248 G8264 Application Guide for ENOS 8.4WRED with ECNWeighted Random Early Detection (WRED) is a congestion avoidance algorithmthat helps prevent a TCP collapse, where a congested port indiscriminately dropspackets from all sessions. The transmitting hosts wait to retransmit resulting in adramatic drop in throughput. Often times, this TCP collapse repeats in a cycle,which results in a saw‐tooth pattern of throughput. WRED selectively dropspackets before the queue gets full, allowing majority of the traffic to flow smoothly.WRED discards packets based on the CoS queues. Packets marked with lowerpriorities are discarded first.Explicit Congestion Notification (ECN) is an extension to WRED. For packets thatare ECN‐aware, the ECN bit is marked to signal impending congestion instead ofdropping packets. The transmitting hosts then slow down sending packets.How WRED/ECN work togetherFor implementing WRED, you must define a profile with minimum threshold,maximum threshold, and a maximum drop probability. The profiles can be definedon a port or a CoS.For implementing ECN, you require ECN‐specific field that has two bits—theECN‐capable Transport (ECT) bit and the CE (Congestion Experienced) bit—in theIP header. ECN is identified and defined by the values in these bits in theDifferentiated Services field of IP Header. Table 19 shows the combination valuesof the ECN bits.Table 19. ECN Bit SettingECT Bit CE Bit Description0 0 Not ECN‐capable0 1 Endpoints of the transport protocol are ECN‐capable1 0 Endpoints of the transport protocol are ECN‐capable1 1 Congestion experienced
