Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!A.ISI.EDU!CERF
From: CERF@A.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Pseudo headers in TCP/UDP checksum -- Why?
Message-ID: <[A.ISI.EDU]17-Feb-89.19:03:51.CERF>
Date: 18 Feb 89 00:03:00 GMT
References: <10010@pasteur.Berkeley.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 14

Mike,

We wanted to make sure that we had true end/end (at TCP level)
checking since the IP level passed through gateways as was potentially
modified en route. We didn't want to literally duplicate information
in the TCP header which was carried in the IP header, so we formed
a pseudo header for efficiency (taking some of the IP fields which
were not supposed to change and making them play a part in the
TCP level checking).

Vint Cerf

p.s. This may sound like a "layer crossing violation" but was
just a way of not duplicating header information.