Subject: How quickly does the queue size change at Internet core routers?

From: Leon Martin <[email-masked]>
Date: Tue, 19 Oct 2004 17:31:26 -0700 (PDT)

Hi Folks,

Does anyone know how quickly the queue size changes at
Internet core routers? Please give me a reference if
possible.

This question is very important to active queue
management (AQM) algorithms. To see this, note that
many AQM algorithms operate on the queue size, and the
action taken by an Internet core router will take
about an RTT to be perceived by a TCP sender.
Therefore, if the queue size changes too fast compared
to an RTT, AQM algorithms will simply not work.

A more detailed description on this question is
available at:
http://www.cs.ucr.edu/~mart/204/CS_204_Project.html

And here I quote the question:
"Active queue management algorithms (like RED) try to
prevent congestion by watching how the queue sizes
fluctuate over time. If the queue size starts to get
large and looks like it will soon grow even larger,
the queue management algorithm attempts to cause the
TCP senders who are using this link to reduce their
respective transmission rates by randomly dropping a
few packets before the buffer is completely full.
However, the question is, can the queue management
algorithm inform the TCP senders of impending buffer
congestion quickly enough to be useful?  The typical
round-trip delay for a TCP session that crosses the
Internet is on the order of 10s or 100s milliseconds.
Thus, if the queue management algorithm tells a TCP
sender to slow down now (or, conversely, that it is OK
to speed up again), the sender will be unable to react
until one round-trip time later. If queue sizes change
very slowly (i.e., the time between episodes of
congestion is much larger than the round-trip delay),
then the queue management algorithm can be quite
effective. However, if queue sizes change very quickly
in comparison to the round-trip delay then queue
management will not work very well. "

Thanks.

- Leon

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