[RP-PPPoE] PPPoE MTU problem

[Wenzhuo Zhang]

Hi,

The PPPoE Access Concentrator of my ISP offers MRU values of both 1492
and 1442. ppp-2.4.5 in Ubuntu 14.04 adopts 1492 as the MTU of the ppp
device, while ppp-2.4.7 in Ubuntu 16.04 (xenial developement) adopts
1442. Choosing the lower MTU on the WAN link (ppp0) exposes users to
the nasty PMTU-D problem, especially for remote VPN clients. I also
reproduced the same problem in openwrt 15.05 and dd-wrt r29085, both
of which come with ppp-2.4.7.

Based on "The Point-to-Point Protocol (PPP), 6.1. Maximum-Receive-Unit
(MRU)"<https://tools.ietf.org/html/rfc1661#page-41> and "A Method for
Transmitting PPP Over Ethernet (PPPoE), 7. LCP
Considerations"<https://tools.ietf.org/html/rfc2516#page-7>, I think
pppd should choose the larger MRU as the MTU of ppp devices.

Please check https://github.com/paulusmack/ppp/issues/48 for detailed
LCP messages. Detailed log is available at
https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1547754.

Wenzhuo