S5/S8 Interface Documentation

GTP-C Communication with SGW-C

OmniPGW by Omnitouch Network Services

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Overview

The S5/S8 interface connects OmniPGW to the SGW-C (Serving Gateway Control plane) using the GTP-C v2 (GPRS Tunnelling Protocol - Control plane) protocol. This interface handles session management signaling between the gateways.

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Protocol Details

GTP-C Version 2

• Protocol: GTP-C v2 (3GPP TS 29.274)
• Transport: UDP
• Port: 2123 (standard)
• Interface Type: Control Plane

TEID (Tunnel Endpoint Identifier)

Each session has a unique TEID for routing messages:

• Local TEID - Allocated by OmniPGW for incoming messages
• Remote TEID - Allocated by SGW-C for outgoing messages

Message Flow:
  SGW-C → OmniPGW: Destination TEID = OmniPGW's Local TEID
  OmniPGW → SGW-C: Destination TEID = SGW-C's Remote TEID

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Configuration

Basic Configuration

# config/runtime.exs
config :pgw_c,
  s5s8: %{
    # Local IPv4 address for S5/S8 interface
    local_ipv4_address: "10.0.0.20",

    # Optional: Local IPv6 address
    local_ipv6_address: nil,

    # Optional: Override default port
    local_port: 2123,

    # GTP-C request timeout in milliseconds (default: 500ms)
    # Timeout per attempt when waiting for GTP-C responses (Create Bearer, Delete Bearer, etc.)
    request_timeout_ms: 500,

    # Number of retry attempts for GTP-C requests (default: 3)
    # Total maximum wait time = request_timeout_ms * request_attempts
    # Example: 500ms * 3 attempts = 1500ms (1.5 seconds) total
    request_attempts: 3
  }

Timeout Configuration

The S5/S8 interface uses configurable timeouts for GTP-C request/response transactions.

Parameters:

• request_timeout_ms - Timeout in milliseconds per retry attempt (default: 500ms)
• request_attempts - Number of retry attempts before giving up (default: 3)

Total Wait Time: request_timeout_ms × request_attempts

Default behavior: 500ms × 3 attempts = 1.5 seconds total maximum wait

Tuning Guidelines:

Network Latency	Recommended request_timeout_ms	Total Wait Time
Low latency (<50ms)	200-300ms	600-900ms (3 attempts)
Normal (50-150ms)	500ms (default)	1.5s (3 attempts)
High latency (>150ms)	1000-2000ms	3-6s (3 attempts)
Unstable/satellite	2000-3000ms	6-9s (3 attempts)

Example - High Latency Network:

s5s8: %{
  local_ipv4_address: "10.0.0.20",
  request_timeout_ms: 1500,  # 1.5 seconds per attempt
  request_attempts: 3         # Total: 4.5 seconds max
}

When timeout occurs:

• OmniPGW logs error: "Create Bearer Request timed out"
• Returns error to PCRF (Diameter Result-Code: 5012 UNABLE_TO_COMPLY)
• Bearer remains in early storage for cleanup via Charging-Rule-Remove

Network Requirements

Firewall Rules:

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

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

Routing:

# Ensure route to SGW-C network
ip route add <sgw_network>/24 via <gateway_ip> dev eth0

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Message Types

The S5/S8 interface handles GTP-C signaling for PDN session management. For detailed session lifecycle and state management, see Session Management Guide.

Session Management

Create Session Request

Direction: SGW-C → OmniPGW

Purpose: Establish a new PDN connection

Key IEs (Information Elements):

IE Name	Type	Description
IMSI	Identity	International Mobile Subscriber Identity
MSISDN	Identity	Mobile phone number
APN	String	Access Point Name (e.g., "internet")
RAT Type	Enum	Radio Access Technology (EUTRAN)
Bearer Context	Grouped	Default bearer information
UE Time Zone	Timestamp	UE's timezone
ULI	Grouped	User Location Information (TAI, ECGI)
Serving Network	PLMN	MCC/MNC of serving network

Example:

Create Session Request
├── IMSI: 310260123456789
├── MSISDN: 14155551234
├── APN: internet
├── RAT Type: EUTRAN (6)
├── Bearer Context
│   ├── EBI: 5
│   ├── Bearer QoS (QCI 9, ARP, bitrates)
│   └── S5/S8 F-TEID (SGW-U tunnel endpoint)
└── ULI
    ├── TAI: MCC 310, MNC 260, TAC 12345
    └── ECGI: MCC 310, MNC 260, ECI 67890

Create Session Response

Direction: OmniPGW → SGW-C

Purpose: Acknowledge session creation

Key IEs:

IE Name	Type	Description
Cause	Result	Success or error code
Bearer Context	Grouped	Bearer information
PDN Address Allocation	IP	Allocated UE IP address (see UE IP Allocation)
APN Restriction	Enum	APN usage restrictions
PCO	Options	Protocol Configuration Options (see PCO Configuration)

Success Response:

Create Session Response
├── Cause: Request accepted (16)
├── PDN Address Allocation
│   └── IPv4: 100.64.1.42
├── Bearer Context
│   ├── EBI: 5
│   ├── Cause: Request accepted
│   └── S5/S8 F-TEID (PGW-U tunnel endpoint from PFCP)
├── APN Restriction: Public-1 (1)
└── PCO
    ├── DNS Server: 8.8.8.8
    ├── DNS Server: 8.8.4.4
    └── Link MTU: 1400

