Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!A.ISI.EDU!CERF
From: CERF@A.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Pseudo-Headers & Checksumming
Message-ID: <[A.ISI.EDU]17-Jan-88.19:53:04.CERF>
Date: 18 Jan 88 00:53:00 GMT
References: <12365632245.20.STEVENS@A.ISI.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 30

Jim,

I am going through a bunch of messages serially so I haven't yet
seen any responses to your checksum query (if any). 

The pseudo-header was an attempt to fashion a true end/end checksum
at the TCP level which included everything necessary at the TCP level
to be sure you were getting a valid packet from the putative origin.

We considered replicating information from the IP header in the TCP
header as a way of making the TCP header easier to checksum, but
the header was already so big, we decided to try the pseudo-header
approach instead. 

It's possible that we just went too far on the "end/end" road and
could have left some of the information covered in the TCP checksum
out (that is, left it to IP level), but at the time, there was
great concern that the IP level would only be checked host-gateway,
gateway-gateway and gateway-host, not really end/end. So many times
we found problems with lower level subsystems by doing end/end checking
that we allowed ourselves the "awkward" luxury of the TCP pseudo-header.

I seem to recall a recent exchange on TCP-IP in which the TCP level
checksum proved very helpful in protecting against some LAN problems
arising at the IP level, but that only underscores the value of
end/end, not necessarily making an argument for the pseudo-header
we chose for TCP. Really, my recollection of the pseudo header was
to avoid making the TCP and IP headers redundant.

Vint Cerf