4. Active Queue Management (AQM)

4. Active Queue Management (AQM)

Random Early Detection (RED) is a mechanism for Active Queue Management (AQM) that has been proposed for detecting incipient congestion [FJ93] and is currently being deployed in the Internet [RFC2309]. AQM is intended to be a general mechanism for congestion indication using one of several alternative approaches, but in the absence of ECN, AQM is limited to using packet dropping as the congestion indication mechanism. AQM drops packets based on the average queue length exceeding a threshold, rather than only when the queue overflows. However, because AQM may drop packets before the queue actually overflows, AQM is not always forced to drop packets due to memory limitations.

AQM can set the Congestion Experienced (CE) codepoint in the packet header instead of dropping a packet, when such a field is provided in the IP header and understood by the transport protocol. Using the CE codepoint with ECN allows the receiver to receive the packet, avoiding the potential for excessive delays in the case where a packet loss must be followed by a retransmission. We use the term 'CE packet' to denote a packet that has the CE codepoint set.