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Versions: 00 01 02 03 04 05 RFC 3973

Internet Engineering Task Force                                   PIM WG
INTERNET DRAFT                       Andrew Adams (NextHop Technologies)
draft-ietf-pim-dm-new-v2-00.txt             Jonathan Nicholas (ITT A/CD)
                                   William Siadak (NextHop Technologies)
                                                       November 21, 2001

         Protocol Independent Multicast - Dense Mode (PIM-DM)
                   Protocol Specification (Revised)

Status of this Document

This document is an Internet Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.

Internet Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet Drafts.

Internet Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet Drafts can be accessed at
http://www.ietf.org/ietf/lid-abstracts.txt.

The list of Internet Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This document is a product of the IETF PIM WG.  Comments should be
addressed to the authors, or the WG's mailing list at
pim@catarina.usc.edu.

                            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 with no group membership
information.















Adams, Nicholas, Siadak                                         [Page 1]


                            Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . .   3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
4. Pseudocode Notation . . . . . . . . . . . . . . . . . . . . . . .   3
5. PIM-DM Protocol Overview. . . . . . . . . . . . . . . . . . . . .   4
6. Protocol Specification. . . . . . . . . . . . . . . . . . . . . .   5
 6.1. PIM Protocol State . . . . . . . . . . . . . . . . . . . . . .   5
  6.1.1. General Purpose State . . . . . . . . . . . . . . . . . . .   6
  6.1.2. (S,G) State . . . . . . . . . . . . . . . . . . . . . . . .   6
  6.1.3. State Summarization Macros. . . . . . . . . . . . . . . . .   6
 6.2. Data Packet Forwarding Rules . . . . . . . . . . . . . . . . .   8
 6.3. Hello Messages . . . . . . . . . . . . . . . . . . . . . . . .   9
  6.3.1. Sending Hello Messages. . . . . . . . . . . . . . . . . . .   9
  6.3.2. Receiving Hello Messages. . . . . . . . . . . . . . . . . .   9
  6.3.3. Hello Message Hold Time . . . . . . . . . . . . . . . . . .   9
  6.3.4. Handling Router Failures. . . . . . . . . . . . . . . . . .   9
  6.3.5.  Reducing Prune Propagation Delay on LANs . . . . . . . . .  10
 6.4. PIM-DM Prune, Join and Graft Messages. . . . . . . . . . . . .  10
  6.4.1. Upstream Prune, Join and Graft Messages . . . . . . . . . .  11
  6.4.2. Downstream Prune, Join and Graft Messages . . . . . . . . .  16
 6.5. State Refresh. . . . . . . . . . . . . . . . . . . . . . . . .  20
  6.5.1. Forwarding of State Refresh Messages. . . . . . . . . . . .  20
  6.5.2. State Refresh Message Origination . . . . . . . . . . . . .  21
 6.6. PIM Assert Messages. . . . . . . . . . . . . . . . . . . . . .  23
  6.6.1. Assert Metrics. . . . . . . . . . . . . . . . . . . . . . .  23
  6.6.2. AssertCancel Messages . . . . . . . . . . . . . . . . . . .  24
  6.6.3. Assert State Macros . . . . . . . . . . . . . . . . . . . .  24
  6.6.4. (S,G) Assert Message State Machine. . . . . . . . . . . . .  25
  6.6.5. Rationale for Assert Rules. . . . . . . . . . . . . . . . .  29
 6.7. PIM Packet Formats . . . . . . . . . . . . . . . . . . . . . .  30
  6.7.1. PIM Header. . . . . . . . . . . . . . . . . . . . . . . . .  30
  6.7.2. Encoded Unicast Address . . . . . . . . . . . . . . . . . .  31
  6.7.3. Encoded Group Address . . . . . . . . . . . . . . . . . . .  31
  6.7.4. Encoded Source Address. . . . . . . . . . . . . . . . . . .  32
  6.7.5. Hello Message Format. . . . . . . . . . . . . . . . . . . .  33
  6.7.6. Join/Prune Message Format . . . . . . . . . . . . . . . . .  35
  6.7.7. Assert Message Format . . . . . . . . . . . . . . . . . . .  37
  6.7.8. Graft Message Format. . . . . . . . . . . . . . . . . . . .  37
  6.7.9. Graft Ack Message Format. . . . . . . . . . . . . . . . . .  37
  6.6.10. State Refresh Message Format . . . . . . . . . . . . . . .  38
 6.8. PIM-DM Timers. . . . . . . . . . . . . . . . . . . . . . . . .  39
  6.8.1. Timer Values. . . . . . . . . . . . . . . . . . . . . . . .  40
7. Protocol Interaction Considerations . . . . . . . . . . . . . . .  43
 7.1. PIM-SM Interactions. . . . . . . . . . . . . . . . . . . . . .  43
 7.2. IGMP Interactions. . . . . . . . . . . . . . . . . . . . . . .  43
 7.3. Source Specific Multicast (SSM) Interactions . . . . . . . . .  43
 7.4. Multicast Group Scope Boundary Interactions. . . . . . . . . .  43
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . .  44
 8.1. PIM Address Family . . . . . . . . . . . . . . . . . . . . . .  44
 8.2. PIM Hello Options. . . . . . . . . . . . . . . . . . . . . . .  44
9. Security Considerations . . . . . . . . . . . . . . . . . . . . .  44
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .  44
11. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . .  45
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . .  45

Adams, Nicholas, Siadak                                         [Page 2]


1. Introduction

This specification defines a multicast routing algorithm for multicast
groups that are densely distributed across a network.  This protocol
does not have a topology discovery mechanism often used by a unicast
routing protocol.  It employs the same packet formats sparse mode PIM
(PIM-SM) uses.  This protocol is called PIM - Dense Mode.  The
foundation of this design was largely built on Deering's early work on
IP multicast routing [1].

