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.

                 4.4.1.1.  Transitions from the Forwarding

                 4.4.1.2.  Transitions from the Pruned

                 4.4.1.3.  Transitions from the AckPending

         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

                 4.4.2.3.  Transitions from the Prune

                 4.5.2.1.  Transitions from the NotOriginator

                 4.5.2.2.  Transitions from the Originator

1 Introduction
2 Terminology
- 2.1 Definitions
- 2.2 Pseudocode Notation
3 PIM-DM Protocol Overview
4 Protocol Specification
5 Protocol Interaction Considerations
6 IANA Considerations
- 6.1 PIM Address Family
- 6.2 PIM Hello Options
7 Security Considerations
8 Acknowledgments
9 References
- 9.1 Normative References
- 9.2 Informative References

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].

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.

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.

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.

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.

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]).

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.

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.

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)

(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)

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)

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}

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.

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.

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.

Hello Messages

This section describes the generation and processing of Hello messages.

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.

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.

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.

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.

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.

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.

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).

      +-------------+                        +-------------+
      |             |     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:

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.

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.

 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.

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

   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.

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.

 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.

 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.

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.

 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

In tabular form, the state machine is as follows:

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.

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

   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.

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.

 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.

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.

 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.

State Refresh

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

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;

 } 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.

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

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

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.

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.

 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).

PIM Assert Messages

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.

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}

}

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.

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.

(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.

     +-------------+                        +-------------+
     |             | 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 | +-------------------------------+------------+------------+------------+ | 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)

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).

 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.

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

   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.

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

   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).

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.

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'.

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 PIM version number is 2.

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].

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.

Unicast Address

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

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.

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.

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.

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.

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.

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.

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

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.

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | . | | . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

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.

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.

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.

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.

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.

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.

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.

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)

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

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).

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.

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: 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.

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 | +------------+--------------------+------------------------------------+

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.

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.

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.

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.

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.

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.

IANA Considerations

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:

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].

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].

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.

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

  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.

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.

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

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.

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.

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.

References

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.

[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.

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.

[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: [email protected]

Jonathan Nicholas ITT Industries Aerospace/Communications Division 100 Kingsland Rd Clifton, NJ 07014

EMail: [email protected]

William Siadak NextHop Technologies 825 Victors Way, Suite 100 Ann Arbor, MI 48108-2738

EMail: [email protected]

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Acknowledgement

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RFC3973

Contents

Introduction

Terminology

Definitions

Pseudocode Notation

PIM-DM Protocol Overview

Protocol Specification

PIM Protocol State

General Purpose State

(S,G) State

State Summarization Macros

Data Packet Forwarding Rules

Hello Messages

Sending Hello Messages

Receiving Hello Messages

Hello Message Hold Time

Handling Router Failures

Reducing Prune Propagation Delay on LANs

PIM-DM Prune, Join, and Graft Messages

Upstream Prune, Join, and Graft Messages

Transitions from the Forwarding (F) State

Transitions from the Pruned (P) State

Transitions from the AckPending (AP) State

Downstream Prune, Join, and Graft Messages

Transitions from the NoInfo State

Transitions from the PrunePending (PP) State

Transitions from the Prune (P) State

State Refresh

Forwarding of State Refresh Messages

State Refresh Message Origination

Transitions from the NotOriginator (NO) State

Transitions from the Originator (O) State

PIM Assert Messages

Assert Metrics

AssertCancel Messages

Assert State Macros

(S,G) Assert Message State Machine

Transitions from NoInfo State

Transitions from Winner State

Transitions from Loser State

Rationale for Assert Rules

PIM Packet Formats

PIM Header

Encoded Unicast Address

Encoded Group Address

Encoded Source Address

Hello Message Format

Hello Hold Time Option

LAN Prune Delay Option

Generation ID Option

State Refresh Capable Option

Join/Prune Message Format

Assert Message Format

Graft Message Format

Graft Ack Message Format

PIM-DM Timers

Protocol Interaction Considerations

PIM-SM Interactions

IGMP Interactions

Source Specific Multicast (SSM) Interactions

Multicast Group Scope Boundary Interactions

IANA Considerations

PIM Address Family

PIM Hello Options

Security Considerations

Attacks Based on Forged Messages

Non-cryptographic Authentication Mechanisms

Authentication Using IPsec

Denial of Service Attacks

Acknowledgments

References

Normative References

Informative References

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