6. Constructing SIP URIs

   In many cases, an element needs to construct a SIP URI for inclusion
   in a Contact header in a REGISTER, or in a Record-Route header in an
   INVITE.  According to RFC 3261prop [1], these URIs have to have the
   property that they resolve to the specific element that inserted
   them.  However, if they are constructed with just an IP address, for
   example:

   sip:1.2.3.4

   then should the element fail, there is no way to route the request or
   response through a backup.

   SRV provides a way to fix this.  Instead of using an IP address, a
   domain name that resolves to an SRV record can be used:

   sip:server23.provider.com

   The SRV records for a particular target can be set up so that there
   is a single record with a low value for the priority field
   (indicating the preferred choice), and this record points to the
   specific element that constructed the URI.  However, there are
   additional records with higher values of the priority field that
   point to backup elements that would be used in the event of failure.
   This allows the constraint of RFC 3261prop [1] to be met while allowing
   for robust operation.