2. Terminology

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

3. Definitions

Multicast Routing Information Base (MRIB)
  This is the multicast topology table, which is typically derived from
  the unicast routing table, or 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 Join/Prune messages, PIM Assert 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 where a data packet is accepted for
  forwarding if it is received on an interface used to reach the source
  in unicast.

Upstream Interface
  Interface towards the source of the datagram.  Also known as the RPF
  Interface.

Downstream Interface
  All interfaces that are not upstream interfaces, including the router
  itself.

(S,G) Pair
  Source S and destination group G associated with an IP packet.

4. Pseudocode Notation

We use set notation in several places in this specification.

A (+) B
  is the union of two sets A and B.



Adams, Nicholas, Siadak                                         [Page 3]


A (-) B
  are the elements of set A that are not in set B.

NULL
  is the empty set or list.

Note that (+) and (-) operators are NOT commutative, and must be
conducted in the order specified.

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.

5. 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 6. 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.

In order 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.










Adams, Nicholas, Siadak                                         [Page 4]


Compared with multicast routing protocols with built in topology
discovery mechanisms (e.g. DVMRP) PIM-DM has a simplified design and is
not hard-wired into a specific topology discovery protocol.  However,
such a 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 [3]).

6. Protocol Specification

The specification of PIM-DM is broken into several parts:

  Section 6.1 details the protocol state stored.
  Section 6.2 specifies the data packet forwarding rules.
  Section 6.3 specifies generation and processing of Hello messages.
  Section 6.4 specifies the Join, Prune and Graft generation and
  processing rules.
  Section 6.5 specifies the State Refresh generation and forwarding
  rules.
  Section 6.6 specifies the Assert generation and processing rules.
  Section 6.7 gives details on PIM-DM Packet Formats
  Section 6.8 summarizes PIM-DM timers and their defaults

6.1. PIM Protocol State

This section specifies all the protocol states that a PIM-DM
implementation should maintain in order 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.

Although we specify precisely the state to be kept, this does not mean
that an implementation of PIM-DM needs to hold the state in this form.
This is actually an abstract state definition, which is needed in order
to specify the router's behavior.  A PIM-DM implementation is free to
hold whatever internal state it requires, and will still be conformant
with this specification so long as it results in the same externally
visible protocol behavior as an abstract router that holds the following
state.




Adams, Nicholas, Siadak                                         [Page 5]


6.1.1. General Purpose State

A router stores the following non-group-specific state:
For each interface:
  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)
  State Refresh Capable

6.1.2. (S,G) State

For every source/group pair (S,G), a router stores the following state:

(S,G) state:
  For each interface:
    Local Membership:
      State: One of {"NoInfo", "Include"}

    PIM (S,G) Prune State:
      State: One of {"NoInfo" (NI), "Pruned" (P), "PrunePending" (PP)}
        Prune Pending Timer (PPT)
        Prune Timer (PT)

    (S,G) Assert Winner State:
      State: One of {"NoInfo" (NI), "I lost Assert" (L),
                     "I won Assert" (W)}
        Assert Timer (AT)
        Assert winner's IP Address
        Assert winner's Assert Metric

  Upstream interface-specific:
    Graft/Prune State:
      State: One of {"NoInfo" (NI), "Pruned" (P), "Forwarding" (F),
                     "AckPending" (AP) }
        GraftRetry Timer (GRT)
        Override Timer (OT)
        Prune Limit Timer (PLT)

    Originator State
      Source Active Timer (SAT)
      State Refresh Timer (SRT)

6.1.3. State Summarization Macros

Using the state defined above, the following "macros" are defined and
will be used in the descriptions of the state machines and pseudocode in
the following sections.

The most important macros are those defining the outgoing interface list
(or "olist") for the relevant state.

Adams, Nicholas, Siadak                                         [Page 6]


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 be forwarded or not 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 has determined that there are local
members on interface I that desire 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 desire 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
desire 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}


Adams, Nicholas, Siadak                                         [Page 7]


The following pseudocode macro definitions are also used in many places
in the specification.  Basically RPF' is the RPF neighbor towards 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 6.6) for (S,G) on interface I is in the "I am Assert Loser"
state.

6.2. Data Packet Forwarding Rules

The PIM-DM packet forwarding rules are defined below in pseudocode.

iif is the incoming interface of the packet.
S is the source address of the packet.
G is the destination address of the packet (group address).
RPF_interface(S) is the interface the MRIB indicates would be used to
route packets to S.

First, an RPF check MUST be performed to determine whether the packet
should be accepted based on TIB state and the interface on which that
the packet arrived.  Packets that fail the RPF check MUST NOT be
forwarded and the router will conduct an assert process for the (S,G)
pair specified in the packet.  Packets for which a route to the source
cannot be found MUST be discarded.

If the RPF check has been passed, an outgoing interface list is
constructed for the packet.  If this list is not empty, then the packet
MUST be forwarded to all listed interfaces.  If the list is empty, then
the router will conduct a prune process for the (S,G) pair specified in
the packet.

On receipt on 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
6.4.1. Upstream Prune, Join and Graft Messages.


Adams, Nicholas, Siadak                                         [Page 8]


6.3. Hello Messages

This section describes the generation and processing of Hello messages.

6.3.1. Sending Hello Messages

PIM-DM uses Hello messages to detect other PIM routers.  Hello messages
are sent periodically on each PIM enabled interface.  Hello messages are
multicast to address 224.0.0.13 (the ALL PIM ROUTERS group).  When PIM
is enabled on an interface or a router first starts, the Hello Timer
(HT) MUST be set to a random value between 0 and Hello_Period.  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.

6.3.2. Receiving Hello Messages

When a Hello message is received, the receiving router SHALL record the
receiving interface and the sender.  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 new value.

