Call Placement Service (CPS) URI Certificate Extension for STI Certificates

This document specifies a non-critical X.509 v3 certificate extension that conveys the HTTPS URI of a Call Placement Service (CPS) associated with the telephone numbers authorized in a STIR Certificate. The extension enables originators and verifiers of STIR PASSporTs to discover, with a single certificate lookup, where Out- of-Band (OOB) PASSporTs can be retrieved. The mechanism only provides a new way to discover the URI of CPS endpoint and is fully backward compatible with existing STIR certificates and OOB APIs.