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Versions: 00 02 03 04 05 06 07 08 09 RFC 2117

Network Working Group                             Steven Deering (XEROX)
Internet Draft                                      Deborah Estrin (USC)
                                                  Dino Farinacci (CISCO)
                                                      Mark Handley (UCL)
                                                       Ahmed Helmy (USC)
                                                      Van Jacobson (LBL)
                                                     Chinggung Liu (USC)
                                                     Puneet Sharma (USC)
                                                    David Thaler (UMICH)
                                                      Liming Wei (CISCO)

draft-ietf-idmr-PIM-SM-spec-02.txt                         May 7, 1996




   Protocol  Independent  Multicast-Sparse   Mode   (PIM-SM):   Protocol
   Specification



   Status of This Memo

   This document is an Internet  Draft.   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.  Internet  Drafts  may  be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to  use  Internet
   Drafts  as  reference  material  or  to  cite  them  other  than as a
   ``working'' draft'' or ``work in progress.''

   Please check the I-D abstract  listing  contained  in  each  Internet
   Draft  directory  to  learn  the  current status of this or any other
   Internet Draft.











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1 Introduction

   This  document  describes  a  protocol  for  efficiently  routing  to
   multicast   groups   that   may  span  wide-area  (and  inter-domain)
   internets.  We  refer  to  the  approach  as   Protocol   Independent
   Multicast--Sparse  Mode  (PIM-SM)  because it is not dependent on any
   particular unicast routing protocol, and because it  is  designed  to
   support  sparse  groups as defined in [1][2]. This document describes
   the protocol details. For the motivation  behind  the  design  and  a
   description  of  the  architecture, see [1][2]. Section  2 summarizes
   PIM-SM  operation.  It  describes  the  protocol   from   a   network
   perspective, in particular, how the participating routers interact to
   create and maintain  the  multicast  distribution  tree.  Section   3
   describes  PIM-SM  operations from the perspective of a single router
   implementing the protocol; this section constitutes the main body  of
   the  protocol  specification.  It  is  organized  according to PIM-SM
   message type; for each message type we  describe  its  contents,  its
   generation, and its processing. Interoperability with other protocols
   will be further discussed in an appendix to this document.

   Section  4 provides packet format details.

   The  most  significant  functional  changes  since  the  January  '95
   version,  are the Rendezvous Point-related mechanisms and the removal
   of the PIM-DM protocol details to a separate [3] (for clarity).


2 PIM-SM Protocol Overview

   In  this  section  we  provide  an  overview  of  the   architectural
   components of PIM-SM.

   A router  [*]

   receives explicit Join/Prune messages from those neighboring  routers
   that  have  downstream  group  members. The router then forwards data
   packets addressed to a multicast group, G, only onto those interfaces
   on which explicit joins have been received.

   A Designated Router (DR) sends periodic Join/Prune messages toward  a
   group-specific  Rendezvous Point (RP) for each group for which it has
   active members. Each router along  the  path  toward  the  RP  builds
   wildcard  (any-source)  forwarding  state  for  the  group  and sends
   messages on toward the RP. The wildcard forwarding  entry's  incoming
_________________________
[*] All routers mentioned in this document are  assumed
to be PIM-SM capable, unless otherwise specified.




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   interface points toward the RP; the outgoing interfaces point to  the
   neighboring  downstream  routers  that  have sent Join/Prune messages
   toward the RP. This forwarding state creates a  shared,  RP-centered,
   distribution  tree that reaches all group members. When a data source
   first sends to a group, its DR unicasts Register messages to  the  RP
   with  the source's data packets encapsulated within. If the data rate
   is high, the RP can send  source-specific  Join/Prune  messages  back
   towards  the  source  and  the  source's data packets will follow the
   resulting forwarding state  and  travel  unencapsulated  to  the  RP.
   Whether  they  arrive  encapsulated  or natively, the RP forwards the
   source's decapsulated data packets down the RP-centered  distribution
   tree toward group members. If the data rate warrants it, routers with
   local  receivers  can  join   a   source-specific,   shortest   path,
   distribution tree, and prune these source's packets off of the shared
   RP-centered tree. Even if all receivers switch to the  shortest  path
   tree,  state  for  that  source  will  be kept at the RP, so that new
   members that join the RP-centered tree will receive data packets from
   the  source.  For low data rate sources, neither the RP, nor last hop
   routers need join a  source-specific  shortest  path  tree  and  data
   packets can be delivered via the shared, RP-tree.

   The following subsections describe SM operation in  more  detail,  in
   particular,  the  control  messages,  and  the  actions they trigger.
   Section  3  describes  protocol  operation   from   an   implementors
   perspective, i.e., the actions performed by a single router.

2.1 Local hosts joining a group

   In order to join a multicast group, G, a host  sends  an  IGMP  Host-
   Membership-Report   message  identifying  the  particular  group.  As
   specified in [4], IGMP Host-Membership-Report messages  are  sent  in
   response  to a directly-connected router's IGMP Host-Membership-Query
   message (see figure 1).   [*]

   From this point on we refer to such a host  as  a  receiver,  R,  (or
   member) of the group G.







_________________________
[*] All figures used in this section are for  illustra-
tion  and are not intended to be complete. For complete
and detailed protocol action see Section 3.




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      Fig. 1  Example: how a receiver joins, and sets up shared tree



   When a DR receives an IGMP Host-Membership-Report for a new group, G,
   the  DR  looks up the associated RP. The DR (e.g., router A in figure
   1) creates a wildcard  multicast  forwarding  entry  for  the  group,
   referred to here as a (*,G) entry; if there is no more specific match
   for a particular source, the packet will be  forwarded  according  to
   this entry.

   The RP address is included in a special field in the forwarding entry
   and  is  included  in  periodic  upstream  Join/Prune  messages.  The
   outgoing  interface  is  set  to  that  over  which  the  IGMP  Host-
   Membership-Report  was  received  from  the  new member. The incoming
   interface is set to the interface used to send unicast packets to the
   RP. An RP-bit associated with this entry is also set, indicating that
   this entry, (*,G), represents state on the shared RP-tree. Each DR on
   the  RP-tree  with  directly  connected members sets a timer for this
   entry. If the timer expires and the DR has neither local members  nor
   downstream receivers, the (*,G) state is deleted. If the DR does have
   local members, it refreshes the (*,G) entry timer each time  it  gets
   an IGMP Host-Membership-Report.

2.2 Establishing the RP-rooted shared tree

   Triggered by the (*,G) state, the DR  creates  a  Join/Prune  message
   with  the  RP address in its join list and the WC-bit and RP-bit set;
   nothing is listed in its prune list. The RP-bit  flags  the  join  as
   being  associated  with  the shared tree and therefore the Join/Prune
   message is propagated along the RP-tree. The  WC-bit  indicates  that
   the address is an RP and the receiver expects to receive packets from
   all sources via this (shared tree) path.

   Each upstream router creates  or  updates  its  multicast  forwarding
   entry  for  (*,G)  when  it receives a Join/Prune with the RP-bit and
   WC-bit set. The interface on which the Join/Prune message arrived  is
   added  to  the list of outgoing interfaces (oifs) for (*,G). Based on
   this entry each upstream router between the receiver and the RP sends
   a  Join/Prune  message  in  which  the join list includes the RP. The
   packet  payload  contains  Multicast-Address=G,  Join=RP,WCbit,RPbit,
   Prune=NULL.




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2.3 Hosts sending to a group

   When a host first sends a multicast data packet to a  group,  its  DR
   must  deliver  the packet to the RP for distribution down the RP-tree
   (see figure 2). This is done by the sender's DR unicasting a Register
   packet  to  the  RP for the group. The data packet is encapsulated in
   the Register packet so that the RP can decapsulate it and deliver  it
   to downstream members.






                Fig. 2  Example: a host sending to a group



   If the data rate of the source warrants  [*]

   the use of a source-specific shortest path tree  (SPT),  the  RP  may
   construct  a  new  multicast forwarding entry that is specific to the
   source, hereafter referred to  as  (S,G)  state,  and  send  periodic
   Join/Prune messages toward the source. The routers between the source
   and the RP build and  maintain  (S,G)  state  in  response  to  these
   messages and send (S,G) messages upstream toward the source.

   The source's DR must stop encapsulating  data  packets  in  Registers
   when (and so long as) it receives Register-Stop messages from the RP.
   The RP triggers Register-Stop messages in response to  Registers,  if
   the  RP  has  no  downstream  receivers  for  the  group (or for that
   particular source), or if the RP has already joined  the  (S,G)  tree
   and is receiving the data packets natively.

2.4 Switching from shared tree (RP-tree)  to  shortest  path  tree  (SP-
   tree)}

   When a router has directly-connected  members,  it  first  joins  the
   shared  RP-tree.  The  router  can switch to a source's shortest path
   tree (SP-tree) after receiving packets  from  that  source  over  the
   shared  RP-tree.  The recommended policy is to initiate the switch to
   the SP-tree after receiving a  significant  number  of  data  packets
_________________________
[*] This decision is a local policy established at  the
RP.  For example, when the Register rate exceeds a con-
figured threshold at the RP, this may warrant  the  use
of the SPT.




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   during a specified time interval from a particular source. To realize
   this  policy  the  router  can  monitor data packets from sources for
   which it has   no  source-specific  multicast  forwarding  entry  and
   initiate  such  an  entry  when  the data rate exceeds the configured
   threshold. As shown in figure 3, router `A' initiates a (S,G) state.






     Fig. 3  Example: Switching from shared tree to shortest path tree



   When a (S,G) entry is activated (and  periodically  so  long  as  the
   state  exists),  a  Join/Prune  message  is sent upstream towards the
   source, S, with S in the join list. The payload  contains  Multicast-
   Address=G,  Join=S,  Prune=NULL. When the (S,G) entry is created, the
   outgoing interface list is copied from (*,G), i.e., all local  shared
   tree branches are replicated in the new shortest path  tree   [*]  In
   this way when a data packet from S arrives and matches on this entry,
   all receivers will continue to receive  the  source's  packets  along
   this  path.  Note that (S,G) state must be maintained in all last-hop
   routers where an SP-tree is maintained. Even  when  (*,G)  and  (S,G)
   overlap,  both  states  are  needed  to  trigger  the source-specific
   Join/Prune messages. (S,G)  state  is  kept  alive  by  data  packets
   arriving  from  that  source.  A timer, S-timer, is set for the (S,G)
   entry and this timer is restarted whenever a data packet for (S,G) is
   forwarded out at least one oif. When the S-timer expires the state is
   deleted.

   Only the RP and routers with local members can initiate switching  to
   the  SP-tree;  intermediate  routers  do  not. Consequently, last hop
   routers create (S,G) state in  response  to  data  packets  from  the
   source,  S;  whereas  intermediate routers only create (S,G) state in
   response to Join/Prune messages from downstream that have  S  in  the
   Join list  [*]
_________________________
[*] In more complicated scenarios, other entries in the
router  have  to be considered. For details see Section 3.
[*]  For  example, to implement the policy that source-
specific  trees  are  only  setup  for  high-data  rate
source,  a last-hop router might not create a (S,G) en-
try until it has received m  data  packets  from  the
source within some interval of n seconds.




