[ns] A question about ns2 TCP cwnd

[Ming Zhang]

Hi all,

 

I am a new comer to ns2. I am struggling with a problem for few days and
really hope that someone here could help me out. Thanks in advance!

 

I would like to monitor TCP cwnd change in ns2. So I set up a simple
topology: 3 wired nodes 0-->1-->2 are linked together and there is a TCP
flow from 0 to 2 (relayed by 1). A FTP server is on top of TCP at 0, which
generates the traffic. I intentionally set link 0->1 with much higher
bandwidth (e.g., 2Mb) than that of 1->2 (e.g., 0.2Mb). I expect that packets
may be dropped (or cause timeout) at the intermediate node 1 because 0 could
send more packets to 1 than 1 could forward to 2. Then, TCP should change
its cwnd to force node 0 to slow down. 

 

However, I could not see the sawtooth graph of cwnd at 0. 

 

Then I set the queue size of link 1->2 to be 10 - now I could see that cwnd
changes as what I expect. However, if I set the queue size to be 20, the
sawtooth is gone and cwnd just keeps rising to a very large number again.

 

My question is that:

1. Which layer is the 1->2 queue size setting? MAC layer?

2. Why will the size 10 and 20 make different stories?

 

Any suggestion is welcome!

 

My TCL script is attached as follows:

 

set ns [new Simulator]

 

set nd [open out.tr w]

$ns trace-all $nd

 

set wnd_trace [open cwnd.tr w]

 

proc record {} {

  global ns tcp wnd_trace

  set time 0.1

  set curr_cwnd [$tcp set cwnd_]

  set now [$ns now]

  puts $wnd_trace "$now $curr_cwnd "

  $ns at [expr $now+$time] "record"

}

 

proc finish {} {

  global ns nd wnd_trace

  $ns flush-trace

  close $nd

  close $wnd_trace

 

  exit 0

}

 

set n0 [$ns node]

set n1 [$ns node]

set n2 [$ns node]

 

$ns duplex-link $n0 $n1 2Mb 10ms DropTail

$ns duplex-link $n1 $n2 0.2Mb 10ms DropTail

 

$ns queue-limit $n1 $n2 20

 

set tcp [new Agent/TCP]

$tcp set class_ 2

$ns attach-agent $n0 $tcp

set sink [new Agent/TCPSink]

$ns attach-agent $n2 $sink

$ns connect $tcp $sink

 

set ftp [new Application/FTP]

$ftp attach-agent $tcp

$ftp set type_ FTP

 

$ns at 1.0 "$ftp start"

$ns at 1.1 "record"

$ns at 100.0 "$ftp stop"

 

$ns at 100.5 "$ns detach-agent $n0 $tcp ; $ns detach-agent $n2 $sink"

 

$ns at 101.0 "finish"

 

$ns run