From: "R. Parameswaran" <redacted>
Date: Thu, 22 Sep 2016 13:52:43 -0700 (PDT)

From ed585bdd6d3d2b3dec58d414f514cd764d89159d Mon Sep 17 00:00:00 2001
From: "R. Parameswaran" <redacted>
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 <redacted>

Reviewed-by: "N. Prachanda" <redacted>,
Reviewed-by: "R. Shearman" <redacted>,
Reviewed-by: "D. Fawcus" <redacted>

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