Can a UAC cache the nonce and avoid the extra round-trip?
It is possible in some cases to cache the nonce that is used in the challenge, so that the Authorization header can be re-used in later SIP requests without necessitating going through the authorization challenge. However, for security reasons, the nonce is valid for only a limited period of time. SPEC SIP assumes that any nonce would have expired and thus the authorization challenge is necessary for each transaction.
