The traditional technique for managing router queue lengths is to set a maximum length (in terms of

packets) for each queue, accept packets for the queue until the maximum length is reached, then reject

(drop) subsequent incoming packets until the queue decreases because a packet from the queue has been

transmitted. This technique is known as “tail drop”, since the packet that arrived most recently (i.e., the one

on the tail of the queue) is dropped when the queue is full. This method has served the Internet well for

2. Full Queues

The tail drop discipline allows queues to maintain a full (or, almost full) status for long periods of time,

since tail drop signals congestion (via a packet drop) only when the queue has become full. It is important

to reduce the steady-state queue size, and this is perhaps queue management’s most important goal.

The naive assumption might be that there is a simple tradeoff between delay and throughput, and that the

recommendation that queues be maintained in a “non-full” state essentially translates to a recommendation

that low end-to-end delay is more important than high throughput. However, this does not take into account

the critical role that packet bursts play in Internet performance. Even though TCP constrains a flow’s

window size, packets often arrive at routers in bursts [Leland94]. If the queue is full or almost full, an

arriving burst will cause multiple packets to be dropped. This can result in a global synchronization of flows

queues we want maintained in the network; instead, they should reflect the size of bursts we need to absorb.