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Synchronized TCP flow

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• Drop Tail Queue Management
  • Internals of a router:

    Notes:

    Packets arrive on the input link The switching fabric inside the router direct the packets to the buffer of the appropriate output link Packets in the buffer are (usually) transmitted/serviced in a First Come First Serve (FCFS or FIFO) manner When the buffer is full, an arriving packet is discarded (dropped)

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  • Because the queue discards the last arriving packet, this type of buffer management is known as: drop tail buffer management

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• Effect of Drop Tail Queue Management on TCP flows
  • Characteristic of Drop Tail buffer management:

    Indiscriminant packet drop I.e., packets from different flows have equal probability to be discarded

    Result:

    It is very likely that packets from multiple TCP flows are dropped.

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  • Consequence:

    The affected TCP flows will perform a slow start (set CWND = 1) if the packet loss results in a time out. Or the affected TCP flows will perform a fast recovery (CWND is halved) if it receives 3 duplicate ACKs

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    Net result:

    Because the packet losses are synchronized , the CWND window decreases are also synchronized The net effect is that after some time, the affected TCP flows are synchronized .

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  • Example:

  • The progression of the CWND of these 2 TCP flows are synchronized:

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• Lesson learned
  • Allowing queues to overflow is not good - it is too late to do anything.

  • It may be better to do something BEFORE congestion happens...