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From: CERF@A.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Using the 4.2 broadcast addr with 4.3 systems
Message-ID: <[A.ISI.EDU].2-Sep-89.12:00:34.CERF>
Date: 2 Sep 89 16:00:00 GMT
References: <8909011945.AA00826@cincsac.arc.nasa.gov>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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Milo,

I understand your point. Something still seems to be missing.
Suppose the intent is to do directed broadcast (we can argue
separately about whether this is a good idea - I'm prepared to
come to the conclusion that it isn't and that the intent is
better served with multicast). The target of the broadcast
may be at the far end of the Internet. One would have to
somehow identify the target net, for routing purposes, and
the also indicate that the packet is to be broadcast there.
This information needs to be contained in the destination IP
address, because that is all that survives as the packet
is encapsulation and decapsulated on its journey through
an indeterminate number of nets and routers.

The only way to achieve this is to have the destination IP
address show the target net and the "rest" show "broadcast."

If the source is on a LAN, the packet clearly should NOT
show a level two "broadcast" indication at the LAN level,
since the intent is to broadcast only at the destination.

Multicasting is more complex because the multicast address
format in IP is not network specific, so more machinery
is needed.

I agree that it is never appropriate to resend or forward
a packet marked "broadcast" at level two.

Are we in disagreement at this point?

Vint