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   The (S,G) entry is initialized with the SPT-bit  cleared,  indicating
   that  the  shortest  path  tree  branch from S has not yet been setup
   completely, and the router can  still  accept  packets  from  S  that
   arrive on the (*,G) entry's iif.

   When a router with a (S,G) entry and  a  cleared  SPT-bit  starts  to
   receive packets from the new source S on the iif for the (S,G) entry,
   and that iif differs from the (*,G) entry's iif, the router sets  the
   SPT-bit,  and  sends  a Join/Prune message towards the RP, indicating
   that the router no longer wants to receive packets  from  S  via  the
   shared RP-tree. The Join/Prune message sent towards the RP includes S
   in the prune list, with the RP-bit set indicating  that  S's  packets
   should  not  be forwarded down this branch of the shared tree. If the
   router receiving the Join/Prune message  has  (S,G)  state  (with  or
   without  the  RPbit  set), it deletes the arriving interface from the
   (S,G) oif list. If the router has only (*,G)  state,  it  creates  an
   (S,G)RP-bit   entry.   The   Join/Prune   message   payload  contains
   Multicast-Address=G, Join=NULL, Prune=S,RPbit.

   If at a later time a new receiver joins  the  RP-tree,  the  negative
   cache  state  on the RP-tree must be eradicated to bring all sources'
   data packets down to the new receiver. Therefore, when a  (*,G)  Join
   arrives  with  a null prune list at a router that has any (S,G)RP-bit
   entries (which is causing it to send  source-specific  prunes  toward
   the  RP),  all RP-bit state for that group has to be updated upstream
   of the router; so as to bring all sources' packets down  to  the  new
   member.  To  accomplish this the router updates all existing (S,G)RP-
   bit entries; it  adds  to  each  (S,G)RP-bit  entry's  oif  list  the
   interface on which the (*,G) join arrived. The router also triggers a
   (*,G) join upstream to cause the same  updating  of  RP-bit  settings
   upstream  and  pull down all active sources' packets. If the arriving
   (*,G) join has some sources included in  its  prune  list,  then  the
   corresponding (S,G)RP-bit entries are left unchanged (i.e., the RPbit
   remains set and no oif is added).


2.5 Steady state maintenance of distribution tree (i.e., router state)}

   In the steady state each router sends  periodic  Join/Prune  messages
   for each active (S,G), (*,G) or (*,*,RP)  [*]

   entry; the Join/Prune messages are sent to the RPF  neighbor  on  the
   iif  of the corresponding entry. These messages are sent periodically
   to capture state, topology,  and  membership  changes.  A  Join/Prune
_________________________
[*] (*,*,RP) entry is introduced for  interoperability,
see Sections 2.10 and 6.




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   message is also sent on an event-triggered  basis  each  time  a  new
   forwarding  entry  is established for some new source (note that some
   damping function may be applied,  e.g.,  a  merge  time).  Join/Prune
   messages  do  not elicit any form of explicit acknowledgment; routers
   recover from lost packets using the periodic refresh mechanism.


2.6 Obtaining RP information

   To obtain the RP information, all routers  collect  RP-Set  messages.
   RP-Set messages are sent hop-by-hop within the domain; originating at
   the domain's bootstrap router (BSR). The BSR is  elected  dynamically
   within each domain.
     [*]


   Routers then use the set of  RPs  to  get  the  proper  Group  to  RP
   mapping. Details are as follows:

2.6.1 Bootstrap Router

   A (small)  set  of  routers,  within  a  domain,  are  configured  as
   candidate  bootstrap  routers.  Initially,  each  of these candidates
   includes its address in `RP-set' messages. Through a simple  election
   mechanism, a single bootstrap router (BSR) is elected for that domain
   (see Section  3.6).
2.6.2 Candidate RPs

   A set of routers within a domain are configured as candidate RPs  (C-
   RPs); typically these will be the same routers that are configured as
   C-BSRs. Candidate RPs periodically unicast Candidate-RP-Advertisement
   messages (C-RP-Advs) to the BSR of that domain. C-RP-Advs include the
   address of the advertising C-RP, as well as an optional group address
   and  a  mask  length field, indicating the group prefix(es) for which
   the candidacy is advertised. The BSR then includes  a  set  of  these
   Candidate-RPs  in  the  RP-Set messages, along with the corresponding
   group prefixes (see Section
    3.6.2). RP-Set messages are periodically sent hop-by-hop  throughout
   the domain.



_________________________
[*] A domain in this context is a multicast  region  in
which  routers  implement PIM-SM. PIM-SM border routers
are assumed to connect a domain to the rest of the  in-
ternet.




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2.6.3 Group to RP mapping

   Routers receive and store RP-Set messages originated by the BSR. When
   a DR receives IGMP Host-Membership-Report (or a data packet) from a
   directly connected host, for a group for which it has no  entry,  the
   DR  uses  a hash function to map the pertinent group to one of the C-
   RPs whose Group-prefix includes the group (see Section  3.7). The  DR
   then  sends  a  Join/Prune message towards (or unicasts Registers to)
   that RP.
     [*]


2.6.4 Providing RP liveness

   The RP-Set message indicates liveness of the RPs included therein; if
   an  RP  is  included in the message, then it is tagged as `up' at the
   routers, while RPs not included in the message are tagged  as  `down'
   and  removed from the list of RPs over which the hash algorithm acts.
   Each router continues to  use  the  contents  of  the  most  recently
   received RP-set message until it receives a new RP-set message.

2.7 Multicast data packet processing


   Data packets are processed in a manner similar to existing  multicast
   schemes.  A  router  first performs a longest match on the source and
   group address in the data packet. A (S,G) entry is matched  first  if
   one  exists;  a  (*,G)  entry  is matched otherwise. If neither state
   exists, then a (*,*,RP) entry match  is  attempted  as  follows:  the
   router  hashes  on  G to identify the RP for group G, and looks for a
   (*,*,RP) entry that has this RP address associated with it.  If  none
   of  the  above  exists,  then  the  packet  is dropped. If a state is
   matched, an incoming interface check (RPF check) is performed on  the
   matching  state  and if it fails the packet is dropped, otherwise the
   packet  is  forwarded  to  all  interfaces  listed  in  the  outgoing
   interface list.

   Some special actions are needed to deliver packets continuously while
   switching  from the shared to shortest-path tree. In particular, when
   a (S,G) entry is matched, incoming packets are forwarded as follows:


        1    If the SPT-bit is set, then:
_________________________
[*] Each intermediate router also uses this  same  hash
function  to  determine the (*,*,RP) match for incoming
data packets.




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             1    if the incoming interface is the same  as  a  matching
                  (S,G)  iif, the packet is forwarded to the oif-list of
                  (S,G).

             2    if the incoming interface is different than a matching
                  (S,G) iif , the packet is discarded.



        2    If the SPT-bit is cleared, then:


             1    if the incoming interface is the same  as  a  matching
                  (S,G)  iif, the packet is forwarded to the oif-list of
                  (S,G). In addition, the SPT bit is set for that  entry
                  if  the  incoming  interface differs from the incoming
                  interface of the (*,G) or (*,*,RP) entry.

             2    if the incoming interface is different than a matching
                  (S,G)  iif, the incoming interface is tested against a
                  matching (*,G) or (*,*,RP) entry. IF the  iif  is  the
                  same  as  one of those, the packet is forwarded to the
                  oif-list of the matching entry.

             3    Otherwise the iif does not match any entry for  G  and
                  the packet is discarded.



        Data packets never trigger prunes.  However,  data  packets  may
        trigger  actions  that in turn trigger prunes. For example, when
        router  B in figure 3 decides to switch to SP-tree at step 3, it
        creates  a  (S,G) entry with SPT-bit set to 0. When data packets
        from S arrive at interface 2 of  B,  B sets  the  SPT-bit  to  1
        since  the  iif for (*,G) is different than that for (S,G). This
        triggers the sending of prunes towards the RP.


     2.8 Operation over Multi-access Networks


        This section describes  a  few  additional  protocol  mechanisms
        needed  to  operate  PIM  over multi-access networks: Designated
        Router election, Assert messages to resolve parallel paths,  and
        the  Joiner  bit  to  suppress  redundant  Joins on multi-access
        networks.





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     2.8.1 Designated router election

        When there are multiple  routers  connected  to  a  multi-access
        network,  one  of  them  should  be  chosen  to  operate  as the
        designated  router  (DR)  at  any  point  in  time.  The  DR  is
        responsible   for  sending  triggered  Join/Prune  and  Register
        messages toward the RP  [*]

        A simple designated router (DR) election mechanism is  used  for
        both SM and traditional IP multicast routing.

        Neighboring routers send  Query  messages  to  each  other.  The
        sender  with the largest IP address assumes the role of DR. Each
        router connected to  the  multi-access  LAN  sends  the  Queries
        periodically in order to adapt to changes in router status.


     2.8.2 Parallel paths to a source or the RP

        If a router receives a multicast datagram on a multi-access  LAN
        from  a source whose corresponding (S,G) outgoing interface list
        includes the interface  to  that  LAN,  the  packet  must  be  a
        duplicate.  In  this  case  a  single forwarder must be elected.
        Using Assert messages addressed to `224.0.0.13' (ALL-PIM-ROUTERS
        group)  on  the LAN, upstream routers can resolve which one will
        act as the forwarder. Downstream routers listen to  the  Asserts
        so  they know which one was elected, and therefore where to send
        subsequent Joins. Typically this is the same as  the  downstream
        router's  RPF  neighbor  but  there are circumstances where this
        might not be  the  case,  e.g.,  when  using  different  unicast
        protocols.

        The upstream router elected is the one  that  has  the  shortest
        distance  to the source. Therefore, when a packet is received on
        an outgoing interface a router sends an Assert  message  on  the
        multi-access  LAN  indicating  what  metric it uses to reach the
        source  of  the  data  packet.  The  router  with  the  smallest
        numerical  metric  (with  ties  broken  by highest address) will
        become the forwarder. All other upstream routers will delete the
        interface  from  their  outgoing  interface list. The downstream
        routers  also  do  the  comparison  in  case  the  forwarder  is
        different than the RPF neighbor.
_________________________
[*] IGMP Queries are sent by a PIMv2 DR if it  supports
IGMPv1.  If  a  PIMv2  router is using IGMPv2 then Host
queries are not sent by the PIMv2 DR but  by  the  IGMP
querier.




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        Associated with the metric is a metric preference value. This is
        provided  to  deal  with the case where the upstream routers may
        run different unicast routing protocols. The numerically smaller
        metric  preference  is  always  preferred. The metric preference
        should be treated as the high-order part  of  an  assert  metric
        comparison.  Therefore,  a  metric  value  can  be compared with
        another metric value provided both metric  preferences  are  the
        same.  A  metric  preference can be assigned per unicast routing
        protocol and needs to be  consistent  for  all  routers  on  the
        multi-access network.

