North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: Nanog list uncharacteristically quiet?

  • From: Paul A Vixie
  • Date: Sun Dec 15 15:38:47 1996

> Sounds like this would make everything be symmetrically routed.

No.  It only means the end system's first hop is symmetrical.  The
intermediate systems might packet-stuff on multiple paths, or choose
a different path based on private data, or whatever.  My goal is not
end to end path symmetry -- hot potato routing makes that impossible
in any case.  My goal is that an end system should be able to accept
connections via multiple providers without needing a routing table
from each of those providers.  (Which can't be made symmetrical even
in the average case, anyway.)

> Has anyone researched whether this can scale?

I havn't.  I know that right now, someone who wants to multihome has
to do way too much work and they cannot reliably send packets back on
the same first-hop as the last-hop they came in on, which makes it
pretty much worthless to multihome unless you break your BGP listener
to use round robin or pseudo random path selection -- and when we did
this it made some things worse rather than better.

> Will you handle the case where the interface a flow uses changes
> during the course of the flow?

If the link (or ARP) for the current next-hop dies, the route rehomes
among the remaining interface defaults and then rehomes again next time
a segment is received on some remaining-up interface.

> I assume this will lead to "round robin default" until an inbound
> packet for the flow is received from a new interface.

That's what I remember saying, yes.

> Will the "round robin default" favor uncongested or highest speed links? 

Not initially.  My test environment has a pair of 10baseT links and the
congestion is all after the first hop, where interface statistcs can't
help with local route selection.  I won't entertain the above question
seriously unless our initial results hint that it will be time well spent.

> I can't wait to hear the results of doing this!

I expect to have an informal report for this mailing list by January 10th.
- - - - - - - - - - - - - - - - -