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SubjectRe: [PATCH net v2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2
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From: "R. Parameswaran" <parameswaran.r7@gmail.com>
Date: Thu, 22 Sep 2016 13:52:43 -0700 (PDT)

> From ed585bdd6d3d2b3dec58d414f514cd764d89159d Mon Sep 17 00:00:00 2001
> From: "R. Parameswaran" <rparames@brocade.com>
> Date: Thu, 22 Sep 2016 13:19:25 -0700
> Subject: [PATCH] L2TP:Adjust intf MTU,factor underlay L3,overlay L2
>
> Take into account all of the tunnel encapsulation headers when setting
> up the MTU on the L2TP logical interface device. Otherwise, packets
> created by the applications on top of the L2TP layer are larger
> than they ought to be, relative to the underlay MTU, leading to
> needless fragmentation once the outer IP encap is added.
>
> Specifically, take into account the (outer, underlay) IP header
> imposed on the encapsulated L2TP packet, and the Layer 2 header
> imposed on the inner IP packet prior to L2TP encapsulation.
>
> Do not assume an Ethernet (non-jumbo) underlay. Use the PMTU mechanism
> and the dst entry in the L2TP tunnel socket to directly pull up
> the underlay MTU (as the baseline number on top of which the
> encapsulation headers are factored in). Fall back to Ethernet MTU
> if this fails.
>
> Signed-off-by: R. Parameswaran <rparames@brocade.com>
>
> Reviewed-by: "N. Prachanda" <nprachan@brocade.com>,
> Reviewed-by: "R. Shearman" <rshearma@brocade.com>,
> Reviewed-by: "D. Fawcus" <dfawcus@brocade.com>

I have to ask, how do other tunnels over UDP such as VXLAN handle
this problem?

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