Gn/Gp Interface Documentation

GTP-C Communication with SGSN (2G/3G Networks)

OmniPGW by Omnitouch Network Services

Overview

The Gn/Gp interface enables OmniPGW to function as a GGSN (Gateway GPRS Support Node) for 2G/3G networks using the GTP-C v1 protocol. This interface handles PDP (Packet Data Protocol) context management between the GGSN and SGSNs.

Gn interface: Connects to SGSNs within the same PLMN (home network)
Gp interface: Connects to SGSNs in other PLMNs (roaming scenarios)

Both interfaces use identical protocols - the distinction is purely topological.

Key Differences from PGW (S5/S8)

Aspect	GGSN (Gn/Gp)	PGW (S5/S8)
Protocol Version	GTP-C v1	GTP-C v2
3GPP Specification	TS 29.060	TS 29.274
Session Concept	PDP Context	PDN Session + Bearers
Bearer Identifier	NSAPI (0-15)	EBI (5-15)
Policy Interface	None (static policy)	Gx (PCRF)
QoS Model	R99 QoS Profile	EPS Bearer QoS (QCI)
Port	2123 (shared)	2123

Protocol Details

GTP-C Version 1

Protocol: GTP-C v1 (3GPP TS 29.060)
Transport: UDP
Port: 2123 (shared with S5/S8 GTP-C v2)
Interface Type: Control Plane

TEID (Tunnel Endpoint Identifier)

Each PDP context has unique TEIDs for control and user plane:

GGSN Control TEID: Allocated by OmniPGW for incoming control messages
GGSN Data TEID: Allocated by UPF for incoming user data
SGSN Control TEID: Provided by SGSN for outgoing control messages
SGSN Data TEID: Provided by SGSN for outgoing user data

Message Flow:
  SGSN → GGSN: Destination TEID = GGSN's Control TEID
  GGSN → SGSN: Destination TEID = SGSN's Control TEID

CUPS Support (Control and User Plane Separation)

OmniPGW supports CUPS architecture for 2G/3G:

GSN Address for Signalling: OmniPGW's control plane IP (from gn.local_ipv4_address)
GSN Address for User Traffic: UPF's IP address (from PFCP Session Establishment Response)

This allows the GGSN-C and PGW-U/UPF to run on separate nodes with different IP addresses, which is standard for CUPS deployments.

NSAPI (Network Service Access Point Identifier)

The NSAPI identifies individual PDP contexts within a subscriber session:

Range: 5-15 (0-4 reserved)
Usage: Multiple PDP contexts per subscriber possible
Equivalent: Similar to EBI in LTE/EPC

Configuration

Basic Configuration

# config/runtime.exs
config :pgw_c,
  # Gn/Gp interface is enabled automatically when gn config is present
  gn: %{
    # Local IPv4 address for Gn interface
    local_ipv4_address: "10.0.0.20",

    # Optional: Local IPv6 address
    local_ipv6_address: nil,

    # Port for GTP-C v1 (shared with S5/S8)
    local_port: 2123
  },

  # DNS servers returned in PCO (shared with PGW)
  dns: %{
    primary_ipv4: {8, 8, 8, 8},
    secondary_ipv4: {8, 8, 4, 4}
  }

Gn Interface Parameters

Parameter	Type	Required	Default	Description
`local_ipv4_address`	String	Yes	-	IPv4 address to bind for Gn interface. This is the GGSN address advertised to SGSNs.
`local_ipv6_address`	String	No	`nil`	Optional IPv6 address. Most 2G/3G networks are IPv4-only.
`local_port`	Integer	No	2123	UDP port for GTP-C. Standard port per 3GPP TS 29.060. Shared with S5/S8.

DNS Parameters

Parameter	Type	Required	Default	Description
`primary_ipv4`	Tuple	No	`{8, 8, 8, 8}`	Primary DNS server IPv4 address returned in PCO.
`secondary_ipv4`	Tuple	No	`{8, 8, 4, 4}`	Secondary DNS server IPv4 address returned in PCO.
`primary_ipv6`	Tuple	No	`nil`	Primary DNS server IPv6 address.
`secondary_ipv6`	Tuple	No	`nil`	Secondary DNS server IPv6 address.

