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RFC 7761

 
 
 

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

Part 3 of 7, p. 29 to 44
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4.3.  Designated Routers (DRs) and Hello Messages

   A shared-media LAN like Ethernet may have multiple PIM-SM routers
   connected to it.  A single one of these routers, the DR, will act on
   behalf of directly connected hosts with respect to the PIM-SM
   protocol.  Because the distinction between LANs and point-to-point
   interfaces can sometimes be blurred, and because routers may also
   have multicast host functionality, the PIM-SM specification makes no
   distinction between the two.  Thus, DR election will happen on all
   interfaces, LAN or otherwise.

   DR election is performed using Hello messages.  Hello messages are
   also the way that option negotiation takes place in PIM, so that
   additional functionality can be enabled, or parameters tuned.

4.3.1.  Sending Hello Messages

   PIM Hello messages are sent periodically on each PIM-enabled
   interface.  They allow a router to learn about the neighboring PIM
   routers on each interface.  Hello messages are also the mechanism
   used to elect a DR, and to negotiate additional capabilities.  A
   router must record the Hello information received from each PIM
   neighbor.

   Hello messages MUST be sent on all active interfaces, including
   physical point-to-point links, and are multicast to the
   'ALL-PIM-ROUTERS' group address ('224.0.0.13' for IPv4 and 'ff02::d'
   for IPv6).

   We note that some implementations do not send Hello messages on
   point-to-point interfaces.  This is non-compliant behavior.  A
   compliant PIM router MUST send Hello messages, even on point-to-point
   interfaces.

   A per-interface Hello Timer (HT(I)) is used to trigger sending Hello
   messages on each active interface.  When PIM is enabled on an
   interface or a router first starts, the Hello Timer of that interface
   is set to a random value between 0 and Triggered_Hello_Delay.  This
   prevents synchronization of Hello messages if multiple routers are
   powered on simultaneously.  After the initial randomized interval,
   Hello messages MUST be sent every Hello_Period seconds.  The
   Hello Timer SHOULD NOT be reset except when it expires.

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   Note that neighbors will not accept Join/Prune or Assert messages
   from a router unless they have first heard a Hello message from that
   router.  Thus, if a router needs to send a Join/Prune or Assert
   message on an interface on which it has not yet sent a Hello message
   with the currently configured IP address, then it MUST immediately
   send the relevant Hello message without waiting for the Hello Timer
   to expire, followed by the Join/Prune or Assert message.

   The DR Priority option allows a network administrator to give
   preference to a particular router in the DR election process by
   giving it a numerically larger DR Priority.  The DR Priority option
   SHOULD be included in every Hello message, even if no DR Priority is
   explicitly configured on that interface.  This is necessary because
   priority-based DR election is only enabled when all neighbors on an
   interface advertise that they are capable of using the DR Priority
   option.  The default priority is 1.

   The Generation Identifier (GenID) option SHOULD be included in all
   Hello messages.  The GenID option contains a randomly generated
   32-bit value that is regenerated each time PIM forwarding is started
   or restarted on the interface, including when the router itself
   restarts.  When a Hello message with a new GenID is received from a
   neighbor, any old Hello information about that neighbor SHOULD be
   discarded and superseded by the information from the new Hello
   message.  This may cause a new DR to be chosen on that interface.

   The LAN Prune Delay option SHOULD be included in all Hello messages
   sent on multi-access LANs.  This option advertises a router's
   capability to use values other than the defaults for the
   Propagation_Delay and Override_Interval, which affect the setting of
   the Prune-Pending, Upstream Join, and Override Timers (defined in
   Section 4.10).

   The Address List option advertises all the secondary addresses
   associated with the source interface of the router originating the
   message.  The option MUST be included in all Hello messages if there
   are secondary addresses associated with the source interface and MAY
   be omitted if no secondary addresses exist.

   To allow new or rebooting routers to learn of PIM neighbors quickly,
   when a Hello message is received from a new neighbor, or a Hello
   message with a new GenID is received from an existing neighbor, a new
   Hello message SHOULD be sent on this interface after a randomized
   delay between 0 and Triggered_Hello_Delay.  This triggered message
   need not change the timing of the scheduled periodic message.  If a
   router needs to send a Join/Prune to the new neighbor or send an
   Assert message in response to an Assert message from the new neighbor
   before this randomized delay has expired, then it MUST immediately

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   send the relevant Hello message without waiting for the Hello Timer
   to expire, followed by the Join/Prune or Assert message.  If it does
   not do this, then the new neighbor will discard the Join/Prune or
   Assert message.

