Как узнать длину пакета по DatagramChannel?

Вот изучаю здесь код: http://cs.ecs.baylor.edu/~donahoo/practical/JavaSockets2/code/UDPEchoServerSelector.java

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.DatagramChannel;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.util.Iterator;
public class UDPEchoServerSelector {
    private static final int TIMEOUT = 3000; // Wait timeout (milliseconds)
    private static final int ECHOMAX = 255; // Maximum size of echo datagram
    public static void main(String[] args) throws IOException {
        if (args.length != 1) // Test for correct argument list
        throw new IllegalArgumentException("Parameter(s): <Port>");
        int servPort = Integer.parseInt(args[0]);
        // Create a selector to multiplex client connections.
        Selector selector = Selector.open();
        DatagramChannel channel = DatagramChannel.open();
        channel.configureBlocking(false);
        channel.socket().bind(new InetSocketAddress(servPort));
        channel.register(selector, SelectionKey.OP_READ, new ClientRecord());
        while (true) { // Run forever, receiving and echoing datagrams
        // Wait for task or until timeout expires
        if (selector.select(TIMEOUT) == 0) {
        System.out.print(".");
        continue;
        }
         // Get iterator on set of keys with I/O to process
        Iterator<SelectionKey> keyIter = selector.selectedKeys().iterator();
        while (keyIter.hasNext()) {
        SelectionKey key = keyIter.next(); // Key is bit mask
    // Client socket channel has pending data?
    if (key.isReadable())
      handleRead(key);
    // Client socket channel is available for writing and
    // key is valid (i.e., channel not closed).
    if (key.isValid() && key.isWritable())
      handleWrite(key);
    keyIter.remove();
    }
    }
    }
public static void handleRead(SelectionKey key) throws IOException {
 DatagramChannel channel = (DatagramChannel) key.channel();
 ClientRecord clntRec = (ClientRecord) key.attachment();
 clntRec.buffer.clear();    // Prepare buffer for receiving
 clntRec.clientAddress = channel.receive(clntRec.buffer);
 if (clntRec.clientAddress != null) {  // Did we receive something?
   // Register write with the selector
   key.interestOps(SelectionKey.OP_WRITE);
 }
}
public static void handleWrite(SelectionKey key) throws IOException {
 DatagramChannel channel = (DatagramChannel) key.channel();
 ClientRecord clntRec = (ClientRecord) key.attachment();
 clntRec.buffer.flip(); // Prepare buffer for sending
 int bytesSent = channel.send(clntRec.buffer, clntRec.clientAddress);
 if (bytesSent != 0) { // Buffer completely written?
  // No longer interested in writes
   key.interestOps(SelectionKey.OP_READ);
  }
 }
static class ClientRecord {
public SocketAddress clientAddress;
public ByteBuffer buffer = ByteBuffer.allocate(ECHOMAX);
}
}

И не понимаю, каким образом можно узнать через DatagramChannel длинну приходящего пакета чтобы из ByteBuffer считать нужное количество?