netdev - Re: [PATCH] l2tp: avoid checksum offload for fragmented packets

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Date:	Mon, 3 Jun 2013 15:44:12 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Tom Parkin <tparkin@...alix.com>
CC:	<netdev@...r.kernel.org>, <jchapman@...alix.com>
Subject: Re: [PATCH] l2tp: avoid checksum offload for fragmented packets

On Mon, 2013-06-03 at 08:49 +0100, Tom Parkin wrote:
> Hardware offload for UDP datagram checksum calculation doesn't work with
> fragmented IP packets -- the device will note the fragmentation and leave the
> UDP checksum well alone.
> 
> As such, if we expect the L2TP packet to be fragmented by the IP layer we need
> to perform the UDP checksum ourselves in software (ref: net/ipv4/udp.c).
>
> This change modifies the L2TP xmit path to fallback to software checksum
> calculation if the L2TP packet + IP header exceeds the tunnel device MTU.
[...]

Surely this should be done in the IP stack when fragmenting, not in any
particular client?

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
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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