8. The Transport Determination Application

   This section more formally defines the NAPTR usage of this
   specification, using the Dynamic Delegation Discovery System (DDDS)
   framework as a guide [7].  DDDS represents the evolution of the NAPTR
   resource record.  DDDS defines applications, which can make use of
   the NAPTR record for specific resolution services.  This application
   is called the Transport Determination Application, and its goal is to
   map an incoming SIP or SIPS URI to a set of SRV records for the
   various servers that can handle the URI.

   The following is the information that DDDS requests an application to
   provide:

      Application Unique String: The Application Unique String (AUS) is
         the input to the resolution service.  For this application, it
         is the URI to resolve.

      First Well Known Rule: The first well known rule extracts a key
         from the AUS.  For this application, the first well known rule
         extracts the host portion of the SIP or SIPS URI.

      Valid Databases: The key resulting from the first well known rule
         is looked up in a single database, the DNS [8].

      Expected Output: The result of the application is an SRV record
         for the server to contact.