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TCP Packet Header
Transmission Control Protocol
TCP source port number
TCP destination port number
TCP connection state flags
The vast majority of network services run over TCP. Messages are sent reliably in both directions as part of an ongoing connection between two programs - without error, without loss or duplication, and delivered in order. Every TCP segment is acknowledged upon receipt. Every TCP segment is identified by a unique sequence number. Flag bits are used to define the state of the connection.
Each individual connection between a given client and server, possibly just one in a set of simultaneous connections to that server (e.g. a web server), is uniquely identified by both the source address and port number of the client in conjunction with the server's IP address and assigned port number.
A TCP packet header contains the source and destination service ports, and sequence and acknowledgement numbers and control flags used to create and maintain a reliable virtual circuit, or ongoing two-way connection.