Understanding GTP-C Signaling in PS Core Networks

What is GTP-C?

The GPRS Tunneling Protocol - Control Plane (GTP-C) is one of the most critical signaling protocols in mobile packet-switched networks. It runs over UDP port 2123 and is responsible for creating, modifying, and deleting GTP tunnels between network elements.

In a typical Huawei PS Core deployment, GTP-C operates between:

• UGW (Universal Gateway) — the user plane gateway
• USN (User Support Node) — session management
• CG (Charging Gateway) — online/offline charging coordination
• SGSN/MME — mobility management

Key GTP-C Message Types

Message	Code	Purpose
Echo Request	1	Path management — keepalive
Echo Response	2	Response to echo
Create PDP Context Request	16	Establish a PDP context (2G/3G)
Create PDP Context Response	17	Accept/reject PDP creation
Update PDP Context Request	18	Modify existing PDP context
Delete PDP Context Request	20	Tear down PDP context
Create Session Request	32	LTE — Establish EPS bearer
Create Session Response	33	Accept/reject session creation
Modify Bearer Request	34	Modify bearer parameters
Delete Session Request	36	Delete EPS bearer

Typical Create Session Flow (LTE)

Here’s the standard flow for a UE attaching to the network:

UE → eNodeB → MME → SGW → PGW/UGW

1. MME sends Create Session Request to SGW
   - IMSI, MEI, RAT type, APN, PDN type
   - Bearer QoS, Teid (S11)

2. SGW forwards to PGW/UGW
   - Adds S5/S8 specific IEs
   - PGW allocates IP address (if not static APN)

3. PGW/UGW responds with Create Session Response
   - Cause = Request Accepted
   - PDN Address Allocation (IP)
   - Bearer ID, TEID for user plane
   - APN-AMBR (Aggregate Maximum Bit Rate)

4. SGW forwards response to MME
   - MME updates location with HSS
   - MME sends Modify Bearer to SGW with eNodeB TEID

Common GTP-C Issues in Huawei PS Core

1. Create Session Failure — Cause #26 (Insufficient Resources)

Symptom: UGW rejects Create Session Request
Root cause: APN configuration limits exceeded
Fix: Check APN profile on UGW — max sessions, PDN type

2. PDP Context Deactivation — Cause #36 (Regular Deactivation)

Symptom: Unexpected session teardown
Root cause: Inactivity timer expiry on UGW
Fix: Review inactivity timer values in APN profile

3. Echo Timeout — Path Management Failure

Symptom: GTP tunnel becomes stale, traffic drops
Root cause: Echo Request/Response not working
Fix: Verify UDP 2123 reachability between nodes

Wireshark Filter Tips for GTP-C

# All GTP-C messages
gtp

# Only Create Session messages
gtp.msg_type == 32 || gtp.msg_type == 33

# Filter by IMSI
gtpv2.imsi == "0010112345678"

# Filter by APN
gtpv2.apn == "internet"

# GTP-C errors
gtpv2.cause_value != 16

Huawei-Specific Debugging

On Huawei UGW/USN, use these commands:

# Display GTP-C statistics
display gtp statistics

# Display active PDP contexts
display gtp pdp-context all

# Debug GTP-C messages
debugging gtp message all

# Display GTP peers
display gtp peer

Summary

Understanding GTP-C is fundamental for any PS Core engineer. Whether you’re troubleshooting a failed session creation or optimizing bearer setup times, knowing the message flows and common failure modes will save hours of debugging.

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Next article: Understanding Diameter Signaling in LTE Charging