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+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 yo 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.