The Diameter protocol was initially developed by the IETF to provide an Authentication, Authorization, and Accounting
(AAA) framework for applications such as remote network access or IP mobility. The
Diameter protocol has several advantages over previous AAA protocols like RADIUS
in that it offers improvements in the areas of reliability, security, scalability,
and flexibility.
The Diameter base protocol provides the following facilities:
- Connection and session management
- User authentication and capabilities negotiation
- Reliable delivery of attribute value pairs (AVPs)
- Agent support for proxy, redirect, and relay servers
- Extensibility, through addition of new commands and AVPs
- Basic accounting services
Diameter sessions consist of exchange of commands and AVPs between authorized Diameter
Clients and Servers. Some of the command values are used by the Diameter protocol
itself, while others deliver data associated with particular applications that employ
Diameter. The Diameter base protocol provides the minimum requirements needed for
AAA protocol, Mobile IPv4, or remote network access applications. In addition to
the base Diameter specification (RFC 3588), the IETF defines several Diameter applications
that use the underlying services.
Diameter in the IMS
The 3GPP standards body has adopted
Diameter as the primary signaling protocol for AAA and mobility management in the
IP Multimedia Subsystem (IMS). The IMS is the service delivery environment for real-time
multimedia services for the 3rd Generation wireless network and is also an ideal
architecture for service convergence across fixed, wireless, and corporate enterprise
networks. IMS offers a secure, mediated access to value-added services and network
resources inside a managed core IP network while providing open interfaces for third
party developers and enterprise-based applications that need access to the trusted
carrier.
The following diagram is an example of how Diameter is used in the IMS network.
The Home Subscriber Server (HSS) is the master database within the IMS for maintaining
subscriber information including user identification, control information for user
authentication and authorization, location information, and user profile data.
The Call Session Control Function (CSCF) performs SIP session management for a user
(or SIP client) requesting access to IMS services. SIP signaling is used to register
with the Serving CSCF in the home network of the user. The S-CSCF uses the Diameter
Cx interface both to request authorization information from the HSS in response
to a SIP registration request and to retrieve subscriber information. The Diameter
Dx interface is used by the CSCF to locate the HSS serving the subscriber using
the Service Location Feature.
The Diameter Sh interface is used by the Application Servers or OSA/Parlay Gateway
to retrieve and update subscriber profiles and shared database information such
as call screening lists, directories, or presence information. This information
is unique to a subscriber but may require shared access by various authorized application
platforms.
IIn addition to the Diameter applications shown above, IMS charging is implemented
using the Diameter Ro and Rf interfaces. These interfaces support both on-line charging
and off-line collection and forwarding of Call Detail Records using a Diameter protocol
interface. The Ro interface is a 3GPP extension to the Diameter Credit-Control draft
specification currently under development in the IETF.
Diameter Reference Guide
For other useful information, including a glossary of Diamter-related terms and
acronyms, please refer to our
Diameter Reference Guide.
Read more about the
Diameter Protocol and additional Diameter applications.