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Versions: 00 01 02 03 04 RFC 3702

Internet Engineering Task Force                                   SIP WG
Internet Draft                                               J. Loughney
                                                                   Nokia
                                                            G. Camarillo
                                                                Ericsson
draft-ietf-sipping-aaa-req-04.txt
December 14, 2003
Expires: May, 2004


            Authentication, Authorization and Accounting
          Requirements for the Session Initiation Protocol

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   As SIP services are deployed on the Internet, there is a need for
   authentication, authorization and accounting of SIP sessions. This
   document sets out the basic requirements for this work.












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                           Table of Contents



   1          Introduction ........................................    4
   1.1        RADIUS ..............................................    4
   1.2        Terminology and Acronyms ............................    5
   1.3        Requirements Language ...............................    5
   2          Requirements ........................................    6
   2.1        Common Requirements .................................    6
   2.1.1      Communication within the Same Domain ................    6
   2.1.2      Communication between Different Domains .............    6
   2.1.3      Discovery ...........................................    6
   2.1.4      Ability to Integrate Different Networks, Services
              and Users ...........................................    6
   2.1.5      Updating SIP Server Entries .........................    6
   2.1.6      SIP Session Changes .................................    7
   2.1.7      Reliable Transfer of Protocol Messages ..............    7
   2.1.8      Call Setup Times ....................................    7
   2.1.9      Security ............................................    7
   2.2        Authentication Requirements .........................    7
   2.2.1      Authentication Based on SIP Requests ................    7
   2.2.2      Flexible Authentication of SIP Requests .............    8
   2.3        Authorization Requirements ..........................    8
   2.3.1      Ability to Authorize SIP Requests ...................    8
   2.3.2      Information Transfer ................................    8
   2.3.3      User De-authorization ...............................    8
   2.3.4      User Re-authorization ...............................    9
   2.3.5      Support for Credit Control ..........................    9
   2.4        Accounting Requirements .............................    9
   2.4.1      Separation of Accounting Information ................    9
   2.4.2      Accounting Information Related to Session
              Progression .........................................   10
   2.4.3      Accounting Information Not Related to Session
              Progression .........................................   10
   2.4.4      Support for One-Time and Session-based Accounting
              Records .............................................   10
   2.4.5      Support for Accounting on Different Media
              Components ..........................................   10
   2.4.6      Configuration of Accounting Generation Parameters ...   10
   2.4.7      Support for Arbitrary Correlations ..................   10
   3          Scenarios ...........................................   11
   3.1        WLAN Roaming Using Third Party Service Providers ....   12





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   3.2        Conditional Authorization ...........................   13
   4          Security Considerations .............................   13
   5          Acknowledgements ....................................   13
   6          Authors' Addresses ..................................   14
   7          Normative References ................................   14
   8          Informative References ..............................   14













































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1 Introduction

   The AAA working group is chartered to work on authentication,
   authorization and accounting solutions for the Internet. This work
   consists of a base protocol, applications, end-to-end security
   application and a general architecture for providing these services
   [3]. The AAA working group has specified applicability of AAA-based
   solutions for a number of protocols (e.g., AAA requirements for
   Mobile IP [4]).

   SIP is a signalling protocol for creating, modifying and terminating
   different types sessions such as Internet phone calls, multimedia
   distribution and multimedia conferences [1]. SIP sessions have needs
   for session authentication, authorization and accounting (AAA.)

   In order to authenticate and authorize users, it is typically more
   convenient for SIP entities to communicate with a AAA sever than to
   attempt to store user credentials and profiles locally. SIP entities
   use the SIP-AAA interface to access the AAA server.

   This document provides requirements on for the interface between SIP
   entities and AAA servers. While accounting requirements are
   discussed, this document does not cover SIP charging or billing
   mechanisms.

   One possible use of this document would be to create a AAA
   application for SIP. Any protocol meeting the requirements outlined
   by this document could be used. Possible candidates, among others,
   are Diameter [3] and XML-based protocols following the web-services
   model.

