Editing Transmission Control Protocol (section)

===Connection termination===<!--[[FIN (TCP)]] redirects here-->
[[File:TCP CLOSE.svg|right|thumbnail|260px|Connection termination]]
[[File:TCP close() - sequence diagram.svg|thumb|Detailed TCP close() sequence diagram]]

The connection termination phase uses a four-way handshake, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges with an ACK. Therefore, a typical tear-down requires a pair of FIN and ACK segments from each TCP endpoint. After the side that sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this state lets the TCP client resend the final acknowledgment to the server in case the ACK is lost in transit. The time duration is implementation-dependent, but some common values are 30 seconds, 1 minute, and 2 minutes. After the timeout, the client enters the CLOSED state and the local port becomes available for new connections.<ref>{{Cite book |last=Kurose |first=James F. |url=https://www.worldcat.org/oclc/936004518 |title=Computer networking : a top-down approach |date=2017 |others=Keith W. Ross |isbn=978-0-13-359414-0 |edition=7th |location=Harlow, England |pages=286 |oclc=936004518}}</ref>

It is also possible to terminate the connection by a 3-way handshake, when host A sends a FIN and host B replies with a FIN & ACK (combining two steps into one) and host A replies with an ACK.<ref>{{cite book|last= Tanenbaum|first= Andrew S.|author-link= Andrew S. Tanenbaum|title= Computer Networks|edition= Fourth|date= 2003-03-17|publisher= Prentice Hall|isbn= 978-0-13-066102-9|url= https://archive.org/details/computernetworks00tane_2}}</ref>

Some operating systems, such as [[Linux]]<ref>{{Cite web |title=linux/net/ipv4/tcp_minisocks.c at master · torvalds/linux |url=https://github.com/torvalds/linux/blob/master/net/ipv4/tcp_minisocks.c |access-date=2025-04-24 |website=GitHub |language=en}}</ref> implement a half-duplex close sequence. If the host actively closes a connection, while still having unread incoming data available, the host sends the signal RST (losing any received data) instead of FIN. This assures that a TCP application is aware there was a data loss.{{sfn|RFC 1122|loc=4.2.2.13. Closing a Connection}}

A connection can be in a [[TCP half-open|half-open]] state, in which case one side has terminated the connection, but the other has not. The side that has terminated can no longer send any data into the connection, but the other side can. The terminating side should continue reading the data until the other side terminates as well.<ref>{{Cite web |date=2020-03-02 |title=TCP (Transmission Control Protocol) – The transmission protocol explained |url=https://www.ionos.com/digitalguide/server/know-how/introduction-to-tcp/ |access-date=2025-04-24 |website=IONOS Digital Guide |language=en}}</ref><ref>{{Cite web |title=The TCP/IP Guide - TCP Connection Termination |url=http://www.tcpipguide.com/free/t_TCPConnectionTermination-2.htm |access-date=2025-04-24 |website=www.tcpipguide.com}}</ref>