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Internet Engineering Task Force                                   PIM WG
INTERNET-DRAFT                                          Mark Handley/UCL
draft-ietf-pim-bidir-06.txt                        Isidor Kouvelas/Cisco
                                                     Tony Speakman/Cisco
                                                  Lorenzo Vicisano/Cisco
                                                           12 April 2004
                                                   Expires: October 2004


       Bi-directional Protocol Independent Multicast (BIDIR-PIM)


Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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/1id-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 discusses Bi-directional PIM, a variant of PIM
     Sparse-Mode [4] that builds bi-directional shared trees
     connecting multicast sources and receivers. Bi-directional
     trees are built using a fail-safe Designated Forwarder (DF)
     election mechanism operating on each link of a multicast
     topology.  With the assistance of the DF, multicast data is



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     natively forwarded from sources to the Rendezvous-Point and
     hence along the shared tree to receivers without requiring
     source-specific state.  The DF election takes place at RP
     discovery time and provides the route to the RP thus
     eliminating the requirement for data-driven protocol events.














































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                           Table of Contents


     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   5
     2. Terminology . . . . . . . . . . . . . . . . . . . . . .   5
      2.1. Definitions. . . . . . . . . . . . . . . . . . . . .   6
      2.2. Pseudocode Notation. . . . . . . . . . . . . . . . .   8
     3. Protocol Specification. . . . . . . . . . . . . . . . .   8
      3.1. BIDIR-PIM Protocol State . . . . . . . . . . . . . .   9
       3.1.1. General Purpose State . . . . . . . . . . . . . .   9
       3.1.2. RPA State . . . . . . . . . . . . . . . . . . . .  10
       3.1.3. Group State . . . . . . . . . . . . . . . . . . .  10
       3.1.4. State Summarization Macros. . . . . . . . . . . .  11
      3.2. PIM Neighbor Discovery . . . . . . . . . . . . . . .  12
      3.3. Data Packet Forwarding Rules . . . . . . . . . . . .  13
       3.3.1. Upstream Forwarding at RP . . . . . . . . . . . .  14
       3.3.2. Source-Only Branches. . . . . . . . . . . . . . .  14
       3.3.3. Directly Connected Sources. . . . . . . . . . . .  15
      3.4. PIM Join/Prune Messages. . . . . . . . . . . . . . .  15
       3.4.1. Receiving (*,G) Join/Prune Messages . . . . . . .  15
       3.4.2. Sending Join/Prune Messages . . . . . . . . . . .  18
      3.5. Designated Forwarder (DF) Election . . . . . . . . .  21
       3.5.1. DF Requirements . . . . . . . . . . . . . . . . .  21
       3.5.2. DF Election description . . . . . . . . . . . . .  22
        3.5.2.1. Bootstrap Election . . . . . . . . . . . . . .  22
        3.5.2.2. Loser Metric Changes . . . . . . . . . . . . .  23
        3.5.2.3. Winner Metric Changes. . . . . . . . . . . . .  24
        3.5.2.4. Winner Loses Path. . . . . . . . . . . . . . .  24
        3.5.2.5. Late Router Starting Up. . . . . . . . . . . .  25
        3.5.2.6. Winner Dies. . . . . . . . . . . . . . . . . .  25
       3.5.3. Election Protocol Specification . . . . . . . . .  25
        3.5.3.1. Election State . . . . . . . . . . . . . . . .  25
        3.5.3.2. Election Messages. . . . . . . . . . . . . . .  26
        3.5.3.3. Election Events. . . . . . . . . . . . . . . .  27
        3.5.3.4. Election Actions . . . . . . . . . . . . . . .  28
        3.5.3.5. Election State Transitions . . . . . . . . . .  28
       3.5.4. Election Reliability Enhancements . . . . . . . .  32
       3.5.5. Missing Pass. . . . . . . . . . . . . . . . . . .  32
       3.5.6. Periodic Winner Announcement. . . . . . . . . . .  32
      3.6. Timers Counters and Constants. . . . . . . . . . . .  32
      3.7. BIDIR PIM Packet Formats . . . . . . . . . . . . . .  36
       3.7.1. DF Election Packet Formats. . . . . . . . . . . .  36
       3.7.2. Backoff Message . . . . . . . . . . . . . . . . .  37
       3.7.3. Pass Message. . . . . . . . . . . . . . . . . . .  38
       3.7.4. Bidir Capable PIM-Hello Option. . . . . . . . . .  39
     4. RP Discovery. . . . . . . . . . . . . . . . . . . . . .  39
     5. Security Considerations . . . . . . . . . . . . . . . .  39
      5.1. Attacks Based on Forged Messages . . . . . . . . . .  39



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       5.1.1. Election of an Incorrect DF . . . . . . . . . . .  40
       5.1.2. Preventing Election Convergence . . . . . . . . .  41
      5.2. Non-cryptographic Authentication Mechanisms. . . . .  41
       5.2.1. Basic Access Control. . . . . . . . . . . . . . .  41
      5.3. Authentication Using IPsec . . . . . . . . . . . . .  41
      5.4. Denial of Service Attacks. . . . . . . . . . . . . .  41
     6. Change history. . . . . . . . . . . . . . . . . . . . .  42
     7. Acknowledgments . . . . . . . . . . . . . . . . . . . .  42
     8. Authors' Addresses. . . . . . . . . . . . . . . . . . .  42
     9. Normative References. . . . . . . . . . . . . . . . . .  43
     10. Informative References . . . . . . . . . . . . . . . .  43
     11. Index. . . . . . . . . . . . . . . . . . . . . . . . .  45







































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1.  Introduction

This document specifies Bi-directional PIM (BIDIR-PIM), a variant of PIM
Sparse-Mode (PIM-SM) [4] that builds bi-directional shared trees
connecting multicast sources and receivers.

PIM-SM constructs uni-directional shared trees that are used to forward
data from senders to receivers of a multicast group.  PIM-SM also allows
the construction of source specific trees, but this capability is not
related to the protocol described in this document.

The shared tree for each multicast group is rooted at a multicast router
called the Rendezvous Point (RP). Different multicast groups can use
separate RPs within a PIM domain.

In unidirectional PIM-SM, there are two possible methods for
distributing data packets on the shared tree. These differ in the way
packets are forwarded from a source to the RP:

o Initially when a source starts transmitting, its first hop router
  encapsulates data packets in special control messages (Registers)
  which are unicast to the RP. After reaching the RP the packets are
  decapsulated and distributed on the shared tree.

o A transition from the above distribution mode can be made at a later
  stage.  This is achieved by building source specific state on all
  routers along the path between the source and the RP.  This state is
  then used to natively forward packets from that source.

Both these mechanisms suffer from problems. Encapsulation results in
significant processing, bandwidth and delay overheads. Forwarding using
source specific state has additional protocol and memory requirements.

Bi-directional PIM dispenses with both encapsulation and source state by
allowing packets to be natively forwarded from a source to the RP using
shared tree state. In contrast to PIM-SM this mode of forwarding does
not require any data-driven events.

The protocol specification in this document assumes familiarity with the
PIM-SM specification in [4]. Portions of the BIDIR-PIM protocol
operation that are identical to that of PIM-SM are only defined by
reference.


2.  Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and



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"OPTIONAL" are to be interpreted as described in RFC 2119 and indicate
requirement levels for compliant BIDIR-PIM implementations.

2.1.  Definitions

This specification uses a number of terms to refer to the roles of
routers participating in BIDIR-PIM.  The following terms have special
significance for BIDIR-PIM:

MRIB  Multicast Routing Information Base.  This is the multicast
      topology table, which is typically derived from the unicast
      routing table, or routing protocols such as MBGP that carry
      multicast-specific topology information. It is used by PIM for
      establishing the RPF interface (used in the forwarding rules). In
      PIM-SM the MRIB is also used to make decisions regarding where to
      forward Join/Prune messages whereas in BIDIR-PIM it is used as a
      source for routing metrics for the DF election process.

Rendezvous Point Address (RPA):
      An RPA is an address that has been configured to be used as the
      root of the distribution tree for a range of multicast groups. The
      RPA must be routable from all routers in the PIM domain. The RPA
      does not need to correspond to an address for an interface of a
      real router. In this respect BIDIR-PIM differs from PIM-SM that
      requires an actual router to be configured as the Rendezvous Point
      (RP). Join messages from receivers for a BIDIR-PIM group propagate
      hop-by-hop towards the RPA.

