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

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Table of Contents

1. Introduction

The Session Initiation Protocol (SIP) [RFC3261] uses the framework used by HTTP [RFC7230] for authenticating users, which is a simple challenge-response authentication mechanism that allows a server to challenge a client request and allows a client to provide authentication information in response to that challenge.¶

OAuth 2.0 [RFC6749] defines a token based authorization framework to allow clients to access resources on behalf of their user.¶

The OpenID Connect 1.0 [OPENID] specifications defines a simple identity layer on top of the OAuth 2.0 protocol, which enables clients to verify the identity of the user based on the authentication performed by a dedicated authorization server, as well as to obtain basic profile information about the user.¶

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

This document defines an mechanism for SIP, that relies on the OAuth 2.0 and OpenID Connect Core 1.0 specifications, to enable the delegation of the user authentication and SIP registration authorization to a dedicated third-party entity that is separate from the SIP network elements that provide the SIP service.¶

¶

2. SIP Procedures

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

¶

3. Access Token Claims

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

If an access token is encoded as a JWT, it might contain a list of claims [RFC7519], some registered and some are application specific claims. The REGISTRAR can grant access to services either based on such claims, using 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], user profile lookups, etc.¶

¶

4. WWW-Authenticate Response Header Field

This section describes the syntax of the WWW-Authenticate Response Header Field when used with the Bearer scheme to challenge the UA for credentials, by extending the 'challnge' header field defined by [RFC3261].¶

    challenge  =/  ("Bearer" LWS bearer-cln *(COMMA bearer-cln))
    bearer-cln = realm / scope / authz-server / error / 
                 auth-param
    authz-server = "authz_server" EQUAL authz-server-value
    authz-server-value = https-URI
    realm = <defined in RFC3261>
    auth-param = <defined in RFC3261>
    scope = <defined in RFC6749>
    error = <defined in RFC6749>
    https-URI = <defined in RFC7230>
  

The authz-server parameters contains the HTTPS URI, as defined in [RFC7230], of the authorization server. The UA 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].¶

As 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 contains a hostname or domain name. It also states that the realm string should be human-readable identifier that can be rendered to the user.¶

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

The scope parameter could 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 authorization server. The values of the scope parameter is 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. Example Flows

  UA                          Registrar                          AS
---------------------------------------------------------------------
  |                               |                               |
  | [1] REGISTER                  |                               |
  |------------------------------>|                               |
  |                               |                               |
  | [2] 401 Unauthorized          |                               |
  |     WWW-Authenticate: Bearer "authz_server"="<authz_server>"  |
  |<------------------------------|                               |
  |                               |                               |
  | [3] The UA 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}            |
  |                               |------------------------------>|
  |                               |                               |
  |                               | [6] 200 OK {metadata}         |
  |                               |<------------------------------|
  |                               |                               |
  | [7] 200 OK                    |                               |
  |<------------------------------|                               |
  |                               |                               |

  UA                          Registrar                          AS
---------------------------------------------------------------------
  |                               |                               |
  | [1] The UA 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}            |
  |                               |------------------------------>|
  |                               |                               |
  |                               | [4] 200 OK {metadata}         |
  |                               |<------------------------------|
  |                               |                               |
  | [5] 200 OK                    |                               |
  |<------------------------------|                               |
  |                               |                               |

6. 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 JSON Web Tokens (JWT) are defined in [RFC7519]. These security considerations also apply to SIP usage of access token as defined in this document.¶

[RFC6749] mandates that Access Tokens are protected with TLS. However, 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 endpoints will have access to it. Endpoints that support this specifications MUST support encrypted JSON Web Tokens (JWT) [RFC7519] for encoding and protecting Access Token when included in SIP requests, unless some other mechanism is used to guarantee that only authorized SIP endpoints have access to the Access Token.¶

¶

7. IANA Considerations

8. Acknowledgments

The authors would like to specially thank Paul Kyzivat for his multiple detailed reviews and suggested text that significanly 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, and Dale Worley.¶

The authors would also like to thank Jean Mahoney for her review, editorial help, and the coversion of the XML source file from v2 to v3.¶

¶

Authors' Addresses