires N/A N/A →AP Send
Graft(S,G)
Set GRT(S,G)

+——————————-+————+————+————+

Receive GraftAck(S,G) from →F →P →F Cancel
RPF'(S) GRT(S,G)

+——————————-+————+————+————+

 The transition event "RcvGraftAck(S,G)" implies receiving a Graft Ack
 message targeted to this router's address on the incoming interface
 for the (S,G) entry.  If the destination address is not correct, the
 state transitions in this state machine must not occur.

Adams, et al. Experimental [Page 16] RFC 3973 PIM - Dense Mode January 2005

4.4.1.1. Transitions from the Forwarding (F) State

 When the Upstream(S,G) state machine is in the Forwarding (F) state,
 the following events may trigger a transition:
   Data Packet arrives on RPF_Interface(S) AND olist(S,G) == NULL AND
   S NOT directly connected
     The Upstream(S,G) state machine MUST transition to the Pruned (P)
     state, send a Prune(S,G) to RPF'(S), and set PLT(S,G) to t_limit
     seconds.
   State Refresh(S,G) Received from RPF'(S)
     The Upstream(S,G) state machine remains in a Forwarding state.
     If the received State Refresh has the Prune Indicator bit set to
     one, this router must override the upstream router's Prune state
     after a short random interval.  If OT(S,G) is not running and the
     Prune Indicator bit equals one, the router MUST set OT(S,G) to
     t_override seconds.
   See Join(S,G) to RPF'(S)
     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).  If the OT(S,G) is running, then it
     means that the router had scheduled a Join to override a
     previously received Prune.  Another router has responded more
     quickly with a Join, so the local router SHOULD cancel its
     OT(S,G), if it is running.  The Upstream(S,G) state machine
     remains in the Forwarding (F) state.
   See Prune(S,G) AND S NOT directly connected
     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).  As this router is in Forwarding state, it must
     override the Prune after a short random interval.  If OT(S,G) is
     not running, the router MUST set OT(S,G) to t_override seconds.
     The Upstream(S,G) state machine remains in Forwarding (F) state.
   OT(S,G) Expires AND S NOT directly connected
     The OverrideTimer (OT(S,G)) expires.  The router MUST send a
     Join(S,G) to RPF'(S) to override a previously detected prune.
     The Upstream(S,G) state machine remains in the Forwarding (F)
     state.
   olist(S,G) -> NULL AND S NOT directly connected
     The Upstream(S,G) state machine MUST transition to the Pruned (P)
     state, send a Prune(S,G) to RPF'(S), and set PLT(S,G) to t_limit
     seconds.

Adams, et al. Experimental [Page 17] RFC 3973 PIM - Dense Mode January 2005

   RPF'(S) Changes AND olist(S,G) is non-NULL AND S NOT directly
   connected
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine MUST transition to the AckPending (AP) state, unicast a
     Graft to the new RPF'(S), and set the GraftRetry Timer (GRT(S,G))
     to Graft_Retry_Period.
   RPF'(S) Changes AND olist(S,G) is NULL
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine MUST transition to the Pruned (P) state.

4.4.1.2. Transitions from the Pruned (P) State

 When the Upstream(S,G) state machine is in the Pruned (P) state, the
 following events may trigger a transition:
   Data arrives on RPF_interface(S) AND PLT(S,G) not running AND S NOT
   directly connected
     Either another router on the LAN desires traffic from S addressed
     to G or a previous Prune was lost.  To prevent generating a
     Prune(S,G) in response to every data packet, the PruneLimit Timer
     (PLT(S,G)) is used.  Once the PLT(S,G) expires, the router needs
     to send another prune in response to a data packet not received
     directly from the source.  A Prune(S,G) MUST be sent to RPF'(S),
     and the PLT(S,G) MUST be set to t_limit.
   State Refresh(S,G) Received from RPF'(S)
     The Upstream(S,G) state machine remains in a Pruned state.  If
     the State Refresh has its Prune Indicator bit set to zero and
     PLT(S,G) is not running, a Prune(S,G) MUST be sent to RPF'(S),
     and the PLT(S,G) MUST be set to t_limit.  If the State Refresh
     has its Prune Indicator bit set to one, the router MUST reset
     PLT(S,G) to t_limit.
   See Prune(S,G) to RPF'(S)
     A Prune(S,G) is seen on RPF_interface(S) to RPF'(S).  The
     Upstream(S,G) state machine stays in the Pruned (P) state.  The
     router MAY reset its PLT(S,G) to the value in the Holdtime field
     of the received message if it is greater than the current value
     of the PLT(S,G).
   olist(S,G)->non-NULL AND S NOT directly connected
     The set of interfaces defined by the olist(S,G) macro becomes
     non-empty, indicating that traffic from S addressed to group G
     must be forwarded.  The Upstream(S,G) state machine MUST cancel
     PLT(S,G), transition to the AckPending (AP) state and unicast a

Adams, et al. Experimental [Page 18] RFC 3973 PIM - Dense Mode January 2005

     Graft message to RPF'(S).  The Graft Retry Timer (GRT(S,G)) MUST
     be set to Graft_Retry_Period.
   RPF'(S) Changes AND olist(S,G) == non-NULL AND S NOT directly
   connected
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine MUST cancel PLT(S,G), transition to the AckPending (AP)
     state, send a Graft unicast to the new RPF'(S), and set the
     GraftRetry Timer (GRT(S,G)) to Graft_Retry_Period.
   RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine stays in the Pruned (P) state and MUST cancel the
     PLT(S,G) timer.
   S becomes directly connected
     Unicast routing changed so that S is directly connected.  The
     Upstream(S,G) state machine remains in the Pruned (P) state.

4.4.1.3. Transitions from the AckPending (AP) State

 When the Upstream(S,G) state machine is in the AckPending (AP) state,
 the following events may trigger a transition:
   State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 1
     The Upstream(S,G) state machine remains in an AckPending state.
     The router must override the upstream router's Prune state after
     a short random interval.  If OT(S,G) is not running and the Prune
     Indicator bit equals one, the router MUST set OT(S,G) to
     t_override seconds.
   State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 0
     The router MUST cancel its GraftRetry Timer (GRT(S,G)) and
     transition to the Forwarding (F) state.
   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).  If the OT(S,G) is running, then it
     means that the router had scheduled a Join to override a
     previously received Prune.  Another router has responded more
     quickly with a Join, so the local router SHOULD cancel its
     OT(S,G), if it is running.  The Upstream(S,G) state machine
     remains in the AckPending (AP) state.

Adams, et al. Experimental [Page 19] RFC 3973 PIM - Dense Mode January 2005

   See Prune(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).  As this router is in AckPending (AP) state, it
     must override the Prune after a short random interval.  If
     OT(S,G) is not running, the router MUST set OT(S,G) to t_override
     seconds.  The Upstream(S,G) state machine remains in AckPending
     (AP) state.
   OT(S,G) Expires
     The OverrideTimer (OT(S,G)) expires.  The router MUST send a
     Join(S,G) to RPF'(S).  The Upstream(S,G) state machine remains in
     the AckPending (AP) state.
   olist(S,G) -> NULL
     The set of interfaces defined by the olist(S,G) macro becomes
     null, indicating that traffic from S addressed to group G should
     no longer be forwarded.  The Upstream(S,G) state machine MUST
     transition to the Pruned (P) state.  A Prune(S,G) MUST be
     multicast to the RPF_interface(S), with RPF'(S) named in the
     upstream neighbor field.  The GraftRetry Timer (GRT(S,G)) MUST be
     cancelled, and PLT(S,G) MUST be set to t_limit seconds.
   RPF'(S) Changes AND olist(S,G) does not become NULL AND S NOT
   directly connected
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine stays in the AckPending (AP) state.  A Graft MUST be
     unicast to the new RPF'(S) and the GraftRetry Timer (GRT(S,G))
     reset to Graft_Retry_Period.
   RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
     Unicast routing or Assert state causes RPF'(S) to change,
     including changes to RPF_Interface(S).  The Upstream(S,G) state
     machine MUST transition to the Pruned (P) state.  The GraftRetry
     Timer (GRT(S,G)) MUST be cancelled.
   S becomes directly connected
     Unicast routing has changed so that S is directly connected.  The
     GraftRetry Timer MUST be cancelled, and the Upstream(S,G) state
     machine MUST transition to the Forwarding(F) state.