1.1 RADIUS

   The main purpose of this document is to provide input to designers
   working on AAA applications using new protocols such as Diameter and
   XML-based protocols. Nevertheless, a few, limited RADIUS [5]
   extensions may meet some of the requirements in this document (for
   instance, some of the authentication requirements). We expect that
   while RADIUS with these limited extensions will meet particular
   functional requirements, it will not meet other requirements that are
   important. The following are some requirements that are not expected
   to be met by RADIUS:

        1.   Section 2.1.3: RADIUS does not support a discovery feature.

        2.   Section 2.1.7: RADIUS does not support reliable message
             delivery.




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   The following list contains the requirements that can be met by
   RADIUS or RADIUS extensions.

        1.   Section 2.1.2: Communication between domains does not scale
             well in RADIUS. As a result, inter-domain communications
             are typically handled using a proxy architecture [6].

        2.   Section 2.1.5: RADIUS clients would need to support Dynamic
             Authorization [7].

        3.   Section 2.1.9: RADIUS clients would need to rely on a
             lower-layer security protocol, such as IPSec, to perform
             mutual authentication.

        4.   Section 2.3.3: RADIUS clients would need to support Dynamic
             Authorization [7].

        5.   Section 2.3.4: RADIUS clients would need to support Dynamic
             Authorization [7].

1.2 Terminology and Acronyms

        AAA: Authentication, Authorization and Accounting

        Accounting: The collection of resource consumption data for the
             purposes of capacity and trend analysis, cost allocation,
             auditing, and billing. Accounting management requires that
             resource consumption be measured, rated, assigned, and
             communicated between appropriate parties [8].

        Accounting with credit control: The application checks the end
             user's account for coverage for the requested service event
             charge prior to execution of that service event.

        Home AAA Server: Server where user with which the user maintains
             an account relationship.

        SIP: Session Initiation Protocol

        SIP proxies: SIP proxies are nodes which forward SIP requests
             and responses as well as make policy decisions.

        UAC: User Agent Client

        UAS: User Agent Server

1.3 Requirements Language




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   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2].

2 Requirements

   In this section, we list the requirements. Protocol solutions are not
   required to fulfill requirements for services that they do not
   support. For example, a solution that provides authentication
   services but not accounting services does not need to fulfill the
   accounting requirements. It is expected that solutions do fulfill the
   general requirements plus the requirements for the specific services
   they are providing.

   Section 2.1 lists general requirements, Section 2.2 lists
   requirements related to authentication, Section 2.3 lists
   requirements related to authorization and Section 2.4 lists
   requirements related to accounting.

2.1 Common Requirements

   This section outlines general requirements on the SIP-AAA interface.

2.1.1 Communication within the Same Domain

   The SIP-AAA interface MUST support communications between a SIP
   entity and a AAA server that belong to the same domain.

2.1.2 Communication between Different Domains

   The SIP-AAA interface MUST support communications between a SIP
   entity in one domain and a AAA server in another domain. This MAY
   involve a proxy or a redirect server architecture between both
   entities.

2.1.3 Discovery

   With the information contained in the SIP messages, the SIP-AAA
   interface SHOULD be able to deduce the particular AAA server that has
   to be queried.

2.1.4 Ability to Integrate Different Networks, Services and Users

   The basic AAA architecture MUST be access independent. Service
   providers have to be able to provide AAA services for SIP,
   irrespective of access method or technology.

2.1.5 Updating SIP Server Entries



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   When required, the SIP-AAA interface MUST allow the AAA server to
   update the information that a SIP entity has about a user.

2.1.6 SIP Session Changes

   The SIP-AAA interface MUST allow a SIP entity to inform the AAA
   server about changes in the SIP session that may affect the
   authorization, authentication or accounting for that SIP session.

2.1.7 Reliable Transfer of Protocol Messages

   The SIP-AAA interface SHOULD provide a reliable transfer of AAA
   protocol messages between the SIP entity and the AAA server.

2.1.8 Call Setup Times

   AAA SHOULD NOT unduly burden call setup times where appropriate. It
   may be reasonable to support some delay during registration, but
   delay during on-going sessions (especially real-time) are
   problematic.

2.1.9 Security

   The SIP-AAA interface is a potential target of an attack. An
   eavesdropper may attempt to obtain confidential data by sniffing
   messages. Additionally, an active attacker may attempt to modify,
   insert, or replay messages between the SIP entity and the AAA server.
   Attackers may also attempt to impersonate legitimate SIP entities or
   AAA servers.

