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 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.