This specification extends the SHAKEN framework to enable the transmission of Personal ASSertion Tokens (PASSporTs), as defined in RFC 8225, Personal Assertion Token, for calls that use TDM signaling and/or TDM switches during transit. This specification adheres to the following core principles:

1. The solution does not place any new requirements on SHAKEN-compliant VoIP service providers.

2. Preferably, the solution supports the most common call scenarios representing a majority of traffic but does not need to support all possible call scenarios.

3. The solution supports and facilitates the long-term industry goal of migrating to VoIP-based networks.

Within the specification, cryptographically signed PASSporT(s) are exchanged out-of-band, that is, separate from the telephone network signaling. The mechanism of exchanging PASSporT(s) out-of-band is based on draft-ietfstir- servprovider-oob-01, Out-of-Band STIR for Service Providers.

It is recommended that ATIS-1000097, Technical Report on Alternatives for Caller Authentication for Non-IP Traffic, evaluating the viability of implementing this call authentication mechanism for TDM networks, be considered along with this document.