        Asserts are also needed for (*,G) entries  since  there  may  be
        parallel  paths  from  the  RP  and  sources  to  a multi-access
        network. When an assert is sent for a (*,G) entry, the first bit
        in the metric preference (RP-bit) is always set to 1 to indicate
        that this path corresponds to the RP tree, and  that  the  match
        should  be  done  on  (*,G) if exits. Furthermore, the RP-bit is
        always cleared for  SP-tree  entries'  metric  preference;  this
        causes  an  SP-tree  path  to always look better than an RP-tree
        path. When the SP-tree and  RPtree  cross  the  same  LAN,  this
        mechanism  eliminates  the  duplicates  that  would otherwise be
        carried over the LAN.

        In case the packet, or the Assert message, matches  on  oif  for
        (*,*,RP) entry, a (*,G) entry is created, and asserts take place
        as if the matching state were (*,G).

        The DR may lose to another router  on  the  LAN  by  the  Assert
        process  if  there are multiple paths to the RP through the LAN.
        From then on, the DR is no longer the last-hop router for  local
        receivers. The winning router becomes the last-hop router and is
        responsible for sending (*,G) join messages to the  RP.  Asserts
        are rate limited.

     2.8.3 Join/Prune suppression

        If a Join/Prune message arrives on the incoming interface for an
        existing  (S,G)  entry,  and  the sender of the Join/Prune has a
        higher IP address than the recipient of the message,  a  Joiner-
        bit  in the multicast routing table entry is cleared to suppress
        further Join/Prune messages. A timer is set for the  Joiner-bit;
        after  it  expires  the  Joiner-bit  is  set  indicating further
        periodic Join/Prunes should be sent for this entry. The  Joiner-
        bit  timer  is  restarted  each  time  a  Join/Prune  message is
        received from a higher-IP-addressed PIM neighbor.






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     2.9 Unicast Routing Changes

        When unicast routing changes, an RPF check is done on all active
        (S,G),  (*,G)  and  (*,*,RP)  entries, and all affected expected
        incoming interfaces are  updated.  In  particular,  if  the  new
        incoming interface appears in the outgoing interface list, it is
        deleted from the outgoing interface list. The previous  incoming
        interface  may  be  added  to  the  outgoing interface list by a
        subsequent  Join/Prune  from  downstream.  Join/Prune   messages
        received   on   the  current  incoming  interface  are  ignored.
        Join/Prune messages  received  on  new  interfaces  or  existing
        outgoing  interfaces  are not ignored. Other outgoing interfaces
        are left as is until they are explicitly  pruned  by  downstream
        routers  or  are timed out due to lack of appropriate Join/Prune
        messages. If the router has a (S,G) entry with the SPT-bit  set,
        and  the  updated  iif(S,G)  does   not  differ from iif(*,G) or
        iif(*,*,RP), then the router resets the SPT-bit.

        The router must send a Join/Prune message with  S  in  the  Join
        list  out  its new incoming interface to inform upstream routers
        that it expects multicast datagrams over the interface.  It  may
        also  send a Join/Prune message with S in the Prune list out the
        old incoming interface, if the link is  operational,  to  inform
        upstream  routers  that  this  part  of the distribution tree is
        going away.



     2.10 Interaction with dense mode  protocols such as DVMRP

        The essential problem in connecting to dense mode  protocols  is
        to  pull  all packets generated within the PIM-SM region down to
        the dense mode routers.  To  do  this,  a  special  entry  type,
        referred  to as (*,*,RP), is introduced. Every (*,*,RP) entry is
        associated with a particular RP in the domain; that RP  is  used
        to  conduct  RPF checks. Border routers initiate the building of
        (*,*,RP) towards all internal Candidate  RPs.  (*,*,RP)  entries
        represent an aggregation of all the groups supported by the RP.

        Most of the mechanisms needed to support  interoperability  with
        dense mode protocols such as DVMRP are implemented in BRs, i.e.,
        special routers that  sit  at  the  boundary  between  a  PIM-SM
        regions  and  the  DVMRP regions and which speak both protocols.
        However, all PIM-SM  routers  must  be  capable  of  supporting
        (*,*,RP)   state  and  interpreting  associated  Join  messages.
        Interaction with non-PIM-SM networks  will  be  discussed  in  a
        separate interoperability appendix.




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     2.11 PIM-SM for Inter-Domain Multicast


        Future documents will address the use of PIM-SM  as  a  backbone
        inter-domain  multicast  routing protocol. Design choices center
        primarily around the distribution and usage  of  RP  information
        for wide area, inter-domain groups.

     2.12 Security

        { Editors Note: This section requires further work.}

        All PIM  control  messages  may  use  [5]  to  address  security
        concerns.





































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     3 Detailed Protocol Description


        This  section  describes  the  protocol  operations   from   the
        perspective   of   an   individual   router  implementation.  In
        particular,  for  each  message  type  we  describe  how  it  is
        generated and processed.

     3.1 Query

        Query messages are sent so neighboring routers can discover each
        other.

     3.1.1 Sending Queries

        Query messages are sent periodically between PIM  neighbors.  By
        default  they  are  transmitted  every  30 seconds. This informs
        routers what interfaces have PIM neighbors. Query  messages  are
        multicast  using address 224.0.0.13 (ALL-PIM-ROUTERS group). The
        packet  includes  the  holdtime  for  neighbors  to   keep   the
        information valid. The recommended holdtime is 3 times the query
        transmission interval. By default the holdtime  is  90  seconds.
        Queries are sent on all types of communication links.


     3.1.2 Receiving queries

        When a router receives a Query packet, it stores the IP  address
        for  that  neighbor,  sets  the  PIM neighbor timer based on the
        Query holdtime, and determines the Designated  Router  (DR)  for
        that  interface.  The highest IP addressed system is elected DR.
        Each query received causes the DR's address to be updated.

        When a router that is the active DR receives a query from a  new
        neighbor  (i.e.,  from  an IP address that is not yet in the DRs
        neighbor  table),  the  DR  unicasts  its  most  recent   RP-set
        information to the new neighbor.



     3.1.3 Timing out neighbor entries

        A periodic process is run to time out PIM  neighbors  that  have
        not sent queries. If the DR has gone down, a new DR is chosen by
        scanning all neighbors on the interface and selecting the new DR
        to  be  the one with the highest IP address. If an interface has
        gone down, the router may optionally time out all PIM  neighbors
        associated with the interface.



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     3.2 Join/Prune

        Join/Prune messages are sent to join or prune a  branch  off  of
        the  multicast distribution tree. A single message contains both
        a join and prune list, either one of which  may  be  null.  Each
        list  contains a set of source addresses, indicating the source-
        specific trees or shared tree that the router wants to  join  or
        prune.

     3.2.1 Sending Join/Prune Messages


        Join/Prune messages are merged such that a  message  sent  to  a
        particular  upstream  neighbor,  N,  includes all of the current
        joined and pruned sources that are reached via N;  according  to
        unicast routing Join/Prune messages are multicast to all routers
        on multi-access networks with the target address set to the next
        hop  router  towards  S  or  RP.  Join/Prune  messages  are sent
        periodically. Currently the period is set to 60 seconds.  [*]


        A router  sends  a  periodic  Join/Prune  message  to  each
        distinct  RPF  neighbor  associated  with  each (S,G), (*,G) and
        (*,*,RP) entry. Join/Prune messages are only  sent  if  the  RPF
        neighbor  is  a PIM neighbor. A periodic Join/Prune message sent
        towards a particular RPF neighbor is constructed as follows:



        1     Each router determines the RP for a (*,G) entry  by  using
             the hash function described. The RP address (with RP and WC
             bits set) is included  in  the  join  list  of  a  periodic
             Join/Prune message under the following conditions:


             1    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor  to  the  RP  for an active (*,G) or (*,*,RP)
                  entry, and

             2    The outgoing interface list in the (*,G)  or  (*,*,RP)
                  entry is non-NULL, or the router is the DR on the same
                  interface as the RPF neighbor.

_________________________
[*] In the  future  we  will  introduce  mechanisms  to
rate-limit  this control traffic on a hop by hop basis,
in order to avoid excessive overhead on small links.




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        2    A particular source address, S, is  included  in  the  join
             list  with  the  RP and WC bits cleared under the following
             conditions:


             1    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor to S, and

             2    There exists an active  (S,G)  entry  with  the  RPbit
                  cleared, and

             3    The oif list in the (S,G) entry is not null.



        3    A particular source address, S, is included  in  the  prune
             list  with  the  RP and WC bits cleared under the following
             conditions:


             1    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor to S, and

             2    There exists an active  (S,G)  entry  with  the  RPbit
                  cleared, and

             3    The oif list in the (S,G) entry is null.



        4    A particular source address, S, is included  in  the  prune
             list  with the RP bit  set and the WC bit cleared under the
             following conditions:


             1    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor  toward the RP and there exists a (S,G) entry
                  with the RPbit set and null oif list, or

             2    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor  toward  the  RP,  there exists a (S,G) entry
                  with the  RPbit  cleared  and  SPT-bit  set,  and  the
                  incoming  interface  toward  S  is  different than the
                  incoming interface toward the RP, or

             3    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor toward the RP, and there exists a (*,G) entry
                  and (S,G) entry for a directly connected source.



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        5    The RP address (with RP and WC bits set) is included in the
             prune list if:



             1    The Join/Prune  message  is  being  sent  to  the  RPF
                  neighbor  toward the RP and there exists a (*,G) entry
                  with a null oif list (see Section  3.5.2).



        In addition to these periodic  messages,  the  following  events
        will  trigger  Join/Prune  messages  (the  contents of triggered
        messages are the same as the periodic, described above)


        1    Receipt of an IGMP  Host-Membership-Report  message  for  a
             group  G  will  cause  building  or modifying corresponding
             (*,G)  state,  and  subsequent   triggering   of   upstream
             Join/Prune messages as follows:


             1    If the receiving router does  not  have  a  forwarding
                  entry for G the router creates a (*,G) entry, with the
                  interface upon which the  IGMP  Host-Membership-Report
                  was  received  included  in  the  oif list. The router
                  sends a Join/Prune message towards the RP with the  RP
                  address and RP-bit and WC-bits set in the join list. A
                  timer is initiated for each interface in the oif list.
                  Or,

             2    If the (*,G) already exists, the interface upon  which
                  the  IGMP Host-Membership-Report was received is added
                  to the oif list (if it was not included  already)  and
                  the timer for that interface is restarted.



        2    Receipt  of  a  Join/Prune  message  for  (S,G),  (*,G)  or
             (*,*,RP)  will  cause  building  or modifying corresponding
             state, and subsequent  triggering  of  upstream  Join/Prune
             messages, in the following cases:


             1    When there is no  current  forwarding  entry,  the  RP
                  address  included in the Join/Prune message is checked
                  against the local RP-Set information. If  it  matches,
                  an  entry will be created. If the router has no RP-Set



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                  information it may discard the message, or  optionally
                  use the RP address included in the message.

                  The  new  entry  will  in  turn  trigger  an  upstream
                  Join/Prune message.


             2    When the outgoing interface list of (S,G) RPbit  entry
                  is null, the triggered Join/Prune message will contain
                  S in the prune list.





