Deltathree This document examines the proposal for a SIP extension to allow authentication of a user-agent through a proxy to a RADIUS server using CHAP put forward by Byerly and Williams [BYERLY]. An alternative method is offered which does not require any changes to the SIP protocol as defined in [SIP]. It also will not require any modification to the way RADIUS clients communicate with RADIUS Servers [RADIUS], although it will change the way some of the fields are interpreted by the RADIUS server.