Here is the detail of RADIUS Operations. Before Client starts communicating with RADIUS Server, it is required that shared secret must be shared between Client and Server and Client must be configured to use RADIUS server to get service.
Once Client is configured properly then :
Client starts with Access-Request.
Server sends either Access-Accept, Access-Reject or Access-Challenge.
Access-Accept keeps all required attribute to provide a service to user.
RADIUS Codes (decimal) are assigned as follows:
12 Status-Server (experimental)
13 Status-Client (experimental)
No Keep Alive concept - Good or Bad ??
Codes 4 and 5 are related to RADIUS Accounting Functionality. Codes 12 and 13 are reserved for possible use, but are not further mentioned here.