R: [e2e] [Fwd: RED-->ECN]

[Saverio Mascolo]

What Hollot says is right. Averaging the queue makes much more difficult to
control the queue level. In control terms is like to add another pole in the
feedback loop.
An interesting paper is also "Reasons not to deploy RED" by J. Bolot et. al.
( at http://citeseer.nj.nec.com/337634.html).
This paper experiments average vs. instantaneous queue RED dropping.

-s


----- Original Message -----
From: C.V. Hollot <hollot at ecs.umass.edu>
To: <slow at caltech.edu>
Cc: Greg Minshall <minshall at redback.com>; Alhussein Abouzeid
<hussein at ee.washington.edu>; <end2end-interest at postel.org>;
<ecn-interest at research.att.com>; <misra at gaia.cs.umass.edu>
Sent: Saturday, February 03, 2001 4:01 PM
Subject: Re: [e2e] [Fwd: RED-->ECN]


> another point about measuring (and using) average queue size to mark
packets:
> average queue size does contain (perhaps useful) information about
> instantaneous queue length and arrival rate.  The real question is how to
> act on this information.  If we use this averaged info to mark packets,
> then we are (implicilty) closing a feedback loop around "stale (averaged)"
> information.
> Queue averaging introduces additional time (phase) delay to the feedback
> (in addition to queuing and propagation delays)- and we know that delayed
> feedback contributes to poor and even unstable feedback behavior.
> For example, consider the task of controlling the level of water in your
> bath tub if you could only measure the "average level of water."
>
> In my opinion it is better to measure instantaneous queue length, and
then,
> with a dynamic model of TCP/AQM in hand,  develop AQM algorithms (based on
> instantaneous queue length) to achieve  network performance objectives.
> for more details on TCP/AQM modelling and PI control see
> http://www-net.cs.umass.edu/~misra/
>
>
> cvh
> --------------------------------------------------
>
> C.V. Hollot
> Associate Professor
> ECE Department
> University of Massachusetts
> Amherst, MA 01003
> ph:  413.545.1586
> fax: 413.545.1993
>