Subject: [Diffserv] Dropping Strategy in AF PHB

From: Jay Wang <[email-masked]>
Date: Thu, 14 Dec 2000 13:10:27 -0800

Lots of efforts have been spent to EF, but little attention
is paid to AF.  I have a specific question regarding AF
as per RFC 2597, in which (Section 4) a statement was made:

"The dropping algorithm MUST be insensitive to the short-term
traffic characteristics of the micro flows using AF class.
That is, flows with different short-term burst but identical 
longer-term packets rates should have packets discarded with
essentially equal probability."

For one thing, I don't know why the authors of the RFC wanted 
to mandate this. It seems logical and OK to me that we drop
packets of a very bursty flow more than one that is "smooth"
even though the long term average rate are the same between the
two. Consider two flows both with a long term average rate r.
Yet one comes in as a CBR and the other has an on-off pattern
which toggles between rates 0 and R, where R>>r.  The rate R can 
be so large that not only I want to drop it (since I consider
it as a ill-behaving to keep it from depleting system resources 
while the flow is on), but I may in fact have no choice but drop 
packets simply due to limited buffer space for that PHB.

What are your thoughts?

- Jay

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From: "Nabil Seddigh" <[email-masked]>
Date: Thu, 14 Dec 2000 17:03:38 -0500 (EST)

>to mandate this. 

There was certainly a lot of attention paid to AF.

If I recall correctly, the intent of the quoted paragraph was to  ensure

that drop-tail was not used for AF. It essentially mandates an AQM
mechanism that takes past history into account when making its drop
decisions. I don't think the intent of that paragraph was
to require per-micro-flow drop decisions.

Best
Nabil

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 14 Dec 2000 16:10:25 -0600

> to require per-micro-flow drop decisions.

I think it was also to avoid trying to redefine RED, which would
have been out of place in a diffserv document. Anyway it was a
carefully debated paragraph.

   Brian

   Brian