Rendezvous Point Link (RPL):
      An RPL for a particular RPA is the physical link to which the RPA
      belongs. In BIDIR-PIM all multicast traffic to groups mapping to a
      specific RPA is forwarded on the RPL of that RPA. The RPL is
      special within a BIDIR-PIM domain as it is the only link on which
      a Designated Forwarder election does not take place (see DF
      definition below).

Upstream
      Towards the root (RPA) of the tree. The direction used by packets
      traveling from sources to the RPL.

Downstream
      Away from the root of the tree. The direction on which packets
      travel from the RPL to receivers.

Designated Forwarder (DF):
      The protocol presented in this document is largely based on the
      concept of a Designated Forwarder (DF). A single DF exists for
      each RPA on every link within a BIDIR-PIM domain (this includes



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      both multi-access and point-to-point links). The only exception is
      the RPL on which no DF exists. The DF is the router on the link
      with the best route to the RPA (determined by comparing MRIB
      provided metrics). A DF for a given RPA is in charge of forwarding
      downstream traffic onto its link, and forwarding upstream traffic
      from its link towards the RPL.  It does this for all the bi-
      directional groups that map to the RPA.  The DF on a link is also
      responsible for processing Join messages from downstream routers
      on the link as well as ensuring that packets are forwarded to
      local receivers (discovered through a local membership mechanism
      such as MLD [3] or IGMP [2]).

RPF Interface
      RPF stands for "Reverse Path Forwarding".  The RPF Interface of a
      router with respect to an address is the interface that the MRIB
      indicates should be used to reach that address.  In the case of a
      BIDIR-PIM multicast group, the RPF interface is determined by
      looking up the RPA in the MRIB. The RPF information determines the
      interface of the router that would be used to send packets towards
      the RPL for the group.

RPF Neighbor
      The RPF Neighbor of a router with respect to an address is the
      neighbor that the MRIB indicates should be used to reach that
      address. Note that in BIDIR-PIM, the RPF neighbor for a group is
      not necessarily the router on the RPF interface that Join messages
      for that group would be directed to (Join messages are only
      directed to the DF on the RPF interface for the group).

TIB   Tree Information Base.  This is the collection of state at a PIM
      router that has been created by receiving PIM Join/Prune messages,
      PIM DF election messages and IGMP or MLD information from local
      hosts.  It essentially stores the state of all multicast
      distribution trees at that router.

MFIB  Multicast Forwarding Information Base.  The TIB holds all the
      state that is necessary to forward multicast packets at a router.
      However, although this specification defines forwarding in terms
      of the TIB, to actually forward packets using the TIB is very
      inefficient.  Instead a real router implementation will normally
      build an efficient MFIB from the TIB state to perform forwarding.
      How this is done is implementation-specific, and is not discussed
      in this document.








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2.2.  Pseudocode Notation

We use set notation in several places in this specification.

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

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

NULL
    is the empty set or list.

In addition we use C-like syntax:

=   denotes assignment of a variable.

==  denotes a comparison for equality.

!=  denotes a comparison for inequality.

Braces { and } are used for grouping.

3.  Protocol Specification

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

o Section 3.1 details the protocol state stored.

o Section 3.2 defines the BIDIR-PIM extensions to the PIM-SM [4]
  neighbour discovery mechanism.

o Section 3.3 specifies the data packet forwarding rules.

o Section 3.4 specifies the BIDIR-PIM Join/Prune generation and
  processing rules.

o Designated Forwarder (DF) election is specified in Section 3.5.

o PIM packet formats are specified in Section 3.7.

o A summary of BIDIR-PIM timers and their default values is given in
  Section 3.6.








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3.1.  BIDIR-PIM Protocol State

This section specifies all the protocol state that a BIDIR-PIM
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 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 BIDIR-PIM 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 BIDIR-PIM implementation is
free to hold whatever internal state it requires, and will still be
conformant with this specification so long as it results in the same
externally visible protocol behavior as an abstract router that holds
the following state.

We divide TIB state into two sections:

RPA state
     State that maintains the DF election information for each RPA.

Group state
     State that maintains a group-specific tree for groups that map to a
     given RPA.

The state that should be kept is described below.  Of course,
implementations will only maintain state when it is relevant to
forwarding operations - for example, the "NoInfo" state might be assumed
from the lack of other state information, rather than being held
explicitly.

3.1.1.  General Purpose State

A router holds the following state that is not specific to a RPA or
group:

     Neighbor State:

          For each neighbor:

               o Neighbor's Gen ID.





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               o Neighbor liveness timer (NLT)

               o Other information from neighbor's Hello

For more information on Hello information look at section 3.2 as well as
the PIM-SM specification in [4].

3.1.2.  RPA State

A router maintains a multicast-group to RPA mapping which is built
through static configuration or by using an automatic RP discovery
mechanism like BSR or AUTO-RP (see section 4). For each BIDIR-PIM RPA a
router holds the following state:

     o RPA (actual address)

     Designated Forwarder (DF) State:

            For each router interface:

            Acting DF information:

                 o DF IP Address

                 o DF metric

            Election information:

                 o Election State

                 o DF election-Timer (DFT)

                 o Message-Count (MC)

                   Current best offer:

                   o IP address of best offering router

                   o Best offering router metric

Designated Forwarder state is described in section 3.5.


3.1.3.  Group State

For every group G a router keeps the following state:





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          Group state:

               For each interface:

               Local Membership:

                    o State: One of {"NoInfo", "Include"}

               PIM Join/Prune State:

                    o State: One of {"NoInfo" (NI), "Join" (J),
                      "PrunePending" (PP)}

                    o Prune Pending Timer (PPT)

                    o Join/Prune Expiry Timer (ET)

          Not interface specific:

               o Upstream Join/Prune Timer (JT)

               o Last RPA Used

Local membership is the result of the local membership mechanism (such
as IGMP [2]) running on that interface. This information is used by the
pim_include(*,G) macro described in section 3.1.4.

PIM Join/Prune state is the result of receiving PIM (*,G) Join/Prune
messages on this interface, and is specified in section 3.4.1. The state
is used by the macros that calculate the outgoing interface list in
section 3.1.4, and in the JoinDesired(G) macro (defined in section
3.4.2) that is used in deciding whether a Join(*,G) should be sent
upstream.

The upstream Join/Prune timer is used to send out periodic Join(*,G)
messages, and to override Prune(*,G) messages from peers on an upstream
LAN interface.

The last RPA used must be stored because if the group to RPA mapping
changes (see RP Set changes in [4]) then state must be torn down and
rebuilt for groups whose RPA changes.



3.1.4.  State Summarization Macros

Using this state, we define the following "macro" definitions which we
will use in the descriptions of the state machines and pseudocode in the



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following sections.


olist(G) =
    RPF_interface(RPA(G)) (+) joins(G) (+) pim_include(G)


RPF_interface(RPA) is the interface the MRIB indicates would be used to
route packets to RPA. The olist(G) is the list of interfaces on which
packets to group G must be forwarded.

The macro pim_include(G) indicates the interfaces to which traffic might
be forwarded because of hosts that are local members on that interface.


pim_include(G) =
    { all interfaces I such that:
      I_am_DF(RPA(G),I) AND  local_receiver_include(G,I) }


The clause "I_am_DF(RPA,I)" is TRUE if the router is in the Win or
Backoff states in the DF election state machine (described in section
3.5) for the given RPA on interface I. Otherwise it is FALSE.

The clause "local_receiver_include(G,I)" is true if the IGMP module, MLD
module or other local membership mechanism has determined that there are
local members on interface I that desire to receive traffic sent to
group G.

The set "joins(G)" is the set of all interfaces on which the router has
received (*,G) Joins:

joins(G) =
    { all interfaces I such that
      I_am_DF(RPA(G),I) AND
      DownstreamJPState(G,I) is either Joined or PrunePending }

DownstreamJPState(G,I) is the state of the finite state machine in
section 3.4.1.

RPF_DF(RPA) is the neighbor that Join messages must be sent to in order
to build the group shared tree rooted at the RPL for the given RPA. This
is the Designated-Forwarder on the RPF_interface(RPA).

