5.3.2. Appearance Numbers and Call Control

When an INVITE is generated to attempt to bridge or take a call (i.e., contains Join or Replaces with a dialog identifier of another dialog in the shared appearance group), the UA MUST first send a PUBLISH to the Appearance Agent. This PUBLISH will contain:

1. The appearance number of the joined or replaced call in the <appearance> element

2. The dialog information from the Join header field in the <joined-dialog> element, if the dialog is being joined

3. The dialog information from the Replaces header field in the <replaced-dialog> element, if the dialog is being replaced

Note that this information is provided to the Appearance Agent so that it can provide proper appearance assignment behavior. If the INVITE Join or Replaces was sent without publishing first, the Appearance Agent might assign a new appearance number to this INVITE, which would be a mistake. With Join, the publication has the <joined-dialog> element to prevent the Appearance Agent from generating a 400 (Bad Request) response due to the reuse of an appearance number. For Replaces, the purpose of the <replaced-dialog> is to prevent a race condition where the BYE could cause the appearance number to be released when it should stay with the replacing dialog.