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From: CERF@A.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Separation of Layers
Message-ID: <[A.ISI.EDU]30-Oct-87.06:57:13.CERF>
Date: Fri, 30-Oct-87 06:57:00 EST
Article-I.D.: <[A.ISI.EDU]30-Oct-87.06:57:13.CERF>
Posted: Fri Oct 30 06:57:00 1987
Date-Received: Thu, 5-Nov-87 23:30:14 EST
References: <8710300305.AA20607@PARIS.MIT.EDU>
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Organization: The ARPA Internet
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Yaqim,

I would be surprised if multi-homed hosts were implemented in such a way
as to make indistinguishable to the source which net was being used.
Generally, in the scenario you describe, the arriving packet with the 
unexpected source IP address might be thought an intruder by the recipient.
How could you tell the difference between a source which is attempting a
spoof of a multi-homed host and a true multi-homed host? Logical
addressing at the IP level seems a more solid way of proceeding - of
course, this leaves unspoken how the various physical addresses are
bound/validated to the logical IP level address - for some networks,
this is done automatically as in the DDN and ARPANET.

Vint Cerf