TCP Flags Definition and Operation

URG This flag indicates that the packet contains urgent data that should be processed as soon as possible. This flag is used to provide two virtual channels within one TCP connection. The URG flag is also known as out-of-band. This is usefful for a sender to present data that can overtake any bytes queued in the send and recieve buffers and be presented directy to the receiving application . For example, in Telnet the interrrupt key sequenece may be pressed and sent to overtake all other traffic and enable the recievd whatever it is doing with the received data.

ACK This flag indicates that the packet is an acknowledgement for a previous packet. It is always set for normal data flow once a connection is established. The ACK bit is also sometimes called the established bit.
PSH This flag, or push bit, tells the operating system to transmit the data at once; the receiving end should also transmit the data to the application right away. If this flag is not set, the operating system is free to buffer TCP transmissions to optimally manage the data.
RST This flag resets the TCP connection when an error occurs. Errors can include an incomplete initial handshake, a timeout at another phase of the connection, or a SYN sent to a port that isn’t listening or is filtering connections.
SYN This flag sets the initial request to synchronize sequence numbers and is used to initiate a connection between two hosts.
FIN This flag is used to gracefully disconnect an existing session, notifying the remote system that there will be no more transmissions.
When a receiving TCP sees the PUSH flag, it must not wait for more data from the sending TCP before passing the data to the receiving process.

Rick Donato is a Network Automation Architect/Evangelist and the founder of Packet Coders.

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