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| '''Weighted random early detection''' (WRED) is a queueing discipline for a [[network scheduler]] suited for [[Network congestion avoidance|congestion avoidance]].<ref>{{cite web |url=http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfconav_ps1835_TSD_Products_Configuration_Guide_Chapter.html |title=Congestion Avoidance Overview |publisher=Cisco |accessdate=2012-05-10}}</ref> It is an extension to [[random early detection]] (RED) where a single queue may have several different queue thresholds. Each queue threshold is associated to a particular [[Traffic_shaping#Traffic_classification|traffic class]].
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| For example, a queue may have lower thresholds for lower priority packet. A queue buildup will cause the lower priority packets to be dropped, hence protecting the higher priority [[network packet|packets]] in the same queue. In this way [[quality of service]] prioritization is made possible for important packets from a pool of packets using the same buffer.<ref>{{cite web |url=http://www.cisco.com/en/US/docs/ios/11_2/feature/guide/wred_gs.html |title=Weighted Random Early Detection (WRED) |publisher=Cisco |accessdate=2011-05-27}}</ref>
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| It is more likely that standard traffic will be dropped instead of higher prioritized traffic.
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| == Restrictions ==
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| On Cisco switches WRED is restricted to
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| * TCP/IP traffic. Only this kind of traffic indicates congestion to the sender to enable a reduction of the transmission rate.
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| Non-IP traffic will be dropped more often than TCP/IP traffic because it is treated with the lowest possible precedence.
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| == Functional Description ==
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| WRED proceeds in this order when a packet arrives:
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| * Calculation of the average queue size.
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| * The arriving packet is queued only if the average queue size is below the minimum queue threshold.
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| * Depending on the packet drop probability the packet is either dropped or queued if the average queue size is between the minimum and maximum queue threshold.
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| * The packet is automatically dropped if the average queue size is greater than the maximum threshold.
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| === Calculation of average queue size ===
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| The average queue size depends on the previous average as well as the current size of the queue. The calculation formula is given below:
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| <math> avg= o*(1-2^{-n}) + c*(2^{-n})\,\!</math> | |
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| where <math>n</math> is the user-configurable exponential weight factor, <math>o</math> is the old average and <math>c</math> is the current queue length. The previous average will be more important for high values of <math>n</math>. Peaks and Lows in queue length will be smoothed by a high value. Low values of n allows the average queue size to stay close to the current queue size.
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| ==References== | |
| {{Reflist}}
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| [[Category:Network performance]]
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| {{Compu-network-stub}}
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| [[da:Undgåelse af datanet-trafikforstoppelse#RED og WRED]]
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