HTTP keep-alive
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：
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：
而启用Keep-Alive后，发送多次HTTP请求无需重新握手：
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）:
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：