Adams, et al. Experimental [Page 20] RFC 3973 PIM - Dense Mode January 2005

   GRT(S,G) Expires
     The GraftRetry Timer (GRT(S,G)) expires for this (S,G) entry.
     The Upstream(S,G) state machine stays in the AckPending (AP)
     state.  Another Graft message for (S,G) SHOULD be unicast to
     RPF'(S) and the GraftRetry Timer (GRT(S,G)) reset to
     Graft_Retry_Period.  It is RECOMMENDED that the router retry a
     configured number of times before ceasing retries.
   See GraftAck(S,G) from RPF'(S)
     A GraftAck is received from  RPF'(S).  The GraftRetry Timer MUST
     be cancelled, and the Upstream(S,G) state machine MUST transition
     to the Forwarding(F) state.

4.4.2. Downstream Prune, Join, and Graft Messages

 The Prune(S,G) Downstream state machine for receiving Prune, Join and
 Graft messages on interface I is given below.  This state machine
 MUST always be in the NoInfo state on the upstream interface.  It
 contains three states.
   NoInfo(NI)
     The interface has no (S,G) Prune state, and neither the Prune
     timer (PT(S,G,I)) nor the PrunePending timer ((PPT(S,G,I)) is
     running.
   PrunePending(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 PrunePending state functions exactly like the
     NoInfo state.
   Pruned(P)
     The router has received a Prune(S,G) on this interface from a
     downstream neighbor, and the Prune was not overridden.  Data from
     S addressed to group G is no longer being forwarded on this
     interface.
 In addition, there are two timers:
   PrunePending Timer (PPT(S,G,I))
     This timer is set when a valid Prune(S,G) is received.  Expiry of
     the PrunePending Timer (PPT(S,G,I)) causes the interface to
     transition to the Pruned state.

Adams, et al. Experimental [Page 21] RFC 3973 PIM - Dense Mode January 2005

   Prune Timer (PT(S,G,I))
     This timer is set when the PrunePending Timer (PT(S,G,I))
     expires.  Expiry of the Prune Timer (PT(S,G,I)) causes the
     interface to transition to the NoInfo (NI) state, thereby
     allowing data from S addressed to group G to be forwarded on the
     interface.
          +-------------+                        +-------------+
          |             |      PPT Expires       |             |
          |PrunePending |----------------------->|   Pruned    |
          |             |                        |             |
          +-------------+                        +-------------+
               |   ^                                      |
               |   |                                      |
               |   |Rcv Prune                             |
               |   |                                      |
               |   |         +-------------+              |
               |   +---------|             |              |
               |             |   NoInfo    |<-------------+
               +------------>|             | Rcv Join/Graft OR
           Rcv Join/Graft OR +-------------+ PT Expires OR
         RPF_Interface(S)->I                 RPF_Interface(S)->I
               Figure 2: Downstream Interface State Machine

Adams, et al. Experimental [Page 22] RFC 3973 PIM - Dense Mode January 2005

 In tabular form, the state machine is as follows:

+——————————-+————————————–+

Previous State

+ +————+————+————+

Event No Info PrunePend Pruned

+——————————-+————+————+————+

Receive Prune(S,G) →PP Set →PP →P Reset
PPT(S,G,I) PT(S,G,I)

+——————————-+————+————+————+

Receive Join(S,G) →NI →NI Cancel →NI Cancel
PPT(S,G,I) PT(S,G,I)

+——————————-+————+————+————+

Receive Graft(S,G) →NI Send →NI Send →NI Send
GraftAck GraftAck GraftAck
Cancel Cancel
PPT(S,G,I) PT(S,G,I)

+——————————-+————+————+————+

PPT(S,G) Expires N/A →P Set N/A
PT(S,G,I)

+——————————-+————+————+————+

PT(S,G) Expires N/A N/A →NI

+——————————-+————+————+————+

RPF_Interface(S) becomes I →NI →NI Cancel →NI Cancel
PPT(S,G,I) PT(S,G,I)

+——————————-+————+————+————+

Send State Refresh(S,G) out I →NI →PP →P Reset
PT(S,G,I)

+——————————-+————+————+————+

 The transition events "Receive Graft(S,G)", "Receive Prune(S,G)", and
 "Receive Join(S,G)" denote receiving a Graft, Prune, or Join message
 in which this router's address on I is contained in the message's
 upstream neighbor field.  If the upstream neighbor field does not
 match this router's address on I, then these state transitions in
 this state machine must not occur.

4.4.2.1. Transitions from the NoInfo State

 When the Prune(S,G) Downstream state machine is in the NoInfo (NI)
 state, the following events may trigger a transition:
   Receive Prune(S,G)
     A Prune(S,G) is received on interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     PrunePending (PP) state.  The PrunePending Timer (PPT(S,G,I))
     MUST be set to J/P_Override_Interval if the router has more than

Adams, et al. Experimental [Page 23] RFC 3973 PIM - Dense Mode January 2005

     one neighbor on I.  If the router has only one neighbor on
     interface I, then it SHOULD set the PPT(S,G,I) to zero,
     effectively transitioning immediately to the Pruned (P) state.
   Receive Graft(S,G)
     A Graft(S,G) is received on the interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I stays in the NoInfo (NI)
     state.  A GraftAck(S,G) MUST be unicast to the originator of the
     Graft(S,G) message.

4.4.2.2. Transitions from the PrunePending (PP) State

 When the Prune(S,G) downstream state machine is in the PrunePending
 (PP) state, the following events may trigger a transition.
   Receive Join(S,G)
     A Join(S,G) is received on interface I with the upstream neighbor
     field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     NoInfo (NI) state.  The PrunePending Timer (PPT(S,G,I)) MUST be
     cancelled.
   Receive Graft(S,G)
     A Graft(S,G) is received on interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     NoInfo (NI) state and MUST unicast a Graft Ack message to the
     Graft originator.  The PrunePending Timer (PPT(S,G,I)) MUST be
     cancelled.
   PPT(S,G,I) Expires
     The PrunePending Timer (PPT(S,G,I)) expires, indicating that no
     neighbors have overridden the previous Prune(S,G) message.  The
     Prune(S,G) Downstream state machine on interface I MUST
     transition to the Pruned (P) state.  The Prune Timer (PT(S,G,I))
     is started and MUST be initialized to the received
     Prune_Hold_Time minus J/P_Override_Interval.  A PruneEcho(S,G)
     MUST be sent on I if I has more than one PIM neighbor.  A
     PruneEcho(S,G) is simply a Prune(S,G) message multicast by the
     upstream router to a LAN, with itself as the Upstream Neighbor.
     Its purpose is to add additional reliability so that if a Join
     that should have overridden the Prune is lost locally on the LAN,
     the PruneEcho(S,G) may be received and trigger a new Join
     message.  A PruneEcho(S,G) is OPTIONAL on an interface with only
     one PIM neighbor.  In addition, the router MUST evaluate any
     possible transitions in the Upstream(S,G) state machine.

Adams, et al. Experimental [Page 24] RFC 3973 PIM - Dense Mode January 2005

   RPF_Interface(S) becomes interface I
     The upstream interface for S has changed.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     NoInfo (NI) state.  The PrunePending Timer (PPT(S,G,I)) MUST be
     cancelled.

4.4.2.3. Transitions from the Prune (P) State

 When the Prune(S,G) Downstream state machine is in the Pruned (P)
 state, the following events may trigger a transition.
   Receive Prune(S,G)
     A Prune(S,G) is received on the interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I remains in the Pruned (P)
     state.  The Prune Timer (PT(S,G,I)) SHOULD be reset to the
     holdtime contained in the Prune(S,G) message if it is greater
     than the current value.
   Receive Join(S,G)
     A Join(S,G) is received on the interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     downstream state machine on interface I MUST transition to the
     NoInfo (NI) state.  The Prune Timer (PT(S,G,I)) MUST be
     cancelled.  The router MUST evaluate any possible transitions in
     the Upstream(S,G) state machine.
   Receive Graft(S,G)
     A Graft(S,G) is received on interface I with the upstream
     neighbor field set to the router's address on I.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     NoInfo (NI) state and send a Graft Ack back to the Graft's
     source.  The Prune Timer (PT(S,G,I)) MUST be cancelled.  The
     router MUST evaluate any possible transitions in the
     Upstream(S,G) state machine.
   PT(S,G,I) Expires
     The Prune Timer (PT(S,G,I)) expires, indicating that it is again
     time to flood data from S addressed to group G onto interface I.
     The Prune(S,G) Downstream state machine on interface I MUST
     transition to the NoInfo (NI) state.  The router MUST evaluate
     any possible transitions in the Upstream(S,G) state machine.
   RPF_Interface(S) becomes interface I
     The upstream interface for S has changed.  The Prune(S,G)
     Downstream state machine on interface I MUST transition to the
     NoInfo (NI) state.  The PruneTimer (PT(S,G,I)) MUST be cancelled.

