Functionality and Testing of Policy Control in IP
Multimedia Subsystem
Skander Chaichee
HUT/Nokia Networks
17.8. 2004
Supervisor:
Professor Raimo Kantola
Instructor: Mikko Kiiski, M. Sc.
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Objectives and Purpose of the Thesis
• Thesis tries to answer the problem of how to test the IMS
Policy Control functionality in GGSN.
• This requires designing and building a test network and
methods how to use it properly.
• It also requires planning a comprehensive set of the test
cases, writing them to test scripts and finding a way to execute
them.
• The second objective is to explain the reader the functionality
of the IMS Policy Control and the interfaces and network
elements taking part of it.
• The Go interface is a new interface and this is the first time it is
implemented in Nokia GGSN. Thus, the functionality of it
deserves to be analyzed and tested.
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GPRS & 3G Network
• GPRS comes as an upgrade to the existing GSM networks. It offers
packet-switched data service and possible connection to the external
networks e.g. Internet. Universal Mobile Telecommunication System
(UMTS) is a 3G wireless system that provides higher data rates and
enhanced services to subscribers compared to the 2.5G mobile
networks.
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SGSN & GGSN
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• The Serving GPRS Support Node (SGSN) provides the
interface between the GPRS core network and the mobile
radio networks. SGSN handles several functions, such as
packet routing and transfer, mobile attach and detach
procedure, location management, assigning channels and time
slots, and authentication.
• The Gateway GPRS Support Node (GGSN) is the place in the
network, which allows the mobile operators to control the
service access and to perform differentiated charging based on
the services and applications that are used.
• GPRS Tunneling Protocol between SGSN and GGSN in the
Gn interface
• A PDP Context can be understood as a ‘call’ in a packet
switched network. A PDP context is a logical connection
between a mobile and an external data network, with predefined and dynamic parameters.
• Secondary PDP contexts
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The Go Interface towards IP Multimedia
Subsystem (IMS)
• The IMS can be understood to be an evolution of 2.5G and 3G
mobile technologies that brings the ability to deliver IP-based
person-to-person real-time multimedia communications.
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Protocol Stack in the Go Interface
• The Policy Decision Function (PDF) is a logical policy decision
element that uses standard IP mechanisms to implement
policy in the IP media layer. The PDF makes decisions in
regard to network based IP policy using policy rules and
communicates these decisions to the PEP in the GGSN.
• The Common Open Policy Service for Policy Provisioning
(COPS-PR) over TCP. PDF = PDP, GGSN = PEP.
• For the purpose of exchanging the required specific Go
information, a 3GPP Go COPS-PR PIB is defined. The Go PIB
defines all objects to carry the information specific to the Go
interface. COPS/COPS-PR/Go PIB.
• The Go interface allows service-based local policy information
to be "pushed" to or requested by the Policy Enforcement Point
(PEP) in the GGSN from a Policy Decision Function (PDF).
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IMS Policy Control/Go Interface
Functionality
•
IMS PDP Context Activation and Modification/ Media
Authorization
•
IMS PDP Context Modification
•
Gating Functionality
•
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Indication of PDP Context Release / Revoke Authorization
for GPRS
and IP resources
•
IMS PDP Context QoS Modification to/from 0 kbps
•
Charging correlation
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Binding Information and Authorized
QoS
• The binding information is used by the GGSN to identify the
correct PDF and subsequently request service-based local
policy information from the PDF.
• The Authorized QoS specifies the maximum QoS that is
authorized for a PDP context for that specific binding
information.
Authorised QoS
Authorised QoS
for uplink
Authorised QoS
for dow nlink
QoS class
Data rate
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Go PIB QoS Class
UMTS Traffic Class
A
B
Conversational
Streaming
C
D
Interactive
E
F
Background
The Flow Specific Policy
• The flow specific policy is
supplied by a PDF for each
direction of an IP flow. There
can be one or more IP flows
to be carried in the one PDP
context.
IMS PDP context
1..n
Gate
Dow nlink gate
Uplink gate
Gate filter:
• Source addr,
• Dest addr,
• Source port,
• Dest port,
• Protocol
Gate status:
• Open/close
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IMS PDP Context Procedures
• IMS PDP Context Activation
• IMS PDP Context Modification
• IMS PDP Context Deactivation
IMS PDP Context Activation Procedure
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Unsolicited (PDF initiated) PDP Context
Modification
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IMS Policy Control Test Tools
• TTCN COPS Tester emulating PDF in the Go interface
• GTPtool: emulates the SGSN. It can be used to send, receive
and process GTP-C/GTP-U messages in the Gn interface.
• Netcat: provides a tool to send downlink user data packets in
Gi interface from any source IP address and port to a wanted
UE IP address and port via GGSN.
• Ethereal protocol analyzer is used in the emulated Gn, Go and
Gi interfaces for verifying the correctness of messages.
• GGSN Internal Tools. There are some tools implemented to
help the testing to verify the status of PDP contexts and
processes running in the GGSN.
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IMS Policy Control Testing Environment
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Further Development – Testing
Automation
• Policy Control functionality found many faults in the code
already in the early phase of the testing. This caused much retesting and regression testing.
• The GGSN build contains also other subsystems. Changes to
another subsystem requires regression testing also for IMS
Policy Control feature
• HIT (Holistic Integration Testing). HIT is widely used for
automation of testing scripts and macros. It can also run scripts
in TTCN Testers.
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IMS Policy Control Test Automation
Network
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Conclusions (1/3)
• The study showed that the Go interface is an interface addition to
the GPRS/3G network causing more signaling via the PDF for the
PDP context activation, modification and deactivation procedures.
• The test planning covered successfully tests for the IMS Policy
Control features to achieve functioning code
• Studies and testing proved that the COPS/COPS-PR/Go PIB is a
functioning protocol stack to provide the IMS Policy Control
functionality for the GGSN.
• The Go PIB/COPS-PR/COPS protocol stack has also its drawbacks.
The stack is over TCP and therefore quite heavy for real-time PDP
context signaling. The Go PIB/COPS-PR/COPS structure contains
three different layers for handling one PIB.
• The Intel COPS Client is more optimal for COPS Differentiated
Services (DiffServ) PIB usage. In this case PDP pushes to PEP
heavy Decisions seldom and real-time factor is not important.
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Conclusions (2/3)
• The tester proved to be efficient tool for the Go interface
testing. Especially, regression and automation testing shows
the usefulness of the TTCN COPS Tester. On the other hand,
the user must make sure that the structure of the Go COPS
message is correct in PRI level.
• A lot of faults were found in the COPS and COPS-PR layer for
the code based on the open source Intel COPS Client SDK.
• As criticism towards the IMS Policy Control we can ask what is
the purpose of controlling in detail what the users can do with
an open network with intelligent terminals.
• GGSN performance problem: The implementation of the Go
interface adds the amount of rules the GGSN has to process
for every IMS PDP context user data packet. ->ASIC based
packet processing?
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Conclusions (3/3)
• Deficiencies in the Specifications: Go interface is a new
interface meaning that the standard [29207] is not yet mature
and there can be faults in it. This can cause misunderstandings
in development and testing process.
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