   To address these threats, the SIP-AAA interface MUST support
   confidentiality, data origin authentication, integrity, and replay
   protection. In addition to this, bi-directional authentication
   between the SIP entity and the AAA server MUST be supported as well.

2.2 Authentication Requirements

   This section outlines requirements on the SIP-AAA interface related
   to authentication.

2.2.1 Authentication Based on SIP Requests

   The home AAA server MUST be able to authenticate a user based on any
   SIP request, except CANCELs and ACKs for non-2xx final responses.

        CANCELs and ACKs for non-2xx final response are hop-by-hop
        requests that can be generated by proxies that do not have
        the user's credentials.



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2.2.2 Flexible Authentication of SIP Requests

   The SIP-AAA interface MUST be flexible enough to accommodate a
   variety of authentication mechanisms used to authenticate SIP
   requests. In particular, the SIP-AAA interface MUST be able to
   accommodate all the authentication mechanisms mandated by the SIP
   specs (e.g., Digest authentication.)

2.3 Authorization Requirements

   This section outlines requirements on the SIP-AAA interface related
   to authorization.

2.3.1 Ability to Authorize SIP Requests

   The SIP-AAA interface MUST allow AAA servers to authorize any SIP
   request, except CANCELs and ACKs for non-2xx final responses.

        CANCELs and ACKs for non-2xx final responses are hop-by-hop
        requests that can be generated by proxies. SIP servers
        receiving a CANCEL or a ACK for a non-2xx final response do
        not challenge them, as they would do with an end-to-end
        request. Instead, they check at the transport or network
        layer that the entity sending the CANCEL or the ACK is the
        same as the one that generated the request being canceled
        or acked.

2.3.2 Information Transfer

   The SIP-AAA interface MUST allow transferring a wide range or set of
   information to be used to make an authorization decision. In
   particular, the SIP-AAA interface MUST allow a AAA server that is
   making an authorization decision to deliver the user profile to the
   SIP entity. Such a user profile may provide further information about
   the authorization decision to the SIP entity.

   For instance, a SIP proxy receives an INVITE from user A addressed to
   user B. The SIP proxy queries a AAA server and gets the following
   answer: user A is authorized to call user B as long as the requests
   are routed through a particular SIP proxy server C. In this case, the
   SIP proxy needs to use SIP loose routing techniques to forward the
   INVITE so that it traverses SIP proxy C before reaching user B.

2.3.3 User De-authorization

   The SIP-AAA interface MUST allow the AAA server to inform a SIP
   entity when a particular user is no longer authorized to perform a
   particular task, even if it is an ongoing task.



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2.3.4 User Re-authorization

   The SIP-AAA interface MUST allow the AAA server to inform a SIP
   entity that a particular authorization has been refreshed, and
   therefore, the user is still authorized to perform a particular task.

2.3.5 Support for Credit Control

   The SIP-AAA interface MUST support credit control. That is, the AAA
   server has to be able to check the end user's account for coverage
   for the requested service event charge before authorizing execution
   of that service event. Note that this requirement is related to
   accounting as well.

   Credit control is useful to implement prepaid services where all
   chargeable events related to a specific account are prevented from
   the end user when the credit of that account is exhausted or expired.

2.4 Accounting Requirements

   This section outlines requirements on the SIP-AAA interface related
   to accounting. Accounting is more than simple charging. Accounting
   may be a simple list of services accessed, servers accessed, duration
   of session, etc. Charging for SIP sessions can be extremely complex
   and requires some additional study. It is not the intent of this
   section to focus on charging.

        The information available to be accounted is different at
        SIP proxies and at SIP UAs. When end-to-end encryption is
        used, proxies do not have access to some parts of the SIP
        messages while UAs have access to the whole messages. In
        addition to this, UAs typically have information about the
        session itself (e.g., number of audio packets exchanged
        during an audio session). Therefore, even if the SIP-AAA
        interface provides a means to transfer a wide range of
        data, some SIP nodes may not have access to it. In order to
        design a network, it is important to analyze which SIP
        nodes will be able to generate the desired account records.

