tcp连接在高负载下随机失败

Question

我们的应用程序使用非阻塞套接字使用连接和选择操作(c代码).pusedo代码如下:

unsigned int ConnectToServer(struct sockaddr_in *pSelfAddr,struct sockaddr_in *pDestAddr)
    {
         int sktConnect = -1;
         sktConnect = socket(AF_INET,SOCK_STREAM,0);
         if(sktConnect == INVALID_SOCKET)
             return -1;
         fcntl(sktConnect,F_SETFL,fcntl(sktConnect,F_GETFL) | O_NONBLOCK);
         if(pSelfAddr != 0)
         {
             if(bind(sktConnect,(const struct sockaddr*)(void *)pSelfAddr,sizeof(*pSelfAddr)) != 0)
             {
                 closesocket(sktConnect);
                 return -1;
             }
         }
         errno = 0;
         int nRc = connect(sktConnect,(const struct sockaddr*)(void *)pDestAddr, sizeof(*pDestAddr));
         if(nrC != -1)
         {
             return sktConnect;
         }
         if(errno != EINPROGRESS)
         {
             int savedError = errno;
             closesocket(sktConnect);
             return -1;
         }
         fd_set scanSet;
         FD_ZERO(&scanSet);
         FD_SET(sktConnect,&scanSet);
         struct timeval waitTime;
         waitTime.tv_sec = 2;
         waitTime.tv_usec = 0;
         int tmp;
         tmp = select(sktConnect +1, (fd_set*)0, &scanSet, (fd_set*)0,&waitTime);
         if(tmp == -1 || !FD_ISSET(sktConnect,&scanSet))
         {
             int savedErrorNo = errno;
             writeLog("Connect %s failed after select, cause %d, error %s",inet_ntoa(pDestAddr->sin_addr),savedErrorNo,strerror(savedErrorNo));
             closesocket(sktConnect);
             return -1;
         } 
    .        .        .     .     .}

有80个这样的节点,应用程序以循环方式连接到所有对等节点.在此阶段,某些节点无法连接(api - connect + select),错误号为115.

在成功方案的下面的日志(tcpdump输出)中,我们可以看到(SYN,SYN + ACK,ACK),但tcpdump日志中的故障节点不存在偶数SYN的条目.

tcpdump日志是:

387937 2012-07-05 07:45:30.646514 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [SYN] Seq=0 Ack=0 Win=5792 Len=0 MSS=1460 TSV=1414450402 TSER=912308224 WS=8 
387947 2012-07-05 07:45:30.780762 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [SYN, ACK] Seq=0 Ack=1 Win=5792 Len=0 MSS=1460 TSV=912309754 TSER=1414450402 WS=8 
387948 2012-07-05 07:45:30.780773 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [ACK] Seq=1 Ack=1 Win=5888 Len=0 TSV=1414450435 TSER=912309754 
All the above three events indicate the success information.
387949 2012-07-05 07:45:30.782652 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [PSH, ACK] Seq=1 Ack=1 Win=5888 Len=320 TSV=1414450436 TSER=912309754 
387967 2012-07-05 07:45:30.915615 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [ACK] Seq=1 Ack=321 Win=6912 Len=0 TSV=912309788 TSER=1414450436 
388011 2012-07-05 07:45:31.362712 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [PSH, ACK] Seq=321 Ack=1 Win=5888 Len=320 TSV=1414450581 TSER=912309788 
388055 2012-07-05 07:45:31.495558 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [ACK] Seq=1 Ack=641 Win=7936 Len=0 TSV=912309933 TSER=1414450581 
388080 2012-07-05 07:45:31.702336 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [PSH, ACK] Seq=1 Ack=641 Win=7936 Len=712 TSV=912309985 TSER=1414450581 
388081 2012-07-05 07:45:31.702350 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [ACK] Seq=641 Ack=713 Win=7424 Len=0 TSV=1414450666 TSER=912309985 
388142 2012-07-05 07:45:32.185612 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [PSH, ACK] Seq=713 Ack=641 Win=7936 Len=320 TSV=912310106 TSER=1414450666 
388143 2012-07-05 07:45:32.185629 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [ACK] Seq=641 Ack=1033 Win=8704 Len=0 TSV=1414450786 TSER=912310106 
388169 2012-07-05 07:45:32.362622 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [PSH, ACK] Seq=641 Ack=1033 Win=8704 Len=320 TSV=1414450831 TSER=912310106 
388212 2012-07-05 07:45:32.494833 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [ACK] Seq=1033 Ack=961 Win=9216 Len=0 TSV=912310183 TSER=1414450831 
388219 2012-07-05 07:45:32.501613 10.137.165.136        10.18.92.173          TCP      8441 > 33728 [PSH, ACK] Seq=1033 Ack=961 Win=9216 Len=356 TSV=912310185 TSER=1414450831 
388220 2012-07-05 07:45:32.501624 10.18.92.173          10.137.165.136        TCP      33728 > 8441 [ACK] Seq=961 Ack=1389 Win=10240 Len=0 TSV=1414450865 TSER=912310185 

应用程序日志通知连接错误(即api - connect + select)

[5258: 2012-07-05 07:45:30]Connect [10.137.165.136 <- 10.18.92.173] success. 
[5258: 2012-07-05 07:45:32]Connect 10.137.165.137 fail after select, cause:115, error Operation now in progress. Check whether remote machine exist and the network is normal or not. 
[5258: 2012-07-05 07:45:32]Connect to server([10.137.165.137 <- 10.18.92.173], port=8441) Failed!

成功日志对应tcpdump的前3个条目.并且在tcpdump中没有事件的故障日志

我的问题是:当客户端为失败的情况启动"连接"api时,我无法在客户端的tcpdump中看到任何事件(甚至是初始SYN).这种随机性的原因是什么？

Answer 1

你已经击中了EINPROGRESS。从connect手册页：

套接字是非阻塞的，连接不能立即完成。通过选择要写入的套接字，可以选择 select(2) 或 poll(2) 来完成。select(2) 指示可写性后，使用 getsockopt(2) 读取 SOL_SOCKET 级别的 SO_ERROR 选项，以确定 connect() 是否成功完成（SO_ERROR 为零）或不成功（SO_ERROR 是此处列出的常见错误代码之一，解释了失败的原因）。

这表明EINPROGRESS内核现在无法完成连接，即使有可用的本地端口和路由缓存条目。当套接字状态尚未转换为“ESTABLISHED”时，似乎会发生这种情况。只需再次等待套接字select，但getsockopt随后调用以查看是否connect已完成。

至于为什么，套接字在连接期间转换为 SYN_SENT 状态，但数据包可能仍在输出队列中，并且尚未真正到达网络设备缓冲区。