1PPPoL2TP-Android plugin
2=======================
3
4This PPPoL2TP-Android plugin was written for AOSP project from scratch. It has
5nothing to do with pppol2tp plugin from upstream ppp project [1].
6
7This plugin adds support for upstream kernel L2TP implementation in pppd
8daemon. Only LAC part of L2TP is implemented, as we don't use Android devices
9in LNS mode.
10
11In general case, the execution flow for VPN startup on Android devices is:
12 1. Run IPSec client (racoon)
13 2. Run L2TP client (mtpd)
14 3. mtpd obtains "remote tunnel ID" and "remote session ID" from L2TP server
15 4. mtpd creates L2TP sockets (see [2] for details):
16    - tunnel management PPPoX socket
17    - session PPPoX data socket
18 5. mtpd runs pppd, passing arguments for pppol2tp-android plugin
19 6. pppd dlopen() pppol2tp-android plugin (because it receives corresponding\
20    arguments from mtpd)
21
22Main task of this plugin is to obtain session_fd socket from mtpd and pass it
23back to pppd when .connect() callback is executed. In this respect it's similar
24to old pppox.c implementation. The differences are next:
25 1. pppol2tp-android also obtains the second socket (for tunnel management)
26    which allows us to handle it properly
27 2. pppol2tp-android handles the setting of MTU/MRU size (which can be passed
28    to the plugin with pppd parameters) via kernel L2TP implementation
29 3. pppol2tp-android plugin is actually loaded dynamically with dlopen(),
30    as opposed to pppox.c which is hard-coded into pppd code
31
32[1] https://github.com/paulusmack/ppp
33[2] https://www.kernel.org/doc/Documentation/networking/l2tp.txt
34