Studying TCP's Congestion Window using NS
-
How to obtain TCP's CWND value
- The most important value that determine the behavior of TCP is the congestion window size or traditionally abreviated as CWND
- In NS, every TCP-type class (Agent/TCP/Tahoe, (Agent/TCP/Reno, etc) has a variable named
cwnd_
that contains the congestion window size of the TCP module
- Recall that we can use the set command to return a value
- Hence, the following command will retrieve the congestion window size of a TCP module:
set tcp1 [new Agent/TCP/Reno] set cwnd1 [ $tcp1 set cwnd_ ] // read variable "cwnd_"
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How to obtain TCP's CWND value PERIODICALLY
- Now that we know how to read the congestion window size of a TCP module once, it is easy to make the NS simulation system repeatedly read the value (say, after every 0.1 sec of simulation time).
- All we need to do is to schedule a read operation repeatedly
- We have seen an example of self-scheduling behavior in the "2 person talking example" (click here)
- We can use a similar self-scheduling procedure to obtain the value of CWND repeated.
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Example: (requires that the Simulator object variable be named $ns)
proc plotWindow {tcpSource outfile} {
global ns set now [$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
- The procedure plotWindow takes a paramter tcpSource which is a TCP agent
- Here is the previous example (click here) which additional code to obtain the congestion window size of the TCP module $tcp1:
(New code is colored as magenta )
#Make a NS simulator
set ns [new Simulator] # Define a 'finish' procedure
proc finish {} {
exit 0
} # Create the nodes:
set n0 [$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
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Example Program: (Demo above code)
- This 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 run the program, use the command:
ns Reno2.tcl
To plot the window progressing from "winfile", do:
- UNIX>> gnuplot
-
- gnuplot>> plot "WinFile" using 1:2 title "Flow 1" with lines 1
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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
- 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.
-
Example: 2 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"
http://www.mathcs.emory.edu/~cheung/Courses/558-old/Syllabus/90-NS/3-Perf-Anal/TCP-CWND.html