3.2.  PIM Neighbor Discovery

PIM routers exchange PIM-Hello messages with their neighboring PIM
routers. These messages are used to update the Neighbor State described



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in section 3.1. The procedures for generating and processing Hello
messages as well as maintaining Neighbor State are specified in the PIM-
SM [4] documentation.

Bidir PIM introduces the Bidir_Capable PIM-Hello option that MUST be
included in all Hello messages from a Bidir-PIM capable router.  The
Bidir_Capable option advertises the router's ability to participate in
the Bidir-PIM protocol. The format of the Bidir_Capable option is
described in section 3.7.

If a Bidir PIM router receives a PIM-Hello message that does not contain
the Bidir_Capable option from one of its neighbours, the error must be
logged to the router administrator in a rate-limited manner.

3.3.  Data Packet Forwarding Rules

For groups mapping to a given RPA, the following responsibilities are
uniquely assigned to the DF for that RPA on each link:

o The DF is the only router that forwards packets traveling downstream
  onto the link.

o The DF is the only router that picks-up upstream traveling packets off
  the link to forward towards the RPL.

Non-DF routers on a link, that use that link as their RPF interface to
reach the RPA, may perform the following forwarding actions for
bidirectional groups:

o Forward packets from the link towards downstream receivers.

o Forward packets from downstream sources onto the link (provided they
  are the DF for the downstream link from which the packet was picked-
  up).

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

     iif is the incoming interface of the packet.
     G is the destination address of the packet (group address).
     RPA is the Rendezvous Point Address for this group.

First we check to see whether the packet should be accepted based on TIB
state and the interface that the packet arrived on. A packet is accepted
if it arrives on the RPF_interface to reach the RPA (downstream
traveling packet) or if the router is the DF on the interface the packet
arrives (upstream traveling packet).





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If the packet should be forwarded we build an outgoing interface list
for the packet.

Finally we remove the incoming interface from the outgoing interface
list we've created, and if the resulting outgoing interface list is not
empty, we forward the packet out of those interfaces.

On receipt on a data to G on interface iif:

 if( iif == RPF_interface(RPA) || I_am_DF(RPA,I) ) {
    oiflist = olist(G) (-) iif
    forward packet on all interfaces in oiflist
 }




3.3.1.  Upstream Forwarding at RP

When configuring a BIDIR-PIM domain it is possible to assign the
Rendezvous Point Address (RPA) such that it does not belong to a
physical box but instead is simply a routable address. Routers that have
interfaces on the RPL that the RPA belongs to will upstream forward
traffic onto the link. Joins from receivers in the domain will propagate
hop-by-hop till they reach one of the routers connected to the RPL where
they will terminate (as there will be no DF elected on the RPL).

If instead the administrator chooses to configure the RPA to be the
address of an interface of a specific router then nothing changes. That
router must still upstream forward traffic on to the RPL and behave no
differently than any other router with an interface on the RPL.

To configure a BIDIR-PIM network to operate in a mode similar to that of
PIM-SM where a single router (the RP) is acting as the root of the
distribution tree, the RPA address can be configured to be the loopback
interface of a router.


3.3.2.  Source-Only Branches

Source-only branches of the distribution tree for a group G are branches
which do not lead to any receivers, but which are used to forward
packets traveling upstream from sources towards the RPL.  Routers along
source-only branches only have the RPF_interface to the RPA in their
olist for G and hence do not need to maintain any group specific state.
Upstream forwarding can be performed using only RPA specific state. An
implementation may decide to maintain group state for source-only
branches for accounting or performance reasons.  However, doing so



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requires data-driven events thus sacrificing one of tha main benefits of
Bidir PIM.


3.3.3.  Directly Connected Sources

 A major advantage of using a Designated Forwarder in BIDIR-PIM compared
to PIM-SM is that special treatment is no longer required for sources
that are directly connected to a router. Data from such sources does not
need to be differentiated from other multicast traffic and will
automatically be picked up by the DF and forwarded upstream. This
removes the need for performing a directly-connected-source check for
data to groups that do not have existing state.

3.4.  PIM Join/Prune Messages

BIDIR-PIM Join/Prune messages are used to construct group specific
distribution trees between receivers and the RPL. Joins are originated
by last-hop routers that are elected as the DF on an interface with
directly connected receivers. The Joins propagate hop-by-hop towards the
RPA till they reach a router connected to the RPL.

A BIDIR-PIM Join/Prune message consists of a list of Joined and Pruned
Groups. When processing a received Join/Prune message, each Joined or
Pruned Group is effectively considered individually by applying the
following state machines.  When considering a Join/Prune message whose
PIM Destination field addresses this router, (*,G) Joins and Prunes can
affect the downstream state machine.  When considering a Join/Prune
message whose PIM Destination field addresses another router, most Join
or Prune entries could affect the upstream state machine.


3.4.1.  Receiving (*,G) Join/Prune Messages

When a router receives a Join(*,G) or Prune(*,G) it must first check to
see whether the RP address in the message matches RPA(G) (the router's
idea of what the Rendezvous Point Address is). If the RP address in the
message does not match RPA(G) the Join or Prune MUST be silently
dropped.

The per-interface state-machine for receiving (*,G) Join/Prune Messages
is given below. There are three states:

     NoInfo (NI)
          The interface has no (*,G) Join state and no timers running.

     Join (J)
          The interface has (*,G) Join state. If the router is the DF on



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          this interface (I_am_DF(RPA(G),I) is TRUE), the Join state
          will cause us to forward packets destined for G on this
          interface.

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

In addition the state-machine uses two timers:

     ExpiryTimer (ET)
          This timer is restarted when a valid Join(*,G) is received.
          Expiry of the ExpiryTimer causes the interface state to revert
          to NoInfo for this group.

     PrunePendingTimer (PPT)
          This timer is set when a valid Prune(*,G) is received.  Expiry
          of the PrunePendingTimer causes the interface state to revert
          to NoInfo for this group.

                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

           Figure 1: Downstream group per-interface state-machine























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In tabular form, the group per-interface state-machine is:

+----------+------------------------------------------------------------+
|          |                     Event                                  |
|          +----------+------------+-----------+------------+-----------+
Prev State |Receive   |Receive     |Prune      |Expiry      Stop Being  |
|          |Join(*,G) |Prune(*,G)  |Pending    |Timer       DF on I     |
|          |          |            |Timer      |Expires     |           |
|          |          |            |Expires    |            |           |
+----------+----------+------------+-----------+------------+-----------+
|          |-> J state|-           |-          |-           +           |
NoInfo     |start     |            |           |            |           |
(NI)       |Expiry    |            |           |            |           |
|          |Timer     |            |           |            |           |
+----------+----------+------------+-----------+------------+-----------+
|          |-> J state|-> PP state |-          |-> NI state +> NI state |
Join (J)   |restart   |start Prune |           |            |           |
|          |Expiry    |Pending     |           |            |           |
|          |Timer     |Timer       |           |            |           |
+----------+----------+------------+-----------+------------+-----------+
|          |-> J state|-> PP state |-> NI state|-> NI state +> NI state |
|          |restart   |            |Send Prune-|            |           |
Prune      |Expiry    |            |Echo(*,G)  |            |           |
Pending    |Timer;    |            |           |            |           |
(PP)       |stop Prune|            |           |            |           |
|          |Pending   |            |           |            |           |
|          |Timer     |            |           |            |           |
+----------+----------+------------+-----------+------------+-----------+

The transition events "Receive Join(*,G)" and "Receive Prune(*,G)" imply
receiving a Join or Prune targeted to this router's address on the
received interface.  If the destination address is not correct, these
state transitions in this state machine must not occur, although seeing
such a packet may cause state transitions in other state machines.

On unnumbered interfaces on point-to-point links, the router's address
should be the same as the source address it chose for the hello packet
it sent over that interface.  However on point-to-point links we also
recommend that PIM messages with a destination address of all zeros are
also accepted.

The transition event "Stop being DF" implies a DF re-election taking
place on this router interface for RPA(G) and the router changing status
from being the active DF to being a non-DF router (the value of the
I_am_DF macro changing to FALSE).

When ExpiryTimer is started or restarted, it is set to the HoldTime from
the triggering received Join/Prune message.



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When PrunePendingTimer is started, it is set to the
J/P_Override_Interval if the router has more than one neighbor on that
interface; otherwise it is set to zero causing it to expire immediately.

