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1 | The Linux kernel GTP tunneling module |
2 | ====================================================================== | |
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3 | Documentation by Harald Welte <laforge@gnumonks.org> and |
4 | Andreas Schultz <aschultz@tpip.net> | |
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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 | ||
bb38ccce | 70 | The official homepage of the module is at |
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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 | ||
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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. | |
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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 osmocom-net-gprs@lists.osmocom.org and the mailman | |
bb38ccce | 123 | interface for managing your subscription is at |
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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. | |
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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. |