Transmission Control Protocol

TCP provides reliable stream delivery of data between Internet hosts.

Like UDP, TCP uses Internet Protocol, the underlying protocol, to transport datagrams, and supports the block transmission of a continuous stream of datagrams between process ports. Unlike UDP, TCP provides reliable message delivery. TCP ensures that data is not damaged, lost, duplicated, or delivered out of order to a receiving process. This assurance of transport reliability keeps applications programmers from having to build communications safeguards into their software.

The following are operational characteristics of TCP:

Item Description
Basic Data Transfer TCP can transfer a continuous stream of 8-bit octets in each direction between its users by packaging some number of bytes into segments for transmission through the Internet system. TCP implementation allows a segment size of at least 1024 bytes. In general, TCP decides when to block and forward packets at its own convenience.
Reliability TCP must recover data that is damaged, lost, duplicated, or delivered out of order by the Internet. TCP achieves this reliability by assigning a sequence number to each octet it transmits and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within the time-out interval, the data is retransmitted. The TCP retransmission time-out value is dynamically determined for each connection, based on round-trip time. At the receiver, the sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments.
Flow Control TCP governs the amount of data sent by returning a window with every ACK to indicate a range of acceptable sequence numbers beyond the last segment successfully received. The window indicates an allowed number of octets that the sender may transmit before receiving further permission.
Multiplexing TCP allows many processes within a single host to use TCP communications facilities simultaneously. TCP receives a set of addresses of ports within each host. TCP combines the port number with the network address and the host address to uniquely identify each socket. A pair of sockets uniquely identifies each connection.
Connections TCP must initialize and maintain certain status information for each data stream. The combination of this information, including sockets, sequence numbers, and window sizes, is called a connection. Each connection is uniquely specified by a pair of sockets identifying its two sides.
Precedence and Security Users of TCP may indicate the security and precedence of their communications. Default values are used when these features are not needed.

The TCP Packet Header figure illustrates these characteristics.

Figure 1. Transmission Control Protocol (TCP) packet header
Transmission Control Protocol (TCP) packet header
This illustration shows what is contained in the TCP packet header. The individual entities are listed in the text below.