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Random Early Detection for Congestion Avoidance

Random Early Detection for Congestion Avoidance. Sally Floyd Van Jacobson. Drop tail Gateways. End to End Congestion control may result in global synchronization. The source takes action only after congestion actually occurs. Droptail routers biased against bursty traffic.

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Random Early Detection for Congestion Avoidance

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  1. Random Early Detection for Congestion Avoidance Sally Floyd Van Jacobson

  2. Drop tail Gateways • End to End Congestion control may result in global synchronization. • The source takes action only after congestion actually occurs. • Droptail routers biased against bursty traffic. • Transient and persistent congestion not distinguished. • Gateway has no say in congestion control.

  3. Introduction. • RED provides congestion avoidance by controlling the queue size at the gateway. • RED notifies the source before the congestion actually happens rather than wait till it actually occurs. • RED provides a mechanism for the gateway to provide some feedback to the source on congestion status.

  4. RED Algorithm. • RED parameters • Minimum queue threshold (minQ) • Maximum queue threshold (maxQ) • Average Queue length (AvgQ). • This is calculated dynamically • Maximum drop probability (maxP) • Drop probability (P) • Calculated dynamically.

  5. RED Algorithm • For Each incoming packet • If AvgQ <= minQ • queue packet • If minQ <= AvgQ < maxQ • Mark packet with probability P • If maxQ <= AvgQ • Mark the packet

  6. Average Queue Length • AvgQ = (1 – weight) X AvgQ + weight x currQ • 0 < Weight < 1 • currQ is the current queue length • AvgQ may be calculated for each new packet arrival or at fixed time intervels.

  7. Drop Probability • TempP = MaxP x (AvgQ – minQ)/(maxQ – minQ) • P = TempP/ (1 – count.TempP) • Count is number of newly arrived packets queued. • This Extra step has been introduced to keep the drops evenly spaced in time.

  8. More on RED • Function of Average Queue length • Small bursts go untouched • Affects long loads • Misbehaving users can be identified and severely penalized. Co-operating users get overall delays.

  9. Advantages of RED gateways • Congestion Avoidance • If the RED gateway drops packets when avgQ reached maxQ, the avgQ will never exceed maxQ. • Appropriate time scales • Source will not be notified of transient congestion. • No Global Synchronization. • All connection wont back off at same time. • Simple • High link utilization • Fair

  10. Other Issues with RED • Fine tuning minQ, maxQ, maxP and weight needed for optimum performance. • RED needs to be deployed at the edge of the network.

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