来源：http://blog.csdn.net/weiwangchao_/article/details/7225338

http://www.cnitblog.com/zouzheng/archive/2007/09/15/33418.html

http://blog.csdn.net/dongfangyu/article/details/5537781

http://www.cppblog.com/aaxron/archive/2011/08/07/152703.html

http://blog.leeyiw.org/tcp-keep-alive/