Internet Draft


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



	             PIM Neighbor Hello GenId Option
		 <draft-ietf-idmr-pim-hello-genid-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 except for the right to 
   produce derivative works.

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Abstract

   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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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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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 
   1998.


Authors' Addresses

   Yunzhou Li
   Nortel Networks
   BL60-304
   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
   BL60-304
   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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