HTTP/1.1协议的RFC2616中8.1.4中讲到：




Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion.



 


使用持久连接的Client不应该维持到同一Server多于2个的连接数，基本上，各个浏览器都依据该规范，实现了自己的限制，如下，引自这里：



折叠复制代码




  1. Browser HTTP/1.1 HTTP/1.0

  2. IE 6,7 2 4

  3. IE 8 6 6

  4. Firefox 2 2 8

  5. Firefox 3 6 6

  6. Safari 3,4 4 4

  7. Chrome 1,2 6 ?

  8. Chrome 3 4 4

  9. Opera 4 4




最杯具的莫过于IE6了，严格按照RFC规范来实现，每Server只能并发2个连接，到了IE8就有了改进，增大到了6个，见这里


Six connections per hostfor broadband scenarios instead of two, and a scriptable property, improve performance by allowing parallelization of downloads in Internet Explorer 8. This also increases functionality by ensuring that a request is not blocked to a host if two connections already exist. Websites can optimize downloads based on thewindow.maxConnectionsPerServerproperty.


这个限制，对于普通的Web应用来讲，影响不算太大，毕竟正常的连接请求都会很快有返回结果的


但如果在Web中要是用长连接的话，就可能会有比较大的问题了，尤其是在IE6里面


长连接会一直占用一个连接数，如果你有2个长连接，那就更杯具了，浏览器这时将无法向同这个Server去request了...


解决办法，看起来只有2个：


1. 修改浏览器的设置，IE和Firefox貌似都可以设置，但我们的Web应用总不能要求所有的用户都去修改那个设置吧


2. 把长连接的请求分发到其它的Server上，不要和普通的请求放在同一个Domain下面，但这样ajax就会有跨域的问题了，也许可以试试Jsonp?