MSDN中TCP keepalive时效的说明以及设置TCP Keepalive的代码

说明：
A TCP keep-alive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection. A host receiving one of these ACKs will respond with an ACK for the current sequence number.
翻译：
一个TCP keep-alive 包是一个简单的ACK,该ACK包内容为 一个比当前连接sequence number 小于一的包。主机接受到这些ACKs会返回一个包含当前sequence number 的ACK包。

说明：
Keep-alives can be used to verify that the computer at the remote end of a connection is still available.
翻译：
Keep-alives一般被用来验证远端连接是否有效。

说明：
TCP keep-alives can be sent once every KeepAliveTime (defaults to 7,200,000 milliseconds or 2 hours), if no other data or higher-level keep-alives have been carried over the TCP connection.
翻译：
如果该连接上没有其他数据被传输，或者更高level 的 keep-alives被传送，keep-alives 在每个KeepAliveTime被发送。（默认是 7,200,000 milliseconds ，也就是2个小时）。

说明：
If there is no response to a keep-alive, it is repeated once every KeepAliveInterval seconds. KeepAliveInterval defaults to 1 second. NetBT connections, such as those used by many parts of the Microsoft networking functionality, send NetBIOS keep-alives more frequently, so usually no TCP keep-alives will be sent on a NetBIOS connection.
原翻译：
如果对于keep-alive连接端没有回应，他继续将在每KeepAliveInterval秒发送，KeepAliveInterval默认为1秒，在NETBT连接，比如Microsoft networking的很多功能性的部分，发送NETBios keep-alives频率高，所以，在一个NetBios连接中，TCP keep-alives不被发送。  

此处感谢 网友of123 修正翻译（给大家造成的误解博主向所有的读者表示歉意）：

引用

如果没有收到 keep-alive 应答，keep-alive 将在每 KeepAliveInterval 秒重发一次。KeepAliveInterval 默认为1秒。如 Microsoft 网络功能中很多部分中采用的 NETBT 连接，更常见的是发送 NETBios keep-alives，所以，在 NetBios 连接中通常不发送TCP keep-alives。

说明：
TCP keep-alives are disabled by default, but Windows Sockets applications can use the SetSockOpt function to enable them.
翻译：
TCP保持连接默认被禁用，但是微软Sockets应用程序可以使用SetSockOpt函数去启用他们。


几点比较重要的说明：

1：TCP 的默认时效是2个小时。
2：如果发送Keep-alive没有回应，将会在KeepAliveInterval再次发送，不过这里是否需要系统来进行判断，并断开连接？

设置keepalive代码如下：

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1.                 tcp_keepalive live,liveout;   
2.                 live.keepaliveinterval=500;    
3.                 live.keepalivetime=3000; //勘误  1分钟是 60000 以此类推   
4.                 live.onoff=TRUE;   
5.                 int iRet = setsockopt(Socket,SOL_SOCKET,SO_KEEPALIVE,(char *)Opt,sizeof(int));   
6.                 if(iRet == 0){   
7.   
8.                         DWORD dw;   
9.                    
10.                         if(WSAIoctl(Socket,SIO_KEEPALIVE_VALS,&live,sizeof(live),&liveout,sizeof(liveout),&dw,NULL,NULL)== SOCKET_ERROR){   
11.   
12.                                 //Delete Client   
13.   
14.                                 return;   
15.                         }   
16.                    }   

                tcp_keepalive live,liveout;                live.keepaliveinterval=500;                 live.keepalivetime=3000; //勘误  1分钟是 60000 以此类推                live.onoff=TRUE;                int iRet = setsockopt(Socket,SOL_SOCKET,SO_KEEPALIVE,(char *)Opt,sizeof(int));                if(iRet == 0){                        DWORD dw;                                        if(WSAIoctl(Socket,SIO_KEEPALIVE_VALS,&live,sizeof(live),&liveout,sizeof(liveout),&dw,NULL,NULL)== SOCKET_ERROR){                                //Delete Client                                return;                        }                   }

判断掉线并处理：

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1. BOOL bReturn = GetQueuedCompletionStatus(   
2.       g_hIOCompletionPort,   
3.       &dwBytesTransfered,   
4.       (LPDWORD)&lpContext,   
5.       &pOverlapped,   
6.       INFINITE);   
7. if (bReturn== false){   
8.    //Client has gone delete client and close socket   
9. }