Internet Core Protocols: The Definitive Guide

Flow Control

When an application needs to send data to another application over TCP, it writes the data to the local TCP provider, which queues the data into a send buffer. Periodically, TCP will package portions of the data into segments and pass them off to IP for delivery to the destination system.

One of the key elements to this process is flow control, where a sending system will adjust the rate at which it tries to send data to the destination system. A change in rate may be required due to a variety of reasons, including the available buffer space on the destination system and the packet-handling characteristics of the network. For this reason, TCP incorporates a variety of flow control mechanisms, allowing the sending system to react to these changes easily.

Originally, RFC 793 proposed only a handful of flow control mechanisms, most of which were focused on the receiving end of the connection. Of these services, the two most important were:

Receive window sizing

TCP can send only as much data as a receiver will allow, based on the amount of space available in the remote system’s receive buffer, the frequency at which the buffers are drained, and other related factors. Therefore, one way for a receiver to adjust the transfer rate is to increase or decrease the size of the buffer being advertised. This in turn controls how much data a sender can transmit at once.

Sliding receive windows

In addition to the Window size being advertised by a receiver, the concept of a “sliding window” ...