Internet Core Protocols: The Definitive Guide

Notes on Determining the Optimal Receive Window Size

Almost all of TCP’s flow control services depend upon the size of the Window field that is advertised by a recipient, since this value dictates the maximum amount of data that can be sent during steady-state operations (i.e., after slow start has fully opened the initial congestion window) without the sender having to stop and wait for an acknowledgment. As such, this value determines the smooth flow of data more than just about any other element, so correctly determining the appropriate size of a system’s receive window is central to achieving efficient throughput on a virtual circuit.

Although an application can set the size of the receive window in use with that application, most applications just use the system-wide default, which is only sometimes appropriate for the typical usage of that system. Therefore, one way to improve performance for any given system is to optimize the system-wide default for the receive window in use on that system, so that it more accurately reflects the typical usage.

In fact, setting this value accurately is crucial to achieving optimal performance. Setting the receive window too small results in an artificial bottleneck, where the receiver’s window is smaller than the amount of data that the network can handle. In this model, the sender has to wait for the recipient to acknowledge data before it can send any more data, even though the network may be idle and have plenty of excess capacity. This ...