Diameter is a Authentication Authorization and Accounting (AAA) protocol. It works on the Application Layer if we consider OSI Layered model. Diameter is a message based protocol, where AAA nodes exchange messages and receive Positive or Negative acknowledgment for each message exchanged between nodes. For message exchange it internally uses the TCP and SCTP which makes diameter reliable. Its technical specifications are given in RFC-6733 Diameter Base Protocol.
Diameter basically a successor of RADIUS (Remote Authentication Dial In User Service) Which is also a AAA protocol based on UDP. UDP does not use implicit hand-shaking mechanism for providing reliability, ordering, or data integrity, Unreliability was the major Flaw in Radius.
Here i have tried to explain various facets of Diameter Base Protocol, Before going to the details of various aspects of the Diameter Base Protocol we will see the improvements of the diameter over RADIUS.