DatagramSocketImpl.join incorrectly handles 224.0.0.0; spec is unclear

Name: mgC56079 Date: 08/04/99


According to the MulticastSocket spec, "A multicast group is specified by a class D
IP address, those in the range 224.0.0.1 to 239.255.255.255".
The spec for MulticastSocket.joinGroup(InetAddress) says an IOException will be thrown
"when the address is not a multicast address".

There are two problems here:

1. The method MulticastSocket.joinGroup doesn't throw an exception for invalid address 224.0.0.0.
   (this method simply calls DatagramSocketImpl.join (the actual implementation is in
   PlainDatagramSocketImpl.join)).

2. The spec for DatagramSocketImpl.join is not consistent with the one of
   MulticastSocket.joinGroup as it says nothing about handling of non-multicast addresses.

An example demonstrating the problem #1:
======== MultiCastTest.java ===========
import java.net.*;
class MultiCastTest {

    public static void main(String args[]) throws Exception {
        InetAddress addr = InetAddress.getByName("224.0.0.0");
        MulticastSocket s = new MulticastSocket(3000);
        try {
            s.joinGroup(addr);
            System.out.println("failed: IOException expected");
        }
        catch(java.io.IOException e) {
            System.out.println("passed");
        }
    }

}

======== sample run ===========
% java MultiCastTest
failed: IOException expected

======================================================================