Transmission Control Protocol

TCP is a connection-oriented, end-to-end reliable protocol designed to fit into a layered hierarchy of protocols which support multi-network applications. The TCP provides for reliable inter-process communication between pairs of processes in host computers attached to distinct but interconnected computer communication networks. Very few assumptions are made as to the reliability of the communication protocols below the TCP layer. TCP assumes it can obtain a simple, potentially unreliable datagram service from the lower level protocols. In principle, the TCP should be able to operate above a wide spectrum of communication systems ranging from hard-wired connections to packet-switched or circuit-switched networks.


TCP was specifically designed to be a reliable end-to-end byte stream transmission protocol over an unreliable network. The IP layer does not provide any guarantees that datagrams will be delivered with any degree of reliability. Hence it is up to the upper-layer protocol to provide this reliability. The key functionality associated with TCP is basic data transfer. Transmission Control Protocol, or TCP as it is commonly referred to, is a transport-layer protocol that runs on top of IP.

Multiplexing. Multiplexing is achieved through the concept of ports. A port is a 16-bit number used by the host-to-host protocol to identify to which higher-level protocol or application process it must deliver incoming messages. Two types of ports exist: (1) Well-known: these ports belong to standard applications servers such as telnet, ftp, and http. The well-known ports are controlled and assigned by the Internet Assigned Numbers Authority (IANA). Well-known ports range from 1 to 1023. (2) Ephemeral: A client can negotiate the use of a port dynamically and such ports can be called ephemeral. These ports are maintained for the duration of the session and then released. Ephemeral ports range from 1024 to 65535. Multiple applications can use the ports as a means of multiplexing for communicating with other nodes.

Reliability. TCP assigns a sequence number to each byte transmitted and expects a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data are retransmitted. Since the data are transmitted in blocks (TCP segments), only the sequence number of the first data byte in the segment is sent to the destination host.
Flow Control. The receiving TCP, when sending an ACK back to the sender, also indicates to the sender the number of bytes it can receive beyond the last received TCP segment, without causing overrun and overflow in its internal buffers. This is sent in the ACK in the form of the highest sequence number it can receive without problems.