This document extends the current error-handling procedures for mapping of verification failure reasons to 4xx codes for Secure Telephone Identity Revisited (STIR) and the Authenticated Identity Management in the Session Initiation Protocol (SIP). It extends the ability to use the Reason header field as an option for conveying an error associated with an Identity header field to the upstream authentication service when local policy dictates that the call should continue in the presence of a verification failure. This document also defines procedures that enable a failure reason to be mapped to a specific Identity header field for scenarios that use multiple Identity header fields, where some may have errors and others may not. The handling of those situations is also defined.

This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9410.

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The STIR framework as described in [RFC 7340] is an authentication framework for asserting a telephone number or URI-based identity using a digital signature and certificate-based framework, as described [RFC 8225] and [RFC 8226], respectively. [RFC 8224] describes the use of the STIR framework in the SIP protocol [RFC 3261]. It defines both a) the authentication service that creates a PASSporT [RFC 8225] and delivers it in an Identity header field, and b) the verification service that correspondingly verifies the PASSporT and embedded originating identity. This document is concerned with errors in validating PASSporTs and Identity header fields and how they are communicated in special cases. This document also defines a solution to help address the potential issue of multiple Identity header fields and the plurality of potential verification errors. Additionally, it addresses the issue of the current 4xx error response, i.e., the call is terminated when a verification error is present. In some deployments, it may be the case that the policy for handling errors dictates that calls should continue even if there is a verification error. For example, in many cases of inadvertent or operational errors that do not represent any type of identity falsification attempt, the preferred policy may be to continue the call despite the unverified identity. In these cases, the authentication service should still be notified of the error so that corrective action can be taken to fix any issues. This specification will discuss the use of the Reason header field in subsequent provisional (1xx) responses in order to deliver the error back to the authentication service or other SIP path network equipment responsible for error handling. To handle multiple Identity header fields where some in a call may be verified while others may not (i.e., they have errors), this document defines a method by which an identifier is added to the header so that the authentication service can uniquely identify which Identity header field is being referred to in the case of an error.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

This document defines a new Reason header field [RFC 3326] protocol, "STIR", for STIR applications using SIP as defined in [RFC 8224]. The use of "STIR" as a Reason header field protocol with the error defined in [RFC 8224] causes codes to allow the use of multiple Reason header fields as detailed in [RFC 3326] and updated in [RFC 9366]. Any provisional SIP response message or final response message, with the exception of a 100 (Trying), MAY contain one or more Reason header fields with a STIR-related cause code defined in [RFC 8224] or future specifications. The use of multiple Reason header fields is discussed in more detail later in the document.

In cases where local policy dictates that a call should continue regardless of any verification errors that may have occurred, including 4xx errors described in Section 6.2.2 of RFC 8224, the verification service MUST NOT send the 4xx as a response. Rather, it should include the error response code and reason phrase in a Reason header field in the next provisional or final response it sends to the authentication service. Example Reason header field:

In cases where a SIP message includes multiple Identity header fields and one of those Identity header fields has an error, the verification service MUST include the error response code and reason phrase associated with the error in a Reason header field, defined in [RFC 3326], in the next provisional or final responses sent to the authentication service. The reason cause in the Reason header field MUST represent the error that occurred when verifying the contents of the Identity header field. For a SIP INVITE containing multiple Identity header fields, the "ppi" parameter for the Reason header field is RECOMMENDED. As defined in [RFC 8224], the STIR error codes used in responses are based on an error associated with a specific Identity header field representing a single error occurring with the verification and processing of that Identity header field. The association of a "ppi" parameter with a Reason header field [RFC 3326] using the protocol value of "STIR" defined in this document MUST only identify a single cause code [RFC 3326] in the context of a call dialog [RFC 3261] corresponding only to the STIR-related error codes defined in [RFC 8224] or future documents defining STIR-related error codes. The associated PASSporT object can be included either in full form or in compact form, where only the signature of the PASSporT is included with two periods as a prefix, as defined in Section 7 of RFC 8225, to identify the reported Identity header field with an error. Compact form is the recommended form, as full form may include information that could have privacy or security implications in some call scenarios; this is discussed in Section 9. Example Reason header field with a full form PASSporT: Example Reason header field with a compact form PASSporT:

If there are multiple Identity header field verification errors being reported, the verification service MUST include a corresponding number of Reason header fields per error. These Reason header fields should include a "ppi" parameter, including the full or compact form of the PASSporT with cause and text parameters identifying each error. As mentioned previously, the potential use of multiple Reason header fields defined in [RFC 3326] is updated in [RFC 9366], allowing multiple Reason header fields with the same protocol value. For this specification, "STIR" should be used for any STIR error defined in [RFC 8224] or future specifications. Example Reason header fields for two identity info errors:

When an authentication service [RFC 8224] receives the Reason header field with a PASSporT it generated as part of an Identity header field and the authentication of a call, it should first follow local policy to recognize and acknowledge the error (e.g., perform operational actions like logging or alarming). Then, it MUST remove the identified Reason header field to avoid the PASSporT information from going upstream to a User Agent Client (UAC) or User Agent Server (UAS) that may not be authorized to see claim information contained in the PASSporT for privacy or other reasons.

IANA has registered the following new protocol value (and associated protocol cause) in the "Reason Protocols" registry under <http://www.iana.org/assignments/sip-parameters>: Table 1 IANA has also registered a new header field parameter name in the "Header Field Parameters and Parameter Values" registry under <https://www.iana.org/assignments/sip-parameters>: Table 2

This specification discusses the use of a PASSporT as an identifier for cases where there are multiple identity header field errors occurring as part of the Reason header field response. For some call scenarios (e.g., diversion-based call flows), the signer of the PASSporT(s) may not be the first-hop initiator of the call. In those cases, there may be some security or privacy concerns associated with PASSporT information that is passed upstream beyond the authentication service that originally signed the PASSporT(s) in the resulting error Reason header field. This specification states that the authentication service MUST remove the Reason header field with the PASSporT to protect the security (e.g., use of a potentially still-fresh PASSporT for replay attacks) and privacy of any potential information that could be passed beyond the authentication service response back in the direction of the call initiator. While this specification allows for both the full and compact form of the PASSporT to be used as the error identifier, use of the compact form is RECOMMENDED to avoid the potential exposure of call information contained in the full form of the PASSporT.

[RFC2119]

S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3261]

J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3326]

H. Schulzrinne, D. Oran, and G. Camarillo, "The Reason Header Field for the Session Initiation Protocol (SIP)", RFC 3326, DOI 10.17487/RFC3326, December 2002,
<https://www.rfc-editor.org/info/rfc3326>.

[RFC8174]

B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

[RFC8224]

J. Peterson, C. Jennings, E. Rescorla, and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 8224, DOI 10.17487/RFC8224, February 2018,
<https://www.rfc-editor.org/info/rfc8224>.

[RFC8225]

C. Wendt, and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.

[RFC8226]

J. Peterson, and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.

[RFC9366]

R. Sparks, "Multiple SIP Reason Header Field Values", RFC 9366, DOI 10.17487/RFC9366, March 2023,
<https://www.rfc-editor.org/info/rfc9366>.

[RFC7340]

J. Peterson, H. Schulzrinne, and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014,
<https://www.rfc-editor.org/info/rfc7340>.

The author would like to thank David Hancock for help identifying these error scenarios, as well as Jon Peterson, Roman Shpount, Robert Sparks, Christer Holmberg, and others in the STIR Working Group for their helpful feedback and discussion.

Chris Wendt