Internet Core Protocols: The Definitive Guide

Name

Control Flags

Synopsis

Since TCP uses a point-to-point virtual circuit for all communications, it needs to be able to manage the flow of information across the virtual circuit at different times. The Control Flags are used to provide circuit-management services to the TCP endpoints.

Size

Six bits.

Notes

There are six different flags in the Control Flags field, with each flag being represented by an “on” or “off” condition.

Each of the Control Flags provide a variety of different service to TCP. Some of the flags provide circuit-management services, while others provide data-management services. The flags and their meanings are listed in Table 7.3.

Table 7.3. The Control Flags and Their Meanings

Control Flag	Usage
Urgent	If the Urgent flag is set, then this segment contains urgent data, up through the sequence number referenced by the Urgent Pointer field. If this flag is not set, then the Urgent Pointer field should be ignored.
Acknowledgment	If the Acknowledgment flag is set, then this segment contains an acknowledgment. Every segment (except for the very first segment that was used to initialize the circuit, and the Reset segments that are used to abort connections) should have this flag set.
Push	If the Push flag is set, then this segment contains data that is being “pushed” by the sending application. Typically, the Push flag is used to indicate that all of the data has been transferred, and is conceptually similar to an end-of-record marker.
Reset	The Reset flag should be seen only when ...