Question

I am implementing a p2p application, in which nodes communicate by UDP packet. The packets read from InputStream are sometime not complete.

Here is my code:

…

protected String key;
    protected Identifier messId; //Identifier hold a BigInteger
    protected String range;
    protected String concat;

….

public ReplicationMessage(DataInput in) throws IOException {
       fromStream(in);
}


public void fromStream(DataInput in)
    try {
    super.fromStream(in);      

    int length=in.readInt();
    byte[] data=new byte[length];
    in.readFully(data);
    concat = new String(data);
    System.out.println("concat: "+concat); 

            messId = new Identifier(in);

    } catch (IOException e) {
        e.printStackTrace();
    }        
  }

public void toStream(DataOutput out) {
    try {

    super.toStream(out);

    byte[] data = concat.getBytes();
    out.writeInt(data.length);
    out.write(data);        

    messId.toStream(out);

    } catch (IOException e) {

e.printStackTrace();
    }   
}

the read packet sometime is complete, for example

concat: 179136678282544:140737488355328

but sometime is not complete, for example

concat: 179136678282544
concat: 179136678282544
concat: 179136678282544

Can any one tell me what the problem is?

Many thanks

Here are the code for sending/receiving the UDP packet
for sending:

private void sendMessage(int comm, Message message, InetAddress ip, int port)
throws IOException {

    ByteArrayOutputStream bout = new ByteArrayOutputStream();

    DataOutputStream dout = new DataOutputStream(bout);

    dout.writeInt(comm);

    dout.writeByte(message.code());

    message.toStream(dout);

    dout.close();

    byte[] data = bout.toByteArray();
    if (data.length > DATAGRAM_BUFFER_SIZE) {
        throw new IOException("Message too big, size="+data.length+
                              " bytes, max="+DATAGRAM_BUFFER_SIZE+" bytes");
    }

    DatagramPacket packet = new DatagramPacket(data, data.length, ip, port);

    socket.send(packet);
}

For receiving the UDP packet

     byte[] buffer = new byte[DATAGRAM_BUFFER_SIZE];

     DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

     socket.receive(packet);

The sending node and receiving node belonging to one machine. the Buffer receiving packet is set to 10*1024 bytes which is much larger than packet length

Here is code for turning incoming datapacket into stream

                ByteArrayInputStream bin = new ByteArrayInputStream(packet.getData(),
                                             packet.getOffset(), packet.getLength());

                DataInputStream din = new DataInputStream(bin);

                int comm = din.readInt();

                byte messCode = din.readByte();

                Message message = factory.createMessage(messCode, din);

Answer 1

UDP does not deliver damaged or partial packets. Either you aren’t sending all the data or you aren’t unpacking it properly. You still haven’t posted all the code concerned so it is not possible to say exactly where.