1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
|
Osmocom kernel-level GTP implementation
=======================================
Andreas Schultz + Harald Welte
:copyright: Andreas Schultz + Harald Welte (Liense: CC-BY-SA)
:backend: slidy
:max-width: 45em
== 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)
[graphviz]
----
digraph G {
label="GPRS Network"
rankdir=LR;
MS0 [label="MS"]
MS0->BTS0 [label="Um", dir=both]
BTS0 [label="BTS + CCU"]
BTS0->PCU [dir=both]
PCU->SGSN [label="Gb", dir=both]
SGSN->GGSN [label="GTP", dir=both]
}
----
== GTP in UMTS
* In UMTS, PDP originally remained between SGSN and GGSN
[graphviz]
----
digraph G {
label="UMTS Network"
rankdir=LR;
UE [label="UE"]
NB [label="NodeB"]
RNC [label="RNC"]
UE->NB [label="Uu", dir=both]
NB->RNC [label="Iub", dir=both]
RNC->SGSN [label="IuPS", dir=both]
SGSN->GGSN [label="GTP", dir=both]
}
----
* 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
[graphviz]
----
digraph G {
label="UMTS Network / Later Release"
rankdir=LR;
UE [label="UE"]
NB [label="NodeB"]
RNC [label="RNC"]
UE->NB [label="Uu", dir=both]
NB->RNC [label="Iub", dir=both]
RNC->SGSN [label="IuPS", dir=both]
SGSN->GGSN [label="GTP-C", dir=both]
NB->GGSN [label="GTP-U", dir=both]
}
----
== 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
[graphviz]
----
digraph G {
label="LTE Network"
rankdir=LR;
UE [label="UE"]
NB [label="eNodeB"]
UE->NB [label="Uu", dir=both]
NB -> MME [label="S1", dir=both]
NB -> SGW [label="S1-U (GTP-U)", dir=both]
MME -> SGW [label="S11", dir=both]
SGW->PGW [label="S5 (GTP-U)", dir=both]
}
----
== 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
[packetdiag]
----
{
colwidth = 32
node_height = 64
0-7: Flags
8-15: Type
16-31: Length
32-63: Tunnel ID
}
----
== 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
[graphviz]
----
digraph G {
rankdir=LR;
MS0 [label="MS 10.0.0.1"];
MS1 [label="MS 10.0.0.2"];
MS2 [label="MS 10.0.0.3"];
GGSN;
tun0 [shape=box];
MS0 -> SGSN [dir=both];
MS1 -> SGSN [dir=both];
MS2 -> SGSN [dir=both];
SGSN -> GGSN [label="GTP-U", dir=both];
GGSN -> tun0 [dir=none, minlen=0];
tun0 -> SRV [label = "10.0.0.1 - 1.2.3.4", dir=both]
tun0 -> SRV [label = "10.0.0.2 - 1.2.3.4", dir=both]
tun0 -> SRV [label = "10.0.0.3 - 1.2.3.4", dir=both]
}
----
== 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?
|
