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Transport Layer protocol "agents" in NS

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• Transport Layer Protocol modules
  • For one reason or another, the transport protocol modules in NS are called agents

  • There are an ever increasing number of sending transport agents (because people are developing new TCP transmission protocols to improve network performance.

  • There are 2 receiving transport agents that sends back ACKs to the sending agent.

  • The most commonly used sending transport protocol modules are:

    Agent/TCP: TCP Tahoe Agent/TCP/Reno: TCP Reno Agent/TCP/Newreno: Improved TCP Reno that paces the transmission interval Agent/TCP/Vegas: TCP Vegas

    Example:

    set tcp1 [new Agent/TCP/Reno]

  • The most commonly used receiving transport protocol modules are:

    Agent/TCPSink: sends ACK packet immediately after receiving a packet Agent/TCPSink/DelAck: ACK is delayed. Receiver sends ACK packet after receiving 2 packets or after a certain delay (tries to reduce number of ACK packets)

    Example:

    set sink1 [new Agent/TCPSink]

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• Connecting Transport Protocol Agents to Nodes
  • Transport agents are end points of communication

    They can be attached to any Node object

  • You can attach multiple Transport agents to one Node object

  • The attach-agent command in a Simulator object is used to associate a Transport agents to a Node:

    $ns attach-agent NODE TCP-Agent

  • Example:

    set ns [new Simulator] set node1 [$ns node] set tcp1 [new Agent/TCP/Reno] $ns attach-agent $node1 $tcp1

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• Connecting a Sending Transport Agent to a Receiving Transport Agent
  • Transport agents are end points of communication

    We need to tell NS the destination of each sending Transport Agent.

    The destination must be a receiving Transport Agent.

  • In order to tell NS where to transmit the packets, we need to connect a sending communication end points to a receiving communication end points

  • This is done through the NS method: connect SOURCE DESTINATION

  • Example:

    set ns [new Simulator] set tcp1 [new Agent/TCP/Reno] set sink1 [new Agent/TCPSink] (tcp1 and sink1 must also be attached to nodes, this step is omitted) $ns connect $tcp1 $sink1

    This will make NS route all packets from the source tcp1 towards the destination sink1

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• Some Parameters of the TCP module
  • After a TCP agent has been created, you can set some of its parameters (using the set command of the TCP agent)

  • The most useful parameters are:
    1. packetSize_: the packet size used by TCP (default packet size is 1000 bytes)

    2. maxcwnd_: the maximum of CWND (0 means infinite)

    3. window_: the Advertised Window size

  • Example:

    set tcp1 [new Agent/TCP/Reno] $tcp1 set packetSize_ 552

  • NOTE: recall that the set method without arguments will return the value of the variable !

    Example:

    set tcp1 [new Agent/TCP/Reno] set pksize [$tcp1 set packetSize_] // get packetzise used