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diff --git a/Documentation/networking/gtp.rst b/Documentation/networking/gtp.rst new file mode 100644 index 000000000..1563fb94b --- /dev/null +++ b/Documentation/networking/gtp.rst @@ -0,0 +1,251 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================================== +The Linux kernel GTP tunneling module +===================================== + +Documentation by + Harald Welte <laforge@gnumonks.org> and + Andreas Schultz <aschultz@tpip.net> + +In 'drivers/net/gtp.c' you are finding a kernel-level implementation +of a GTP tunnel endpoint. + +What is GTP +=========== + +GTP is the Generic Tunnel Protocol, which is a 3GPP protocol used for +tunneling User-IP payload between a mobile station (phone, modem) +and the interconnection between an external packet data network (such +as the internet). + +So when you start a 'data connection' from your mobile phone, the +phone will use the control plane to signal for the establishment of +such a tunnel between that external data network and the phone. The +tunnel endpoints thus reside on the phone and in the gateway. All +intermediate nodes just transport the encapsulated packet. + +The phone itself does not implement GTP but uses some other +technology-dependent protocol stack for transmitting the user IP +payload, such as LLC/SNDCP/RLC/MAC. + +At some network element inside the cellular operator infrastructure +(SGSN in case of GPRS/EGPRS or classic UMTS, hNodeB in case of a 3G +femtocell, eNodeB in case of 4G/LTE), the cellular protocol stacking +is translated into GTP *without breaking the end-to-end tunnel*. So +intermediate nodes just perform some specific relay function. + +At some point the GTP packet ends up on the so-called GGSN (GSM/UMTS) +or P-GW (LTE), which terminates the tunnel, decapsulates the packet +and forwards it onto an external packet data network. This can be +public internet, but can also be any private IP network (or even +theoretically some non-IP network like X.25). + +You can find the protocol specification in 3GPP TS 29.060, available +publicly via the 3GPP website at http://www.3gpp.org/DynaReport/29060.htm + +A direct PDF link to v13.6.0 is provided for convenience below: +http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.06.00_60/ts_129060v130600p.pdf + +The Linux GTP tunnelling module +=============================== + +The module implements the function of a tunnel endpoint, i.e. it is +able to decapsulate tunneled IP packets in the uplink originated by +the phone, and encapsulate raw IP packets received from the external +packet network in downlink towards the phone. + +It *only* implements the so-called 'user plane', carrying the User-IP +payload, called GTP-U. It does not implement the 'control plane', +which is a signaling protocol used for establishment and teardown of +GTP tunnels (GTP-C). + +So in order to have a working GGSN/P-GW setup, you will need a +userspace program that implements the GTP-C protocol and which then +uses the netlink interface provided by the GTP-U module in the kernel +to configure the kernel module. + +This split architecture follows the tunneling modules of other +protocols, e.g. PPPoE or L2TP, where you also run a userspace daemon +to handle the tunnel establishment, authentication etc. and only the +data plane is accelerated inside the kernel. + +Don't be confused by terminology: The GTP User Plane goes through +kernel accelerated path, while the GTP Control Plane goes to +Userspace :) + +The official homepage of the module is at +https://osmocom.org/projects/linux-kernel-gtp-u/wiki + +Userspace Programs with Linux Kernel GTP-U support +================================================== + +At the time of this writing, there are at least two Free Software +implementations that implement GTP-C and can use the netlink interface +to make use of the Linux kernel GTP-U support: + +* OpenGGSN (classic 2G/3G GGSN in C): + https://osmocom.org/projects/openggsn/wiki/OpenGGSN + +* ergw (GGSN + P-GW in Erlang): + https://github.com/travelping/ergw + +Userspace Library / Command Line Utilities +========================================== + +There is a userspace library called 'libgtpnl' which is based on +libmnl and which implements a C-language API towards the netlink +interface provided by the Kernel GTP module: + +http://git.osmocom.org/libgtpnl/ + +Protocol Versions +================= + +There are two different versions of GTP-U: v0 [GSM TS 09.60] and v1 +[3GPP TS 29.281]. Both are implemented in the Kernel GTP module. +Version 0 is a legacy version, and deprecated from recent 3GPP +specifications. + +GTP-U uses UDP for transporting PDUs. The receiving UDP port is 2151 +for GTPv1-U and 3386 for GTPv0-U. + +There are three versions of GTP-C: v0, v1, and v2. As the kernel +doesn't implement GTP-C, we don't have to worry about this. It's the +responsibility of the control plane implementation in userspace to +implement that. + +IPv6 +==== + +The 3GPP specifications indicate