linux-kernel - Re: [RFC PATCH] lro: ip fragment checking

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Date:	Mon, 24 Nov 2008 16:51:08 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jan-Bernd Themann <ossthema@...ibm.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	tklein@...ibm.com, Christoph Raisch <raisch@...ibm.com>,
	jb.billaud@...il.com, hering2@...ibm.com
Subject: Re: [RFC PATCH] lro: ip fragment checking

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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