Adams, et al. Experimental [Page 25] RFC 3973 PIM - Dense Mode January 2005

   Send State Refresh(S,G) out interface I
     The router has refreshed the Prune(S,G) state on interface I.
     The router MUST reset the Prune Timer (PT(S,G,I)) to the Holdtime
     from an active Prune received on interface I.  The Holdtime used
     SHOULD be the largest active one but MAY be the most recently
     received active Prune Holdtime.

4.5. State Refresh

 This section describes the major portions of the state refresh
 mechanism.

4.5.1. Forwarding of State Refresh Messages

 When a State Refresh message, SRM, is received, it is forwarded
 according to the following pseudo-code.
 if (iif != RPF_interface(S))
   return;
 if (RPF'(S) != srcaddr(SRM))
   return;
 if (StateRefreshRateLimit(S,G) == TRUE)
   return;
 for each interface I in pim_nbrs {
   if (TTL(SRM) == 0 OR (TTL(SRM) - 1) < Threshold(I))
     continue;     /* Out of TTL, skip this interface */
   if (boundary(I,G))
     continue;     /* This interface is scope boundary, skip it */
   if (I == iif)
     continue;     /* This is the incoming interface, skip it */
   if (lost_assert(S,G,I) == TRUE)
     continue;     /* Let the Assert Winner do State Refresh */
   Copy SRM to SRM';   /* Make a copy of SRM to forward */
   if (I contained in prunes(S,G)) {
     set Prune Indicator bit of SRM' to 1;
     if StateRefreshCapable(I) == TRUE
       set PT(S,G) to largest active holdtime read from a Prune
       message accepted on I;

Adams, et al. Experimental [Page 26] RFC 3973 PIM - Dense Mode January 2005

   } else {
     set Prune Indicator bit of SRM' to 0;
   }
   set srcaddr(SRM') to my_addr(I);
   set TTL of SRM' to TTL(SRM) - 1;
   set metric of SRM' to metric of unicast route used to reach S;
   set pref of SRM' to preference of unicast route used to reach S;
   set mask of SRM' to mask of route used to reach S;
   if (AssertState == NoInfo) {
     set Assert Override of SRM' to 1;
   } else {
     set Assert Override of SRM' to 0;
   }
   transmit SRM' on I;
 }
 The pseudocode above employs the following macro definitions.
 Boundary(I,G) is TRUE if an administratively scoped boundary for
 group G is configured on interface I.
 StateRefreshCapable(I) is TRUE if all neighbors on an interface use
 the State Refresh option.
 StateRefreshRateLimit(S,G) is TRUE if the time elapsed since the last
 received StateRefresh(S,G) is less than the configured
 RefreshLimitInterval.
 TTL(SRM) returns the TTL contained in the State Refresh Message, SRM.
 This is different from the TTL contained in the IP header.
 Threshold(I) returns the minimum TTL that a packet must have before
 it can be transmitted on interface I.
 srcaddr(SRM) returns the source address contained in the network
 protocol (e.g., IPv4) header of the State Refresh Message, SRM.
 my_addr(I) returns this node's network (e.g., IPv4) address on
 interface I.

Adams, et al. Experimental [Page 27] RFC 3973 PIM - Dense Mode January 2005

4.5.2. State Refresh Message Origination

 This section describes the origination of State Refresh messages.
 These messages are generated periodically by the PIM-DM router
 directly connected to a source.  One Origination(S,G) state machine
 exists per (S,G) entry in a PIM-DM router.
 The Origination(S,G) state machine has the following states:
   NotOriginator(NO)
     This is the starting state of the Origination(S,G) state machine.
     While in this state, a router will not originate State Refresh
     messages for the (S,G) pair.
   Originator(O)
     When in this state the router will periodically originate State
     Refresh messages.  Only routers directly connected to S may
     transition to this state.
 In addition, there are two state machine specific timers:
   State Refresh Timer (SRT(S,G))
     This timer controls when State Refresh messages are generated.
     The timer is initially set when that Origination(S,G) state
     machine transitions to the O state.  It is cancelled when the
     Origination(S,G) state machine transitions to the NO state.  This
     timer is normally set to StateRefreshInterval (see 4.8).
   Source Active Timer (SAT(S,G))
     This timer is first set when the Origination(S,G) state machine
     transitions to the O state and is reset on the receipt of every
     data packet from S addressed to group G.  When it expires, the
     Origination(S,G) state machine transitions to the NO state.  This
     timer is normally set to SourceLifetime (see 4.8).
          +-------------+  Rcv Directly From S   +-------------+
          |             |----------------------->|             |
          |NotOriginator|                        | Originator  |
          |             |<-----------------------|             |
          +-------------+     SAT Expires OR     +-------------+
                           S NOT Direct Connect
                   Figure 3: State Refresh State Machine

Adams, et al. Experimental [Page 28] RFC 3973 PIM - Dense Mode January 2005

 In tabular form, the state machine is defined as follows:

+———————————————————————-+

Previous State
Event NotOriginator Originator

+———————————-+—————+——————-+

Receive Data from S AND →O →O Reset
S directly connected Set SRT(S,G) SAT(S,G)
Set SAT(S,G)

+———————————-+—————+——————-+

SRT(S,G) Expires N/A →O Send
StateRefresh(S,G)
Reset SRT(S,G)

+———————————-+—————+——————-+

SAT(S,G) Expires N/A →NO Cancel
SRT(S,G)

+———————————-+—————+——————-+

S no longer directly connected →NO →NO
Cancel SRT(S,G)
Cancel SAT(S,G)

+———————————-+—————+——————-+

4.5.2.1. Transitions from the NotOriginator (NO) State

 When the Originating(S,G) state machine is in the NotOriginator (NO)
 state, the following event may trigger a transition:
   Data Packet received from directly connected Source S addressed to
   group G
     The router MUST transition to an Originator (O) state, set
     SAT(S,G) to SourceLifetime, and set SRT(S,G) to
     StateRefreshInterval.  The router SHOULD record the TTL of the
     packet for use in State Refresh messages.

4.5.2.2. Transitions from the Originator (O) State

 When the Originating(S,G) state machine is in the Originator (O)
 state, the following events may trigger a transition:
   Receive Data Packet from S addressed to G
     The router remains in the Originator (O) state and MUST reset
     SAT(S,G) to SourceLifetime.  The router SHOULD increase its
     recorded TTL to match the TTL of the packet, if the packet's TTL
     is larger than the previously recorded TTL.  A router MAY record
     the TTL based on an implementation specific sampling policy to
     avoid examining the TTL of every multicast packet it handles.

Adams, et al. Experimental [Page 29] RFC 3973 PIM - Dense Mode January 2005

   SRT(S,G) Expires
     The router remains in the Originator (O) state and MUST reset
     SRT(S,G) to StateRefreshInterval.  The router MUST also generate
     State Refresh messages for transmission, as described in the
     State Refresh Forwarding rules (Section 4.5.1), except for the
     TTL.  If the TTL of data packets from S to G are being recorded,
     then the TTL of each State Refresh message is set to the highest
     recorded TTL.  Otherwise, the TTL is set to the configured State
     Refresh TTL.  Let I denote the interface over which a State
     Refresh message is being sent.  If the Prune(S,G) Downstream
     state machine is in the Pruned (P) state, then the Prune-
     Indicator bit MUST be set to 1 in the State Refresh message being
     sent over I. Otherwise, the Prune-Indicator bit MUST be set to 0.
   SAT(S,G) Expires
     The router MUST cancel the SRT(S,G) timer and transition to the
     NotOriginator (NO) state.
   S is no longer directly connected
     The router MUST transition to the NotOriginator (NO) state and
     cancel both the SAT(S,G) and SRT(S,G).

4.6. PIM Assert Messages

4.6.1. Assert Metrics

 Assert metrics are defined as follows:
 struct assert_metric {
   metric_preference;
   route_metric;
   ip_address;
 };
 When assert_metrics are compared, the metric_preference and
 route_metric field are compared in order, where the first lower value
 wins.  If all fields are equal, the IP address of the router that
 sourced the Assert message is used as a tie-breaker, with the highest
 IP address winning.

