Keepalive

https://en.wikipedia.org/wiki/Keepalive

Description

A keepalive signal is often sent at predefined intervals, and plays an important role on the Internet.

After a signal is sent, if no reply is received the link is assumed to be down and future data will be routed via another path until the link is up again.

A keepalive signal can also be used to indicate to Internet infrastructure that the connection should be preserved.

Without a keepalive signal, intermediate NAT-enabled routers can drop the connection after timeout.

Since the only purpose is to find links that don't work or to indicate links that should be preserved, keepalive messages tend to be short and not take much bandwidth.

However, their precise明确的，精确的 format and usage terms depend on the communication protocol.

TCP keepalive

Transmission Control Protocol (TCP) keepalives are an optional feature, and if included must default to off.^[1]

The keepalive packet contains null data.

In an Ethernet network, a keepalive frame length is 60 bytes, while the server response to this, also a null data frame, is 54 bytes.^{[citation needed]}

There are three parameters related to keepalive:

Keepalive time is the duration between two keepalive transmissions in idle空闲 condition状态. 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.

Keepalive on higher layers

Since TCP keepalive is optional, various protocols (e.g. SMB^[2] and TLS^[3]) add a similar feature on top of it.

This is also common for protocols which maintain a session over a connectionless protocol, e.g. OpenVPN over UDP

Other uses

HTTP keepalive

The Hypertext Transfer Protocol uses the keyword "Keep-Alive" in the "Connection" header to signal that the connection should be kept open for further messages (this is the default in HTTP 1.1, but in HTTP 1.0 the default was to use a new connection for each request/reply pair).^[6] Despite the similar name, this function is entirely unrelated.

http://web.archive.org/web/20110605181241/http://www.io.com/~maus/HttpKeepAlive.html

HTTP is a stateless protocol - this means that every request is independent of every other. Keep alive doesn’t change that. Additionally, there is no guarantee that the client or the server will keep the connection open. Even in 1.1, all that is promised is that you will probably get a notice that the connection is being closed. So keepalive is something you should not write your application to rely upon.

https://en.wikipedia.org/wiki/HTTP_persistent_connection