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10.2.1.2 Preferences among Contact Addresses
If more than one Contact is sent in a REGISTER request, the
registering UA intends to associate all of the URIs in these Contact
header field values with the address-of-record present in the To
field. This list can be prioritized with the "q" parameter in the
Contact header field. The "q" parameter indicates a relative
preference for the particular Contact header field value compared to
other bindings for this address-of-record. Section 16.6 describes
how a proxy server uses this preference indication.
10.2.2 Removing Bindings
Registrations are soft state and expire unless refreshed, but can
also be explicitly removed. A client can attempt to influence the
expiration interval selected by the registrar as described in Section
10.2.1. A UA requests the immediate removal of a binding by
specifying an expiration interval of "0" for that contact address in
a REGISTER request. UAs SHOULD support this mechanism so that
bindings can be removed before their expiration interval has passed.
The REGISTER-specific Contact header field value of "*" applies to
all registrations, but it MUST NOT be used unless the Expires header
field is present with a value of "0".
Use of the "*" Contact header field value allows a registering UA
to remove all bindings associated with an address-of-record
without knowing their precise values.
10.2.3 Fetching Bindings
A success response to any REGISTER request contains the complete list
of existing bindings, regardless of whether the request contained a
Contact header field. If no Contact header field is present in a
REGISTER request, the list of bindings is left unchanged.
10.2.4 Refreshing Bindings
Each UA is responsible for refreshing the bindings that it has
previously established. A UA SHOULD NOT refresh bindings set up by
other UAs.
The 200 (OK) response from the registrar contains a list of Contact
fields enumerating all current bindings. The UA compares each
contact address to see if it created the contact address, using
comparison rules in Section 19.1.4. If so, it updates the expiration
time interval according to the expires parameter or, if absent, the
Expires field value. The UA then issues a REGISTER request for each
of its bindings before the expiration interval has elapsed. It MAY
combine several updates into one REGISTER request.
A UA SHOULD use the same Call-ID for all registrations during a
single boot cycle. Registration refreshes SHOULD be sent to the same
network address as the original registration, unless redirected.
10.2.5 Setting the Internal Clock
If the response for a REGISTER request contains a Date header field,
the client MAY use this header field to learn the current time in
order to set any internal clocks.