6.3.3. Hello Message Hold Time

The Hold Time in the Hello Message should be set to a value that can
reasonably be expected to keep the Hello active until a new Hello
message is received.  On most links, this will be 3.5 times the value of
Hello_Period.

If the Hold Time is set to '0xffff', the receiving router MUST NOT time
out that Hello message.  This feature might be used for on-demand links
to avoid keeping the link up with periodic Hello messages.

If a Hold Time of '0' is received, the corresponding neighbor state is
expired immediately. When an interface goes 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.

6.3.4. Handling Router Failures

If a Hello message is received from an active downstream neighbor with a
different Generation ID (GenID), the neighbor has restarted and may not
contain the correct (S,G) state. 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 at t_override (see 6.8.1).




Adams, Nicholas, Siadak                                         [Page 9]


Upon startup, a router MAY use any State Refresh messages received
within J/P_Override_Interval 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 will set the PruneLimitTimer to
Prune_Holdtime and set the PruneTimer on all downstream interfaces to
the State Refresh's Interval times two.  The router will then propagate
the State Refresh as described in section 6.5.1.

6.3.5.  Reducing Prune Propagation Delay on LANs

If all routers on a LAN support the LAN Prune Delay option, then the PIM
routers on that LAN SHOULD use the values received to adjust its
J/P_Override_Interval on that interface.  Briefly, to avoid
synchronization of Prune Override (Join) messages when multiple
downstream routers share a multi-access link, sending of such messages
is delayed by a small random amount of time. The period of randomization
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 SHOULD 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 SHOULD 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 as well as
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.

6.4. PIM-DM Prune, Join and Graft Messages

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







Adams, Nicholas, Siadak                                        [Page 10]


6.4.1. Upstream Prune, Join and Graft Messages

The Upstream(S,G) state machine for sending Prune, Graft and Join
messages is given below.  There are three states.

  Forwarding (F)
    This is the starting state of the Upsteam(S,G) state machine.  The
    state machine is in this state if it just started or if
    oiflist(S,G) != NULL.

  Pruned(P)
    The set, olist(S,G), is empty   The router will not forward data
    from S addressed to group G.

  AckPending(AP)
    The router was in the Pruned(P) state but a transition has occurred
    in the Downstream(S,G) state machine for one of this (S,G) entry's
    outgoing interfaces indicating that traffic from S addressed to G
    should again be forwarded.  A Graft message has been sent to RPF'(S)
    but a Graft Ack message has not yet been received.

In addition there are three state-machine-specific timers:

  GraftRetry Timer (GRT(S,G))
    This timer is set when a Graft is sent upstream.  If a corresponding
    GraftAck is not received before the timer expires, then another
    Graft is sent and the GraftRetry Timer is reset.  The timer is
    stopped when a Graft Ack message is received. This timer is normally
    set to Graft_Retry_Period (see 6.8.1).

  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 6.8.1).

  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 6.8.1)

         [For State Machine Figure refer to Postscript Version]

               Figure 1  Upstream Interface State Machine











Adams, Nicholas, Siadak                                        [Page 11]


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

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
+                               +------------+------------+------------+
|            Event              | Forwarding |   Pruned   | AckPending |
+-------------------------------+------------+------------+------------+
| Data packet arrives on        | ->P Send   | ->P Send   | N/A        |
| RPF_Interface(S) AND          | Prune(S,G) | Prune(S,G) |            |
| olist(S,G) == NULL AND        |Set PLT(S,G)|Set PLT(S,G)|            |
| PLT(S,G) not running          |            |            |            |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received   | ->F  Set   | ->P Reset  |->AP  Set   |
| from RPF'(S) AND              |    OT(S,G) |  PLT(S,G)  |    OT(S,G) |
| Prune Indicator == 1          |            |            |            |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received   | ->F        | ->P Send   |->F Cancel  |
| from RPF'(S) AND              |            | Prune(S,G) |  GRT(S,G)  |
| Prune Indicator == 0 AND      |            |Set PLT(S,G)|            |
| PLT(S,G) not running          |            |            |            |
+-------------------------------+------------+------------+------------+
| See Join(S,G) to RPF'(S)      | ->F Cancel | ->P        |->AP Cancel |
|                               |    OT(S,G) |            |    OT(S,G) |
+-------------------------------+------------+------------+------------+
| See Prune(S,G)                | ->F Set    | ->P        |->AP Set    |
|                               |    OT(S,G) |            |    OT(S,G) |
+-------------------------------+------------+------------+------------+
| OT(S,G) Expires               | ->F Send   | N/A        |->AP Send   |
|                               |  Join(S,G) |            |  Join(S,G) |
+-------------------------------+------------+------------+------------+
| olist(S,G)->NULL              | ->P Send   | N/A        |->P Send    |
|                               | Prune(S,G) |            | Prune(S,G) |
|                               |Set PLT(S,G)|            |Set PLT(S,G)|
+-------------------------------+------------+------------+------------+
| olist(S,G)->non-NULL          | N/A        | ->AP Send  | N/A        |
|                               |            | Graft(S,G) |            |
|                               |            |Set GRT(S,G)|            |
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND           | ->AP Send  | ->AP Send  |->AP Send   |
| olist(S,G) != NULL            | Graft(S,G) | Graft(S,G) | Graft(S,G) |
|                               |Set GRT(S,G)|Set GRT(S,G)|Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND           | ->P        | ->P Cancel |->P Cancel  |
| olist(S,G) == NULL            |            |  PLT(S,G)  |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+
| S becomes directly connected  | ->F        | ->P        |->F Cancel  |
|                               |            |            |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+
| GRT(S,G) Expires              | N/A        | N/A        |->AP Send   |
|                               |            |            | Graft(S,G) |
|                               |            |            |Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| Receive GraftAck(S,G) from    | ->F        | ->P        |->F Cancel  |
| RPF'(S)                       |            |            |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+

Adams, Nicholas, Siadak                                        [Page 12]


The transition event 'Receive GraftAck(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.

