netdev - Re: [PATCH net v2] vxlan: calculate correct header length for GPE

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Date: Fri, 21 Jul 2023 13:46:56 +0100
From: Simon Horman <simon.horman@...igine.com>
To: Jiri Benc <jbenc@...hat.com>
Cc: netdev@...r.kernel.org, Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Eyal Birger <eyal.birger@...il.com>
Subject: Re: [PATCH net v2] vxlan: calculate correct header length for GPE

On Thu, Jul 20, 2023 at 11:05:56AM +0200, Jiri Benc wrote:
> VXLAN-GPE does not add an extra inner Ethernet header. Take that into
> account when calculating header length.
> 
> This causes problems in skb_tunnel_check_pmtu, where incorrect PMTU is
> cached.
> 
> In the collect_md mode (which is the only mode that VXLAN-GPE
> supports), there's no magic auto-setting of the tunnel interface MTU.
> It can't be, since the destination and thus the underlying interface
> may be different for each packet.
> 
> So, the administrator is responsible for setting the correct tunnel
> interface MTU. Apparently, the administrators are capable enough to
> calculate that the maximum MTU for VXLAN-GPE is (their_lower_MTU - 36).
> They set the tunnel interface MTU to 1464. If you run a TCP stream over
> such interface, it's then segmented according to the MTU 1464, i.e.
> producing 1514 bytes frames. Which is okay, this still fits the lower
> MTU.
> 
> However, skb_tunnel_check_pmtu (called from vxlan_xmit_one) uses 50 as
> the header size and thus incorrectly calculates the frame size to be
> 1528. This leads to ICMP too big message being generated (locally),
> PMTU of 1450 to be cached and the TCP stream to be resegmented.
> 
> The fix is to use the correct actual header size, especially for
> skb_tunnel_check_pmtu calculation.
> 
> Fixes: e1e5314de08ba ("vxlan: implement GPE")
> Signed-off-by: Jiri Benc <jbenc@...hat.com>
> ---
> v2: more verbose patch description

Reviewed-by: Simon Horman <simon.horman@...igine.com>