通而不达网络的发现与解决方案

通而不达网络又称受限网络。若连接到这样的网络，手机收到的是网络正常通知，手机状态栏出现正常的wifi标志。若已经连接上后切换到这种网络，select函数监控不到异常（正常的响应超时，网络错误码errno有时间为45有时间为22，但是我们的socket在正常的情况下有时间errno为9，有时间为22，所以根据errno判断不出来这种异常），但是收不到服务器发回的所有响应（类似于 iphone手机里设置的弱网测试，丢失率为100%的设置情况）。在这种网络下发起connect 连接，那么会出现典型的75秒超时的情况。所以通而不达的网络典型特征是connect 75秒超时。通而不达网络通常是网络防火墙禁止了用户随意访问外网，进行了身份识别和登录。
既然网络通知，select函数， connect函数及网络错误码（errno）都不能准确定位出这类异常网络，那么解决这样的通而不达网络的办法代价很大，只能用心跳的方式发现这类网络。毕竟这类的网络很罕见，一般通过连接3次或超过75＊2秒没有连接上，或没75秒发送一次心跳，若服务器无法实时响应（10秒）就判定它为通而不达网络。
aibinjiang这个网络在登录连接上切换到后台，到了无信号区自动切换到4g网络，后进入aibinjiang信号区，在次离开该信号区，自动切换到4g网络。在connect连接时1毫秒给予响应，结果为1（0为正常，非零为失败），返回的错误码是65（网关或主机不可达）,53(软件引起的连接异常)。

连接aibinjiang公交wifi网络异常日志：

15/12/03 08:13:18:422  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1807 Debug:connect before:_connectTime = 1449101598421, Thread Sno: 18
2015/12/03 08:13:18:422  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1811 Debug:connect after:_connectTime = 1449101598422, Thread Sno: 18
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2586 Debug:ret = 1
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2598 Debug:pipe data:GuardThreadStartMonitor
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1865 Debug:errno:65
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2613 Debug:收到守护线程开始监控的消息，开始实时监控长连接线程:GuardThreadStartMonitor
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1880 Debug:i= -1
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1882 Debug:error : 65
2015/12/03 08:13:18:423  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2586 Debug:ret = 1
2015/12/03 08:13:18:424  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:2530 Debug:socket err 
2015/12/03 08:13:18:424  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2598 Debug:pipe data:GuardThreadIdle
2015/12/03 08:13:18:424  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:2549 Debug:sleep: 1秒
2015/12/03 08:13:18:424  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2604 Debug:收到守护线程空闲的消息，停止实时监控长连接线程:GuardThreadIdle
2015/12/03 08:13:19:372  Singleton.m:-[Singleton getLatitudeAll]:392 Debug:函数
2015/12/03 08:13:19:372  Singleton.m:-[Singleton getLongitudeAll]:404 Debug:函数
2015/12/03 08:13:19:372  Singleton.m:-[Singleton getLatitudeAll]:392 Debug:函数
2015/12/03 08:13:19:372  Singleton.m:-[Singleton getLongitudeAll]:404 Debug:函数
2015/12/03 08:13:19:372  Singleton.m:-[Singleton getDistanceWithStartLat:startLng:endLat:endLng:]:564 Debug:函数
2015/12/03 08:13:19:489  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1650 Debug:SKIP fd= 19
2015/12/03 08:13:19:489  Singleton.m:-[Singleton getLoginOrderStat]:265 Debug:函数
2015/12/03 08:13:19:489  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1735 Debug:socket sucess 
2015/12/03 08:13:19:489  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1736 Debug:g_fd= 19
2015/12/03 08:13:19:489  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1741 Debug:main loop 
2015/12/03 08:13:19:490  Singleton.m:-[Singleton getLoginOrderStat]:265 Debug:函数
2015/12/03 08:13:19:490  SingleAsyncSocket.m:-[SingleAsyncSocket getNetworkStatus]:54 Debug:函数
2015/12/03 08:13:19:490  SingleAsyncSocket.m:-[SingleAsyncSocket getNetworkStatus]:54 Debug:函数
2015/12/03 08:13:19:490  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1807 Debug:connect before:_connectTime = 1449101599489, Thread Sno: 18
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2586 Debug:ret = 1
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1811 Debug:connect after:_connectTime = 1449101599491, Thread Sno: 18
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2598 Debug:pipe data:GuardThreadStartMonitor
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1865 Debug:errno:65
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2613 Debug:收到守护线程开始监控的消息，开始实时监控长连接线程:GuardThreadStartMonitor
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2586 Debug:ret = 1
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2598 Debug:pipe data:GuardThreadIdle
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__32-[SingleAsyncSocket guardThread]_block_invoke:2604 Debug:收到守护线程空闲的消息，停止实时监控长连接线程:GuardThreadIdle
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1880 Debug:i= -1
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:1882 Debug:error : 65
2015/12/03 08:13:19:492  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:2530 Debug:socket err 
2015/12/03 08:13:19:493  SingleAsyncSocket.m:__36-[SingleAsyncSocket socketBSDThread]_block_invoke:2549 Debug:sleep: 1秒

iphone手机弱设置（用来模拟测试应用在不通网络环境下的健壮性），在设置->开发者->Status(Network Link Condtioner)里设置。