        3    Receipt of a packet on  a  (S,G)  entry  whose  SPT-bit  is
             cleared triggers the following if the packet arrived on the
             correct incoming interface and there is a (*,G) or (*,*,RP)
             entry with a different incoming RPF neighbor: a) setting of
             the SPT-bit on (S,G) entry, and  b)  sending  a  Join/Prune
             message  towards  the RP with S,RP-bit in the prune list if
             the iif of (S,G) is different from  the  iif  of  (*,G)  or
             (*,*,RP).


        4    When a Join/Prune message is received for a  group  G,  the
             prune  list  is  checked. If it contains a source for which
             the receiving router  has  a  corresponding  active  (S,G),
             (*,G) or (*,*,RP) entry, and whose iif is that on which
             the Join/Prune was received, then a join for  (S,G),  (*,G)
             or   (*,*,RP)   is   triggered   to   override  the  prune,
             respectively. (This is necessary in the  case  of  parallel
             downstream routers connected to a multi-access network.)



        5    When the RP fails, the RP will not be included in  the  RP-
             Set  messages sent to the receivers' last-hop routers. This
             triggers the last-hop routers to send (*,G)  joins  towards
             the  new  RP for the group, as determined by the RP-Set and
             the hash function  [*]


_________________________
[*] PIM Multicast Border Routers (PMBRs), handling  in-
teroperability  functionality,  trigger  (*,*,RP) joins
towards each RP in the RP-Set.




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        We do not trigger prunes onto interfaces for SM groups based  on
        data  packets.  Data  packets  that arrive on the wrong incoming
        interface for an SM group are silently dropped.

        It is possible that a Join/Prune message  constructed  according
        to  the  preceeding  rules could exceed the MTU of a network. In
        this  case,  the  message  can  undergo  semantic  fragmentation
        whereby  information  corresponding  to  different groups can be
        sent in different messages. However,  if  a  Join/Prune  message
        must be fragmented the following rule must be followed:


        1    The complete prune list corresponding to a group G must  be
             included  in  the same Join/Prune message as the associated
             RP-tree Join for G.


     3.2.2 Receiving  Join/Prune  Messages  When  a  router  receives  a
        Join/Prune message, it processes it as follows:


        1    The receiver of the Join/Prune notes the interface on which
             the PIM message arrived, call it I. The router accepts this
             Join/Prune message if this Join/Prune message is  addressed
             to  the router itself. If the Join/Prune is for this router
             the following actions are taken:



             1    If an address Sj in the join list has RP-bit  and  WC-
                  bit  set,  then  Sj  is  the  RP  address  used by the
                  downstream router and the following actions are taken:


                  1    If Sj is not the same as the  receiving  router's
                       RP mapping for G, the receiving router may ignore
                       that group entry in the  Join/Prune  message.  If
                       the  router does not have any RP-Set information,
                       it  may  use  the  address  Sj  included  in  the
                       Join/Prune message as the RP for the group.

                  2    If Sj is the same as the  receiving  router's  RP
                       mapping   for  G,  it  adds  I  to  the  outgoing
                       interface list of the (*,G) forwarding entry  and



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                       sets the timer for that interface (if there is no
                       (*,G) entry, the router creates one first). If  a
                       (*,*,RP)  exists,  for  the RP associated with G,
                       then the oif list of the newly created  (*,G)  is
                       copied   from   that  (*,*,RP)  state,  excluding
                       iif(*,G),


                  3    For each (Si,G) entry associated with group G, if
                       Si is not included in the prune list, and if I is
                       not the iif then interface I is added  to  the
                       oif  list and the timers are restarted for that
                       interface in each affected entry. If the G in the
                       join message is `*'  [*] , then every  (*,G)  and
                       (S,G) entry, whose group address hashes to the RP
                       indicated  in  the  (*,*,RP)  join  message,   is
                       updated accordingly,


                  4    If the (Si,G) entry is an RP-bit entry and its
                       oif  list  is  the  same as (*,G) oif list,
                       then the (Si,G,RPbit) entry is deleted,


                  5    The incoming interface is set  to  the  interface
                       used  to  send  unicast  packets to the RP in the
                       (*,G) forwarding entry, i.e.,  RPF  interface  to
                       the RP.





             2    For each address Si in the join list whose RP-bit  and
                  WC-bit  are   not  set,  and  for  which  there  is no
                  existing (Si,G) forwarding entry, the router initiates
                  one.
                    [*]
_________________________
[*] A `*' in the  group  field  of  the  Join/Prune  is
represented  by  a  group address 224.0.0.0 and a group
mask length of 4, indicating a (*,*,RP) Join.
[*] The router creates a (S,G)  entry  and  copies  all
outgoing   interfaces,  excluding  iif(S,G),  from  the
(S,G)RP-bit, (*,G), or (*,*,RP), entry, if  it  exists.
If  a router does not copy all outgoing interfaces from
the (*,G), or (*,*,RP) entry, all receivers on RP-tree,
downstream  from outgoing interfaces other than the one



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                  1    The outgoing interface for (Si,G) is  set  to  I.
                       The  incoming  interface for (Si,G) is set to the
                       interface used to  send  unicast  packets  to  Si
                       (i.e., the RPF neighbor).


                  2    If the interface used to reach Si is the same  as
                       the  outgoing  interface  being  built,  I,  this
                       represents an error and the Join/Prune should not
                       be processed.




             3    For any Si included in the join list of the Join/Prune
                  message,   for  which  there  is  an  existing  (Si,G)
                  forwarding entry,



                  1    If the RP-bit is not set for  Si  listed  in  the
                       Join/Prune  message, but the RP-bit is set on the
                       existing (Si,G) entry, the router clears the  RP-
                       bit  on (Si,G) entry, sets the incoming interface
                       to point towards Si for that  (Si,G)  entry,  and
                       sends a Join/Prune to the new incoming interface;
                       and


                  2    The  router  adds  I  to  the  list  of  outgoing
                       interfaces  if  I is not the same as the existing
                       incoming interface; the timer for I is restarted.


                  3    The (Si,G) SPT bit is initialized to  be  cleared
                       until data comes down the shortest path tree.



             4    For each address Si in the prune list, with the RP-bit
                  is either set or cleared, and the WC-bit cleared:
_________________________
newly added to (S,G), will  not  receive  packets  from
source  S.  Data packets of S arriving from the RP will
match the (S,G) entry instead of  (*,G),  or  (*,*,RP),
entry,  and will be dropped because the incoming inter-
face is incorrect.




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                  1    If there is an existing (Si,G) forwarding  entry,
                       the  router  schedules  a  deletion of I from the
                       list of outgoing interfaces by lowering that  oif
                       timer  to 5 seconds (unless it is already lower).
                       The deletion is not  executed  until  this  timer
                       expires, allowing for other downstream routers on
                       a multi-access LAN to override the prune.


                  2    If the router has a current (*,G),  or  (*,*,RP),
                       forwarding  entry,  and  if  a (Si,G)RP-bit entry
                       also  exists  then  the  (Si,G)RP-bit  entry   is
                       maintained even if its outgoing interface list is
                       null.




             5    For any Si in the prune list that has the RP-bit  set,
                  and the WC-bit cleared:




                  1    If  (*,G),  or  corresponding   (*,*,RP),   state
                       exists,   but   there  is  no  (Si,G)  entry,  an
                       (Si,G)RP-bit entry  is  created  .  The  outgoing
                       interface  list  is  copied  from  the  (*,G), or
                       (*,*,RP), entry, with the interface, I, on  which
                       the  prune was received deleted. Packets from the
                       pruned source, Si, match on this  state  and  are
                       not forwarded toward the pruned receivers.


                  2    If there exists a (Si,G) entry, with  or  without
                       the  RPbit  set,  the  iif on which the prune was
                       received, I, is deleted from the oif  list,
                       and the entry timer is restarted.




             6    For each address Si in the prune list, with the RP-bit
                  and the WC-bit set:







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                  1    If there is an existing (*,G) entry, with  Si  as
                       the  RP for G, the router schedules a deletion of
                       I  from  the  list  of  outgoing  interfaces   by
                       lowering  that  oif timer to 5 seconds (unless it
                       is already lower). The deletion is  not  executed
                       until  this  timer  expires,  allowing  for other
                       downstream  routers  on  a  multi-access  LAN  to
                       override the prune.



                  2    If the corresponding (*,*,RP) state  exists,  but
                       there  is  no  (*,G)  entry,  a  (*,G)  entry  is
                       created. The outgoing interface  list  is  copied
                       from  (*,*,RP)  entry,  with the interface, I, on
                       which the prune was received, deleted.


                  3    If there exists a (*,G) entry, the  interface  on
                       which  the prune was received, I, is deleted from
                       the oif  list,  and  the  entry  timer   is
                       restarted.






        2    If the received Join/Prune does not indicate the router  as
             its  target, then if the Join/Prune is for a (S,G) pair for
             which the router has an active  (S,G)  entry,  and  if  the
             Join/Prune  arrived on the iif for that entry, then the
             router compares the IP address  of  the  generator  of  the
             Join/Prune, to its own IP address.



             1    If its own IP address is higher, the Joiner-bit in the
                  (S,G) entry is set.

             2    If its own IP address is lower, the Joiner-bit in  the
                  (S,G)  entry  is  cleared, and the Joiner-bit timer is
                  activated.

             After the timer expires the Joiner-bit  is  set  indicating
             further periodic Join/Prunes should be sent for this entry.
             The Joiner-bit timer is restarted each  time  a  Join/Prune
             message   is   received   from  a  higher-IP-addressed  PIM



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             neighbor.


             For any new (S,G), (*,G) or (*,*,RP) entry  created  by  an
             incoming  Join/Prune message, the Joiner-bit is set and the
             SPT-bit is cleared.


     3.3 Register and Register-Stop

        When a source first starts sending to a group  its  packets  are
        encapsulated  in  Register  messages  and sent to the RP. If the
        data rate warrants source-specific paths, the RP sets up  source
        specific  state  and  starts  sending  (S,G) Join/Prune messages
        toward the source.



     3.3.1 Sending Registers and Receiving Register-Stops

        Register messages are sent as follows:



        1    When a DR receives  a  packet  from  a  directly  connected
             source, S  [*] :


             1    If there is no  corresponding  (S,G)  entry,  and  the
                  router has RP-Set information, the DR creates one with
                  the Register-bit set to  1  and  the  RP  address  set
                  according   to  the  hash  function  mapping  for  the
                  corresponding   group.   The   Register-bit-timer   is
                  initialized  to  zero;  the Register-bit-timer is non-
                  zero only when the Register-bit is set to 0.



             2    If there is a (S,G) entry in existence, the DR  simply
                  restarts the corresponding S-timer (entry timer).
_________________________
[*] When a border router (e.g., a router that  connects
the  PIM-SM region to a dense mode region running DVMRP
or PIM-DM) receives a packet from a source in the dense
mode region, the router treats the packet as if it were
from a directly connected source. See the  Appendix  on
Interoperability for more details.




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        2    If the new  or  previously-existing  (S,G)  entry  has  the
             Register-bit  set,  the  data  packet  is encapsulated in a
             Register message and unicast to the RP for that group.  The
             data  packet  is also forwarded according to (S,G) state in
             the DR if the oif list is not null; since  a  receiver  may
             join  the  SP-tree while the DR is still registering to the
             RP.



