[ih] Why did congestion happen at all? Re: why did CC happen at all?

[Brian E Carpenter]

On 01/09/2014 12:43, Miles Fidelman wrote:
> Tony Li wrote:
>> On Aug 31, 2014, at 1:20 PM, Miles Fidelman
>> <mfidelman at meetinghouse.net> wrote:
>>
>>> Last time I looked, bandwidth and delay were part of the metrics used
>>> in at least some routing tables (e.g.,  Cisco EGIRP) - which are at
>>> least indirect measures of congestion.  Or am I wrong here?
>>
>> Yes, but that’s maximum bandwidth and propagation delay, not queueing
>> delay and folks don’t actually enable that part of the metric anyway. 
>> Nothing dynamic here.
>>
>> Oh, and the last poor soul who did enable all of the dynamic features
>> of (E)IGRP ended up with a violently unstable network.
>>
>>
> 
> So is it really the case that there's no dynamic adaptation in the net,
> except if there's a major cable cut or some such?  I guess I haven't
> been paying attention of late.

"Interface down" is a much clearer signal than "Path might be congested".
The former *must* be dealt with; if you shift load as a result of the
latter, you shift the congestion, and the result is oscillation between
one congested path and another.

Some people do believe that an ECMP scenario can benefit from
flow analysis:
http://tools.ietf.org/html/draft-ietf-opsawg-large-flow-load-balancing
but this doesn't seem prone to oscillation, doesn't fundamentally
change routing topoloogy, and is a modest step.

    Brian