Adams, et al. Experimental [Page 30] RFC 3973 PIM - Dense Mode January 2005

 An Assert metric for (S,G) to include in (or compare against) an
 Assert message sent on interface I should be computed by using the
 following pseudocode:
 assert_metric
 my_assert_metric(S,G,I) {
   if (CouldAssert(S,G,I) == TRUE) {
     return spt_assert_metric(S,G,I)
   } else {
     return infinite_assert_metric()
   }
 }
 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_addr(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_addr(I) is simply the
 router's network (e.g., IP) address associated with the local
 interface I.
 infinite_assert_metric() gives the Assert metric we need to send an
 Assert but doesn't match (S,G) forwarding state:
 assert_metric
 infinite_assert_metric() {
   return {1,infinity,infinity,0}
 }

4.6.2. AssertCancel Messages

 An AssertCancel(S,G) message is simply an Assert message for (S,G)
 with infinite metric.  The Assert winner sends this message when it
 changes its upstream interface to this interface.  Other routers will
 see this metric, causing those with forwarding state to send their
 own Asserts and re-establish an Assert winner.
 AssertCancel messages are simply an optimization.  The original
 Assert timeout mechanism will eventually allow a subnet to become
 consistent; the AssertCancel mechanism simply causes faster
 convergence.  No special processing is required for an AssertCancel
 message, as it is simply an Assert message from the current winner.

Adams, et al. Experimental [Page 31] RFC 3973 PIM - Dense Mode January 2005

4.6.3. Assert State Macros

 The macro lost_assert(S,G,I), is used in the olist computations of
 Section 4.1.3, and is defined as follows:
 bool lost_assert(S,G,I) {
   if ( RPF_interface(S) == I ) {
     return FALSE
   } else {
     return (AssertWinner(S,G,I) != me  AND
             (AssertWinnerMetric(S,G,I) is better than
              spt_assert_metric(S,G,I)))
   }
 }
 AssertWinner(S,G,I) defaults to NULL, and AssertWinnerMetric(S,G,I)
 defaults to Infinity when in the NoInfo state.

4.6.4. (S,G) Assert Message State Machine

 The (S,G) Assert state machine for interface I is shown in Figure 4.
 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 via
     interface I.
   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.
 In addition, an Assert Timer (AT(S,G,I)) is used to time out the
 Assert state.

Adams, et al. Experimental [Page 32] RFC 3973 PIM - Dense Mode January 2005

       +-------------+                        +-------------+
       |             | Rcv Pref Assert or SR  |             |
       |   Winner    |----------------------->|    Loser    |
       |             |                        |             |
       +-------------+                        +-------------+
            ^   |                                  ^   |
            |   |                Rcv Pref Assert or|   |
            |   |AT Expires OR        State Refresh|   |
            |   |CouldAssert->FALSE                |   |
            |   |                                  |   |
            |   |         +-------------+          |   |
            |   +-------->|             |----------+   |
            |             |   No Info   |              |
            +-------------|             |<-------------+
     Rcv Data from dnstrm +-------------+ Rcv Inf Assert from Win OR
   OR Rcv Inferior Assert                 Rcv Inf SR from Winner OR
       OR Rcv Inferior SR                 AT Expires OR
                                          CouldAssert Changes OR
                                          Winner's NLT Expires
                   Figure 4: Assert State Machine
 In tabular form, the state machine is defined as follows:

+——————————-+————————————–+

Previous State
Event No Info Winner Loser

+——————————-+————+————+————+

An (S,G) Data packet received →W Send →W Send →L
on downstream interface Assert(S,G) Assert(S,G)
Set Set
AT(S,G,I) AT(S,G,I)

+——————————-+————————————–+

Receive Inferior (Assert OR N/A N/A →NI Cancel
State Refresh) from Assert AT(S,G,I)
Winner

+——————————-+————————————–+

Receive Inferior (Assert OR →W Send →W Send →L
State Refresh) from non-Assert Assert(S,G) Assert(S,G)
Winner AND CouldAssert==TRUE Set Set
AT(S,G,I) AT(S,G,I)

+——————————-+————————————–+

Adams, et al. Experimental [Page 33] RFC 3973 PIM - Dense Mode January 2005

+——————————-+————————————–+

Previous State
Event No Info Winner Loser

+——————————-+————+————+————+

Receive Preferred Assert OR →L Send →L Send →L Set
State Refresh Prune(S,G) Prune(S,G) AT(S,G,I)
Set Set
AT(S,G,I) AT(S,G,I)

+——————————-+————————————–+

Send State Refresh →NI →W Reset N/A
AT(S,G,I)

+——————————-+————————————–+

AT(S,G) Expires N/A →NI →NI

+——————————-+————————————–+

CouldAssert → FALSE →NI →NI Cancel →NI Cancel
AT(S,G,I) AT(S,G,I)

+——————————-+————————————–+

CouldAssert → TRUE →NI N/A →NI Cancel
AT(S,G,I)

+——————————-+————————————–+

Winner's NLT(N,I) Expires N/A N/A →NI Cancel
AT(S,G,I)

+——————————-+————————————–+

Receive Prune(S,G), Join(S,G) →NI →W →L Send
or Graft(S,G) Assert(S,G)

+——————————-+————————————–+

 Terminology: A "preferred assert" is one with a better metric than
 the current winner.  An "inferior assert" is one with a worse metric
 than my_assert_metric(S,G,I).
 The state machine uses the following macro:
 CouldAssert(S,G,I) = (RPF_interface(S) != I)

4.6.4.1. Transitions from NoInfo State

 In the NoInfo state, the following events may trigger transitions:
   An (S,G) data packet arrives on downstream interface I
     An (S,G) data packet arrived on a downstream interface.  It is
     optimistically assumed that this router will be the Assert winner
     for this (S,G).  The Assert state machine MUST transition to the
     "I am Assert Winner" state, send an Assert(S,G) to interface I,
     store its own address and metric as the Assert Winner, and set
     the Assert_Timer (AT(S,G,I) to Assert_Time, thereby initiating
     the Assert negotiation for (S,G).

Adams, et al. Experimental [Page 34] RFC 3973 PIM - Dense Mode January 2005

   Receive Inferior (Assert OR State Refresh) AND
   CouldAssert(S,G,I)==TRUE
     An Assert or State Refresh is received for (S,G) that is inferior
     to our own assert metric on interface I. The Assert state machine
     MUST transition to the "I am Assert Winner" state, send an
     Assert(S,G) to interface I, store its own address and metric as
     the Assert Winner, and set the Assert Timer (AT(S,G,I)) to
     Assert_Time.
   Receive Preferred Assert or State Refresh
     The received Assert or State Refresh has a better metric than
     this router's, and therefore the Assert state machine MUST
     transition to the "I am Assert Loser" state and store the Assert
     Winner's address and metric.  If the metric was received in an
     Assert, the router MUST set the Assert Timer (AT(S,G,I)) to
     Assert_Time.  If the metric was received in a State Refresh, the
     router MUST set the Assert Timer (AT(S,G,I)) to three times the
     received State Refresh Interval.  If CouldAssert(S,G,I) == TRUE,
     the router MUST also multicast a Prune(S,G) to the Assert winner
     with a Prune Hold Time equal to the Assert Timer and evaluate any
     changes in its Upstream(S,G) state machine.

4.6.4.2. Transitions from Winner State

 When in "I am Assert Winner" state, the following events trigger
 transitions:
   An (S,G) data packet arrives on downstream interface I
     An (S,G) data packet arrived on a downstream interface.  The
     Assert state machine remains in the "I am Assert Winner" state.
     The router MUST send an Assert(S,G) to interface I and set the
     Assert Timer (AT(S,G,I) to Assert_Time.
   Receive Inferior Assert or State Refresh
     An (S,G) Assert is received containing a metric for S that is
     worse than this router's metric for S.  Whoever sent the Assert
     is in error.  The router MUST send an Assert(S,G) to interface I
     and reset the Assert Timer (AT(S,G,I)) to Assert_Time.
   Receive Preferred Assert or State Refresh
     An (S,G) Assert or State Refresh is received that has a better
     metric than this router's metric for S on interface I.  The
     Assert state machine MUST transition to "I am Assert Loser" state
     and store the new Assert Winner's address and metric.  If the
     metric was received in an Assert, the router MUST set the Assert
     Timer (AT(S,G,I)) to Assert_Time.  If the metric was received in
     a State Refresh, the router MUST set the Assert Timer (AT(S,G,I))
     to three times the State Refresh Interval.  The router MUST also

Adams, et al. Experimental [Page 35] RFC 3973 PIM - Dense Mode January 2005

     multicast a Prune(S,G) to the Assert winner, with a Prune Hold
     Time equal to the Assert Timer, and evaluate any changes in its
     Upstream(S,G) state machine.
   Send State Refresh
     The router is sending a State Refresh(S,G) message on interface
     I.  The router MUST set the Assert Timer (AT(S,G,I)) to three
     times the State Refresh Interval contained in the State
     Refresh(S,G) message.
   AT(S,G,I) Expires
     The (S,G) Assert Timer (AT(S,G,I)) expires.  The Assert state
     machine MUST transition to the NoInfo (NI) state.
   CouldAssert(S,G,I) -> FALSE
     This router's RPF interface changed, making CouldAssert(S,G,I)
     false.  This router can no longer perform the actions of the
     Assert winner, so the Assert state machine MUST transition to
     NoInfo (NI) state, send an AssertCancel(S,G) to interface I,
     cancel the Assert Timer (AT(S,G,I)), and remove itself as the
     Assert Winner.