        3    If the (S,G) entry has the Register-bit cleared,  the  data
             packet  is  not  sent  in  a  Register  message, it is just
             forwarded according to the (S,G) oif list.



        The DR processes Register-Stop messages as follows:




        1    The DR clears the Register-bit and restarts  the  Register-
             bit-timer in the corresponding (S,G) entry(ies).


        When a Register-bit-timer expires, the corresponding  entry(ies)
        Register-bit  is  set  to 1 to reinstigate encapsulation of data
        packets in Register messages.

     3.3.2 Receiving Register Messages and Sending Register-Stops

        When a router (i.e., the RP) receives a  Register  message,  the
        router does the following:



        1    Decapsulates  the   data   packet,   and   checks   for   a
             corresponding (S,G) entry.



             1    If a (S,G) entry exists, the packet is  forwarded  but
                  the  SPT bit is left cleared (0). If the SPT bit is 1,
                  the packet is dropped, and Register-Stop messages  are
                  triggered. Register-Stops are rate limited.  [*]
_________________________
[*] Register-Stops should be rate limited  so  that  no
more  than  a  few  are  sent per round trip time. This



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             2    If there is no (S,G)  entry,  but  there  is  a  (*,G)
                  entry,  or  a (*,*,RP) entry with the RP corresponding
                  to G, the packet is forwarded according to that entry.


             3    If there is a (*,*,RP) entry but  no  (*,G)  entry,  a
                  (*,G)  or (S,G) entry is created and the oif is copied
                  from the (*,*,RP) entry to the new entry.

             4    If there is no G or (*,*,RP) entry corresponding to G,
                  the   packet   is  dropped,  and  a  Register-Stop  is
                  triggered.


             5    A ``Border bit'' bit is added to the Register message,
                  to  facilitate  interoperability  mechanisms. PIM MBRs
                  set this bit when  registering  for  external  sources
                  (see  Sections  2.10 and  6). If the ``Border bit'' is
                  set in the Register, the RP does the following:



                  1    If there is  no  matching  (S,G)  state,  the  RP
                       creates  one,  with  a  `PMBR'  field. This field
                       holds the source of the Register (i.e. the  outer
                       IP  address  of  the  register  packet).  The  RP
                       triggers a (S,G) join towards the source  of  the
                       data packet, and clears the SPT bit for the (S,G)
                       entry, else


                  2    If the `PMBR' field for the  corresponding  (S,G)
                       entry  matches the source of the Register packet,
                       the decapsulated packet is forwarded to  the  oif
                       list of that entry, else


                  3    The packet is dropped,  and  a  Register-stop  is
                       triggered towards the source of the Register.


_________________________
prevents  a  high  datarate  stream  of  packets   from
triggering  a  large  number  of Register-stop messages
between the time that the first packet is received  and
the  time  when the source receives the first Register-
Stop.




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             The (S,G) state timer is restarted  by  Registers  arriving
             from that source to that group.


        2    If the matching (S,G) or (*,G) state contains  a  null  oif
             list, the RP unicasts a Register-Stop message to the source
             of the Register message; in the latter  case,  the  source-
             address  field, within the Register-Stop message, is set to
             the wildcard value (all 0's). This message is not processed
             by   intermediate   routers,   hence   no  (S,G)  state  is
             constructed between the RP and the source.


        3    If the Register message arrival rate warrants it and  there
             is  no  existing  (S,G)  entry,  the  RP  sets  up  a (S,G)
             forwarding  entry  with  the   outgoing   interface   list,
             excluding   iif(S,G),   copied   from  the  (*,G)  outgoing
             interface list, its SPT-bit is initialized to 0. If a (*,G)
             entry  does  not  exist,  but there exists a (*,*,RP) entry
             with the RP corresponding to G , the oif list for (S,G)  is
             copied -excluding the iif- from that (*,*,RP) entry.

             A timer is set for  the  (S,G)  entry  and  this  timer  is
             restarted  by  receipt of data packets for (S,G). The (S,G)
             entry causes the RP to send a Join/Prune  message  for  the
             indicated group towards the source of the register message.

             If the (S,G) oif list  becomes  null,  Join/Prune  messages
             will not be sent towards the source, S.



     3.4 Multicast Data Packet Forwarding

        Processing a multicast data packet involves the following steps:


        1    Lookup forwarding state based on a longest match
               [*]

             of  the  source  address,  and  an  exact  match   of   the
             destination  address in the data packet and compare the RPF
             check on the source address in the packet header with the
_________________________
[*] The longest match is  performed  in  the  following
order:  (1)  (S,G),  (2)  (*,G). If neither is matched,
then a lookup is performed on (*,*,RP) entries.




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             iif specified in the forwarding entry.



        2    If the  packet  arrived  on  the  interface  found  in  the
             matching-entry's iif field, and the oif list is not
             null:


             1    Forward the packet to the oif list for that  entry
                  and  restarted the entry's timer if the matching entry
                  is (S,G)  [*]




             2    If the entry is a (S,G) entry with a cleared  SPT-bit,
                  and  a  (*,G) or associated (*,*,RP) also exists whose
                  incoming interface is different than that  for  (S,G),
                  set  the  SPT-bit  for  the (S,G) entry and trigger an
                  (S,G) RP-bit prune towards the RP.


             3    If the source of the packet  is  a  directly-connected
                  host  and  the  router  is  the  DR  on a multi-access
                  network, check the Register-bit  associated  with  the
                  (S,G)   entry.   If   it   is  set,  then  the  router
                  encapsulates the data packet in a register message and
                  sends it to the RP.


             This covers the common case of a packet arriving on the RPF
             interface  to  the  source or RP and being forwarded to all
             joined branches. It also detects when packets arrive on the
             SP-tree, and triggers their pruning from the RP-tree. If it
             is  the  DR  for  the  source,  it   sends   data   packets
             encapsulated in Registers to the RPs.


        3    If the packet matches to an entry but did not arrive on the
             interface  found  in  the  entry's iif field, check the
             SPT-bit of the entry. If  the  SPT-bit  is  set,  drop  the
             packet.  If  the SPT-bit is cleared, then lookup the (*,G),
             or (*,*,RP), entry for G. If the packet arrived  on  the
_________________________
[*] Optionally, the (S,G) timer  may  be  restarted  by
periodic checking of the matching packet count.




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             iif  found  in  (*,G),  or  the  corresponding  (*,*,RP),
             forward  the  packet  to  the  oif list of the matching
             entry. This covers the case when a data packet matches on a
             (S,G)  entry  for  which  the  SP-tree  has  not  yet  been
             completely established upstream.


        4    If the packet does not match to any entry, but  the  source
             of the data packet is a local, directly-connected host, and
             the router is the DR on a multi-access LAN and  has  RP-Set
             information, the DR uses the hash function to determine the
             RP associated with the destination group, G.  The  DR  then
             checks  the  Register-bit  associated with the local sender
             (if there is no such a Register-bit, a new  register  flag,
             associated  with the local sender, is created and set), and
             encapsulates the data packet  in  a  Register  message  and
             unicasts it to the RP.


        5    If the packet does not match to any entry, and it is not  a
             local host or the router is not the DR, drop the packet.




     3.4.1 Data triggered switch to shortest path tree (SP-tree)

        Different criteria can be applied to trigger switching over from
        the  RP-based  shared  tree  to  source-specific,  shortest path
        trees.

        One proposed example is  to  do  so  based  on  data  rate.  For
        example,  when  a  (*,G),  or  corresponding  (*,*,RP), entry is
        created, a data rate counter may be initiated  at  the  last-hop
        routers.  The  counter  is  incremented  with  every data packet
        received for directly connected members of an SM group,  if  the
        longest  match  is  (*,G) or (*,*,RP). If and when the data rate
        for the group exceeds a certain configured threshold  (t1),  the
        router  initiates  `source-specific'  data rate counters for the
        following data packets. Then, each  counter  for  a  source,  is
        incremented  when  packets  matching  on (*,G), or (*,*,RP), are
        received from that source. If the data rate from the  particular
        source  exceeds  a  configured  threshold (t2), a (S,G) entry is
        created and a Join/Prune message is sent towards the source.  If
        the RPF interface for (S,G) is
         not the same as that for (*,G) -or (*,*,RP), then  the  SPT-bit
        is cleared in the (S,G) entry.




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        Other configured rules may  be  enforced  to  cause  or  prevent
        establishment of (S,G) state.





     3.5 Assert

        Asserts are used  to  resolve  which  of  the  parallel  routers
        connected  to  a  multi-access LAN is responsible for forwarding
        packets onto the LAN.

     3.5.1 Sending Asserts

        The following Assert rules are provided when a multicast  packet
        is received on an outgoing multi-access interface of an existing
        (S,G) entry:



        1    Do unicast routing table lookup on source IP  address  from
             data  packet,  and  send  assert on interface for source IP
             address  in  data  packet;  include  metric  preference  of
             routing protocol and metric from routing table lookup.


        2    If route is not found, use metric preference of  0x7fffffff
             and metric 0xffffffff.


        3    When an assert is sent for a (*,G) entry, the first bit  in
             the  metric preference (the RP-bit) is set to 1, indicating
             the data packet is routed down the RP-tree.


        Asserts are rate-limited by the router.

     3.5.2 Receiving Asserts


        When an assert is received the router performs a  longest  match
        on  the  source  and  group  address  in the assert message. The
        router checks the first bit of the metric  preference  (RP-bit).
        If  the  RP-bit  is  set,  the  router does a match on (*,G), or
        (*,*,RP), entries, otherwise, the router matches (S,G)  entries.
        If  the  matching  entry is (*,*,RP), the router creates a (*,G)
        entry.



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        If the interface that received the Assert message is  in  the
        oif  list  of  the  matched  entry, then this assert should be
        processed by this router as follows:


        1    Compare the metric received in the Assert with the one  the
             router  would  have  advertised  in  an  assert. The metric
             preference should be treated as the high-order part  of  an
             assert  metric  comparison.  If  the value in the assert is
             less than the router's value, delete the interface from the
             entry.  If  the value is the same, compare IP addresses, if
             the routers address is less than the assert sender,  delete
             the interface.


        2    If the router has won the election and there  are  directly
             connected members on the multi-access LAN, the router keeps
             the interface in its outgoing interface list.  It  acts  as
             the forwarder for the LAN.



        3    If the router won the election but there  are  no  directly
             connected  members  on  the  multi-access  LAN,  the router
             schedules to delete the interface. The LAN might be a  stub
             LAN  with  no  members  (and  no downstream routers). If no
             subsequent Join/Prunes are received, the router deletes the
             interface  from  the  outgoing interface list; otherwise it
             keeps the interface in its outgoing interface and  acts  as
             the forwarder for the LAN.


        The winning router should send out an assert  message  including
        its own metric to that outgoing interface. This will cause other
        routers on the LAN to prune that interface from their forwarding
        entries.

