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INFORMATIONAL

Network Working Group                                        J. Loughney
Request for Comments: 3702                                         Nokia
Category: Informational                                     G. Camarillo
                                                                Ericsson
                                                           February 2004


             Authentication, Authorization, and Accounting
         Requirements for the Session Initiation Protocol (SIP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   As Session Initiation Protocol (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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  RADIUS . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.2.  Terminology and Acronyms . . . . . . . . . . . . . . . .  4
       1.3.  Requirements Language. . . . . . . . . . . . . . . . . .  4
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.  Common Requirements. . . . . . . . . . . . . . . . . . .  5
             2.1.1.  Communication within the Same Domain . . . . . .  5
             2.1.2.  Communication between Different Domains. . . . .  5
             2.1.3.  Discovery. . . . . . . . . . . . . . . . . . . .  5
             2.1.4.  Ability to Integrate Different Networks,
                     Services and Users . . . . . . . . . . . . . . .  5
             2.1.5.  Updating SIP Server Entries. . . . . . . . . . .  5
             2.1.6.  SIP Session Changes. . . . . . . . . . . . . . .  5
             2.1.7.  Reliable Transfer of Protocol Messages . . . . .  5
             2.1.8.  Call Setup Times . . . . . . . . . . . . . . . .  6
             2.1.9.  Security . . . . . . . . . . . . . . . . . . . .  6
       2.2.  Authentication Requirements. . . . . . . . . . . . . . .  6
             2.2.1.  Authentication Based on SIP Requests . . . . . .  6
             2.2.2.  Flexible Authentication of SIP Requests. . . . .  6



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       2.3.  Authorization Requirements . . . . . . . . . . . . . . .  6
             2.3.1.  Ability to Authorize SIP Requests. . . . . . . .  7
             2.3.2.  Information Transfer . . . . . . . . . . . . . .  7
             2.3.3.  User De-authorization. . . . . . . . . . . . . .  7
             2.3.4.  User Re-authorization. . . . . . . . . . . . . .  7
             2.3.5.  Support for Credit Control . . . . . . . . . . .  7
       2.4.  Accounting Requirements. . . . . . . . . . . . . . . . .  8
             2.4.1.  Separation of Accounting Information . . . . . .  8
             2.4.2.  Accounting Information Related to Session
                     Progression. . . . . . . . . . . . . . . . . . .  8
             2.4.3.  Accounting Information Not Related to Session
                     Progression. . . . . . . . . . . . . . . . . . .  9
             2.4.4.  Support for One-Time and Session-based
                     Accounting Records . . . . . . . . . . . . . . .  9
             2.4.5.  Support for Accounting on Different Media
                     Components . . . . . . . . . . . . . . . . . . .  9
             2.4.6.  Configuration of Accounting Generation
                      Parameters. . . . . . . . . . . . . . . . . . .  9
             2.4.7.  Support for Arbitrary Correlations . . . . . . .  9
   3.  Scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.  WLAN Roaming Using Third Party Service Providers . . . . 11
       3.2.  Conditional Authorization. . . . . . . . . . . . . . . . 12
   4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 12
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 13
       6.2.  Informative References . . . . . . . . . . . . . . . . . 13
   7.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15

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 of sessions, such as Internet phone calls, multimedia
   distribution, and multimedia conferences [1].  SIP sessions have
   needs for session authentication, authorization, and accounting
   (AAA).






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   In order to authenticate and authorize users, it is typically more
   convenient for SIP entities to communicate with an 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 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 an 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 important
   requirements.  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.

   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.





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      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

   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 BCP 14, RFC 2119
   [2].

2.  Requirements

   In this section, we list the requirements.  Protocol solutions are
   not required to satisfy 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 will fulfill
   the general requirements, plus the requirements for the specific
   services they are providing.



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   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 an 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 an 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

   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.



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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) is 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 responses are hop-by-hop
      requests that can be generated by proxies that do not have the
      user's credentials.

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
   specifications (e.g., Digest authentication).

2.3.  Authorization Requirements

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





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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 an 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 an 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.

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.



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   Credit control is useful to implement prepaid services where all
   chargeable events related to a specific account are withheld 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
   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).









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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
   purposes 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
   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, an AAA server that
   receives accounting information about a SIP dialog may be interested
   in knowing the Call-ID of the SIP dialog.







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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 an 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 A's 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 a 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

   Security is a critical requirement of the SIP-AAA Interface.  Section
   2.1.9 describes the threats and security requirements.  Sections 2.2
   and 2.3 elaborate on the authentication and authorization
   requirements.

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
   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" document, as it provided a basis for
   some of the information contained in this document.



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6.  References

6.1.  Normative References

   [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
       Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
       Session Initiation Protocol", RFC 3261, June 2002.

   [2] Bradner, S., "Key words for use in RFCs to indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

6.2.  Informative References

   [3] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
       "Diameter Base Protocol", RFC 3588, September 2003.

   [4] Glass, S., Hiller, T., Jacobs, S. and C. Perkins, "Mobile IP
       Authentication, Authorization, and Accounting Requirements", RFC
       2977, October 2000.

   [5] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
       Authentication Dial in User Service (RADIUS)", RFC 2865, June
       2000.

   [6] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
       Implementation in Roaming", RFC 2607, June 1999.

   [7] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B. Aboba,
       "Dynamic Authorization Extensions to Remote Authentication Dial
       in User Service (RADIUS)", RFC 3576, July 2003.

   [8] Aboba, B., Arkko, J. and D. Harrington, "Introduction to
       Accounting Management", RFC 2975, October 2000.


















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7.  Authors' Addresses

   John Loughney
   Nokia
   Itamerenkatu 11-13
   00180 Helsinki
   Finland

   EMail:  John.Loughney@nokia.com


   Gonzalo Camarillo
   Ericsson
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   Finland

   EMail:  Gonzalo.Camarillo@ericsson.com

































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8.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology
   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
   represent that it has made any independent effort to identify any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









Loughney & Camarillo         Informational                     [Page 15]


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