netdev - Re: [RFC/PATCH] SO_NO

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Date:	Thu, 22 Nov 2007 17:13:06 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jeff@...zik.org
Cc:	yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, herbert@...dor.apana.org.au
Subject: Re: [RFC/PATCH] SO_NO_CHECK for IPv6

From: Jeff Garzik <jeff@...zik.org>
Date: Wed, 21 Nov 2007 19:17:40 -0500

> YOSHIFUJI Hideaki / 吉藤英明 wrote:
> > In article <20071121124532.GA17263@...oc.gtf.org> (at Wed, 21 Nov 2007 07:45:32 -0500), Jeff Garzik <jeff@...zik.org> says:
> > 
> >> SO_NO_CHECK support for IPv6 appeared to be missing. This is presented,
> >> based on a reading of net/ipv4/udp.c.
> > 
> > Disagree. UDP checksum is mandatory in IPv6.
> 
> Ah, you mean that I need to turn off UDP checksum on receive end as well 
> in IPv6...  true.
> 
> For those interested, I am dealing with a UDP app that already does very 
> strong checksumming and encryption, so additional software checksumming 
> at the lower layers is quite simply a waste of CPU cycles.  Hardware 
> checksumming is fine, as long as its "free."

Regardless of whatever verifications your application is doing
on the data, it is not checksumming the ports and that's what
the pseudo-header is helping with.

You cannot disable checksums in ipv6/UDP, they are not optional and
with %99.9999999 of cards doing the checksum in hardware, and even if
we do have to compute it it's free during the copy during recvmsg().
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