tcp 长连接和心跳

http://www.cnblogs.com/diyunpeng/archive/2011/07/21/2112380.html

http://blog.csdn.net/channels_net/article/details/4146225

http://www.felix021.com/blog/read.php?2076

http://www.tldp.org/HOWTO/html_single/TCP-Keepalive-HOWTO/