原文
RFC 2616: Hypertext Transfer Protocol – HTTP/1.1 (deprecated)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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.
翻译
HTTP持久连接有以下优点：
- 通过开启和关闭更少HTTP连接，节约路由器和主机 (包括：客户端、服务器、代理、网关、隧道和缓存) 的CPU时间，且主机中TCP协议控制块内存占用量也有所减少。
- 在同一个连接中可管道化传输HTTP请求和HTTP响应。管道能让客户端在不等待每个响应到达的情况下发送多个HTTP请求，令一个独立的TCP连接在更短经历时间中得到更高效重用。
- 减少TCP建立引起数据包数量，缓解网络拥塞，也给TCP留出充足时间确定网络拥堵的状况。
- 没有TCP开启连接握手的时间消耗，可降低后续请求的延迟。
- 错误报告不再受到HTTP连接关闭带来的不良后果，让HTTP可以更加优雅地演进。客户端使用未来的HTTP版本时可能会乐观地尝试新特性，但如果与之建立通讯的是一个更旧的服务器，报告错误之后会重试旧版的语义。