TCP

Transmission Control Protocol (TCP) is a core transport-layer protocol of the Internet protocol suite. It provides reliable, ordered, and error-checked delivery of a stream of bytes between applications on hosts communicating over an IP network. TCP is connection-oriented: a logical connection is established between two endpoints before data can be exchanged, using a handshake to negotiate parameters and state.

Key features include reliable data transfer with in-order delivery, error detection by checksums, data integrity through retransmission of lost segments, and flow control via a sliding window to match the receiver's capacity. TCP uses multiplexing through port numbers to allow multiple applications to share a single network connection between hosts.

Connections are established with a three-way handshake (SYN, SYN-ACK, ACK) and terminated with a four-way sequence (FIN exchange). TCP implements congestion control and avoidance (such as slow start and congestion avoidance) and mechanisms like fast retransmit and fast recovery to respond to network congestion. Optional features include the Nagle algorithm and delayed acknowledgments.

TCP operates over IP and supports both IPv4 and IPv6. It negotiates parameters such as maximum segment size and employs techniques like path MTU discovery. It is the underlying transport protocol for many application-layer protocols, including HTTP, HTTPS, FTP, SMTP, and SSH.

Developed in the 1970s as part of the early Internet protocol suite by Vinton Cerf and Robert Kahn, TCP was standardized in RFC 793 and has since been extended by numerous updates. It remains the dominant transport protocol for reliable communication on the Internet.