Controlled Delay (CoDel) Active Queue Management

Nearly three decades after it was first diagnosed, the “persistently full buffer problem,” recently exposed as part of bufferbloat,^6,7 is still with us and made increasingly critical by two trends. First, cheap memory and a “more is better” mentality have led to the inflation and proliferation of buffers. Second, dynamically varying path characteristics are much more common today and are the norm at the consumer Internet edge. Reasonably sized buffers become extremely oversized when link rates and path delays fall below nominal values.

The solution for persistently full buffers, AQM (active queue management), has been known for two decades but has not been widely deployed because of implementation difficulties and general misunderstanding about Internet packet loss and queue dynamics. Unmanaged buffers are more critical today since buffer sizes are larger, delay-sensitive applications are more prevalent, and large (streaming) downloads common. The continued existence of extreme delays at the Internet’s edge can impact its usefulness and hamper the growth of new applications.

This article aims to provide part of the bufferbloat solution, proposing an innovative approach to AQM suitable for today’s Internet called CoDel (for Controlled Delay, pronounced like “coddle”). This is a “no-knobs” AQM that adapts to changing link rates and is suitable for deployment and experimentation in Linux-based routers (as well as silicon).

http://www.pollere.net/CoDel.html

http://queue.acm.org/detail.cfm?id=2209336