GTP: Add some basic documentation about drivers/net/gtp.c

In order to clarify what the module actually does, and how to use it,
let's add some basic documentation to the kernel tree, together with
pointers to related specs and projects.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/gtp.txt b/Documentation/networking/gtp.txt
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+The Linux kernel GTP tunneling module
+======================================================================
+Documentation by Harald Welte <laforge@gnumonks.org>
+
+In 'drivers/net/gtp.c' you are finding a kernel-level implementation
+of a GTP tunnel endpoint.
+
+== What is GTP ==
+
+GTP is the Generic Tunnel Protocol, which is a 3GPP protocol used for
+tunneling User-IP payload between a mobile station (phone, modem)
+and the interconnection between an external packet data network (such
+as the internet).
+
+So when you start a 'data connection' from your mobile phone, the
+phone will use the control plane to signal for the establishment of
+such a tunnel between that external data network and the phone.  The
+tunnel endpoints thus reside on the phone and in the gateway.  All
+intermediate nodes just transport the encapsulated packet.
+
+The phone itself does not implement GTP but uses some other
+technology-dependent protocol stack for transmitting the user IP
+payload, such as LLC/SNDCP/RLC/MAC.
+
+At some network element inside the cellular operator infrastructure
+(SGSN in case of GPRS/EGPRS or classic UMTS, hNodeB in case of a 3G
+femtocell, eNodeB in case of 4G/LTE), the cellular protocol stacking
+is translated into GTP *without breaking the end-to-end tunnel*.  So
+intermediate nodes just perform some specific relay function.
+
+At some point the GTP packet ends up on the so-called GGSN (GSM/UMTS)
+or P-GW (LTE), which terminates the tunnel, decapsulates the packet
+and forwards it onto an external packet data network.  This can be
+public internet, but can also be any private IP network (or even
+theoretically some non-IP network like X.25).
+
+You can find the protocol specification in 3GPP TS 29.060, available
+publicly via the 3GPP website at http://www.3gpp.org/DynaReport/29060.htm
+
+A direct PDF link to v13.6.0 is provided for convenience below:
+http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.06.00_60/ts_129060v130600p.pdf
+
+== The Linux GTP tunnelling module ==
+
+The module implements the function of a tunnel endpoint, i.e. it is
+able to decapsulate tunneled IP packets in the uplink originated by
+the phone, and encapsulate raw IP packets received from the external
+packet network in downlink towards the phone.
+
+It *only* implements the so-called 'user plane', carrying the User-IP
+payload, called GTP-U.  It does not implement the 'control plane',
+which is a signaling protocol used for establishment and teardown of
+GTP tunnels (GTP-C).
+
+So in order to have a working GGSN/P-GW setup, you will need a
+userspace program that implements the GTP-C protocol and which then
+uses the netlink interface provided by the GTP-U module in the kernel
+to configure the kernel module.
+
+This split architecture follows the tunneling modules of other
+protocols, e.g. PPPoE or L2TP, where you also run a userspace daemon
+to handle the tunnel establishment, authentication etc. and only the
+data plane is accelerated inside the kernel.
+
+Don't be confused by terminology:  The GTP User Plane goes through
+kernel accelerated path, while the GTP Control Plane goes to
+Userspace :)
+
+The official homepge of the module is at
+https://osmocom.org/projects/linux-kernel-gtp-u/wiki
+
+== Userspace Programs with Linux Kernel GTP-U support ==
+
+At the time of this writing, there are at least two Free Software
+implementations that implement GTP-C and can use the netlink interface
+to make use of the Linux kernel GTP-U support:
+
+* OpenGGSN (classic 2G/3G GGSN in C):
+  https://osmocom.org/projects/openggsn/wiki/OpenGGSN
+
+* ergw (GGSN + P-GW in Erlang):
+  https://github.com/travelping/ergw
+
+== Userspace Library / Command Line Utilities ==
+
+There is a userspace library called 'libgtpnl' which is based on
+libmnl and which implements a C-language API towards the netlink
+interface provided by the Kernel GTP module:
+
+http://git.osmocom.org/libgtpnl/
+
+== Protocol Versions ==
+
+There are two different versions of GTP-U: v0 and v1.  Both are
+implemented in the Kernel GTP module.  Version 0 is a legacy version,
+and deprecated from recent 3GPP specifications.
+
+There are three versions of GTP-C: v0, v1, and v2.  As the kernel
+doesn't implement GTP-C, we don't have to worry about this.  It's the
+responsibility of the control plane implementation in userspace to
+implement that.
+
+== IPv6 ==
+
+The 3GPP specifications indicate either IPv4 or IPv6 can be used both
+on the inner (user) IP layer, or on the outer (transport) layer.
+
+Unfortunately, the Kernel module currently supports IPv6 neither for
+the User IP payload, nor for the outer IP layer.  Patches or other
+Contributions to fix this are most welcome!
+
+== Mailing List ==
+
+If yo have questions regarding how to use the Kernel GTP module from
+your own software, or want to contribute to the code, please use the
+osmocom-net-grps mailing list for related discussion. The list can be
+reached at osmocom-net-gprs@lists.osmocom.org and the mailman
+interface for managign your subscription is at
+https://lists.osmocom.org/mailman/listinfo/osmocom-net-gprs
+
+== Issue Tracker ==
+
+The Osmocom project maintains an issue tracker for the Kernel GTP-U
+module at
+https://osmocom.org/projects/linux-kernel-gtp-u/issues
+
+== History / Acknowledgements ==
+
+The Module was originally created in 2012 by Harald Welte, but never
+completed.  Pablo came in to finish the mess Harald left behind.  But
+doe to a lack of user interest, it never got merged.
+
+In 2015, Andreas Schultz came to the rescue and fixed lots more bugs,
+extended it with new features and finally pushed all of us to get it
+mainline, where it was merged in 4.7.0.