path: root/doc/rfc/rfc8898.txt

Internet Engineering Task Force (IETF)                    R. Shekh-Yusef
Request for Comments: 8898                                         Auth0
Updates: 3261                                                C. Holmberg
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                               V. Pascual
                                                                   Nokia
                                                          September 2020


  Third-Party Token-Based Authentication and Authorization for Session
                       Initiation Protocol (SIP)

Abstract

   This document defines the "Bearer" authentication scheme for the
   Session Initiation Protocol (SIP) and a mechanism by which user
   authentication and SIP registration authorization is delegated to a
   third party, using the OAuth 2.0 framework and OpenID Connect Core
   1.0.  This document updates RFC 3261 to provide guidance on how a SIP
   User Agent Client (UAC) responds to a SIP 401/407 response that
   contains multiple WWW-Authenticate/Proxy-Authenticate header fields.

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/rfc8898.

Copyright Notice

   Copyright (c) 2020 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
   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.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Applicability
     1.3.  Token Types and Formats
     1.4.  Example Flows
       1.4.1.  Registration
       1.4.2.  Registration with Preconfigured AS
   2.  SIP Procedures
     2.1.  UAC Behavior
       2.1.1.  Obtaining Tokens and Responding to Challenges
       2.1.2.  Protecting the Access Token
       2.1.3.  REGISTER Request
       2.1.4.  Non-REGISTER Request
     2.2.  User Agent Server (UAS) and Registrar Behavior
     2.3.  Proxy Behavior
   3.  Access Token Claims
   4.  WWW-Authenticate Response Header Field
   5.  Security Considerations
   6.  IANA Considerations
     6.1.  New Proxy-Authenticate Header Field Parameters
     6.2.  New WWW-Authenticate Header Field Parameters
   7.  Normative References
   8.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] uses the same
   framework as HTTP [RFC7230] to authenticate users: a simple
   challenge-response authentication mechanism that allows a SIP User
   Agent Server (UAS), proxy, or registrar to challenge a SIP User Agent
   Client (UAC) request and allows the UAC to provide authentication
   information in response to that challenge.

   OAuth 2.0 [RFC6749] defines a token-based authorization framework to
   allow an OAuth client to access resources on behalf of its user.

   The OpenID Connect Core 1.0 specification [OPENID] defines a simple
   identity layer on top of the OAuth 2.0 protocol, which enables OAuth/
   OpenID clients to verify the identity of the user based on the
   authentication performed by a dedicated authorization server (AS),
   referred to as OpenID Provider (OP), as well as to obtain basic
   profile information about the user.

   This document defines the "Bearer" authentication scheme for SIP and
   a mechanism by which user authentication and SIP registration
   authorization is delegated to a third party, using the OAuth 2.0
   framework and OpenID Connect Core 1.0.  This kind of user
   authentication enables single sign-on, which allows the user to
   authenticate once and gain access to both SIP and non-SIP services.

   This document also updates [RFC3261] by defining the UAC procedures
   when a UAC receives a 401/407 response with multiple WWW-
   Authenticate/Proxy-Authenticate header fields, providing challenges
   using different authentication schemes for the same realm.


1.1.  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.

1.2.  Applicability

   This document covers cases where grants that allow the UAC to obtain
   an access token from the AS are used.  Cases where the UAC is not
   able to obtain an access token (e.g., in the case of an authorization
   code grant) are not covered.


1.3.  Token Types and Formats

   The tokens used in third-party authorization depend on the type of
   AS.

   An OAuth AS provides the following tokens to a successfully
   authorized UAC:

   Access Token:
      The UAC will use this token to gain access to services by
      providing the token to a SIP server.

   Refresh Token:
      The UAC will present this token to the AS to refresh a stale
      access token.

   An OP returns an additional token:

   ID Token:
      This token contains a SIP URI associated with the user and other
      user-specific details that will be consumed by the UAC.

   Tokens can be represented in two different formats:

   Structured Token:
      A token that consists of a structured object that contains the
      claims associated with the token, e.g., JSON Web Token (JWT), as
      defined in [RFC7519].

   Reference Token:
      A token that consists of an opaque string that is used to obtain
      the details of the token and its associated claims, as defined in
      [RFC6749].

