Internet Core Protocols: The Definitive Guide

Sliding receive windows

Even though large window sizes can help to increase overall throughput, they do not provide for sustained levels of throughput. In particular, if a situation required the use of a synchronous “send-and-wait” design that required a system to send data and then stop to wait for an acknowledgment, the network would be quite jerky, with bursts of writes followed by long pauses. This problem is most noticeable on networks with high levels of latency that cause extended periods of delay between the two endpoints.

In an effort to avoid this type of scenario, RFC 1122 states that a recipient should issue an acknowledgment for every two segments that it receives, if not more often. This design causes the receiver to issue acknowledgments quickly. In turn, these acknowledgments arrive back to the sender quickly.

Once an acknowledgment has arrived back at the sending system, the outstanding data is cleared from the send queue, thereby letting the sender transmit more data. In effect, the sending system can “slide” the window over by the number of segments that have been successfully acknowledged, allowing it to transmit more data, even though not all of the segments have been acknowledged yet.

As long as a sender continues receiving acknowledgments, it is able to continue sending data, with the maximum amount of outstanding segments being determined by the size of the recipient’s receive buffer. This concept is illustrated in Figure 7.10, which shows how a sender can ...