总得有个时间吧，或者接受客户端请求的个数
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。市场上的大部分Web服务器，包括iPlanet、IIS和Apache，都支持HTTP Keep-Alive。
The Keep-Alive extension to HTTP, as defined by the HTTP/1.1 draft, allows persistent connections. These long-lived HTTP sessions allow multiple requests to be send over the same TCP connection, and in some cases have been shown to result in an almost 50% speedup in latency times for HTML documents with lots of images.
Wiki:
[quote]Keepalive messages were not officially supported in HTTP 1.0. In HTTP 1.1 all connections are considered persistent, unless declared otherwise[1]. However, the default keepalive timeout of Apache 2.0 httpd is as little as [b]15 seconds[/b]] and for Apache 2.2 only [b]5 seconds[/b]. The advantage of a short timeout is the ability to deliver multiple components of a web page quickly while not tying up multiple server processes or threads for too long.[/quote]
另外，HTTP/1.1里面有个keep-alive的头可以设置连接的timeout，不过这不是标准的部分，并不能保证所有客户端都支持该响应头。