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From: bostic@ucbvax.BERKELEY.EDU (Keith Bostic)
Newsgroups: comp.bugs.4bsd
Subject: Re: 4.[23]BSD TCP checksum problem
Message-ID: <21219@ucbvax.BERKELEY.EDU>
Date: Fri, 9-Oct-87 15:11:33 EDT
Article-I.D.: ucbvax.21219
Posted: Fri Oct  9 15:11:33 1987
Date-Received: Sun, 11-Oct-87 15:59:43 EDT
References: <1985@encore.UUCP>
Organization: University of California at Berkeley
Lines: 20
Keywords: don't turn it off
Summary: checksum of zero is meaningless

In article <1985@encore.UUCP>, loverso@encore.UUCP (John LoVerso) writes:
> Index:	netinet/tcp_input.c 4.[23]BSD +FIX
> 
> Description:
> 	If you turn off checking of TCP checksums (via the tcpcksum
> 	variable in tcp_input.c), then checksums on outgoing datagrams
> 	are computed incorrectly.  When checking is off, the checksum
> 	on an incoming frame isn't recalculated, and thus doesn't get
> 	set to zero.  When tcp_respond() (of tcp_subr.c) is called, it
> 	reuses the tcpiphdr of the received frame and a garbage TCP
> 	checksum gets calculated.

A checksum value of zero has no special meaning to tcp, i.e. if you
choose not to compute a checksum there is no way to indicate this.
The only reasonable alternative here to to change the meaning of
tcp_cksum to be that checksums are generated on outgoing packets but
not validated on incoming packets, otherwise running with tcp_cksum
disabled is non-standard.

--keith