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 .. 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.

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