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Versions: 00 01 02 03                                                   
Network Working Group                            Stephen Deering (Cisco)
Internet Draft                                      Deborah Estrin (USC)
                                                  Dino Farinacci (Cisco)
                                                    Van Jacobson (Cisco)
                                                       Ahmed Helmy (USC)
                                                     David Meyer (Cisco)
                                                      Liming Wei (Cisco)

draft-ietf-pim-v2-dm-03.txt                            June 7, 1999

   Protocol Independent Multicast Version 2 Dense Mode Specification

   Status of This Memo

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

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

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

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Internet Draft   PIM Version 2 Dense Mode Specification         Mar 1999

1 Introduction

  This specification defines a multicast routing algorithm efficient for
  multicast  groups  that are densely distributed across a network. This
  protocol does not have a topology discovery mechanism often used by  a
  unicast  routing  protocol. It employs the same packet formats sparse-
  mode PIM [PIMSM] uses. This protocol is  called  dense-mode  PIM.  The
  foundation of this design was largely built on Deering's early work on
  IP multicast routing [Deering91].

2 Terminology

  The key words ``MUST'', ``MUST NOT'', ``REQUIRED'', ``SHALL'', ``SHALL
  NOT'',  ``SHOULD'',  ``SHOULD  NOT'',  ``RECOMMENDED'',  ``MAY'',  and
  ``OPTIONAL'' in this document are to be interpreted  as  described  in
  RFC 2119.

3 Glossary

      Reverse Path Forwarding (RPF) / RPF interface / RPF lookup

           RPF is a multicast forwarding mode where  a  data  packet  is
           accepted  for  forwarding  if  it is received on an interface
           used to reach the source in unicast.  The  interface  passing
           this check is called the RPF interface.

           An RPF lookup for a source returns the RPF interface and  the
           next-hop  information,  as  if a route lookup is done for the
           source on a unicast routing table.

      Topology Discovery Mechanism/Protocol

           This mechanism provides  sufficient  topological  information
           for  a  router  to  determine  whether  a  neighbor system is
           upstream with respect to each multicast source. Some topology
           discovery  mechanism  can  also  determine  if  a neighbor is
           downstream with respect to each multicast source. An existing
           unicast  routing  protocol or a clone of it is sometimes used
           for this purpose (such as in DVMRP[DVMRP]). When a  multicast
           routing protocol has a topology discovery mechanism built-in,
           the topology discovery mechanism is sometimes referred to  as
           the  unicast  routing part of the protocol (even though it is

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Internet Draft   PIM Version 2 Dense Mode Specification         Mar 1999

           not used for forwarding unicast packets).

4 PIM-DM Protocol Overview

  Dense-mode  PIM  assumes  that  when  a  source  starts  sending,  all
  downstream  systems  want  to  receive multicast datagrams. Initially,
  multicast datagrams are flooded to all areas of the network.  If  some
  areas  of  the  network do not have group members, dense-mode PIM will
  prune off the forwarding branch by setting up prune state.  The  prune
  state  has  an  associated  timer,  which on expiration will turn into
  forward state, allowing data to go down the branch previously in prune

  The prune state contains source and group address information. When  a
  new member appears in a pruned area, a router can ``graft'' toward the
  source for the group, turning the pruned branch into forward state.

  The forwarding branches form a tree rooted at the  source  leading  to
  all members of the group. This tree is called a source rooted tree.

  The broadcast of datagrams followed by pruning of unwanted branches is
  often  referred  to  as  a broadcast-and-prune cycle, typical of dense
  mode protocols. The broadcast-and-prune mechanism in  dense  mode  PIM
  uses  a  technique  called  reverse  path forwarding (RPF), in which a
  multicast datagram is forwarded if the receiving interface is the  one
  used to forward unicast datagrams to the source of the datagram.

