The proliferation of services on today's internet has made it necessary to develop a new solution to manage growing network traffic. A successor to the RADIUS protocol, Diameter was created as a signaling protocol to manage the interconnection of servers in core networks. Diameter is a packet-based system that uses TCP or SCTP in an all-IP network. Deployments in LTE networks have given telecom providers significant advantages over legacy technologies.

Diameter Signaling Protocol

Diameter is a signaling protocol designed to meet authentication, authorization and accounting (AAA) needs in computer networks. As new access technologies developed, it became clear that more robust support was needed to handle the scale and complexity of AAA networks. RFC 6733 (which superseded RFC 3588 in 2012) provides the standard for the Diameter Signaling Protocol. It describes the significant advantages of Diameter over its predecessor RADIUS (RFC 2685). The protocol is designed to meet these network access requirements as spelled out in RFC 2989:

  • Failover
  • Transmission-level security
  • Reliable transport
  • Agent support
  • Server-initiated messages
  • Transition support

A protocol facilitates a conversation between two network elements. Diameter makes this conversation with the use of Attribute-Value Pairs (AVPs). The exchange of data provides access to a broad array of technologies in today's connected world. Diameter is extensible and provides reliable peer-to-peer networking to meet a growing demand for services. (For more on networking, see The Growing Demand for Networking Pros.)

Advantages of Diameter

Diameter is a more robust protocol than RADIUS. Initially used for dial-up PPP and terminal service access, RADIUS enabled users to get onto the internet and access services. To overcome the limitations of RADIUS, Diameter was created. (This was a play on words. A diameter is equal to twice the radius.) Diameter has several advantages over RADIUS:

  • Advanced Processes – Error notification and AVPs are part of Diameter signaling. AVPs permit application developers to include customizable controls for signaling streams. A Diameter application, defined by the Application-ID field in the AVP, allows for communication dedicated to a specific purpose.
  • Reliability – While RADIUS uses the unreliable transport protocol UDP, Diameter makes use of the reliable protocols TCP or SCTP on port 3868. The hop-by-hop retransmission mechanism of Diameter ensures reliability of transport and continuity of traffic flow.
  • Extensibility – Diameter makes it possible for third parties, such as other IETF working groups, to define standard attributes which enable new services.

Diameter in the Network

RFC 2989 says that “the AAA protocol must be capable of supporting millions of users and tens of thousands of simultaneous requests.” Not only that, “the AAA architecture and protocols must be capable of supporting tens of thousands of devices, AAA servers, proxies and brokers.” These are high demands. Diameter was designed to address the exponential growth of network services.

All-IP networks, such as LTE, have become the perfect arena for the Diameter protocol. With the tremendous increase of internet services and users, AAA solutions providers saw the need for a Diameter-based traffic cop. A common term for this device is “Diameter Signaling Router.” Use cases include charging proxy, policy proxy and core routing. This means that servers performing these functions could be connected in a robust signaling environment. These devices are given different names by the providers:

  • Oracle – Diameter Signaling Router
  • F5 – Traffic SDC
  • Diametriq – Diameter Routing Engine
  • Ericsson – Diameter Signaling Controller
  • Sonus – Diameter Signaling Controller

Exponential growth of Diameter traffic is expected as subscribers move to IMS-based networks. The IMS (IP Multimedia Subsystem) architecture is a 3GPP specification originally designed to provide IP multimedia to mobile users. It functions as a core network to provide voice, text and multimedia across large networks.

Along with Session Initiation Protocol (SIP), Diameter has become a key signaling protocol for IMS. Working together, Diameter and IMS have been employed to gain control of expanding IP services. Components such as Home Subscriber System (HSS), Application Server (AS), Packet Gateway (PGW) and Mobility Management Entity (MME) are connected by well-defined interfaces within the network. Diameter works well in the integration of IMS into the Evolved Packet Core (EPC) of Long-Term Evolution (LTE) networks. (For more on LTE, see The Real Score on 4G Wireless.)

Diameter routers can be dedicated as Policy and Charging Rules Function (PCRF) devices. PCRF can be used to limit usage, charge based on usage or control roaming or bandwidth. Diameter also allows for interoperability with legacy networks, such as UMTS. In the peer-to-peer Diameter architecture, Diameter devices can function as relay agents, proxy agents, redirect agents or translation agents. These network elements become nodes in the Diameter network, providing reliable sessions across TCP or SCTP links. Diameter nodes negotiate capabilities and provide security within the messages they send, providing significant benefits over the legacy RADIUS protocol.

The Diameter signaling protocol is a capable and versatile part of modern IP networks. Its scalability and potential use cases make it essential to the growing IP universe. It is likely that further adaptations of the protocol will continue to be developed for some time to come.