What is GTP / GTP in GPRS

GTP, the GPRS Tunneling Protocol was first introduced with GPRS, the packet-switched add-on to GSM networks
PDP contexts are sessions with an external packet data network (IP) which provide a tunnel between the phone and that external network
GTP is used betwen the SGSN and GGSN to tunnel the User-plane (IP) packet data of a PDP Context
latest definition in (3GPP TS 29.281 version 12.1.0 Release 12)

GTP in UMTS

In UMTS, PDP originally remained between SGSN and GGSN

With HSPA in later releases (Rel7+), user plane is moved out of SGSN and NodeB generates GTP directly
hNodeB (femtocell) architecture also originates GTP directly in hNodeB

GTP in LTE

LTE uses GTP at even more interfaces between network elements
LTE introduces GTP-C Version 2 for control plane, but user plane remains GTP-U v1 like before.
What used to be the GGSN is now called PDN-GW in LTE

GTP

control (GTP-C) and user (GTP-U) plane protocol
UDP based on well known ports (3386, 2152)
per PDP context/tunnel identifiers (TID)
- one for each direction(!) in GTP v1
- is the only identifier for tunnel, not IP/Port tuples!
support for packet sequnce numbers and reordering

GTP-C

control protocol for:

exchanging meta data
assigning parameters to client:
- IPv4 addresses
- IPv6 prefixes
- DNS servers
- …
establishing and removing tunnels
moving tunnel endpoints to other SGW’s

GTP-U

two main jobs:

per client tunnelling of IP packets
path maintenance
- echo requests/replies
- error reporting

Problems with Userspace GTP

Userspace GTP was fine with GPRS speeds
- Packet rates / bandwidths small compared to routing
With HSPA and particularly LTE, the world has changed
- Signalling / Control Plane traffic is very limited
- User Plane traffic is very high bandwidth

Osmocom kernel-level GTP

Move bandwidth-critical User Plane into Linux kernel
Keep Control Plane stays in Userspace (OpenGGSN)
Use netlink based API to control user plane in kernel
Started by Harald Welte + Pablo Neira @ sysmocom in 2012
- unfortunately abandoned due to customer disappearing
picked up by Andreas Schultz @ travelping in 2015
- fixed Harald + Pablo’s bugs
- introduce network namespace support

Design

netlink API built on top of genl
initialization:
- create GTP socket in userspace
- create tun like network interface
- bind GTP sockets to it
for each PDP context, user space adds one record
- IP address assigned to MS for this PDP context
- TEIDs to identify mobile-originated packets
- SGSN IP for GTP header of mobile-terminated packets

Design

netlink API built on top of genl
all PDP contexts share one tun device
- one tun device for all subscribers, not one per subscriber

STATUS

working
- GTP-U v0 and v1 for IPv4 over IPv4
- multiple tunnels supported
- only single APN (multiple IP ranges not permited)
Limitations
- no IPv6 support
- no offload support
existing users (both GPLv2)
- OpenGGSN (http://cgit.osmocom.org/openggsn/)
- new ergw Erlang GGSN / P-GW (https://github.com/travelping/ergw)

TODO

TODO:
- final round of clean-up, submission for mainline
- fix or remove IPv6 support
discussion items:
- path MTU discovery
- implications by/for offloading faetures (csum / gso / …)
wishlist
- light-weight tunneling integration

The End

Questions?