RFC9410: Handling of Identity Header Errors for Secure Telephone Identity Revisited (STIR)

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Internet Engineering Task Force (IETF)                          C. Wendt
Request for Comments: 9410                                    Somos Inc.
Category: Standards Track                                      July 2023
ISSN: 2070-1721

    Handling of Identity Header Errors for Secure Telephone Identity
                            Revisited (STIR)


   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.

Status of This Memo

   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

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Reason Header Field Protocol "STIR"
   4.  Use of Provisional Response to Signal Errors without
           Terminating the Call
   5.  Handling of a Verification Error When There Are Multiple
           Identity Header Fields
   6.  Handling Multiple Verification Errors
   7.  Removal of the Reason Header Field by Authentication Service
   8.  IANA Considerations
   9.  Security Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Author's Address

1.  Introduction

   The STIR framework as described in [RFC7340] is an authentication
   framework for asserting a telephone number or URI-based identity
   using a digital signature and certificate-based framework, as
   described [RFC8225] and [RFC8226], respectively.  [RFC8224] describes
   the use of the STIR framework in the SIP protocol [RFC3261].  It
   defines both a) the authentication service that creates a PASSporT
   [RFC8225] 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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Reason Header Field Protocol "STIR"

   This document defines a new Reason header field [RFC3326] protocol,
   "STIR", for STIR applications using SIP as defined in [RFC8224].  The
   use of "STIR" as a Reason header field protocol with the error
   defined in [RFC8224] causes codes to allow the use of multiple Reason
   header fields as detailed in [RFC3326] and updated in [RFC9366].  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 [RFC8224] or future
   specifications.  The use of multiple Reason header fields is
   discussed in more detail later in the document.

4.  Use of Provisional Response to Signal Errors without Terminating the

   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 [RFC8224], 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:

   Reason: STIR ;cause=436 ;text="Bad Identity Info"

5.  Handling of a Verification Error When There Are Multiple Identity
    Header Fields

   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
   [RFC3326], 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 [RFC8224], 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 [RFC3326]
   using the protocol value of "STIR" defined in this document MUST only
   identify a single cause code [RFC3326] in the context of a call
   dialog [RFC3261] corresponding only to the STIR-related error codes
   defined in [RFC8224] 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
   [RFC8225], 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:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi= \
   "eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1I \
   joiaHR0cHM6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJ \
   kZXN0Ijp7InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdC \
   I6IjE0NDMyMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.r \
   q3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \

   Example Reason header field with a compact form PASSporT:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi= \
   "..rq3pjT1akEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \

6.  Handling Multiple Verification Errors

   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 [RFC3326] is updated in [RFC9366], allowing
   multiple Reason header fields with the same protocol value.  For this
   specification, "STIR" should be used for any STIR error defined in
   [RFC8224] or future specifications.

   Example Reason header fields for two identity info errors:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi=     \
   "..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFY \

   Reason: STIR ;cause=438 ;text="Invalid Identity Header" ;ppi=  \
   "..rJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYsq3pjT1hoRwakEGjHCnWSwUnsh \

7.  Removal of the Reason Header Field by Authentication Service

   When an authentication service [RFC8224] 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.

8.  IANA Considerations

   IANA has registered the following new protocol value (and associated
   protocol cause) in the "Reason Protocols" registry under

             | Protocol Value | Protocol Cause  | Reference |
             | STIR           | STIR Error code | [RFC8224] |

                                 Table 1

   IANA has also registered a new header field parameter name in the
   "Header Field Parameters and Parameter Values" registry under

     | Header Field | Parameter Name | Predefined Values | Reference |
     | Reason       | ppi            | No                | RFC 9410  |

                                  Table 2

9.  Security Considerations

   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.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
              Header Field for the Session Initiation Protocol (SIP)",
              RFC 3326, DOI 10.17487/RFC3326, December 2002,

   [RFC8174]  Leiba, B., "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]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

   [RFC9366]  Sparks, R., "Multiple SIP Reason Header Field Values",
              RFC 9366, DOI 10.17487/RFC9366, March 2023,

10.2.  Informative References

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,


   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.

Author's Address

   Chris Wendt
   Somos Inc.
   Email: chris-ietf@chriswendt.net