iOS: network reachability

https://developer.apple.com/library/ios/#samplecode/Reachability/Introduction/Intro.html

http://www.raddonline.com/blogs/geek-journal/iphone-sdk-testing-network-reachability/

http://www.yifeiyang.net/iphone-web-development-skills-of-the-article-4-make-sure-the-network-environment-3gwifi/

http://www.aslibra.com/blog/post/iphone_network_check.php

http://blog.csdn.net/favormm/article/details/5352656

http://blog.csdn.net/xbiii3s/article/details/6612139

http://www.cocoachina.com/bbs/read.php?tid-33789.html

http://www.switchonthecode.com/tutorials/iphone-snippet-detecting-network-status

http://tympanus.net/codrops/2009/09/14/checking-network-reachability-iphone/