4.6.4.3. Transitions from Loser State

 When in "I am Assert Loser" state, the following transitions can
 occur:
   Receive Inferior Assert or State Refresh from Current Winner
     An Assert or State Refresh is received from the current Assert
     winner that is worse than this router's metric for S (typically,
     the winner's metric became worse).  The Assert state machine MUST
     transition to NoInfo (NI) state and cancel AT(S,G,I).  The router
     MUST delete the previous Assert Winner's address and metric and
     evaluate any possible transitions to its Upstream(S,G) state
     machine.  Usually this router will eventually re-assert and win
     when data packets from S have started flowing again.
   Receive Preferred Assert or State Refresh
     An Assert or State Refresh is received that has a metric better
     than or equal to that of the current Assert winner.  The Assert
     state machine remains in Loser (L) state.  If the metric was
     received in an Assert, the router MUST set the Assert Timer
     (AT(S,G,I)) to Assert_Time.  If the metric was received in a
     State Refresh, the router MUST set the Assert Timer (AT(S,G,I))
     to three times the received State Refresh Interval.  If the
     metric is better than the current Assert Winner, the router MUST

Adams, et al. Experimental [Page 36] RFC 3973 PIM - Dense Mode January 2005

     store the address and metric of the new Assert Winner, and if
     CouldAssert(S,G,I) == TRUE, the router MUST multicast a
     Prune(S,G) to the new Assert winner.
   AT(S,G,I) Expires
     The (S,G) Assert Timer (AT(S,G,I)) expires.  The Assert state
     machine MUST transition to NoInfo (NI) state.  The router MUST
     delete the Assert Winner's address and metric.  If CouldAssert ==
     TRUE, the router MUST evaluate any possible transitions to its
     Upstream(S,G) state machine.
   CouldAssert -> FALSE
     CouldAssert has become FALSE because interface I has become the
     RPF interface for S.  The Assert state machine MUST transition to
     NoInfo (NI) state, cancel AT(S,G,I), and delete information
     concerning the Assert Winner on I.
   CouldAssert -> TRUE
     CouldAssert has become TRUE because interface I used to be the
     RPF interface for S, and now it is not.  The Assert state machine
     MUST transition to NoInfo (NI) state, cancel AT(S,G,I), and
     delete information concerning the Assert Winner on I.
   Current Assert Winner's NeighborLiveness Timer Expires
     The current Assert winner's NeighborLiveness Timer (NLT(N,I)) has
     expired.  The Assert state machine MUST transition to the NoInfo
     (NI) state, delete the Assert Winner's address and metric, and
     evaluate any possible transitions to its Upstream(S,G) state
     machine.
   Receive Prune(S,G), Join(S,G), or Graft(S,G)
     A Prune(S,G), Join(S,G), or Graft(S,G) message was received on
     interface I with its upstream neighbor address set to the
     router's address on I.  The router MUST send an Assert(S,G) on
     the receiving interface I to initiate an Assert negotiation.  The
     Assert state machine remains in the Assert Loser(L) state.  If a
     Graft(S,G) was received, the router MUST respond with a
     GraftAck(S,G).

Adams, et al. Experimental [Page 37] RFC 3973 PIM - Dense Mode January 2005

4.6.5. Rationale for Assert Rules

 The following is a summary of the rules for generating and processing
 Assert messages.  It is not intended to be definitive (the state
 machines and pseudocode provide the definitive behavior).  Instead,
 it provides some rationale for the behavior.
 1. The Assert winner for (S,G) must act as the local forwarder for
    (S,G) on behalf of all downstream members.
 2. PIM messages are directed to the RPF' neighbor and not to the
    regular RPF neighbor.
 3. An Assert loser that receives a Prune(S,G), Join(S,G), or
    Graft(S,G) directed to it initiates a new Assert negotiation so
    that the downstream router can correct its RPF'(S).
 4. An Assert winner for (S,G) sends a cancelling assert when it is
    about to stop forwarding on an (S,G) entry.  Example: If a router
    is being taken down, then a canceling assert is sent.

4.7. PIM Packet Formats

 All PIM-DM packets use the same format as PIM-SM packets.  In the
 event of a discrepancy, PIM-SM [4] should be considered the
 definitive specification.  All PIM control messages have IP protocol
 number 103.  All PIM-DM messages MUST be sent with a TTL of 1.  All
 PIM-DM messages except Graft and Graft Ack messages MUST be sent to
 the ALL-PIM-ROUTERS group.  Graft messages SHOULD be unicast to the
 RPF'(S).  Graft Ack messages MUST be unicast to the sender of the
 Graft.
 The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13.  The IPv6 ALL-PIM-
 ROUTERS group is 'ff02::d'.

4.7.1. PIM Header

 All PIM control messages have the following header:
  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.

Adams, et al. Experimental [Page 38] RFC 3973 PIM - Dense Mode January 2005

 Type
   Types for specific PIM messages.  Available types are as follows:
   0 = Hello
   1 = Register (PIM-SM only)
   2 = Register Stop (PIM-SM only)
   3 = Join/Prune
   4 = Bootstrap (PIM-SM only)
   5 = Assert
   6 = Graft
   7 = Graft Ack
   8 = Candidate RP Advertisement (PIM-SM only)
   9 = State Refresh
 Reserved
   Set to zero on transmission.  Ignored upon receipt.
 Checksum
   The checksum is the standard IP checksum; i.e., the 16 bit one's
   complement of the one's complement sum of the entire PIM message.
   For computing checksum, the checksum field is zeroed.
   For IPv6, the checksum also includes the IPv6 "pseudo-header", as
   specified in RFC 2460, Section 8.1 [5].

4.7.2. Encoded Unicast Address

 An Encoded Unicast Address has 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 [9].  Values 128 - 250 are reserved to be
   assigned by the IANA for PIM specific Address Families.  Values 251
   - 255 are designated for private use.  As there is no assignment
   authority for this space; collisions should be expected.
 Encoding Type
   The type of encoding used with a specific Address Family.  The
   value '0' is reserved for this field and represents the native
   encoding of the Address Family.

Adams, et al. Experimental [Page 39] RFC 3973 PIM - Dense Mode January 2005

 Unicast Address
   The unicast address as represented by the given Address Family and
   Encoding Type.

4.7.3. Encoded Group Address

 An Encoded Group address has 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
   As described above.
 Encoding Type
   As described above.
 B
   Indicates that the group range should use Bidirectional PIM [16].
   Transmitted as zero; ignored upon receipt.
 Reserved
   Transmitted as zero.  Ignored upon receipt.
 Z
   Indicates that the group range is an admin scope zone.  This is
   used in the Bootstrap Router Mechanism [18] only.  For all other
   purposes, this bit is set to zero and ignored on receipt.
 Mask Len
   The mask length field is 8 bits.  The value is the number of
   contiguous left justified one bits used as a mask, which, combined
   with the address, describes a range of addresses.  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 address
   then the mask length MUST equal the address length.  PIM-DM routers
   MUST only send for a single address.
 Group Multicast Address
   The address of the multicast group.

Adams, et al. Experimental [Page 40] RFC 3973 PIM - Dense Mode January 2005

4.7.4. Encoded Source Address

 An Encoded Source address has 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
   As described above.
 Encoding Type
   As described above.
 Rsrvd
   Reserved.  Transmitted as zero.  Ignored upon receipt.
 S
   The Sparse Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.
 W
   The Wild Card Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.
 R
   The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
   receipt.
 Mask Len
   As described above.  PIM-DM routers MUST only send for a single
   source address.
 Source Address
   The source address.

