拥塞控制是一种用来调整传输控制协议（TCP）连接单次发送的分组数量（单次发送量，在英文文献和进程代码中常叫做cwnd）的算法。它通过增减单次发送量逐步调整，使之逼近当前网络的承载量。如果单次发送量为1，此协议就退化为停等协议。单次发送量是以字节来做单位的；但是如果假设TCP每次传输都是按照最大报文段来发送数据的，那幺也可以把数据包个数当作单次发送量的单位，所以有时我们说单次发送量增加1也就是增加相当于1个最大报文段的字节数。

Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of new connections. A consequence of the latter two effects is that an incremental increase in offered load leads either only to a small increase or even a decrease in network throughput.