Hi David,

Thanks for the reply, please see inline:

On Tue, 27 Sep 2016, David Miller wrote:

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

quoted

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?

Specific to Vxlan, it appears to behave similarly.  I haven't functionally 
tested fragmentation on vxlan interfaces, but looking at the
code, it seems to account for the headers involved:


When the vxlan interface is created, from vxlan_dev_create(), in 
vxlan_setup(), it initially starts off with an ethernet MTU:

vxlan_setup(struct net_device *dev)
{
...
...
        ether_setup(dev); <<<<<<< Will set device MTU to 1500


Later, in vxlan_dev_configure(), called from vxlan_dev_create(), it gets 
adjusted to account for the headers:

vxlan_dev_configure():
...
 if (!conf->mtu)
                        dev->mtu = lowerdev->mtu - (use_ipv6 ? 
VXLAN6_HEADROOM : VXLAN_HEADROOM);


where VXLAN_HEADROOM is defined as follows: 

/* IP header + UDP + VXLAN + Ethernet header */
#define VXLAN_HEADROOM (20 + 8 + 8 + 14)
/* IPv6 header + UDP + VXLAN + Ethernet header */
#define VXLAN6_HEADROOM (40 + 8 + 8 + 14)


This seems to match what I see with hand config:

sudo ip link add vxlan0 type vxlan id 42 group 239.1.1.1 dev eth0 dstport 
4789 <<<< (eth0 has an MTU of 1500)


sudo ip -d link show vxlan0
36: vxlan0: <BROADCAST,MULTICAST> mtu 1450 qdisc noop state DOWN mode 
DEFAULT group default <<<< (1450 = 1500 -50)
    link/ether e2:b8:2d:f4:f7:ae brd ff:ff:ff:ff:ff:ff promiscuity 0
    vxlan id 42 group 239.1.1.1 dev eth0 srcport 32768 61000 dstport 4789 
ageing 300

thanks,

Ramkumar