   Access tokens are represented in one of the above two formats.
   Refresh tokens usually are represented in a reference format, as this
   token is consumed only by the AS that issued the token.  The ID token
   is defined as a structured token in the form of a JWT.


1.4.  Example Flows

1.4.1.  Registration

   Figure 1 below shows an example of a SIP registration where the
   registrar informs the UAC about the AS from which the UAC can obtain
   an access token.

     UAC                         Registrar                         AS/OP
   ---------------------------------------------------------------------
     |                               |                               |
     | [1] REGISTER                  |                               |
     |------------------------------>|                               |
     |                               |                               |
     | [2] 401 Unauthorized          |                               |
     |     WWW-Authenticate: Bearer "authz_server"="<authz_server>"  |
     |<------------------------------|                               |
     |                               |                               |
     | [3] The UAC interacts with the AS and obtains tokens using    |
     |     some out-of-scope mechanism.                              |
     |<=============================================================>|
     |                               |                               |
     | [4] REGISTER                  |                               |
     |     Authorization: Bearer <access_token>                      |
     |------------------------------>|                               |
     |                               | [5] HTTP POST /introspect     |
     |                               |     {access_token}            |
     |                               |       (OPTIONAL)              |
     |                               |------------------------------>|
     |                               |                               |
     |                               | [6] 200 OK {metadata}         |
     |                               |       (OPTIONAL)              |
     |                               |<------------------------------|
     |                               |                               |
     | [7] 200 OK                    |                               |
     |<------------------------------|                               |
     |                               |                               |

                    Figure 1: Example Registration Flow

   In step [1], the UAC starts the registration process by sending a SIP
   REGISTER request to the registrar without any credentials.

   In step [2], the registrar challenges the UA by sending a SIP 401
   (Unauthorized) response to the REGISTER request.  In the response,
   the registrar includes information about the AS to contact in order
   to obtain a token.

   In step [3], the UAC interacts with the AS via an out-of-scope
   mechanism, potentially using the OAuth Native App mechanism defined
   in [RFC8252].  The AS authenticates the user and provides the UAC
   with the tokens needed to access the SIP service.

   In step [4], the UAC retries the registration process by sending a
   new REGISTER request that includes the access token that the UAC
   obtained in the step above.

   The registrar validates the access token.  If the access token is a
   reference token, the registrar MAY perform an introspection
   [RFC7662], as in steps [5] and [6], in order to obtain more
   information about the access token and its scope, per [RFC7662].
   Otherwise, after the registrar validates the token, it inspects its
   claims and acts upon it.

   In step [7], once the registrar has successfully verified and
   accepted the access token, it sends a 200 (OK) response to the
   REGISTER request.


1.4.2.  Registration with Preconfigured AS

   Figure 2 shows an example of a SIP registration where the UAC has
   been preconfigured with information about the AS from which to obtain
   the access token.

     UAC                         Registrar                         AS/OP
   ---------------------------------------------------------------------
     |                               |                               |
     | [1] The UAC interacts with the AS and obtains tokens using    |
     |     some out-of-scope mechanism.                              |
     |<=============================================================>|
     |                               |                               |
     | [2] REGISTER                  |                               |
     |     Authorization: Bearer <access_token>                      |
     |------------------------------>|                               |
     |                               | [3] HTTP POST /introspect     |
     |                               |     {access_token}            |
     |                               |       (OPTIONAL)              |
     |                               |------------------------------>|
     |                               |                               |
     |                               | [4] 200 OK {metadata}         |
     |                               |       (OPTIONAL)              |
     |                               |<------------------------------|
     |                               |                               |
     | [5] 200 OK                    |                               |
     |<------------------------------|                               |
     |                               |                               |

     Figure 2: Example Registration Flow - AS Information Preconfigured

   In step [1], the UAC interacts with the AS using an out-of-scope
   mechanism, potentially using the OAuth Native App mechanism defined
   in [RFC8252].  The AS authenticates the user and provides the UAC
   with the tokens needed to access the SIP service.

   In step [2], the UAC initiates the registration process by sending a
   new REGISTER request that includes the access token that the UAC
   obtained in the step above.

