keep alive 长连接

http://xsk.tehon.org/den/index.php/category/tech/short-or-long-tcp-connection-and-keepalive.html 短连接、长连接与keep-alive

http://www.cnblogs.com/liuyong/archive/2011/07/01/2095487.html  TCP长连接与短连接的区别 长连接的保活功能