Keepalive

A keepalive (KA) is a message sent by one device to another to check that the link between the two is operating, or to prevent this link from being broken.

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 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. There are three parameters related to keepalive:

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.[4]

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).[5] Despite the similar name, this function is entirely unrelated.

See also

References


This article is issued from Wikipedia - version of the Saturday, November 28, 2015. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.