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

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.