Re: [Idr] Progressing draft-bhatia-bgp-multiple-next-hops-01.txt

[Paul Jakma <paul at clubi.ie>]

On Thu, 24 Aug 2006, John G. Scudder wrote:

> I don't understand your apparent concern about how to choose the 
> path identifier.  Surely this is a problem that can be solved by 
> any moderately skilled undergraduate.

:)

Yes, it undoubtedly is. And they should reach the following 
conclusion:

  for readvertisement of multiple-paths to the same destination to
  be feasible you need to be able to determine the identity of the
  path from the path itself

As you can not construct the additional path identifier solely from 
the identity of the speaker from whom you received the path, 
obviously, if they are to advertise multiple-paths.

Hence it follows that some additional aspect of each path itself must 
be used to discriminate between paths to the same destination (e.g. 
the hash value of certain key values, or even one key value directly, 
such as nexthop)[1].

Our point therefore is that we should simply use next-hop as this 
additional path-identity, as it is emminently suited to the task.

There is no value in leaving the additional identifier to local 
implementations, as they're going to have to come to the same 
conclusion, with the risk that if they do not[2] they will unable to 
cope with RR clusters and various other multiple-readvertisement 
scenarios.

1. Note that this means the stability of the Add-Path-ID depends on
   the stability of whatever aspect of the path is chosen to mix into the
   Add-Path-ID.

   Consider this with the point raised against Multiple-Hop by Enke
   that if next-hop changes, Multiple-Hop must withdraw and
   reannounce. The same likely will be true for add-path implementations.

2. E.g. by choosing to use identity of the speaker (Router-ID,
   etc..) from whom a path was received solely as the Add-Path-ID, as
   the add-path draft hints at, and as Enke gave as an example in
   this thread.

regards,
--
Paul Jakma	paul at clubi.ie	paul at jakma.org	Key ID: 64A2FF6A
Fortune:
Oh, `no attitude,' eh?  Not `in your face,' huh?  Well, you can cram it
with walnuts, ugly!

		-- Homer Simpson
		   The Itchy & Scratchy & Poochie Show

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