Re: [Diffserv] problems with Virtual Wire and RFC2598

Jean-Yves Le Boudec wrote:
> 
> I have some problems understanding RFC2598 and
> draft-ietf-diffserv-ba-vw-00
> 
> a) RFC2598 requires that the service given to an aggregate flow
> guarantees a rate down to a small time scale and further gives as
> example
> "A simple priority queue will give the appropriate behavior as long as
> there is no higher priority queue that could preempt the EF for more
> than a packet time at the configured rate. "
> 
> I have the impression that this statement ignores the phenomenon of
> jitter accumulation due to aggregate multiplexing. When several flows
> share their fate in a multiplex, then there is accumulation of jitter
> from network node to network node, well beyond the bounds given in
> this internet draft. The fact that flows are shaped at the network
> input does not guarantee a bound on jitter after several hops. Some
> bounds do exist, but they are not straightforward, and it is not even
> clear whether finite bounds exist in all cases where the maximum
> network utilization factor is less than 1. In the particular case
> where the network topology is a ring, the answer is yes, however, in
> general, it is unknown (as far as I know). See for example the
> references below.
> 

So this is what a lot of the discussion in the vw draft is
attempting to address. We've tried to spell out what
the assumptions are and how they get used. I'm hoping
that in today's meeting we can figure out what is
missing in the explanation so the next roll can make
this clearer. Anna Charney's work uses a different set
of assumptions.

There are some university folks who believe they can work
out the analysis under the assumptions we work under. 
Hopefully, they'll get something worked out and made
publically available.

> It seems to me that the only way to guarantee the requirement
> described in RFC2598 is to shape every aggregate at the output of
> *every* network node, a solution that requires per aggregate
> information which is not compatible with diffserv.

This comes out under Charney's assumptions since they are
no more restrictive than the rules we assume for individual
domains. Thus this is like the rules for shaping at the
edge of every domain.

	Kathie

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