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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

 Copyright (c) 2021 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 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
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 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 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

iat

orig

rph

sph

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