1 .. SPDX-License-Identifier: GPL-2.0 2 3 ===================================== 4 The Linux kernel GTP tunneling module 5 ===================================== 6 7 Documentation by 8 Harald Welte <laforge@gnumonks.org> and 9 Andreas Schultz <aschultz@tpip.net> 10 11 In 'drivers/net/gtp.c' you are finding a kernel-level implementation 12 of a GTP tunnel endpoint. 13 14 What is GTP 15 =========== 16 17 GTP is the Generic Tunnel Protocol, which is a 3GPP protocol used for 18 tunneling User-IP payload between a mobile station (phone, modem) 19 and the interconnection between an external packet data network (such 20 as the internet). 21 22 So when you start a 'data connection' from your mobile phone, the 23 phone will use the control plane to signal for the establishment of 24 such a tunnel between that external data network and the phone. The 25 tunnel endpoints thus reside on the phone and in the gateway. All 26 intermediate nodes just transport the encapsulated packet. 27 28 The phone itself does not implement GTP but uses some other 29 technology-dependent protocol stack for transmitting the user IP 30 payload, such as LLC/SNDCP/RLC/MAC. 31 32 At some network element inside the cellular operator infrastructure 33 (SGSN in case of GPRS/EGPRS or classic UMTS, hNodeB in case of a 3G 34 femtocell, eNodeB in case of 4G/LTE), the cellular protocol stacking 35 is translated into GTP *without breaking the end-to-end tunnel*. So 36 intermediate nodes just perform some specific relay function. 37 38 At some point the GTP packet ends up on the so-called GGSN (GSM/UMTS) 39 or P-GW (LTE), which terminates the tunnel, decapsulates the packet 40 and forwards it onto an external packet data network. This can be 41 public internet, but can also be any private IP network (or even 42 theoretically some non-IP network like X.25). 43 44 You can find the protocol specification in 3GPP TS 29.060, available 45 publicly via the 3GPP website at http://www.3gpp.org/DynaReport/29060.htm 46 47 A direct PDF link to v13.6.0 is provided for convenience below: 48 http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.06.00_60/ts_129060v130600p.pdf 49 50 The Linux GTP tunnelling module 51 =============================== 52 53 The module implements the function of a tunnel endpoint, i.e. it is 54 able to decapsulate tunneled IP packets in the uplink originated by 55 the phone, and encapsulate raw IP packets received from the external 56 packet network in downlink towards the phone. 57 58 It *only* implements the so-called 'user plane', carrying the User-IP 59 payload, called GTP-U. It does not implement the 'control plane', 60 which is a signaling protocol used for establishment and teardown of 61 GTP tunnels (GTP-C). 62 63 So in order to have a working GGSN/P-GW setup, you will need a 64 userspace program that implements the GTP-C protocol and which then 65 uses the netlink interface provided by the GTP-U module in the kernel 66 to configure the kernel module. 67 68 This split architecture follows the tunneling modules of other 69 protocols, e.g. PPPoE or L2TP, where you also run a userspace daemon 70 to handle the tunnel establishment, authentication etc. and only the 71 data plane is accelerated inside the kernel. 72 73 Don't be confused by terminology: The GTP User Plane goes through 74 kernel accelerated path, while the GTP Control Plane goes to 75 Userspace :) 76 77 The official homepage of the module is at 78 https://osmocom.org/projects/linux-kernel-gtp-u/wiki 79 80 Userspace Programs with Linux Kernel GTP-U support 81 ================================================== 82 83 At the time of this writing, there are at least two Free Software 84 implementations that implement GTP-C and can use the netlink interface 85 to make use of the Linux kernel GTP-U support: 86 87 * OpenGGSN (classic 2G/3G GGSN in C): 88 https://osmocom.org/projects/openggsn/wiki/OpenGGSN 89 90 * ergw (GGSN + P-GW in Erlang): 91 https://github.com/travelping/ergw 92 93 Userspace Library / Command Line Utilities 94 ========================================== 95 96 There is a userspace library called 'libgtpnl' which is based on 97 libmnl and which implements a C-language API towards the netlink 98 interface provided by the Kernel GTP module: 99 100 http://git.osmocom.org/libgtpnl/ 101 102 Protocol Versions 103 ================= 104 105 There are two different versions of GTP-U: v0 [GSM TS 09.60] and v1 106 [3GPP TS 29.281]. Both are implemented in the Kernel GTP module. 107 Version 0 is a legacy version, and deprecated from recent 3GPP 108 specifications. 109 110 GTP-U uses UDP for transporting PDUs. The receiving UDP port is 2151 111 for GTPv1-U and 3386 for GTPv0-U. 112 113 There are three versions of GTP-C: v0, v1, and v2. As the kernel 114 doesn't implement GTP-C, we don't have to worry about this. It's the 115 responsibility of the control plane implementation in userspace to 116 implement that. 117 118 IPv6 119 ==== 120 121 The 3GPP specifications indicate either IPv4 or IPv6 can be used both 122 on the inner (user) IP layer, or on the outer (transport) layer. 123 124 Unfortunately, the Kernel module currently supports IPv6 neither for 125 the User IP payload, nor for the outer IP layer. Patches or other 126 Contributions to fix this are most welcome! 