  Compared with  multicast  routing  protocols  with  built-in  topology
  discovery  mechanisms  (e.g.  DVMRP  with its own RIP-like ``unicast''
  routing protocol), dense mode PIM has simplified design,  and  is  not
  hard-wired  into  a  specific  type  of  topology  discovery protocol.
  However, such  simplification  does  incur  more  overhead  and  cause
  broadcast-and-prune  to  occur  on some links that could be avoided if
  sufficient topology information is available, e.g. to  decide  whether
  each  interface  leads  to  any  downstream neighbors for a particular
  source. We chose to accept the additional overhead  in  favor  of  the
  simplification  and  flexibility gained by not depending on a specific
  type of topology discovery protocol.

  In  relation  to  sparse-mode  PIM,  dense-mode  PIM  differs  in  two
  essential  ways:  1)  there  are  no  periodic joins transmitted, only
  explicit triggered grafts/prunes, and 2) there is no Rendezvous  Point

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5 Protocol Description

  Dense mode PIM initiates forwarding state in  routers  when  a  source
  begins  to send. A source does not give any prior notifications to the
  network when it sends multicast datagrams to a group G. If a receiving
  router does not already have a forwarding entry, it creates it for the
  source and group G. This forwarding entry is called a (S,G) entry.  It
  includes  the  following  contents: source address, group address, the
  incoming interface, a list of outgoing interfaces, a few flags  and  a
  few  timers.  The incoming interface for (S,G) is determined by an RPF
  lookup in the unicast routing table. The (S,G) outgoing interface list
  contains  interfaces that have PIM routers present or host members for
  group G.

  If a router creates a (S,G) entry with  an  empty  outgoing  interface
  list after receiving a multicast datagram, it must trigger a PIM-Prune
  message toward the source S. This type of entry is called  a  negative
  cache  entry. Negative cache entries can be found on leaf routers with
  no local group members,  or  on  routers  where  prune  messages  were
  received  from  downstream  routers that caused the outgoing interface
  list to become NULL.

  Dense mode PIM routers  send  periodic  Hello  messages  out  of  each
  interface  and  keep  track  of  neighbors  based  on  received  Hello
  messages. The Hello message  has  a  Holdtime  field  that  tells  the
  neighbor  to delete neighbor information if it is not refreshed before

  The following sections describe in detail how leaf network is  defined
  and  detected;  how  pruning is done on multi-access LANs; and actions
  related  to  new  members  joining  an  existing  group;  issues  with
  designated   router  election;  parallel  path  resolution;  multicast
  forwarding entry expiration and the adaptation to topology changes.

5.1 Leaf network detection

  In dense mode PIM,  prune  state  is  first  instantiated  on  routers
  connected  to  leaf  networks  without  group  members. A network on a
  router interface is deemed a leaf if there is no other  PIM  neighbors
  on  that  network.  The  notion of leaf network here might be slightly
  different from that defined in other protocols.   [*]
[*] E.g. In DVMRP  a  leaf  network  for  a  source  is
defined   as  one  without  downstream  neighbor  DVMRP
routers. Each DVMRP router is able to decide whether it
has  a  downstream  DVMRP neighbor with respect to each
source. This is due to the RIP-like mechanism  used  in

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  A router determines it is a leaf by not receiving PIM Hello  messages.
  A  leaf router can detect that there are no members downstream when it
  is the only router on a network and  there  are  no  IGMP  Host-Report
  messages   received  from  hosts.  A  router  should  not  populate  a
  forwarding  entry's  outgoing  interface  list  with  a  leaf  network
  interface without downstream members.

  It should be noted that a non-leaf router in  PIM  dense-mode  is  not
  necessarily a non-leaf node on a particular multicast forwarding tree.
  There are topologies where two parallel routers  are  connected  to  a
  common  LAN  where  none  of the routers uses the other one to reach a
  source. Therefore both routers will be leaves  of  the  shortest  path
  tree  rooted  at  the  source. When there are no group members on this
  LAN, both routers must not forward packets onto it. This  is  achieved
  by an assert process. See section 5.5 for more details.