   Before an interface goes down or changes primary IP address, a Hello
   message with a zero HoldTime SHOULD be sent immediately (with the old
   IP address if the IP address changed).  This will cause PIM neighbors
   to remove this neighbor (or its old IP address) immediately.  After
   an interface has changed its IP address, it MUST send a Hello message
   with its new IP address.  If an interface changes one of its
   secondary IP addresses, a Hello message with an updated Address List
   option and a non-zero HoldTime SHOULD be sent immediately.  This will
   cause PIM neighbors to update this neighbor's list of secondary
   addresses immediately.

4.3.2.  DR Election

   When a PIM Hello message is received on interface I, the following
   information about the sending neighbor is recorded:

      neighbor.interface
            The interface on which the Hello message arrived.

      neighbor.primary_ip_address
            The IP address that the PIM neighbor used as the source
            address of the Hello message.

      neighbor.genid
            The Generation ID of the PIM neighbor.

      neighbor.dr_priority
            The DR Priority field of the PIM neighbor, if it is present
            in the Hello message.

      neighbor.dr_priority_present
            A flag indicating if the DR Priority field was present in
            the Hello message.

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      neighbor.timeout
            A timer value to time out the neighbor state when it becomes
            stale, also known as the Neighbor Liveness Timer.

            The Neighbor Liveness Timer (NLT(N,I)) is reset to
            Hello_Holdtime (from the Hello Holdtime option) whenever a
            Hello message is received containing a Holdtime option, or
            to Default_Hello_Holdtime if the Hello message does not
            contain the Holdtime option.

            Neighbor state is deleted when the neighbor timeout expires.

   The function for computing the DR on interface I is:

     host
     DR(I) {
         dr = me
         for each neighbor on interface I {
             if ( dr_is_better( neighbor, dr, I ) == TRUE ) {
                 dr = neighbor
             }
         }
         return dr
     }

   The function used for comparing DR "metrics" on interface I is:

     bool
     dr_is_better(a,b,I) {
         if( there is a neighbor n on I for which n.dr_priority_present
                 is false ) {
             return a.primary_ip_address > b.primary_ip_address
         } else {
             return ( a.dr_priority > b.dr_priority ) OR
                    ( a.dr_priority == b.dr_priority AND
                      a.primary_ip_address > b.primary_ip_address )
         }
     }

   The trivial function I_am_DR(I) is defined to aid readability:

     bool
     I_am_DR(I) {
        return DR(I) == me
     }

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   The DR Priority is a 32-bit unsigned number, and the numerically
   larger priority is always preferred.  A router's idea of the current
   DR on an interface can change when a PIM Hello message is received,
   when a neighbor times out, or when a router's own DR Priority
   changes.  If the router becomes the DR or ceases to be the DR, this
   will normally cause the DR Register state machine to change state.
   Subsequent actions are determined by that state machine.

      We note that some PIM implementations do not send Hello messages
      on point-to-point interfaces and thus cannot perform DR election
      on such interfaces.  This is non-compliant behavior.  DR election
      MUST be performed on ALL active PIM-SM interfaces.

4.3.3.  Reducing Prune Propagation Delay on LANs

   In addition to the information recorded for the DR Election, the
   following per-neighbor information is obtained from the LAN Prune
   Delay Hello option:

      neighbor.lan_prune_delay_present
            A flag indicating if the LAN Prune Delay option was present
            in the Hello message.

      neighbor.tracking_support
            A flag storing the value of the T bit in the LAN Prune Delay
            option if it is present in the Hello message.  This
            indicates the neighbor's capability to disable Join message
            suppression.

      neighbor.propagation_delay
            The Propagation Delay field of the LAN Prune Delay option
            (if present) in the Hello message.

      neighbor.override_interval
            The Override_Interval field of the LAN Prune Delay option
            (if present) in the Hello message.

   The additional state described above is deleted along with the DR
   neighbor state when the neighbor timeout expires.

