Internet Engineering Task Force (IETF) R. Singh
Request for Comments: 8443 Vencore Labs
Category: Standards Track M. Dolly
ISSN: 2070-1721 AT&T
S. Das
Vencore Labs
A. Nguyen
Office of Emergency Communication/DHS
August 2018
Personal Assertion Token (PASSporT) Extension for Resource Priority
Authorization
Abstract
This document extends the Personal Assertion Token (PASSporT)
specification defined in RFC 8225 to allow the inclusion of
cryptographically signed assertions of authorization for the values
populated in the Session Initiation Protocol (SIP) 'Resource-
Priority' header field, which is used for communications resource
prioritization.
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/rfc8443.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PASSporT "rph" Claim . . . . . . . . . . . . . . . . . . . . 3
4. "rph" in SIP . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Authentication Service Behavior . . . . . . . . . . . . . 5
4.2. Verification Service Behavior . . . . . . . . . . . . . . 5 6
5. Further Information Associated with the SIP "Resource-
Priority" 'Resource-
Priority' Header Field . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. JSON Web Token Claims . . . . . . . . . . . . . . . . . . 7
6.2. PASSporT Types . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7.1. Avoidance of Replay and Cut-and-Paste Attacks . . . . . . 7
7.2. Solution Considerations . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
PASSporT [RFC8225] is a token format based on JSON Web Token (JWT)
[RFC7519] for conveying cryptographically signed information about
the identities involved in personal communications. PASSporT with
Secure Telephone Identity Revisited (STIR) [RFC8224] provides a
mechanism by which an authority on the originating side of a call,
using a protocol like SIP [RFC3261], can provide a cryptographic
assurance of the validity of the calling party telephone number in
order to prevent impersonation attacks.
[RFC4412] defines a mechanism to prioritize access to SIP-signaled
resources during periods of communications resource scarcity using
the SIP 'Resource-Priority' header field for
communications Resource-Priority. header. As specified in [RFC4412], the
SIP 'Resource-Priority' header field may be used by SIP user agents
(UAs) [RFC3261] (including Public Switched Telephone Network (PSTN)
gateways and SIP proxy servers) to influence prioritization afforded
to communication sessions, including PSTN calls (e.g., to manage
scarce network resources during network congestion scenarios).
However, the SIP 'Resource-Priority' header field could be spoofed
and abused by unauthorized entities, the threat models and use cases
of which are described in [RFC7375] and [RFC7340], respectively.
Compromise of the SIP 'Resource-Priority' header field [RFC4412]
could lead to misuse of network resources (i.e., during congestion
scenarios), impacting the application services supported using the
SIP 'Resource-Priority' header field.
[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 can use a PASSPorT claim to
cryptographically sign the SIP 'Resource-Priority' header field and
convey assertion of the authorization for the SIP 'Resource-Priority'
header field. A signed SIP 'Resource-Priority' header field will
allow a receiving entity (including entities located in different
network domains/boundaries) to verify the validity of assertions
authorizing the SIP 'Resource-Priority' header field and to act on
the information with confidence that the information has not been
spoofed or compromised.
This specification documents an extension to PASSporT and the
associated STIR mechanisms to provide a function to cryptographically
sign the SIP 'Resource-Priority' header field. This PASSporT object
is used to provide attestation of a calling-user authorization for
priority communications. This is necessary in addition to the
PASSporT object that is used for calling-user telephone-number
attestation. How this extension to PASSporT is used for real-time
communications supported using the SIP 'Resource-Priority' header
field is outside the scope of this document. In addition, the
PASSPorT extension defined in this document is intended for use in
environments where there are means to verify that the signer of the
SIP 'Resource-Priority' header field 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. PASSporT "rph" Claim
This specification defines a new JSON Web Token claim for "rph" that
provides an assertion for information in the SIP 'Resource-Priority'
header field.