  A dense mode PIM implementation MUST take proper actions when  a  non-
  leaf  router  becomes  a leaf router. When the last PIM neighbor on an
  interface goes away  and  turns  the  connected  subnet  into  a  leaf
  network,  the  router  SHOULD  delete that interface from the outgoing
  interface lists of all groups without attached group members.

5.2 Pruning of branches

5.2.1 Pruning on multi-access LANs and prune-override

  PIM prune  message  storms  can  be  generated  if  an  implementation
  responds  to  received multicast packets aggressively, e.g. by sending
  one prune for each data packet matching a  negative  cache  entry.  An
  implementation  SHOULD  either  respond  to  such  packets  in  a very
  conservative rate-limited manner, or not responding to such packets at
  all,  and  rely  on  the  expiration  and  recreation  of the entry to
  generate a prune. The latter is sufficient  in  most  cases  when  the
  probability of prune message loss is low.

  A prune is sent upstream when a router creates a (S,G) negative  cache
  entry,   or  when  the  entry  turns  into  a  negative  cache.  Prune
  information is flushed periodically. This causes  multicast  datagrams
  to be sent in RPF mode again which in turn triggers prune messages.

  When a prune message is sent onto an upstream LAN,  it  is  data  link
  multicast  and  IP  addressed  to  the  all  PIM routers group address The router to process  the  prune  will  be  indicated  by
  inclusion  of  its address in the "Address" field of the message. This
its   topology   discovery   (or  ``unicast  routing'')

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  address is obtained by an RPF look up for the source. When  the  prune
  message  is  sent,  the expected upstream router will schedule a prune
  request of the LAN from its outgoing interfaces for the  (S,G)  entry.
  The suggested default delay time before deletion is 3 seconds.

  Other routers on the LAN will hear the prune message and respond  with
  a  join  if  they  still  expect multicast datagrams from the expected
  upstream router. The Join  message  is  data  link  multicast  and  IP
  addressed  to the all PIM routers group address The router
  to process the join will be indicated by inclusion of its  address  in
  the  "Address"  field  of the message. The address is determined by an
  RPF lookup for the source. When the expected router receives the  join
  message,  it  will  cancel  the  prune request. This process is called

  Routers that need to send prune-override joins will randomly  generate
  a  join  message  delay  timer. If a join is heard from another router
  before a router sends its own, it will cancel sending  its  own  join.
  This  will  reduce  control traffic on the LAN. The maximum join delay
  timer should be equal to or smaller than the prune delay timer  value.
  The default is 3 seconds.

  If the expected upstream router does not receive any PIM Join messages
  before  the  scheduled  expiration  time  for the deletion request, it
  prunes the outgoing LAN interface from the (S,G) multicast  forwarding
  entry.  The  prune  timer SHOULD take into account the 3 seconds delay
  after the prune was scheduled. At the time of the prune,  by  default,
  the  timer  SHOULD  expire in [Data-Timeout - 3 seconds]. This reduces
  the chance of join latency in case a  new  member  immediately  joined
  after the last prune.

  However, if the prune-override join message is lost,  the  prune  will
  occur  and  there  will be no data delivery for the amount of time the
  interface remains in prune state. To reduce the  probability  of  this
  occurence,  a  router  that  has  scheduled to prune the interface off
  should immediately send a prune onto the interface. This  gives  other
  routers another chance to hear the prune, and to respond with a join.

5.2.2 Pruning a Point-to-point link

  Prune messages received on a point  to  point  link  are  not  delayed
  before  processing  as  they are in the LAN procedure. If the prune is
  received on an interface in the outgoing interface list, it is  pruned

  Prunes  may  be  rate-limited  on  point-to-point  interfaces  when  a
  multicast  datagram  is received for a negative cache entry, or if the

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  point-to-point  interface  receiving  the  datagram  is  not  the  RPF
  interface toward the source.

