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Network Working Group                                         F. Adrangi
Request for Comments: 4372                                         Intel
Category: Standards Track                                        A. Lior
                                                     Bridgewater Systems
                                                             J. Korhonen
                                                             J. Loughney
                                                            January 2006

                        Chargeable User Identity

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes a new Remote Authentication Dial-In User
   Service (RADIUS) attribute, Chargeable-User-Identity.  This attribute
   can be used by a home network to identify a user for the purpose of
   roaming transactions that occur outside of the home network.

Table of Contents

   1. Introduction ....................................................2
      1.1. Motivation .................................................3
      1.2. Terminology ................................................4
   2. Operation .......................................................5
      2.1. Chargeable-User-Identity (CUI) Attribute ...................5
      2.2. CUI Attribute ..............................................6
   3. Attribute Table .................................................7
   4. Diameter Consideration ..........................................7
   5. IANA Considerations .............................................7
   6. Security Considerations .........................................7
   7. Acknowledgements ................................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................8

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1.  Introduction

   Some authentication methods, including EAP-PEAP, EAP-TTLS, EAP-SIM
   and EAP-AKA, can hide the true identity of the user from RADIUS
   servers outside of the user's home network.  In these methods, the
   User-Name(1) attribute contains an anonymous identity (e.g.,
   @example.com) sufficient to route the RADIUS packets to the home
   network but otherwise insufficient to identify the user.  While this
   mechanism is good practice in some circumstances, there are problems
   if local and intermediate networks require a surrogate identity to
   bind the current session.

   This document introduces an attribute that serves as an alias or
   handle (hereafter, it is called Chargeable-User-Identity) to the real
   user's identity.  Chargeable-User-Identity can be used outside the
   home network in scenarios that traditionally relied on User-Name(1)
   to correlate a session to a user.

   For example, local or intermediate networks may limit the number of
   simultaneous sessions for specific users; they may require a
   Chargeable-User-Identity in order to demonstrate willingness to pay
   or otherwise limit the potential for fraud.

   This implies that a unique identity provided by the home network
   should be able to be conveyed to all parties involved in the roaming
   transaction for correlating the authentication and accounting

   Providing a unique identity, Chargeable-User-Identity (CUI), to
   intermediaries, is necessary to fulfill certain business needs.  This
   should not undermine the anonymity of the user.  The mechanism
   provided by this document allows the home operator to meet these
   business requirements by providing a temporary identity representing
   the user and at the same time protecting the anonymity of the user.

   When the home network assigns a value to the CUI, it asserts that
   this value represents a user in the home network.  The assertion
   should be temporary -- long enough to be useful for the external
   applications and not too long such that it can be used to identify
   the user.

   Several organizations, including WISPr, GSMA, 3GPP, Wi-Fi Alliance,
   and IRAP, have been studying mechanisms to provide roaming services,
   using RADIUS.  Missing elements include mechanisms for billing and
   fraud prevention.

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   The CUI attribute is intended to close operational loopholes in
   RADIUS specifications that have impacted roaming solutions
   negatively.  Use of the CUI is geared toward EAP methods supporting
   privacy (such as PEAP and EAP-TTLS), which are, for the most part,
   recent deployments.  A chargeable identity reflecting the user
   profile by the home network is needed in such roaming scenarios.

1.1.  Motivation

   Some other mechanisms have been proposed in place of the CUI
   attribute.  These mechanisms are insufficient or cause other
   problems.  It has been suggested that standard RADIUS Class(25) or
   User-Name(1) attributes could be used to indicate the CUI.  However,
   in a complex global roaming environment where there could be one or
   more intermediaries between the NAS [RFC4282] and the home RADIUS
   server, the use of aforementioned attributes could lead to problems
   as described below.

      - On the use of RADIUS Class(25) attribute:

      [RFC2865] states: "This Attribute is available to be sent by the
      server to the client in an Access-Accept packet and SHOULD be sent
      unmodified by the client to the accounting server as part of the
      Accounting-Request packet if accounting is supported.  The client
      MUST NOT interpret the attribute locally."  So RADIUS clients or
      intermediaries MUST NOT interpret the Class(25) attribute, which
      precludes determining whether it contains a CUI.  Additionally,
      there could be multiple class attributes in a RADIUS packet, and
      since the contents of Class(25) attribute is not to be interpreted
      by clients, this makes it hard for the entities outside the home
      network to determine which one contains the CUI.

