Diameter Signaling in LTE Charging

Introduction

The Diameter protocol is the backbone of authentication, authorization, and charging (AAA) in LTE networks. It replaced the legacy RADIUS protocol and provides more robust features including reliable transport, peer-to-peer communication, and extensible AVPs (Attribute-Value Pairs).

In Huawei PS Core networks, Diameter is used across multiple interfaces:

• Gx — Between PGW/UGW and PCRF (Policy and Charging Rules Function)
• Gy — Between PGW/UGW and OCS (Online Charging System)
• S6a — Between MME and HSS (Home Subscriber Server)
• S13 — Between MME and EIR (Equipment Identity Register)

Diameter Message Structure

Every Diameter request and response follows this structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Version (1)                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Message Length                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Command Flags |                  Command Code                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Application ID                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Hop-by-Hop ID                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        End-to-End ID                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          AVPs ...                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Gx Interface — Policy & Charging Control

The Gx interface connects the PGW/UGW to the PCRF. It’s responsible for:

1. Policy decisions — QoS rules, bandwidth limits
2. Charging rules — How to charge for specific services
3. Traffic detection — Deep packet inspection rules

Key Messages

Command	Code	Direction	Purpose
CCR (Credit Control Request)	272	PGW → PCRF	Report usage, request rules
CCA (Credit Control Answer)	272	PCRF → PGW	Provide charging/policy rules
RAR (Re-Auth-Request)	258	PCRF → PGW	Push new rules (push mode)
RAA (Re-Auth-Answer)	258	PGW → PCRF	Acknowledge rule change

Typical Gx Flow

UE → PGW/UGW → PCRF

1. UE attaches, PGW sends CCR-I (Initial)
   - Session-Id, Subscription-Id (IMSI)
   - Framed-IP-Address
   - Called-Station-Id (APN)

2. PCRF responds with CCA-I
   - Charging-Rule-Definition
   - QoS-Information (QCI, ARP, MBR/GBR)
   - Flow-Description (IP filters)

3. UE makes data connection → PGW sends CCR-U (Update)
   - Used-Service-Unit (bytes used)
   - Rating-Group

4. PCRF responds with CCA-U
   - Updated rules (if policy changed)

Gy Interface — Online Charging

The Gy interface connects the PGW/UGW to the OCS. It implements online credit control — meaning the network checks if the user has credit BEFORE allowing data usage.

Key Messages

Command	Code	Direction	Purpose
CCR (Credit Control Request)	272	PGW → OCS	Request quota
CCA (Credit Control Answer)	272	OCS → PGW	Grant/deny quota

Gy Flow — Quota Management

PGW/UGW → OCS

1. CCR-I (Initial): Request credit for new session
   - Requested-Service-Unit (RSU)
   - Used-Service-Unit (USU) = 0
   - Service-Context-Id

2. CCA-I: OCS grants initial quota
   - Granted-Service-Unit (GSU)
   - Result-Code = 2001 (SUCCESS)

3. PGW tracks usage → CCR-U when quota low
   - Used-Service-Unit = bytes consumed
   - Requested-Service-Unit = additional quota

4. CCA-U: OCS grants more quota
   - Or Result-Code = 4012 (QUOTA_EXHAUSTED)

Common Diameter Issues

1. CCR-I Timeout

Symptom: PGW cannot establish charging session
Cause: OCS overloaded or unreachable
Check: TCP/SCTP port 3868 connectivity

2. Result-Code 5012 (UNABLE_TO_COMPLY)

Symptom: OCS rejects credit request
Cause: Missing or invalid AVP
Check: Subscription data, rating group config

3. Gx Push Failure

Symptom: PCRF pushes rules but PGW doesn't apply
Cause: Session-Id mismatch
Check: Diameter session state on PGW

Wireshark Analysis

# All Diameter messages
diameter

# Filter CCR messages
diameter.cmd.ccr

# Filter by Result-Code
diameter.result_code == 2001

# Filter by Application-Id
diameter.application_id == 16777238  # Gx
diameter.application_id == 4          # Charging

# Show Diameter AVPs
diameter.avp

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