The action "Send PruneEcho(*,G)" is triggered when the router stops
forwarding on an interface as a result of a prune.  A PruneEcho(*,G) is
simply a Prune(*,G) message sent by the upstream router to itself on a
LAN.  Its purpose is to add additional reliability so that if a Prune
that should have been overridden by another router is lost locally on
the LAN, then the PruneEcho may be received and cause the override to
happen.  A PruneEcho(*,G) need not be sent when the router has only one
neighbour on the link.


3.4.2.  Sending Join/Prune Messages

The downstream per-interface state-machines described above hold join
state from downstream PIM routers. This state then determines whether a
router needs to propagate a Join(*,G) upstream towards the RPA.  Such
Join(*,G) messages are sent on the RPF_interface towards the RPA and are
targeted at the DF on that interface.

If a router wishes to propagate a Join(*,G) upstream, it must also watch
for messages on its upstream interface from other routers on that
subnet, and these may modify its behavior.  If it sees a Join(*,G) to
the correct upstream neighbor, it should suppress its own Join(*,G).  If
it sees a Prune(*,G) to the correct upstream neighbor, it should be
prepared to override that prune by sending a Join(*,G) almost
immediately.  Finally, if it sees the Generation ID (see PIM-SM
specification [4]) of the correct upstream neighbor change, it knows
that the upstream neighbor has lost state, and it should be prepared to
refresh the state by sending a Join(*,G) almost immediately.

In addition changes in the next hop towards the RPA trigger a prune off
from the old next hop, and join towards the new next hop. Such a change
can be caused by the following two events:

     o The MRIB indicates that the RPF Interface towards the RPA has
       changed.  In this case the DF on the new RPF_interface becomes
       the new RPF Neighbour.

     o There is a DF re-election on the RPF_interface and a new router
       emerges as the DF.

The upstream (*,G) state-machine only contains two states:

Not Joined
     The downstream state-machines indicate that the router does not



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     need to join the RPA tree for this group.

Joined
     The downstream state-machines indicate that the router would like
     to join the RPA tree for this group.

In addition, one timer JT(G) is kept which is used to trigger the
sending of a Join(*,G) to the upstream next hop towards the RPA (the DF
on the RPF_interface for RPA(G)).

                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

                   Figure 2: Upstream group state-machine




































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In tabular form, the state machine is:

+----------------------+------------------------------------------------+
|                      |                     Event                      |
|  Prev State          +------------------------+-----------------------+
|                      |    JoinDesired(G)      |    JoinDesired(G)     |
|                      |    ->True              |    ->False            |
+----------------------+------------------------+-----------------------+
|                      |    -> J state          |    -                  |
|  NotJoined (NJ)      |    Send Join(*,G);     |                       |
|                      |    Set Timer to        |                       |
|                      |    t_periodic          |                       |
+----------------------+------------------------+-----------------------+
|  Joined (J)          |    -                   |    -> NJ state        |
|                      |                        |    Send Prune(*,G)    |
+----------------------+------------------------+-----------------------+

In addition, we have the following transitions which occur within the
Joined state:

+-----------------------------------------------------------------------+
|                         In Joined (J) State                           |
+-----------------+-----------------+-----------------+-----------------+
|Timer Expires    | See Join(*,G)   | See Prune(*,G)  | RPF_DF(RPA(G))  |
|                 | to              | to              | changes         |
|                 | RPF_DF(RPA(G))  | RPF_DF(RPA(G))  |                 |
+-----------------+-----------------+-----------------+-----------------+
|Send             | Increase Timer  | Decrease Timer  | Decrease Timer  |
|Join(*,G); Set   | to              | to t_override   | to t_override   |
|Timer to         | t_suppressed    |                 |                 |
|t_periodic       |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

+-----------------------------------------------------------------------+
|                         In Joined (J) State                           |
+-------------------------------------+---------------------------------+
|     Change of RPF_DF(RPA(G))        |       RPF_DF(RPA(G)) GenID      |
|                                     |       changes                   |
+-------------------------------------+---------------------------------+
|     Send Join(*,G) to new           |       Decrease Timer to         |
|     DF; Send Prune(*,G) to          |       t_override                |
|     old DF; set Timer to            |                                 |
|     t_periodic                      |                                 |
+-------------------------------------+---------------------------------+







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This state machine uses the following macro:

  bool JoinDesired(G) {
     if (olist(G) (-) RPF_interface(RPA(G))) != NULL
         return TRUE
     else
         return FALSE
  }


3.5.  Designated Forwarder (DF) Election

This section presents a fail-safe mechanism for electing a per-RPA
designated router on each link in a BIDIR-PIM domain. We call this
router the Designated Forwarder (DF). The DF election does not take
place on the RPL for a RPA.


3.5.1.  DF Requirements

The DF election chooses the best router on a link to assume the
responsibility of forwarding traffic between the RPL and the link for
the range of multicast groups served by the RPA. Different multicast
groups that share a common RPA share the same upstream direction.
Hence, the election of an upstream forwarder on each link does not have
to be a group specific decision but instead can be RPA-specific. As the
number of RPAs is typically small, the number of elections that have to
be performed is significantly reduced by this observation.

To optimise tree creation, it is desirable that the winner of the
election process should be the router on the link with the "best"
unicast routing metric (as reported by the MRIB) to reach the RPA.  When
comparing metrics from different unicast routing protocols, we use the
same comparison rules used by the PIM-SM assert process [4].

The election process needs to take place when information on a new RPA
initially becomes available. The result can be re-used as new bidir
groups that map to the same RPA are encountered. There are however some
conditions under which an update to the election is required:

     o There is a change in unicast metric to reach the RPA for any of
       the routers on the link.

     o The interface on which the RPA is reachable (RPF Interface)
       changes to an interface for which the router was previously the
       DF.





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     o A new PIM neighbor starts up on a link that must participate in
       the elections and be informed of current outcome.

     o The elected DF fails (detected through neighbor information
       timeout or MRIB RPF change at downstream router).

The election process has to be robust enough to ensure with very high
probability that all routers on the link have a consistent view of the
DF. This is because with the forwarding rules described in section 3.3
if multiple routers end-up thinking that they should be responsible for
forwarding, loops may result. To reduce the possibility of this
occurrence to a minimum, the election algorithm has been biased towards
discarding DF information and suspending forwarding during periods of
ambiguity.


3.5.2.  DF Election description

This section gives an outline of the DF election process. It does not
provide the definitive specification for the DF election. If any
discrepancy exists between section 3.5.3 and this section, the
specification in section 3.5.3 is to be assumed correct.

To perform the election of the DF for a particular RPA, routers on a
link need to exchange their unicast routing metric information for
reaching the RPA. Routers advertise their own metrics in Offer, Winner,
Backoff and Pass messages. The advertised metric is calculated using the
RPF Interface and metric to reach the RPA available through the MRIB.
When a router is participating in a DF election for an RPA on the
interface that its MRIB indicates as the RPF Interface then that router
MUST always advertise an infinite metric in its election messages. When
a router is participating in a DF election on an interface other than
the MRIB indicated RPF Interface then it MUST advertise the MRIB
provided metrics in its election messages.

In the election protocol described below, many message exchanges are
repeated Election_Robustness times for reliability. In all those cases
the message retransmissions are spaced in time by a small random
interval. All of the following description is specific to the election
on a single link for a single RPA.


3.5.2.1.  Bootstrap Election

Initially when no DF has been elected, routers finding out about a new
RPA start participating in the election by sending Offer messages.
Offer messages include the router's metric to reach the RPA. Offers are
periodically retransmitted with a period of Offer_Interval.



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If a router hears a better offer than its own from a neighbor, it stops
participating in the election for a period of Election_Robustness *
Offer_Interval thus giving a chance to the neighbour with the better
metric to be elected DF. If during this period no winner is elected, the
router restarts the election from the beginning. If at any point during
the initial election a router receives an out of order offer with worse
metrics than its own, then it restarts the election from the beginning.

The result should be that all routers except the best candidate stop
advertising their offers.

A router assumes the role of the DF after having advertised its metrics
Election_Robustness times without receiving any offer from any other
neighbor. At that point it transmits a Winner message which declares to
every other router on the link the identity of the winner and the
metrics it is using.

Routers receiving a winner message stop participating in the election
and record the identity and metrics of the winner. If the local metrics
are better than those of the winner then the router records the identity
of the winner (accepting it as the acting DF) but re-initiates the
election to try and take over.


