TCP (Transmission Control Protocol)

PSH

(Flag) If set to 1, it indicates that data should be sent to an application without buffering.

ACK

(Flag) If set to 1, the Acknowledgement field contains information for the receiver. (If set to 0, the receiver will ignore the Acknowledgement field.)

URG

(Flag) If set to 1, the Urgent pointer field contains information for the receiver.

FIN

(Flag) If set to 1, the segment is the last in a sequence and the communication should be closed.

SYN

(Flag) If set to 1, the sender is requesting a synchronization of the sequence numbers between two nodes. Used when TCP requests a connection to set the initial sequence number.

RST

(Flag) If set to 1, the sender is requesting that the data be reset.

Reserved

A 6-bit field that is for later use.

Flags

A collection of six 1-bit fields that signal special conditions and are available for the sender's use.

Checksum

Allows the receiving node to determine whether the TCP segment became corrupted during transmission. The size of this field can vary from 0-32 bits.

Padding

Contains filler information to ensure that the size of the TCP header is a multiple of 32 bits. The size of this field varies, but is most often 0 (Zero).

Data

Contains information originally sent by the source node. This data field size depends upon how much information needs to be transmitted, the constraints on the TCP segment imposed by the network type, and the limitation that the segment must fit within an IP datagram.

Sequence Number

Identifies the data segment's position in the stream of data segments already sent. This field is 32 bits long.

Urgent Pointer

Indicates a location in the data field where urgent data resides. This field is 16 bits long.

Sliding-Window Size (or window)

Indicates how many bytes the sender can issue to a receiver while acknowledgement for this segment is outstanding. This field performs flow control, preventing the receiver from being deluged with bytes. This field is 16 bits long.

Destination Port

Indicates port number at the destination node. This port field is 16 bits long.

Source Port

Indicates port number at the source node. This port field is 16 bits long.

TCP Header Length

Indicates the length of the TCP header. This field is 4 bits long.

Options

Specifies special options, such as maximum segment size a network can handle. This bit varies in size from 0-32 bits.