Editing Transmission Control Protocol (section)

====Congestion control====
{{Main|TCP congestion control}}
The final main aspect of TCP is [[congestion control]]. TCP uses a number of mechanisms to achieve high performance and avoid [[congestive collapse]], a gridlock situation where network performance is severely degraded. These mechanisms control the rate of data entering the network, keeping the data flow below a rate that would trigger collapse. They also yield an approximately [[max-min fair]] allocation between flows.

Acknowledgments for data sent, or the lack of acknowledgments, are used by senders to infer network conditions between the TCP sender and receiver. Coupled with timers, TCP senders and receivers can alter the behavior of the flow of data. This is more generally referred to as congestion control or congestion avoidance.

Modern implementations of TCP contain four intertwined algorithms: [[TCP congestion control#Slow start|slow start]], [[TCP congestion avoidance algorithm|congestion avoidance]], [[fast retransmit]], and [[fast recovery]].{{sfn|RFC 5681}}

In addition, senders employ a ''retransmission timeout'' (RTO) that is based on the estimated [[round-trip time]] (RTT) between the sender and receiver, as well as the variance in this round-trip time.{{sfn|RFC 6298}} There are subtleties in the estimation of RTT. For example, senders must be careful when calculating RTT samples for retransmitted packets; typically they use [[Karn's Algorithm]] or TCP timestamps.{{sfn|RFC 7323}} These individual RTT samples are then averaged over time to create a smoothed round trip time (SRTT) using [[Jacobson's algorithm]]. This SRTT value is what is used as the round-trip time estimate.

Enhancing TCP to reliably handle loss, minimize errors, manage congestion and go fast in very high-speed environments are ongoing areas of research and standards development. As a result, there are a number of [[TCP congestion avoidance algorithm]] variations.