   The registrar validates the access token.  If the access token is a
   reference token, the registrar MAY perform an introspection
   [RFC7662], as in steps [4] and [5], in order to obtain more
   information about the access token and its scope, per [RFC7662].
   Otherwise, after the registrar validates the token, it inspects its
   claims and acts upon it.

   In step [5], once the registrar has successfully verified and
   accepted the access token, it sends a 200 (OK) response to the
   REGISTER request.


2.  SIP Procedures

   Section 22 of [RFC3261] defines the SIP procedures for the Digest
   authentication mechanism.  The same procedures apply to the "Bearer"
   authentication mechanism, with the changes described in this section.

2.1.  UAC Behavior

2.1.1.  Obtaining Tokens and Responding to Challenges

   When a UAC sends a request without credentials (or with invalid
   credentials), it could receive either a 401 (Unauthorized) response
   with a WWW-Authenticate header field or a 407 (Proxy Authentication
   Required) response with a Proxy-Authenticate header field.  If the
   WWW-Authenticate or Proxy-Authenticate header field indicates
   "Bearer" scheme authentication and contains an address to an AS, the
   UAC contacts the AS in order to obtain tokens and includes the
   requested scopes, based on a local configuration (Figure 1).  The UAC
   MUST check the AS URL received in the 401/407 response against a list
   of trusted ASs configured on the UAC in order to prevent several
   classes of possible vulnerabilities when a client blindly attempts to
   use any provided AS.

   The detailed OAuth2 procedure to authenticate the user and obtain
   these tokens is out of scope of this document.  The address of the AS
   might already be known to the UAC via configuration.  In such cases,
   the UAC can contact the AS for tokens before it sends a SIP request
   (Figure 2).  Procedures for native applications are defined in
   [RFC8252].  When using the mechanism defined in [RFC8252], the user
   of the UAC will be directed to interact with the AS using a web
   browser, which allows the AS to prompt the user for multi-factor
   authentication, to redirect the user to third-party identity
   providers, and to enable the use of single sign-on sessions.

   The tokens returned to the UAC depend on the type of AS; an OAuth AS
   provides an access token and, optionally, a refresh token [RFC6749].
   The refresh token is only used between the UAC and the AS.  If the AS
   provides a refresh token to the UAC, the UAC uses it to request a new
   access token from the AS before the currently used access token
   expires ([RFC6749], Section 1.5).  If the AS does not provide a
   refresh token, the UAC needs to reauthenticate the user in order to
   get a new access token before the currently used access token
   expires.  An OP returns an additional ID token that contains claims
   about the authentication of the user by an authorization server.  The
   ID token can potentially include other optional claims about the
   user, e.g., the SIP URI, that will be consumed by the UAC and later
   used to register with the registrar.

   If the UAC receives a 401/407 response with multiple WWW-
   Authenticate/Proxy-Authenticate header fields, providing challenges
   using different authentication schemes for the same realm, the UAC
   provides credentials for one of the schemes that it supports, based
   on local policy.

      |  NOTE: At the time of writing, detailed procedures for the cases
      |  where a UAC receives multiple different authentication schemes
      |  had not been defined.  A future specification might define such
      |  procedures.

      |  NOTE: The address of the AS might be known to the UAC, e.g.,
      |  using means of configuration, in which case the UAC can contact
      |  the AS in order to obtain the access token before it sends SIP
      |  request without credentials.

2.1.2.  Protecting the Access Token

   [RFC6749] mandates that access tokens are protected with TLS when in
   transit.  However, SIP makes use of intermediary SIP proxies, and TLS
   only guarantees hop-to-hop protection when used to protect SIP
   signaling.  Therefore, the access token MUST be protected in a way so
   that only authorized SIP servers will have access to it.  SIP
   endpoints that support this document MUST use encrypted JWTs
   [RFC7519] for encoding and protecting access tokens when they are
   included in SIP requests, unless some other mechanism is used to
   guarantee that only authorized SIP endpoints have access to the
   access token.  TLS can still be used for protecting traffic between
   SIP endpoints and the AS, as defined in [RFC6749].


