TCP congestion control

Transmission Control Protocol (TCP) uses a congestion control algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and a congestion window (CWND), to achieve congestion avoidance. The TCP congestion-avoidance algorithm is the primary basis for congestion control in the Internet. Per the end-to-end principle, congestion control is largely a function of internet hosts, not the network itself. There are several variations and versions of the algorithm implemented in protocol stacks of operating systems of computers that connect to the Internet.

To avoid congestive collapse, TCP uses a multi-faceted congestion-control strategy. For each connection, TCP maintains a CWND, limiting the total number of unacknowledged packets that may be in transit end-to-end. This is somewhat analogous to TCP's sliding window used for flow control.

The additive increase/multiplicative decrease (AIMD) algorithm is a closed-loop control algorithm. AIMD combines linear growth of the congestion window with an exponential reduction when congestion occurs. Multiple flows using AIMD congestion control will eventually converge to use equal amounts of a contended link.

This is the algorithm that is described in RFC 5681 for the congestion avoidance state.

In TCP, the congestion window (CWND) is one of the factors that determines the number of bytes that can be sent out at any time. The congestion window is maintained by the sender and is a means of preventing a link between the sender and the receiver from becoming overloaded with too much traffic. This should not be confused with the sliding window maintained by the sender which exists to prevent the receiver from becoming overloaded. The congestion window is calculated by estimating how much congestion there is on the link.

When a connection is set up, the congestion window, a value maintained independently at each host, is set to a small multiple of the maximum segment size (MSS) allowed on that connection. Further variance in the congestion window is dictated by an additive increase/multiplicative decrease (AIMD) approach. This means that if all segments are received and the acknowledgments reach the sender on time, some constant is added to the window size. It will follow different algorithms.

A system administrator may adjust the maximum window size limit, or adjust the constant added during additive increase, as part of TCP tuning.

The flow of data over a TCP connection is also controlled by the use of the receive window advertised by the receiver. A sender can send data less than its own congestion window and the receive window.