  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.

  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.

  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 and 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.

  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.

  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.






Adams, Nicholas, Siadak                                        [Page 13]


  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.

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:

  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 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.

  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 greater than the current value of the
    PLT(S,G).

  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.

  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.  In order 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.


Adams, Nicholas, Siadak                                        [Page 14]


  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 Indicated 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 Indicated bit set to one, the router MUST reset PLT(S,G) to
    t_limit.

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:

  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 unicasted to RPF'(S) and
    the GraftRetry Timer (GRT(S,G)) reset to Graft_Retry_Period.  Note
    that RPF’(S) may have changed since the previous Graft.

  RPF'(S) Changes AND olist(S,G) does not become NULL
    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.

  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.

  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.

  olist(S,G) -> NULL
    The set of interfaces defined by the olist(S,G) macro becomes null
    indicating 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.

  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.




Adams, Nicholas, Siadak                                        [Page 15]


  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 and 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.

  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.

  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.

6.4.2. Downstream Prune, Join and Graft Messages

The Prune(S,G) Downstream state machine for receiving Prune, Join and
Graft messages on interface I is given below.  This state machine MUST
always be in the NoInfo state on the upstream interface.  It contains
three states.

  NoInfo(NI)
    The interface has no (S,G) Prune state and neither the Prune timer
    (PT(S,G,I)) nor the PrunePending timer ((PPT(S,G,I)) is running.

  PrunePending(PP)
    The router has received a Prune(S,G) on this interface from a
    downstream neighbor and is waiting to see whether the prune will be
    overridden by another downstream router.  For forwarding purposes,
    the PrunePending state functions exactly like the NoInfo state.

  Pruned(P)
    The router has received a Prune(S,G) on this interface from a
    downstream neighbor and the Prune was not overridden.  Data from S
    addressed to group G is no longer being forwarded on this interface.

In addition there are two timers:

  PrunePending Timer (PPT(S,G,I))
    This timer is set when a valid Prune(S,G) is received.  Expiry of
    the PrunePending Timer (PPT(S,G,I)) causes the interface to
    transition to the Pruned state.


Adams, Nicholas, Siadak                                        [Page 16]


  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.

         [For State Machine Figure refer to Postscript Version]

             Figure 2: Prune(S,G) Downstream State Machine

In tabular form, the state machine is:

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
+                               +------------+------------+------------+
|            Event              |  No Info   | PrunePend  |   Pruned   |
+-------------------------------+------------+------------+------------+
| Receive Prune(S,G)            |->PP  Set   |->PP        |->P Reset   |
|                               | PPT(S,G,I) |            |  PT(S,G,I) |
+-------------------------------+--------------------------------------+
| Receive Join(S,G)             |->NI        |->NI Cancel |->NI Cancel |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+--------------------------------------+
| Receive Graft(S,G)            |->NI Send   |->NI Send   |->NI Send   |
|                               |  GraftAck  |  GraftAck  |  GraftAck  |
|                               |            |  Cancel    |  Cancel    |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+--------------------------------------+
| PPT(S,G) Expires              | N/A        |->P Set     | N/A        |
|                               |            |  PT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| PT(S,G) Expires               | N/A        | N/A        |->NI        |
+-------------------------------+--------------------------------------+
| RPF_Interface(S) becomes I    |->NI        |->NI Cancel |->NI Cancel |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+--------------------------------------+

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.













Adams, Nicholas, Siadak                                        [Page 17]


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 unicasted 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 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.

  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.

  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, then 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.


Adams, Nicholas, Siadak                                        [Page 18]


  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 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.

  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 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.

  PT(S,G,I) Expires
    The Prune Timer (PT(S,G,I)) expires indicating that it is again time
    to flood data from S addressed to group G onto interface I.  The
    Prune(S,G) Downstream state machine on interface I MUST transition
    to the NoInfo (NI) state.  The router MUST evaluate any possible
    transitions in the Upstream(S,G) state machine.

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












Adams, Nicholas, Siadak                                        [Page 19]


6.5. State Refresh

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

6.5.1. Forwarding of State Refresh Messages

When a State Refresh message, SRM, is received, it is forwarded
according to the following pseudo-code.

if (iif != RPF_interface(S))
  return;
if (RPF'(S) != srcaddr(SRM))
  return;

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 */

  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 prference 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) evaluates to TRUE if an administratively scoped boundary
for group G is configured on interface I.


Adams, Nicholas, Siadak                                        [Page 20]


StateRefreshCapable(I) evaluates to TRUE if all neighbors on an
interface use the State Refresh option.

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 of interface
I.

6.5.2. State Refresh Message Origination

This section describes the origination of State Refresh messages.  These
messages are generated periodically by the PIM-DM router that is
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 which are directly connected to S
    may transition to this state.

In addition there are two state-machine-specific timers:

  StateRefresh Timer (SRT(S,G))
    This timer is 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 6.8.1).

  SourceActive 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 6.8.1).