2.1.3.  REGISTER Request

   The procedures in this section apply when the UAC has received a
   challenge that contains a "Bearer" scheme and the UAC has obtained a
   token, as specified in Section 2.1.1.

   The UAC sends a REGISTER request with an Authorization header field
   containing the response to the challenge, including the "Bearer"
   scheme carrying a valid access token in the request, as specified in
   [RFC6750].

   Note that if there were multiple challenges with different schemes,
   then the UAC may be able to successfully retry the request using
   non-"Bearer" credentials.

   Typically, a UAC will obtain a new access token for each new binding.
   However, based on local policy, a UAC MAY include an access token
   that has been used for another binding associated with the same
   Address Of Record (AOR) in the request.

   If the access token included in a REGISTER request is not accepted
   and the UAC receives a 401 response or a 407 response, the UAC
   follows the procedures in Section 2.1.1.


2.1.4.  Non-REGISTER Request

   The procedures in this section apply when the UAC has received a
   challenge that contains a "Bearer" scheme and the UAC has obtained a
   token, as specified in Section 2.1.1.

   When the UAC sends a request, it MUST include an Authorization header
   field with a "Bearer" scheme carrying a valid access token obtained
   from the AS indicated in the challenge in the request, as specified
   in [RFC6750].  Based on local policy, the UAC MAY include an access
   token that has been used for another dialog, or for another stand-
   alone request, if the target of the new request is the same.

   If the access token included in a request is not accepted and the UAC
   receives a 401 response or a 407 response, the UAC follows the
   procedures in Section 2.1.1.


2.2.  User Agent Server (UAS) and Registrar Behavior

   When a UAS or registrar receives a request that fails to contain
   authorization credentials acceptable to it, the UAS/registrar SHOULD
   challenge the request by sending a 401 (Unauthorized) response.  If
   the UAS/registrar chooses to challenge the request and is willing to
   accept an access token as a credential, it MUST include a WWW-
   Authenticate header field in the response that indicates a "Bearer"
   scheme and includes an AS address, encoded as an https URI [RFC7230],
   from which the UAC can obtain an access token.

   When a UAS or registrar receives a SIP request that contains an
   Authorization header field with an access token, the UAS/registrar
   MUST validate the access token using the procedures associated with
   the type of access token (structured or reference) used, e.g.,
   [RFC7519].  If the token provided is an expired access token, then
   the UAS/registrar MUST reply with a 401 (Unauthorized) response, as
   defined in Section 3 of [RFC6750].  If the validation is successful,
   the UAS/registrar can continue to process the request using normal
   SIP procedures.  If the validation fails, the UAS/registrar MUST
   reply with a 401 (Unauthorized) response.


2.3.  Proxy Behavior

   When a proxy receives a request that fails to contain authorization
   credentials acceptable to it, it SHOULD challenge the request by
   sending a 407 (Proxy Authentication Required) response.  If the proxy
   chooses to challenge the request and is willing to accept an access
   token as a credential, it MUST include a Proxy-Authenticate header
   field in the response that indicates a "Bearer" scheme and includes
   an AS address, encoded as an https URI [RFC7230], from which the UAC
   can obtain an access token.

   When a proxy wishes to authenticate a received request, it MUST
   search the request for Proxy-Authorization header fields with 'realm'
   parameters that match its realm.  It then MUST successfully validate
   the credentials from at least one Proxy-Authorization header field
   for its realm.  When the scheme is "Bearer", the proxy MUST validate
   the access token using the procedures associated with the type of
   access token (structured or reference) used, e.g., [RFC7519].


3.  Access Token Claims

   The type of services to which an access token grants access can be
   determined using different methods.  The methods used and the access
   provided by the token are based on local policy agreed between the AS
   and the registrar.

   If an access token is encoded as a JWT, it will contain a list of
   claims [RFC7519], including both registered and application-specific
   claims.  The registrar can grant access to services based on such
   claims, some other mechanism, or a combination of claims and some
   other mechanism.  If an access token is a reference token, the
   registrar will grant access based on some other mechanism.  Examples
   of such other mechanisms are introspection [RFC7662] and user profile
   lookups.


