GPRS Tunneling Protocol (GTP)

Agenda

GPRS/UMTS Overview

GTP Overview

APNs

GTP Details

GiGn

Gc

Gp

Signalling and Data TransferInterface

Signalling Interface

TEPDN

Iu

UTRANTE MT

Gr or Gc

HLR

Other PLMN

SGSN GGSN

SGSN

GTP-MAPprotocol

convertingGSN

Gn

Iu

TE MT UTRAN

SGSN

Gn

BSS

GbTE MT

Gn

GPRS/UMTS Logical Architecture

User Plan for GPRS/UMTS

Relay

NetworkService

GTP-U

Application

IP / X.25

SNDCP

LLC

RLC

MAC

GSM RF

SNDCP

LLC

BSSGP

L1bis

RLC

MAC

GSM RF

BSSGP

L1bis

Relay

L2

L1

IP

L2

L1

IP

GTP-U

IP / X.25

Um Gb Gn GiMS BSS SGSN GGSN

NetworkService

UDP /TCP

UDP /TCP

L1

RLC

PDCP

MAC

IP,PPP,OSP

Application

L1

RLC

PDCP

MAC

ATM

UDP/IP

GTP-U

AAL5

Relay

L1

UDP/IP

L2

GTP-U

IP,PPP,OSP

3G-SGSNUTRANMS

Iu-PSUu Gn Gi

3G-GGSN

ATM

UDP/IP

GTP-U

AAL5

L1

UDP/IP

GTP-U

L2

Relay

GPRS Tunneling Protocol is

• The protocol between GPRS Support Nodes• Includes both the GTP signaling (GTP-C) & data transfer (GTP-U) procedures • Is defined for the Gn interface, i.e. the interface between

GSNs within a PLMN &• For the Gp interface between GSNs in different PLMNs• Allows multi-protocol packets to be tunneled through the UMTS/GPRS backbone

GTP Overview

GTP Overview (Cont.) GTP-C is• the signaling plane and specifies a tunnel control and management protocol• is used to create, delete modify tunnels.

GTP-U is • the transmission plan & uses a tunneling mechanism (GTP-U) to provide a service for carrying user data packets

DefinitionsT-PDU – Original data packetG-PDU – Original data packet (T-PDU) + GTP header

GTP Header

Version PT (*) E S PN

Message Type

Length (1st Octet)

Length (2nd Octet)

Tunnel Endpoint Identifier (1st Octet)

Tunnel Endpoint Identifier (2nd Octet)

Tunnel Endpoint Identifier (3rd Octet)

Tunnel Endpoint Identifier (4th Octet)

Sequence Number (1st Octet)

Sequence Number (2nd Octet)

N-PDU number

Next Extension Header Type

8 7 6 5 4 3 2 1Octets123456789101112

SGSN specific

GTP Messages

GTP defines a set of messages. These are classified into

• Path Management messages

• Tunnel Management messages

• Location Management messages

• Mobility Management messages

Call set up and tear down

Create PDP Context req.

Create PDP Context resp.

G-PDU (data transfer)

Delete PDP Context req.

Delete PDP Context resp.

SGSN GGSN

Tunnel Management Messages• Create PDP Context Request• Create PDP Context Response

• Delete PDP Context Request• Delete PDP Context Response

• Update PDP Context request• Update PDP Context Response

• PDU Notification request • PDU Notification response• PDU Notification reject request• PDU Notification reject response

Used for n/w initiated PDP context

Used for SGSN handoffDHCP relay of PDP address

Access Point Name

An APN identifies a PDN that is configured on and accessible from A GGSN. APN has two parts.

• APN Network Identifier • APN Operator Identifier

The APN network identifier must correspond to a fully qualified name in DNS e.g. starentnetworks.com The access Points that are supported by the GGSN are preconfigured on the GGSN. APN is sent in the create PDP context request message to GGSN. APN has several attributes associated associated with its configuration which specify how the user can access the network at the entry point.

APN AttributesSome of the attributes are - APN Network Identifier - Access Mode – Transparent (no authentication) / Non-transparent- IP address pool- Primary DHCP server- Secondary DHCP server- DHCP gateway address- RADIUS server- Backup RADIUS server- IP address of the next hop for the Gi interfaceAttributes to get logs- Current allocated IP address count- Current PDP context count

Path Management Messages

Path management messages can be sent between any type of GSN’s.