        Note that when an Assert is received,  the  router  performs  an
        exact  match  based on the source address, group address and the
        RP-bit of the metric preference in the assert message.  This  is
        not  a longest match; only exact state will be matched. If there
        is no such state, then the  router  drops  the  Assert  message.
        Otherwise, If the interface that received the Assert matches the
        incoming interface of the exactly matched entry, then the Assert
        message is processed as follows:


        1    Downstream routers will select the upstream router with the



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             smallest  metric  as their RPF neighbor. If two metrics are
             the same, the highest IP address is  chosen  to  break  the
             tie.  [*]




        2    If the downstream routers  have  downstream  members,  they
             must  schedule  a  join  to inform the upstream router that
             packets should be forwarded on  the  multi-access  network.
             This  will  cause  the  upstream  forwarder  to  cancel its
             scheduled deletion of the interface.



























_________________________
[*] This is important so that downstream  routers  send
subsequent  Joins/Prunes  (in SM) to the correct neigh-
bor. An Assert timer is initiated when changing the RPF
neighbor  to  the Assert winner. When the timer expires
the router resets its RPF neighbor according to its un-
icast  routing tables to capture failures of the Assert
winner.




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     3.6 Candidate-RP-Advertisements and RP-Set messages

        Candidate-RP-Advertisements   (C-RP-Advs)   are   periodic   PIM
        messages  unicast  by  those  routers  that  are  configured  as
        Candidate-RPs (C-RPs).

        RP-Set messages are periodic  PIM  messages  originated  by  the
        Bootstrap router (BSR) within a domain, and forwarded hop-by-hop
        to distribute the current RP-set to all routers in that  domain.
        The RP-Set messages also support a simple mechanism by which the
        Candidate BSR (C-BSR)  with  the  highest  BSR-priority  and  IP
        address (referred to as the preferred BSR) is elected as the BSR
        for the domain  [*]

     3.6.1 Sending Candidate-RP-Advertisements

        C-RPs periodically unicast C-RP-Advs to the BSR for that domain.
        The  interval  for  sending  these  messages is subject to local
        configuration at the C-RP. A recommended  default  value  is  60
        seconds.

        Candidate-RP-Advertisements carry group address and  group  mask
        fields.  This  enables  the  advertising  router  to  limit  the
        advertisement to certain  prefixes  or  scopes  of  groups.  The
        advertising  router  may  enforce  this  scope  acceptance  when
        receiving Registers or Join/Prune messages.

     3.6.2 Receiving C-RP-Advs and Originating RP-Set

        Upon receiving a C-RP-Adv, a router does the following:


        1    If the router is  not  the  elected  BSR,  it  ignores  the
             message, else


        2    The BSR adds the RP address to its local pool of  candidate
             RPs,  according  to  the associated group prefix(es) in the
             C-RP-Adv message  [*]  The  BSR  may  override  the  prefix
             indicated in a C-RP-Adv.


_________________________
[*] We recommend that each router configured as a  C-RP
also be configured as a C-BSR.
[*] The BSR may apply  a  local  policy  to  limit  the
number of Candidate RPs included in the RP-Set message.




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        The BSR keeps an RP-timer per RP in its local  RP-set.  The  RP-
        timer  is initialized to three times the holdtime in the RP's C-
        RP-Adv. When the timer expires, the corresponding RP is  removed
        from the RP-set. The RP-timer is restarted by the C-RP-Advs from
        the corresponding RP.

        The BSR also keeps an  RP-Set  timer  to  send  RP-Set  messages
        periodically.  In particular, when the RP-Set timer expires, the
        BSR originates an RP-Set message on each of its interfaces.  The
        message  is sent with a TTL of 1 to the `ALL-PIM-ROUTERS' group.
        In steady state, the BSR originates  RP-Set  messages  every  60
        seconds.  At  startup,  the  RP-Set  timer is initialized to 180
        seconds, causing the first RP-Set message to be originated after
        180  seconds,  when/if  the timer expires. For timer details see
        Section  3.6.3. A DR unicasts  an  RP-Set  message  to  new  PIM
        neighbors  starting  up,  after  receiving their Query messages.
        (since after DR election the new neighbor  may  become  the  new
        DR.)

        The RP-Set message is subdivided into sets  of  group-prefix,RP-
        Count,RP-addresses.  The  format  of  the  RP-Set message allows
        `semantic fragmentation', if the length of the  original  RP-Set
        message  exceeds the packet maximum boundaries (see Section  4).
        However, we recommend against  configuring  a  large  number  of
        routers as C-RPs, to reduce the semantic fragmentation required.

     3.6.3 Receiving and Forwarding RP-Set

        Each router keeps an RP-Set timer, initialized to 180 seconds at
        startup.

        When a router receives RP-Set message sent to  `ALL-PIM-ROUTERS'
        group, it performs the following:


        1    If the message was not sent by the RPF neighbor towards the
             BSR address included, the message is dropped. Else


        2    If the included BSR is  not preferred over, and  not  equal
             to, the currently active BSR:


             1    If the RP-Set timer is  not yet  expired,  or  if  the
                  receiving  router  is a C-BSR, then the RP-Set message
                  is dropped. Else

             2    The RP-Set timer  is expired and the receiving  router



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                  is

                  not a C-BSR, so the receiving router stores the RP-Set
                  and  BSR  address  found  in  the  message. The RP-Set
                  message is then  forwarded  out  all  PIM  interfaces,
                  excluding  the  one over which the message arrived, to
                  `ALL-PIM-ROUTERS' group, with a TTL of 1.



        3     If the RP-Set message includes  a  BSR  address  that   is
             preferred  over, or equal to, the currently active BSR, the
             router resets its RP-Set timer to 180 seconds,  and  stores
             the  BSR address and RP-Set information. The RP-Set message
             is then forwarded out all PIM interfaces, excluding the one
             over which the message arrived, to `ALL-PIM-ROUTERS' group,
             with a TTL of 1.

        If the receiving router has no current RP  set  information  and
        the RP-set was unicast to it from a directly connected neighbor,
        the router stores the information as its new RP-set. This covers
        the  startup  condition  when  a newly booted router obtains the
        RP-Set and BSR address from its DR.

        When a router receives a new RP-Set it checks if each of the RPs
        referred  to  by  existing  state  (i.e., by (*,G), (*,*,RP), or
        (S,G)RPbit entries) is in the new RP-Set. If an RP is not in the
        new  RP-set,  that  RP  is  considered  unreachable and the hash
        algorithm (see  below)  is  re-performed  for  each  group  with
        locally  active  state  that  previously hashed to that RP. This
        will cause those groups to be distributed  among  the  remaining
        RPs. When the new RP-Set contains a new RP, the value of the new
        RP is calculated for each group covered by  that  C-RP's  Group-
        prefix.  Any  group for which the new RP's value is greater than
        the previously active RP's value is switched over to the new RP.




     3.7 Hash Function

        The hash function is used by all routers within a domain, to map
        a  group  to  one of the C-RPs from the RP-Set. For a particular
        group, G, the hash function uses only those C-RPs  whose  Group-
        prefix covers G. The algorithm takes as input the group address,
        and the addresses of the Candidate RPs, and gives as output  one
        RP address to be used.




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        The protocol requires that all  routers  hash  to  the  same  RP
        within  a  domain  (except  for  transients). The following hash
        function must be used in each router:


        1    For each candidate RP address Ci  in  the  Candidate-RP-
             Set,  whose  Group-prefix  covers  G,  compute  a  value:
             Value(G,M,Ci) =
              1103515245 ((1103515245 (G&M)+12345) XOR Ci)+ 12345 mod 2^31
             where M is a hash-mask included in RP-Set messages.
             This hash-mask  allows  a   small   number   of
             consecutive groups (e.g., 4) to always hash to the same RP.
             For instance, hierarchically-encoded data can  be  sent  on
             consecutive  group  addresses  to  get  the  same delay and
             fate-sharing characteristics.



             In standard C, this corresponds to:

           srand(G & M);
           srand(rand() ^ Ci);
           value = rand();



        2    The candidate with the  highest  resulting  value  is  then
             chosen  as the RP for that group, and its identity and hash
             value are stored with the entry created.

             Ties between C-RPs having the same hash value,  are  broken
             in advantage of the highest address.



        The hash function algorithm is invoked by a DR,  upon  reception
        of  a  packet,  or IGMP Host-Membership-Report, for a group, for
        which the DR has no entry. It is invoked by any router that  has
        (*,*,RP)  state  when a packet is received for which there is no
        corresponding  (S,G)  or  (*,G)  entry.  Furthermore,  the  hash
        function  is  invoked by all routers upon receiving a Join/Prune
        message with WC-bit set.



     3.8 Processing Timer Events

        { Editors Note: Timers are also discussed  individually  in  the
        sections  that  pertain  to  the  protocol  messages  that  they



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        trigger/affect. Until we finalize this section, if discrepencies
        exist,   then   assume   that   the   individual   sections  are
        authoritative over this table.}

        In this subsection, we  enumerate  all  timers  that  have  been
        discussed  or  implied. Since some critical timer events are not
        associated with the receipt or sending of messages, they are not
        fully covered by earlier subsections.

        In many cases, the values for timers come from  Holdtime  fields
        in  PIM  control messages, in which case the default values used
        in  these  Holdtime  fields  are  shown  in  the  tables  below.
        Otherwise,  the  default  value  used  when setting the timer is
        shown.  In  general,  the  default  timeout  value   for   state
        information  is  three  times  the  refresh period. For example,
        Queries refresh  Neighbor  state  and  the  default  Query-timer
        period is 30 seconds, so a default Neighbor-timer duration of 90
        seconds is included in the Holdtime field of the Queries.

        In this version of the  spec  we  suggest  particular  numerical
        timer  settings.  A  future  version  of  the specification will
        specify a mechanism for timers to be set as a  function  of  the
        outgoing link bandwidth.

        bsubsection*Timers related to tree maintenance

        Each (S,G),  (*,G),  and  (*,*,RP)  entry  has  multiple  timers
        associated  with  it:  one  for  each  interface in the outgoing
        interface list, one for the multicast routing entry itself,  and
        one  for  the Joiner-bit. Each (S,G) and (*,G) entry also has an
        Assert timer and an Assert-rate-limit timer. In  addition,  DR's
        have  a Register-bit-timer for each (S,G) entry and every router
        has a single Join/Prune timer.

