Question about freeradius and Cisco VoIP router

Hello All I am writing a C program as an external program for freeradius to handle authentication requests recieved from a Cisco VoIP router. How can I pack attribute-value pairs and send them to the router? I can determine the attribute-value pairs transmitted by the router through environment variables, but I don't know how to send the attribute-value pairs which the router expects to recieve. Best Regards Ali