EAP TLS: how the radius server track clients

[Jim Shi]

Hi, I have a question about how the radius server keep track of clients in EAP TLS.
My understanding is that EAP TLS involves a series of  requests/responses. 
Hence the radius server needs to keep track of status for each client during the handshake.
How does the radius server keep track of clients? Based on 'User-Name' in the radius packet request?
Thanks
Jim