5.3 New members joining an existing group

  If a router is directly connected to a host that  wants  to  become  a
  member  of  a  group, the router may send a Graft message toward known
  sources. This allows join latency to be reduced below  that  indicated
  by the relatively large timeout value suggested for prune information.

  The host indicates its interest in group G by sending an IGMP  report.
  If  a receiving router has state for group G, it adds the interface on
  which the IGMP Report or Graft was received for all known (S,G)'s.  If
  the  (S,G)  entry was a negative cache entry, the router sends a Graft
  message upstream toward S.

  A router receiving the Graft message adds the received interface  into
  the  matching  (S,G)  entry's  outgoing  interface  list. If the entry
  transitions to forward state due to this added outgoing interface, the
  router must send a Graft message toward the source.

  Routers with no group state do nothing on  receipt  of  IGMP  reports,
  since   dense-mode   PIM  will  deliver  multicast  datagrams  to  all
  interfaces when creating state for a group.

  Graft  message  is  the  only  PIM  message  that  uses   a   positive
  acknowledgment  strategy.  Senders  of  Graft messages unicast them to
  their upstream RPF neighbors. The neighbor processes  each  (S,G)  and
  immediately  acknowledges  each  (S,G)  in a GraftAck message. This is
  relatively easy, since the receiver simply  changes  the  PIM  message
  type  from Graft to Graft-Ack and unicasts the original packet back to
  the source. The sender periodically retransmits the Graft message  for
  any  (S,G)  that  has not been acknowledged. The interval between each
  retransmission is 3 seconds. Note that the  sender  need  not  keep  a
  retransmission  list  for  each neighbor since Grafts are only sent to
  the RPF neighbor.  Only  the  (S,G)  entry  needs  to  be  tagged  for

5.4 Designated Router Election

  Dense mode PIM itself does not  need  the  function  of  a  designated
  router  (DR).  But  a  DR  is needed on multi-access LANs running IGMP
  version 1, which relies on a routing protocol to select a query router
  for  the  purpose  of sending IGMP Host-Query messages. Dense-mode PIM

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  designated router (DR) election has the same procedure sparse-mode PIM

  Each PIM router connected to a multi-access  LAN  should  periodically
  transmit  Hello  messages  onto  the  LAN.  The period is specified as
  [Hello-Timer] in the sparse mode specification (default  30  seconds).
  The  highest  IP  addressed  router becomes the DR. The discovered PIM
  routers should  be  timed  out  after  [Neighbor-Timer]  (default  105
  seconds) in the PIM-SM specification. If the DR goes down, a new DR is

  DR election is only necessary on multi-access networks.

5.5 Parallel paths to a source

  Two or more routers may receive the same multicast datagram replicated
  upstream.  In  particular,  if  two routers have equal cost paths to a
  source and are connected on a common multi-access  network,  duplicate
  datagrams  will  travel  downstream  onto the LAN. Dense-mode PIM will
  detect such a situation and will not let it persist.

  If a router receives a multicast datagram on an outgoing interface  on
  a  multi-access  LAN,  the  packet must be a duplicate. In this case a
  single forwarder must be elected.  The  upstream  routers  can  decide
  which  one  becomes  the forwarder, using Assert messages addressed to on the LAN. Downstream routers listen  to  the  Asserts  so
  they  know  which  one  was elected (typically this is the same as the
  downstream router's RPF neighbor  but  there  are  circumstances  when
  using different unicast protocols where this might not be the case).

  The upstream router elected is the one that has the best metric to the
  source.  When  a packet is received on an outgoing interface, a router
  will send an Assert packet on the LAN indicating what metric  it  uses
  to reach the source of the data packet. If metrics are comparable, the
  router with the best metric will become  the  forwarder.  Incomparable
  metrics will be discussed later in this section when metric preference
  is described. All other upstream routers will prune the interface from
  their  outgoing  interface  list.  The  downstream routers also do the
  comparison in case the forwarder is different than the  RPF  neighbor.
  This  is  important  so  downstream  routers send subsequent Prunes or
  Grafts to the correct neighbor.

