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From: wayne@ultra.com (Wayne Hathaway)
Newsgroups: comp.protocols.tcp-ip
Subject: Question on forwarding broadcasts
Summary: Two nets on one cable == Trouble!
Message-ID: <1990Sep28.143945.7440@ultra.com>
Date: 28 Sep 90 14:39:45 GMT
Reply-To: wayne@ultra.com (Wayne Hathaway)
Distribution: comp
Organization: Ultra Network Technologies
Lines: 62

I have a little "IP forwarding of broadcasts" question.  The following
example configuration is absurdly contrived, of course, but it is the
simplest that will illustrate the problem.  The problem did surface in
a much more complex (and realistic) situation, but I don't want to
bore everybody with that.

Anyway, consider:

    +------------+    +------------------------+    +------------+
    |   Host A   |    |     Default Router     |    |   Host B   |
    |            |    |                        |    |            |
    | 192.40.1.1 |    | 192.40.1.2  192.40.2.2 |    | 192.40.2.1 |
    +------+-----+    +------+----------+------+    +-----+------+
           |                 |          |                 |
===========#=================#==========#=================#===========
                               Ethernet

where both Host A and Host B are configured to forward things they
cannot handle to the Default Router.  Note that there are TWO
DIFFERENT NETWORKS active on a SINGLE ETHERNET CABLE (192.40.1 and
192.40.2).  (This is of course key to the problem, but it is a
perfectly legal configuration, no?) 

Now assume a user on Host A does a broadcast to network 192.40.1 (its
own network).  No problem, Host A simply puts the message out with the
Ethernet MAC layer broadcast address, and it is picked up by everybody
on network 192.40.1.  But Host B will also pick up this message, and
will find that it is not destined for him.  Host B will then forward
it to the Default Router over network 192.40.2.  The Default Router
will receive this forwarded packet, and will see that it is a
broadcast for a directly-connected network, so he will broadcast it
out on network 192.40.1, by putting it out with the Ethernet MAC layer
broadcast address.  But Host B will also pick up this message, and
will find ...  [repeat as needed].

Clearly what Host B SHOULD do with the "misdelivered broadcast" is say
"This is not for me AND it came in as a MAC layer broadcast; therefore
just discard it (do NOT forward it)."  But unfortunately that requires
IP to know what MAC layer address a datagram came in on, and (at least
in any BSD-derivative implementation I know of) that information is
not available to IP.

Finally, note that saying "Host B should recognize this is a broadcast
from the destination address and not forward it" is NOT a solution,
since it appears to Host B to be a perfectly legal DIRECTED broadcast
to network 192.40.1 from somewhere else, and as far as Host B knows,
he is the only way for that directed broadcast to get through; he HAS
to forward it.

Any comments/suggestions/flames?  Are there non-BSD implementations
out there that handle this?  Mods to BSD?  Future plans from vendors?
Anything else?

[This IS a real problem in any situation where there are multiple
independent networks or subnets active on a single cable, and we in
particular have a lot of those situations. Any real live help will be
most appreciated!]

    Wayne Hathaway               domain: wayne@Ultra.COM
    Ultra Network Technologies     uucp: ...!ames!ultra!wayne
    101 Daggett Drive             phone: 408-922-0100 x132
    San Jose, CA 95134           direct: Hey, you!