Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!NSIPO.ARC.NASA.GOV!medin
From: medin@NSIPO.ARC.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Using the 4.2 broadcast addr with 4.3 systems
Message-ID: <8909040456.AA21609@nsipo.arc.nasa.gov>
Date: 4 Sep 89 04:56:06 GMT
References: <[A.ISI.EDU].2-Sep-89.12:00:34.CERF>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 38


Vint, I understand your point, and don't think we disagree.  A packet
received via link level broadcast should NEVER be forwarded.  I don't
think anyone disagrees with that.  But the only way I can think of to verify
that is to tag the packet at link layer as recieved by broadcast.

As for what happens at the remote end of the packet's travel, well, that
gets hairy.  If you support the concept if directed broadcast (which, as
I've said before, I do not), then the packet's destination IP address 
should probably be set to the all 1's network broadcast or subnet broadcast
address.  Clearly, you can't send to a subnet broadcast address without
knowing the mask, and while you there are hacks you can perform, such
as sending an ICMP netmask request message to one of the gateways on
that net (or a host for that matter), they are all inadequate.  For
one thing, you have to deal with the existance of variable length subnet
masks, and that means that there may not be one subnet mask for the entire 
network.  While this typically hasn't been the case so far, routing protocols
like OSPF are being implemented that support that functionality.  Then
you have to worry about how to find a gateway or host on the piece of plumbing
you are trying to get to, and you may not be able to figure that without
a priori knowledge...  All this means you basically have to settle for
directed network broadcast, if that.

I think all this shows is that directed broadcast is a flawed concept, at
least given the current way addressing works in the Internet.  I
think multicasting is really the right way to go, and gateway vendors
should put their emphasis into that approach, and put the issue of
directed broadcast to bed.  

All this directed broadcast discussion is really orthogonal to the 
issue of link level broadcast tagging.  You can still have directed 
broadcast even if you do link level tagging.  As far as I am concerned,
there is no way you can make sure you don't forward broadcast packets
without doing that tagging.  Gateway vendors who don't implement tagging
aren't being 1009 compliant.

						Thanks,
						    Milo