Wred With Ecn; How Wred/Ecn Work Together - Lenovo RackSwitch G8264 Application Manual

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WRED with ECN

G8264 Application Guide for ENOS 8.4

Weighted Random Early Detection (WRED) is a congestion avoidance algorithm

that helps prevent a TCP collapse, where a congested port indiscriminately drops

packets from all sessions. The transmitting hosts wait to retransmit resulting in a

dramatic drop in throughput. Often times, this TCP collapse repeats in a cycle,

which results in a saw‐tooth pattern of throughput. WRED selectively drops

packets before the queue gets full, allowing majority of the traffic to flow smoothly.

WRED discards packets based on the CoS queues. Packets marked with lower

priorities are discarded first.

Explicit Congestion Notification (ECN) is an extension to WRED. For packets that

are ECN‐aware, the ECN bit is marked to signal impending congestion instead of

dropping packets. The transmitting hosts then slow down sending packets.

For implementing WRED, you must define a profile with minimum threshold,

maximum threshold, and a maximum drop probability. The profiles can be defined

on a port or a CoS.

For implementing ECN, you require ECN‐specific field that has two bits—the

ECN‐capable Transport (ECT) bit and the CE (Congestion Experienced) bit—in the

IP header. ECN is identified and defined by the values in these bits in the

Differentiated Services field of IP Header. Table

of the ECN bits.

ECN Bit Setting

Not ECN‐capable

Endpoints of the transport protocol are ECN‐capable

Congestion experienced

19 shows the combination values

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