Evaluation of the Fraunhofer Open Source IMS Core platform with special focus on the Call Session Control Function (CSCF)

Table of Contents

1. Purpose of this Thesis

2. Introduction

I UNDERSTANDING IMS

3. Principles of the IP Multimedia Subsystem

3.1. What is the IP Multimedia Subsystem?

3.2. IMS Specification Bodies: 3GPP and IETF

3.3. High Level Requirements

3.3.1. Differences to RFC 3261 SIP

3.3.2. Registration

3.3.3. Session Setup & Control

3.4. Private Header Extensions to SIP for 3GPP

3.4.1. P-Access-Network-Info Header

3.4.2. P-Asserted-Identity Header

3.4.3. P-Associated-URI Header

3.4.4. P-Called-Party-ID Header

3.4.5. P-Charging-Function-Addresses Header

3.4.6. P-Charging-Vector Header

3.4.7. P-Media-Authorization Header

3.4.8. P-Preferred-Identity Header

3.4.9. P-Visited-Network-ID Header

3.4.10. Path Header

3.4.11. Security-Client Header

3.4.12. Security-Server Header

3.4.13. Security-Verify Header

3.4.14. Service-Route Header

4. The IP Multimedia Subsystem Architecture

4.1. Home Subscriber Server

4.1.1. Private User Identity

4.1.2. Public User Identity

4.2. Subscription Locator Function

4.3. Application Server

4.4. Media Gateway Control Function

4.5. IP Multimedia Subsystem Media Gateway

4.6. Signaling Gateway

4.7. Multimedia Resource Function

4.7.1. Multimedia Resource Function Controller

4.7.2. Multimedia Resource Function Processor

4.8. Breakout Gateway Control Function

4.9. IMS Application Layer Gateway

4.10. Transition Gateway

5. The Call Session Control Function

5.1. Proxy Call Session Control Function

5.1.1. P-CSCF Discovery

5.1.2. Discovery of I-CSCF in Home Network Domain

5.1.3. Confidentiality and Integrity Protection of SIP Signaling

5.1.4. Signaling Compression

5.1.5. Treatment for SIP Dialogs and Transactions

5.1.6. Bearer Authorization & Quality of Service Management

5.1.7. Subscription to reg event state at the S-CSCF

5.1.8. Charging

5.1.9. Emergency Services

5.2. Interrogating Call Session Control Function

5.2.1. User Location Query Procedure

5.2.2. User Registration Status Query

5.2.3. S-CSCF Assignment during IMS Registration

5.2.4. Topology Hiding Inter-network Gateway

5.2.5. Network Domain Security

5.2.6. IMS-Application Layer Gateway

5.2.7. Charging

5.3. Service Call Session Control Function

5.3.1. Subscriber Authentication

5.3.2. S-CSCF Registration Notification

5.3.3. Notification Server for “reg”-event

5.3.4. Treatment for SIP Dialogs and Transactions

5.3.5. Charging

5.3.6. Timer Supervision

II IMPLEMENTATION

6. Testbed Specification

6.1. Introduction

6.2. Architecture

6.3. Hardware and Software Components

6.4. Open Source IMS Core

7. Open IMS Core Installation

7.1. Prerequisites

7.2. SIP Express Router

7.3. Fraunhofer HSS

8. Open IMS Core Configuration

8.1. Domain Name Service

8.2. Proxy Call Session Control Function

8.3. Interrogating Call Session Control Function

8.4. Serving Call Session Control Function

8.5. Home Subscriber Server

8.6. User Endpoints

8.6.1. Snom Phone

8.6.2. eyeBeam Software Client

8.6.3. SIPp performance testing tool

9. The Open IMS Core CSCF Modules

9.1. The Proxy CSCF Module

9.1.1. REGISTER Request

9.1.2. REGISTER Response

9.1.3. NOTIFY Request

9.1.4. Mobile-Originating Request

9.1.5. Mobile-Terminating Request

9.2. The Interrogating CSCF Module

9.2.1. REGISTER Request

9.2.2. Initial Request

9.3. The Serving CSCF Module

9.3.1. REGISTER Request

9.3.2. SUBSCRIBE reg event Request

9.3.3. Mobile-Originating Request

9.3.4. Mobile-Terminating Request

III DISCUSSION

10. Conclusions

11. Future Work

Research Objectives and Key Themes

The primary goal of this thesis is to evaluate the Open Source IMS Core platform developed by the Fraunhofer Institute FOKUS. The research examines how the SIP Express Router (SER) functions within the IP Multimedia Subsystem (IMS) and analyzes its specific modules and routing logic to act as a Call Session Control Function (CSCF).

