Documentation/networking/gtp.rst

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.. SPDX-License-Identifier: GPL-2.0

=====================================
The Linux kernel GTP tunneling module
=====================================

Documentation by
		 Harald Welte <laforge@gnumonks.org> and
		 Andreas Schultz <aschultz@tpip.net>

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

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