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From: barmar@think.com (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: TCP checksums
Message-ID: <1991May28.221045.27724@Think.COM>
Date: 28 May 91 22:10:45 GMT
References: <3270025@hpctdlb.HP.COM>
Sender: news@Think.COM
Reply-To: barmar@think.com
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 18

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?

You must be talking about the *UDP* checksum, which is optional.  The TCP
checksum isn't optional.

Most BSD-derived Unix systems have a kernel variable that controls whether
UDP checksums are generated and checked (it's generally called something
like "udp_cksum", but I've seen variants without the underscore).  In
recent SunOS releases it's configurable in a header file at kernel build
time.  SunOS defaults to having checksums off.
-- 
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar