Transport Layer - Freie Universität Berlin
Transport Layer - Freie Universität Berlin
Transport Layer - Freie Universität Berlin
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Timer Management with TCP: Retransmission Timer<br />
● Solution: dynamic algorithm, which adapts the timer by current<br />
measurements of the network performance.<br />
● Algorithm of Jacobson (1988)<br />
● TCP manages a variable RTT (Round Trip Time) for each connection<br />
● RTT is momentarily the best estimation of the round trip time<br />
● When sending a segment, a timer is started which measures the time the<br />
acknowledgement needs and initiates a retransmission if necessary.<br />
● If the acknowledgement arrives before expiration of the timer (after a<br />
time unit sampleRTT), RTT is updated:<br />
● a is a smoothing factor, typically 0.875<br />
RTT = a × RTT + (1 – a) × sampleRTT<br />
● The choice of the timeout is based on RTT<br />
Timeout = b × RTT<br />
● At the beginning, b was chosen as 2, but this was too inflexible<br />
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 8: <strong>Transport</strong> <strong>Layer</strong><br />
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