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To: Ruslan Ermilov <ru@FreeBSD.ORG>
Cc: Jonathan Lemon <jlemon@flugsvamp.com>,
	Jonathan Lemon <jlemon@FreeBSD.ORG>, net@FreeBSD.ORG
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From: "Louis A. Mamakos" <louie@TransSys.COM>
Subject: Re: Delayed checksums commit broke UDP checksum calculation 
References: <20001116120936.A45755@sunbay.com> <20001116091954.A19895@prism.flugsvamp.com> <20010307123156.A19829@sunbay.com> 
In-reply-to: Your message of "Wed, 07 Mar 2001 12:31:56 +0200."
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> > So that the same logic applies to TCP packets as well.  Currently, we
> > can send a TCP packet with a checksum of 0, which is legal.  Of possible
> > interest is that Linux doesn't do this; they alwyas send a non-zero
> > checksum in the TCP case, if a checksum was computed.
> > 
> Hmm, but why would we do this for TCP?  This violates RFC 793.
> AFAIK, only UDP checksums are special.

0x0000 and 0xFFFF are both 16-bit 1's complement representations of
zero, so you could send either and still have the remote TCP validate
the checksum.

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