3.5.2.2.  Loser Metric Changes

Whenever the unicast metric to a RPA changes at a non-DF router to a
value that is better than that previously advertised by the acting DF,
the router with the new better metric should take action to eventually
assume forwarding responsibility. When the metric change is detected,
the non-DF router with the now better metric restarts the DF election
process by sending Offer messages with this new metric.  Note that at
any point during an election if no response is received after
Election_Robustness retransmissions of an offer, a router assumes the
role of the DF following the usual Winner announcement procedure.

Upon receipt of an offer that is worse than its current metric, the DF
will respond with a Winner message declaring its status and advertising
its better metric. Upon receiving the Winner message, the originator of
the Offer records the identity of the DF and aborts the election.

Upon receipt of an offer that is better than its current metric, the DF
records the identity and metrics of the offering router and responds
with a Backoff message. This instructs the offering router to hold off
for a short period of time while the unicast routing stabilises and
other routers get a chance to put in their offers. The Backoff message
includes the offering router's new metric and address.  All routers on
the link that have pending offers with metrics worse than those in the



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backoff message (including the original offering router) will hold
further offers for a period of time defined in the Backoff message.

If during the Backoff_Period, a third router sends a new better offer,
the Backoff message is repeated for the new offer and the Backoff_Period
restarted.

Before the Backoff_Period expires, the acting DF nominates the router
having made the best offer as the new DF using a Pass message.  This
message includes the IDs and metrics of both the old and new DFs.  The
old DF stops performing its tasks at the time the Pass message
transmission is made. The new DF assumes the role of the DF as soon as
it receives the Pass message. All other routers on the link take note of
the new DF and its metric. Note that this event constitutes an RPF
Neighbour change which may trigger Join messages to the new DF (see
section 3.4).


3.5.2.3.  Winner Metric Changes

If the DF's routing metric to reach the RPA changes to a worse value, it
sends a set of Election_Robustness randomly spaced Winner messages on
the link, advertising the new metric. Routers that receive this
announcement but have a better metric may respond with an Offer message
which results in the same handoff procedure described above.  All
routers assume the DF has not changed until they see a Pass or Winner
message indicating the change.

There is no pressure to make this handoff quickly if the acting DF still
has a path to the RPL. The old path may now be suboptimal but it will
still work while the re-election is in progress.

If the routing metric at the DF changes to a better value, a single
Winner message is sent advertising the new metric.


3.5.2.4.  Winner Loses Path

If a router's RPF Interface to the RPA switches to be on a link for
which it is acting as the DF, then it can no longer provide forwarding
services for that link. It therefore immediately stops being the DF and
restarts the election. As its path to the RPA is through the link, an
infinite metric is used in the Offer message it sends.

Note: At this stage the old DF will have a hint at a possible RPF
neighbor on the link indicated by the new MRIB next-hop. The old DF
could use this next-hop hint in a Pass message but this adds unnecessary
complication to the election process.



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3.5.2.5.  Late Router Starting Up

A late router starting up after the DF election process has completed
will have no immediate knowledge of the election outcome. As a result,
it will start advertising its metric in Offer messages. As soon as this
happens, the currently elected DF will respond with a Winner message if
its metric is better than the metric in the Offer message, or with a
Backoff message if its metric worse than the metric in the Offer
message.


3.5.2.6.  Winner Dies

Whenever the DF dies, a new DF has to be elected. The speed at which
this can be achieved depends on whether there are any downstream routers
on the link.

If there are downstream routers, typically their MRIB reported next-hop
before the DF dies will be the DF itself. They will therefore notice
either a change in the metric for the route to the RPA or a change in
next-hop away from the DF and can restart the election by transmitting
Offer messages. If according to the MRIB the RPA is now reachable
through the same link via another upstream router, an infinite metric
will be used in the Offer.

If no downstream routers are present, the only way for other upstream
routers to detect a DF failure is by the timeout of the PIM neighbor
information, which will take significantly longer.


3.5.3.  Election Protocol Specification

This section provides the definitive specification for the DF election
process. If any discrepancy exists between section 3.5.2 and this
section, the specification in this section is to be assumed correct.


3.5.3.1.  Election State

The DF election state is maintained per RPA for each multicast enabled
interface I on the router as introduced in section 3.1.

The state machine has the following four states:

     Offer
          Initial election state. When in the Offer state a router
          thinks it can eventually become the winner and periodically
          generates Offer messages.



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     Lose In this state the router knows that there either is a
          different election winner or that no router on the link has a
          path to the RP.

     Winner
          The router is the acting DF without any contest.

     Backoff
          The router is the acting DF but another router has made a bid
          to take over.

In the state machine a router is considered to be an acting DF if it is
in the Win or Backoff states.

The operation of the election protocol makes use of the variables and
timers described below:

     Acting DF information
          Used to store the identity and advertised metrics of the
          election winner that is the currently acting DF.

     DF election-Timer (DFT)
          Used to schedule transmission of Offer, Winner and Pass
          messages.

     Message-Count (MC)
          Used to maintain the number of times an Offer or Winner
          message has been transmitted.

     Best-Offer
          Used by the DF to record the identity and advertised metrics
          of the router has made the last offer for use when sending the
          Pass message.


3.5.3.2.  Election Messages

The election process uses the following PIM control messages the packet
format of which is described in section 3.7:

     Offer (OfferingID, Metric)
          Sent by routers that believe they have a better metric to the
          RPA than the metric that has been on offer so far.

     Winner (DF-ID, DF-Metric)
          Sent by a router when assuming the role of the DF or when re-
          asserting in response to worse offers.




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     Backoff (DF-ID, DF-Metric, OfferingID, OfferMetric,
          BackoffInterval)
          Used by the DF to acknowledge better offers. It instructs
          other routers with equal or worse offers to wait till the DF
          passes responsibility to the sender of the offer.

     Pass (Old-DF-ID, Old-DF-Metric, New-DF-ID, New-DF-Metric)
          Used by the old DF to pass forwarding responsibility to a
          router that has previously made an offer.  The Old-DF-Metric
          is the current metric of the DF at the time the pass is sent.

Note that when a router is participating in a DF election for an RPA on
the interface that its MRIB indicates as the RPF Interface then that
router MUST always advertise an infinite metric in its election
messages. When a router is participating in a DF election on an
interface other than the MRIB indicated RPF Interface then it MUST
advertise the MRIB provided metrics in its election messages.


3.5.3.3.  Election Events

During protocol operation the following events can take place:

     Control message reception
          Reception of one of the four control DF election messages
          (Offer, Winner, Backoff and Pass). When a control message is
          received and actions are specified on a condition that metrics
          are Better or Worse the comparison must be performed as
          follows:

          o On receipt of an Offer or Winner message compare our current
            metrics for the RPA with the metrics advertised for the
            sender of the message.

          o On receipt of a Backoff or Pass message compare our current
            metrics for the RPA with the metrics advertised for the
            target of the message.

     Path to RPA lost
          Losing the path to the RPA can happen in two ways. The first
          happens when the route learned through the MRIB is withdrawn
          and the MRIB no longer reports an available route to reach the
          RPA. The second case happens when the next-hop information
          reported by the MRIB changes to indicate a next-hop that is
          reachable through the router interface under consideration.
          Clearly as the router is using the interface as its RPF
          Interface it cannot offer forwarding services towards the RPL
          to other routers on that link.



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     Metric reported by the MRIB to reach the RPA changes
          This event is triggered when the MRIB supplied information for
          the RPA changes and the new information provides a path to the
          RPA. If the new MRIB information either reports no route or
          reports a next-hop interface through the interface for which
          the DF election is taking place then the "Path to RPA lost"
          event triggers instead. In specific states the event may be
          further filtered by specifying whether it is expected of the
          metric to become better or worse and which stored metric the
          new MRIB information must be compared against. The new
          information must be compared with either the router's old
          metric, the stored DF metric or the stored Best Offer metric.

     Election-Timer (DFT) Expiration
          Expiration of the DFT election timer can cause message
          transmission and state transitions. The event might be further
          qualified by specifying the value of the Message Count (MC) as
          well as the current existence of a path to the RPA (as defined
          above).

     Detection of DF failure
          Detection of DF failure can occur through the timeout of PIM
          neighbor state.

