Subject: Re: [RFC PATCH] lro: ip fragment checking
From: Ben Hutchings <bhutchings@solarflare.com>
To: Jan-Bernd Themann <ossthema@de.ibm.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, tklein@de.ibm.com,
       Christoph Raisch <raisch@de.ibm.com>, jb.billaud@gmail.com,
       hering2@de.ibm.com
In-Reply-To: <492ACEC4.3020702@de.ibm.com>
References: <492ACEC4.3020702@de.ibm.com>
Content-Type: text/plain
Organization: Solarflare Communications
Date: Mon, 24 Nov 2008 16:51:08 +0000
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On Mon, 2008-11-24 at 16:56 +0100, Jan-Bernd Themann wrote:
> Currently there is no checking in the LRO receive path whether
> TCP packets are ip fragmented. We should not consider
> those packets for aggregation.
> I'm not sure if this checking is actually required. Does anyone
> know if it is possible to get fragmented TCP packets without
> the tcp stack changing the MSS size?
> This patch introduces explicit checking. Any objections?

LRO depends on the hardware performing TCP checksum offload, and the TCP
checksum cannot be verified for IP fragments in isolation.  So I think
drivers should not be passing fragments into inet_lro or should reject
them in its get_frag_header() or get_skb_header() method.  Certainly sfc
doesn't pass fragments into inet_lro because they have not been
checksummed.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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