Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 The Linux kernel GTP tunneling module 2 ====================================================================== 3 Documentation by Harald Welte <email@example.com> and 4 Andreas Schultz <firstname.lastname@example.org> 5 6 In 'drivers/net/gtp.c' you are finding a kernel-level implementation 7 of a GTP tunnel endpoint. 8 9 == What is GTP == 10 11 GTP is the Generic Tunnel Protocol, which is a 3GPP protocol used for 12 tunneling User-IP payload between a mobile station (phone, modem) 13 and the interconnection between an external packet data network (such 14 as the internet). 15 16 So when you start a 'data connection' from your mobile phone, the 17 phone will use the control plane to signal for the establishment of 18 such a tunnel between that external data network and the phone. The 19 tunnel endpoints thus reside on the phone and in the gateway. All 20 intermediate nodes just transport the encapsulated packet. 21 22 The phone itself does not implement GTP but uses some other 23 technology-dependent protocol stack for transmitting the user IP 24 payload, such as LLC/SNDCP/RLC/MAC. 25 26 At some network element inside the cellular operator infrastructure 27 (SGSN in case of GPRS/EGPRS or classic UMTS, hNodeB in case of a 3G 28 femtocell, eNodeB in case of 4G/LTE), the cellular protocol stacking 29 is translated into GTP *without breaking the end-to-end tunnel*. So 30 intermediate nodes just perform some specific relay function. 31 32 At some point the GTP packet ends up on the so-called GGSN (GSM/UMTS) 33 or P-GW (LTE), which terminates the tunnel, decapsulates the packet 34 and forwards it onto an external packet data network. This can be 35 public internet, but can also be any private IP network (or even 36 theoretically some non-IP network like X.25). 37 38 You can find the protocol specification in 3GPP TS 29.060, available 39 publicly via the 3GPP website at http://www.3gpp.org/DynaReport/29060.htm 40 41 A direct PDF link to v13.6.0 is provided for convenience below: 42 http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.06.00_60/ts_129060v130600p.pdf 43 44 == The Linux GTP tunnelling module == 45 46 The module implements the function of a tunnel endpoint, i.e. it is 47 able to decapsulate tunneled IP packets in the uplink originated by 48 the phone, and encapsulate raw IP packets received from the external 49 packet network in downlink towards the phone. 50 51 It *only* implements the so-called 'user plane', carrying the User-IP 52 payload, called GTP-U. It does not implement the 'control plane', 53 which is a signaling protocol used for establishment and teardown of 54 GTP tunnels (GTP-C). 55 56 So in order to have a working GGSN/P-GW setup, you will need a 57 userspace program that implements the GTP-C protocol and which then 58 uses the netlink interface provided by the GTP-U module in the kernel 59 to configure the kernel module. 60 61 This split architecture follows the tunneling modules of other 62 protocols, e.g. PPPoE or L2TP, where you also run a userspace daemon 63 to handle the tunnel establishment, authentication etc. and only the 64 data plane is accelerated inside the kernel. 65 66 Don't be confused by terminology: The GTP User Plane goes through 67 kernel accelerated path, while the GTP Control Plane goes to 68 Userspace :) 69 70 The official homepge of the module is at 71 https://osmocom.org/projects/linux-kernel-gtp-u/wiki 72 73 == Userspace Programs with Linux Kernel GTP-U support == 74 75 At the time of this writing, there are at least two Free Software 76 implementations that implement GTP-C and can use the netlink interface 77 to make use of the Linux kernel GTP-U support: 78 79 * OpenGGSN (classic 2G/3G GGSN in C): 80 https://osmocom.org/projects/openggsn/wiki/OpenGGSN 81 82 * ergw (GGSN + P-GW in Erlang): 83 https://github.com/travelping/ergw 84 85 == Userspace Library / Command Line Utilities == 86 87 There is a userspace library called 'libgtpnl' which is based on 88 libmnl and which implements a C-language API towards the netlink 89 interface provided by the Kernel GTP module: 90 91 http://git.osmocom.org/libgtpnl/ 92 93 == Protocol Versions == 94 95 There are two different versions of GTP-U: v0 [GSM TS 09.60] and v1 96 [3GPP TS 29.281]. Both are implemented in the Kernel GTP module. 97 Version 0 is a legacy version, and deprecated from recent 3GPP 98 specifications. 99 100 GTP-U uses UDP for transporting PDUs. The receiving UDP port is 2151 101 for GTPv1-U and 3386 for GTPv0-U. 102 103 There are three versions of GTP-C: v0, v1, and v2. As the kernel 104 doesn't implement GTP-C, we don't have to worry about this. It's the 105 responsibility of the control plane implementation in userspace to 106 implement that. 107 108 == IPv6 == 109 110 The 3GPP specifications indicate either IPv4 or IPv6 can be used both 111 on the inner (user) IP layer, or on the outer (transport) layer. 