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   Just like the DR Priority option, the information provided in the LAN
   Prune Delay option is not used unless all neighbors on a link
   advertise the option.  The function below computes this state:

     bool
     lan_delay_enabled(I) {
         for each neighbor on interface I {
             if ( neighbor.lan_prune_delay_present == false ) {
                 return false
             }
         }
         return true
     }

   The Propagation Delay inserted by a router in the LAN Prune Delay
   option expresses the expected message propagation delay on the link
   and SHOULD be configurable by the system administrator.  It is used
   by upstream routers to figure out how long they should wait for a
   Join override message before pruning an interface.

   PIM implementers SHOULD enforce a lower bound on the permitted values
   for this delay to allow for scheduling and processing delays within
   their router.  Such delays may cause received messages to be
   processed later as well as triggered messages to be sent later than
   intended.  Setting this Propagation Delay to too low a value may
   result in temporary forwarding outages because a downstream router
   will not be able to override a neighbor's Prune message before the
   upstream neighbor stops forwarding.

   When all routers on a link are in a position to negotiate a
   Propagation Delay different from the default, the largest value from
   those advertised by each neighbor is chosen.  The function for
   computing the Effective Propagation Delay of interface I is:

     time_interval
     Effective_Propagation_Delay(I) {
         if ( lan_delay_enabled(I) == false ) {
             return Propagation_delay_default
         }
         delay = Propagation_Delay(I)
         for each neighbor on interface I {
             if ( neighbor.propagation_delay > delay ) {
                 delay = neighbor.propagation_delay
             }
         }
         return delay
     }

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   To avoid synchronization of override messages when multiple
   downstream routers share a multi-access link, the sending of such
   messages is delayed by a small random amount of time.  The period of
   randomization should represent the size of the PIM router population
   on the link.  Each router expresses its view of the amount of
   randomization necessary in the Override Interval field of the LAN
   Prune Delay option.

   When all routers on a link are in a position to negotiate an Override
   Interval different from the default, the largest value from those
   advertised by each neighbor is chosen.  The function for computing
   the Effective Override Interval of interface I is:

     time_interval
     Effective_Override_Interval(I) {
         if ( lan_delay_enabled(I) == false ) {
             return t_override_default
         }
         delay = Override_Interval(I)
         for each neighbor on interface I {
             if ( neighbor.override_interval > delay ) {
                 delay = neighbor.override_interval
             }
         }
         return delay
     }

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   Although the mechanisms are not specified in this document, it is
   possible for upstream routers to explicitly track the join
   membership of individual downstream routers if Join suppression is
   disabled.  A router can advertise its willingness to disable Join
   suppression by using the T bit in the LAN Prune Delay Hello option.
   Unless all PIM routers on a link negotiate this capability, explicit
   tracking and the disabling of the Join suppression mechanism are not
   possible.  The function for computing the state of Suppression on
   interface I is:

     bool
     Suppression_Enabled(I) {
         if ( lan_delay_enabled(I) == false ) {
             return true
         }
         for each neighbor on interface I {
             if ( neighbor.tracking_support == false ) {
                 return true
             }
         }
         return false
     }

   Note that the setting of Suppression_Enabled(I) affects the value of
   t_suppressed (see Section 4.11).

4.3.4.  Maintaining Secondary Address Lists

   Communication of a router's interface secondary addresses to its PIM
   neighbors is necessary to provide the neighbors with a mechanism for
   mapping next_hop information obtained through their MRIB to a primary
   address that can be used as a destination for Join/Prune messages.
   The mapping is performed through the NBR macro.  The primary address
   of a PIM neighbor is obtained from the source IP address used in its
   PIM Hello messages.  Secondary addresses are carried within the Hello
   message in an Address List Hello option.  The primary address of the
   source interface of the router MUST NOT be listed within the Address
   List Hello option.

   In addition to the information recorded for the DR Election, the
   following per-neighbor information is obtained from the Address List
   Hello option:

      neighbor.secondary_address_list
            The list of secondary addresses used by the PIM neighbor on
            the interface through which the Hello message was
            transmitted.

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   When processing a received PIM Hello message containing an Address
   List Hello option, the list of secondary addresses in the message
   completely replaces any previously associated secondary addresses for
   that neighbor.  If a received PIM Hello message does not contain an
   Address List Hello option, then all secondary addresses associated
   with the neighbor MUST be deleted.  If a received PIM Hello message
   contains an Address List Hello option that includes the primary
   address of the sending router in the list of secondary addresses
   (although this is not expected), then the addresses listed in the
   message, excluding the primary address, are used to update the
   associated secondary addresses for that neighbor.

