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
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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
