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136 lines
5.4 KiB
Plaintext
136 lines
5.4 KiB
Plaintext
The Linux kernel GTP tunneling module
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======================================================================
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Documentation by Harald Welte <laforge@gnumonks.org>
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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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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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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 homepge 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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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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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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There are two different versions of GTP-U: v0 and v1. Both are
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implemented in the Kernel GTP module. Version 0 is a legacy version,
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and deprecated from recent 3GPP specifications.
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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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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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If yo 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 managign 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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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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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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