Protocol Operation
TCP protocol operations may be divided into three phases. Connections must be properly established in a multi-step handshake process (connection establishment) before entering the data transfer phase. After data transmission is completed, the connection termination closes established virtual circuits and releases all allocated resources.
A TCP connection is managed by an operating system through a programming interface that represents the local end-point for communications, the Internet socket. During the lifetime of a TCP connection the local end-point undergoes a series of state changes:
- LISTEN
- (server) represents waiting for a connection request from any remote TCP and port.
- SYN-SENT
- (client) represents waiting for a matching connection request after having sent a connection request.
- SYN-RECEIVED
- (server) represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.
- ESTABLISHED
- (both server and client) represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
- FIN-WAIT-1
- (both server and client) represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.
- FIN-WAIT-2
- (both server and client) represents waiting for a connection termination request from the remote TCP.
- CLOSE-WAIT
- (both server and client) represents waiting for a connection termination request from the local user.
- CLOSING
- (both server and client) represents waiting for a connection termination request acknowledgment from the remote TCP.
- LAST-ACK
- (both server and client) represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request).
- TIME-WAIT
- (either server or client) represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request.
- CLOSED
- (both server and client) represents no connection state at all.
Read more about this topic: Transmission Control Protocol
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