         [For State Machine Figure refer to Postscript Version]

           Figure 3 Per-interface State Refresh State Diagram



Adams, Nicholas, Siadak                                        [Page 21]


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

+----------------------------------------------------------------------+
|                                  |           Previous State          |
|                                  +---------------+-------------------+
|            Event                 | NotOriginator |    Originator     |
+----------------------------------+---------------+-------------------+
| Receive Data from S AND          | ->O           | ->O Reset         |
|  S directly connected            | Set SRT(S,G)  |     SAT(S,G)      |
|                                  | Set SAT(S,G)  |                   |
+----------------------------------+---------------+-------------------+
| SRT(S,G) Expires                 | N/A           | ->O    Send       |
|                                  |               | StateRefresh(S,G) |
|                                  |               |  Reset SRT(S,G)   |
+----------------------------------+---------------+-------------------+
| SAT(S,G) Expires                 | N/A           | ->NO  Cancel      |
|                                  |               |       SRT(S,G)    |
+----------------------------------+---------------+-------------------+
| S no longer directly connected   | ->NO          | ->NO              |
|                                  |               |   Cancel SRT(S,G) |
|                                  |               |   Cancel SAT(S,G) |
+----------------------------------+---------------+-------------------+

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:

  SRT(S,G) Expires
    The router remains in the Originator (O) state and resets SRT(S,G)
    to StateRefreshInterval.  The router also generates State Refresh
    messages for transmission over each interface on which there are
    PIM-DM neighbors except for the interface by which S is reached.  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 TTL of the interface over which the
    State Refresh message will be sent.  Let I denote the interface over
    which a State Refresh message is being sent.  If the Prune(S,G)
    Downstream state machine for I is in the NoInfo (NI) state, then the
    Prune-Indicator bit should be set to 0 in the State Refresh message
    being sent over I.  Otherwise the Prune-Indicator bit should be set
    to 1.


Adams, Nicholas, Siadak                                        [Page 22]


  SAT(S,G) Expires
    The router cancels the SRT(S,G) timer and transitions to the
    NotOriginator (NO) state.

  Receive Data Packet from S addressed to G
    The router remains in the Originator (O) state and resets 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.

  S is no longer directly connected
    The router remains transitions to the NotOriginator (NO) state and
    cancels both the SAT(S,G) and SRT(S,G).

6.6. PIM Assert Messages

6.6.1. Assert Metrics

Assert metrics are defined as:

struct assert_metric {
  metric_preference;
  route_metric;
  ip_address;
};

When comparing assert_metrics, 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 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_ip_address(I)}
}





Adams, Nicholas, Siadak                                        [Page 23]


MRIB.pref(X) and MRIB.metric(X) are the routing preference and routing
metrics associated with the route to a particular (unicast) destination
X, as determined by the MRIB.  my_ip_address(I) is simply the router's
IP address that is 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,infinity}
}

6.6.2. AssertCancel Messages

An AssertCancel(S,G) message is simply an Assert message for (S,G) with
infinite metric.  The Assert winner sends such a 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 allow a subnet to eventually become consistent;
the AssertCancel mechanism simply causes faster convergence.  No special
processing is required for an AssertCancel message, since it is simply
an Assert message from the current winner.

6.6.3. Assert State Macros

The macro lost_assert(S,G,I), is used in the olist computations of
section 6.1.3, 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.












Adams, Nicholas, Siadak                                        [Page 24]


6.6.4. (S,G) Assert Message State Machine

The (S,G) Assert state machine for interface I is shown in Figure 4.

There are three states:

  NoInfo (NI)
    This router has no (S,G) Assert state on interface I.

  I am Assert Winner (W)
    This router has won an (S,G) Assert on interface I.  It is now
    responsible for forwarding traffic from S destined for G via
    interface I.

  I am Assert Loser (L)
    This router has lost an (S,G) Assert on interface I.  It must not
    forward packets from S destined for G onto interface I.

In addition there is also an Assert Timer (AT(S,G,I)) that is used to
time out Assert state on the Assert losers and to resend Assert messages
on the Assert winner.

         [For State Machine Figure refer to Postscript Version]

           Figure 4: Per-interface (S,G) Assert state machine































Adams, Nicholas, Siadak                                        [Page 25]


In tabular form the state machine is defined as follows:

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
+                               +------------+------------+------------+
|            Event              |  No Info   |   Winner   |    Loser   |
+-------------------------------+------------+------------+------------+
| An (S,G) Data packet received | ->W Send   | ->W Send   | ->L        |
| on downstream interface       | Assert(S,G)| Assert(S,G)|            |
|                               |    Set     |    Set     |            |
|                               |  AT(S,G,I) |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR   | N/A        | N/A        |->NI Cancel |
| State Refresh) from Assert    |            |            |  AT(S,G,I) |
| Winner                        |            |            |            |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR   | ->W Send   | ->W Send   | ->L        |
| State Refresh) from non-Assert| Assert(S,G)| Assert(S,G)|            |
| Winner AND CouldAssert==TRUE  |    Set     |    Set     |            |
|                               |  AT(S,G,I) |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| 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  | ->L        |
|                               |            |  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 macros:

CouldAssert(S,G,I) = (RPF_interface(S) != I)



Adams, Nicholas, Siadak                                        [Page 26]


The first line accounts for (S,G) join information received on I that
might cause the router to be interested in Asserts on I.
The last line accounts for the fact that a router must keep track of
Assert information on upstream interfaces in order to send Grafts and
Prunes to the proper neighbor.

Transitions from NoInfo State

When in NoInfo state, the following events may trigger transitions:

  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 - Assert_Override_Interval).

  An (S,G) data packet arrives on downstream interface I
    An (S,G) data packet arrived on a downstream interface which is
    contained in immediate_olist(S,G).  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 - Assert_Override_Interval), thereby
    initiating the Assert negotiation for (S,G).

  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.  The router MUST also multicast a Prune(S,G) to
    the Assert winner 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:

  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.

  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.

Adams, Nicholas, Siadak                                        [Page 27]


  Receive Inferior Assert
    An (S,G) Assert is received containing a metric for S that is worse
    metric 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 -
    Assert_Override_Interval).

  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 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 and evaluate any changes in its
    Upstream(S,G) state machine.

