[Diffserv] problems with Virtual Wire and RFC2598

[Jean-Yves Le Boudec <[e-mail]>]

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.

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.

b) I don't understand why we would need to bound the network jitter to the
interval called "jitter window" in draft-ietf-diffserv-ba-vw-00.  It is
possible to guarantee a VW behaviour even if the jitter is larger. It is well
known that if the delay jitter is bounded by D, then a buffer of size 2RD is
able to recreate a periodic flow with 0 jitter.


JY

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References:
 

A. Charny's work at

ftp://ftpeng.cisco.com/ftp/acharny/aggregate_delay_v4.ps

or mine at

http://dscwww.epfl.ch./EN/publications/ps_files/tr00_002.ps

or 

J. Wroclawski and A. Charny's internet-draft:

http://ana.lcs.mit.edu/ISSLL/drafts/draft-ietf-issll-ds-map-00.txt