Newsgroups: comp.protocols.tcp-ip
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: TCP checksums
Message-ID: <1991May28.213132.11627@zoo.toronto.edu>
Date: Tue, 28 May 1991 21:31:32 GMT
References: <3270025@hpctdlb.HP.COM>
Organization: U of Toronto Zoology

In article <3270025@hpctdlb.HP.COM> fish@hpctdlb.HP.COM (Dave Fish - Marketing) writes:
>I'm interested in how common it is for TCP implementations to use all zeros
>for the TCP header checksum.  I know that some HP machines do this but 
>how common is this in the real world?

Not very, I hope.  It has never been legal.  UDP allows omission of the
checksum by this means; TCP does not.  RFC 1122:

         4.2.2.7  TCP Checksum: RFC-793 Section 3.1

            Unlike the UDP checksum (see Section 4.1.3.4), the TCP
            checksum is never optional.  The sender MUST generate it and
            the receiver MUST check it.
-- 
"We're thinking about upgrading from    | Henry Spencer @ U of Toronto Zoology
SunOS 4.1.1 to SunOS 3.5."              |  henry@zoo.toronto.edu  utzoo!henry