• Echo Request – No elements

• Echo Response – Recovery

• Version not supported – No elements

• Supported Extension Header Notifications – Extension header type list

Echo Request/Response Message

• Echo request is send on a path to another GSN to find out if the Peer GSN is alive.• The periodicity is implementation dependent but is > 60 secs.• The Echo response is sent with the local restart counter of the GSN.• The GSN receiving the response would compare the restart counter value to the previous stored value to check if the GSN has restarted.• The receiving GSN clears off all the PDP context belonging to the restarted GSN.• Every GSN maintains its own restart counter in non-volatile memory and its peers restart counters in volatile memory.

Tunnel Management Messages (cont.)Create PDP Context Request

IMSI Conditional

Recovery Optional

Selection Mode Conditional

Tunnel End Point Identifies Data 1 Mandatory

Tunnel Endpoint Identifier Control Plan Conditional

NSAPI Mandatory

Linked NSAPI Conditional

Charging Characteristics Optional

Trace Reference Optional

Trace Type Optional

End User Address Conditional

Access Point Name Conditional

Protocol Configuration Options Conditional

SGSN Address for signaling Mandatory

SGSN Address for user traffic Mandatory

MSISDN Conditional

Quality of service Profile Mandatory

TFT Conditional

Trigger ID Optional

OMC identity Optional

Private Extension Optional

Tunnel Management Messages (cont.)Create PDP Context response

Information Element Presence required

Cause Mandatory

Reordering required Conditional

Recovery Optional

Tunnel Endpoint Identifier Data 1 Conditional

Tunnel Endpoint Id. control Plane Conditional

Changing ID Conditional

End User Address Conditional

Protocol Configuration Options Optional

GGSN Address for Control Plane Conditional

GGSN Address for Use Traffic Conditional

Quality of Service profile Conditional

Charging Gateway Address Optional

Private Extension Optional

* Private extension contains vendor or operator specific info

Tunnel Management Messages (cont.)

Cause: Indicates if the PDP context was created. Some of the values are- Request accepted- No resource available- All dynamic PDP addresses are occupied- No memory available- Missing or unknown APN etc.

If the PDP context request fails the SGSN tries to create the PDP context with the next GGSN in its GGSN list.

Tunnel Management Messages(cont)Error Indication msg.

Information Element Presence

Tunnel Endpoint Identifier data Mandatory

GSN Address Mandatory

Private extension Optional

• Sent from GSN if no active PDP context exists for the received G-PDU. • When an error indication is received, the receiving GSN deletes the corresponding PDP context.• The elements TEID and GSN address are derived from the data message and uniquely specify the PDP context.

Location Management Messages

Used for network initiated PDP context request when there is no SS7 MAP interface at the GGSN

Gn+

UDP

L2 IP

GGSN

L1 L2 IP

UDP

SGSN

GTP

L1 MTP2 MTP3 SCCP TCAP MAP

L1 MTP2 MTP3 SCCP

HLR

TCAP

MAP

L1

GTP

Gc

GTP-MAP Interworking

Location Management Messages (cont)• Send Routing Information for GPRS Request• Send Routing Information for GPRS Response

• Failure Report Request• Failure Report Response

• Note MS GPRS Present Request• Note MS GPRS Present Response

Mobility Management Messages:

• Sent between SGSN’s at the GPRS attach and Inter SGSN routing update procedures.

Reliable Delivery of Signaling Messages

• Each path maintains a queue with Signaling msg. to be sent• A request (with a response) shall be sent with a sequence number and shall be held in a path list till the response comes. • Each path has its own list• The sequence number is unique for each outstanding req. msg.

• T3-Response timer – Max time a requesting entity would wait before retransmission • N3-Requests times – Max number of retransmissions allowed. these two are configurable parameters on the stack.

Handling of Sequence Number (optional)

For GTP-C• Used in request-response type of signaling messages as transaction Identity. In the response message the seq. no. of request message is copied.

For GTP-U• An increasing number of T-PDUs are transmitted via GTP-U tunnel when transmission order must be preserved.• The receiving GGSN would reorder the incoming T-PDUs in seq. if the reordering required flag in the PDP context is set. In this case if required the GGSN would maintain a window and a timers to make sure that a G-PDU was lost.• The sequence number is kept unchanged irrespective of the tunnels it travels before the destination.