Managing Interface Congestion with Tail Drop

Router interfaces experience congestion when the output queue is full:

- Additional incoming packets are tail-dropped

- Dropped packets may cause significant application performance degradation

- Tail drop has significant drawbacks

Router interfaces experience congestion when the output queue is full:

- Additional incoming packets are tail-dropped

- Dropped packets may cause significant application performance degradation

- Tail drop has significant drawbacks

When an interface on a router cannot transmit a packet immediately, the packet is queued, either in an interface transmit (Tx) ring, or the interface output hold queue, depending on the switching path that is used. Packets are then taken out of the queue and eventually transmitted on the interface.

If the arrival rate of packets to the output interface exceeds the router capability to buffer and forward traffic, the queues increase to their maximum length and the interface becomes congested. Tail drop is the default queuing response to congestion. Tail drop treats all traffic equally and does not differentiate between classes of service. Applications may suffer performance degradation due to packet loss caused by tail drop. When the output queue is full and tail drop is in effect, all packets trying to enter (at the tail of) the queue are dropped until the congestion is eliminated and the queue is no longer full.

Weighted fair queuing (WFQ), if configured on an interface, has a more elaborate scheme for dropping traffic. It is able to punish the most aggressive flows via its congestive discard threshold (CDT)-based dropping algorithm. Unfortunately, WFQ does not scale to backbone speeds.

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