From: Zarko Coklin <[EMAIL PROTECTED]>

   1. Can Registrar have liberty to change realm in 407 between
   REGISTER requests?

   2. How should UA react if it gets different realms for 2 different
   REGISTERs?  Some UA cached REALM1 and used it all the time not
   respecting that Registrar required REALM2 to be used.

   My take would be that Registrar can change realm for different
   REGISTER requests. Also, I expect UAs to use all parameters found
   in the last 407 received and not to cache anything.

I'd say you are right on these points.

The only additional complexity is the possibility that that getting
the REGISTER accepted might require *two* sets of credentials.  If the
REGISTER with credentials for REALM1 gets a 407 for REALM2, and the UA
re-sends the REGISTER with credentials for REALM2 (only), it should be
prepared to cope with receiving a 407 for REALM1, by sending the
REGISTER with both sets of credentials.

As an optimization, when sending a request to a destination, the UA
should re-use any credentials that it needed to send a request to that
destination previously, in hopes of avoiding a round-trip for 401/407.
Ideally, a registrar should issue nonces with longer lifetimes than
the registrations it provides, and issue a nextnonce in every 200
response.  If done right, each REGISTER (except the first) succeeds
the first time.

Dale
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