  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.

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  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 PIM routers on the LAN.

  Asserts are rate-limited. The recommended minimum interval between two
  subsequent  asserts  out the same outgoing interface of an (S,G) entry
  is 1 second.

  The following Assert rules must be observed:

  Multicast packet received on outgoing interface:

 1    Do unicast routing table lookup on source  IP  address  from  data

 2    Send Assert on interface for source IP address from  data  packet.
      The  assert message includes metric preference of routing protocol
      and metric from routing table lookup.

 3    If route is not found, Use metric  preference  of  0x7fffffff  and
      metric 0xffffffff.

  Asserts received on outgoing interface:

 1    Compare metric received in Assert with  the  one  you  would  have
      advertised  in  an Assert. If the value in the Assert is less than
      your value, prune the interface. If the value  is  the  same,  and
      your address is less than the Assert sender, prune the interface.

 2    If you have won the election  and  there  are  directly  connected
      members  on the LAN, keep the interface in your outgoing interface
      list. You are the forwarder for the LAN.

 3    If you have won the election but there are no  directly  connected
      members on the LAN, schedule to prune the interface. The LAN might
      be a stub LAN with no members (and no downstream routers).  If  no
      subsequent  Joins  are  received,  prune  the  interface  from the
      outgoing interface list. Otherwise  keep  the  interface  in  your
      outgoing interface. You are the forwarder for the LAN.

 4    The assert winner SHOULD send an assert with its own metric on the

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      LAN, so that all other routers know who the winner is.

  Asserts received on incoming interface:

 1    Downstream routers  will  select  the  upstream  router  with  the
      smallest  metric  as  their  RPF  neighbor. If two metrics are the
      same, the highest IP address is chosen to break the tie.

      If the upstream router selected is different from the RPF neighbor
      selected  natively,  it  should  start  an Assert-Timer. When this
      timer expires, it restores the RPF information obtained by the RPF

 2    If the downstream  routers  have  downstream  members,  they  must
      schedule  a  join  to inform the upstream router packets should be
      forwarded on the LAN. This will cause the  upstream  forwarder  to
      cancel its delayed pruning of the interface.

 3    When the assert state times out, the downstream router may  switch
      from the assert winner to the original RPF neighbor, if different.

5.6 Timers in Multicast Forwarding Entries

  An (S,G) entry  has  a  number  of  associated  timers.  One  for  the
  multicast  routing  entry itself, one for each pruned interface in the
  outgoing interface list and one to time out information about upstream
  assert  winner  (Assert_timer). An outgoing interface in forward state
  does not time  out  or  change  state  without  external  events.  The
  outgoing interface stays in forward state in the list as long as there
  is a group member connected, or there is  a  downstream  PIM  neighbor
  that  has  not  sent  a prune to it. The interface is deleted from the
  outgoing interface list if it is on a leaf network  and  there  is  no
  connected member. The interface timer for a pruned interface should be
  started with the holdtime in the prune message (also  referred  to  as
  the prune timer).

  When a (S,G) entry is in forwarding state, its expiration timer is set
  to   [Data-Timeout],   as  specified  in  the  companion  sparse  mode
  specification [PIMSM], default is 210 seconds. This timer is restarted
  when  a data packet is forwarded. The (S,G) entry is deleted when this
  timer expires.

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  Once all interfaces in the outgoing interface  list  are  pruned,  the
  (S,G)'s  expiration  timer  should  be  set to the maximum prune timer
  among all its outgoing interfaces. During this time the entry is known
  as a negative cache entry.

  If the prune timers on different outgoing  interfaces  are  different,
  the  pruned  interface  with  the  shortest remaining timer may expire
  first and turn the negative cache entry into  forwarding  state.  When
  this  happens,  a  graft should be triggered upstream. When a negative
  cache entry turns into a forward entry,  the  entry  timer  should  be
  restarted  with  the  default  expiration  timer. Once the (S,G) entry
  times out, it will be recreated when the next multicast packet or join

  If the prune timer expires at the exactly same time as the (S,G) entry
  timer, a graft should still be triggered upstream.

