DatagramPacket to read non-constant sized UDP message

Hi,

I need to receive UDP packages through DatagramPacket.

Since the message's length is not constant, I need to read the length first.

I know the protocol definition for this customized package, say,

-- id 2 bytes
-- length 2 bytes
-- body n bytes, (n = length)


I want to read the length first, then read the whole message (or the rest of the message, ie. the body).

The question is, after reading the header( first 4 bytes), when I call the readMsg again, should I read the length of the whole message or just the body ? ( Is the header still in the buffer ?)

I tried, it seems the former one works, but I don't understand why. If the header is still there after being read out, when the buffer will be cleared for the next UDP message ?

And, what if the sender sends very fast, even faster than the receiver can read. Where will the data that are not read be ?

Thanks,

Code:

---

        public byte[] readMsg(int num)throws IOException {
               
               
                // create a packet buffer to store data from packets received.
                byte inBuf[] = new byte[num];
                DatagramPacket inPkt = new DatagramPacket(inBuf, num);

                // receive the reply from server.
       
               
                dgSocket.receive(inPkt);

       
                return inPkt.getData();
        }

---