231 lines
9.1 KiB
Plaintext
231 lines
9.1 KiB
Plaintext
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 homepge 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 managign 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.
|