  The prune message sent upstream contains a holdtime. Its default value
  is  [Data-Timeout],  except when the Assert timer is also running, the
  holdtime in the prune message is set to the smaller value between  the
  prune  holdtime  the system uses, and the remaining assert timer value
  before its expiration. The Assert Timer  is  started  with  a  default
  value of 210 seconds.

5.7 Adapting to unicast route changes

  When unicast route changes occur, the RPF  interface  for  many  (S,G)
  entries  will  also  change.  The  following  should  be  done for the
  affected entries:

 1    A prune should be sent toward the old incoming interface;

 2    A graft should be sent to the new RPF neighbor.

6 Generation Identifier (GenID)

  When a router goes  offline  and  restarts,  it  loses  all  multicast
  forwarding  state. If the upstream router has (S,G) entries in forward
  state, data will come down and recreate state in the restarted router.
  However,  if  the  upstream router is in negative cache state, no data
  packet can flow down until the upstream negative cache state  expires.
  This  results  in  join  latencies  for  new members joining after the
  router went down.

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  To reduce this join latency, all PIM Hello  messages  SHOULD  carry  a
  ``Generation  ID''(GenID)  option.  The  GenID is a randomly generated
  32-bit  number,  that  MUST  be  different  between  different  router

  When a new GenID is detected from a PIM  neighbor,  the  local  router
  will need to go through its multicast forwarding table, and attempt to
  recover the prune state for the PIM neighbor.

  If a new GenID is received on a pruned outgoing interface of  a  (S,G)
  entry,  that  interface  SHOULD  be  turned into forward state. If the
  (S,G) entry transitions into forward state from pruned state, a  graft
  MUST be sent to the upstream RPF neighbor.

  If the new GenID is sent by the RPF neighbor of a negative cache (S,G)
  entry,  a (S,G) prune SHOULD be immediately sent to that RPF neighbor,
  as if the local state just transitioned into prune state.

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7 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
  `' (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  |   reserved    |           Checksum            |

    PIM Ver
          PIM Version number is 2.

    Type  Types for specific PIM messages.  PIM Types are:

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

          Set to zero. Ignored upon receipt.

          The checksum is the  16-bit  one's  complement  of  the  one's

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         complement  sum  of  the  entire PIM message. For computing the
         checksum, the checksum field is zeroed.

     { For all the packet format details, refer to the  PIM  sparse-mode

7.1 PIM-Hello Message

  It is sent periodically by PIM routers on all interfaces.

  The Generation ID option has OptionType of 20, and OptionLength of 4.

 |   OptionType = 20 (GenID)     |      OptionLength = 4         |
 |                       Random Number                           |

7.2 PIM-SM-Register Message

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

7.3 PIM-SM-Register-Stop Message

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

7.4 Join/Prune Message

  Joins are sent by routers toward upstream sources. A join  creates  or
  refreshes forwarding state and a prune negates forwarding state. It is
  processed as if a graft is received, except no ack packet is required.
  Refer to PIM sparse-mode specification.

7.5 PIM-SM-Bootstrap Message

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

7.6 PIM-Assert Message

  The PIM-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. This message is used  in  both  dense-mode
  and  sparse-mode  PIM.  For  packet  format,  refer to PIM sparse-mode

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7.7 PIM-Graft Message

  This message is sent by a downstream router to a neighboring  upstream
  router  to reinstate a previously pruned branch of a source tree. This
  is done for dense-mode groups only.  The  format  is  the  same  as  a
  Join/Prune  message, except that the value in the type field is 6. The
  source address should be included in the join section of the  message.
  The holdtime field is unused, and is ignored when a graft is received.

