title: A problem about using MPI_Send
body: <p>I'm learning MPI_Send, but I'm confused about this method. I wrote a simple pingpong program which the rank-0 node send the message to rank-1 node, and then the latter one returns a message to the former one.</p>
  
  <pre><code>if (rank == 0) {   /* Send Ping, Receive Pong */
    dest = 2;
    source = 2;
    rc = MPI_Send(pingmsg, strlen(pingmsg)+1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    rc = MPI_Recv(buff, strlen(pongmsg)+1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &amp;Stat);
     printf("Rank0 Sent: %s &amp; Received: %s\n", pingmsg, buff);
    }
  else if (rank == 2) { /* Receive Ping, Send Pong */
    dest = 0;
    source = 0;
    rc = MPI_Recv(buff, strlen(pingmsg)+1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &amp;Stat);
    printf("Rank1 received: %s &amp; Sending: %s\n", buff, pongmsg);
    rc = MPI_Send(pongmsg, strlen(pongmsg)+1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
    }
  </code></pre>
  
  <p>I run this program on a 3 nodes environment. However, the system displays:</p>
  
  <pre><code>Fatal error in MPI_Send: Other MPI error, error stack:
  MPI_Send(173)..............: MPI_Send(buf=0xbffffb90, count=10, MPI_CHAR, dest=2, tag=1, MPI_COMM_WORLD) failed
  MPID_nem_tcp_connpoll(1811): Communication error with rank 2: Unknown error 4294967295
  </code></pre>
  
  <p>I'm wondering why I can send a message from rank-0 node to rank-1 node, but an error occurs when changed from rank-0 node to rank-1 node? Thanks.</p>