   All the advertised secondary addresses in received Hello messages
   must be checked against those previously advertised by all other PIM
   neighbors on that interface.  If there is a conflict and the same
   secondary address was previously advertised by another neighbor, then
   only the most recently received mapping MUST be maintained, and an
   error message SHOULD be logged to the administrator in a rate-limited
   manner.

   Within one Address List Hello option, all the addresses MUST be of
   the same address family.  It is not permitted to mix IPv4 and IPv6
   addresses within the same message.  In addition, the address family
   of the fields in the message SHOULD be the same as the IP source and
   destination addresses of the packet header.

4.4.  PIM Register Messages

   The Designated Router (DR) on a LAN or point-to-point link
   encapsulates multicast packets from local sources to the RP for the
   relevant group unless it recently received a Register-Stop message
   for that (S,G) or (*,G) from the RP.  When the DR receives a
   Register-Stop message from the RP, it starts a Register-Stop Timer to
   maintain this state.  Just before the Register-Stop Timer expires,
   the DR sends a Null-Register message to the RP to allow the RP to
   refresh the Register-Stop information at the DR.  If the
   Register-Stop Timer actually expires, the DR will resume
   encapsulating packets from the source to the RP.

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4.4.1.  Sending Register Messages from the DR

   Every PIM-SM router has the capability to be a DR.  The state machine
   below is used to implement Register functionality.  For the purposes
   of specification, we represent the mechanism to encapsulate packets
   to the RP as a Register-Tunnel interface, which is added to or
   removed from the (S,G) olist.  The tunnel interface then takes part
   in the normal packet forwarding rules as specified in Section 4.2.

   If register state is maintained, it is maintained only for directly
   connected sources and is per-(S,G).  There are four states in the
   DR's per-(S,G) Register state machine:

      Join (J)
            The register tunnel is "joined" (the join is actually
            implicit, but the DR acts as if the RP has joined the DR on
            the tunnel interface).

      Prune (P)
            The register tunnel is "pruned" (this occurs when a
            Register-Stop is received).

      Join-Pending (JP)
            The register tunnel is pruned but the DR is contemplating
            adding it back.

      NoInfo (NI)
            No information.  This is the initial state, and the state
            when the router is not the DR.

   In addition, a Register-Stop Timer (RST) is kept if the state machine
   is not in the NoInfo state.

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           Figure 1: Per-(S,G) Register State Machine at a DR

+----------++----------------------------------------------------------+
|          ||                          Event                           |
|          ++----------+-----------+-----------+-----------+-----------+
|Prev State||Register- | Could     | Could     | Register- | RP changed|
|          ||Stop Timer| Register  | Register  | Stop      |           |
|          ||expires   | ->True    | ->False   | received  |           |
+----------++----------+-----------+-----------+-----------+-----------+
|NoInfo    ||-         | -> J state| -         | -         | -         |
|(NI)      ||          | add reg   |           |           |           |
|          ||          | tunnel    |           |           |           |
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-         | -         | -> NI     | -> P state| -> J state|
|          ||          |           | state     |           |           |
|          ||          |           | remove reg| remove reg| update reg|
|Join (J)  ||          |           | tunnel    | tunnel;   | tunnel    |
|          ||          |           |           | set       |           |
|          ||          |           |           | Register- |           |
|          ||          |           |           | Stop      |           |
|          ||          |           |           | Timer(*)  |           |
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-> J state| -         | -> NI     | -> P state| -> J state|
|          ||          |           | state     |           |           |
|Join-     ||add reg   |           |           | set       | add reg   |
|Pending   ||tunnel    |           |           | Register- | tunnel;   |
|(JP)      ||          |           |           | Stop      | cancel    |
|          ||          |           |           | Timer(*)  | Register- |
|          ||          |           |           |           | Stop Timer|
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-> JP     | -         | -> NI     | -         | -> J state|
|          ||state     |           | state     |           |           |
|          ||set       |           |           |           | add reg   |
|Prune (P) ||Register- |           |           |           | tunnel;   |
|          ||Stop      |           |           |           | cancel    |
|          ||Timer(**);|           |           |           | Register- |
|          ||send Null-|           |           |           | Stop Timer|
|          ||Register  |           |           |           |           |
+----------++----------+-----------+-----------+-----------+-----------+

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   Notes:

   (*)  The Register-Stop Timer is set to a random value chosen
        uniformly from the interval ( 0.5 * Register_Suppression_Time,
        1.5 * Register_Suppression_Time) minus Register_Probe_Time.

