Date: Sun, 21 Apr 1996 23:51:03 +1000 (EST) From: Darren Reed <darrenr@vitruvius.arbld.unimelb.edu.au> To: hackers@freebsd.org Subject: one less checksum ? (fwd) Message-ID: <199604211351.XAA10529@vitruvius.arbld.unimelb.EDU.AU>
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Is there any reason why the result of in_cksum() is stored in ip_sum in
ipintr() ?
A small gain can be observed if this is not done, for forwarded packets,
by altering the checksum in ip_output() if IP_FORWARDING is set rather
than recalculating the entire header checksum. Is this worthwhile ?
darren
*** ip_input.c.orig Wed Sep 6 20:31:35 1995
--- ip_input.c Sun Apr 21 12:12:53 1996
***************
*** 197,204 ****
}
ip = mtod(m, struct ip *);
}
! ip->ip_sum = in_cksum(m, hlen);
! if (ip->ip_sum) {
ipstat.ips_badsum++;
goto bad;
}
--- 201,207 ----
}
ip = mtod(m, struct ip *);
}
! if (in_cksum(m, hlen)) {
ipstat.ips_badsum++;
goto bad;
}
*** ip_output.c.orig Wed Sep 6 20:31:40 1995
--- ip_output.c Sun Apr 21 19:41:53 1996
***************
*** 319,326 ****
if ((u_short)ip->ip_len <= ifp->if_mtu) {
ip->ip_len = htons((u_short)ip->ip_len);
ip->ip_off = htons((u_short)ip->ip_off);
! ip->ip_sum = 0;
! ip->ip_sum = in_cksum(m, hlen);
error = (*ifp->if_output)(ifp, m,
(struct sockaddr *)dst, ro->ro_rt);
goto done;
--- 338,352 ----
if ((u_short)ip->ip_len <= ifp->if_mtu) {
ip->ip_len = htons((u_short)ip->ip_len);
ip->ip_off = htons((u_short)ip->ip_off);
! if (flags & IP_FORWARDING) {
! u_long sum = (u_long)ip->ip_sum;
! sum++;
! sum += (sum >> 16);
! ip->ip_sum = (u_short)(sum & 0x0000ffff);
! } else {
! ip->ip_sum = 0;
! ip->ip_sum = in_cksum(m, hlen);
! }
error = (*ifp->if_output)(ifp, m,
(struct sockaddr *)dst, ro->ro_rt);
goto done;
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