TCP Traffic Before and After RED

This topic describes the effects of using RED on TCP traffic by comparing TCP traffic flows both before and after the application of RED.

TCP Traffic Before RED

TCP synchronization prevents average link utilization close to the link bandwidth.

Tail drops cause TCP sessions to go into slow-start.

The figure shows TCP throughput behavior compared to link bandwidth in a congested network scenario where the tail-drop mechanism is in use on a router. The global synchronization phenomenon causes all sessions to slow down when congestion occurs. All sessions are penalized when tail drop is used because it drops packets with no discrimination between individual flows.

When all sessions slow down, congestion on the router interface is removed and all TCP sessions restart their transmission at roughly the same time. Again, the router interface quickly becomes congested, causing tail drop. As a result, all TCP sessions back off again. This behavior cycles constantly, resulting in a link that is generally underutilized.

TCP Traffic After RED

Average link utilization is much closer to link bandwidth. Random drops cause TCP sessions to reduce window sizes.

This figure shows TCP throughput behavior compared to link bandwidth in a congested network scenario in which RED has been configured on a router. RED randomly drops packets, influencing a small number of sessions at a time, before the interface reaches congestion. Overall throughput of sessions is increased, as well as average link utilization. Global synchronization is very unlikely to occur, due to selective, but random, dropping of adaptive traffic.

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