Subject: OSPF Hitless Restart

From: Acee Lindem <[email-masked]>
Date: Fri, 1 Nov 2002 00:22:45 -0500

In working toward the goal of getting to WG last
call for this document, I've incorporated many
of your comments. I tried to include items
that made sense and were not a radical departure from John's
original draft. Here is an excerpt from the change log:

   Changes from 03 to 04 version:

      1. Add change log (Appendix B).
      2. Document that the grace period is restricted to
         LSRefreshTime (Section 2.1).
      3. Document an alternative to saving cryptographic sequence
         numbers in non-volatile storage (Section 2.1).
      4. Document that an implementation may disable graceful
         restart helper termination when the link-state database
         changes (Section 3.2).
      5. In the case of an unplanned restart, document that
         grace LSAs should be flooded to AllSPFRouters on
         broadcast networks (Section 5).
      6. Remove MOSPF from future work. Add Vishwas's suggested
         technique for less conservative helper mode termination as
         possible future work (Section 7).
      7. Change references and citations to meet prevailing IETF
         standards.

I have not incorporated Padma's suggested change that the
grace LSA apply to all neighbor adjacencies with the router ID
of the restarting router. In a nutshell, here are the arguments
for making the change versus leaving the document as is:

  Arguments for changing the behavior so that reception of
  a grace LSA for router ID X causes a router to enter helper
  mode for all neighbor adjacencies with router X.

  1. Restart applies to the OSPF process so the grace LSA should
     be applied to all neighbor adjacencies for the restarting
     router.

  2. Slighty better chance that grace LSA originations and purges
     will be received by a helping router with multiple
     adjacencies.

  Arguments for leaving the document as is with the grace LSA
  applying only to neighbor adjacencies on the link received.

  1. Since the grace LSA is link local scope, it is more
     consistent to apply it only to neighbor adjacencies on
     that link.

  2. Simplifies changes to grace LSA TLVs. The current draft
     just states that new TLV values (grace LSA and reason)
     replace the old ones. The rules for handling
     differences would need to be defined and would be more
     complex.

  3. Slightly less overhead when a grace LSA is received since
     you don't have to search through all your neighbor
     adjacencies looking for the restart router ID (although
     there may be other reasons to maintain these parallel
     adjacencies in a separate list ;^).

All in all, there doesn't appear to be that compelling an
argument of one approach over the other. I would have been
inclined to reject the change had Padma not felt so
strongly. One thing I am strongly against is trying to
define determiniation of which of multiple instances
of *different* LSAs is more recent - we just don't want
to go there.

Please voice your opinions on the list of changes and the
last issue. The 03 version of the draft is available:

http://www.ietf.org/internet-drafts/draft-ietf-ospf-hitless-restart-03.txt

I don't intend to post the complete 04 version to the list
or re-spin it for a while.

-----
Thanks,
Acee