Adams, et al. Experimental [Page 41] RFC 3973 PIM - Dense Mode January 2005

4.7.5. Hello Message Format

 The PIM Hello message, as defined by PIM-SM [4], has 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |PIM Ver| Type  |   Reserved    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Option Type          |         Option Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Option Value                          |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               .                               |
 |                               .                               |
 |                               .                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Option Type          |         Option Length         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         Option Value                          |
 |                              ...                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 PIM Ver, Type, Reserved, Checksum
   Described above.
 Option Type
   The type of option given in the Option Value field.  Available
   types are as follows:
     0              Reserved
     1              Hello Hold Time
     2              LAN Prune Delay
     3 - 16         Reserved
     17             To be assigned by IANA
     18             Deprecated and SHOULD NOT be used
     19             DR Priority (PIM-SM Only)
     20             Generation ID
     21             State Refresh Capable
     22             Bidir Capable
     23 - 65000     To be assigned by IANA
     65001 - 65535  Reserved for Private Use [9]
   Unknown options SHOULD be ignored.

Adams, et al. Experimental [Page 42] RFC 3973 PIM - Dense Mode January 2005

4.7.5.1. Hello Hold Time Option

  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          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            Hold Time          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Hold Time is the number of seconds a receiver MUST keep the neighbor
 reachable.  If the Hold Time 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.  Furthermore, if the Holdtime is set to '0', the
 information is timed out immediately.  The Hello Hold Time option
 MUST be used by PIM-DM routers.

4.7.5.2. LAN Prune Delay Option

  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|       LAN Prune Delay       |       Override Interval       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The LAN_Prune_Delay option is used to tune the prune propagation
 delay on multi-access LANs.  The T bit is used by PIM-SM and SHOULD
 be set to 0 by PIM-DM routers and ignored upon receipt.  The LAN
 Delay and Override Interval fields are time intervals in units of
 milliseconds and are used to tune the value of the J/P Override
 Interval and its derived timer values.  Section 4.3.5 describes how
 these values affect the behavior of a router.  The LAN Prune Delay
 SHOULD be used by PIM-DM routers.

Adams, et al. Experimental [Page 43] RFC 3973 PIM - Dense Mode January 2005

4.7.5.3. Generation ID Option

  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 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.  The Generation
 ID option MAY be used by PIM-DM routers.

4.7.5.4. State Refresh Capable Option

  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 = 21           |           Length = 4          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Version = 1  |   Interval    |            Reserved           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Interval field is the router's configured State Refresh Interval
 in seconds.  The Reserved field is set to zero and ignored upon
 receipt.  The State Refresh Capable option MUST be used by State
 Refresh capable PIM-DM routers.

Adams, et al. Experimental [Page 44] RFC 3973 PIM - Dense Mode January 2005

4.7.6. Join/Prune Message Format

 PIM Join/Prune messages, as defined in PIM-SM [4], have 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |PIM Ver| Type  |   Reserved    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Upstream Neighbor Address (Encoded Unicast Format)     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Reserved    |  Num Groups   |          Hold Time            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         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)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               .                               |
 |                               .                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Adams, et al. Experimental [Page 45] RFC 3973 PIM - Dense Mode January 2005

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Joined Source Address n (Encoded Source Format)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Pruned Source Address 1 (Encoded Source Format)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               .                               |
 |                               .                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Pruned Source Address n (Encoded Source Format)       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 PIM Ver, Type, Reserved, Checksum
   Described above.
 Upstream Neighbor Address
   The address of the upstream neighbor.  The format for this address
   is given in the Encoded Unicast address in Section 4.7.2.  PIM-DM
   routers MUST set this field to the RPF next hop.
 Reserved
   Transmitted as zero.  Ignored upon receipt.
 Hold Time
   The number of seconds a receiving PIM-DM router MUST keep a Prune
   state alive, unless removed by a Join or Graft message.  If the
   Hold Time is '0xffff', the receiver MUST NOT remove the Prune state
   unless a corresponding Join or Graft message is received.  The Hold
   Time is ignored in Join messages.
 Number of Groups
   Number of multicast group sets contained in the message.
 Multicast Group Address
   The multicast group address in the Encoded Multicast address format
   given in Section 4.7.3.
 Number of Joined Sources
   Number of Join source addresses listed for a given group.
 Number of Pruned Sources
   Number of Prune source addresses listed for a given group.

Adams, et al. Experimental [Page 46] RFC 3973 PIM - Dense Mode January 2005

 Join Source Address 1..n
   This list contains the sources from which the sending router wishes
   to continue to receive multicast messages for the given group on
   this interface.  The addresses use the Encoded Source address
   format given in Section 4.7.4.
 Prune Source Address 1..n
   This list contains the sources from which the sending router does
   not wish to receive multicast messages for the given group on this
   interface.  The addresses use the Encoded Source address format
   given in Section 4.7.4.

4.7.7. Assert Message Format

 PIM Assert Messages, as defined in PIM-SM [4], have 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |PIM Ver| Type  |   Reserved    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Multicast Group Address (Encoded Group Format)        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Source Address (Encoded Unicast Format)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |R|                     Metric Preference                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             Metric                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 PIM Ver, Type, Reserved, Checksum
   Described above.
 Multicast Group Address
   The multicast group address in the Encoded Multicast address format
   given in Section 4.7.3.
 Source Address
   The source address in the Encoded Unicast address format given in
   Section 4.7.2.
 R
   The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
   receipt.

Adams, et al. Experimental [Page 47] RFC 3973 PIM - Dense Mode January 2005

 Metric Preference
   The preference value assigned to the unicast routing protocol that
   provided the route to the source.
 Metric
   The cost metric of the unicast route to the source.  The metric is
   in units applicable to the unicast routing protocol used.

4.7.8. Graft Message Format

 PIM Graft messages use the same format as Join/Prune messages, except
 that the Type field is set to 6.  The source address MUST be in the
 Join section of the message.  The Hold Time field SHOULD be zero and
 SHOULD be ignored when a Graft is received.

4.7.9. Graft Ack Message Format

 PIM Graft Ack messages are identical in format to the received Graft
 message, except that the Type field is set to 7.  The Upstream
 Neighbor Address field SHOULD be set to the sender of the Graft
 message and SHOULD be ignored upon receipt.

4.7.10. State Refresh Message Format

 PIM State Refresh Messages have 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |PIM Ver| Type  |   Reserved    |           Checksum            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Multicast Group Address (Encoded Group Format)        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Source Address (Encoded Unicast Format)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Originator Address (Encoded Unicast Format)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |R|                     Metric Preference                       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             Metric                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Masklen    |    TTL        |P|N|O|Reserved |   Interval    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 PIM Ver, Type, Reserved, Checksum
   Described above.

Adams, et al. Experimental [Page 48] RFC 3973 PIM - Dense Mode January 2005

 Multicast Group Address
   The multicast group address in the Encoded Multicast address format
   given in Section 4.7.3.
 Source Address
   The address of the data source in the Encoded Unicast address
   format given in Section 4.7.2.
 Originator Address
   The address of the first hop router in the Encoded Unicast address
   format given in Section 4.7.2.
 R
   The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
   receipt.
 Metric Preference
   The preference value assigned to the unicast routing protocol that
   provided the route to the source.
 Metric
   The cost metric of the unicast route to the source.  The metric is
   in units applicable to the unicast routing protocol used.
 Masklen
   The length of the address mask of the unicast route to the source.
 TTL
   Time To Live of the State Refresh message.  Decremented each time
   the message is forwarded.  Note that this is different from the IP
   Header TTL, which is always set to 1.
 P
   Prune indicator flag.  This MUST be set to 1 if the State Refresh
   is to be sent on a Pruned interface.  Otherwise, it MUST be set to
   0.
 N
   Prune Now flag.  This SHOULD be set to 1 by the State Refresh
   originator on every third State Refresh message and SHOULD be
   ignored upon receipt.  This is for compatibility with earlier
   versions of state refresh.
 O
   Assert Override flag.  This SHOULD be set to 1 by upstream routers
   on a LAN if the Assert Timer (AT(S,G)) is not running and SHOULD be
   ignored upon receipt.  This is for compatibility with earlier
   versions of state refresh.

Adams, et al. Experimental [Page 49] RFC 3973 PIM - Dense Mode January 2005

 Reserved
   Set to zero and ignored upon receipt.
 Interval
   Set by the originating router to the interval (in seconds) between
   consecutive State Refresh messages for this (S,G) pair.