  CouldAssert(S,G,I) -> FALSE
    This router's RPF interface changed so as to make CouldAssert(S,G,I)
    become false.  This router can no longer perform the actions of the
    Assert winner, and so the Assert state machine MUST transition to
    NoInfo (NI) state, send an AssertCancel(S,G) to interface 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 Preferred Assert or State Refresh
    An Assert or State Refresh is received that has a better metric
    than that of the current Assert winner.  The Assert state machine
    remains in Loser (L) state and MUST store the address and metric
    of the new Assert Winner. 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 == TRUE, the router MUST multicast
    a Prune(S,G) to the new Assert winner.

  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.










Adams, Nicholas, Siadak                                        [Page 28]


  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.

  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.

  My metric becomes better than the assert winner's metric AND
  CouldAssert == TRUE
    my_assert_metric(S,G,I) has changed so that now this router's Assert
    metric for (S,G) is better than the metric it has stored for current
    Assert winner.  This might happen when the underlying routing metric
    changes, or when CouldAssert(S,G,I) becomes true. The Assert state
    machine MUST transition to NoInfo (NI) state, delete the Assert
    Winner's address and metric, and evaluate any possible transitions
    to its Upstream(S,G) state machine.

  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.

6.6.5. Rationale for Assert Rules

The following is a summary of the rules for generating and processing
Assert messages.  It is not intended to be definitive (the state
machines and pseudocode provide the definitive behavior).  Instead it
provides some rationale for the behavior.

1. The Assert winner for (S,G) must act as the local forwarder for (S,G)
   on behalf all downstream members.
2. PIM messages are directed towards 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 the downstream
   router can correct its RPF'(S).

Adams, Nicholas, Siadak                                        [Page 29]


4. An assert winner for (S,G) sends a canceling 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.

6.7. PIM Packet Formats

All PIM-DM packets use the same format as PIM-SM packets.  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 (224.0.0.13).  Graft
messages SHOULD be unicast to the RPF'(S).  Graft Ack messages MUST be
unicast to the sender of the Graft.

6.7.1. PIM Header

All PIM control messages have the following header:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type  |   Reserved    |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

PIM Ver
PIM version number is 2.

Type
Types for specific PIM messages.  Available types are:
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 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.










Adams, Nicholas, Siadak                                        [Page 30]


6.7.2. Encoded Unicast Address

An Encoded Unicast Address has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Family  | Encoding Type |     Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

Addr Family

The PIM Address Family of the 'Unicast Address' field of this
address.

Values of 0-127 are as assigned by the IANA for Internet Address
Families in [6].  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.

6.7.3. Encoded Group Address

An Encoded Group address has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Family  | Encoding Type |   Reserved    |  Mask Len     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Group Multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

Addr Family
As described above.

Encoding Type
As described above.

Reserved
Transmitted as zero.  Ignored upon receipt.




Adams, Nicholas, Siadak                                        [Page 31]


Mask Len
The mask length field is 8 bits.  The value is the number of
contiguous on bits left justified 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.

6.7.4. Encoded Source Address

An Encoded Source address has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Family  | Encoding Type |  Rsrvd  |S|W|R|  Mask Len     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

Addr Family
As described above.

Encoding Type
As described above.

Rsrvd
Reserved.  Transmitted as zero.  Ignored upon receipt.

S
The Sparse Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.

W
The Wild Card Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.

R
The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored  upon
receipt.

Mask Len
As described above.  PIM-DM routers MUST only send for a single
source address.

Source Address
The source address.







Adams, Nicholas, Siadak                                        [Page 32]


6.7.5. Hello Message Format

The PIM Hello message 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:
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
22-65000        To be assigned by IANA
65001-65535     Reserved for Private Use [7]

Unknown options SHOULD be ignored.

6.7.5.1. Hello Hold Time Option

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type = 1           |           Length = 2          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Hold Time          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Adams, Nicholas, Siadak                                        [Page 33]


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.

6.7.5.2. LAN Prune Delay Option

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type = 2           |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|       LAN Prune Delay       |       Override Interval       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The LAN_Prune_Delay option is used to tune the prune propagation delay
on multi-access LANs. The T bit is used by PIM-SM and SHOULD be set to 0
by PIM-DM routers and ignored upon receipt. The LAN Delay and Override
Interval fields are time intervals in units of milliseconds and are used
to tune the value of the J/P Override Interval and its derived timer
values. Section 6.3.5 describes how these values affect the behavior of
a router. The LAN Prune Delay SHOULD be used by PIM-DM routers.

6.7.5.3. Generation ID Option

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type = 20           |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Generation ID                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Generation ID is a random value for the interface on which the Hello
message is sent.  The Generation ID is regenerated whenever PIM
forwarding is started or restarted on the interface.  The Generation ID
option MAY be used by PIM-DM routers.

6.7.5.4. State Refresh Capable Option

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Type = 21           |           Length = 4          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Version = 1  |   Interval    |            Reserved           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Interval field is the router's configured State Refresh Interval in
seconds.  The Reserved field is set to zero and ignored upon reception.
The State Refresh Capable option MUST be used by State Refresh capable
PIM-DM routers.


Adams, Nicholas, Siadak                                        [Page 34]


6.7.6. Join/Prune Message Format

PIM Join/Prune 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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Upstream Neighbor Address (Encoded Unicast Format)     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Reserved    |  Num Groups   |          Hold Time            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Multicast Group Address 1 (Encoded Group Format)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Number of Joined Sources    |   Number of Pruned Sources    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Joined Source Address 1 (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Joined Source Address n (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Pruned Source Address 1 (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Pruned Source Address n (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Multicast Group Address m (Encoded Group Format)      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Number of Joined Sources    |   Number of Pruned Sources    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Joined Source Address 1 (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Joined Source Address n (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Pruned Source Address 1 (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Pruned Source Address n (Encoded Source Format)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Adams, Nicholas, Siadak                                        [Page 35]


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 6.7.2.  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 6.7.3.

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 6.7.4.

