Editing Transmission Control Protocol (section)

====Flow control====
TCP uses an end-to-end [[flow control (data)|flow control]] protocol to avoid having the sender send data too fast for the TCP receiver to receive and process it reliably. Having a mechanism for flow control is essential in an environment where machines of diverse network speeds communicate. For example, if a PC sends data to a smartphone that is slowly processing received data, the smartphone must be able to regulate the data flow so as not to be overwhelmed.<ref name=comer/>

TCP uses a [[sliding window]] flow control protocol. In each TCP segment, the receiver specifies in the ''receive window'' field the amount of additionally received data (in bytes) that it is willing to buffer for the connection. The sending host can send only up to that amount of data before it must wait for an acknowledgment and receive window update from the receiving host.

[[File:Tcp.svg|right|thumbnail|250px|TCP sequence numbers and receive windows behave very much like a clock. The receive window shifts each time the receiver receives and acknowledges a new segment of data. Once it runs out of sequence numbers, the sequence number loops back to 0.]]

When a receiver advertises a window size of 0, the sender stops sending data and starts its ''persist timer''. The persist timer is used to protect TCP from a [[deadlock (computer science)|deadlock]] situation that could arise if a subsequent window size update from the receiver is lost, and the sender cannot send more data until receiving a new window size update from the receiver. When the persist timer expires, the TCP sender attempts recovery by sending a small packet so that the receiver responds by sending another acknowledgment containing the new window size.

If a receiver is processing incoming data in small increments, it may repeatedly advertise a small receive window. This is referred to as the [[silly window syndrome]], since it is inefficient to send only a few bytes of data in a TCP segment, given the relatively large overhead of the TCP header.