draft-ietf-pim-refresh-02.txt

BCAST Implementation for NS2

        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            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Encoded-Group Address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Encoded-Unicast-Source Address                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Encoded-Unicast-Originator Address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |R|                        Metric Preference                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Metric                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Masklen    |      TTL      |P|N|O|Reserved |   Interval    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        PIM Version, Reserved, Checksum
            Described in [2].

        Type
            State-Refresh message type value is 9. See [2] for types of
            other PIM control messages.

        Encoded-Group Address
            The group address to which the data packets were addressed,
            and which triggered the State-Refresh-Timer. Format
            described in [2].

        Encoded-Unicast-Source Address
            The address of the data packet source. Format described in
            [2].

        Encoded-Unicast-Originator Address
            The address of the first hop router that originated the
            State-Refresh message.  Format described in [2].

        Metric Preference, Metric, Masklen
            Preference value assigned to the unicast routing protocol
            that provided the route to Host address, the metric in units
            applicable to the unicast routing protocol and the mask
            length used (needed for assert logic as described in [1]).

        TTL
            This is set by the originating router to either a locally
            configured value or the TTL observed in the data packets for
            the group and is decremented each time the State-Refresh



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            message is forwarded.

        P
            The Prune-Indicator flag. This is set if the State-Refresh
            message was forwarded on a pruned interface and cleared oth-
            erwise.

        N
            The Prune-Now flag. This is set by the State-Refresh origi-
            nator on one out of three transmitted messages and is used
            by downstream routers on LANs to rate-control Prune
            transmission.

        O
            The Assert-Override flag. This is set by candidate forward-
            ers on a LAN if a State-Refresh message has not been heard
            by the assert winner over the period of three times the
            [Refresh-Interval].

        Reserved
            Set to zero and ignored upon receipt.

        Interval
            Set by the originating router to the interval (in seconds)
            between consecutive State-Refresh messages for this source
            [Refresh-Interval].

6. Handling Router Failures

   PIM Hello messages will contain a Generation ID (GenID) in a Hello
   option [3].  When a PIM Hello is received from an existing neighbor
   and the GenID differs from the previous ID, the neighbor has res-
   tarted and may not contain (S,G) state. In order to recreate the
   missing state, for each (S,G), all routers upstream of the failed
   router (i.e. those receiving the Hello on a non-iif) can send a new
   (S,G) PIM State-Refresh message on the interface that the Hello mes-
   sage was received.  In order to avoid a burst of incoming State-
   Refresh messages at the recovering router, transmission of messages
   for different (S,G) entries has to be randomly spaced over a period
   of time.  The duration of this period can be configured locally and a
   default value of 3 seconds is recommended.  The Prune-Indicator flag
   of the State-Refresh message should be set to indicate if the recov-
   ering router is on a forwarding or pruned branch of the (S,G) tree.

7. Compatibility with Legacy PIM Routers

   In order to enable incremental deployment of State-Refresh capable
   routers, additional mechanisms have to be used to prevent holes in



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   the distribution tree. These holes can be created because downstream
   routers without the State-Refresh capability will not send PIM grafts
   when (S,G) prune state times out. Upstream state-refresh capable
   routers will maintain (S,G) prune state.  If a new receiver joins on
   a legacy branch, data will never reach this receiver.

   Legacy routers are detected through the use of a new capability indi-
   cator in PIM Hello messages that can be used to inform neighbors
   whether a router is State-Refresh capable. The format of this option
   is as follows:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       OptionType = 21         |       OptionLength = 4        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------------------------+
       |  Version = 1  |   Interval    |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Interval field is used to advertise the [Refresh-Interval] used
   by the router for originating SR messages for directly connected
   sources on this interface. Using this field, inconsistencies in ori-
   gination intervals between first-hop routers for the same source can
   be detected.

   The only protocol modification that is required to enable interopera-
   bility with detected legacy routers is in the procedures for packet
   reception:

   o When a State-Refresh message is received on the (S,G) incoming
     interface from the upstream neighbor (i.e, RPF neighbor or Assert
     winner), then all (S,G) outgoing interface prune timers are
     refreshed except those leading to directly connected legacy
     routers. Further if all outgoing interfaces leading to State-
     Refresh capable routers are pruned then the entry timer is
     refreshed to its default value.


   This will allow the prune state of the outgoing interface leading to
   the legacy router to timeout and change to forwarding state. As the
   entry timer will be updated by State-Refresh messages, the entry will
   persist even after the transition. If the entry was a negative cache
   entry a graft will be sent upstream as a result.

   The above modifications will enable prune state to persist in sub-
   trees of a source distribution tree that fulfill the following two
   conditions:




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   a) The subtree is entirely State-Refresh capable.

   b) The path from the source to the subtree in entirely State-Refresh
      capable.

   A subtree of the source distribution tree rooted at a legacy router
   as well as the path from the source to the subtree will not benefit
   from State-Refresh messages and will experience traditional dense
   mode flood and prune behavior.










































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8. References

   [1] Deering, et al., "Protocol Independent Multicast Version 2 Dense Mode
       Specification", draft-ietf-pim-v2-dm-01.txt, November 1998.
   [2] Estrin, et al., "Protocol Independent Multicast-Sparse Mode (PIM-
       SM): Protocol Specification", RFC 2362, June 1998.
   [3] Li, et al., "PIM Neighbor Hello GenId Option",
       draft-ietf-idmr-pim-hello-genid-00.txt, February 1999.

9. Acknowledgments

   The authors would like to acknowledge Liming Wei (cisco), Tony Speak-
   man (cisco) and John Zwiebel (cisco) for their comments and contribu-
   tions to this specification.

10. Author Information

   Dino Farinacci
   Procket Networks
   [e-mail]

   Isidor Kouvelas
   cisco Systems, Inc.
   [e-mail]

   Kurt Windisch
   cisco Systems, Inc.
   [e-mail]























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