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.
  • 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" }

    1. 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.

    2. The procedure plotWindow takes an output file ID outfile
       
       You should first open an output file (or use "stdout") in the main program

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• Examining progressing of CWND in TCP (Reno)
  • 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

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• 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.
  • 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