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Date: Thu, 16 Nov 2000 09:19:54 -0600
From: Jonathan Lemon <jlemon@flugsvamp.com>
To: Jonathan Lemon <jlemon@FreeBSD.org>, net@FreeBSD.org
Subject: Re: Delayed checksums commit broke UDP checksum calculation
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On Thu, Nov 16, 2000 at 12:09:36PM +0200, Ruslan Ermilov wrote:
> Hi!
> 
> RFC768> If the computed checksum is zero, it is transmitted as all ones
> RFC768> (the equivalent in one's complement arithmetic).  An all zero
> RFC768> transmitted checksum value means that the transmitter generated
> RFC768> no checksum.
> 
> This (0x0000 -> 0xFFFF) apparently got broken in udp_usrreq.c,v 1.65.

Actually, it got broken in a different fashion; I had originally moved
the above logic within in_cksum itself, and when that got taken out,
the UDP case wasn't updated.


> Index: ip_output.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/netinet/ip_output.c,v
> retrieving revision 1.116
> diff -u -p -r1.116 ip_output.c
> --- ip_output.c	2000/11/01 01:59:28	1.116
> +++ ip_output.c	2000/11/16 10:05:06
> @@ -974,6 +974,8 @@ in_delayed_cksum(struct mbuf *m)
>  	ip = mtod(m, struct ip *);
>  	offset = IP_VHL_HL(ip->ip_vhl) << 2 ;
>  	csum = in_cksum_skip(m, ip->ip_len, offset);
> +	if (m->m_pkthdr.csum_flags & CSUM_UDP && csum == 0)
> +		csum = 0xffff;			/* per RFC768 */
>  	offset += m->m_pkthdr.csum_data;	/* checksum offset */
>  
>  	if (offset + sizeof(u_short) > m->m_len) {


This would work.  Alternatively, you could change it to:

	if (csum == 0)
		csum = 0xffff;

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.
--
Jonathan


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