3.5.3.4.  Election Actions

The DF election state machine action descriptions use the following
notation in addition to the pseudocode notation described earlier in
this spec.

     ?=  denotes the operation of lowering a timer to a new value. If
         the timer is not running then it is started using the new
         value. If the timer is running with an expiration lower than
         the new value, then the timer is not altered.

When an action of "set DF to Sender or Target" is encountered during
receipt of a Winner, Pass or Backoff message it means the following:

     o On receipt of a Winner message set the DF to be the originator of
       the message and record its metrics.

     o On receipt of a Pass message set the DF to be the target of the
       message and record its metrics.

     o On receipt of a Backoff message set the DF to be the originator
       of the message and record its metrics.





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3.5.3.5.  Election State Transitions

When a Designated Forwarder election is initiated the starting state is
the Offer state, the message counter (MC) is set to zero and the DF
election Timer (DFT) is set to OPlow (see section 3.6 for a definition
of timer values).


                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

            Figure 3: Designated Forwarder election state-machine


In tabular form, the state machine is:

+-------------++--------------------------------------------------------+
|             ||                         Event                          |
| Prev State  ++------------------+------------------+------------------+
|             || Recv better      |  Recv better     |  Recv better     |
|             || Pass / Win       |  Backoff         |  Offer           |
+-------------++------------------+------------------+------------------+
|             || -> Lose          |  -               |  -               |
| Offer       || DF = Sender or   |  DFT = BOperiod  |  DFT = OPhigh;   |
|             || Target; Stop     |  + OPlow; MC =   |  MC = 0          |
|             || DFT              |  0               |                  |
+-------------++------------------+------------------+------------------+
|             || -                |  -               |  -> Offer        |
| Lose        || DF = Sender or   |  DF = Sender     |  DFT = OPhigh;   |
|             || Target           |                  |  MC = 0          |
+-------------++------------------+------------------+------------------+
|             || -> Lose          |  -> Lose         |  -> Backoff      |
|             || DF = Sender or   |  DF = Sender;    |  Set Best to     |
| Win         || Target; Stop     |  Stop DFT        |  Sender; Send    |
|             || DFT              |                  |  Backoff; DFT =  |
|             ||                  |                  |  BOperiod        |
+-------------++------------------+------------------+------------------+
|             || -> Lose          |  -> Lose         |  -               |
|             || DF = Sender or   |  DF = Sender;    |  Set Best to     |
| Backoff     || Target; Stop     |  Stop DFT        |  Sender; Send    |
|             || DFT              |                  |  Backoff; DFT =  |
|             ||                  |                  |  BOperiod        |
+-------------++------------------+------------------+------------------+







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+-----------++----------------------------------------------------------+
|           ||                          Event                           |
|           ++-------------+--------------+--------------+--------------+
|Prev State ||Recv Backoff | Recv Pass    | Recv Worse   | Recv worse   |
|           ||for us       | for us       | Pass / Win / | Offer        |
|           ||             |              | Backoff      |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-            | -> Win       | -            | -            |
|           ||DFT =        | Stop DFT     | Set DF to    | DFT ?=       |
|Offer      ||BOperiod +   |              | Sender or    | OPlow; MC =  |
|           ||OPlow; MC =  |              | Target; DFT  | 0            |
|           ||0            |              | ?= OPlow; MC |              |
|           ||             |              | = 0          |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -> Offer     |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | DFT = OPlow; |
|Lose       ||DFT = OPlow; | DFT = OPlow; | or Target;   | MC = 0       |
|           ||MC = 0       | MC = 0       | DFT = OPlow; |              |
|           ||             |              | MC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -            |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | Send Winner  |
|Win        ||DFT = OPlow; | DFT = OPlow; | or Target;   |              |
|           ||MC = 0       | MC = 0       | DFT = OPlow; |              |
|           ||             |              | MC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -> Win       |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | Send Winner; |
|Backoff    ||DFT = OPlow; | DFT = OPlow; | or Target;   | Stop DFT     |
|           ||MC = 0       | MC = 0       | DFT = OPlow; |              |
|           ||             |              | MC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+


+-----------------------------------------------------------------------+
|                            In Offer State                             |
+-----------------------+-----------------------+-----------------------+
| DFT Expires and MC    |  DFT Expires and MC   |   DFT Expires and MC  |
| is less than          |  is equal to          |   is equal to         |
| Robustness            |  Robustness and we    |   Robustness and      |
|                       |  have path to RPA     |   there is no path    |
|                       |                       |   to RPA              |
+-----------------------+-----------------------+-----------------------+
| -                     |  -> Win               |   -> Lose             |
| Send Offer; DFT =     |  Send Winner          |   Set DF to None      |
| OPlow; MC = MC + 1    |                       |                       |
+-----------------------+-----------------------+-----------------------+




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+-----------------------------------------------------------------------+
|                           In Offer State                              |
+-----------------------------------------------------------------------+
|                   Metric changes and is now worse                     |
+-----------------------------------------------------------------------+
|                   DFT ?= OPlow                                        |
|                   MC = 0                                              |
+-----------------------------------------------------------------------+


+-----------------------------------------------------------------------+
|                            In Lose State                              |
+--------------------------------+--------------------------------------+
|     Detect DF Failure          |        Metric changes and now        |
|                                |        is better than DF             |
+--------------------------------+--------------------------------------+
|     -> Offer                   |        -> Offer                      |
|     DF = None; DFT =           |        DFT = OPlow_int; MC = 0       |
|     OPlow_int; MC = 0          |                                      |
+--------------------------------+--------------------------------------+


+-----------------------------------------------------------------------+
|                             In Win State                              |
+-----------------------+------------------------+----------------------+
| Metric changes and    |  Timer Expires and     |   Path to RPA lost   |
| is now worse          |  MC is less than       |                      |
|                       |  Robustness            |                      |
+-----------------------+------------------------+----------------------+
| -                     |  -                     |   -> Offer           |
| DFT = OPlow; MC =     |  Send Winner; DFT =    |   Set DF to None;    |
| 0                     |  OPlow; MC = MC + 1    |   DFT = OPlow; MC =  |
|                       |                        |   0                  |
+-----------------------+------------------------+----------------------+


+-----------------------------------------------------------------------+
|                          In Backoff State                             |
+-----------------------+------------------------+----------------------+
| Metric changes and    |   Timer Expires        |  Path to RPA lost    |
| is now better than    |                        |                      |
| Best                  |                        |                      |
+-----------------------+------------------------+----------------------+
| -> Win                |   -> Lose              |  -> Offer            |
| Stop Timer            |   Send Pass; Set DF    |  Set DF to None;     |
|                       |   to stored Best       |  DFT = OPlow; MC =   |
|                       |                        |  0                   |
+-----------------------+------------------------+----------------------+



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3.5.4.  Election Reliability Enhancements

For the correct operation of BIDIR-PIM it is very important to avoid
situations where two routers consider themselves to be Designated
Forwarders for the same link. The two precautions below are not required
for correct operation but can help diagnose anomalies and correct them.

3.5.5.  Missing Pass

After a DF has been elected, a router whose metrics change to become
better than the DF will attempt to take over. If during the re-election
the acting DF has a condition that causes it to lose all of the election
messages (like a CPU overload), the new candidate will transmit three
offers and assume the role of the forwarder resulting in two DFs on the
link. This situation is pathological and should be corrected by fixing
the overloaded router. It is desirable that such an event can be
detected by a network administrator.

When a router becomes the DF for a link without receiving a Pass message
from the known old DF, the PIM neighbor information for the old DF can
be marked to this effect. Upon receiving the next PIM Hello message from
the old DF, the router can retransmit Winner messages for all the RPAs
for which it is acting as the DF. The anomaly may also be logged by the
router in a rate-limited manner to alert the operator.


3.5.6.  Periodic Winner Announcement

An additional degree of safety can be achieved by having the DF for each
RPA periodically announce its status in a Winner message.  Transmission
of the periodic Winner message can be restricted to occur only for RPAs
which have active groups, thus avoiding the periodic control traffic in
areas of the network without senders or receivers for a particular RPA.

3.6.  Timers Counters and Constants

BIDIR-PIM maintains the following timers, as discussed in section 3.1.
All timers are countdown timers - they are set to a value and count down
to zero, at which point they typically trigger an action.  Of course
they can just as easily be implemented as count-up timers, where the
absolute expiry time is stored and compared against a real-time clock,
but the language in this specification assumes that they count downwards
to zero.


