In TCP Networks, the most important factor that determines behavior is its congestion window size. In NS2, we can handle it with using cwnd command. In NS, every TCP type classes have a variable named "cwnd" that contains the congestion window size of the TCP Module. In here we can use 'set' command to return a value.
We can write it as;
set tcp1 [new Agent/TCP/Reno]
set cwnd1 [ $tcp1 set cwnd_ ]
To obtain TCP's CWND value frquently;
- We can easily make the NS simulation system repeatedly read the value (say, after every 0.1 sec of simulation time).
- Schedule a read operation repeatedly
Proc plotWindow {tcpSource outfile} {
global ns
set now [$ns now]
set cwnd [$tcpSource set cwnd_]
# Print TIME CWND for gnuplot to plot progressing on CWND
puts $outfile "$now $cwnd"
$ns at [expr $now+0.1] "plotWindow $tcpSource $outfile"
}
- The procedure plotWindow takes a paramter tcpSource which is a TCP agent.So you can use the procedure to plot the CWND from any number of TCP flows.
- The procedure plotWindow takes an output file ID outfile.You should first open an output file (or use "stdout") in the main program.
Examining progressing of CWND in TCP (Reno)
#Make a NS simulator
set ns [new Simulator]
# Define a 'finish' procedure
proc finish {} {
exit 0
}
# Create the nodes:
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]
set n5 [$ns node]
# Create the links:
$ns duplex-link $n0 $n2 2Mb 10ms DropTail
$ns duplex-link $n1 $n2 2Mb 10ms DropTail
$ns duplex-link $n2 $n3 0.3Mb 200ms DropTail
$ns duplex-link $n3 $n4 0.5Mb 40ms DropTail
$ns duplex-link $n3 $n5 0.5Mb 30ms DropTail
# Add a TCP sending module to node n0
set tcp1 [new Agent/TCP/Reno]
$ns attach-agent $n0 $tcp1
# Add a TCP receiving module to node n4
set sink1 [new Agent/TCPSink]
$ns attach-agent $n4 $sink1
# Direct traffic from "tcp1" to "sink1"
$ns connect $tcp1 $sink1
# Setup a FTP traffic generator on "tcp1"
set ftp1 [new Application/FTP]
$ftp1 attach-agent $tcp1
$ftp1 set type_ FTP (no necessary)
# Schedule start/stop times
$ns at 0.1 "$ftp1 start"
$ns at 100.0 "$ftp1 stop"
# Set simulation end time
$ns at 125.0 "finish" (Will invoke "exit 0")
##################################################
## Obtain CWND from TCP agent
##################################################
proc plotWindow {tcpSource outfile} {
global ns
set now [$ns now]
set cwnd [$tcpSource set cwnd_]
###Print TIME CWND for gnuplot to plot progressing on CWND
puts $outfile "$now $cwnd"
$ns at [expr $now+0.1] "plotWindow $tcpSource $outfile"
}
$ns at 0.0 "plotWindow $tcp1 stdout" // Start the probe !!
# Run simulation !!!!
$ns run
his NS Prog prints the (time, cwnd) to the terminal: click here
This NS Prog prints the (time, cwnd) to the output file "WinFile": click here
To plot the window progressing from "winfile", do:
UNIX>> gnuplot
gnuplot>> plot "WinFile" using 1:2 title "Flow 1" with lines 1
Note:
In case you wonder why the cwnd plot look so different, It's because the setting of some parameters.
Add the following statements to the simulation to get the one I used in class:
# ########################################################
# Set Queue Size of link (n2-n3) to 10 (default is 50 ?)
# ########################################################
$ns queue-limit $n2 $n3 10
# ########################################################
# TCP parameters:
# ########################################################
$tcp1 set window_ 8000
$tcp1 set packetSize_ 552
This NS Prog will draw the CWND: click here
Postscript: Analyzing multiple TCP flows
The easiest way to analyze the behavior of multiple TCP is to open one file to store the progression of one TCP agent's variable values.
TCP Agents
set tcp1 [new Agent/TCP/Reno]
...
set tcp2 [new Agent/TCP/Reno]
...
set outfile1 [open "WinFile1" w]
set outfile2 [open "WinFile2" w]
$ns at 0.0 "plotWindow $tcp1 $outfile1"
$ns at 0.0 "plotWindow $tcp2 $outfile2"
Plot data of TCP 1 will be store in file "WinFile1"
Plot data of TCP 2 will be store in file "WinFile2"
