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.
• Transient and persistent congestion not
distinguished.
• Gateway has no say in congestion control.
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.
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.
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
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.
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.
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.
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
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.