RFC 9888: Out-of-Band Secure Telephone Identity Revisited (STIR) for Service Providers

The Secure Telephone Identity Revisited (STIR) framework defines means of carrying its Personal Assertion Tokens (PASSporTs) either in-band, within the headers of a Session Initiation Protocol (SIP) request, or out-of-band, through a service that stores PASSporTs for retrieval by relying parties. This specification defines a way that the out-of-band conveyance of PASSporTs can be used to support large service providers for cases in which in-band STIR conveyance is not universally available.