4.8. PIM-DM Timers

 PIM-DM maintains the following timers.  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 downward
 towards zero.
 Global Timers
   Hello Timer: HT
   Per interface (I):
     Per neighbor (N):
       Neighbor Liveness Timer: NLT(N,I)
     Per (S,G) Pair:
       (S,G) Assert Timer: AT(S,G,I)
       (S,G) Prune Timer: PT(S,G,I)
       (S,G) PrunePending Timer: PPT(S,G,I)
     Per (S,G) Pair:
       (S,G) Graft Retry Timer: GRT(S,G)
       (S,G) Upstream Override Timer: OT(S,G)
       (S,G) Prune Limit Timer: PLT(S,G)
       (S,G) Source Active Timer: SAT(S,G)
       (S,G) State Refresh Timer: SRT(S,G)

Adams, et al. Experimental [Page 50] RFC 3973 PIM - Dense Mode January 2005

 When timer values are started or restarted, they are set to default
 values.  The following tables summarize those default values.

Timer Name: Hello Timer (HT) +———————-+——–+————————————–+

Value Name Value Explanation

+———————-+——–+————————————–+

Hello_Period 30 sec Periodic interval for hello messages

+———————-+——–+————————————–+

Triggered_Hello_Delay 5 sec Random interval for initial Hello
message on bootup or triggered Hello
message to a rebooting neighbor

+———————-+——–+————————————–+

 Hello messages are sent on every active interface once every
 Hello_Period seconds.  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

+——————-+—————–+——————————–+

Hello Holdtime From message Hold Time from Hello Message

+——————-+—————–+——————————–+

Timer Name: PrunePending Timer (PPT(S,G,I)) +———————–+—————+——————————+

Value Name Value Explanation

+———————–+—————+——————————+

J/P_Override_Interval OI(I) + PD(I) Short time after a Prune to
allow other routers on the
LAN to send a Join

+———————–+—————+——————————+

 The J/P_Override_Interval is the sum of the interface's
 Override_Interval (OI(I)) and Propagation_Delay (PD(I)).  If all
 routers on a LAN are using the LAN Prune Delay option, both
 parameters MUST be set to the largest value on the LAN.  Otherwise,
 the Override_Interval (OI(I)) MUST be set to 2.5 seconds, and the
 Propagation_Delay (PD(I)) MUST be set to 0.5 seconds.

Adams, et al. Experimental [Page 51] RFC 3973 PIM - Dense Mode January 2005

Timer Name: Prune Timer (PT(S,G,I)) +—————-+—————-+————————————+

Value Name Value Explanation

+—————-+—————-+————————————+

Prune Holdtime From message Hold Time read from Prune Message

+—————-+—————-+————————————+

Timer Name: Assert Timer (AT(S,G,I)) +————————–+———+———————————+

Value Name Value Explanation

+————————–+———+———————————+

Assert Time 180 sec 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.  If all members of a LAN are state refresh
 enabled, the Assert Time will be three times the received
 RefreshInterval(S,G).

Timer Name: Graft Retry Timer (GRT(S,G)) +——————–+——-+—————————————–+

Value Name Value Explanation

+——————–+——-+—————————————–+

Graft_Retry_Period 3 sec In the absence of receipt of a GraftAck
message, the time before retransmission
of a Graft message

+——————–+——-+—————————————–+

Timer Name: Upstream Override Timer (OT(S,G)) +————+—————-+—————————————-+

Value Name Value Explanation

+————+—————-+—————————————-|

t_override rand(0, OI(I)) Randomized delay to prevent response
implosion when sending a join message
to override someone else's prune

+————+—————-+—————————————-+

 t_override is a random value between 0 and the interface's
 Override_Interval (OI(I)).  If all routers on a LAN are using the LAN
 Prune Delay option, the Override_Interval (OI(I)) MUST be set to the
 largest value on the LAN.  Otherwise, the Override_Interval (OI(I))
 MUST be set to 2.5 seconds.

Adams, et al. Experimental [Page 52] RFC 3973 PIM - Dense Mode January 2005

Timer Name: Prune Limit Timer (PLT(S,G)) +————+——————–+————————————+

Value Name Value Explanation

+————+——————–+————————————|

t_limit Default: 210 secs Used to prevent Prune storms on a
LAN

+————+——————–+————————————+

Timer Name: Source Active Timer (SAT(S,G)) +—————-+——————-+———————————+

Value Name Value Explanation

+—————-+——————-+———————————+

SourceLifetime Default: 210 secs Period of time after receiving
a multicast message a directly
attached router will continue
to send State Refresh messages

+—————-+——————-+———————————+

Timer Name: State Refresh Timer (SRT(S,G)) +—————–+——————+———————————+

Value Name Value Explanation

+—————–+——————+———————————+

RefreshInterval Default: 60 secs Interval between successive
state refresh messages

+—————–+——————+———————————+

5. Protocol Interaction Considerations

 PIM-DM is designed to be independent of underlying unicast routing
 protocols and will interact only to the extent needed to perform RPF
 checks.  It is generally assumed that multicast area and autonomous
 system boundaries will correspond to the same boundaries for unicast
 routing, though a deployment that does not follow this assumption is
 not precluded by this specification.
 In general, PIM-DM interactions with other multicast routing
 protocols should be in compliance with RFC 2715 [7].  Other specific
 interactions are noted below.

5.1. PIM-SM Interactions

 PIM-DM is not intended to interact directly with PIM-SM, even though
 they share a common packet format.  It is particularly important to
 note that a router cannot differentiate between a PIM-DM neighbor and
 a PIM-SM neighbor based on Hello messages.

Adams, et al. Experimental [Page 53] RFC 3973 PIM - Dense Mode January 2005

 In the event that a PIM-DM router becomes a neighbor of a PIM-SM
 router, the two will effectively form a simplex link, with the PIM-DM
 router sending all multicast messages to the PIM-SM router while the
 PIM-SM router sends no multicast messages to the PIM-DM router.
 The common packet format permits a hybrid PIM-SM/DM implementation
 that would use PIM-SM when a rendezvous point is known and PIM-DM
 when one is not.  Such an implementation is outside the scope of this
 document.

5.2. IGMP Interactions

 PIM-DM will forward received multicast data packets to neighboring
 host group members in all cases except when the PIM-DM router is in
 an Assert Loser state on that interface.  Note that a PIM Prune
 message is not permitted to prevent the delivery of messages to a
 network with group members.
 A PIM-DM Router MAY use the DR Priority option described in PIM-SM
 [14] to elect an IGMP v1 querier.

5.3. Source Specific Multicast (SSM) Interactions

 PIM-DM makes no special considerations for SSM [15].  All Prunes and
 Grafts within the protocol are for a specific source, so no
 additional checks have to be made.

5.4. Multicast Group Scope Boundary Interactions

 Although multicast group scope boundaries are generally identical to
 routing area boundaries, it is conceivable that a routing area might
 be partitioned for a particular multicast group.  PIM-DM routers MUST
 NOT send any messages concerning a particular group across that
 group's scope boundary.

6. IANA Considerations

6.1. 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.  When the
 PIM packet format was designed, only 15 values were assigned for
 Address Families, and large numbers of new Address Families were not
 envisioned; 8 bits seemed large enough.  However, the IANA assigns
 Address Families in a 16 bit value.  Therefore, the PIM Address
 Family is allocated as follows:

Adams, et al. Experimental [Page 54] RFC 3973 PIM - Dense Mode January 2005

 Values 0 - 127 are designated to have the same meaning as IANA
 assigned Address Family Numbers [9].
 Values 128 - 250 are designated to be assigned by the IANA based on
 IESG approval, as defined in [8].
 Values 251 - 255 are designated for Private Use, as defined in [8].

6.2. PIM Hello Options

 Values 17 - 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
 [8].  Assignments are valid for one year and may be renewed.
 Permanent assignments require a specification, as defined in [8].

7. Security Considerations

 The IPsec authentication header [10] MAY be used to provide data
 integrity protection and groupwise data origin authentication of PIM
 protocol messages.  Authentication of PIM messages can protect
 against unwanted behaviors caused by unauthorized or altered PIM
 messages.  In any case, a PIM router SHOULD NOT accept and process
 PIM messages from neighbors unless a valid Hello message has been
 received from that neighbor.
 Note that PIM-DM has no rendezvous point, and therefore no single
 point of failure that may be vulnerable.  Because PIM-DM uses unicast
 routes provided by an unknown routing protocol, it may suffer
 collateral effects if the unicast routing protocol is attacked.

