From owner-freebsd-hackers  Sun Apr 21 13:31:35 1996
Return-Path: owner-hackers
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.3/8.7.3) id NAA05019
          for hackers-outgoing; Sun, 21 Apr 1996 13:31:35 -0700 (PDT)
Received: from labinfo.iet.unipi.it (labinfo.iet.unipi.it [131.114.9.5])
          by freefall.freebsd.org (8.7.3/8.7.3) with SMTP id NAA05012
          for <hackers@FreeBSD.ORG>; Sun, 21 Apr 1996 13:31:28 -0700 (PDT)
Received: from localhost (luigi@localhost) by labinfo.iet.unipi.it (8.6.5/8.6.5) id WAA29029; Sun, 21 Apr 1996 22:26:10 +0200
From: Luigi Rizzo <luigi@labinfo.iet.unipi.it>
Message-Id: <199604212026.WAA29029@labinfo.iet.unipi.it>
Subject: Re: one less checksum ? (fwd)
To: darrenr@vitruvius.arbld.unimelb.edu.au (Darren Reed)
Date: Sun, 21 Apr 1996 22:26:10 +0200 (MET DST)
Cc: hackers@FreeBSD.ORG
In-Reply-To: <199604211351.XAA10529@vitruvius.arbld.unimelb.EDU.AU> from "Darren Reed" at Apr 21, 96 11:50:44 pm
X-Mailer: ELM [version 2.4 PL23]
Content-Type: text
Sender: owner-hackers@FreeBSD.ORG
X-Loop: FreeBSD.org
Precedence: bulk

> 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 ?

TCP/IP Illustrated vol.1 suggests something similar, so I'd say it is
worthwhile. Garret Wollman should be looking after this code right in
these days.

But I don't understand how you relate your code (which looks correct)
with the above comment. It looks like your code is computing the
checksum in the same place, just with a more efficient algorithm.

The same code fragments suggest some more optimizations when
if_forwarding is enabled, e.g. don't convert header fields (ip_len and
ip_off) for back and forth from network format.

	Luigi