      - On the use of RADIUS User-Name(1) attribute:

      The User-Name(1) attribute included in the Access-Request packet
      may be used for the purpose of routing the Access-Request packet,
      and in the process may be rewritten by intermediaries.  As a
      result, a RADIUS server receiving an Access-Request packet relayed
      by a proxy cannot assume that the User-Name(1) attribute remained

      On the other hand, rewriting of a User-Name(1) attribute sent
      within an Access-Accept packet occurs more rarely, since a
      Proxy-State(33) attribute can be used to route the Access-Accept
      packet without parsing the User-Name(1) attribute.  As a result, a
      RADIUS server cannot assume that a proxy stripping routing
      information from a User-Name(1) attribute within an Access-Request
      packet will add this information to a User-Name(1) attribute

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      included within an Access-Accept packet.  The result is that when
      a User-Name(1) attribute is sent in an Access-Accept packet, it is
      possible that the Access-Request packet and Accounting-Request
      packets will follow different paths.  Where this outcome is
      undesirable, the RADIUS client should use the original
      User-Name(1) in accounting packets.  Therefore, another mechanism
      is required to convey a CUI within an Access-Accept packet to the
      RADIUS client, so that the CUI can be included in the accounting

   The CUI attribute provides a solution to the above problems and
   avoids overloading RADIUS User-Name(1) attribute or changing the
   usage of existing RADIUS Class(25) attribute.  The CUI therefore
   provides a standard approach to billing and fraud prevention when EAP
   methods supporting privacy are used.  It does not solve all related
   problems, but does provide for billing and fraud prevention.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The following acronyms are used:

      3GPP - Third Generation Partnership Project
      AAA - Authentication, Authorization, and Accounting
      AKA - Authentication and Key Agreement
      CUI - Chargeable-User-Identity
      GSMA - GSM Association
      IRAP - International Roaming Access Protocols Program
      NAS - Network Access Server
      PEAP - Protected Extensible Authentication Protocol
      SIM - Subscriber Identity Modules
      TTLS - Tunneled Transport Layer Security
      WISPr - Wireless ISP Roaming
      WPA - Wi-Fi Protected Access

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2.  Operation

   This document assumes that the RADIUS protocol operates as specified
   in [RFC2865] and [RFC2866], dynamic authorization as specified in
   [RFC3576], and the Diameter protocol as specified in [RFC3588].

2.1.  Chargeable-User-Identity (CUI) Attribute

   The CUI attribute serves as an alias to the user's real identity,
   representing a chargeable identity as defined and provided by the
   home network as a supplemental or alternative information to
   User-Name(1).  Typically, the CUI represents the identity of the
   actual user, but it may also indicate other chargeable identities
   such as a group of users.  RADIUS clients (proxy or NAS) outside the
   home network MUST NOT modify the CUI attribute.

   The RADIUS server (a RADIUS proxy, home RADIUS server) may include
   the CUI attribute in the Access-Accept packet destined to a roaming
   partner.  The CUI support by RADIUS infrastructure is driven by the
   business requirements between roaming entities.  Therefore, a RADIUS
   server supporting this specification may choose not to send the CUI
   in response to an Access-Request packet from a given NAS, even if the
   NAS has indicated that it supports CUI.

   If an Access-Accept packet without the CUI attribute was received by
   a RADIUS client that requested the CUI attribute, then the
   Access-Accept packet MAY be treated as an Access-Reject.

   If the CUI was included in an Access-Accept packet, RADIUS clients
   supporting the CUI attribute MUST ensure that the CUI attribute
   appears in the RADIUS Accounting-Request (Start, Interim, and Stop).
   This requirement applies regardless of whether the RADIUS client
   requested the CUI attribute.

   RFC 2865 includes the following statements about behaviors of RADIUS
   client and server with respect to unsupported attributes:

      - "A RADIUS client MAY ignore Attributes with an unknown Type."
      - "A RADIUS server MAY ignore Attributes with an unknown Type."

   Therefore, RADIUS clients or servers that do not support the CUI may
   ignore the attribute.

   A RADIUS client requesting the CUI attribute in an Access-Accept
   packet MUST include within the Access-Request packet a CUI attribute.
   For the initial authentication, the CUI attribute will include a
   single NUL character (referred to as a nul CUI).  And, during

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   re-authentication, the CUI attribute will include a previously
   received CUI value (referred to as a non-nul CUI value) in the

   Upon receiving a non-nul CUI value in an Access-Request, the home
   RADIUS server MAY verify that the value of CUI matches the CUI from
   the previous Access-Accept.  If the verification fails, then the
   RADIUS server SHOULD respond with an Access-Reject message.

