查看“︁HTTP keep-alive”︁的源代码

HTTP persistent connection, also called HTTP keep-alive, or HTTP connection reuse, is the idea of using a single TCP connection to send and receive multiple HTTP requests/responses, as opposed to opening a new connection for every single request/response pair. The newer HTTP/2 protocol uses the same idea and takes it further to allow multiple concurrent requests/responses to be multiplexed over a single connection.

= Implementations ==
== HTTP 1.1 ==

HTTP1.0 通过在请求头中添加Connection: keep-alive启用长连接（非官方做法），而HTTP 1.1默认使用Keep-Alive机制（也叫持久化连接），可以使得多个HTTP请求共用一个底层的TCP连接。
正常情况下，客户端发送单个HTTP请求后，会发送RST报文主动断开连接：

[[Image:http-rst.png|600px|border]]

如果不启用Keep-Alive或者因某种原因没有生效（比如Go语言中没有读取完Body内容时，该连接无法复用），发送多个HTTP请求时会导致多次握手建立连接：

[[Image:http-multi-connection.png|600px|border]]

而启用Keep-Alive后，发送多次HTTP请求无需重新握手：

[[Image:http-share-connection.png|600px|border]]

当客户端Keep-Alive超时时间小于服务器超时时间时，会发送Keep-Alive报文（ACK）:

[[Image:http-ack.png|600px|border]]

当服务器Keep-Alive超时时间小于客户端超时时间时，则会主动发送FIN断开连接。再次发送数据时，需要重新握手建立连接：

[[Image:http-fin.png|600px|border]]

== HTTP 1.1 ==

== HTTP 2.0 ==

== See also ==

* [[HTTP/1.1]]
* [[HTTP/2]]

[[Category:Network]]
[[Category:HTTP]]
[[Category:Protocol]]
[[Category:RFC]]