        Subtracting off Register_Probe_Time is a bit unnecessary because
        it is really small compared to Register_Suppression_Time, but
        this was in the old specification and is kept for compatibility.

   (**) The Register-Stop Timer is set to Register_Probe_Time.

   The following three actions are defined:

   Add Register Tunnel

      A Register-Tunnel virtual interface, VI, is created (if it doesn't
      already exist) with its encapsulation target being RP(G).
      DownstreamJPState(S,G,VI) is set to Join state, causing the tunnel
      interface to be added to immediate_olist(S,G) and
      inherited_olist(S,G).

   Remove Register Tunnel

      VI is the Register-Tunnel virtual interface with encapsulation
      target of RP(G).  DownstreamJPState(S,G,VI) is set to NoInfo
      state, causing the tunnel interface to be removed from
      immediate_olist(S,G) and inherited_olist(S,G).  If
      DownstreamJPState(S,G,VI) is NoInfo for all (S,G), then VI can be
      deleted.

   Update Register Tunnel

      This action occurs when RP(G) changes.

      VI_old is the Register-Tunnel virtual interface with encapsulation
      target old_RP(G).  A Register-Tunnel virtual interface, VI_new, is
      created (if it doesn't already exist) with its encapsulation
      target being new_RP(G).  DownstreamJPState(S,G,VI_old) is set to
      NoInfo state, and DownstreamJPState(S,G,VI_new) is set to Join
      state.  If DownstreamJPState(S,G,VI_old) is NoInfo for all (S,G),
      then VI_old can be deleted.

      Note that we cannot simply change the encapsulation target of
      VI_old because not all groups using that encapsulation tunnel will
      have moved to the same new RP.

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   CouldRegister(S,G)

      The macro "CouldRegister" in the state machine is defined as:

         bool CouldRegister(S,G) {
            return ( I_am_DR( RPF_interface(S) ) AND
                     KeepaliveTimer(S,G) is running AND
                     DirectlyConnected(S) == TRUE )
         }

      Note that on reception of a packet at the DR from a directly
      connected source, KeepaliveTimer(S,G) needs to be set by the
      packet forwarding rules before computing CouldRegister(S,G) in the
      register state machine, or the first packet from a source won't be
      registered.

   Encapsulating Data Packets in the Register Tunnel

      Conceptually, the Register Tunnel is an interface with a smaller
      MTU than the underlying IP interface towards the RP.  IP
      fragmentation on packets forwarded on the Register Tunnel is
      performed based upon this smaller MTU.  The encapsulating DR may
      perform Path MTU Discovery to the RP to determine the effective
      MTU of the tunnel.  Fragmentation for the smaller MTU should take
      both the outer IP header and the PIM register header overhead into
      account.  If a multicast packet is fragmented on the way into the
      Register Tunnel, each fragment is encapsulated individually so it
      contains IP, PIM, and inner IP headers.

      In IPv6, the DR MUST perform Path MTU Discovery, and an ICMP
      Packet Too Big message MUST be sent by the encapsulating DR if it
      receives a packet that will not fit in the effective MTU of the
      tunnel.  If the MTU between the DR and the RP results in the
      effective tunnel MTU being smaller than 1280 (the IPv6 minimum
      MTU), the DR MUST send Fragmentation Required messages with an MTU
      value of 1280 and MUST fragment its PIM register messages as
      required, using an IPv6 fragmentation header between the outer
      IPv6 header and the PIM Register header.

      The TTL of a forwarded data packet is decremented before it is
      encapsulated in the Register Tunnel.  The encapsulating packet
      uses the normal TTL that the router would use for any locally
      generated IP packet.

      The IP Explicit Congestion Notification (ECN) bits should be
      copied from the original packet to the IP header of the
      encapsulating packet.  They SHOULD NOT be set independently by the
      encapsulating router.

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      The Diffserv Code Point (DSCP) should be copied from the original
      packet to the IP header of the encapsulating packet.  It MAY be
      set independently by the encapsulating router, based upon static
      configuration or traffic classification.  See [12] for more
      discussion on setting the DSCP on tunnels.

