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This article mainly explains "what is the congestion control of TCP". The content of the explanation is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the congestion control of TCP".

In a certain period of time, if the demand for a resource in the network exceeds the available part of the resource, the performance of the network will deteriorate. This situation is called congestion. Congestion control is to prevent too much data from being injected into the network, so that routers or links in the network will not be overloaded. Congestion control has a premise that the network can bear the existing network load. Congestion control is a global process, involving all hosts, all routers, and all factors related to reducing network transmission performance. On the contrary, flow control is often the control of point-to-point traffic, which is an end-to-end problem. What flow control needs to do is to restrain the rate of data sent by the sender so that the receiver can receive it in time.

For congestion control, the TCP sender maintains a state variable of the congestion window (cwnd). The size of the congestion control window depends on the degree of congestion of the network and changes dynamically. The sender takes its sending window as the smaller one of the congestion window and the receiver's acceptance window.

Four algorithms are used in TCP congestion control, namely, slow start, congestion avoidance, fast retransmission and fast recovery. In the network layer, routers can also adopt appropriate packet dropping strategies (such as active queue management AQM) to reduce the occurrence of network congestion.

Slow start: the idea of the slow start algorithm is that when a host starts sending data, if a large number of data bytes are immediately injected into the network, it may cause network congestion, because the compliance of the network is not yet known. Experience shows that the better method is to detect it first, that is, gradually increase the sending window from small to large, that is, gradually increase the congestion window value from small to large. The initial value of cwnd is 1, and the cwnd is doubled for each propagation round.

Congestion avoidance: the idea of congestion avoidance algorithm is to make the congestion window cwnd increase slowly, that is, each round trip time RTT increases the sent cwnd by 1.

Fast retransmission and fast recovery: in TCP/IP, Fast retransmission and recovery (fast retransmit and recovery,FRR) is a congestion control algorithm that can quickly recover lost packets. There is no FRR, and if the packet is lost, TCP will use a timer to request a transmission pause. During the pause period, no new or copied packets were sent. With FRR, if the receiver receives a segment that is out of order, it immediately sends a repeat acknowledgement to the sender. If the sender receives three duplicate acknowledgements, it assumes that the segments indicated by the confirmations are missing and immediately retransmits the missing segments. With FRR, it will not be delayed by the pause required during retransmission. Fast retransmission and recovery (FRR) works best when individual packets are lost. When multiple data packets are lost in a short period of time, it does not work very effectively.

Thank you for reading, the above is the content of "what is the congestion control of TCP". After the study of this article, I believe you have a deeper understanding of what the congestion control of TCP is, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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