CoDel

In network routing, CoDel (pronounced 'coddle') for controlled delay is a scheduling algorithm for the network scheduler developed by Van Jacobson and Kathleen Nichols. It is designed to overcome bufferbloat in network links, such as routers, by setting limits on the delay network packets experience as they pass through the buffer. CoDel aims at improving on the overall performance of the random early detection (RED) algorithm by addressing some of its fundamental misconceptions, as perceived by Jacobson, and by being easier to manage. In network routing, CoDel (pronounced 'coddle') for controlled delay is a scheduling algorithm for the network scheduler developed by Van Jacobson and Kathleen Nichols. It is designed to overcome bufferbloat in network links, such as routers, by setting limits on the delay network packets experience as they pass through the buffer. CoDel aims at improving on the overall performance of the random early detection (RED) algorithm by addressing some of its fundamental misconceptions, as perceived by Jacobson, and by being easier to manage. In 2012, an implementation of CoDel was written by Dave Täht and Eric Dumazet for the Linux kernel and dual licensed under the GNU General Public License and the 3-clause BSD license. Dumazet's variant of CoDel is called fq_codel, standing for 'fair queuing controlled delay'; it was adopted as the standard active queue management (AQM) and packet scheduling solution in the OpenWrt release called 'Barrier Breaker'. From there, CoDel and fq_codel have migrated into various downstream projects such as Tomato, dd-wrt and OPNsense. The theory behind CoDel is based on observations of packet behavior in packet-switched networks under the influence of data buffers. Some of these observations are about the fundamental nature of queueing and the causes of bufferbloat, others relate to weaknesses of alternative queue management algorithms. CoDel was developed as an attempt to address the problem of bufferbloat. The flow of packets slows down while travelling through a network link between a fast and a slow network, especially at the start of a TCP session, when there is a sudden burst of packets and the slower network may not be able to accept the burst quickly enough. Buffers exist to ease this problem by giving the fast network a place to store packets to be read by the slower network at its own pace. In other words, buffers act like shock absorbers to convert bursty arrivals into smooth, steady departures. However, a buffer has limited capacity. The ideal buffer is sized so it can handle a sudden burst of communication and match the speed of that burst to the speed of the slower network. Ideally, the shock absorbing situation is characterized by a temporary delay for packets in the buffer during the transmission burst, after which the delay rapidly disappears and the network reaches a balance in offering and handling packets. The TCP congestion control algorithm relies on packet drops to determine the available bandwidth between two communicating devices. It speeds up the data transfer until packets start to drop, and then slows down the transmission rate. Ideally it keeps speeding up and slowing down as it finds an equilibrium at the speed of the link. For this to work, the packet drops must occur in a timely manner so that the algorithm can responsively select a suitable transfer speed. With packets held in an overly-large buffer, the packets will arrive at their destination but with a higher latency but no packets are dropped so TCP does not slow down. Under these conditions, TCP may even decide that the path of the connection has changed and repeat the search for a new equilibrium.