With the early launch of the Mobile Internet with 2.5G and 3G the main AAA (Authentication, Authorisation and Accounting) protocol used was RADIUS (Remote Authentication Dialled Up User Service). It was straight from the Wireline environment.
Due to limitations (e.g. Security, Scalability, Availability, AVP extensions, etc..) with RADIUS another protocol has to be defined for 3GPP IMS, etc.. This protocol was known as Diameter. It was built from the existing RADIUS stack and extended.
IETF RFC 6733 defines Diameter. Note that the earlier RFC 3588 is still seen in a number of documents / specifications. Note that RFC 6733 replaces RFC 3588. For online charging RFC 4006 (Diameter Credit Control Application) is used.
For more information look at Gy, Gx, Sy, Ro, Rf, etc..