Re: Subject: Re: [TE-wg] TE use in today's networks

[Yakov Rekhter <yakov@juniper.net>]

Noel,

>     > From: Jennifer Rexford <jrex@research.att.com>
> 
>     >> If one *does* need dynamic response, it should be intuitively
>     >> obvious that even if one can find a set of weights that produces
>     >> the result one wants in the base case, then the resulting traffic
>     >> pattern, once chance has taken some random set of elements out of
>     >> service, is probably not going to be one that comes anywhere near
>     >> meeting whatever goals one has.
> 
>     > experiments suggest that in practice the weights that are good in
>     > the base case often work pretty well in the failure case; when they
>     > don't, a change or two does the trick.
> 
> This may be related to a generic problem with min-path routing
> architectures (i.e. those which pick src->dest paths which minimize a
> fixed metric), which is that in many topologies, you see "hot links" which
> want to be overutilized (and "cold links" which want to be underutilized).
> 
> The old ARPAnet had this problem to some degree, only there they had
> load-based (actually delay-based) metrics in the routing which tended to
> ameliorate it - when one link loaded up, traffic tended to divert around
> that link. (Of course, then they had problems with oscillatory behaviour,
> but that's a different problem!)
> 
> One obvious way to fix this (other than either varying the capacity of
> links based on observed loads, or changing the topology so that it doesn't
> have as many hot-spot problems) is to tweak the metrics on hot-spot links
> up, to move traffic off them. So perhaps this part of what's going on.
> 
> 
>     > What if you don't have a screwdriver and the hammer works pretty
>     > well?
> 
> That's a perfectly appropriate attitude for operational people who have to
> deal with what's available off the shelf.

Just to add, screwdrivers are available, and whether "the hammer works
pretty well" depends on the definition of "pretty well".

> However, I'd hope that when it comes to basic engineering, there's more of
> a sense of "how do we do this right", not "how do we bludgeon existing
> stuff into doing more or less what we want".

There are some folks whose arguments against a screwdriver is based
on the dogma that all you need is a hammer, and everything else
(including a screwdriver) is "evil", and should not be used. Since
this is a fairly irrational attitude, no amount of rational arguments
is going to change it.

Yakov.