   Handling Register-Stop(*,G) Messages at the DR

      An old RP might send a Register-Stop message with the source
      address set to all zeros.  This was the normal course of action in
      RFC 2362 when the Register message matched against (*,G) state at
      the RP, and it was defined as meaning "stop encapsulating all
      sources for this group".  However, the behavior of such a
      Register-Stop(*,G) is ambiguous or incorrect in some
      circumstances.

      We specify that an RP should not send Register-Stop(*,G) messages,
      but for compatibility, a DR should be able to accept one if it is
      received.

      A Register-Stop(*,G) should be treated as a Register-Stop(S,G) for
      all (S,G) Register state machines that are not in the NoInfo
      state.  A router should not apply a Register-Stop(*,G) to sources
      that become active after the Register-Stop(*,G) was received.

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4.4.2.  Receiving Register Messages at the RP

   When an RP receives a Register message, the course of action is
   decided according to the following pseudocode:

   packet_arrives_on_rp_tunnel( pkt ) {
       if( outer.dst is not one of my addresses ) {
           drop the packet silently.
           # Note: This may be a spoofing attempt.
       }
       if( I_am_RP(G) AND outer.dst == RP(G) ) {
             sentRegisterStop = FALSE;
             if ( SPTbit(S,G) OR
              ( SwitchToSptDesired(S,G) AND
                ( inherited_olist(S,G) == NULL ))) {
               send Register-Stop(S,G) to outer.src
               sentRegisterStop = TRUE;
             }
             if ( SPTbit(S,G) OR SwitchToSptDesired(S,G) ) {
                  if ( sentRegisterStop == TRUE ) {
                       set KeepaliveTimer(S,G) to RP_Keepalive_Period;
                  } else {
                       set KeepaliveTimer(S,G) to Keepalive_Period;
                  }
             }
             if( !SPTbit(S,G) AND ! pkt.NullRegisterBit ) {
                  decapsulate and forward the inner packet to
                  inherited_olist(S,G,rpt) # Note (+)
             }
       } else {
           send Register-Stop(S,G) to outer.src
           # Note (*)
       }
   }

   outer.dst is the IP destination address of the encapsulating header.

   outer.src is the IP source address of the encapsulating header, i.e.,
   the DR's address.

   I_am_RP(G) is true if the group-to-RP mapping indicates that this
   router is the RP for the group.

   Note (*): This may block traffic from S for Register_Suppression_Time
             if the DR learned about a new group-to-RP mapping before
             the RP did.  However, this doesn't matter unless we figure
             out some way for the RP also to accept (*,G) joins when it
             doesn't yet realize that it is about to become the RP

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             for G.  This will all get sorted out once the RP learns the
             new group-to-RP mapping.  We decided to do nothing about
             this and just accept the fact that PIM may suffer
             interrupted (*,G) connectivity following an RP change.

   Note (+): Implementations SHOULD NOT make this a special case, but
             SHOULD arrange that this path rejoin the normal packet
             forwarding path.  All of the appropriate actions from the
             "On receipt of data from S to G on interface iif"
             pseudocode in Section 4.2 should be performed.

   KeepaliveTimer(S,G) is restarted at the RP when packets arrive on the
   proper tunnel interface and the RP desires to switch to the SPT or
   the SPTbit is already set.  This may cause the upstream (S,G) state
   machine to trigger a join if the inherited_olist(S,G) is not NULL.

   An RP should preserve (S,G) state that was created in response to a
   Register message for at least ( 3 * Register_Suppression_Time );
   otherwise, the RP may stop joining (S,G) before the DR for S has
   restarted sending registers.  Traffic would then be interrupted until
   the Register-Stop Timer expires at the DR.

   Thus, at the RP, KeepaliveTimer(S,G) should be restarted to ( 3 *
   Register_Suppression_Time + Register_Probe_Time ).

   When forwarding a packet from the Register Tunnel, the TTL of the
   original data packet is decremented after it is decapsulated.

   The IP ECN bits should be copied from the IP header of the Register
   packet to the decapsulated packet.

   The DSCP should be copied from the IP header of the Register packet
   to the decapsulated packet.  The RP MAY retain the DSCP of the inner
   packet or re-classify the packet and apply a different DSCP.
   Scenarios where each of these might be useful are discussed in [12].



(page 44 continued on part 4)

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