The Session Initiation Protocol (SIP) has seen widespread deployment
within individual domains, typically supporting voice and video
communications. Though it was designed from the outset to support
inter-domain federation over the public Internet, such federation has
not materialized. The primary reasons for this are the complexities
of inter-domain phone number routing and concerns over security.
This document reviews this problem space, outlines requirements, and
then describes a new model and technique for inter-domain federation
with SIP involving the Public Switched Telephone Network (PSTN),
called Verification Involving PSTN Reachability (VIPR). VIPR
addresses the problems that have prevented inter-domain federation
over the Internet. It provides fully distributed inter-domain
routing for phone numbers, authorized mappings from phone numbers to
domains, a new technique for automated SIP anti-spam, and privacy of
number ownership, all while preserving the trapezoidal model of SIP.