Internet Core Protocols: The Definitive Guide

Receive window size adjustments

In Section 7.1.5 I first mentioned the TCP header’s Window field, suggesting that it provided an insight into the size of the receive buffer in use on a destination system. Although this is an accurate assessment when looking at TCP’s segment sizing process, the primary purpose of the Window field is to provide the receiving system with flow control management services. The Window field is used to tell a sender how much data a recipient can handle. In this model, the recipient dictates flow control.

According to RFC 793, the window field “specifies the number of octets ... that the receiving TCP is currently prepared to receive.” In this scenario, a sending system can transmit only as much data as will fit within the recipient’s receive buffer (as specified by the Window field) before an acknowledgment is required. Once the sender has transmitted enough data to fill the receive buffer, it must stop sending data and wait for an acknowledgment from the recipient before sending any more data.

Therefore, one way to speed up and slow down the data transfer rate between the two endpoint systems is for the receiving system to change the buffer size being advertised in the Window field. If a system that had been advertising an eight-kilobyte window suddenly started advertising a 16-kilobyte window, the sender could pump twice as much data through the circuit before having to wait for an acknowledgment.

Conversely, if the recipient started advertising a four-kilobyte ...