Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!ucbvax!VENERA.ISI.EDU!braden
From: braden@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: UDP Checksumming of IP addresses
Message-ID: <9011080038.AA11962@braden.isi.edu>
Date: 8 Nov 90 00:38:34 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 16

There are two answers, one historical and the other architectural.

Historically, the early versions of TCP/IP followed the original
Cerk&Kahn paper in having a single protocol.  Only later (version 4)
was it split into an IP and a Transport layer.  When it was split,
the addressing information was split between the two layers.  But
who wants to change the checksum definition...?

The architectural justification was that the transport layer should be
able to verify the integrity of its addressing information no matter
what lower-layer protocol it runs over.  "Don't trust ANYBODY".  The
TCP and UDP specs say, "This gives protection against misrouted
segments".  Actually, all it probably gives you is protection
against software bugs.

Bob Braden