2.4.1 Separation of Accounting Information

   AAA accounting messages MUST be able to provide granular information
   based on different parameters.

   For example, it should be possible to separate "session duration"
   information from other information generated via additional services
   (e.g., 3-way calling). Separating accounting information makes it
   possible to provide accounting information to different parties based



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   upon different aspects of the session.

2.4.2 Accounting Information Related to Session Progression

   There MUST be support in the SIP-AAA interface for accounting
   transfers where the information contained in the accounting data has
   a direct bearing on the establishment, progression and termination of
   a session (e.g., reception of a BYE request).

2.4.3 Accounting Information Not Related to Session Progression

   There MUST be support in the SIP-AAA interface for accounting
   transfers where the information contained in the accounting data does
   NOT have a direct bearing on the establishment, progression and
   termination of a session (e.g., an instant MESSAGE that is not
   related to any session).

2.4.4 Support for One-Time and Session-based Accounting Records

   The SIP-AAA interface MUST allow SIP servers to provide relevant
   accounting information for billing and inter-network settlement
   purpose to the AAA servers. Both one-time event accounting records
   and session based (START, INTERIM, STOP records) accounting MUST be
   supported.

2.4.5 Support for Accounting on Different Media Components

   The SIP-AAA interface MUST support accounting per media component
   (e.g., voice and video). That is, the SIP-AAA interface MUST be able
   to provide the AAA server with the types (e.g., voice and video) of
   the media streams of a given session.

   Note, however, that some SIP entities do not have access to this
   information, which is typically carried in session descriptions. An
   example of a SIP entity with access to this information is a SIP UA
   (e.g., a gateway towards the PSTN.)

   The SIP-AAA interface MUST enable different parties to be charged per
   media component.

2.4.6 Configuration of Accounting Generation Parameters

   The SIP-AAA interface MUST allow AAA servers to communicate
   parameters for accounting generation.

2.4.7 Support for Arbitrary Correlations

   Some networks need to be able to relate accounting information to



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   some aspect of the SIP messages involved. So, the SIP-AAA interface
   MUST allow the AAA server to correlate a particular AAA session with
   any aspect of the SIP messages. For example, a AAA server that
   receives accounting information about a SIP dialog may be interested
   in knowing the Call-ID of the SIP dialog.

3 Scenarios

   This section outlines some possible scenarios for SIP and AAA
   interaction. These are purely illustrative examples, and do not
   impose any requirements.

   Figure 1 shows the typical call flow between a SIP proxy that
   communicates to a AAA server that performs authentication and
   authorization. All the examples are based on this flow.



          SIP            SIP            AAA
          UAC           Proxy          Server

           |              |              |
           |---METHOD---->|              |
           |              |--Is it OK?-->|
           |              |              |
           |              |<-----OK------|
           |              |              |
           |              |              |




   Figure 1: Call flow over the SIP-AAA interface



   The SIP proxy receives a request with certain credentials. The SIP
   UAC that generated the request may have included the credentials
   after having been challenged by the proxy using a 407 (Proxy
   Authentication Required) response. The SIP proxy sends a request to
   the AAA server asking if it is OK to provide a particular service for
   this request. The service may be simply routing forward the request
   or may consist of a more complex service. The AAA server checks that
   the credentials are correct (authentication), and checks the user
   profile. The user profile indicates that it is OK to provide the
   service, and responds to the SIP proxy. The SIP proxy provides the
   service requested by the SIP UAC.




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3.1 WLAN Roaming Using Third Party Service Providers

   User A wants to establish a voice session over the Internet with user
   B. User A wants its SIP signalling to be routed through SIP proxy C,
   because it provides a call log service (i.e., SIP proxy C sends an
   email to user A once a month with the duration of all the calls made
   during the month.)



