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The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It operates on the transport layer, providing reliable, ordered, and error-checked delivery of a stream of bytes between applications running on hosts communicating over an IP network. TCP is connection-oriented, meaning that a connection is established and maintained until the applications at each end have finished exchanging messages. This is in contrast to the User Datagram Protocol (UDP), which is connectionless.

TCP uses a three-way handshake to establish a connection. This involves the exchange of three messages between the sender and receiver to synchronize sequence numbers and ensure that both are ready to transmit data. Once the connection is established, TCP segments the data into smaller packets, each with a sequence number. These packets are then transmitted over the network, and the receiver acknowledges receipt of each packet. If a packet is lost or corrupted, TCP will retransmit the packet until it is successfully received.

TCP also includes flow control and congestion control mechanisms. Flow control ensures that a sender does not overwhelm a receiver with too much data at once, while congestion control prevents the network from becoming congested, which can lead to packet loss and delays. These mechanisms help to maintain the reliability and efficiency of TCP.

TCP is widely used in various applications, including web browsing, email, file transfers, and remote login. It is the protocol that underlies the Hypertext Transfer Protocol (HTTP), which is used for transmitting web pages over the internet. TCP is also used in many other protocols, such as the File Transfer Protocol (FTP) and the Secure Shell (SSH) protocol.