|The Linux kernel GTP tunneling module
|Documentation by Harald Welte <firstname.lastname@example.org> and
| Andreas Schultz <email@example.com>
|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:
|== 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
|The official homepage of the module is at
|== 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):
|* ergw (GGSN + P-GW in Erlang):
|== 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:
|== 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
|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
|== 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 firstname.lastname@example.org and the mailman
|interface for managing your subscription is at
|== Issue Tracker ==
|The Osmocom project maintains an issue tracker for the Kernel GTP-U
|== 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.
| * 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
|[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
|Correct is that the Gi/SGi reference point defines the interworking
|between +the 3GPP packet domain (PDN) based on GTP-U tunnel and IP
|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
|> 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.