either IPv4 or IPv6 can be used both +on the inner (user) IP layer, or on the outer (transport) layer. + +Unfortunately, the Kernel module currently supports IPv6 neither for +the User IP payload, nor for the outer IP layer. Patches or other +Contributions to fix this are most welcome! + +Mailing List +============ + +If you have questions regarding how to use the Kernel GTP module from +your own software, or want to contribute to the code, please use the +osmocom-net-grps mailing list for related discussion. The list can be +reached at osmocom-net-gprs@lists.osmocom.org and the mailman +interface for managing your subscription is at +https://lists.osmocom.org/mailman/listinfo/osmocom-net-gprs + +Issue Tracker +============= + +The Osmocom project maintains an issue tracker for the Kernel GTP-U +module at +https://osmocom.org/projects/linux-kernel-gtp-u/issues + +History / Acknowledgements +========================== + +The Module was originally created in 2012 by Harald Welte, but never +completed. Pablo came in to finish the mess Harald left behind. But +doe to a lack of user interest, it never got merged. + +In 2015, Andreas Schultz came to the rescue and fixed lots more bugs, +extended it with new features and finally pushed all of us to get it +mainline, where it was merged in 4.7.0. + +Architectural Details +===================== + +Local GTP-U entity and tunnel identification +-------------------------------------------- + +GTP-U uses UDP for transporting PDU's. The receiving UDP port is 2152 +for GTPv1-U and 3386 for GTPv0-U. + +There is only one GTP-U entity (and therefor SGSN/GGSN/S-GW/PDN-GW +instance) per IP address. Tunnel Endpoint Identifier (TEID) are unique +per GTP-U entity. + +A specific tunnel is only defined by the destination entity. Since the +destination port is constant, only the destination IP and TEID define +a tunnel. The source IP and Port have no meaning for the tunnel. + +Therefore: + + * when sending, the remote entity is defined by the remote IP and + the tunnel endpoint id. The source IP and port have no meaning and + can be changed at any time. + + * when receiving the local entity is defined by the local + destination IP and the tunnel endpoint id. The source IP and port + have no meaning and can change at any time. + +[3GPP TS 29.281] Section 4.3.0 defines this so:: + + The TEID in the GTP-U header is used to de-multiplex traffic + incoming from remote tunnel endpoints so that it is delivered to the + User plane entities in a way that allows multiplexing of different + users, different packet protocols and different QoS levels. + Therefore no two remote GTP-U endpoints shall send traffic to a + GTP-U protocol entity using the same TEID value except + for data forwarding as part of mobility procedures. + +The definition above only defines that two remote GTP-U endpoints +*should not* send to the same TEID, it *does not* forbid or exclude +such a scenario. In fact, the mentioned mobility procedures make it +necessary that the GTP-U entity accepts traffic for TEIDs from +multiple or unknown peers. + +Therefore, the receiving side identifies tunnels exclusively based on +TEIDs, not based on the source IP! + +APN vs. Network Device +====================== + +The GTP-U driver creates a Linux network device for each Gi/SGi +interface. + +[3GPP TS 29.281] calls the Gi/SGi reference point an interface. This +may lead to the impression that the GGSN/P-GW can have only one such +interface. + +Correct is that the Gi/SGi reference point defines the interworking +between +the 3GPP packet domain (PDN) based on GTP-U tunnel and IP +based networks. + +There is no provision in any of the 3GPP documents that limits the +number of Gi/SGi interfaces implemented by a GGSN/P-GW. + +[3GPP TS 29.061] Section 11.3 makes it clear that the selection of a +specific Gi/SGi interfaces is made through the Access Point Name +(APN):: + + 2. each private network manages its own addressing. In general this + will result in different private networks having overlapping + address ranges. A logically separate connection (e.g. an IP in IP + tunnel or layer 2 virtual circuit) is used between the GGSN/P-GW + and each private network. + + In this case the IP address alone is not necessarily unique. The + pair of values, Access Point Name (APN) and IPv4 address and/or + IPv6 prefixes, is unique. + +In order to support the overlapping address range use case, each APN +is mapped to a separate Gi/SGi interface (network device). + +.. note:: + + The Access Point Name is purely a control plane (GTP-C) concept. + At the GTP-U level, only Tunnel Endpoint Identifiers are present in + GTP-U packets and network devices are known + +Therefore for a given UE the mapping in IP to PDN network is: + + * network device + MS IP -> Peer IP + Peer TEID, + +and from PDN to IP network: + + * local GTP-U IP + TEID -> network device + +Furthermore, before a received T-PDU is injected into the network +device the MS IP is checked against the IP recorded in PDP context. |