Internet Core Protocols: The Definitive Guide

A note on local blocking

Although there are a variety of flow control mechanisms found with TCP, the simplest form of flow control is “local blocking,” whereby a sending system refuses to accept data from a local application. This feature is needed whenever TCP knows that it cannot deliver any data to a specific destination system—perhaps due to problems with the receiver or the network—and the local send buffer is already full. Having nowhere to send the data, TCP must refuse to accept any new data from the sending application.

Note that TCP cannot block incoming network traffic (coming from IP). Since TCP is unable to tell which application a segment is destined for until its contents have been examined, TCP must accept every segment that it gets from IP. However, TCP may be unable to deliver the data to the destination application, due to a full queue or some other temporary condition. If this happens, TCP could choose to discard the segment, thereby causing the sender to retry the operation later (an effort which may or may not succeed).