Diameter is a computer networking protocol for AAA (Authentication, Authorization and Accounting). It is a successor to RADIUS.
Upgrade from RADIUS
The name is a pun on the RADIUS protocol, which is the predecessor (a diameter is twice the radius). Diameter is not directly backwards compatible, but provides an upgrade path for RADIUS. The main differences are as follows:
Reliable transport protocols (TCP or SCTP, not UDP)
Network or transport level security (IPsec or TLS)
Transition support for RADIUS, although Diameter is not fully compatible with RADIUS
Larger address space for attribute-value pairs (AVPs) and identifiers (32 bits instead of 8 bits)
Client-server protocol, with exception of supporting some server-initiated messages as well
Both stateful and stateless models can be used
Dynamic discovery of peers (using DNSSRV and NAPTR)
Capability negotiation
Supports application layer acknowledgements, defines failover methods and state machines (RFC 3539)
More easily extended; new commands and attributes can be defined
Aligned on 32-bit boundaries
Basic support for user-sessions and accounting
Protocol description
The Diameter base protocol is defined by RFC 3588, and defines the minimum requirements for an AAA protocol. Diameter Applications can extend the base protocol, by adding new commands and/or attributes. An application is not a program, but a protocol based on Diameter. Diameter security is provided by IPSEC or TLS, both well-regarded protocols.
Packet format
Commands
Each command is assigned a command code, which is used for both requests and answers.
Diameter Application is not a software application, but a protocol based on the Diameter base protocol (defined in RFC 3588). Each application is defined by an application identifier and can add new command codes and/or new mandatory AVPs. Adding a new optional AVP does not require a new application.
Examples of Diameter applications :
Diameter Mobile IPv4 Application (MobileIP, RFC 4004)
Diameter Network Access Server Application (NASREQ, RFC 4005)
http://www.cisco.com/en/US/products/ps6638/products_data_sheet09186a00804fe332.html Cisco page outlining differences between RADIUS and DIAMETER
http://sourceforge.net/projects/openblox/ Open Source Java IMS optimised Full Diameter solution
http://www.diva-portal.org/diva/getDocument?urn_nbn_se_liu_diva-1195-1__fulltext.pdf Paper about Diameter by Håkan Ventura
http://www.csg.uzh.ch/staff/morariu/opendiameter/ OpenDiameter Debian and Ubuntu repository
http://www.amazon.com/AAA-Network-Security-Mobile-Access/dp/0470011947/ref=sr_1_1?ie=UTF8&s=books&qid=1203514099&sr=1-1 AAA and Network Security for Mobile Access: Radius, Diameter, EAP, PKI and IP Mobility
