For more information on the standardization of Diameter in IETF, the reader is referred to the website for the IETF AAA working group
For information on open source implementations of Diameter, the following website is a good starting point:
One of the major third generation cellular network standards (3GPP) is using Diameter for AAA procedures. The standardization is performed partly at IETF in documents such as [DIA3G3589] and partly in 3GPP, such as [3GPPDIA]. This would mean most Diameter activities can be traced by following the activities around 3GPP cellular networks.