Per Rendezvous-Point Address (RPA):





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     Per interface (I):

          DF Election Timer: DFT(RPA,I)

Per Group (G):

     Upstream Join Timer: JT(G)

     Per interface (I):

          Join Expiry Timer: ET(G,I)

          PrunePending Timer: PPT(G,I)

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



































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Timer Name: DF Election Timer (DFT)


+--------------------+-------------------------+------------------------+
|  Value Name        |  Value                  |   Explanation          |
+--------------------+-------------------------+------------------------+
|  Offer_Period      |  100 ms                 |   Interval to wait     |
|                    |                         |   between repeated     |
|                    |                         |   Offer and Winner     |
|                    |                         |   messages.            |
+--------------------+-------------------------+------------------------+
|  Backoff_Period    |  1 sec                  |   Period that acting   |
|                    |                         |   DF waits between     |
|                    |                         |   receiving a better   |
|                    |                         |   Offer and sending    |
|                    |                         |   the Pass message     |
|                    |                         |   to transfer DF       |
|                    |                         |   responsibility.      |
+--------------------+-------------------------+------------------------+
|  OPLow             |  rand(0.5, 1) *         |   Range of actual      |
|                    |  Offer_Period           |   randomised value     |
|                    |                         |   used between         |
|                    |                         |   repeated messages.   |
+--------------------+-------------------------+------------------------+
|  OPHigh            |  Election_Robustness    |   Interval to wait     |
|                    |  * Offer_Period         |   in order to give a   |
|                    |                         |   chance to a router   |
|                    |                         |   with a better        |
|                    |                         |   Offer to become      |
|                    |                         |   the DF.              |
+--------------------+-------------------------+------------------------+

Timer Names: Join Expiry Timer (ET(G,I))


+----------------+----------------+-------------------------------------+
| Value Name     | Value          |  Explanation                        |
+----------------+----------------+-------------------------------------+
| J/P HoldTime   | from message   |  Hold Time from Join/Prune Message  |
+----------------+----------------+-------------------------------------+











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Timer Names: Prune Pending Timer (PPT(G,I))


+--------------------------+--------------------+-----------------------+
| Value Name               |  Value             |   Explanation         |
+--------------------------+--------------------+-----------------------+
| J/P Override Interval    |  Default: 3 secs   |   Short period after  |
|                          |                    |   a join or prune to  |
|                          |                    |   allow other         |
|                          |                    |   routers on the LAN  |
|                          |                    |   to override the     |
|                          |                    |   join or prune       |
+--------------------------+--------------------+-----------------------+

Note that the value of the J/P Override Interval is interface specific
and depends on both the Propagation_Delay and the Override_Interval
values that may change when Hello messages are received [4].

Timer Names: Upstream Join Timer (JT(G))


+-------------+--------------------+------------------------------------+
|Value Name   |Value               |Explanation                         |
+-------------+--------------------+------------------------------------+
|t_periodic   |Default: 60 secs    |Period between Join/Prune Messages  |
+-------------+--------------------+------------------------------------+
|t_suppressed |rand(1.1 *          |Suppression period when someone     |
|             |t_periodic, 1.4 *   |else sends a J/P message so we      |
|             |t_periodic)         |don't need to do so.                |
+-------------+--------------------+------------------------------------+
|t_override   |rand(0, 0.9 * J/P   |Randomized delay to prevent         |
|             |Override Interval)  |response implosion when sending a   |
|             |                    |join message to override someone    |
|             |                    |else's prune message.               |
+-------------+--------------------+------------------------------------+

For more information about these values refer to the PIM-SM [4]
documentation.













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Constant Name: DF Election Robustness


+--------------------------+-------------------+------------------------+
|  Constant Name           |    Value          |    Explanation         |
+--------------------------+-------------------+------------------------+
|  Election_Robustness     |    Default: 3     |    Minimum number of   |
|                          |                   |    election messages   |
|                          |                   |    that must be lost   |
|                          |                   |    in order for        |
|                          |                   |    election to fail.   |
+--------------------------+-------------------+------------------------+

3.7.  BIDIR PIM Packet Formats

This section describes the details of the packet formats for BIDIR-PIM
control messages. BIDIR-PIM shares a number of control messages in
common with PIM-SM [4]. These include the Hello and Join/Prune messages
as well as the format for the Encoded-Unicast address. For details on
the format of these packets please refer to the PIM-SM documentation.
Here we will only define the additional packets that are introduced by
BIDIR-PIM.  These are the packets used in the DF election process as
well as the Bidir_Capable PIM-Hello option.

3.7.1.  DF Election Packet Formats

All PIM control messages have IP protocol number 103.

BIDIR-PIM messages are multicast with TTL 1 to the `ALL-PIM-ROUTERS'
group `224.0.0.13'.

All DF election BIDIR-PIM control messages share the common header
below:

 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  |Subtype| Rsvd  |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Encoded-Unicast-RP-Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Sender Metric Preference                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sender Metric                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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PIM Ver
     PIM Version number is 2.

Type All DF-Election PIM control messages share the PIM message Type of
     10.

Subtype
     Subtypes for DF election messages are:

               1 = Offer
               2 = Winner
               3 = Backoff
               4 = Pass


Rsvd 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 the checksum, the checksum field is zeroed.

RP-Address
     The bidir RPA for which the election is taking place (note that the
     length of this field is more than 32 bits).

Sender Metric Preference
     Preference value assigned to the unicast routing protocol that the
     message sender used to obtain the route to the RPA.

Sender Metric
     The unicast routing table metric used by the message sender to
     reach the RPA. The metric is in units applicable to the unicast
     routing protocol used.

In addition to the fields defined above the Backoff and Pass messages
have the extra fields described below.


3.7.2.  Backoff Message

The Backoff message uses the following fields in addition to the common
election message format described above.








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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Encoded-Unicast-Offering-Address                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Offering Metric Preference                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Offering Metric                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Interval           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Offering Address
     The address of the router that made the last (best) Offer (note
     that the length of this field is more than 32 bits).

Offering Metric Preference
     Preference value assigned to the unicast routing protocol that the
     offering router used to obtain the route to the RPA.

Offering Metric
     The unicast routing table metric used by the offering router to
     reach the RPA. The metric is in units applicable to the unicast
     routing protocol used.

Interval
     The backoff interval in milliseconds to be used by routers with
     worse metrics than the offering router.


3.7.3.  Pass Message

The Pass message uses the following fields in addition to the common
election fields described above.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Encoded-Unicast-New-Winner-Address               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 New Winner Metric Preference                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      New Winner Metric                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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New Winner Address
     The address of the router that made the last (best) Offer (note
     that the length of this field is more than 32 bits).

New Winner Metric Preference
     Preference value assigned to the unicast routing protocol that the
     offering router used to obtain the route to the RPA.

New Winner Metric
     The unicast routing table metric used by the offering router to
     reach the RPA. The metric is in units applicable to the unicast
     routing protocol used.

3.7.4.  Bidir Capable PIM-Hello Option

BIDIR-PIM introduces one new PIM-Hello option.

o OptionType 22: Bidir Capable

   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 = 22            |         Length = 0            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.  RP Discovery

Routers discover that a range of multicast group addresses operates in
bi-directional mode and the address of the Rendezvous-Point address
(RPA) serving the group range either through static configuration or
using an automatic RP discovery mechanism like the PIM Bootsrtap
mechanism (BSR).  [9] or Auto-RP.

5.  Security Considerations

The IPsec [5] authentication header MAY be used to provide data
integrity protection and group-wise data origin authentication of BIDIR-
PIM protocol messages. Authentication of BIDIR-PIM messages can protect
against unwanted behaviour caused by unauthorized or altered BIDIR-PIM
messages.

5.1.  Attacks Based on Forged Messages

As in PIM Sparse-Mode, the extent of possible damage depends on the type
of counterfeit messages accepted. BIDIR-PIM only uses link-local
multicast messages sent to the ALL_PIM_ROUTERS address, hence attacks
can only be carried out by directly connected nodes, or with the



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complicity of directly connected routers.

