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 <firstname.lastname@example.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 email@example.com 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.