7.8 PIM-Graft-Ack Message

  Sent in response to a received Graft message. This message is sent for
  dense-mode  groups only. The format is the same as Join/Prune message,
  except that the value of the message type field is 7.  The  ``Encoded-
  Unicast-Upstream  Neighbor  Address'' field is unused and needs not be
  checked when this message is  received.  The  holdtime  field  in  the
  packet is ignored when received.

8 Security Considerations

  Security issues are discussed in  detail  in  the  companion  document
  ``Authenticating PIM Version 2 Messages'' [PIMAU].

9 Acknowledgement

  Thanks to Manoj  Leelanivas,  Sangeeta  Mukherjee,  Nitin  Jain,  Bill
  Fenner  and  many  members  of  the  PIM/IDMR  working group for their
  helpful comments.

10 References

  [Deering91]  S.E.   Deering.   Multicast   Routing   in   a   Datagram
  Internetwork.  PhD  thesis,  Electrical  Engineering  Dept.,  Stanford
  University, December 1991.

  [DVMRP]  RFC  1075,  Distance  Vector  Multicast   Routing   Protocol.
  Waitzman, D., Partridge, C., Deering, S.E, November 1988

  [PIMSM] Protocol Independent Multicast Sparse-Mode (PIM-SM):  Protocol
  Specification.  D.  Estrin,  D.  Farinacci,  A.  Helmy,  D. Thaler, S.
  Deering, M. Handley, V. Jacobson, C.  Liu,  P.Sharma,  L.  Wei  draft-

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  [PIMARCH] An Architecture for Wide-Area Multicast Routing, S. Deering,
  D.  Estrin,  D.  Farinacci,  V. Jacobson, G. Liu,L. Wei, USC Technical
  Report, available from authors, Nov 1996.

  [RFC1112] Host Extensions for IP Multicasting, Network Working  Group,
  RFC 1112, S. Deering, August 1989

  [PIMAU] Authenticating PIM Version 2 Messages,  L.  Wei.  <draft-ietf-
  pim-v2-auth-00.txt>, Nov, 1999

11 Authors' Addresses

Stephen Deering
Cisco Systems Inc
170 West Tasman Drive,
San Jose, CA 95134

Deborah Estrin
Computer Science Department/ISI
University of Southern California
Los Angeles, CA 90089

Dino Farinacci
cisco Systems Inc
170 West Tasman Drive
San Jose, CA 95134

Van Jacobson
cisco Systems Inc
170 West Tasman Drive
San Jose, CA 95134

Ahmed Helmy
Computer Science Department
University of Southern California
Los Angeles, CA 90089

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David Meyer
cisco Systems, Inc
170 West Tasman Drive
San Jose, CA95134

Liming Wei
cisco Systems Inc
170 West Tasman Drive
San Jose, CA 95134

12 Appendix. Changes Since The Last Revision

 *    Section 5.2.1 Deleted the paragraph about use of  in  the
      prune message.

 *    Section 5.6 Made it explicit that graft needs to be sent even when
      prune timer and entry timer all expire at the same time.

 *    Section 7.8. Deleted sentence saying graft-ack is sent only if  it
      is received on a non-RPF interface.

 *    Section 5.2.1. Reworded the  ``rate-limiting  prunes''  paragraph.
      Made it more explicit what actions are permitted.

 *    Section 7.4. Reworded and added clarification for join processing:
      ``A join creates or refreshes forwarding state and a prune negates
      forwarding state. It is processed  as  if  a  graft  is  received,
      except no ack packet is required.''

 *    Section 5.2.1. Added sentence to deal with 3 seconds join  latency
      immediately after prune over a multi-access LAN: ``The prune timer
      SHOULD take into account the 3 seconds delay after the  prune  was
      scheduled.  At the time of the prune, by default, the timer SHOULD

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      expire in [Data-Timeout - 3 seconds]. This reduces the  chance  of
      join  latency  in  case  a new member immediately joined after the
      last prune.

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