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Internet Engineering Task Force (IETF)                          M. Dolly
Request for Comments: 9027                                          AT&T
Category: Standards Track                                       C. Wendt
ISSN: 2070-1721                                                  Comcast
                                                               June 2021

Assertion Values for Resource Priority Header and SIP Priority Header

Claims in Support of Emergency Services Networks

Abstract

This document adds new assertion values for a Resource Priority Header ("rph") claim and a new SIP Priority Header ("sph") claim for protection of the "psap-callback" value as part of the "rph" Personal Assertion Token (PASSporT) extension in support of the security of emergency services networks for emergency call origination and callback.

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 https://www.rfc-editor.org/info/rfc9027.

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

   1.  Introduction
   2.  Terminology
   3.  New Assertion Values for "rph" Claim
   4.  The SIP Priority Header ("sph") Claim
   5.  Order of Claim Keys
   6.  Compact Form of PASSporT
   7.  IANA Considerations
     7.1.  JSON Web Token Claims
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1. Introduction

"Personal Assertion Token (PASSporT) Extension for Resource Priority Authorization" [RFC8443] extended the Personal Assertion Token (PASSporT) specification defined in [RFC8225] to allow the inclusion of cryptographically signed assertions of authorization for the values populated in the Session Initiation Protocol (SIP) 'Resource- Priority' header field [RFC4412]. [EMERGENCY-CALLS] introduces the need and justification for the protection of both the SIP 'Resource- Priority' and 'Priority' header fields, used for categorizing the priority use of the call in the telephone network, specifically for emergency calls.

Compromise of the SIP 'Resource-Priority' or 'Priority' header fields could lead to misuse of network resources (i.e., during congestion scenarios), impacting the application services supported using the SIP 'Resource-Priority' header field and the handling of Public Safety Answering Point (PSAP) callbacks.

[RFC8225] allows extensions by which an authority on the originating side verifying the authorization of a particular communication for the SIP 'Resource-Priority' header field or the SIP 'Priority' header field can use PASSporT claims to cryptographically sign the information associated with either the SIP 'Resource-Priority' or the 'Priority' header field and convey assertion of those values by the signing party authorization. A signed SIP 'Resource-Priority' or 'Priority' header field will allow a receiving entity (including entities located in different network domains/boundaries) to verify the validity of assertions to act on the information with confidence that it has not been spoofed or compromised.

This document adds new "auth" array key values for a Resource Priority Header ("rph") claim defined in [RFC8443], in support of emergency services networks for emergency call origination and callback. This document additionally defines a new PASSporT claim, "sph", including protection of the SIP 'Priority' header field for the indication of an emergency service callback assigned the value "psap-callback", as defined in [RFC7090]. The use of the newly defined claim and key values corresponding to the SIP 'Resource- Priority' and 'Priority' header fields for emergency services is introduced in [EMERGENCY-CALLS] but otherwise is out of scope of this document. In addition, the PASSporT claims and values defined in this document are intended for use in environments where there are means to verify that the signer of the SIP 'Resource-Priority' and 'Priority' header fields is authoritative.

2. Terminology

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. New Assertion Values for "rph" Claim

This specification defines the ability to sign the SIP 'Resource- Priority' header field namespace for local emergency communications defined in [RFC7135] and represented by the string "esnet.x", where x is the priority level allowed in the esnet namespace. As of the writing of this specification, the priority level is between 0 and 4, inclusive, but may be extended by future specifications.

Similar to the values defined by [RFC8443] for the "auth" JSON object key inside the "rph" claim, the string "esnet.x" with the appropriate value should be used when resource priority is required for local emergency communications corresponding and exactly matching the SIP 'Resource-Priority' header field representing the namespace invoked in the call.

When using "esnet.x" as the "auth" assertion value in emergency- service-destined calls, the "orig" claim of the PASSporT MUST represent the calling party number that initiates the call to emergency services. The "dest" claim MUST be either a country- or region-specific dial string (e.g., "911" for North America or a "112" GSM-defined string used in Europe and other countries) or "urn:service:sos", as defined in [RFC5031], representing the emergency services destination of the call.

The following is an example of an "rph" claim for the SIP 'Resource- Priority' header field with an "esnet.1" assertion:

     {
       "dest":{"uri":["urn:service:sos"]},
       "iat":1615471428,
       "orig":{"tn":"12155551212"},
       "rph":{"auth":["esnet.1"]}
     }

For emergency services callbacks, the "orig" claim of the "rph" PASSporT MUST represent the Public Safety Answering Point (PSAP) telephone number. The "dest" claim MUST be the telephone number representing the original calling party of the emergency service call that is being called back.

The following is an example of an "rph" claim for the SIP 'Resource- Priority' header field with an "esnet.0" assertion:

     {
       "dest":{"tn":["12155551212"]},
       "iat":1615471428,
       "orig":{"tn":"12155551213"},
       "rph":{"auth":["esnet.0"]}
     }

After the header and claims PASSporT objects have been constructed, their signature is generated normally per the guidance in [RFC8225], using the full form of PASSporT. The credentials (i.e., Certificate) used to create the signature must have authority over the namespace of the "rph" claim, and there is only one authority per claim. The authority MUST use its credentials associated with the specific service supported by the resource priority namespace in the claim. If r-values are added or dropped by the intermediaries along the path, the intermediaries must generate a new "rph" identity header and sign the claim with their own authority.