4.  WWW-Authenticate Response Header Field

   This section uses ABNF [RFC5234] to describe the syntax of the WWW-
   Authenticate header field when used with the "Bearer" scheme to
   challenge the UAC for credentials by extending the 'challenge'
   parameter defined by [RFC3261].

   challenge  =/  ("Bearer" LWS bearer-cln *(COMMA bearer-cln))
   bearer-cln = realm / scope-param / authz-server-param / error-param /
                auth-param
   realm = <defined in RFC 3261>
   scope-param = "scope" EQUAL DQUOTE scope DQUOTE
   scope = <defined in RFC 6749>
   authz-server-param = "authz_server" EQUAL DQUOTE authz-server DQUOTE
   authz-server = https-URI
   https-URI = <defined in RFC 7230>
   error-param = "error" EQUAL DQUOTE error DQUOTE
   error = <defined in RFC 6749>
   auth-param = <defined in RFC 3261>

                      Figure 3: "Bearer" Scheme Syntax

   The authz_server parameter contains the HTTPS URI, as defined in
   [RFC7230], of the AS.  The UAC can discover metadata about the AS
   using a mechanism like the one defined in [RFC8414].

   The realm and auth-param parameters are defined in [RFC3261].

   Per [RFC3261], "the realm string alone defines the protection
   domain".  [RFC3261] states that the realm string must be globally
   unique and recommends that the realm string contain a hostname or
   domain name.  It also states that the realm string should be a human-
   readable identifier that can be rendered to the user.

   The scope and error parameters are defined in [RFC6749].

   The scope parameter can be used by the registrar/proxy to indicate to
   the UAC the minimum scope that must be associated with the access
   token to be able to get service.  As defined in [RFC6749], the value
   of the scope parameter is expressed as a list of space-delimited,
   case-sensitive strings.  The strings are defined by the AS.  The
   values of the scope parameter are out of scope of this document.  The
   UAC will use the scope provided by the registrar to contact the AS
   and obtain a proper token with the requested scope.

   The error parameter could be used by the registrar/proxy to indicate
   to the UAC the reason for the error, with possible values of
   "invalid_token" or "invalid_scope".


5.  Security Considerations

   The security considerations for OAuth are defined in [RFC6749].  The
   security considerations for "Bearer" tokens are defined in [RFC6750].
   The security considerations for JWTs are defined in [RFC7519].  These
   security considerations also apply to SIP usage of access tokens, as
   defined in this document.

   [RFC6749] mandates that access tokens are protected with TLS when in
   transit.  However, SIP makes use of intermediary SIP proxies, and TLS
   only guarantees hop-to-hop protection when used to protect SIP
   signaling.  Therefore, the access token MUST be protected in a way so
   that only authorized SIP servers will have access to it.  SIP
   endpoints that support this document MUST use encrypted JWTs
   [RFC7519] for encoding and protecting access tokens when they are
   included in SIP requests, unless some other mechanism is used to
   guarantee that only authorized SIP endpoints have access to the
   access token.  TLS can still be used for protecting traffic between
   SIP endpoints and the AS, as defined in [RFC6749].

   Single Sign-On (SSO) enables the user to use one set of credentials
   to authenticate once and gain access to multiple SIP and non-SIP
   services using access token(s).  If the SSO login is compromised,
   that single point of compromise has a much broader effect than is the
   case without SSO.  Further, an attacker can often use a compromised
   account to set up Single Sign-On for other services that the victim
   has not established an account with and sometimes can even switch a
   dedicated account into SSO mode, creating a still broader attack.

   Because of that, it is critical to make sure that extra security
   measures be taken to safeguard credentials used for Single Sign-On.
   Examples of such measures include a long passphrase instead of a
   password, enabling multi-factor authentication, and the use of the
   native platform browser when possible, as defined in [RFC8252].

   Although this is out of scope for this document, it is important to
   carefully consider the claims provided in the tokens used to access
   these services to make sure of the privacy of the user accessing
   these services.  As mentioned above, this document calls for
   encrypting JWTs representing the access token.

   It is important that both parties participating in SSO provide
   mechanisms for users to sever the SSO relationship so that it is
   possible without undue difficulty to mitigate a compromise that has
   already happened.

