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