There are two approaches to solving this problem. This document proposes extending the SIP list of authentication schemes to support a CHAP-Password. A nonce is a octet string that is uniquely generated each time a request is made. It is recommended that a nonce be constructed to exhibit global and temporal uniqueness. A sequence number is a monotonically increasing integer.
Sequence numbers allow detection of replays.
When a SIP proxy is used for user authentication an analogy can be drawn from PPP message flows. A string indicating the authentication method to be used. A string containing the user name. The CHAP Identifier is a one octet sequence number. A string of 32 hex digits.