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OTcl: Object-Oriented Tcl

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• OTcl: Object-Oriented Tcl
  • OTcl is an extension to Tcl with object-oriented programming.

  • Features of Otcl:

    dynamically extensible, (just like Tcl) builds on Tcl syntax and concepts compact yet powerful object programming system            fairly portable implementation (2000 lines of C)

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• OTcl and the network simulator NS
  • The network simulator NS is actually a special version of a OTcl programming language interpreter

  • The NS network simulation system:

    NS is compiled from a OTcl language interpreter extended with a built-in library of network simulation classes

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  • The NS2 system has built-in OTcl classes that simulate:

    Routers (node) Links TCP (Tahoe, Reno, Vegas, and many other flavors) FTP etc., etc.

  • Example: Execute this command in NS:

    Class info instances

    You will see a long list of classes that are already defined in OTcl:

    QueueMonitor/ED/Flow/TSW RandomVariable/ParetoII Queue/RED/Semantic Agent/rtProto/Dummy Est/ExpAvg Agent/IVS/Receiver Application/Worm Agent/TCP PacketHeader/ARP Agent/Message Agent/TCP/BayFullTcp Agent/SCTP/MultipleFastRtx Queue/dsRED/core PagePool/ProxyTrace/epa ConnTimer Trace/Enque Connector/AddSR PacketQueue HandoffManager/Sat RandomVariable/Normal Agent/TCP/Sack1 mrtObject Queue/dsRED/edge PagePool/EmpWebTraf Http/Server Application/Telnet Agent/UDP Agent/PGM QueueMonitor/ED/Flow Node/SatNode Classifier/Addr/MIPDecapsulator SnoopQueue/In Application/Traffic/CBR_PP Channel/WirelessChannel PagePool PacketHeader/GAF Classifier/Hash/Dest/Bcast SplitObject FSM/RenoAck RtModule/Mcast PagePool/ProxyTrace Est/PointSample PktInTranMonitor Application/DiffApp/RmstFilter Agent/LossMonitor/PLM PacketHeader/aSRM RandomVariable/Constant Connector/RoutingHelper Queue/GK Phy/Repeater .....

    Notes:

    Agent/TCP is TCP Tahoe             Agent/UDP is UDP

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  • Next, we will to learn how to:

    Create objects in from a built-in class in OTcl             How to invoke methods in an OTcl object

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• Creating an object in OTcl
  • The most common way to create an object is to use the new command:

    new ClassName

    The new command returns an object handle (reference)

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  • Example: Create a TCP Reno object

    set myTCPAgent [new Agent/TCP/Reno]

    Note:

    The class Agent/TCP/Reno simulates the TCP Reno protocol.

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• Using an object in OTcl
  • Just like Java, after you have created an object, you can invoke methods defined in that object using:

    $OTclObjectVar METHODNAME ARG1 ARG2 ...

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• Accessing variables in an object
  • The set method is used to read/write variables in an OTcl object

  • Assign a value to a variable in an object:

    $OTclObjectVar set varName VALUE

  • Reading a value to a variable in an object:

    $OTclObjectVar set varName Usually: set $x [ $OTclObjectVar set varName ]

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• What you need to learn in order to use NS
  • Programming in NS does not happen in the OTcl language

  • All the network protocol modules (e.g., TCP protocols) have been implemented as OTcl classes in NS

    (So you don't need to write network modules (like TCP, FTP, etc) to simulate a network)

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  • All you need to learn is:
    • The name of the OTcl class that implements the network module that you want to use

    • The names of the methods of the class

      The methods implement the behavior of the protocol modules.

    • The names of the variables of the class - setting of some variables can affect the behavior of the protocol module (e.g., packet size, queue length, etc)

  • Once you know these, you can use OTcl to create the modules you need and connect them

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• Further reading
  • OTcl Tutorial: click here