Delete Session Request

Direction: SGW-C → OmniPGW

Purpose: Terminate PDN connection

Key IEs:

IE Name	Description
EBI	EPS Bearer ID to delete
Linked EBI	Related bearer (optional)

Delete Session Response

Direction: OmniPGW → SGW-C

Purpose: Acknowledge session deletion

Key IEs:

IE Name	Description
Cause	Success or error code

Bearer Management

Create Bearer Request

Direction: OmniPGW → SGW-C

Purpose: Create dedicated bearer (initiated by PCRF policy)

Triggered by:

• PCRF sends new PCC rule requiring dedicated bearer
• OmniPGW requests SGW-C to establish bearer

Delete Bearer Request

Direction: OmniPGW → SGW-C or SGW-C → OmniPGW

Purpose: Delete dedicated bearer

Scenarios:

• PGW-initiated: PCRF policy change removes dedicated bearer
• SGW-initiated: Radio resource release

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Message Flows

Session Establishment

Session Termination

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Cause Codes

Success

Code	Name	Description
16	Request accepted	Successful operation

Errors (Permanent Failures)

Code	Name	When Used
65	User Unknown	PCRF rejected (IMSI not found)
66	No resources available	IP pool exhausted
93	Service not supported	Invalid APN
94	Semantic error in TFT	Invalid traffic flow template

Errors (Transient Failures)

Code	Name	When Used
72	Remote peer not responding	PCRF/PGW-U timeout
73	Collision with network initiated request	Simultaneous operations

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Monitoring

S5/S8 Metrics

# Message counters
s5s8_inbound_messages_total{message_type="create_session_request"}
s5s8_inbound_messages_total{message_type="delete_session_request"}

# Error counters
s5s8_inbound_errors_total

# Message handling latency
s5s8_inbound_handling_duration_bucket

# Active TEIDs
teid_registry_count

Useful Queries

Session Creation Rate:

rate(s5s8_inbound_messages_total{message_type="create_session_request"}[5m])

Error Rate:

rate(s5s8_inbound_errors_total[5m])

Latency (p95):

histogram_quantile(0.95,
  rate(s5s8_inbound_handling_duration_bucket{request_message_type="create_session_request"}[5m])
)

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Troubleshooting

Issue: No Response from OmniPGW

Symptoms:

• SGW-C sends Create Session Request
• No response received
• Timeout at SGW-C

Causes:

1. Network connectivity issue
2. OmniPGW not listening on configured IP
3. Firewall blocking UDP 2123
4. Wrong TEID in request

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 firewall
iptables -L -n | grep 2123

Issue: Session Creation Fails

Symptoms:

• Create Session Response with error cause
• Session not established

Common Causes:

Cause 65 (User Unknown):
  → PCRF rejected subscriber
  → Check IMSI in HSS/SPR

Cause 66 (No resources):
  → IP pool exhausted
  → Check: curl http://pgw:9090/metrics | grep address_registry_count
  → Expand IP pool

Cause 72 (Remote peer not responding):
  → PCRF timeout or PGW-U down
  → Check Gx connectivity
  → Check PFCP association

Issue: TEID Collision

Symptoms:

• Message routed to wrong session
• Unexpected behavior

Cause:

• TEID reused before cleanup
• Bug in TEID allocation

Resolution:

• Ensure unique TEID allocation
• Check TEID registry for leaks

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Best Practices

Network Design

1. Dedicated Network Interface

   • Use separate VLAN for S5/S8
   • Isolate from management traffic

2. MTU Optimization

   • Ensure MTU supports GTP headers
   • Minimum MTU: 1500 bytes (1464 payload + 36 GTP)

3. Redundancy

   • Multiple OmniPGW instances
   • DNS-based load balancing from SGW-C

Performance

1. UDP Buffer Sizes

   • Increase socket buffers for high load
   • Typical: 4-8 MB per socket

2. Connection Limits

   • Plan for expected session count
   • Monitor TEID registry count

Security

1. IP Filtering

   • Only allow GTP-C from known SGW-C IPs
   • Use iptables or network ACLs

2. Message Validation

   • OmniPGW validates all incoming messages
   • Rejects malformed GTP-C packets

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Related Documentation

Core Functions

• Configuration Guide - S5/S8 interface configuration, local IP setup
• Session Management - PDN session lifecycle, bearer establishment
• UE IP Allocation - IP address delivery via Create Session Response
• PCO Configuration - PCO parameters in GTP-C messages

Related Interfaces

• Gn/Gp Interface - GTP-C v1 for 2G/3G GGSN functionality
• PFCP Interface - User plane coordination with S5/S8 control plane
• Diameter Gx Interface - Policy integration with bearer setup
• Diameter Gy Interface - Charging integration with bearer management

Operations

• Monitoring Guide - S5/S8 GTP-C metrics, message tracking
• Data CDR Format - CDR generation from GTP-C sessions

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Back to Operations Guide

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OmniPGW S5/S8 Interface - by Omnitouch Network Services