What to do about UDP packets arriving in pieces

Question

I have a Java application that is receiving UDP packets in a loop:

while (running) {
    try {
        // Block until a packet is received on the port
        byte[] buffer = new byte[1024];
        DatagramPacket receivePacket = new DatagramPacket(buffer, buffer.length);
        serverSocket.receive(receivePacket);
        ByteArrayInputStream byteIn = new ByteArrayInputStream(receivePacket.getData(), 0, receivePacket.getLength());
        DataInputStream dataIn = new DataInputStream(byteIn);

        // Print packet
        String packetString = "";
        while((packetString = dataIn.readLine()) != null) {
            Log.d("WiFi", packetString);
        }

    } catch (IOException ex) {
        Log.d("WiFi", "Problem receiving: " + ex.toString());
    }
}

When I get packets, many of them arrive as expected, but every once in a while, I get a packet that is broken up into two pieces:

103, -5.84, 5.64, 140.72, #A-=-22.53,-50.18,248.83,  #G-=52.00,-69.00,-18.00,   #M-=-146.66,-155.18,151.06,   2575012
103, -6.51, 5.62, 140.41, #A-=-22.53,-51.20,248.83,  #G-=23.00,44.00,-7.00,   #M-=-146.21,-156.13,151.06,   2575036
103, -7.23, 5.65, 140.09, #A-=-19.46,-49.15,243.71,  #G-=17.00,68.00,-2.00,   #M-=-144.85,-155.18,151.06,   2575063
103, -7.76, 5.60, 139.81, #A-=-17.41,-50.18,240.64,  #G-=17.00,71.00,0.00,   #M-=-143.94,-155.65,153
.47,   2575090
103, -8.51, 5.34, 139.40, #A-=-12.29,-47.10,233.47,  #G-=31.00,-1.00,-5.00,   #M-=-144.85,-154.70,151.87,   2575135
103, -9.20, 4.76, 138.95, #A-=-15.36,-50.18,239.62,  #G-=22.00,30.00,-3.00,   #M-=-146.66,-156.13,151.87,   2575200
103, -9.82, 4.46, 138.61, #A-=-15.36,-47.10,239.62,  #G-=33.00,1.00,-3.00,   #M-=-145.76,-156.13,152.27,   2575266
103, -10.38, 4.07, 138.23, #A-=-14.34,-48.13,235.52,  #G-=30.00,4.00,-7.00,   #M-=-144.85,-155.65,152.27,   2575372
103, -10.90, 3.76, 137.90, #A-=-14.34,-48.13,237.57,  #G-=29.00,19.00,-4.00,   #M-=-145.30,-156.13,151.47,   2575520
103, -11.41, 3.53, 137.44, #A-=-15.36,-48.13,235.52,  #G-=25.00,18.00,-4.00,   #M-=-146.21,-155.18,151.47,   2576917

As you can see on lines 4 and 5, the last trailing characters of 4 have been cut off and a new packet of the remaining characters have arrived. From my reading about UDP, everyone warns that UDP does not guarantee the arrival of packets. In my case that is not a problem; a dropped packet hear or there isn't going to break my application. However, I haven't found anything about packets not necessarily arriving as complete. In fact, I have read that this does not happen.

How is this dealt with usually? It is difficult for me to know if a packet that has arrived is complete or not in which I could just wait for the next packet and append the data.

Answer 1

What you describe is impossible. UDP datagrams arrive intact and complete or not at all. The server must be sending the data in two datagrams.