Page "Router (computing)" Paragraph 37

Besides making decision as which interface a packet is forwarded to, which is handled primarily via the routing table, a router also has to manage congestion, when packets arrive at a rate higher than the router can process.

Three policies commonly used in the Internet are tail drop, random early detection ( RED ), and weighted random early detection ( WRED ).

Tail drop is the simplest and most easily implemented ; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router.

RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until a pre-determined max, when it becomes tail drop.

WRED requires a weight on the average queue size to act upon when the traffic is about to exceed the pre-configured size, so that short bursts will not trigger random drops.