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 6.7.4.














Adams, Nicholas, Siadak                                        [Page 36]


6.7.7. Assert Message Format

PIM Assert 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)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 6.7.3.

Source Address
The source address in the Encoded Source address format given in section
6.7.4.

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.

6.7.8. Graft Message Format

PIM Graft messages use the same format as Join/Prune messages except 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.

6.7.9. Graft Ack Message Format

PIM Graft Ack messages are identical in format to the received Graft
message except 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.


Adams, Nicholas, Siadak                                        [Page 37]


6.7.10. 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 6.7.3.

Source Address
The address of the data source in the Encoded Source address format
given in section 6.7.4.

Originator Address
The address of the first hop router in the Encoded Source address format
given in section 6.7.4.

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.





Adams, Nicholas, Siadak                                        [Page 38]


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 a State Refresh has not been heard from the assert winner over
the period of three times RefreshInterval(S,G) 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.

6.8. PIM-DM Timers

PIM-DM maintains the following timers.  All timers are countdown timers
- they are set to a value and count down to zero, at which point they
typically trigger an action.  Of course they can just as easily be
implemented as count-up timers, where the absolute expiry time is store
and compared against a real-time clock, but the language in this
specification assumes that they count downwards towards zero.

Global Timers
  Hello Timer: HT

  Per interface (I):
    Propagation Delay: PD(I)
    Override Interval: OI(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)





Adams, Nicholas, Siadak                                        [Page 39]


  Per (S,G) Pair:
    (S,G) Graft Retry Timer: GT(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)

6.8.1. Timer Values

When timer values are started or restarted, they are set to default
values.  This section summarizes those default values.

Timer Name: Hello Timer (HT)

+----------------------+--------+--------------------------------------+
| Value Name           | Value  | Explanation                          |
+----------------------+--------+--------------------------------------+
|Hello_Period          | 30 sec | Periodic interval for hello messages |
+----------------------+--------+--------------------------------------+
|Triggered_Hello_Delay | 5 sec  | Random interval for initial Hello    |
|                      |        | message on bootup or triggered Hello |
|                      |        | message to a rebooting neighbor      |
+----------------------+--------+--------------------------------------+

Hello message 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).  Hello_Period corresponds to the PIM MIB
object pimInterfaceHelloInterval.

Timer Name: Propagation Delay (PD(I))

+-------------------------+----------------+---------------------------+
| Value Name              | Value          | Explanation               |
+-------------------------+----------------+---------------------------+
| LAN_delay_default       | 0.5 sec        | Expected propagation      |
|                         |                | over the local link.      |
+-------------------------+----------------+---------------------------+

If all routers on a LAN are using the LAN Prune Delay option, PD(I) will
be set to the largest LAN Delay on the LAN.  Otherwise, PD(I) will be
set to LAN_delay_default.

Timer Name: Override Interval (OI(I))

+--------------------------+-----------------+-------------------------+
| Value Name               | Value           | Explanation             |
+--------------------------+-----------------+-------------------------+
| t_override_default       | 2.5 sec         | Default delay interval  |
|                          |                 | over which to randomize |
|                          |                 | when scheduling a Join/ |
|                          |                 | Prune Override message. |
+--------------------------+-----------------+-------------------------+


Adams, Nicholas, Siadak                                        [Page 40]


If all routers on a LAN are using the LAN Prune Delay option, OI(I) will
be set to the largest Override Interval on the LAN.  Otherwise, OI(I)
will be set to t_override_default.

Timer Name: Neighbor Liveness Timer (NLT(N,I))

+-------------------+-----------------+--------------------------------+
| Value Name        | Value           | Explanation                    |
+-------------------+-----------------+--------------------------------+
| Hello Holdtime    | From message    | Hold Time from Hello Message   |
+-------------------+-----------------+--------------------------------+

Timer Name: PrunePending Timer (PPT(S,G,I))

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

Timer Name: Prune Timer (PT(S,G,I))

+----------------+----------------+------------------------------------+
| Value Name     | Value          | Explanation                        |
+----------------+----------------+------------------------------------+
| Prune Holdtime | From message   | Hold Time read from Prune Message  |
+----------------+----------------+------------------------------------+

Prune Holdtime corresponds to PIM MIB object
pimInterfaceJoinPruneInterval

Timer Name: Assert Timer (AT(S,G,I))

+--------------------------+---------+---------------------------------+
| Value Name               | Value   | Explanation                     |
+--------------------------+---------+---------------------------------+
| Assert Override Interval | 3 sec   | Short interval before an assert |
|                          |         | times out where the assert      |
|                          |         | winner resends an assert        |
|                          |         | message                         |
+--------------------------+---------+---------------------------------+
| Assert Time              | 180 sec | Period after last assert before |
|                          |         | assert state is timed out       |
+--------------------------+---------+---------------------------------+

Note that for historical reasons, the Assert message lacks a Holdtime
field.  Thus changing the Assert Time from the default value is not
recommended.  If all members of a LAN are state refresh enabled, the
Assert Time will be three times RefreshInterval(S,G) and Assert Override
Interval will not be needed.




