TCP非正常断开-检测方法

如果主机崩溃，write是否阻塞取决于内核的tcp缓冲区，但read将一直阻塞，直到超时ETIMEOUT，或由于某些中间路由器的原因返回EHOSTUNREACH/ENETUNREACH。select不能检测到该情况。
如果主机崩溃并重起，客户的write到达主机时主机响应RST，客户的read将返ECONNRESET。

此处的”非正常断开”指TCP连接不是以优雅的方式断开,如网线故障等物理链路的原因,还有突然主机断电等原因
有两种方法可以检测:1.TCP连接双方定时发握手消息 2.利用TCP协议栈中的KeepAlive探测
第二种方法简单可靠,只需对TCP连接两个Socket设定KeepAlive探测,所以本文只讲第二种方法在Linux,Window2000下的实现(在其它的平台上没有作进一步的测试)

Windows平台

//定义结构及宏
struct TCP_KEEPALIVE {
    u_longonoff;
    u_longkeepalivetime;
    u_longkeepaliveinterval;
} ;
#define SIO_KEEPALIVE_VALS _WSAIOW(IOC_VENDOR,4)
//KeepAlive实现
TCP_KEEPALIVE inKeepAlive = {0}; //输入参数
unsigned long ulInLen = sizeof(TCP_KEEPALIVE);
TCP_KEEPALIVE outKeepAlive = {0}; //输出参数
unsigned long ulOutLen = sizeof(TCP_KEEPALIVE);
unsigned long ulBytesReturn = 0;
//设置socket的keep alive为5秒，并且发送次数为3次
inKeepAlive.onoff = 1;
inKeepAlive.keepaliveinterval = 5000; //两次KeepAlive探测间的时间间隔
inKeepAlive.keepalivetime = 5000; //开始首次KeepAlive探测前的TCP空闭时间
if (WSAIoctl((unsigned int)s, SIO_KEEPALIVE_VALS,
(LPVOID)&inKeepAlive, ulInLen,
(LPVOID)&outKeepAlive, ulOutLen,
&ulBytesReturn, NULL, NULL) == SOCKET_ERROR)
{
    ACE_DEBUG ((LM_INFO,
    ACE_TEXT ("(%P|%t) \WSAIoctl failed. error code(%d)!\n"), WSAGetLastError()));
}

Linux平台

#include
……
KeepAlive实现
//下面代码要求有ACE,如果没有包含ACE,则请把用到的ACE函数改成linux相应的接口
int keepAlive = 1;//设定KeepAlive
int keepIdle = 5;//开始首次KeepAlive探测前的TCP空闭时间
int keepInterval = 5;//两次KeepAlive探测间的时间间隔
int keepCount = 3;//判定断开前的KeepAlive探测次数
if(setsockopt(s,SOL_SOCKET,SO_KEEPALIVE,(void*)&keepAlive,sizeof(keepAlive)) == -1)
{
    ACE_DEBUG ((LM_INFO,
    ACE_TEXT ("(%P|%t) setsockopt SO_KEEPALIVE error!\n")));
}
if(setsockopt(s,SOL_TCP,TCP_KEEPIDLE,(void *)&keepIdle,sizeof(keepIdle)) == -1)
{
    ACE_DEBUG ((LM_INFO,
    ACE_TEXT ("(%P|%t) setsockopt TCP_KEEPIDLE error!\n")));
}
if(setsockopt(s,SOL_TCP,TCP_KEEPINTVL,(void *)&keepInterval,sizeof(keepInterval)) == -1)
{
    ACE_DEBUG ((LM_INFO,
    ACE_TEXT ("(%P|%t) setsockopt TCP_KEEPINTVL error!\n")));
}
if(setsockopt(s,SOL_TCP,TCP_KEEPCNT,(void *)&keepCount,sizeof(keepCount)) == -1)
{
    ACE_DEBUG ((LM_INFO,
    ACE_TEXT ("(%P|%t)setsockopt TCP_KEEPCNT error!\n")));
}

:-)