112 113 Unfortunately, the Kernel module currently supports IPv6 neither for 114 the User IP payload, nor for the outer IP layer. Patches or other 115 Contributions to fix this are most welcome! 116 117 == Mailing List == 118 119 If yo have questions regarding how to use the Kernel GTP module from 120 your own software, or want to contribute to the code, please use the 121 osmocom-net-grps mailing list for related discussion. The list can be 122 reached at email@example.com and the mailman 123 interface for managign your subscription is at 124 https://lists.osmocom.org/mailman/listinfo/osmocom-net-gprs 125 126 == Issue Tracker == 127 128 The Osmocom project maintains an issue tracker for the Kernel GTP-U 129 module at 130 https://osmocom.org/projects/linux-kernel-gtp-u/issues 131 132 == History / Acknowledgements == 133 134 The Module was originally created in 2012 by Harald Welte, but never 135 completed. Pablo came in to finish the mess Harald left behind. But 136 doe to a lack of user interest, it never got merged. 137 138 In 2015, Andreas Schultz came to the rescue and fixed lots more bugs, 139 extended it with new features and finally pushed all of us to get it 140 mainline, where it was merged in 4.7.0. 141 142 == Architectural Details == 143 144 === Local GTP-U entity and tunnel identification === 145 146 GTP-U uses UDP for transporting PDU's. The receiving UDP port is 2152 147 for GTPv1-U and 3386 for GTPv0-U. 148 149 There is only one GTP-U entity (and therefor SGSN/GGSN/S-GW/PDN-GW 150 instance) per IP address. Tunnel Endpoint Identifier (TEID) are unique 151 per GTP-U entity. 152 153 A specific tunnel is only defined by the destination entity. Since the 154 destination port is constant, only the destination IP and TEID define 155 a tunnel. The source IP and Port have no meaning for the tunnel. 156 157 Therefore: 158 159 * when sending, the remote entity is defined by the remote IP and 160 the tunnel endpoint id. The source IP and port have no meaning and 161 can be changed at any time. 162 163 * when receiving the local entity is defined by the local 164 destination IP and the tunnel endpoint id. The source IP and port 165 have no meaning and can change at any time. 166 167 [3GPP TS 29.281] Section 4.3.0 defines this so: 168 169 > The TEID in the GTP-U header is used to de-multiplex traffic 170 > incoming from remote tunnel endpoints so that it is delivered to the 171 > User plane entities in a way that allows multiplexing of different 172 > users, different packet protocols and different QoS levels. 173 > Therefore no two remote GTP-U endpoints shall send traffic to a 174 > GTP-U protocol entity using the same TEID value except 175 > for data forwarding as part of mobility procedures. 176 177 The definition above only defines that two remote GTP-U endpoints 178 *should not* send to the same TEID, it *does not* forbid or exclude 179 such a scenario. In fact, the mentioned mobility procedures make it 180 necessary that the GTP-U entity accepts traffic for TEIDs from 181 multiple or unknown peers. 182 183 Therefore, the receiving side identifies tunnels exclusively based on 184 TEIDs, not based on the source IP! 185 186 == APN vs. Network Device == 187 188 The GTP-U driver creates a Linux network device for each Gi/SGi 189 interface. 190 191 [3GPP TS 29.281] calls the Gi/SGi reference point an interface. This 192 may lead to the impression that the GGSN/P-GW can have only one such 193 interface. 194 195 Correct is that the Gi/SGi reference point defines the interworking 196 between +the 3GPP packet domain (PDN) based on GTP-U tunnel and IP 197 based networks. 198 199 There is no provision in any of the 3GPP documents that limits the 200 number of Gi/SGi interfaces implemented by a GGSN/P-GW. 201 202 [3GPP TS 29.061] Section 11.3 makes it clear that the selection of a 203 specific Gi/SGi interfaces is made through the Access Point Name 204 (APN): 205 206 > 2. each private network manages its own addressing. In general this 207 > will result in different private networks having overlapping 208 > address ranges. A logically separate connection (e.g. an IP in IP 209 > tunnel or layer 2 virtual circuit) is used between the GGSN/P-GW 210 > and each private network. 211 > 212 > In this case the IP address alone is not necessarily unique. The 213 > pair of values, Access Point Name (APN) and IPv4 address and/or 214 > IPv6 prefixes, is unique. 215 216 In order to support the overlapping address range use case, each APN 217 is mapped to a separate Gi/SGi interface (network device). 218 219 NOTE: The Access Point Name is purely a control plane (GTP-C) concept. 220 At the GTP-U level, only Tunnel Endpoint Identifiers are present in 221 GTP-U packets and network devices are known 222 223 Therefore for a given UE the mapping in IP to PDN network is: 224 * network device + MS IP -> Peer IP + Peer TEID, 225 226 and from PDN to IP network: 227 * local GTP-U IP + TEID -> network device 228 229 Furthermore, before a received T-PDU is injected into the network 230 device the MS IP is checked against the IP recorded in PDP context.