9. Interoperability with Non-shared Appearance UAs
It is desirable to allow a basic UA that does not directly support shared appearance to be part of a shared appearance group. To support this, the Proxy must collaborate with the Appearance Agent. This is not required in the basic shared appearance architecture; consequently, shared appearance interoperability with non-shared appearance UAs will not be available in all shared appearance deployments.
First, a UA that does not support dialog events or the shared appearances feature will be discussed. Then, a UA that does support dialog events but not the shared appearances feature will be discussed.
9.1. Appearance Assignment
A UA that has no knowledge of appearances will only have appearance numbers for outgoing calls if assigned by the Appearance Agent. If the non-shared appearance UA does not support Join or Replaces, all dialogs SHOULD be marked "exclusive" to indicate that these options are not available. Marking these dialogs "exclusive" provides a better user experience and avoids extra SIP messaging failures.
9.2. Appearance Release
In all cases, the Appearance Agent must be aware of the dialog lifetime to release appearances back into the group.
It is also desirable that any dialog state changes (such as hold, etc.) be made available to other UAs in the group through the Dialog Event Package. If the Appearance Agent includes a proxy that Record-Routes for dialogs from the non-shared-appearance-aware UA, the Appearance Agent will know about the state of dialogs including hold, etc. This information could be determined from inspection of non-end-to-end-encrypted INVITE and re-INVITE messages and added to the dialog information conveyed to other UAs.
9.3. UAs Supporting Dialog Events but Not Shared Appearance
Interoperability with UAs that support dialog events but not the shared appearances feature is more straightforward. As before, all appearance number assignments must be done by the Appearance Agent. The Appearance Agent SHOULD still include appearance information in NOTIFYs -- this UA will simply ignore this extra information. This type of UA will also ignore appearance number limitations and may attempt to join or replace dialogs marked exclusive. As a result, the Proxy or UAs need to reject such requests or the dialogs will be joined or taken.