Diameter is Growing Up

The online charging mechanisms supported with IN services and CAMEL are not enough to handle real-time rating. For that reason, the 3GPP’s IMS documentation now heavily concentrates on online charging capabilities. At the core of that work is Diameter Base Protocol, which was developed to overcome limitations of Radius.

Diameter has become the base protocol in IMS for charging and credit control. It has been designed to meet the IETF’s requirements for authentication, authorization and accounting. Whether fail-over, transmission-level security or reliable transport, Diameter offers algorithms, IPSec support and mechanisms for transport.

Diameter is logically divided into a base protocol and applications. The base protocol provides basic services needed by all applications, such as peer discovery and session handling, and can also be used for accounting without any additional Diameter applications. Authentication and authorization, in contrast, need Diameter applications in addition to the base protocol.

The IETF’s AAA Working Group has been honing Diameter Base Protocol so that AAA is coordinated across multifarious technologies and platforms. That allows usage and event information to be passed on for rating and billing purposes, regardless of the environment.

In the last couple of years, a mechanism known as the Diameter Credit Control Application (CCA) has gained momentum as a component of Diameter Base Protocol. CCA is designed to provide real-time credit control to the service charging in IP networks. The bidirectional credit control functionality of CCA fosters on-line charging support via AAA infrastructure. That means service charging with credit reservation, direct debiting and refunding, service price inquiry and account balance checks are all possible in real time.

The integration of Diameter and the CCA complies with the 3GPP’s IMS charging descriptions, defined in the documents TS 32.225 and TS 32.296.

Online Charging Capabilities

The 3GPP has recently put a large amount of energy into its IMS architecture support for online charging, the goal of which is to enable pre- and post-paid convergence. That convergence is desired by operators who want to mitigate fraud and credit risks to service providers. If IMS could foster real-time credit controls, carriers would have the structure to do real-time credit balance checks, charges, discounts and promotions. Additionally, more personalized advice of charge and credit limit controls could engender more compelling services.

So far, the 3GPP has defined interfaces for online charging under IMS (known as Ro). Ro will work with the existing Diameter interface for offline charging (Rf) to open the door to pre- and post-paid convergence. The existing Rf interface enables SIP network elements to use Diameter to transmit accounting data to charging collector functions (CCFs), which in turn provide charging information in the form of CDRs.

The documentation for online charging defines the Session Charging Function (SCF), which is responsible for signaling a Serving Call/Session Control Function (S-CSCF) to terminate sessions whenever credit runs dry during a session. The S-CSCF and a Proxy-Call Session Control Function (PCSCF) will then generate charging records related to session events, such as inception and termination of a call.

3GPP has released a document known as TS 32.260, which is the IMS charging document specifying offline and online charging. The charging description includes the offline and online charging architecture and scenarios specific to IMS, as well as the mapping of common 3GPP charging architecture (specified in document TS 32.240).

These documents are complemented by ongoing work for online charging underway with such organizations as JCP (JAIN SLEE, JAIN APIs), Parlay (all Parlay SCF v5) and OMA.

“Online charging is a single document that is divided into subdocuments under the IMS system work conducted by 3GPP,” says Betsy Covell, chair of 3GPP2. “We are in the process of harmonizing work in 3GPP, 3GPP2 and OMA so that operators will have an easier time evolving systems as they develop applications running on multiple environments. We want to ensure charging is coordinated across different environments.”

The ongoing work for online charging will focus on documents to specify the content and functionality of CDRs per domain and per subsystem for offline and online charging, as well as the interfaces used in the charging framework to transfer charging data.

The goal is to collect service usage data or authorization requests for voice and data over wireline and wireless, whether pre-paid and post-paid going over GSM, CDMA or UMTS interfaces.

QoS and Mobility Under Diameter

“For operators looking at fixed/mobile convergence, mobility and QoS will be a big issue,” says Hannes Tschofenig, a scientist in Siemens’ research division. He is now co-chairing a new IETF working group called Diameter Maintenance and Extensions (DIME) together with John Loughney, principal engineer at Nokia’s Helsinki-based research center. This working group will follow up on the work done in the IETF’s AAA working group.

The group will revisit the previous work on Diameter Base Protocol and will work on Diameter Mobile IPv6 and Diameter Quality of Service (QoS) signaling. “There’s a need for additional extensions and new functionality, specifically around Mobile IPv6,” says Tschofenig.

QoS has been an important topic in the 3GPP, the 3GPP2, ETSI TISPAN and the ITU-T. “We hope the DIME working group will be a place to discuss Diameter extensions for QoS, as the AAA group wasn’t the right place to work on these mechanisms,” says Tschofenig. “For the interaction between management entities [policy functions and application servers] to network elements [routers], Diameter will be an important part of the entire QoS story.”

The DIME WG will look at how to reuse existing infrastructure in such a way that systems work together for service differentiation and prioritization of voice or video. “The IMS consists of SIP signaling, and the network elements do the prioritization of data traffic,” Tschofenig says, “but they need to be told which traffic is to be treated differently.”

Thus far, the 3GPP has come up with a way to take call flows for SIP sessions so that elements can be extracted to describe bandwidth and other QoS parameters. The information is then conveyed to routers so they know how to treat the traffic according to parameters and rules.

“Whether video streams or voice signaling, you want to be able to create rules on the fly,” says Tschofenig, who envisions a very dynamic scenario. “Operators need to know where they want QoS treatment for traffic that travels through their network.”

To address the mobility aspects, the DIME and MIP6 working groups will look how the AAA infrastructure can support mobility signaling procedures. The MIP6 WG is currently investigating a bootstrapping solution. (Bootstrapping is the process that allows a mobility anchor point, such as the Home Agent, to obtain essential configuration parameters—such as keying material and addresses—and to trigger AAA procedures.) In short, bootstrapping enables an easier deployment of Mobile IPv6 services for operators.

“Mobility protocols require an interaction with the AAA protocols; the front-end protocols need to work together with the back-end protocols,” adds Tschofenig.

The base protocol and extensions will be tested in an interoperability event to be held in April. “We will be testing how boxes from different vendors interact with each other in order to determine bugs and other problems with our specifications,” says Tschofenig.