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Network Working Group                                     Dino Farinacci
Internet Draft                                             cisco Systems
Expiration Date: August, 1999                                I. Kouvelas
                                                           cisco Systems
                                                             K. Windisch
                                                    University of Oregon
                                                       February 23, 1999


                        State Refresh in PIM-DM
                  <draft-kouvelas-pim-refresh-00.txt>


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-Drafts.

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

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).


1. Introduction

   This proposal extends the PIM-DM [1] protocol specification by intro-
   ducing the PIM State-Refresh control message.

   When an (S,G) entry is created in a router for a directly connected
   source, if the interface directly connected to the source is the
   incoming interface for the entry, a new timer is started: the State-
   Refresh-Timer [SRT(S,G)]. The State-Refresh-Timer controls periodic
   transmission of the PIM State-Refresh message, which is propagated
   hop-by-hop down the (S,G) RPF tree. When received by a router on the
   RPF interface, the State-Refresh message causes existing prune state
   to be refreshed.

   Addition of this heartbeat message solves many of the current



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Internet Draft            PIM-DM State Refresh             February 1999


   problems with PIM-DM. It prevents the periodic timeout of prune state
   in routers, greatly reducing the re-flooding of multicast traffic
   down the pruned branches that expire periodically. It also causes
   topology changes to be realised quicker than the traditional 3 minute
   timeout.

2. Sending State-Refresh

   For a given (S,G) tree, State-Refresh messages will be originated by
   all routers that use an interface directly connected to the source as
   the RPF interface for the source. Upon expiry of their (S,G) State-
   Refresh-Timers the PIM State-Refresh message will be sent on all
   PIM-DM interfaces with active PIM neighbors, except the interface
   connecting the source.

   In addition, when the SRT(S,G) expires, the following timers are
   refreshed:  SRT(S,G) is restarted with it's default value, and all
   (S,G) pruned interface timers are refreshed.

   Stopping transmission of State-Refresh messages is controlled by the
   expiry of the (S,G) entry timer. The entry timer is not reset upon
   expiry of the SRT(S,G) timer and is only updated when data packets
   for the group are received, as in normal PIM-DM operation.

   All other routers will send refresh only when receiving one from a
   neighbor, as described below.

   State-Refresh messages are multicast using address 224.0.0.13 (ALL-
   PIM ROUTERS group) with protocol number equal to PIMv2 and a TTL of
   1. The IP source address is set to the outgoing interface address and
   is rewritten hop-by-hop when forwarding.

   The State-Refresh message also contains the source and group the mes-
   sage is referring to, the originator address (for debugging pur-
   poses), routing information required by the assert mechanism, a TTL
   value for scope control (different from header TTL) and a Prune-
   Indicator flag described below. The routing information, TTL and
   branch indicator can be rewritten hop-by-hop.

   The TTL value in the message is initially set by the originating
   router to the value of the largest TTL observed in data packets from
   the source so far. The TTL value will be decremented by downstream
   routers forwarding the State-Refresh message. Routers will only for-
   ward the State-Refresh message if the value of the TTL in the message
   is greater than 0 and larger than the configured local threshold.
   This will prevent State-Refresh messages from reaching areas of the
   network where data packets have not already created (S,G) state.




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Internet Draft            PIM-DM State Refresh             February 1999


   The Prune-Indicator flag is cleared when the message is transmitted
   on an outgoing interface in forwarding state and set when the message
   is transmitted on a pruned interface. This mechanism is required to
   recover from situations where loss of consecutive refresh messages
   has caused an inconsistency in prune state on a branch of the (S,G)
   tree.

3. Receiving State-Refresh

   PIM State-Refresh messages are RPF flooded down the (S,G) tree using
   the data source address included in the message to determine the RPF
   neighbor. When a PIM State-Refresh message is received for a given
   (S,G), the following steps are taken:

   o Whenever a (S,G) State-Refresh message is received on the interface
     for RPF(S) by a router with no existing (S,G) entry, an (S,G) entry
     should be created. If the Prune-Indicator flag in the message indi-
     cates a forwarding branch, then all non-iif interfaces with PIM
     neighbors are set to forwarding state in the new entry. Otherwise,
     the new entry is created with prune state on all non-iif inter-
     faces.

