pppd/README.pppol2tp

PPPoL2TP-Android plugin
=======================

This PPPoL2TP-Android plugin was written for AOSP project from scratch. It has
nothing to do with pppol2tp plugin from upstream ppp project [1].

This plugin adds support for upstream kernel L2TP implementation in pppd
daemon. Only LAC part of L2TP is implemented, as we don't use Android devices
in LNS mode.

In general case, the execution flow for VPN startup on Android devices is:
 1. Run IPSec client (racoon)
 2. Run L2TP client (mtpd)
 3. mtpd obtains "remote tunnel ID" and "remote session ID" from L2TP server
 4. mtpd creates L2TP sockets (see [2] for details):
    - tunnel management PPPoX socket
    - session PPPoX data socket
 5. mtpd runs pppd, passing arguments for pppol2tp-android plugin
 6. pppd dlopen() pppol2tp-android plugin (because it receives corresponding\
    arguments from mtpd)

Main task of this plugin is to obtain session_fd socket from mtpd and pass it
back to pppd when .connect() callback is executed. In this respect it's similar
to old pppox.c implementation. The differences are next:
 1. pppol2tp-android also obtains the second socket (for tunnel management)
    which allows us to handle it properly
 2. pppol2tp-android handles the setting of MTU/MRU size (which can be passed
    to the plugin with pppd parameters) via kernel L2TP implementation
 3. pppol2tp-android plugin is actually loaded dynamically with dlopen(),
    as opposed to pppox.c which is hard-coded into pppd code

[1] https://github.com/paulusmack/ppp
[2] https://www.kernel.org/doc/Documentation/networking/l2tp.txt