TCP connection and graceful termination

TCP connection

A TCP connection is a logical, full-duplex, reliable virtual circuit between two application processes. It is identified by the pair of sockets (source IP, source port, destination IP, destination port). It has three phases: establishment (3-way handshake), data transfer, and termination.

Graceful termination

Because the connection is full duplex, each direction is closed separately, which needs four segments. This is called a four-way handshake.

 Client (active close)             Server
   |--- FIN, seq=u ---------------->|  client: FIN_WAIT_1
   |<-- ACK, ack=u+1 ---------------|  client: FIN_WAIT_2
   |      (server may still send data: half-close)
   |<-- FIN, seq=v ----------------|  server: LAST_ACK
   |--- ACK, ack=v+1 -------------->|
 TIME_WAIT (2 x MSL)             CLOSED
 CLOSED

Steps

1. The client application calls close(). TCP sends a FIN segment. State: FIN_WAIT_1.
2. The server acknowledges the FIN (ACK u+1). The client enters FIN_WAIT_2. The server is in CLOSE_WAIT and can still send remaining data (half-close).
3. When the server application has finished, it sends its own FIN. State: LAST_ACK.
4. The client acknowledges it with an ACK and enters TIME_WAIT. It waits 2 × MSL (MSL = maximum segment lifetime, typically 30 s to 2 min, so the wait is 1 to 4 min) so that the last ACK, if lost, can be resent and delayed duplicate segments disappear. Then the connection is closed.

Steps 2 and 3 may be combined into one FIN+ACK segment if the server has no more data (three segments only). If an error occurs, TCP can terminate abruptly using an RST segment.