   o If the (S,G) State-Refresh message was received on an interface
     other than RPF(S) by a router with no existing (S,G) entry, then
     the message is ignored.  If the receiving interface corresponds to
     a LAN the message may still be processed according to the normal
     PIM Assert rules described in [1].

   o If the State-Refresh message was received on a (S,G) non-iif inter-
     face then the message is ignored. If the receiving interface
     corresponds to a LAN the message may still be processed according
     to the normal PIM Assert rules described in [1].

   o If the State-Refresh was received on the (S,G) incoming interface
     from a PIM router other than the upstream neighbor (i.e, RPF neigh-
     bor or Assert winner), then the State-Refresh message is ignored.
     However, the message is still processed according to the normal PIM
     Assert rules described in [1].

   o If the State-Refresh was received on the (S,G) incoming interface
     from the upstream neighbor (i.e, RPF neighbor or Assert winner),
     then all (S,G) pruned interface timers are refreshed.  Further, if
     (S,G) is a negative cache entry, then the entry timer is also
     refreshed to its default value.

   o If the State-Refresh was received on the (S,G) incoming interface
     from the upstream neighbor (i.e, RPF neighbor or Assert winner) and
     the Prune-Indicator flag in the message is set, indicating that it



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Internet Draft            PIM-DM State Refresh             February 1999


     was forwarded down a pruned branch, but the local (S,G) entry is
     not a negative cache entry, then the Prune-Indicator flag in the
     message is cleared and a Graft is sent upstream.

   o If the State-Refresh was received on the (S,G) incoming interface
     from the upstream neighbor (i.e, RPF neighbor or Assert winner),
     then the Refresh message is retransmitted on all PIM interfaces
     other than the (S,G) incoming interface, provided that the TTL in
     the message is greater than 0 and larger then the configured thres-
     hold for the interface and that the interface does not have multi-
     cast boundary addresses configured for the group specified in the
     message. The IP header specifies the outgoing interface address as
     the source and the Refresh Packet is rewritten with the local
     router's preference, metric and mask for reaching S. If the (S,G)
     entry has prune state for the interface on which the refresh mes-
     sage is being sent, the Prune-Indicator flag in the message is set
     to indicate a pruned branch. The TTL in the forwarded message is
     one less than that of the received message.

4. State-Refresh Message Packet Format

   This section described the details of the packet format for the PIM
   DM State-Refresh Message. As with all PIM control messages, the
   State-Refresh message uses protocol number 103. It is multicast hop-
   by-hop to the `ALL-PIM-ROUTERS' group `224.0.0.13'.

        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|          Reserved           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

        Type



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Internet Draft            PIM-DM State Refresh             February 1999


            State-Refresh message type value is TBD. 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 the TTL observed in
            the data packets for the group and is decremented each time
            the State-Refresh 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.

        Reserved
            Set to zero and ignored upon receipt.

5. Handling Router Failures

   PIM Hello messages will contain a Generation ID (GenID) in a Hello
   option.  When a PIM Hello is received from an existing neighbor and
   the GenID differs from the previous ID, the neighbor has restarted
   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 message was
   received.  In order to avoid a burst of incoming State-Refresh mes-
   sages at the recovering router, transmission of messages for dif-
   ferent (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



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Internet Draft            PIM-DM State Refresh             February 1999


   value of 3 seconds is recommended.  The Prune-Indicator flag of the
   State-Refresh message should be set to indicate if the recovering
   router is on a forwarding or pruned branch of the (S,G) tree.

6. 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
   the distribution tree. These can be created when grafts are not ori-
   ginated from legacy routers that have timed out prune state whereas
   State-Refresh capable routers higher up the tree have maintained
   prune state for the branch.

   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 only protocol modification that is required to enable interopera-
   bility 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) prune timers are refreshed except those
     leading to legacy routers. Further if all outgoing interfaces lead-
     ing 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:

   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 routed 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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Internet Draft            PIM-DM State Refresh             February 1999


7. 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.

8. Acknowledgments

   The authors would like to acknowledge Tony Speakman (cisco) and Lim-
   ing Wei (cisco) for their comments and contributions to this specifi-
   cation.

9. Author Information

   Dino Farinacci
   cisco Systems, Inc.
   dino@cisco.com

   Isidor Kouvelas
   cisco Systems, Inc.
   kouvelas@cisco.com

   Kurt Windisch
   Advanced Network Technolgy Center
   University of Oregon
   kurtw@antc.uoregon.edu
























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