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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.