Transmission Control Protocol

   Abbreviated TCP. The transport-level protocol used in the TCP/IP suite of protocols. It works above IP in the protocol stack and provides reliable data delivery over connection-oriented links.

   TCP adds a header to the datagram that contains the information needed to get the datagram to its destination. The source port number and the destination port number allow data to be sent back and forth to the correct processes running on each computer. A sequence number allows the datagrams to be rebuilt in the correct order in the receiving computer, and a checksum verifies that the data received is the same as the data sent. In addition to these fields, the TCP header contains the following information:

   • Acknowledgment number

   Indicates that the data was received successfully. If the datagram is damaged in transit, the receiver discards the data and does not send an acknowledgment to the sender. After a specified timeout expires, the sender retransmits data for which no acknowledgment has been received.

   • Offset

   Specifies the length of the header.

   • Reserved

   Variables set aside for future use.

   • Flags

   Indicate that this packet is the end of the data or that the data is urgent.

   • Window size

   Provides a way to increase packet size, which can improve efficiency in data transfers.

   • Urgent pointer

   Gives the location of urgent data.

   • Options

   Reserved for future use or for special options as defined by the protocol.

   • Padding

   Ensures that the header ends on a 32-bit boundary.

   The data immediately follow this header information.

   See also Internet Protocol; Transmission Control Protocol/Internet Protocol; User Datagram Protocol.