Adams, Nicholas, Siadak                                        [Page 41]


Timer Name: Graft Retry Timer (GRT(S,G))

+--------------------+-------+-----------------------------------------+
| Value Name         | Value | Explanation                             |
+--------------------+-------+-----------------------------------------+
| Graft_Retry_Period | 3 sec | In the absence of receipt of a GraftAck |
|                    |       | message, the time before retransmission |
|                    |       | of a Graft message                      |
+-----------------------+---------------+------------------------------+

Timer Name: Upstream Override Timer (OT(S,G))

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

Timer Name: Prune Limit Timer (PLT(S,G))

+-----------+--------------------+-------------------------------------+
|Value Name | Value              | Explanation                         |
+-----------+--------------------+-------------------------------------|
|t_limit    | Equal to the Prune | Used to prevent Prune storms on a   |
|           | Holdtime sent      | LAN                                 |
+-----------+--------------------+-------------------------------------|

Timer Name: Source Activity Timer (SAT(S,G))

+----------------+-------------------+---------------------------------+
| Value Name     | Value             | Explanation                     |
+----------------+-------------------+---------------------------------+
| SourceLifetime | Default: 210 secs | Period of time after receiving  |
|                |                   | a multicast message a directly  |
|                |                   | attached router will continue   |
|                |                   | to send State Refresh messages  |
+----------------+-------------------+---------------------------------+

Timer Name: State Refresh Timer (SRT(S,G))

+-----------------+------------------+---------------------------------+
| Value Name      | Value            | Explanation                     |
+-----------------+------------------+---------------------------------+
| RefreshInterval | Default: 60 secs | Interval between successive     |
|                 |                  | state refresh messages          |
+-----------------+------------------+---------------------------------+








Adams, Nicholas, Siadak                                        [Page 42]


7. 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 which 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 [13].  Other specific interactions
are noted below.

7.1. PIM-SM Interactions

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

In the event that a PIM-DM router becomes a neighbor of a PIM-SM router
they 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.

7.2. IGMP Interactions

PIM-DM will forward received multicast data packets to neighboring host
group members in all cases except when the PIM-DM router is in an Assert
Loser state on that interface.  Note that a PIM Prune message is not
permitted to prevent the delivery of messages to a network with group
members.

A PIM-DM Router MAY use the DR Priority option described in [3] to elect
an IGMP v1 querier.

7.3. Source Specific Multicast (SSM) Interactions

PIM-DM makes no special considerations for SSM [11].  All Prunes and
Grafts within the protocol are for a specific source, so no additional
checks need be made.

7.4. Multicast Group Scope Boundary Interactions

While 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.



Adams, Nicholas, Siadak                                        [Page 43]


8. IANA Considerations

8.1. PIM Address Family

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

Values 0 through 127 are designated to have the same meaning as IANA
assigned Address Family Numbers [6].

Values 128 through 250 are designated to be assigned by the IANA based
upon IESG approval as defined in [7].

Values 251 through 255 are designated for Private Use, as defined in
[7].

8.2. PIM Hello Options

Values 17 through 65000 are to be assigned by the IANA.  Since the space
is large, they may be assigned as First Come First Served as defined in
[7].  Such assignments are valid for one year, and may be renewed.
Permanent assignments require a specification as defined in [7].

9. Security Considerations

All PIM control messages MAY use IPsec [8] to address security concerns.
The authentication methods are addressed in a companion document [9].
Keys may be distributed as described in [10].  In any case, PIM router
SHOULD NOT accept and process PIM messages from neighbors unless a valid
Hello message has been received from that neighbor.

We should note that PIM-DM has no rendezvous point, and therefore no
single point of failure that may be vulnerable.  However, since PIM-DM
assumes that multicast messages are desired throughout the network, it
may be particularly vulnerable to denial of service attacks.
It is further worth noting that because PIM-DM uses unicast routes
provided by an unknown routing protocol, it may suffer collateral
effects if the unicast routing protocol is attacked.

10. Authors' Addresses

Andrew Adams
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
ala@nexthop.com





Adams, Nicholas, Siadak                                        [Page 44]


Jonathan Nicholas
ITT Aerospace/Communications Division
100 Kingsland Rd
Clifton, NJ  07014
jonathan.nicholas@itt.com

William Siadak
NextHop Technologies
825 Victors Way, Suite 100
Ann Arbor, MI 48108-2738
wfs@nexthop.com

11. 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.

12. References

[1]  S.E. Deering,  "Multicast Routing in a Datagram Internetwork",
     Ph.D. Thesis, Electrical Engineering Dept., Stanford University,
     December 1991.
[2]  D. Waitzman, B.Partridge, S.Deering, "Distance Vector Multicast
     Routing Protocol", November 1988, RFC 1075
[3]  W. Fenner,  M. Handley, H.Holbrook, I. Kouvelas, "Protocol
     Independent Multicast - Sparse Mode (PIM-SM)", draft-ietf-pim-sm-
     v2-new-03.txt, work in progress.
[4]  S.E. Deering, "Host Extensions for IP Multicasting", August 1989,
     RFC 1112.
[5]  W.Fenner, "Internet Group Management Protocol, Version 2",
     November 1997, RFC 2236.
[6]  IANA, "Address Family Numbers", linked from
     http://www.iana.org/numbers.html.
[7]  T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
     Considerations Section in RFCs", RFC 2434.
[8]  S. Kent, R. Atkinson, "Security Architecture for the Internet
     Protocol", RFC 2401.
[9]  L. Wei, "Authenticating PIM Version 2 Messages", draft-ietf-pim-
     v2-auth-01.txt, work in progress.
[10] T. Hardjono, B. Cain, "Simple Key Management Protocol for PIM",
     draft-ietf-pim-simplekmp-01.txt, work in progress.
[11] H.Holbrook, B. Cain, "Source Specific Multicast for IP", draft-
     holbrook-ssm-00.txt, work in progress.
[12] B. Cain, S. Deering, B. Fenner, I. Kouvelas, A. Thyagarajan,
     "Internet Group Management Protocol, Version 3", draft-ietf-idmr-
     igmp-v3-07.txt, work in progress.
[13] D. Thaler, "Interoperability Rules for Multicast Routing
     Protocols", October 1999, RFC 2715.
[14] K.McCloghrie, D.Farinacci, D.Thaler, B.Fenner, "Protocol
     Independent Multicast MIB for IPv4", October 2000, RFC 2934

Adams, Nicholas, Siadak                                        [Page 45]


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