Identity Header Error Handling

in Section 6.2.2 discusses future specifications for enhancement of how errors are communicated and the handling of multiple Identity header fields. This specification provides some additional mechanisms for solutions to address these problems. In some deployments of STIR and specifically using SIP as defined by , one issue with the current error handling, specifically with the use of the defined 4xx error responses, is that when an error occurs with the verification of the Identity header field or the PASSporT contained in the Identity header field and a 4xx response is returned, the call is then terminated. It may be the case that the policy for handling errors dictates that calls should continue even if there is a verification error, in the case of, for example inadvertent errors, however the authentication service should still be notified of the error so that corrective action can be taken. This specification will discuss the use of the Reason header field in subsequent provisional (1xx) responses in order to accomplish this. For the handling of multiple Identity header fields and the potential situation that some of the Identity header fields in a call may pass verification but others may have errors, this document provides a mechanism to add an identifier so that the authentication service can identify which Identity header field is being referred to in the case of an error.

The keywords “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 when, and only when, they appear in all capitals, as shown here.

This specification defines a new Reason header field protocol “STIR” for STIR applications using SIP as defined in . This will differentiate current protocols, specifically “SIP” which is currently in wide industry usage, from the defined error cause codes and the potential use of multiple Reason header fields defined in and updated in [upcoming document TBD] allowing multiple Reason header fields with the same “STIR” protocol string. The use of multiple Reason header field 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 occured, including 4XX errors described in Section 6.2.2, then the verification service SHOULD NOT send the 4XX as a response, but rather include the error response code and reason phrase in a Reason header field, defined in , in the next provisional or final responses sent 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 SHOULD include the error response code and reason phrase associated with the error in a Reason header field, defined in , in the next provisional or final responses sent to the authentication service. The reason cause in the Reason header field SHOULD represent the error that occurred when verifying the contents of the Identity header field. The association of a Reason header field and error to a specific Identity header field is accomplished by adding a “ppt” parameter containing the PASSporT that generated the error to the Reason header field. The “ppt” parameter for the Reason header field is optional, but RECOMMENDED, in particular for cases that a SIP INVITE contains multiple Identity header fields. The PASSporT can be included in full form, or optionally in compact form, where only the signature of the PASSporT is used to identify the reported Identity header field with an error. Example Reason header field with full form PASSporT:

Example Reason header field with compact form PASSporT: ~~~~~~~~~~~ Reason: STIR ;cause=436 ;text=”Bad Identity Info” ;ppt= \ “..rq3pjT1akEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \ ojNCpTzO3QfPOlckGaS6hEck7w” ~~~~~~~~~~~

If there are multiple Identity header field verification errors being reported the verification service SHOULD include corresponding Reason header fields with “ppt” parameters including 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 is updated in allowing multiple Reason header fields with the same protocol value, for this specification being “STIR”. Example Reason header fields for two identity info errors:

When an Authentication Service 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), but then MUST remove the identified Reason header field to avoid the PASSporT information from going upstream to a UAC or UAS that may not be authorized to see claim information contained in the PASSporT for privacy or other reasons.

This document requests the definition of a new protocol value (and associated protocol cause) to be registered by the IANA into the “Reason Protocols” sub-registry under http://www.iana.org/assignments/sip-parameter as follows:

Would like to thank David Hancock for help to identify these error scenarios and Jon Peterson, Roman Shpount, Robert Sparks and STIR working group for helpful feedback and discussion.

This specification discusses the use of a PASSporT as an identifier for cases where there is multiple identity header errors occuring 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 beyond the authentication service that originally signed the PASSporT(s) in the resulting error Reason header field. This specification states the authentication service MUST remove the Reason header field with the PASSporT to protect the security (e.g. use of 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. This is just to reinforce this MUST as a security consideration that should be followed.