        Because some of the outgoing interfaces in an  (S,G)  entry  are
        copied from the (*,G) outgoing interface list, they may not have
        explicit (S,G) join messages from some of the downstream routers
        (i.e.,  where members are joining to the (*,G) tree only). Thus,
        when a timer is reset for an  outgoing  interface  listed  in  a
        (*,G)  entry,  the  timers  are reset for that interface in each
        existing (S,G) entry whose oif list contains that interface  [*]
_________________________
[*] If there are sources in the prune list of the (*,G)
join,  then  the timers for the arriving interface will
first be reset for those sources, and then this  inter-
face will be deleted from these same entries; producing
a correct result, even though the updating of  the  ti-
mers  was unnecessary. An implementation could optimize



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        The same rule applies to (*,G) and (S,G) entries when  resetting
        an oif timer on a (*,*,RP) entry.










































_________________________
this by checking the prune list before  processing  the
join list.




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   Timer              DefVal Notes

   Joiner-bit            90  Started : When Joiner bit is cleared
   per route entry           Reset by: Receiving Join from higher-IP neighbor on iif
                             Action  : Set Joiner bit

   Join/Prune            60  Started : When booting
                             Reset by: Nothing
                             Action  : Send Join/Prune to each RPF neighbor, restart timer

   oif                  180  Started : When adding oif to oiflist
   per (*,*,RP) oif          Restarted by: Receiving (*,*,RP) Join on that iface
                             Action  : Remove oif from oiflist

   oif                  180  Started : When adding oif to oiflist
   per (*,G) oif             Restarted by: Receiving (*,G) Join or IGMP
                             Host-Membership-Report for G on that iface, or
                             restartedting oif timer in (*,*,RP).
                             Action  : Remove oif from oiflist

   oif                  180  Started : When adding oif to oiflist
   per (S,G) oif             Restarted by: Receiving (S,G) Join on that
                             iface, or restartedting oif timer in (*,G) or
                             (*,*,RP).
                             Action  : Remove oif from oiflist

   (*,*,RP) entry       180  Started : When entry is created
   per (*,*,RP)              Restarted by: Restartedting timer on any oif
                             Action  : Delete entry

   (*,G) entry          180  Started : When entry is created
   per (*,G)                 Restarted by: Receiving (*,G) prune,
                             restarting timer on any oif, or receiving an
                             Assert with RP-bit set.
                             Action  : Delete entry and any associated
                             (S,G)RP-bit entries

   (S,G) entry          180  Started : When entry is created
   aka S-timer               Restarted by: Forwarding data packet,
   per (S,G)                 receiving Register, receiving (S,G) RP-bit
                             prune, restarting timer on any oif,
                             or receiving an Assert without RP-bit set.
                             Action  : Delete entry

   Register-bit          60  Started : When Register bit is cleared by
   per (S,G)                 receiving a Register-Stop
                             Restarted by: Receiving Register-Stop



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                             Action  : Set Register bit

   Assert               180  Started : Receiving an Assert where the
   per (S,G)                 upstream RPF neighbor is not your unicast RPF
   and (*,G)                 neighbor.
                             Restarted by: Receiving an Assert where the
                             upstream RPF neighbor is not your unicast
                             RPF neighbor.
                             Action  : Change RPF neighbor to unicast RPF neighbor

   Assert-Rate-limit      5  Started : When an Assert is sent
   per (S,G)                 Restarted by: Nothing
   and (*,G)                 Action  : Allow asserts to be triggered by
                             data packets



   *Timers relating to neighbor discovery


   Timer              DefVal Notes

   Query                 30  Started : When booting
                             Restarted by: Nothing
                             Action  : Send Query on all ifaces, restart timer

   Neighbor              90  Started : When receive first Query from neighbor
   per neighbor              Restarted by: When receive subsequent Queries
                             Action  : Delete neighbor entry


   *Timers relating to RP information



















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   Timer              DefVal Notes

   C-RP-Adv              60  Started : When booting if you're a Cand-RP
                             Restarted by: Nothing
                             Action  : Send C-RP-Adv, restart C-RP-Adv timer

   RP                   180  Started : When adding an RP to the RP-Set if
   per RP                    you are BSR
                             Restarted by: Receiving C-RP-Adv
                             Action  : Remove RP from RP-Set

   RP-Set            180/60  Started : Set to 180 when booting if
                             you're a C-BSR
                             Restarted by: Restarted to 180 when receive
                             RP-Set from preferred router if you're a C-BSR
                             Action  : Send RP-Set and restart  timer to 60 secs






     3.9 Summary of flags used

        Following is a summary of all the flags used in our scheme.

   Bit      Used in      Definition

   Border   Register     Register is coming from a PIM border router.
   Joiner   Route entry  Periodic Join/Prunes should be sent for this entry.
   Register (S,G) entry  Encapsulate packets from directly connected
                         sources in Register messages unicast to the RP
                         for that group.
   RP       Route entry  Entry represents state on the RP-tree.
   RP       Join/Prune   Join is associated with the shared tree and therefore
                         the Join/Prune message is propagated along the RP-tree.
   RP       Assert       The data packet was routed down the shared tree; thus,
                         the path indicated corresponds to the RP tree.
   SPT      (S,G) entry  Packets have arrived on the iif towards S,
                         and the iif is different from the (*,G) iif.
   WC       Join         Included address is an RP and the receiver expects to
                         receive packets from all sources via this (shared tree)
                         path.  Thus, the Join/Prune applies to a (*,G) entry.
   WC       Route entry  Wildcard entry; if there is no more specific match for
                         a particular source, packets will be forwarded according
                         to this entry.




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     3.10 Security

        { Editors Note: this section is to be completed.}

        All PIM  control  messages  may  use  [5]  to  address  security
        concerns.













































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     4 Packet Formats

        This section describes the details of the packet formats for PIM
        control messages.

        All PIM control messages have protocol number 103.

        Basically, PIM messages are either unicast (e.g.  Registers  and
        Register-Stop),  or  multicast  hop-by-hop  to `ALL-PIM-ROUTERS'
        group `224.0.0.13' (e.g. Join/Prune, Asserts, etc.).


     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  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






        PIM Ver
              PIM Version number is 2.


        Type  Types for specific PIM messages. PIM Types are:



           0 = Query
           1 = Register
           2 = Register-Stop
           3 = Join/Prune
           4 = RP-Set
           5 = Assert
           6 = Graft (used in PIM-DM only)
           7 = Graft-Ack (used in PIM-DM only)
           8 = Candidate-RP-Advertisement



        Addr length
              Address length in  bytes.  Throughout  this  section  this
             would  indicate the number of bytes in the Address field of
             an address, including unicast and group addresses.




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        Checksum
              The checksum is the 16-bit one's complement of  the  one's
             complement  sum  of  the entire PIM message, (excluding the
             data portion in the Register message).  For  computing  the
             checksum, the checksum field is zeroed.














































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     4.1 Encoded Source and Group Address formats



        1    Unicast address: Only the address is included.  The  length
             of  the  unicast address in bytes is specified in the `Addr
             length' field in the header.


        2    Encoded-Group-Address: Takes the following format:


          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |     Reserved  |  Mask Len     | Group multicast Address ...   |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | ...Group multicast Address ...|
         +-+-+-+-+-+-+-+-+-+-+~+~+~+~+~+~+




             Reserved
                   Transmitted as zero. Ignored upon receipt.

             Mask Len
                   The Mask length is 8 bits. The value is the number of
                  contiguous  bits  left  justified used as a mask which
                  describes the address. It is less  than  or  equal  to
                  Addr  length  * 8. If the message is sent for a single
                  group then the Mask length should equal Addr length  *
                  8 (i.e. 32 for IPv4 and 128 for IPv6).

             Group multicast Address
                   contains the group address, and has number  of  bytes
                  equal to that specified in the Addr length field.



        3    Encoded-Source-Address: Takes the following format:










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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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         | Rsrvd   |S|W|R|  Mask Len     | Source Address ...            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |  ...  Source Address          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+~+~+-+





             Reserved
                   Transmitted as zero, ignored on receipt.

             S,W,R See Section7 ef{Join_format} for details.

             Mask Length
                   Mask length is 8 bits. The value  is  the  number  of
                  contiguous  bits  left  justified used as a mask which
                  describes the address. The mask length  must  be  less
                  than  or  equal  to Addr Length * 8. If the message is
                  sent for a single source then the Mask  length  should
                  equal  Addr  length  *  8.  In version 2 of PIM, it is
                  strongly recommended that this field be set to 32  for
                  IPv4.

             Source Address
                   The address length is indicated from the Addr  length
                  field  at  the  beginning of the header. For IPv4, the
                  address length is 4 octets.



















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     4.2 Query Message

        It is sent periodically by routers on all interfaces.


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




        PIM Version, Type, Addr length, Checksum
              Described above.

        Reserved
              Transmitted as zero, ignored on receipt.

        Holdtime
              The amount of time a receiver  should  keep  the  neighbor
             reachable, in seconds.


























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     4.3 Register Message

        It is sent by the Designated  Router  (DR)  to  the  RP  when  a
        multicast  packet needs to be transmitted on the RP-tree. Source
        IP address is set to the  address  of  the  DR,  destination  IP
        address is to the RP's address.


     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  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |B|                         Reserved                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
                          Multicast data packet
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




        PIM Version, Type, Addr length, Checksum
              Described above.  { Note that the checksum  for  Registers
             is  done  only on the PIM header, excluding the data packet
             portion.}

        B     The Border bit. Set to zero by all DRs. Set to `1' by  the
             PIM Multicast Border Routers, when registering for external
             sources.


        Multicast data packet
              The original packet sent by the source.
















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     4.4 Register-Stop Message

        A Register-Stop is unicast from the RP  to  the  sender  of  the
        Register  message. Source IP address is the address to which the
        register was addressed. Destination IP  address  is  the  source
        address of the register message.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |PIM Ver| Type  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Encoded-Group Address                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Unicast-Source Address                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




        PIM Version, Type, Addr length, Checksum
              Described above.

        Encoded-Group Address
              Format described above. Note that for  Register-Stops  the
             Mask  Len  field  should  contain  Addr  length * 8 (32 for
             IPv4), if the message is sent for a single group.

        Unicast-Source Address
              IP host address of source from multicast  data  packet  in
             register. The length of this field in bytes is specified in
             the Addr length field. A special wild card value (0.0.0.0),
             can be used to indicate any source.

















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     4.5 Join/Prune Message

        It is sent by routers towards upstream sources and RPs.  A  join
        creates  forwarding state and a prune destroys forwarding state.
        Joins are sent to build shared trees (RP trees) or source  trees
        (SPT).  Prunes are sent to prune source trees when members leave
        groups as well as sources that do not use the shared tree.












































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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |PIM Ver| Type  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Unicast-Upstream Neighbor Address                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Reserved     | Num groups    |          Holdtime             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Encoded-Multicast Group Address-1                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Number of Joined  Sources   |   Number of Pruned Sources    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Joined Source Address-1                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             .                                 |
    |                             .                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Joined Source Address-n                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Pruned Source Address-1                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             .                                 |
    |                             .                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Pruned Source Address-n                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           .                                   |
    |                           .                                   |
    |                           .                                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Encoded-Multicast Group Address-n              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Number of Joined  Sources   |   Number of Pruned Sources    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Joined Source Address-1                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             .                                 |
    |                             .                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Joined Source Address-n                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Encoded-Pruned Source Address-1                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             .                                 |
    |                             .                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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    |               Encoded-Pruned Source Address-n                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




        PIM Version, Type, Addr length, Checksum
              Described above.

        Upstream Neighbor Address
              The IP address of the RPF or upstream neighbor.

        Reserved
              Transmitted as zero, ignored on receipt.

        Holdtime
              The amount of time a receiver should keep  the  Join/Prune
             state alive, in seconds.

        Number of Groups
              The number  of  multicast  group  sets  contained  in  the
             message.

        Encoded-Multicast group address
              For format description see Section
              4.1. A wild card group in the (*,*,RP) join is represented
             by  a  224.0.0.0  in the group address field and `4' in the
             mask length field. A (*,*,RP) join also has the WC-bit  and
             the RP-bit set.


        Number of Joined Sources
              Number of join source addresses listed for a given group.


