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TCP States

A TCP socket can be in one of the following states:

• TCP_S_CLOSED. The socket has been closed and may be freed.
• TCP_S_LISTEN. The socket is listening (waiting for the remote host to send a SYNC message).
• TCP_S_TWAIT. The closing handshaking has completed, waiting CFG_TWAITTIME before releasing the socket.
• TCP_S_SYNR. Listen transitions to SYNR when a SYNC message is received. A SYNC message has also been sent and the socket is waiting for an acknowledgment of its SYNC to transition to the established state.
• TCP_S_SYNS. A SYNC message has been sent (active open).
• TCP_S_EST. The connection is established.
• TCP_S_CWAIT. A FIN has been received (and an ack has been sent), but the application has not done a closesocket (we have not sent a FIN).
• TCP_S_FW1. A FIN has been sent (the application called closesocket), but the remote host has not acknowledged the FIN; a FIN has not been received from the remote host.
• TCP_S_FW2. A FIN has been sent (the application called closesocket) and acknowledged; a FIN has not been received from the remote host.
• TCP_S_CLOSING. Waiting for ACK of our FIN before completing close. A FIN has been received and acknowledged. In CLOSING, we send a FIN before receiving a FIN from the remote host.
• TCP_S_LAST. Waiting for ACK of our FIN before completing close. A FIN has been received and acknowledged. In the LAST state, we received a FIN before sending a FIN to the remote host.
• TCP_S_ALLOCED. Special state. The socket has been allocated, i.e. socket has been called, but the handshaking has not been started. A master socket is always in the ALLOCATED state, but its slave sockets are set to the LISTEN state immediately after being allocated.
• TCP_S_FREE. Special state. Not allocated.

The following diagram illustrates transitions between the various TCP socket states:

TCP State Transitions

TCP

Closing a TCP Connection

TCP Data Transfer