From owner-freebsd-hackers  Tue Nov 23 12:10:57 1999
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Date: Tue, 23 Nov 1999 12:09:55 -0800 (PST)
From: Julian Elischer <julian@whistle.com>
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To: Clinton Xavier Berni <xavier@cs.duke.edu>
Cc: "Parthasarathy M. Aji" <partha@cs.duke.edu>,
	freebsd-hackers@FreeBSD.ORG
Subject: Re: ip checksum 
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On Tue, 23 Nov 1999, Clinton Xavier Berni wrote:

> Hey,
> I am sort of confused with the parameters for IPsumReplaceShort function,
>  I understand that I will pass the &ip-ip_sum for the cksump parameter.
> 
> but I am not able to understand wht  oldvap & newvalp stand for..

cksump points to the existing checksum IN THE PACKET
oldvalp points to the old value that will be replaecd IN THE PACKET
newval is the value that will replace the old value.

The function will replace the old value with the new value, and 
update the checksum accordigly.

julian






> 
> Xavier.
> 
> 
> 
> On Mon, 22 Nov 1999, Julian Elischer wrote:
> 
> > How many bytes have you changed?
> > 
> > is it possible that some of the values have already been ntohs()'d
> > or something similar?
> > 
> > rather than recalculate the whole packet, just update the exisitng
> > value.
> > 
> > there is an rfc for this but it took me a while to get 
> > the code right in C on a 386. The trick is getting the 1s complement
> > arithmetic right.
> > 
> > 
> > 
> >  #define FIXSUM16(c, op, np)                                           \
> >     do {                                                                \
> >       (c) -= (u_int16_t) ~*((u_int16_t *) (op));                        \
> >       if ((c) < 0) {                                                    \
> >         (c) += 0xffff;                                                  \
> >       }                                                                 \
> >       (c) -= (u_int16_t)  *((u_int16_t *) (np));                        \
> >       if ((c) < 0) {                                                    \
> >         (c) += 0xffff;                                                  \
> >       }                                                                 \
> >     } while (0)
> > 
> > 
> > /* 
> >  * IpsumReplaceShort()
> >  * 
> >  * Replace a 16 bit aligned (relative to the checksum) 16 bit value
> >  * in a packet and change the IP/TCP/UDP checksum at the same time.
> >  * 
> >  * Works with both big and little endian machines(!)
> >  *    
> >  * If for some wierd reason you want to replace a nonaligned value,
> >  * you need to byteswap it and the old value before doing the
> >  * subtractions.
> >  */
> >       
> > void
> > IpsumReplaceShort(u_int16_t *cksump, u_int16_t *oldvalp, u_int16_t newval)
> > {
> >   register int  cksum;
> >   
> >   cksum = *cksump;
> >   FIXSUM16(cksum, oldvalp, &newval);
> >   *cksump = cksum;
> >   *oldvalp = newval;
> > } 
> > 
> > 
> > On Tue, 23 Nov 1999, Parthasarathy M. Aji wrote:
> > 
> > > Hey,
> > > 
> > > I am trying to recompute the checksum of an IP packet. I use
> > > netinet/in_chksum.c to do this. The values returned are not correct. I've
> > > reset the ip_sum field to 0 before doing the sum. Is there something
> > > missing?   
> > > 
> > > thanks
> > > 
> > > 
> > > 
> > > To Unsubscribe: send mail to majordomo@FreeBSD.org
> > > with "unsubscribe freebsd-hackers" in the body of the message
> > > 
> > 
> > 
> 
> 



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