        Join Source Address-1 .. n
              This list contains the sources  that  the  sending  router
             will  forward  multicast  datagrams  for if received on the
             interface this message is sent on.

             See format section  4.1. The  fields  explanation  for  the
             Encoded-Source-Address format follows:



             Reserved
                   Described above.




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             S     The Sparse bit is a 1 bit value, set to 1 for PIM-SM.
                  It is used for PIM v.1 compatability.

             W     The WC bit is a 1 bit value. If 1, the join or  prune
                  applies to the (*,G) or (*,*,RP) entry. If 0, the join
                  or prune applies to the (S,G) entry where S is  Source
                  Address.  Joins  and prunes sent towards the RP should
                  have this bit set.

             R     The RP bit is a 1 bit value. If  1,  the  information
                  about  (S,G)  is  sent  towards  the  RP.  If  0,  the
                  information should be sent about (S,G) toward S, where
                  S is Source Address.

             Mask Length, Source Address
                   Described above.



             Represented in the form of
             < WCbit >< RPbit >< Mask length>< Source address>:

             A source address could be a host IP address :

              < 0 >< 0 >< 32 >< 192.1.1.17 >

             A source address could be the RP's IP address :

              < 1 >< 1 >< 32 >< 131.108.13.111 >

             A source address could be a subnet address  to  prune  from
             the RP-tree :

              < 0 >< 1 >< 28 >< 192.1.1.16 >

             A source address could be a general aggregate :

              < 0 >< 0 >< 16 >< 192.1.0.0 >

        Number of Pruned Sources
              Number of prune source addresses listed for a group.

        Prune Source Address-1 .. n
              This list contains the sources  that  the  sending  router
             does  not  want  to  forward  multicast  datagrams for when
             received on the interface this message is sent on  [*]
_________________________
[*] If the Join/Prune message boundary exceeds the max-



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     4.6 RP-Set

        The RP-Set messages are multicast  to  `ALL-PIM-ROUTERS'  group,
        out  all interfaces having PIM neighbors (excluding the one over
        which the message was received). RP-Set messages are  sent  with
        TTL  value  of  1. RP-Set messages originate at the BSR, and are
        forwarded by intermediate routers.

        RP-Set message is divided up into `semantic fragments',  if  the
        original message exceeds the maximum packet size boundaries.

        The semantics of a single `fragment' is given below:
































_________________________
imum packet size, then the join and prune lists for the
same group must be included in the same packet.




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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |PIM Ver| Type  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Fragment Tag          | Hash Mask len | BSR-priority  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-BSR-Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Encoded-Group Address-1               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | RP-Count-1    | Frag RP-Cnt-1 |         Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-RP-Address-1                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               .                               |
    |                               .                               |
    |                               .                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-RP-Address-m                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               .                               |
    |                               .                               |
    |                               .                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Encoded-Group Address-n               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | RP-Count-m    | Frag RP-Cnt-m |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-RP-Address-1                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               .                               |
    |                               .                               |
    |                               .                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-RP-Address-m                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






        PIM Version, Type, Addr length, Checksum
              Described above.

        Fragment Tag



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              A randomly  generated  number,  acts  to  distinguish  the
             fragments belonging to different RP-Set messages; fragments
             belonging to same RP-Set message carry the  same  `Fragment
             Tag'.

        Hash Mask len
              The length (in bits) of  the  mask  to  use  in  the  hash
             function.  For IPv4 we recommend a value of 30. For IPv6 we
             recommend a value of 126.

        BSR-priority
              Contains the BSR priority value of the included BSR.  This
             field is considered as a high order byte when comparing BSR
             addresses.

        Unicast-BSR-Address
              The IP address of the bootstrap router for the domain. The
             length of this field in bytes is specified in Addr length.

        Encoded-Group Address-1..n
              The  group  prefix  (address  and  mask)  with  which  the
             Candidate RPs are associated. Format previously described.


        RP-Count-1..n
              The number of Candidate RP addresses included in the whole
             RP-Set message for the corresponding group prefix  [*]


        Frag RP-Cnt-1..m
              The number of Candidate  RP  addresses  included  in  this
             fragment of the RP-Set message, for the corresponding group
             prefix. The `Frag RP-Cnt' field facilitates parsing of  the
             RP-Set  for  a  given  group prefix, when carried over more
             than one fragment.


        Unicast-RP-address-1..m
              The address of the Candidate RPs,  for  the  corresponding
_________________________
[*] A router does not replace its old  RP-Set  for  a
given  group prefix until/unless it receives `RP-Count'
addresses for that prefix; the addresses could be  car-
ried over several fragments. If only part of the RP-Set
for a given group prefix was received, the router  dis-
cards it, without updating that specific group prefix's
RP-Set.




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             group  prefix.  The  length  of  this  field  in  bytes  is
             specified in Addr length.

















































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     4.7 Assert Message

        The Assert message is sent  when  a  multicast  data  packet  is
        received  on an outgoing interface corresponding to the (S,G) or
        (*,G) associated with the source.


     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  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Encoded-Group Address                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Unicast-Source Address                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |R|                        Metric Preference                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Metric                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




        PIM Version, Type, Addr length, Checksum
              Described above.

        Encoded-Group Address
              The group address to which the data packet was  addressed,
             and   which   triggered   the   Assert.  Format  previously
             described.

        Unicast-Source Address
              Source  IP  address  from  IP  multicast   datagram   that
             triggered  the Assert packet to be sent. The length of this
             field in bytes is specified in Addr length.

        R     RP bit is a 1 bit value. If the IP multicast datagram that
             triggered  the  Assert  packet  is routed down the RP tree,
             then the RP bit is 1;  if  the  IP  multicast  datagram  is
             routed down the SPT, it is 0.

        Metric Preference
              Preference value assigned to the unicast routing  protocol
             that provided the route to Host address.

        Metric The unicast routing table metric. The metric is in  units
             applicable to the unicast routing protocol used.



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     4.8 Graft Message

        Used in dense-mode. Refer to PIM dense mode specification.

     4.9 Graft-Ack Message

        Used in dense-mode. Refer to PIM dense mode specification.












































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     4.10 Candidate-RP-Advertisement

        Candidate-RP-Advertisements are periodically  unicast  from  the
        C-RPs to the BSR.


     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  | Addr length   |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Prefix-Cnt    | Reserved      |             Holdtime          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Unicast-RP-Address                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Encoded-Group Address-1               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               .                               |
    |                               .                               |
    |                               .                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Encoded-Group Address-n               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






        PIM Version, Type, Addr length, Checksum
              Described above.

        Prefix-Cnt
              The number of encoded  group  addresses  included  in  the
             message;  indicating  the group prefixes for which the C-RP
             is advertising. A Prefix-Cnt of `0'  implies  a  prefix  of
             224.0.0.0 with mask length of 4; i.e. all multicast groups.
             If  the  C-RP   is   not   configured   with   Group-prefix
             information,  the  C-RP puts a default value of `0' in this
             field.

        Holdtime
              The amount of time the advertisement is valid. This  field
             allows advertisements to be aged out.

        Unicast-RP-Address
              The address of the interface to advertise as  a  Candidate
             RP.  The length of this field in bytes is specified in Addr



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             length.

        Encoded-Group Address-1..n
              The group prefixes for  which  the  C-RP  is  advertising.
             Format previously described.














































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     5 Appendix I: Changes and Updates to the Spec

        This appendix populates the major changes in  the  specification
        document as compared to `draft-ietf-idmr-pim-spec-01.ps,txt'.

     5.1 Major Changes

        List of changes since March '96 IETF:


        1. (*,*,RP) Joins state and data forwarding check;  replaces  (*,G-
        Prefix)  Joins  state for interoperability. (*,G) negative cache
        introduced for the (*,*,RP) state supporting mechanisms.

        2. Semantic fragmentation for the RP-Set message.

        3. Appendix  II  on  interoperability   details   with   DVMRP   in
        preparation.

        List of changes incurred since version 1 of the spec:

        1. Proposal and  refinement  of  bootstrap  router  (BSR)  election
        mechanisms

        2. Introduction of hash functions for Group to RP mapping

        3. New  RP-liveness  indication  mechanisms  based  upon  the   the
        Bootstrap Router (BSR) and the RP-Set messages.

        4. Removal of reachability messages, RP reports  and  multiple  RPs
        per group.


     5.2 Packet Format Changes

        Packet Format incurred updates to accommodate different  address
        lengths, and address aggregation.




        1    The `Addr length' field was added to the PIM  fixed  header



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             to  specify  the  address length in bytes of the underlying
             protocol, see section  4.

        2    The  Encoded  source  and  group   address   formats   were
             introduced,  with the use of a `Mask length' field to allow
             aggregation, section  4.1.

        3    Packet formats are no  longer  IGMP  messages;  rather  PIM
             messages.



        PIM message types and formats were also modified:


        [{ Note: most changes were made to the May  95  version,  unless
        otherwise specified}].




        1    Obsolete messages:

            a. Register-Ack [Feb. 96]

            b. Poll and Poll Response [Feb. 96]

            c. RP-Reachability [Feb. 96]

            d. RPlist-Mapping [Feb. 96]



        2    New messages:

            a. Candidate-RP-Advertisement [change made in  October  95]

            b. RP-Set [Feb. 96]

        3    Modified messages:



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            a. Join/Prune [Feb. 96]

            b. Register [Feb. 96]

            c. Register-Stop [Feb. 96]














































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     6 Appendix II: Interoperability with Dense Mode Protocols

        { Editors Note: This section is to be completed.}
















































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     7 Acknowledgments

        Tony Ballardie, Scott Brim, Jon  Crowcroft,  Bill  Fenner,  Paul
        Francis,   Joel  Halpern,  Horst  Hodel,  Polly  Huang,  Stephen
        Ostrowski,  and  Lixia  Zhang  provided  detailed  comments   on
        previous  drafts.  The authors of [6] and membership of the IDMR
        WG provided many of the  motivating  ideas  for  this  work  and
        useful feedback on design details.

        This work was supported  by  the  National  Science  Foundation,
        ARPA, cisco Systems and Sun Microsystems.




        References


   1.   S.Deering,  D.Estrin,  D.Farinacci,  V.Jacobson,  C.Liu,  L.Wei,
        P.Sharma,  and  A.Helmy.  Protocol independent multicast (pim) :
        Motivation and architecture.
         Internet Draft, May 1995.


   2.   S.Deering, D.Estrin, D.Farinacci, V.Jacobson, C.Liu, and  L.Wei.
        The pim architecture for wide-area multicast routing.
         ACM Transactions on Networks, April 1996.


   3.   D.Estrin, D.Farinacci, V.Jacobson, C.Liu, L.Wei,  P.Sharma,  and
        A.Helmy.  Protocol  independent  multicast-dense mode (pim-dm) :
        Protocol specification.  Internet Draft, November 1995.


   4.   S.Deering.  Host  extensions  for  ip  multicasting,  aug  1989.
        RFC1112.


   5.   R.Atkinson. Security architecture  for  the  internet  protocol,
        August 1995. RFC-1825.


   6.   A.J. Ballardie, P.F. Francis, and J.Crowcroft. Core based trees.
        In  Proceedings of the ACM SIGCOMM, San Francisco, 1993.







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