                          SIP               AAA
        User A          Proxy C            Server           User B

          |                |                 |                |
          |----INVITE----->|                 |                |
          |                |                 |                |
          |<-----407-------|                 |                |
          |                |                 |                |
          |------ACK------>|                 |                |
          |                |                 |                |
          |----INVITE----->|                 |                |
          |                |---Is this OK?-->|                |
          |                |                 |                |
          |                |<------OK--------|                |
          |                |                 |                |
          |                |---------INVITE------------------>|
          |                |                 |                |
          |                |-Accounting msg->|                |
          |                |                 |                |



   Figure 2: WLAN roaming user



   User A accesses the Internet using a WLAN access outside his home
   domain. User A, user B, SIP proxy C and the home AAA server of user A
   are all in different domains.

   SIP proxy C challenges the initial INVITE from user A with a 407
   (Proxy Authentication Required) response, and user A reissues the
   INVITE including his credentials. SIP proxy C consults user's A home
   AAA server, which confirms that the credentials belong to user A and
   that SIP proxy C can go ahead and provide its service for that call.
   SIP proxy C routes the INVITE forward towards user B and sends an
   accounting message to the AAA server, which will be used later to
   charge user A for the service provided by SIP proxy C.



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3.2 Conditional Authorization

   User A is not in his home domain, but he still uses SIP proxy C
   (which is in user's A home domain) as the outbound proxy for an
   INVITE. SIP proxy C consults the home AAA server, which indicates
   that requests from user A have to be routed through SIP proxy D. SIP
   proxy C uses SIP loose routing so that the INVITE traverses D before
   reaching its destination. SIP proxy D will provide call log service
   for user A.



                          SIP                    AAA         SIP
        User A          Proxy C                 Server     Proxy D

          |                |                      |           |
          |----INVITE----->|                      |           |
          |                |                      |           |
          |<-----407-------|                      |           |
          |                |                      |           |
          |------ACK------>|                      |           |
          |                |                      |           |
          |----INVITE----->|                      |           |
          |                |------Is this OK?---->|           |
          |                |                      |           |
          |                |<-OK if routed thru D-|           |
          |                |                      |           |
          |                |---------INVITE------------------>|
          |                |                      |           |



   Figure 3: Conditional Authorization



4 Security Considerations

   This document is informational in nature, so it does not directly
   affect the security of the Internet. However, security is a basic
   requirement of this work. Section 2.1.9 contains security
   requirements related to the SIP-AAA interface.

5 Acknowledgements

   The authors would like to thank the participants of the SIP interim
   meeting, May 2002 for their comments. The authors would also thank
   Harri Hakala, Mary Barns, Pete McCann, Jari Arkko, Aki Niemi, Juha



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   Heinanen, Henry Sinnreich, Allison Mankin and Bernard Aboba for their
   comments.

   The authors would like to thank the authors of the "AAA Requirements
   for IP Telephony/Multimedia" draft, which some of the information in
   this document is based on.

6 Authors' Addresses

   John Loughney
   Nokia
   Itamerenkatu 11-13
   00180 Helsinki
   Finland
   electronic mail:  John.Loughney@nokia.com

   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland
   electronic mail:  Gonzalo.Camarillo@ericsson.com

7 Normative References

   [1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June
   2002.

   [2] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

8 Informative References

   [3] P. Calhoun, J. Loughney, E. Guttman, G. Zorn, and J. Arkko,
   "Diameter base protocol," RFC 3588, Internet Engineering Task Force,
   Sept. 2003.

   [4] S. Glass, T. Hiller, S. Jacobs, and C. E. Perkins, "Mobile IP
   authentication, authorization, and accounting requirements," RFC
   2977, Internet Engineering Task Force, Oct. 2000.

   [5] C. Rigney, S. Willens, A. Rubens, and W. Simpson, "Remote
   authentication dial in user service (RADIUS)," RFC 2865, Internet
   Engineering Task Force, June 2000.

   [6] B. Aboba and J. Vollbrecht, "Proxy chaining and policy



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   implementation in roaming," RFC 2607, Internet Engineering Task
   Force, June 1999.

   [7] M. Chiba, G. Dommety, M. Eklund, D. Mitton, and B. Aboba,
   "Dynamic authorization extensions to remote authentication dial in
   user service (RADIUS)," RFC 3576, Internet Engineering Task Force,
   July 2003.

   [8] B. Aboba, J. Arkko, and D. Harrington, "Introduction to
   accounting management," RFC 2975, Internet Engineering Task Force,
   Oct. 2000.



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