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Internet Engineering Task Force                               Yunzhou Li
INTERNET-DRAFT                                           Nortel Networks
                                                                Billy Ng
                                                         Nortel Networks
                                                        25 February 1999

                     PIM Neighbor Hello GenId Option

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except for the right to
   produce derivative works.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   This memo addresses an issue with the current PIM in Sparse Mode. In
   the case of router reboot, PIM networks have to converge slowly and
   suffer long outage of traffic flows.

   This memo proposes a Generation Identifier (GenId) Option in PIM
   Hello messages. It enables neighbors to quickly detect router reboot
   and thus to synchronize RP-Set information and forwarding states by
   triggering Bootstrap and Join/Prune messages to the rebooted router.
   The rebooted router then is able to quickly recover from the reboot.

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Internet Draft      PIM Neighbor Hello GenId Option     25 February 1999

1. Introduction

   The current default PIM Hello interval is 30 seconds and its holdtime
   is 105 seconds. If a neighbor is rebooted within this holdtime, all
   other neighbors on the LAN will not be able to detect this
   transition.  Thus, in the case of PIM Sparse Mode, the rebooted
   neighbor will take longer to learn its RP-Set information causing
   longer outage of traffic flows through this neighbor. In the worst
   case, the rebooted neighbor was the elected BSR or a Candidate BSR
   in which case the BSR election process will take place unnecessarily
   even if its configuration stay the same.

   In particular, if the DR for a source network is rebooted, other
   routers on the same network will not transit to be a new DR. The DR
   will not forward any data packets until it learns RP-Set information
   60 seconds later (in the worst case).

   In the case of an upstream router reboot, in the worst case, the
   rebooted router has to take 60 seconds to learn RP-Set information,
   and then to take 60 seconds to receive Join/Prune from downstream
   neighbors. As a result, downstream members will suffer 120 second
   outage of traffic flows.

   We propose a GenId option in the PIM Hello message. A GenId is
   randomly selected when the router boots and remains the same as long
   as the router is up.  By including this GenId as an option in the
   Hello packet, a neighbor reboot can easily be detected if its GenId
   is different from before.  When such an event happens, the DR on the
   LAN unicasts its most recent RP-Set information to the rebooted
   neighbor.  If the rebooted neighbor was the DR, the next
   highest ip address (or the next highest DR priority if this option is
   enabled) neighbor will unicast the information.

2. Generation Identifier Option

   The GenId is a 32-bit unsigned number. This number is randomly
   assigned when the router boots up and remains the same for the
   router's up time.  This is usually taken from the router's wall clock
   in seconds.

   A router may intentionally regenerate a new GenId in order to
   synchronize its RP-set information and forwarding states with its
   neighboring routers.

   If no GenId option is specified in a Hello message, the Hello sender
   is deemed not capable of handling the GenId option. When such a hello
   message is received, the receiver will just treat it as zero. This
   way new systems can interoperate with older systems in the old way.

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Internet Draft      PIM Neighbor Hello GenId Option     25 February 1999

   The GenId received in a Hello is kept until the next Hello from the
   the same system arrives. The newly received GenId replaces the cached
   GenId for the same neighbor if its GenId has changed.

   An implementation capable of doing this option should always include
   it in the Hellos even if no GenId option is explicitly configured.

   The following is the format of this option.

   OptionType:   20
   OptionLength: 4

    |    OptionType = 20            |      OptionLength = 4         |
    |                             GenId                             |

   GenId: 32-bit GenId value

3. Triggering Hello Message

   In order to minimize delay, a Hello message should immediately be
   sent upon boot up. After learning a new GenId from a neighbor, a
   router should unicast a Hello message to the neighbor after a random
   delay. This is to trigger the neighbor to establish neighborship with
   all routers as soon as possible.

4. Triggering Behaviours in PIM-SM

   After learning a new GenId from a neighbor, if it determines itself
   is the DR (or the next available DR if the neighbor was the DR), the
   router should unicast a Bootstrap message to the neighbor after
   sending the above Hello message. Aside, for each forwarding
   state with the neighbor as the upstream, the router should
   subsequently send Join/Prune messages to this neighbor.

5. Acknowledgments

   Brad Cain and Hal Sandick commented on this draft.

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Internet Draft      PIM Neighbor Hello GenId Option     25 February 1999

6. References

[PIM-SM] D. Estrin et al, Protocol Independent Multicast Sparse-Mode
   (PIM-SM):  Protocol Specification.  RFC 2362, June 1998.

[PIM-DM] S. Deering et al, Protocol Independent Multicast Version 2
   Dense Mode Specification. Internet Draft, work in progress, November

Authors' Addresses

   Yunzhou Li
   Nortel Networks
   600 Technology Park Drive
   Billerica, MA 01821

   Phone:  1-978-916-1130
   Fax:    1-978-670-8760
   E-mail: yunli@NortelNetworks.COM

   Billy Ng
   Nortel Networks
   600 Technology Park Drive
   Billerica, MA 01821

   Phone:  1-978-916-8412
   Fax:    1-978-670-8760
   E-mail: bng@NortelNetworks.COM

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