7.1. 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. A forged PIM-DM message is link local and 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 Hello message can cause multicast traffic to be delivered
    to links where there are no legitimate requestors, potentially
    wasting bandwidth on that link.  On a multi-access LAN, the
    effects are limited without the capability to forge a Join
    message, as other routers will Prune the link if the traffic is
    not desired.
 2. A forged Join/Prune message can cause multicast traffic to be
    delivered to links where there are no legitimate requestors,
    potentially wasting bandwidth on that link.  A forged Prune

Adams, et al. Experimental [Page 55] RFC 3973 PIM - Dense Mode January 2005

    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 Prune with a Join before the upstream router
    stops forwarding data to the LAN.
 3. A forged Graft message can cause multicast traffic to be delivered
    to links where there are no legitimate requestors, potentially
    wasting bandwidth on that link.  In principle, Graft messages
    could be sent multiple hops because they are unicast to the
    upstream router.  This should not be a problem, as the remote
    forger should have no way to get a Hello message to the target of
    the attack.  Without a valid Hello message, the receiving router
    SHOULD NOT accept the Graft.
 4. A forged GraftAck message has no impact, as it will be ignored
    unless the router has recently sent a Graft to its upstream
    router.
 5. By forging an Assert message on a multi-access LAN, an attacker
    could cause the legitimate forwarder to stop forwarding traffic to
    the LAN.  Such a forgery would prevent any hosts downstream of
    that LAN from receiving traffic.
 6. A forged State Refresh message on a multi-access LAN would have
    the same impact as a forged Assert message, having the same
    general functions.  In addition, forged State Refresh messages
    would be propagated downstream and might be used in a denial of
    service attack.  Therefore, a PIM-DM router SHOULD rate limit
    State Refresh messages propagated.

7.2. Non-cryptographic Authentication Mechanisms

 A PIM-DM router SHOULD provide an option to limit the set of
 neighbors from which it will accept PIM-DM messages.  Either static
 configuration of IP addresses or an IPSec security association may be
 used.  All options that restrict the range of addresses from which
 packets are accepted MUST default to allowing all packets.
 Furthermore, a PIM router SHOULD NOT accept protocol messages from a
 router from which it has not yet received a valid Hello message.

7.3. Authentication Using IPsec

 The IPSec [10] transport mode using the Authentication Header (AH) is
 the recommended method to prevent the above attacks in PIM.  The
 specific AH authentication algorithm and parameters, including the
 choice of authentication algorithm and the choice of key, are
 configured by the network administrator.  The Encapsulating Security

Adams, et al. Experimental [Page 56] RFC 3973 PIM - Dense Mode January 2005

 Payload (ESP) MAY also be used to provide both encryption and
 authentication of PIM protocol messages.  When IPsec authentication
 is used, a PIM router SHOULD reject (drop without processing) any
 unauthorized PIM protocol messages.
 To use IPSec, the administrator of a PIM network configures each PIM
 router with one or more Security Associations and associated Security
 Parameters Indices that are used by senders to authenticate PIM
 protocol messages and are used by receivers to authenticate received
 PIM protocol messages.  This document does not describe protocols for
 establishing Security Associations.  It assumes that manual
 configuration of Security Associations is performed, but it does not
 preclude the use of some future negotiation protocol such as GDOI
 [17] to establish Security Associations.
 The network administrator defines a Security Association (SA) and
 Security Parameters Index (SPI) to be used to authenticate all PIM-DM
 protocol messages from each router on each link in a PIM-DM domain.
 In order to avoid the problem of allocating individual keys for each
 neighbor on a link to each individual router, it is acceptable to
 establish only one authentication key for all PIM-DM routers on a
 link.  This will not specifically authenticate the individual router
 sending the message, but will ensure that the sender is a PIM-DM
 router on that link.  If this method is used, the receiver of the
 message MUST ignore the received sequence number, thus disabling
 anti-replay mechanisms.  The effects of disabling anti-replay
 mechanisms are essentially the same as the effects of forged
 messages, described in Section 7.1, with the additional protection
 that the forger can only reuse legitimate messages.
 The Security Policy Database at a PIM-DM router should be configured
 to ensure that all incoming and outgoing PIM-DM packets use the SA
 associated with the interface to which the packet is sent.  Note
 that, according to [10], there is nominally a different Security
 Association Database (SAD) for each router interface.  Thus, the
 selected Security Association for an inbound PIM-DM packet can vary
 depending on the interface on which the packet arrived.  This fact
 allows the network administrator to use different authentication
 methods for each link, even though the destination address is the
 same for most PIM-DM packets, regardless of interface.

Adams, et al. Experimental [Page 57] RFC 3973 PIM - Dense Mode January 2005

7.4. 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 data traffic.  Authenticating PIM protocol
 traffic prevents some, but not all, of these attacks.  The possible
 attacks include the following:
  • Sending packets to many different group addresses quickly can

amount to a denial of service attack in and of itself. These

   messages will initially be flooded throughout the network before
   they are pruned back.  The maintenance of state machines and State
   Refresh messages will be a continual drain on network resources.
  • Forged State Refresh messages sent quickly could be propagated by

downstream routers, creating a potential denial of service attack.

   Therefore, a PIM-DM router SHOULD limit the rate of State Refresh
   messages propagated.

8. Acknowledgments

 The major features of PIM-DM were originally designed by Stephen
 Deering, Deborah Estrin, Dino Farinacci, Van Jacobson, Ahmed Helmy,
 David Meyer, and Liming Wei.  Additional features for state refresh
 were designed by Dino Farinacci, Isidor Kouvelas, and Kurt Windisch.
 This revision was undertaken to incorporate some of the lessons
 learned during the evolution of the PIM-SM specification and early
 deployments of PIM-DM.
 Thanks the PIM Working Group for their comments.

9. References

9.1. Normative References

 [1]  Deering, S., "Host extensions for IP multicasting", STD 5, RFC
      1112, August 1989.
 [2]  Fenner, W., "Internet Group Management Protocol, Version 2", RFC
      2236, November 1997.
 [3]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
      Thyagarajan, "Internet Group Management Protocol, Version 3",
      RFC 3376, October 2002.

Adams, et al. Experimental [Page 58] RFC 3973 PIM - Dense Mode January 2005

 [4]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
      Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei,
      "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
      Specification", RFC 2362, June 1998.
 [5]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.
 [6]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
      Discovery (MLD) for IPv6", RFC 2710, October 1999.
 [7]  Thaler, D., "Interoperability Rules for Multicast Routing
      Protocols", RFC 2715, October 1999.
 [8]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 [9]  IANA, "Address Family Numbers", linked from
      http://www.iana.org/numbers.html.
 [10] Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

 [12] Deering, S.E., "Multicast Routing in a Datagram Internetwork",
      Ph.D. Thesis, Electrical Engineering Dept., Stanford University,
      December 1991.
 [13] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
      Multicast Routing Protocol", RFC 1075, November 1988.
 [14] Fenner,  W., Handley, M., Holbrook, H., and I. Kouvelas,
      "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
      Specification (Revised)", Work in Progress.
 [15] Holbrook, H. and B. Cain, "Source Specific Multicast for IP",
      Work in Progress.
 [16] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, "Bi-
      directional Protocol Independent Multicast", Work in Progress.
 [17] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group
      Domain of Interpretation", RFC 3547, July 2003.

Adams, et al. Experimental [Page 59] RFC 3973 PIM - Dense Mode January 2005

 [18] Fenner, W., Handley, M., Kermode, R., and D. Thaler, "Bootstrap
      Router (BSR) Mechanism for PIM Sparse Mode", Work in Progress.

Authors' Addresses

 Andrew Adams
 NextHop Technologies
 825 Victors Way, Suite 100
 Ann Arbor, MI 48108-2738
 EMail: ala@nexthop.com
 Jonathan Nicholas
 ITT Industries
 Aerospace/Communications Division
 100 Kingsland Rd
 Clifton, NJ  07014
 EMail: jonathan.nicholas@itt.com
 William Siadak
 NextHop Technologies
 825 Victors Way, Suite 100
 Ann Arbor, MI 48108-2738
 EMail: wfs@nexthop.com

Adams, et al. Experimental [Page 60] RFC 3973 PIM - Dense Mode January 2005

Full Copyright Statement

 Copyright (C) The Internet Society (2005).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the IETF's procedures with respect to rights in IETF Documents can
 be found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Adams, et al. Experimental [Page 61]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3973.txt · Last modified: 2005/01/20 19:29 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki