Keep-Alive header

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2015⁩.

The HTTP Keep-Alive request and response header allows the sender to hint how a connection may be used in terms of a timeout and a maximum amount of requests.

Note: For Keep-Alive to have any effect, the message must also include a Connection: keep-alive header.

HTTP/1.0 closes the connection after each request/response interaction by default, so persistent connections in HTTP/1.0 must be explicitly negotiated. Some clients and servers might wish to be compatible with previous approaches to persistent connections, and can do this with a Connection: keep-alive request header. Additional parameters for the connection can be requested with the Keep-Alive header.

Warning: Connection-specific header fields such as Connection and Keep-Alive are prohibited in HTTP/2 and HTTP/3. Chrome and Firefox ignore them in HTTP/2 responses, but Safari conforms to the HTTP/2 specification requirements and does not load any response that contains them.

Header type	Request header, Response header
Forbidden request header	Yes

Syntax

http

Keep-Alive: <parameters>

Directives

<parameters>

A comma-separated list of parameters, each consisting of an identifier and a value separated by the equal sign (=). The following identifiers are possible:

timeout: An integer that is the time in seconds that the host will allow an idle connection to remain open before it is closed. A connection is idle if no data is sent or received by a host. A host may keep an idle connection open for longer than timeout seconds, but the host should attempt to retain a connection for at least timeout seconds.
max: An integer that is the maximum number of requests that can be sent on this connection before closing it. Unless 0, this value is ignored for non-pipelined connections as another request will be sent in the next response. An HTTP pipeline can use it to limit the pipelining.

Examples

A response containing a Keep-Alive header:

http

HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Encoding: gzip
Content-Type: text/html; charset=utf-8
Date: Thu, 11 Aug 2016 15:23:13 GMT
Keep-Alive: timeout=5, max=200
Last-Modified: Mon, 25 Jul 2016 04:32:39 GMT
Server: Apache

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