The creator of a PASSporT object adds a "ppt" value of "rph" to the
header of a PASSporT object. The PASSporT claims MUST contain an
"rph" claim, and any entities verifying the PASSporT object will be
required to understand the "ppt" extension in order to process the
PASSporT in question. A PASSPorT header with the "ppt" included will
look as follows:
{
"typ":"passport",
"ppt":"rph",
"alg":"ES256",
"x5u":"https://www.example.org/cert.cer"
}
The "rph" claim will provide an assertion of authorization, "auth",
for information in the SIP 'Resource-Priority' header field based on
[RFC4412]. The syntax is:
{
Resource-Priority = "Resource-Priority" : r-value,
r-value= namespace "." r-priority
}
Specifically, the "rph" claim includes an assertion of the priority
level of the user to be used for a given communication session. The
value of the "rph" claim is an object with one or more keys. Each
key is associated with a JSON array. These arrays contain strings
that correspond to the r-values indicated in the SIP 'Resource-
Priority' header field.
The following is an example "rph" claim for a SIP 'Resource-Priority'
header field with one r-value of "ets.0" and with another r-value of
"wps.0":
{
"orig":{"tn":"12155550112"},
"dest":{["tn":"12125550113"]},
"iat":1443208345,
"rph":{"auth":["ets.0", "wps.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" header and sign
the claim with their own authority.
The use of the compact form of PASSporT is not specified in this
document.
4. "rph" in SIP
This section specifies SIP-specific usage for the "rph" claim in
PASSporT.
4.1. Authentication Service Behavior
The Authentication Service will create the "rph" claim using the
values discussed in Section 3 of this document that are based on
[RFC4412]. The construction of the "rph" claim follows the steps
described in Section 4.1 of [RFC8224].
The resulting Identity header for "rph" might look as follows
(backslashes shown for line folding only):
Identity:eyJhbGciOiJFUzI1NiIsInBwdCI6InJwaCIsInR5cCI6InBhc3Nwb3J0\
IiwieDV1IjoiaHR0cHM6Ly93d3cuZXhhbXBsZS5jb20vY2VydC5jZXIifQo.eyJkZ\
XN0Ijp7WyJ0biI6IjEyMTI1NTUwMTEzIl19LCJpYXQiOiIxNDQzMjA4MzQ1Iiwib3\
JpZyI6eyJ0biI6IjEyMTU1NTUwMTEyIn0sInJwaCI6eyJhdXRoIjpbImV0cy4wIiw\
id3BzLjAiXX19Cg.s37S6VC8HM6Dl6YzJeQDsrZcwJ0lizxhUrA7f_98oWBHvo-cl\
-n8MIhoCr18vYYFy3blXvs3fslM_oos2P2Dyw;info=<https://www.example.\
org/cert.cer>;alg=ES256;ppt="rph"
A SIP authentication service will derive the value of "rph" from the
SIP 'Resource-Priority' header field based on policy associated with
service-specific use of the "namespace r-values, defined as follows in [RFC4412]:
r-value = namespace "." r-priority" for r-values
based on [RFC4412]. r-priority
The authentication service derives the value of the PASSPorT claim by
verifying the authorization for the SIP 'Resource-Priority' header
field (i.e., verifying a calling-user privilege for the SIP
'Resource-Priority' header field based on its identity). The
authorization might be derived from customer-profile data or access
to external services.
[RFC4412] allows multiple "namespace "." priority value" pairs,
either in a single SIP 'Resource-Priority' header field or across
multiple SIP 'Resource-Priority' header fields. An authority is
responsible for signing all the content of a SIP 'Resource-Priority'
header field for which it has the authority.
4.2. Verification Service Behavior
[RFC8224], Section 6.2, Step 5 requires that specifications defining
"ppt" values describe any additional verifier behavior. The behavior
specified for the "ppt" values of "rph" is as follows:
The verification service MUST extract the value associated with the
"auth" key in a full-form PASSPorT with a "ppt" value of "rph". If
the signature validates, then the verification service can use the
value of the "rph" claim as validation that the calling party is
authorized for SIP 'Resource-Priority' header fields as indicated in
the claim. This value would, in turn, be used for priority treatment
in accordance with local policy for the associated communication
service. If the signature validation fails, the verification service
should infer that the calling party is not authorized for SIP
'Resource-Priority' header fields as indicated in the claim. In such
cases, the priority treatment for the associated communication
service is handled as per the local policy of the verifier. In such
scenarios, the SIP 'Resource-Priority' header field SHOULD be
stripped from the SIP request, and the network entities should treat
the call as an ordinary call.
