Editing Keepalive (section)

==TCP keepalive==
[[Transmission Control Protocol]] (TCP) keepalives are an optional feature, and if included must default to off.<ref>{{cite journal|url=http://tools.ietf.org/html/rfc1122#page-101|journal=IETF|title=Requirements for Internet Hosts - Communication Layers|date=October 1989|accessdate=November 8, 2013|last1=Braden|first1=Robert T.|editor-first1=R. |editor-last1=Braden |doi=10.17487/RFC1122 |doi-access=free}}</ref> The keepalive packet contains no data. In an [[Ethernet]] network, this results in frames of minimum size (64 bytes<ref name="IEEE 802.3 Clause 3.1.1">{{cite book
 | publisher = IEEE Standard for Ethernet, 802.3-2015 – section one
 | year = 2016
 | page = 108 | doi = 10.1109/IEEESTD.2016.7428776 | isbn = 978-1-5044-0078-7 | title = IEEE Standard for Ethernet
 }}</ref>). There are three parameters<ref>{{Cite web|url=http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html|title=Using TCP keepalive under Linux|website=tldp.org|access-date=2016-07-29}}</ref> related to keepalive:
* '''Keepalive time''' is the duration between two keepalive transmissions in idle condition. TCP keepalive period is required to be configurable and by default is set to no less than 2 hours.
* '''Keepalive interval''' is the duration between two successive keepalive retransmissions, if acknowledgement to the previous keepalive transmission is not received.
* '''Keepalive retry''' is the number of retransmissions to be carried out before declaring that remote end is not available
When two hosts are connected over a network via TCP/IP,  TCP Keepalive Packets can be used to determine if the connection is still valid, and terminate it if needed.

Most hosts that support TCP also support TCP Keepalive. Each host (or peer) periodically sends a TCP packet to its peer which solicits a response. If a certain number of keepalives are sent and no response (ACK) is received, the sending host will terminate the connection from its end.
If a connection has been terminated due to a TCP Keepalive time-out and the other host eventually sends a packet for the old connection, the host that terminated the connection will send a packet with the RST flag set to signal the other host that the old connection is no longer active. This will force the other host to terminate its end of the connection so a new connection can be established.

Typically, TCP Keepalives are sent every 45 or 60 seconds on an idle TCP connection, and the connection is dropped after 3 sequential ACKs are missed. This varies by host, e.g. by default, Windows PCs send the first TCP Keepalive packet after 7200000ms (2 hours), then send 5 Keepalives at 1000ms intervals, dropping the connection if there is no response to any of the Keepalive packets.
Linux hosts send the first TCP Keepalive packet after 2 hours (default since Linux 2.2), then send 9 Keepalive probes (default since Linux 2.2) at 75 seconds (default since Linux 2.4) intervals, dropping the connection if there is no response to any of the Keepalive packets.<ref>{{cite web|url=https://man7.org/linux/man-pages/man7/tcp.7.html|title=tcp(7) - Linux manual page|accessdate=August 14, 2024}}</ref>