   The operator of an SSO authentication system has access to private
   information about sites and services that their users log into and
   even, to some extent, their usage patterns.  It's important to call
   these out in privacy disclosures and policies and to make sure that
   users can be aware of the trade-offs between convenience and privacy
   when they choose to use SSO.

   When a registrar chooses to challenge a REGISTER request, if the
   registrar can provide access to different levels of services, it is
   RECOMMENDED that the registrar include a scope in the response in
   order to indicate the minimum scope needed to register and access
   basic services.  The access token might include an extended scope
   that gives the user access to more advanced features beyond basic
   services.  In SIP, the AS administrator will typically decide what
   level of access is provided for a given user.

   The UAC MUST check the AS URL received in the 401/407 response
   against a list of trusted ASs configured on the UAC in order to
   prevent several classes of possible vulnerabilities when a client
   blindly attempts to use any provided AS.


6.  IANA Considerations

6.1.  New Proxy-Authenticate Header Field Parameters

   This section defines new SIP header field parameters in the "Header
   Field Parameters and Parameter Values" subregistry of the "Session
   Initiation Protocol (SIP) Parameters" registry:
   <https://www.iana.org/assignments/sip-parameters>

            +================+===================+===========+
            | Parameter Name | Predefined Values | Reference |
            +================+===================+===========+
            | authz_server   | No                | RFC 8898  |
            +----------------+-------------------+-----------+
            | error          | No                | RFC 8898  |
            +----------------+-------------------+-----------+
            | scope          | No                | RFC 8898  |
            +----------------+-------------------+-----------+

                Table 1: Header Field: Proxy-Authenticate

6.2.  New WWW-Authenticate Header Field Parameters

   This section defines new SIP header field parameters in the "Header
   Field Parameters and Parameter Values" subregistry of the "Session
   Initiation Protocol (SIP) Parameters" registry:
   <https://www.iana.org/assignments/sip-parameters>

            +================+===================+===========+
            | Parameter Name | Predefined Values | Reference |
            +================+===================+===========+
            | authz_server   | No                | RFC 8898  |
            +----------------+-------------------+-----------+
            | error          | No                | RFC 8898  |
            +----------------+-------------------+-----------+
            | scope          | No                | RFC 8898  |
            +----------------+-------------------+-----------+

                 Table 2: Header Field: WWW-Authenticate

7.  Normative References

   [OPENID]   Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0", February 2014.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

   [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>.

8.  Informative References

   [RFC8252]  Denniss, W. and J. Bradley, "OAuth 2.0 for Native Apps",
              BCP 212, RFC 8252, DOI 10.17487/RFC8252, October 2017,
              <https://www.rfc-editor.org/info/rfc8252>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

Acknowledgments

   The authors would like to specially thank Paul Kyzivat for his
   multiple detailed reviews and suggested text that significantly
   improved the quality of the document.

   The authors would also like to thank the following for their review
   and feedback on this document:

   Olle Johansson, Roman Shpount, Dale Worley, and Jorgen Axell.

   The authors would also like to thank the following for their review
   and feedback of the original document that was replaced with this
   document:

   Andrew Allen, Martin Dolly, Keith Drage, Paul Kyzivat, Jon Peterson,
   Michael Procter, Roy Radhika, Matt Ryan, Ivo Sedlacek, Roman Shpount,
   Robert Sparks, Asveren Tolga, Dale Worley, and Yehoshua Gev.

   Roman Danyliw, Benjamin Kaduk, Erik Kline, Barry Leiba, Eric Vyncke,
   and Magnus Westerlund provided feedback and suggestions for
   improvements as part of the IESG evaluation of the document.  Special
   thanks to Benjamin Kaduk for his detailed and comprehensive reviews
   and comments.

   The authors would also like to specially thank Jean Mahoney for her
   multiple reviews, editorial help, and the conversion of the XML
   source file from v2 to v3.

Authors' Addresses

   Rifaat Shekh-Yusef
   Auth0
   Ottawa Ontario
   Canada

   Email: rifaat.s.ietf@gmail.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland

   Email: christer.holmberg@ericsson.com


   Victor Pascual
   Nokia
   Barcelona
   Spain

   Email: victor.pascual_avila@nokia.com