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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	The Linux kernel GTP tunneling module
2	======================================================================
3	Documentation by Harald Welte <laforge@gnumonks.org>
4	
5	In 'drivers/net/gtp.c' you are finding a kernel-level implementation
6	of a GTP tunnel endpoint.
7	
8	== What is GTP ==
9	
10	GTP is the Generic Tunnel Protocol, which is a 3GPP protocol used for
11	tunneling User-IP payload between a mobile station (phone, modem)
12	and the interconnection between an external packet data network (such
13	as the internet).
14	
15	So when you start a 'data connection' from your mobile phone, the
16	phone will use the control plane to signal for the establishment of
17	such a tunnel between that external data network and the phone.  The
18	tunnel endpoints thus reside on the phone and in the gateway.  All
19	intermediate nodes just transport the encapsulated packet.
20	
21	The phone itself does not implement GTP but uses some other
22	technology-dependent protocol stack for transmitting the user IP
23	payload, such as LLC/SNDCP/RLC/MAC.
24	
25	At some network element inside the cellular operator infrastructure
26	(SGSN in case of GPRS/EGPRS or classic UMTS, hNodeB in case of a 3G
27	femtocell, eNodeB in case of 4G/LTE), the cellular protocol stacking
28	is translated into GTP *without breaking the end-to-end tunnel*.  So
29	intermediate nodes just perform some specific relay function.
30	
31	At some point the GTP packet ends up on the so-called GGSN (GSM/UMTS)
32	or P-GW (LTE), which terminates the tunnel, decapsulates the packet
33	and forwards it onto an external packet data network.  This can be
34	public internet, but can also be any private IP network (or even
35	theoretically some non-IP network like X.25).
36	
37	You can find the protocol specification in 3GPP TS 29.060, available
38	publicly via the 3GPP website at http://www.3gpp.org/DynaReport/29060.htm
39	
40	A direct PDF link to v13.6.0 is provided for convenience below:
41	http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.06.00_60/ts_129060v130600p.pdf
42	
43	== The Linux GTP tunnelling module ==
44	
45	The module implements the function of a tunnel endpoint, i.e. it is
46	able to decapsulate tunneled IP packets in the uplink originated by
47	the phone, and encapsulate raw IP packets received from the external
48	packet network in downlink towards the phone.
49	
50	It *only* implements the so-called 'user plane', carrying the User-IP
51	payload, called GTP-U.  It does not implement the 'control plane',
52	which is a signaling protocol used for establishment and teardown of
53	GTP tunnels (GTP-C).
54	
55	So in order to have a working GGSN/P-GW setup, you will need a
56	userspace program that implements the GTP-C protocol and which then
57	uses the netlink interface provided by the GTP-U module in the kernel
58	to configure the kernel module.
59	
60	This split architecture follows the tunneling modules of other
61	protocols, e.g. PPPoE or L2TP, where you also run a userspace daemon
62	to handle the tunnel establishment, authentication etc. and only the
63	data plane is accelerated inside the kernel.
64	
65	Don't be confused by terminology:  The GTP User Plane goes through
66	kernel accelerated path, while the GTP Control Plane goes to
67	Userspace :)
68	
69	The official homepge of the module is at
70	https://osmocom.org/projects/linux-kernel-gtp-u/wiki
71	
72	== Userspace Programs with Linux Kernel GTP-U support ==
73	
74	At the time of this writing, there are at least two Free Software
75	implementations that implement GTP-C and can use the netlink interface
76	to make use of the Linux kernel GTP-U support:
77	
78	* OpenGGSN (classic 2G/3G GGSN in C):
79	  https://osmocom.org/projects/openggsn/wiki/OpenGGSN
80	
81	* ergw (GGSN + P-GW in Erlang):
82	  https://github.com/travelping/ergw
83	
84	== Userspace Library / Command Line Utilities ==
85	
86	There is a userspace library called 'libgtpnl' which is based on
87	libmnl and which implements a C-language API towards the netlink
88	interface provided by the Kernel GTP module:
89	
90	http://git.osmocom.org/libgtpnl/
91	
92	== Protocol Versions ==
93	
94	There are two different versions of GTP-U: v0 and v1.  Both are
95	implemented in the Kernel GTP module.  Version 0 is a legacy version,
96	and deprecated from recent 3GPP specifications.
97	
98	There are three versions of GTP-C: v0, v1, and v2.  As the kernel
99	doesn't implement GTP-C, we don't have to worry about this.  It's the
100	responsibility of the control plane implementation in userspace to
101	implement that.
102	
103	== IPv6 ==
104	
105	The 3GPP specifications indicate either IPv4 or IPv6 can be used both
106	on the inner (user) IP layer, or on the outer (transport) layer.
107	
108	Unfortunately, the Kernel module currently supports IPv6 neither for
109	the User IP payload, nor for the outer IP layer.  Patches or other
110	Contributions to fix this are most welcome!
111	
112	== Mailing List ==
113	
114	If yo have questions regarding how to use the Kernel GTP module from
115	your own software, or want to contribute to the code, please use the
116	osmocom-net-grps mailing list for related discussion. The list can be
117	reached at osmocom-net-gprs@lists.osmocom.org and the mailman
118	interface for managign your subscription is at
119	https://lists.osmocom.org/mailman/listinfo/osmocom-net-gprs
120	
121	== Issue Tracker ==
122	
123	The Osmocom project maintains an issue tracker for the Kernel GTP-U
124	module at
125	https://osmocom.org/projects/linux-kernel-gtp-u/issues
126	
127	== History / Acknowledgements ==
128	
129	The Module was originally created in 2012 by Harald Welte, but never
130	completed.  Pablo came in to finish the mess Harald left behind.  But
131	doe to a lack of user interest, it never got merged.
132	
133	In 2015, Andreas Schultz came to the rescue and fixed lots more bugs,
134	extended it with new features and finally pushed all of us to get it
135	mainline, where it was merged in 4.7.0.
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