Network Working Group
Internet draft                          Mitchell Erblich
                                        November 2004
Category: Informational
Document: <draft-erblich-ospf-nbr-rexmit-robust-enhance-00.txt>

        Robust Enhancement to the Neighbor's Retransmission
        List when one or more LSA Checksum and length are in
        Error


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.


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Abstract

   The ability to process LSAs within a Update packet
   requires that the length field be correct to generate
   the next offset within the packet. During the rare
   times that a checksum error and length LSA fields are
   incorrect, the beginning of later LSAs header's can't
   be determined. This draft specifies a transparent
   method to allow all valid LSAs to be processed even
   when these corrupted LSAs exist on the neighbor's
   retransmission list.



Table of Contents

 1. Introduction ...............                                2
 2. Neighbor retransmission list robust enhancement             2
 3. Alternative neighbor retransmission robust enhancement      2
 4. References .................                                3
 5. Security Considerations ....                                3
 6. Author's Address ...........                                3


. Introduction

   RFC [2328] for OSPFv2 specifies that on page 142
   all the LSAs that exist within the Update packet be
   processed. In the event that LSAs within this packet
   are not acknowledged, the non acknowledged LSAs will
   be retransmitted. These non acknowledged LSAs are
   repeatedly retrieved for retransmission from the neighbor's
   retransmission list.

   In the past, networks have been moderated by router's
   bandwidth, memory, and other limitations and have
   summarized their links. This has kept link-state databases
   from expanding to excessive levels. However, this has
   come with a resultant loss of metric information. With
   current and near future technology, LSDBs can expand
   to current levels.

   What was once rare checksum and length errors within
   moderately sized LSDBs can become common place when
   a router is dealing with millions of LSAs.

   To allow the OSPF specifications to scale when dealing
   with the once rare event of a checksum and length LSA
   error existing on the neighbor retransmission list,
   rare error events need to be handled.


. Neighbor retransmission list robust enhancement

   With the assumption that a a corrupted LSA with a bad
   checksum and length is on the neighbor retransmission
   list.  If later non corrupted LSAs are ordered after
   this corrupted LSA, they can never be processed.

   A transparent change to the receiver requires that the
   LSA transmitter alter the order of LSAs that are appearing
   on the Update packet.

   The method chosen was to rotate the order by one position
   on each neighbor retransmission as if the LSAs were in a
   circular buffer. This allows all the non corrupted LSAs to
   be processed by the receiver over time.


. Alternative neighbor retransmission robust enhancement

    With the assumption that retransmission of LSAs may
    infrequently occur due to checksum / length failures
    and that the problem occurred before the LSAs was
    copied to the neighbor retransmission LSA list.



    An alternative method is for the sender to periodically
    run a checksum scan on the retransmission LSA list
    and remove corrupted LSAs from the retransmission list.

    The suggested period is 30 seconds to prevent normal
    retransmissions from having additional checksum overhead.


. References

[] Moy, J., "OSPF Version 2", RFC 2328, April 1998.


. Security Considerations

   This memo does not create any new security issues for
   the OSPF protocol.


. Authors Address

   Mitchell Erblich
   erblichs@earthlink.net


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