netdev - Re: [PATCH] lro: IP fragment checking

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Message-ID: <49347B0B.8030705@myri.com>
Date:	Mon, 01 Dec 2008 19:02:19 -0500
From:	Andrew Gallatin <gallatin@...i.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	David Miller <davem@...emloft.net>, ossthema@...ibm.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	tklein@...ibm.com, raisch@...ibm.com, jb.billaud@...il.com,
	hering2@...ibm.com
Subject: Re: [PATCH] lro: IP fragment checking

Ben Hutchings wrote:
> On Mon, 2008-12-01 at 16:53 -0500, Andrew Gallatin wrote:
>> David Miller wrote:
>>> From: Andrew Gallatin <gallatin@...i.com>
>>> Date: Mon, 01 Dec 2008 12:50:15 -0500
>>>
>>>> As to whether or not to do it in the drivers/hardware or in the
>>>> LRO code, I favor doing it in the LRO code just so that it is not
>>>> missed in some driver.
>>> Then there is no point in the hardware doing the check, if
>>> we're going to check it anyways.
>>>
>>> That's part of my point about why this check doesn't belong
>>> here.
>> What hardware does an explicit check for fragmentation?
> 
> Any that implements TCP/UDP checksumming properly.

How many do?

>> In most cases, aren't we just relying on the hardware checksum
>> to be wrong on fragmented packets? That works 99.999% of the time,
>> but the TCP checksum is pretty weak, and it is possible to
>> have a fragmented packet where the first fragment has the same
>> checksum as the entire packet.
> [...]
> 
> If your hardware/firmware wrongly claims to be able to verify the
> TCP/UDP checksum for an IP fragment, it seems to me you should deal with
> that in your driver or fix the firmware.

We do partial checksums.

Drew
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