Program: 5631
Project: 5631
Buggy source:
public void handlePacket(IPv6Packet packet){
  TCPPacket tcpPacket=(TCPPacket)packet.getIPPayload();
  TCPConnection connection=findConnection(packet,tcpPacket);
  if (connection == null) {
    connection=findListenConnection(packet,tcpPacket);
    if (connection == null) {
      System.out.println("String_Node_Str");
    }
 else {
      if ((tcpPacket.flags & TCPPacket.SYN) > 0) {
        TCPPacket tcpReply=createAck(tcpPacket,TCPPacket.SYN);
        TCPConnection tc=new TCPConnection(ipStack,packet.netInterface);
        tc.externalIP=packet.sourceAddress;
        tc.externalPort=tcpPacket.sourcePort;
        tc.localIP=ipStack.myIPAddress;
        tc.localPort=tcpPacket.destinationPort;
        tc.state=TCPConnection.SYN_RECEIVED;
        tc.receiveNext=tcpPacket.seqNo + 1;
        tc.sentUnack=tc.sendNext=tcpReply.seqNo=(int)(System.currentTimeMillis() * 7);
        addConnection(tc);
        tcpReply.ackNo=tcpPacket.seqNo + 1;
        tc.send(tcpReply);
        tc.sentUnack=tc.sendNext=tc.sendNext + 1;
        connection.newConnection(tc);
      }
    }
  }
 else {
switch (connection.state) {
case TCPConnection.SYN_RECEIVED:
      if (tcpPacket.isAck()) {
        System.out.println("String_Node_Str");
        connection.state=TCPConnection.ESTABLISHED;
      }
    break;
case TCPConnection.ESTABLISHED:
  if (tcpPacket.isAck() && (tcpPacket.payload == null || tcpPacket.payload.length == 0)) {
    return;
  }
if (tcpPacket.isFin()) {
  connection.state=TCPConnection.CLOSE_WAIT;
}
connection.receive(tcpPacket);
break;
case TCPConnection.LAST_ACK:
if (tcpPacket.isAck()) {
connection.state=TCPConnection.CLOSED;
}
break;
case TCPConnection.FIN_WAIT_1:
if (tcpPacket.isAck()) {
connection.state=TCPConnection.FIN_WAIT_2;
}
break;
case TCPConnection.FIN_WAIT_2:
if (tcpPacket.isFin()) {
System.out.println("String_Node_Str");
connection.state=TCPConnection.TIME_WAIT;
connection.lastSendTime=System.currentTimeMillis();
}
break;
case TCPConnection.CLOSE_WAIT:
break;
}
}
}

Fixed version (ground truth):
public void handlePacket(IPv6Packet packet){
  TCPPacket tcpPacket=(TCPPacket)packet.getIPPayload();
  TCPConnection connection=findConnection(packet,tcpPacket);
  if (connection == null) {
    connection=findListenConnection(packet,tcpPacket);
    if (connection == null) {
      System.out.println("String_Node_Str");
    }
 else {
      if ((tcpPacket.flags & TCPPacket.SYN) > 0) {
        TCPPacket tcpReply=createAck(tcpPacket,TCPPacket.SYN);
        TCPConnection tc=new TCPConnection(ipStack,packet.netInterface);
        tc.externalIP=packet.sourceAddress;
        tc.externalPort=tcpPacket.sourcePort;
        tc.localIP=ipStack.myIPAddress;
        tc.localPort=tcpPacket.destinationPort;
        tc.state=TCPConnection.SYN_RECEIVED;
        tc.receiveNext=tcpPacket.seqNo + 1;
        tc.sentUnack=tc.sendNext=tcpReply.seqNo=(int)(System.currentTimeMillis() * 7);
        addConnection(tc);
        tcpReply.ackNo=tcpPacket.seqNo + 1;
        tc.send(tcpReply);
        tc.sentUnack=tc.sendNext=tc.sendNext + 1;
        connection.newConnection(tc);
      }
    }
  }
 else {
switch (connection.state) {
case TCPConnection.SYN_RECEIVED:
      if (tcpPacket.isAck()) {
        System.out.println("String_Node_Str");
        connection.state=TCPConnection.ESTABLISHED;
      }
    break;
case TCPConnection.ESTABLISHED:
  if (tcpPacket.isFin()) {
    connection.state=TCPConnection.CLOSE_WAIT;
  }
connection.receive(tcpPacket);
break;
case TCPConnection.LAST_ACK:
if (tcpPacket.isAck()) {
connection.state=TCPConnection.CLOSED;
}
break;
case TCPConnection.FIN_WAIT_1:
if (tcpPacket.isAck()) {
connection.state=TCPConnection.FIN_WAIT_2;
}
break;
case TCPConnection.FIN_WAIT_2:
if (tcpPacket.isFin()) {
System.out.println("String_Node_Str");
connection.state=TCPConnection.TIME_WAIT;
connection.lastSendTime=System.currentTimeMillis();
}
break;
case TCPConnection.CLOSE_WAIT:
break;
}
}
}