Subject: Receiving multicast messages in UDP sockets question.

From: Grigory Kljuchnikov <[email-masked]>
Date: Tue, 16 Oct 2001 18:56:51 +0400 (MSK)

Hello,

RFC 2553 says:
Note that to receive multicast datagrams a process must join the
   multicast group and bind the UDP port to which datagrams will be
   sent.  Some processes also bind the multicast group address to the
   socket, in addition to the port, to prevent other datagrams destined
   to that same port from being delivered to the socket.

Please, clarify the quotation.  How does socket address affect messages
receiving?
There are four possible cases.
1.  Socket is bound to a port (address left unspecified) and joined
several multicast groups.
2.  Socket is bound to a unicast address and a port and joined several
multicast groups.
3.  Socket is bound to a multicast address and a port and joined several
multicast groups, one group has the address that the socket is bound to.
4.  Socket is bound to a multicast address and a port and joined several
multicast groups, and none of the groups has the address that the socket
is bound to.

In the first case socket receives all multicast messages, that are sent to
addresses of it's groups (and received on corresponding interfaces) and
the socket's port. Right?
In the second case socket receives unicast packets sent to the socket's
address+port AND all multicast messages sent to addresses of it's groups
(and received on corresponding interfaces) and the socket's port. Right?

QUESTION: What messages should the socket receive in the third and
fourth cases?

Grigory Klyuchnikov, System Engineer,
Institute for System Programming
Russian Academy of Sciences

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From: Ville Nummela <[email-masked]>
Date: Thu, 18 Oct 2001 09:29:01 +0300 (EET DST)

On Tue, 16 Oct 2001, Grigory Kljuchnikov wrote:

(sorry for the long quote)

> the socket's port. Right?

I believe the socket receives all packets destined to that port, since the
address hasn't been specified. That means the multicast messages AND the
unicast messages.

> (and received on corresponding interfaces) and the socket's port. Right?

I believe here we would only receive the messages for the unicast address,
as that is the one we have bound.

> QUESTION: What messages should the socket receive in the third and

Case 3: The messages for that multicast group we've bound.
Case 4: Nothing.

I just woke up, so forgive me my mistakes :-)

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