• Evaluation of the Fraunhofer Open Source IMS Core (OSIMS) platform.
• Technical analysis of the Call Session Control Function (CSCF) roles: P-CSCF, I-CSCF, and S-CSCF.
• Examination of SIP protocol extensions required for 3GPP IMS compliance.
• Implementation of a functional IMS testbed on a single-node architecture.
• Analysis of signaling flows and security mechanisms within the IMS environment.

Excerpt from the Book

Summary of Chapters

1. Purpose of this Thesis: Outlines the motivation behind the research into IMS as a universal service delivery platform for Next Generation Networks (NGN) and defines the objective to evaluate the FOKUS Open Source IMS Core.

2. Introduction: Provides a roadmap of the thesis, detailing the structure into theoretical background, practical implementation of the testbed, and final discussion of results.

3. Principles of the IP Multimedia Subsystem: Reviews the history, requirements, and the collaboration between 3GPP and IETF, while discussing essential private SIP header extensions for IMS.

4. The IP Multimedia Subsystem Architecture: Describes the functional entities of the IM Core Network and their layered architectural design, including the role of the Home Subscriber Server (HSS).

5. The Call Session Control Function: Offers a deep dive into the P-CSCF, I-CSCF, and S-CSCF, analyzing their specific procedures for registration, session management, and security.

6. Testbed Specification: Details the planning and architecture of the implemented testbed, using a single-machine approach to simulate a complete IMS environment.

7. Open IMS Core Installation: Provides a step-by-step installation guide for the required software packages, SER, and the Fraunhofer HSS.

8. Open IMS Core Configuration: Explains the setup of critical components like DNS, the various CSCFs, and user endpoints for testing.

9. The Open IMS Core CSCF Modules: Analyzes the specific SER modules and routing logic that enable the components to function as an IMS core.

Keywords

IP Multimedia Subsystem, IMS, Call Session Control Function, CSCF, P-CSCF, I-CSCF, S-CSCF, Session Initiation Protocol, SIP, 3GPP, Fraunhofer FOKUS, SIP Express Router, SER, Next Generation Networks, NGN

Frequently Asked Questions

What is the core focus of this diploma thesis?

The work primarily focuses on the evaluation of the Fraunhofer Open Source IMS Core (OSIMS) platform, specifically analyzing how the SIP Express Router (SER) is extended to act as a Call Session Control Function (CSCF) within an IMS environment.

Which functional entities of the IMS are analyzed?

The thesis provides a detailed examination of the three primary types of CSCFs: the Proxy CSCF (P-CSCF), the Interrogating CSCF (I-CSCF), and the Serving CSCF (S-CSCF).

What is the primary objective or research question?

The objective is to evaluate the functionality of the Open Source IMS Core platform from the Fraunhofer Institute FOKUS and to analyze the specific modules and functions required for the SIP Express Router to operate effectively in an IMS network.

Which scientific methodology is applied in this paper?

The thesis utilizes a combination of theoretical analysis based on 3GPP Technical Specifications and IETF RFCs, followed by a practical, empirical approach involving the implementation and configuration of a functional IMS testbed on a Linux-based platform.

What topics are covered in the main section of the thesis?

The main part covers the theoretical foundations of IMS principles, the system architecture, the implementation and installation of the testbed environment, and a detailed module-by-module analysis of the CSCF logic.

Which keywords characterize this work?

Key terms include IP Multimedia Subsystem (IMS), CSCF, SIP, 3GPP, Fraunhofer FOKUS, SER, and Next Generation Networks.

How does the I-CSCF perform its load distribution role?

The I-CSCF acts as a point of contact for peer networks. It queries the HSS using Diameter interfaces (Cx/Dx) to identify the appropriate S-CSCF for a user's registration or session request and routes the traffic accordingly.

What are the identified limitations of the FOKUS testbed?

The evaluation identifies several limitations, including the lack of support for session timers (RFC 4028), the absence of persistent memory storage for registered user data, and limited implementation of the Dx-interface and Quality of Service (QoS) management.