   If a home RADIUS server that supports the CUI attribute receives an
   Access-Request packet containing a CUI (set to nul or otherwise), it
   MUST include the CUI attribute in the Access-Accept packet.
   Otherwise, if the Access-Request packet does not contain a CUI, the
   home RADIUS server SHOULD NOT include the CUI attribute in the
   Access-Accept packet.  The Access-Request may be sent either in the
   initial authentication or during re-authentication.

   A NAS that requested the CUI during re-authentication by including
   the CUI in the Access-Request will receive the CUI in the
   Access-Accept.  The NAS MUST include the value of that CUI in all
   Accounting Messages.

2.2.  CUI Attribute

   A summary of the RADIUS CUI attribute is given below.

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |     Type      |    Length     | String...

   Type: 89 for Chargeable-User-Identity.

   Length: >= 3


      The string identifies the CUI of the end-user.  This string value
      is a reference to a particular user.  The format and content of
      the string value are determined by the Home RADIUS server.  The
      binding lifetime of the reference to the user is determined based
      on business agreements.  For example, the lifetime can be set to
      one billing period.  RADIUS entities other than the Home RADIUS
      server MUST treat the CUI content as an opaque token, and SHOULD
      NOT perform operations on its content other than a binary equality
      comparison test, between two instances of CUI.  In cases where the

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      attribute is used to indicate the NAS support for the CUI, the
      string value contains a nul character.

3.  Attribute Table

   The following table provides a guide to which attribute(s) may be
   found in which kinds of packets, and in what quantity.

   Request Accept Reject Challenge Accounting #     Attribute
     0-1    0-1     0        0        0-1    89 Chargeable-User-Identity

   Note: If the Access-Accept packet contains CUI, then the NAS MUST
   include the CUI in Accounting Requests (Start, Interim, and Stop)

4.  Diameter Consideration

   Diameter needs to define an identical attribute with the same Type
   value.  The CUI should be available as part of the NASREQ application

5.  IANA Considerations

   This document uses the RADIUS [RFC2865] namespace; see
   http://www.iana.org/assignments/radius-types.  The IANA has assigned
   a new RADIUS attribute number for the CUI attribute.

   CUI 89

6.  Security Considerations

   It is strongly recommended that the CUI format used is such that the
   real user identity is not revealed.  Furthermore, where a reference
   is used to a real user identity, it is recommended that the binding
   lifetime of that reference to the real user be kept as short as

   The RADIUS entities (RADIUS proxies and clients) outside the home
   network MUST NOT modify the CUI or insert a CUI in an Access-Accept.
   However, there is no way to detect or prevent this.

   Attempting theft of service, a man-in-the-middle may try to insert,
   modify, or remove the CUI in the Access-Accept packets and Accounting
   packets.  However, RADIUS Access-Accept and Accounting packets
   already provide integrity protection.

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   If the NAS includes CUI in an Access-Request packet, a
   man-in-the-middle may remove it.  This will cause the Access-Accept
   packet to not include a CUI attribute, which may cause the NAS to
   reject the session.  To prevent such a denial of service (DoS)
   attack, the NAS SHOULD include a Message-Authenticator(80) attribute
   within Access-Request packets containing a CUI attribute.

7.  Acknowledgements

   The authors would like to thank Jari Arkko, Bernard Aboba, David
   Nelson, Barney Wolff, Blair Bullock, Sami Ala-Luukko, Lothar Reith,
   David Mariblanca, Eugene Chang, Greg Weber, and Mark Grayson for
   their feedback and guidance.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [RFC4005]  Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
              "Diameter Network Access Server Application", RFC 4005,
              August 2005.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

8.2.  Informative References

   [RFC3576]  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.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

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RFC 4372                Chargeable User Identity            January 2006

Authors' Addresses

   Farid Adrangi
   Intel Corporation
   2111 N.E. 25th Avenue
   Hillsboro, OR  97124

   Phone: +1 503-712-1791
   EMail: farid.adrangi@intel.com

   Avi Lior
   Bridgewater Systems Corporation
   303 Terry Fox Drive
   Ottawa, Ontario  K2K 3J1

   Phone: +1 613-591-9104
   EMail: avi@bridgewatersystems.com

   Jouni Korhonen
   Teliasonera Corporation
   P.O.Box 970
   FIN-00051,   Sonera

   Phone: +358405344455
   EMail: jouni.korhonen@teliasonera.com

   John Loughney
   Itamerenkatu 11-13
   FIN-00180,   Helsinki

   Phone: +358504836342
   EMail: john.loughney@nokia.com

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RFC 4372                Chargeable User Identity            January 2006

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