TCP Header Flags

Flags

Let’s have a quick look at what each flag is meant for.

The first four discussed below, namely ACK, RST, SYN, and FIN are used in the establishment, maintenance, and tear-down of a TCP connection.

ACK

This flag is set to $1$ in a segment to acknowledge a segment that was received previously. This is an important part of the protocol. In other words, when a receiver wants to acknowledge some received data, it sends a TCP segment with the ACK flag and the acknowledgment number field appropriately set. This flag is also used in connection establishment and termination as we will see in more detail later.

Note: These acknowledgment messages can and often do contain data as well!

RST

The reset flag immediately terminates a connection. This is sent due to the result of some confusion, such as if the host doesn’t recognize the connection, if the host has crashed, or if the host refuses an attempt to open a connection.

SYN

The synchronization flag initiates a connection establishment with a new host. The details will be covered later in the lesson on connection establishment.

FIN

This flag is used to terminate or finish a connection with a host.

TCP Connection Establishment & Termination

The slides below give a very high level overview of how these flags are used to establish and terminate a TCP connection.

The rest of the flags, given below, are not very well-known. However, it doesn’t hurt to know about them.

CWR & ECN

These flags, Congestion Window Reduced and Explicit Congestion Notification are used to handle congestion. To put it very simply, the ECN flag is set by the receiver, so that the sender knows that congestion is occurring. The sender sets the CWR flag in response to this so that the receiver knows that the receiver has reduced its congestion window to compensate for congestion and the sender is sending data at a slower rate. Originally, CWR and ECN were not part of the TCP header. In RFC 3168, these two additional flags were introduced by borrowing 2-bit space from the Reserved field.

PSH

The default behavior of TCP is in the interest of efficiency; if multiple small TCP segments were received, the receiving TCP will combine them before handing them over to the application layer. However, when the Push (PSH) flag is set, the receiving end immediately flushes the data from its buffer to the application instead of waiting for the rest of it to arrive.

This is usually used for applications like Telnet, where every keystroke is a command. It would not make sense to say, buffer 50 keystrokes and send them to the application layer at once, so, every keystroke is pushed.

URG

The Urgent flag marks some data within a message as urgent. Upon receipt of an urgent segment, the receiving host forwards the urgent data to the application with an indication that the data is marked as urgent by the sender. The rest of the data in the segment is processed normally.

This would be used when suppose a large file is being transferred but the sender realizes that it’s the wrong file and sends a command to stop transfer. It wouldn’t make sense to have the file finish transferring first, hence the command to stop transfer is marked as urgent and is executed before the file is done transferring.

Quick Quiz!

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Let’s finish off looking at the rest of the headers in the next lesson!