Network Requirements

Firewall Rules:

# Allow GTP-C from SGSN network
iptables -A INPUT -p udp --dport 2123 -s <sgsn_network>/24 -j ACCEPT

# Allow outbound GTP-C to SGSN
iptables -A OUTPUT -p udp --dport 2123 -d <sgsn_network>/24 -j ACCEPT

Message Types

PDP Context Management

Create PDP Context Request

Direction: SGSN → GGSN

Purpose: Establish a new PDP context for data connectivity

Key IEs (Information Elements):

IE Name	Type	Description
IMSI	Identity	International Mobile Subscriber Identity
NSAPI	Integer	Network Service Access Point Identifier (5-15)
TEID Data I	Integer	SGSN's user plane tunnel endpoint
TEID Control	Integer	SGSN's control plane tunnel endpoint
End User Address	Address	Requested PDN type and optional IP
Access Point Name	String	APN for the connection
GSN Address (Signalling)	IP	SGSN signalling plane address
GSN Address (User)	IP	SGSN user plane address
QoS Profile	Binary	Requested QoS parameters
MSISDN	Identity	Mobile phone number (optional)
Selection Mode	Enum	APN selection mode
Protocol Configuration Options	Options	Additional protocol options

Example:

Create PDP Context Request
├── IMSI: 310260123456789
├── NSAPI: 5
├── TEID Data I: 0x12345678
├── TEID Control: 0x87654321
├── End User Address: IPv4 (dynamic)
├── APN: internet
├── GSN Address (Signalling): 10.1.1.100
├── GSN Address (User): 10.1.1.100
├── QoS Profile: [R99 QoS parameters]
└── PCO: [DNS request, etc.]

Create PDP Context Response

Direction: GGSN → SGSN

Purpose: Acknowledge PDP context creation with allocated resources

Key IEs:

IE Name	Type	Description
Cause	Result	Success or error code
Reordering Required	Boolean	Whether reordering is needed
Recovery	Counter	GGSN restart counter
TEID Data I	Integer	GGSN's user plane tunnel endpoint
TEID Control	Integer	GGSN's control plane tunnel endpoint
NSAPI	Integer	Confirmed NSAPI
Charging ID	Binary	Charging correlation ID
End User Address	Address	Allocated UE IP address
GSN Address (Signalling)	IP	GGSN signalling plane address
GSN Address (User)	IP	GGSN user plane address
QoS Profile	Binary	Negotiated QoS parameters
Protocol Configuration Options	Options	DNS servers, etc.

Success Response:

Create PDP Context Response
├── Cause: Request Accepted (128)
├── TEID Data I: 0xAABBCCDD
├── TEID Control: 0xDDCCBBAA
├── NSAPI: 5
├── Charging ID: 0x11223344
├── End User Address
│   └── IPv4: 100.64.1.42
├── GSN Address (Signalling): 10.0.0.20
├── GSN Address (User): 10.0.0.20
├── QoS Profile: [Negotiated QoS]
└── PCO
    ├── Primary DNS: 8.8.8.8
    └── Secondary DNS: 8.8.4.4

Update PDP Context Request

Direction: SGSN → GGSN

Purpose: Modify an existing PDP context (e.g., SGSN relocation, QoS change)

Common scenarios:

Inter-SGSN handover (routing area update)
QoS renegotiation
SGSN address change

Key IEs:

IE Name	Description
NSAPI	PDP context identifier
TEID Data I	New SGSN user plane TEID
GSN Address	New SGSN addresses
QoS Profile	Updated QoS (optional)

Update PDP Context Response

Direction: GGSN → SGSN

Purpose: Acknowledge PDP context update

Key IEs:

IE Name	Description
Cause	Success or error code
TEID Data I	GGSN user plane TEID
GSN Address	GGSN addresses
QoS Profile	Negotiated QoS

Delete PDP Context Request

Direction: SGSN → GGSN

Purpose: Terminate a PDP context

Key IEs:

IE Name	Description
NSAPI	PDP context to delete
Teardown Ind	Delete all contexts for this subscriber

Delete PDP Context Response

Direction: GGSN → SGSN

Purpose: Acknowledge PDP context deletion

Key IEs:

IE Name	Description
Cause	Success or error code

Path Management

Echo Request / Response

Direction: Bidirectional

Purpose: Path keepalive and restart detection

Key IEs:

IE Name	Description
Recovery	Restart counter for detecting node restarts

Message Flows

PDP Context Activation

PDP Context Deactivation

Inter-SGSN Handover

Shared Infrastructure

OmniPGW's GGSN functionality shares infrastructure with PGW mode:

UE IP Address Allocation

Uses the same AddressRegistry and subnet configuration as PGW. See UE IP Allocation for configuration details.