127 128 Mailing List 129 ============ 130 131 If you have questions regarding how to use the Kernel GTP module from 132 your own software, or want to contribute to the code, please use the 133 osmocom-net-grps mailing list for related discussion. The list can be 134 reached at osmocom-net-gprs@lists.osmocom.org and the mailman 135 interface for managing your subscription is at 136 https://lists.osmocom.org/mailman/listinfo/osmocom-net-gprs 137 138 Issue Tracker 139 ============= 140 141 The Osmocom project maintains an issue tracker for the Kernel GTP-U 142 module at 143 https://osmocom.org/projects/linux-kernel-gtp-u/issues 144 145 History / Acknowledgements 146 ========================== 147 148 The Module was originally created in 2012 by Harald Welte, but never 149 completed. Pablo came in to finish the mess Harald left behind. But 150 doe to a lack of user interest, it never got merged. 151 152 In 2015, Andreas Schultz came to the rescue and fixed lots more bugs, 153 extended it with new features and finally pushed all of us to get it 154 mainline, where it was merged in 4.7.0. 155 156 Architectural Details 157 ===================== 158 159 Local GTP-U entity and tunnel identification 160 -------------------------------------------- 161 162 GTP-U uses UDP for transporting PDU's. The receiving UDP port is 2152 163 for GTPv1-U and 3386 for GTPv0-U. 164 165 There is only one GTP-U entity (and therefore SGSN/GGSN/S-GW/PDN-GW 166 instance) per IP address. Tunnel Endpoint Identifier (TEID) are unique 167 per GTP-U entity. 168 169 A specific tunnel is only defined by the destination entity. Since the 170 destination port is constant, only the destination IP and TEID define 171 a tunnel. The source IP and Port have no meaning for the tunnel. 172 173 Therefore: 174 175 * when sending, the remote entity is defined by the remote IP and 176 the tunnel endpoint id. The source IP and port have no meaning and 177 can be changed at any time. 178 179 * when receiving the local entity is defined by the local 180 destination IP and the tunnel endpoint id. The source IP and port 181 have no meaning and can change at any time. 182 183 [3GPP TS 29.281] Section 4.3.0 defines this so:: 184 185 The TEID in the GTP-U header is used to de-multiplex traffic 186 incoming from remote tunnel endpoints so that it is delivered to the 187 User plane entities in a way that allows multiplexing of different 188 users, different packet protocols and different QoS levels. 189 Therefore no two remote GTP-U endpoints shall send traffic to a 190 GTP-U protocol entity using the same TEID value except 191 for data forwarding as part of mobility procedures. 192 193 The definition above only defines that two remote GTP-U endpoints 194 *should not* send to the same TEID, it *does not* forbid or exclude 195 such a scenario. In fact, the mentioned mobility procedures make it 196 necessary that the GTP-U entity accepts traffic for TEIDs from 197 multiple or unknown peers. 198 199 Therefore, the receiving side identifies tunnels exclusively based on 200 TEIDs, not based on the source IP! 201 202 APN vs. Network Device 203 ====================== 204 205 The GTP-U driver creates a Linux network device for each Gi/SGi 206 interface. 207 208 [3GPP TS 29.281] calls the Gi/SGi reference point an interface. This 209 may lead to the impression that the GGSN/P-GW can have only one such 210 interface. 211 212 Correct is that the Gi/SGi reference point defines the interworking 213 between +the 3GPP packet domain (PDN) based on GTP-U tunnel and IP 214 based networks. 215 216 There is no provision in any of the 3GPP documents that limits the 217 number of Gi/SGi interfaces implemented by a GGSN/P-GW. 218 219 [3GPP TS 29.061] Section 11.3 makes it clear that the selection of a 220 specific Gi/SGi interfaces is made through the Access Point Name 221 (APN):: 222 223 2. each private network manages its own addressing. In general this 224 will result in different private networks having overlapping 225 address ranges. A logically separate connection (e.g. an IP in IP 226 tunnel or layer 2 virtual circuit) is used between the GGSN/P-GW 227 and each private network. 228 229 In this case the IP address alone is not necessarily unique. The 230 pair of values, Access Point Name (APN) and IPv4 address and/or 231 IPv6 prefixes, is unique. 232 233 In order to support the overlapping address range use case, each APN 234 is mapped to a separate Gi/SGi interface (network device). 235 236 .. note:: 237 238 The Access Point Name is purely a control plane (GTP-C) concept. 239 At the GTP-U level, only Tunnel Endpoint Identifiers are present in 240 GTP-U packets and network devices are known 241 242 Therefore for a given UE the mapping in IP to PDN network is: 243 244 * network device + MS IP -> Peer IP + Peer TEID, 245 246 and from PDN to IP network: 247 248 * local GTP-U IP + TEID -> network device 249 250 Furthermore, before a received T-PDU is injected into the network 251 device the MS IP is checked against the IP recorded in PDP context.
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