The definition of transmission control protocol (TCP) is a transport-layer protocol that provides applications with reliable, in-order delivery of data as a byte stream. In plain language, TCP helps two applications exchange data while detecting missing or corrupted portions and retransmitting lost data when needed.
What TCP does
TCP operates between application processes on networked hosts. An application writes data to a TCP connection and reads data from it as a continuous stream. TCP divides that stream into segments, which are carried inside IP datagrams. The segment boundaries are transport details: they do not mark the start and end of the messages an application may care about.
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The Internet Engineering Task Force’s current TCP specification says, “TCP provides a reliable, in-order, byte-stream service to applications.” RFC 9293 is the current consolidated specification.
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TCP uses sequence numbers to track data in the stream and checksums to help detect corruption. Acknowledgments tell the sender what data has arrived; if data is missing, TCP can retransmit it. The receiver also advertises how much data it is prepared to accept, supporting flow control so a sender does not overwhelm the receiver.
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These mechanisms support reliable, ordered delivery, but they do not turn TCP into an application messaging format. If an application needs distinct messages, it must define or use its own framing on top of the byte stream.
What “connection-oriented” means
TCP endpoints establish and maintain connection state for an exchange, and a connection supports communication in both directions. An application can use that connection for traffic in only one direction if its design calls for it. Port numbers identify application services and help distinguish traffic flows on a host.
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Connection-oriented does not mean TCP continuously confirms that the remote host or application is still alive. RFC 9293 states that TCP “does not inherently include a liveness detection capability.” An application that needs timely failure detection must use an appropriate mechanism at the application or another layer.
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TCP and IP have different jobs
IP addresses and carries datagrams between hosts. TCP runs above IP to provide the application-facing transport service: the ordered byte stream, acknowledgments, retransmission, and flow control. Together, they address different parts of moving application data across a network.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which TCP specification is current?
For current TCP rules, cite RFC 9293, published as an Internet Standard in August 2022. It consolidates standards-track changes and clarifications accumulated since the original specification and obsoletes RFC 793. RFC 793, published in September 1981, is useful for historical design context rather than as the current consolidated specification.
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- Data Communications and Networking with TCP/IP Protocol Suite 6th Edition by Behrouz A. Forouzan
- Data Communications and Networking with TCP/IP Protocol Suite 6th Edition
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