UPF/PFCP Session Management

Uses the same PFCP session establishment as PGW:

PDR/FAR/QER rules created per PDP context
UPF selection via the same upf_selection configuration
See PFCP Interface for details

Online Charging (Gy)

Optional Diameter Gy integration available:

Credit control for prepaid subscribers
Same Gy configuration as PGW mode
See Diameter Gy Interface for configuration

CDR Generation

Uses the same CDR generation as PGW. See Data CDR Format for details.

Cause Codes

Success

Code	Name	Description
128	Request Accepted	Successful operation

Rejection Causes

Code	Name	When Used
192	Non-existent	PDP context not found
193	Invalid message format	Malformed GTP message
194	IMSI not known	Subscriber not found
195	MS is GPRS Detached	MS not attached
196	MS is not GPRS Responding	MS unreachable
197	MS Refuses	MS rejected request
198	Version not supported	GTP version mismatch
199	No resources available	Resource exhaustion
200	Service not supported	Unsupported service
201	Mandatory IE incorrect	IE validation failed
202	Mandatory IE missing	Required IE absent
203	Optional IE incorrect	Optional IE invalid
204	System failure	Internal error
205	Roaming restriction	Roaming not allowed
206	P-TMSI Signature mismatch	Security failure
207	GPRS connection suspended	Connection suspended
208	Authentication failure	Auth failed
209	User authentication failed	User auth failed

Monitoring

Gn Interface Metrics

# Message counters
gn_inbound_messages_total{message_type="create_pdp_context_request"}
gn_inbound_messages_total{message_type="update_pdp_context_request"}
gn_inbound_messages_total{message_type="delete_pdp_context_request"}

gn_outbound_messages_total{message_type="create_pdp_context_response"}
gn_outbound_messages_total{message_type="update_pdp_context_response"}
gn_outbound_messages_total{message_type="delete_pdp_context_response"}

# Transaction latency
gn_outbound_transaction_duration_bucket

Useful Queries

PDP Context Creation Rate:

rate(gn_inbound_messages_total{message_type="create_pdp_context_request"}[5m])

Success Rate:

sum(rate(gn_outbound_messages_total{message_type="create_pdp_context_response"}[5m]))
  / sum(rate(gn_inbound_messages_total{message_type="create_pdp_context_request"}[5m]))

Active PDP Contexts:

gn_inbound_messages_total{message_type="create_pdp_context_request"}
  - gn_inbound_messages_total{message_type="delete_pdp_context_request"}

Troubleshooting

Issue: No Response to Create PDP Context

Symptoms:

SGSN sends Create PDP Context Request
No response received
Timeout at SGSN

Possible causes:

Network connectivity issue
OmniPGW not listening on configured IP
Firewall blocking UDP 2123
UPF/PFCP establishment failure

Debug:

# Check OmniPGW is listening
netstat -ulnp | grep 2123

# Check for incoming packets
tcpdump -i any -n port 2123

# Verify configuration
grep "local_ipv4_address" config/runtime.exs

# Check PFCP connectivity
curl http://pgw:9090/metrics | grep pfcp

Issue: PDP Context Creation Rejected

Symptoms:

Create PDP Context Response with error cause
PDP context not established

Common causes:

Cause 199 (No resources available):
  → IP pool exhausted
  → Check: curl http://pgw:9090/metrics | grep address_registry_count
  → Expand IP pool in ue.subnet_map

Cause 202 (Mandatory IE missing):
  → SGSN not sending required IEs
  → Check SGSN configuration
  → Review message in tcpdump

Cause 204 (System failure):
  → UPF/PFCP failure
  → Check PFCP association status
  → Review OmniPGW logs

Issue: Update PDP Context Fails

Symptoms:

Update PDP Context Response with error cause
Handover fails

Possible causes:

PDP context not found (different TEID)
NSAPI mismatch
Session already deleted

Debug:

# Check active sessions
curl http://pgw:9090/metrics | grep session_count

# Review logs for context lookup
journalctl -u omnipgw | grep "PDP Context"

Best Practices

Network Design

Shared Port with S5/S8
- Gn and S5/S8 share UDP port 2123
- OmniPGW auto-detects GTP version from message
- Single IP can serve both 2G/3G and 4G
SGSN Connectivity
- Ensure all SGSNs can reach GGSN IP
- Consider dedicated VLAN for Gn traffic
- Plan for roaming (Gp) if applicable
IP Address Planning
- Same IP pools serve both PGW and GGSN modes
- Consider separate APN-based pools for 2G/3G

Performance

Session Capacity
- Monitor session_count metric
- Plan for expected 2G/3G subscriber count
- Consider that 2G/3G typically has fewer concurrent sessions
UDP Buffer Sizes
- Same recommendations as S5/S8
- Typical: 4-8 MB per socket

Migration Considerations

When migrating from standalone GGSN to OmniPGW:

APN Configuration
- Ensure APNs match existing GGSN configuration
- Verify DNS server settings in PCO
IP Pool Migration
- Plan IP pool cutover carefully
- Consider gradual migration per APN
QoS Mapping
- R99 QoS profiles accepted as-is
- No QCI translation needed for 2G/3G

Core Functions

Configuration Guide - Complete configuration reference
Session Management - Session lifecycle details
UE IP Allocation - IP address management
PCO Configuration - Protocol Configuration Options

S5/S8 Interface - 4G/LTE equivalent interface
PFCP Interface - User plane coordination
Diameter Gy Interface - Online charging

Operations

Monitoring Guide - Metrics and alerting
Troubleshooting - Common issues
Data CDR Format - CDR generation

References

3GPP TS 29.060 - GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface
3GPP TS 23.060 - General Packet Radio Service (GPRS); Service description
3GPP TS 24.008 - Mobile radio interface Layer 3 specification (QoS profiles)

Back to Documentation Index

OmniPGW Gn/Gp Interface - by Omnitouch Network Services

Overview​

Key Differences from PGW (S5/S8)​

Protocol Details​

GTP-C Version 1​

TEID (Tunnel Endpoint Identifier)​

CUPS Support (Control and User Plane Separation)​

NSAPI (Network Service Access Point Identifier)​

Configuration​

Basic Configuration​

Gn Interface Parameters​

DNS Parameters​

Network Requirements​

Message Types​

PDP Context Management​

Create PDP Context Request​

Create PDP Context Response​

Update PDP Context Request​

Update PDP Context Response​

Delete PDP Context Request​

Delete PDP Context Response​

Path Management​

Echo Request / Response​

Message Flows​

PDP Context Activation​

PDP Context Deactivation​

Inter-SGSN Handover​

Shared Infrastructure​

UE IP Address Allocation​

UPF/PFCP Session Management​

Online Charging (Gy)​

CDR Generation​

Cause Codes​

Success​

Rejection Causes​

Monitoring​

Gn Interface Metrics​

Useful Queries​

Troubleshooting​

Issue: No Response to Create PDP Context​

Issue: PDP Context Creation Rejected​

Issue: Update PDP Context Fails​

Best Practices​

Network Design​

Performance​

Migration Considerations​

Related Documentation​

Core Functions​

Related Interfaces​

Operations​

References​

Overview

Key Differences from PGW (S5/S8)

Protocol Details

GTP-C Version 1

TEID (Tunnel Endpoint Identifier)

CUPS Support (Control and User Plane Separation)

NSAPI (Network Service Access Point Identifier)

Configuration

Basic Configuration

Gn Interface Parameters

DNS Parameters

Network Requirements

Message Types

PDP Context Management

Create PDP Context Request

Create PDP Context Response

Update PDP Context Request

Update PDP Context Response

Delete PDP Context Request

Delete PDP Context Response

Path Management

Echo Request / Response

Message Flows

PDP Context Activation

PDP Context Deactivation

Inter-SGSN Handover

Shared Infrastructure

UE IP Address Allocation

UPF/PFCP Session Management

Online Charging (Gy)

CDR Generation

Cause Codes

Success

Rejection Causes

Monitoring

Gn Interface Metrics

Useful Queries

Troubleshooting

Issue: No Response to Create PDP Context

Issue: PDP Context Creation Rejected

Issue: Update PDP Context Fails

Best Practices

Network Design

Performance

Migration Considerations

Related Documentation

Core Functions

Related Interfaces

Operations

References