4. The SIP Priority Header ("sph") Claim

As defined in [RFC7090], the SIP 'Priority' header field may be set to the value "psap-callback" for emergency services callback calls. Because some SIP networks may act on this value and provide priority or other special routing based on this value, it is important to protect and validate the authoritative use associated with it.

Therefore, we define a new claim key as part of the "rph" PASSporT, "sph". This is an optional claim that MUST only be used with an "auth" claim with an "esnet.x" value indicating an authorized emergency callback call and corresponding to a SIP 'Priority' header field with the value "psap-callback".

The value of the "sph" claim key should only be "psap-callback", which MUST match the SIP 'Priority' header field value for authorized emergency services callbacks. If the value is anything other than "psap-callback", the PASSporT validation MUST be considered a failure case.

Note that because the intended use of this specification is only for emergency services, there is also an explicit assumption that the signer of the "rph" PASSporT can authoritatively represent both the content of the 'Resource-Priority' header field and 'Priority' header field information associated specifically with an emergency services callback case where both could exist. This document is not intended to be a general mechanism for protecting the SIP 'Priority' header fields; this could be accomplished as part of future work with a new PASSporT extension or new claim added to either an existing PASSporT or PASSporT extension usage.

The following is an example of an "sph" claim for the SIP 'Priority' header field with the value "psap-callback":

     {
       "dest":{"tn":["12155551212"]},
       "iat":1615471428,
       "orig":{"tn":"12155551213"},
       "rph":{"auth":["esnet.0"]},
       "sph":"psap-callback"
     }

5. Order of Claim Keys

The order of the claim keys MUST follow the rules of Section 9 of [RFC8225], which defines the deterministic JSON serialization used for signature generation (and validation); the claim keys MUST appear in lexicographic order. Therefore, the claim keys discussed in this document appear in the PASSporT Payload in the following order:

dest

orig

6. Compact Form of PASSporT

The use of the compact form of PASSporT is not specified in this document or recommended for "rph" PASSporTs.

7. IANA Considerations

7.1. JSON Web Token Claims

This specification requests that the IANA add one new claim to the "JSON Web Token Claims" registry, as defined in [RFC7519].

   Claim Name:  sph
   Claim Description:  SIP Priority header field
   Change Controller:  IESG
   Specification Document(s):  RFC 9027

8. Security Considerations

   The security considerations discussed in [RFC8224], [RFC8225], and
   [RFC8443] are applicable here.

9. References

9.1. 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,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4412]  Schulzrinne, H. and J. Polk, "Communications Resource
              Priority for the Session Initiation Protocol (SIP)",
              RFC 4412, DOI 10.17487/RFC4412, February 2006,
              <https://www.rfc-editor.org/info/rfc4412>.
   
   [RFC5031]  Schulzrinne, H., "A Uniform Resource Name (URN) for
              Emergency and Other Well-Known Services", RFC 5031,
              DOI 10.17487/RFC5031, January 2008,
              <https://www.rfc-editor.org/info/rfc5031>.
   
   [RFC7090]  Schulzrinne, H., Tschofenig, H., Holmberg, C., and M.
              Patel, "Public Safety Answering Point (PSAP) Callback",
              RFC 7090, DOI 10.17487/RFC7090, April 2014,
              <https://www.rfc-editor.org/info/rfc7090>.
   
   [RFC7135]  Polk, J., "Registering a SIP Resource Priority Header
              Field Namespace for Local Emergency Communications",
              RFC 7135, DOI 10.17487/RFC7135, May 2014,
              <https://www.rfc-editor.org/info/rfc7135>.
   
   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.
   
   [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,
              <https://www.rfc-editor.org/info/rfc8224>.
   
   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.
   
   [RFC8443]  Singh, R., Dolly, M., Das, S., and A. Nguyen, "Personal
              Assertion Token (PASSporT) Extension for Resource Priority
              Authorization", RFC 8443, DOI 10.17487/RFC8443, August
              2018, <https://www.rfc-editor.org/info/rfc8443>.

9.2. Informative References

[EMERGENCY-CALLS]

              Rosen, B., "Non-Interactive Emergency Calls", Work in
              Progress, Internet-Draft, draft-rosen-stir-emergency-
              calls-00, 9 March 2020, <https://tools.ietf.org/html/
              draft-rosen-stir-emergency-calls-00>.

Acknowledgements

The authors would like to thank Brian Rosen, Terry Reese, and Jon Peterson for helpful suggestions, comments, and corrections.

Authors' Addresses

Martin Dolly
AT&T

Email:

          md3135@att.com

Chris Wendt
Comcast
Comcast Technology Center
Philadelphia, PA 19103
United States of America

   Email: chris-ietf@chriswendt.net

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