[ns] Problem detected with TCP and bottleneck links

[Julian Waldby <waldby@uiuc.edu>]

I want to test TCP performance over a bottleneck link, which is
basically a low bandwidth link somewhere in the path of the connection.
I have a problem with ns-2.1b6's simulation of TCP, as follows.

When the queue at the bottleneck link ($bottleQueueSize in attached
code) is of size 16, TCP performs as expected. However, when the queue
size is increased to 17, suddenly TCP performs in an erroneous fashion -
the congestion window grows without bound.

Please try the following TCL script with
"set bottleQueueSize 16"
and
"set bottleQueueSize 17"

I find that with the larger queue, TCP's congestion window grows
steadily, never backing off.  Eg, when $totalTime is  3000.0 seconds,
cwnd_ grows to more than 700.  Code is attached.  It uses standard
building blocks of ns-2.1b6.

Julian
--
http://www.comm.csl.uiuc.edu/~waldby/

tcp-3L-1S.tcl