Re: Network layer reqt? [was Re: Transport level multihoming]

[Joe Abley <jabley-ietf@automagic.org>]

On Wed, Apr 04, 2001 at 05:16:44PM -0500, Brian E Carpenter wrote:
> Margaret Wasserman wrote:
> > 
> > >No. But today, connections with TCP/IPv4 and typical IP multi-homing get
> > >dropped and data gets lost when the prefered path fails. Traffic is lost
> > >until the routing reconverges which is usually long enough to back TCP off
> > >into oblivion.
> > 
> > Is this true?  Can other operators or admins of large sites
> > comment on this statement, in particular?
> 
> Well, we've had many anecdotes of very long (>100 seconds) BGP 
> convergence times- Abha reported on some actual *facts* in the 
> IETF plenary- see http://www.merit.edu/ipma/. TCP doesn't
> react well.

I'm not sure there is a direct one-to-one correspondence here.

Edge networks will often route by default in the absense of completed
BGP convergence. Using Abha's taxonomy, instability in covered routes
in the core need not necessarily cause packet loss since packets will
be forwarded according to shorter-prefix aggregates.

Hence packets may still be delivered (perhaps over sub-optimal paths)
even while BGP is taking time to converge.

In my experience it is usually only severe turbulance which triggers
an ICMP back to an endpoint that causes sessions to die; multi-minute
packet loss due to loops in response to re-homing is not altogether
usual, I think.


Joe