Subject: [Diffserv] ID-VW-BA, questions on jitter

From: Siamack Ayandeh <[email-masked]>
Date: Wed, 22 Mar 2000 15:27:44 -0500

I appreciate if the authors of the draft would comment
on the following factors impacting jitter for virtual wire
behavior aggregate (VW-BA).

1. Neither RFC 2598 nor VW deal with the jitter induced by inband
control
and routing traffic which may be of higher priority than EF. Relative
to "network" precedence is EF

a)  at a  higher priority?
b)  Is it of the same priority where each Class Selector CP
      is limited to emitting one packet using a token bucket.
c)  Or of lower priority.

2. Related question is that ID-VW-BA page-5 talks of "EF's priority
queuing model".  RFC 2598 does not impose such a requirement.

3.  Given that the MTU size is likely to be >> than (fixed) EF packet
size (S),  "n" in equation-1 should be at least 2 x MTU/S.
Leading to the need for fragmentation on slow links carrying EF.
This limits the plausible share of EF to 5% of link bandwidth or
less.

4. Does this residual service time of  (2 x MTU/S)  also arise in the
context
of non-prority schedulers such as non-bursty WFQ?  Is the new
limit k x 2 xMTU/S where k+1 is the number of scheduling classes.

5. Given that "k" may be of order of 10s if not hundreds (of
class-queues),
what guidelines do authors suggest should be followed in terms of
limiting
"k".

6. If this limit on "k" is not acceptable, then is EF-PHB and the VW
behavior
aggregate  limiting  scheduler implementations?

7. Given that S, the packet size would be different for various EF
flows, the
jitter window discussed in the draft is an upper bound based on the
largest
packet size.  How does this bound on jitter window effect the idea of a
virtual wire for flows carrying the smaller packets?

Many thanks, Siamack

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