HTTP协议 keep-alive连接与 BS(firefox-thttpd)实验

Posted 2020-07-16 LightSong@计海拾贝

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什么是 keep-alive 连接

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

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.

https://www.byvoid.com/blog/http-keep-alive-header

我们知道HTTP协议采用“请求-应答”模式，当使用普通模式，即非KeepAlive模式时，每个请求/应答客户和服务器都要新建一个连接，完成之后立即断开连接（HTTP协议为无连接的协议）；当使用Keep-Alive模式（又称持久连接、连接重用）时，Keep-Alive功能使客户端到服务器端的连接持续有效，当出现对服务器的后继请求时，Keep-Alive功能避免了建立或者重新建立连接。

http 1.0中默认是关闭的，需要在http头加入"Connection: Keep-Alive"，才能启用Keep-Alive；http 1.1中默认启用Keep-Alive，如果加入"Connection: close "，才关闭。目前大部分浏览器都是用http1.1协议，也就是说默认都会发起Keep-Alive的连接请求了，所以是否能完成一个完整的Keep- Alive连接就看服务器设置情况。

keep-alive 连接的优点

https://www.w3.org/Protocols/rfc2616/rfc2616-sec8.html#sec8.1

Persistent HTTP connections have a number of advantages:

      - By opening and closing fewer TCP connections, CPU time is saved
        in routers and hosts (clients, servers, proxies, gateways,
        tunnels, or caches), and memory used for TCP protocol control
        blocks can be saved in hosts.

      - HTTP requests and responses can be pipelined on a connection.
        Pipelining allows a client to make multiple requests without
        waiting for each response, allowing a single TCP connection to
        be used much more efficiently, with much lower elapsed time.

      - Network congestion is reduced by reducing the number of packets
        caused by TCP opens, and by allowing TCP sufficient time to
        determine the congestion state of the network.

      - Latency on subsequent requests is reduced since there is no time
        spent in TCP\'s connection opening handshake.

      - HTTP can evolve more gracefully, since errors can be reported
        without the penalty of closing the TCP connection. Clients using
        future versions of HTTP might optimistically try a new feature,
        but if communicating with an older server, retry with old
        semantics after an error is reported.

HTTP implementations SHOULD implement persistent connections.

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

Advantages

Lower CPU and memory usage (because fewer connections are open simultaneously).
Enables HTTP pipelining of requests and responses.
Reduced network congestion (fewer TCP connections).
Reduced latency in subsequent requests (no handshaking).
Errors can be reported without the penalty of closing the TCP connection.

These advantages are even more important for secure HTTPS connections, because establishing a secure connection needs much more CPU time and network round-trips.

1、较低的CPU和内存消耗。

2、启用HTTP管道线功能

3、减少网络拥塞

4、减少后续请求的延迟

5、遇到错误，可以报告错误，不用强行关闭。

对于 https 连接效果显著，因为https的握手流程很是耗时！

keep-alive 连接的缺点

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

Disadvantages

If the client does not close the connection when all of the data it needs has been received, the resources needed to keep the connection open on the server will be unavailable for other clients. How much this affects the server\'s availability and how long the resources are unavailable depend on the server\'s architecture and configuration.

浏览器支持性

Use in web browsers

All modern web browsers use persistent connections, including Google Chrome, Firefox, Internet Explorer (since 4.01), Opera (since 4.0)^[8] and Safari.
By default, Internet Explorer versions 6 and 7 use two persistent connections while version 8 uses six.^[9] Persistent connections time out after 60 seconds of inactivity which is changeable via the Windows Registry.^[10]
In Firefox, the number of simultaneous connections can be customized (per-server, per-proxy, total). Persistent connections time out after 115 seconds (1.92 minutes) of inactivity which is changeable via the configuration.^[11]