In addition, [RFC8224], Section 6.2, Step 4 requires the "iat" value
in "rph" claim to be verified.
The behavior of a SIP UA upon receiving an INVITE containing a
PASSporT object with an "rph" claim will largely remain a matter of
implementation policy for the specific communication service. In
most cases, implementations would act based on confidence in the
veracity of this information.
5. Further Information Associated with the SIP "Resource-Priority" 'Resource-Priority'
Header Field
There may be additional information about the calling party or the
call that could be relevant to authorization for the SIP 'Resource-
Priority' header field. This may include information related to the
device subscription of the caller, to any institutions that the
caller or device is associated with, or even to categories of
institutions. All of these data elements would benefit from the
secure attestations provided by the STIR and PASSporT frameworks.
The specification of the "rph" claim could entail the optional
presence of one or more such additional information fields applicable
to the SIP 'Resource-Priority' header field.
A new IANA registry has been defined to hold potential values of the
"rph" array; see Section 6.2. The definition of the "rph" claim may
have one or more such additional information field(s). Details of
how an "rph" claim encompasses other data elements are left for
future specifications.
6. IANA Considerations
6.1. JSON Web Token Claims
IANA has added a new claim to the "JSON Web Token Claims" registry as
defined in [RFC7519].
o Claim Name: "rph"
o Claim Description: Resource Priority Header Authorization
o Change Controller: IESG
o Specification Document(s): Section 3 of RFC 8443
6.2. PASSporT Types
IANA has created a new entry in the "Personal Assertion Token
(PASSporT) Extensions" registry for the type "rph", which is
specified in this document. In addition, the "PASSporT Resource
Priority Header (rph) Types" registry has been created in which each
entry must contain two fields: the name of the "rph" type and the
specification in which the type is described. This registry has been
initially populated with the single value for "auth", which is
specified in this document. Registration of new "rph" types shall be
under the Specification Required policy[RFC8126].
7. Security Considerations
The security considerations discussed in [RFC8224], Section 12, are
applicable here.
7.1. Avoidance of Replay and Cut-and-Paste Attacks
The PASSporT extension with a "ppt" value of "rph" MUST only be sent
with SIP INVITE when the SIP 'Resource-Priority' header field is used
to convey the priority of the communication, as defined in [RFC4412].
To avoid replay and cut-and-paste attacks, the recommendations
provided in Section 12.1 of [RFC8224] MUST be followed.
7.2. Solution Considerations
Using extensions to PASSporT tokens with a "ppt" value of "rph"
requires knowledge of the authentication, authorization, and
reputation of the signer to attest to the identity being asserted,
including validating the digital signature and the associated
certificate chain to a trust anchor. The following considerations
should be recognized when using PASSporT extensions with a "ppt"
value of "rph":
o A signer is only allowed to sign the content of a SIP 'Resource-
Priority' header field for which it has the proper authorization.
Before signing tokens, the signer MUST have a secure method for
authentication of the end user or the device being granted a
token.
o The verification of the signature MUST include means of verifying
that the signer is authoritative for the signed content of the
resource priority namespace in the PASSporT.
8. References
8.1. 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>.
[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,
<https://www.rfc-editor.org/info/rfc3261>.
[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>.
[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>.
8.2. 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,
<https://www.rfc-editor.org/info/rfc7340>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<https://www.rfc-editor.org/info/rfc7375>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Acknowledgements
We would like to thank STIR Working Group members, the ATIS/SIP Forum
Task Force on IPNNI members, and the NS/EP Priority Services
community for contributions to this problem statement and
specification. We would also like to thank David Hancock and Ning
Zhang for their valuable inputs.
Authors' Addresses
Ray P. Singh
Vencore Labs
150 Mount Airy Road
New Jersey, NJ 07920
United States of America
Email: rsingh@vencorelabs.com
Martin Dolly
AT&T
200 Laurel Avenue
Middletown, NJ 07748
United States of America
Email: md3135@att.com
Subir Das
Vencore Labs
150 Mount Airy Road
New Jersey, NJ 07920
United States of America
Email: sdas@vencorelabs.com
An Nguyen
Office of Emergency Communication/DHS
245 Murray Lane, Building 410
Washington, DC 20528
United States of America
Email: an.p.nguyen@HQ.DHS.GOV