Some of the BIDIR-PIM protocol messages (Join/Prune and Hello) are
identical, both in format and functionality, to the respective messages
used in PIM-SM. Security considerations for these messages are to be
found in [4]. Other messages (DF-election messages) are specific to
BIDIR-PIM and will be discussed in the following paragraphs.

By forging DF-election messages an attacker can disrupt the election of
the Designated Forwarder on a link in two different ways:

5.1.1.  Election of an Incorrect DF

An attacker can force its election as DF by participating in a regular
election and advertising the best metric to reach the RPA.  An attacker
can also try to force the election of another router as DF by sending an
Offer, Winner or Pass message and impersonating another router. In some
cases (e.g. the Offer) multiple messages might be needed to carry out an
attack.

In the case of Offer or Winner messages the attacker will have to
impersonate the node that it wants to have become the DF. In the case of
the Pass it will have to impersonate the current DF. This type of attack
causes the wrong DF to be recorded in all nodes apart from the one that
is being impersonated. This node typically will be able to detect the
anomaly and, possibly, restart a new election.

A more sophisticated attacker might carry out a concurrent DoS attack on
the node being impersonated, so that it will not be able to detect the
forged packets and/or take countermeasures.

All attacks based on impersonation can be detected by all routers and
avoided if the source of DF-election messages can be authenticated.
When authentication is available, spoofed messages MUST be discarded and
a rate-limited warning message SHOULD be logged.

A more subtle attacker could use MAC-level addresses to partition the
set of recipients of DF-election messages and create an inconsistent DF
view on the link.  For example the attacker could use unicast MAC
addresses for its forged DF-election messages.  To prevent this type of
attack, BIDIR-PIM routers SHOULD check the destination MAC address of
received DF-election messages.  This however is ineffective on links
that do not support layer-2 multicast delivery.

Source authentication is also sufficient to prevent this kind of attack.






Handley/Kouvelas/Speakman/Vicisano             Section 5.1.1.  [Page 40]

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5.1.2.  Preventing Election Convergence

By forging DF election messages, an attacker can prevent the election
from converging thus disrupting the establishment of multicast
forwarding trees. There are many ways to achieve this. The simplest is
by sending an infinite sequence of Offer messages (the metric used in
the messages is not important).

5.2.  Non-cryptographic Authentication Mechanisms

A BIDIR-PIM router SHOULD provide an option to limit the set of
neighbors from which it will accept Join/Prune, Assert, and DF-election
messages.  Either static configuration of IP addresses or an IPsec
security association may be used.  Furthermore, a PIM router SHOULD NOT
accept protocol messages from a router from which it has not yet
received a valid Hello message.

5.2.1.  Basic Access Control

In a PIM-SM domain, when all routers are trusted, it is possible to
implement a basic form of access control for both sources and receivers:
Receivers can be validated by the last-hop DR and sources can be
validated by the first-hop DR and/or the RP.

In BIDIR-PIM this is generally feasible only for receivers, as sources
can send to the multicast group without the need for routers to detect
their activity and create source-specific state. However it is possible
to modify the standard BIDIR-PIM behaviour, in a backward compatible
way, to allow per-source access control. The tradeoff would be protocol
simplicity, memory and processing requirements.

5.3.  Authentication Using IPsec

The IPsec [5] transport mode using the Authentication Header (AH) is the
RECOMMENDED method to prevent the above attacks against BIDIR-PIM.

It is RECOMMENDED that IPsec authentication be applied to all BIDIR-PIM
protocol messages. The specification on how this is done is to be found
in [4]. specifically the authentication of PIM-SM link-local messages,
described in [4] applies to all BIDIR-PIM messages as well.

5.4.  Denial of Service Attacks

The denial of service attack based on forged Join described in [4] also
apply to BIDIR-PIM.






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6.  Change history

>From 05 to 06:

Minor editorial corrections.

>From 03 to 05:

RP concept replaced by RP Address (RPA) and RP Link (RPL). No DF
election on RPL. RP forwards upstream on RPL. Accept joins even if not
DF but do not forward. Added event description for DF election state
machine. Security considerations by Lorenzo.Removed comparison with
Dino's draft.

>From 02 to 03:

Consistency fixes in DF election tables to match state transition
diagram pointed out by Apoorva.

>From 00 to 01:

The differences between this version (-01) of the BIDIR-PIM
specification and draft-ietf-pim-bidir-new-00.txt are mostly in the
format of the information presented. As BIDIR-PIM has many similarities
in operation to Sparse-Mode PIM, the earlier version of this spec relied
heavily on the now obsolete PIM-SM [8] specification. This revision
removes this dependency and instead references the new Sparse-Mode
documentation [4] where necessary. In addition the method in which the
protocol specification is presented has been updated to follow the
format of [4].

7.  Acknowledgments

The bidir proposal in this draft is heavily based on the ideas and text
presented by Estrin and Farinacci in [7]. The main difference between
the two proposals is in the method chosen for upstream forwarding.

We would also like to thank John Zwiebel at Procket, Deborah Estrin at
ISI/USC as well as Nidhi Bhaskar, Yiqun Cai, Toerless Eckert, Apoorva
Karan, Rajitha Sumanasekera and Beau Williamson at cisco for their
contributions and comments to this draft.










Handley/Kouvelas/Speakman/Vicisano                 Section 8.  [Page 42]

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8.  Authors' Addresses

     Mark Handley
     Computer Science Department
     University College London
     M.Handley@cs.ucl.ac.uk


     Isidor Kouvelas
     Cisco Systems
     kouvelas@cisco.com


     Tony Speakman
     Cisco Systems
     speakman@cisco.com


     Lorenzo Vicisano
     Cisco Systems
     lorenzo@cisco.com


9.  Normative References

[1] S.E. Deering, "Host extensions for IP multicasting", RFC 1112, Aug
     1989.

[2] B. Cain, S Deering, W. Fenner, I Kouvelas, A. Thyagarajan, "Internet
     Group Management Protocol, Version 3", RFC 3376.

[3] S. Deering, W. Fenner, B. Haberman, "Multicast Listener Discovery
     (MLD) for IPv6", RFC 2710.

[4] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas "Protocol
     Independent Multicast - Sparse Mode (PIM-SM):  Protocol
     Specification (Revised)", Work In Progress, <draft-ietf-pim-sm-
     v2-new-09.txt>, 2004.

[5] S. Kent, R. Atkinson, "Security Architecture for the Internet
     Protocol.", RFC 2401.


10.  Informative References

[6] T. Bates , R. Chandra , D. Katz , Y. Rekhter, "Multiprotocol
     Extensions for BGP-4", RFC 2283




Handley/Kouvelas/Speakman/Vicisano                Section 10.  [Page 43]

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[7] D. Estrin, D. Farinacci, "Bi-directional Shared Trees in PIM-SM",
     <draft-farinacci-bidir-pim-01.txt>, May 1999.

[8] D. Estrin et al, "Protocol Independent Multicast-Sparse Mode (PIM-
     SM): Protocol Specification", RFC 2362, Nov 1999.

[9] W. Fenner, M. Handley, R. Kermode and D. Thaler, "Bootstrap Router
     (BSR) Mechanism for PIM Sparse Mode", Work in progress <draft-ietf-
     pim-sm-bsr-03.txt>, 2003.










































Handley/Kouvelas/Speakman/Vicisano                Section 10.  [Page 44]

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11.  Index
DF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7,21
Downstream . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
DownstreamJPState(G,I) . . . . . . . . . . . . . . . . . . . . . . .  12
ET(G,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,16,34
ET(RPA,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
I_am_DF(RPA,I) . . . . . . . . . . . . . . . . . . . . . . . . .12,14,17
J/P_HoldTime . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
J/P_Override_Interval. . . . . . . . . . . . . . . . . . . . . . . 18,35
JoinDesired(G) . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
joins(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
JT(*,G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
JT(G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11,35
local_receiver_include(G,I). . . . . . . . . . . . . . . . . . . . .  12
MFIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
NLT(N,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
Offer_Period . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
olist(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . .12,14,20
OT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
pim_include(G) . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
PPT(G,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,16,35
RPA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
RPF_interface(RPA) . . . . . . . . . . . . . . . . . . . . . . . . 12,14
RPL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
TIB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
t_override . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35
t_periodic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35
t_suppressed . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,35
Upstream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6






















Handley/Kouvelas/Speakman/Vicisano                Section 11.  [Page 45]


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