Network Working Group                                         F. Adrangi
Internet-Draft                                                     Intel
Expires: June 9, 2005                                            A. Lior
                                                     Bridgewater Systems
                                                             J. Korhonen
                                                             J. Loughney
                                                        December 9, 2004

                        Chargeable User Identity

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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Copyright Notice

   Copyright (C) The Internet Society (2004).


   This document describes a new RADIUS attribute, Chargeable User
   Identity.  This attribute can be used by a home network to identity a

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   user for the purpose of roaming transactions that occur outside of
   the home network.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1   Chargeable User Identity (CUI) Attribute . . . . . . . . .  5
   3.  Diameter RADIUS Interoperability . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security considerations  . . . . . . . . . . . . . . . . . . .  8
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1   Normative references . . . . . . . . . . . . . . . . . . .  8
     7.2   Informative references . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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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., 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 user identity in order
   to enforce usage policies.

   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 an authenticated and 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 packets.

   Providing a unique identity, called the 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 draft allows the home operator to meet
   these business requirements by providing a temporal identity
   representing the subscriber and at the same time protecting the
   anonymity of the subscriber.

1.1  Motivation

   Several organizations, including WISPr, GSMA, 3GPP, Wi-Fi Alliance,
   IRAP, have been studying mechanisms to provide roaming services,
   using RADIUS.  A mechanism for providing the current deployments with
   the capacity to deploy, bill and oversee WPA networks against fraud.

   The CUI attribute has been designed to close operational loopholes in
   RADIUS specifications that have impacted roaming solutions
   negatively, especially when tunneled protocols with multiple
   identities, such as PEAP or TTLS, are used.  A chargeable identity
   reflecting the user profile authenticated by the home network is
   needed in such roaming scenarios.

   Existing RADIUS servers that do not understand the CUI attribute
   SHOULD silently discard the attribute.  Use of the CUI is geared to
   multi-identity EAP authentications which are, for the most part,
   recent deployments.

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   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 Chargeable User
   Identity.  However, in a complex global roaming environment where
   there could be one or more intermediaries between the NAS and the
   home RADIUS server, the use of aforementioned attributes could lead
   to problems as described below.

      - On 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 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 for
      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 with
      unspecified ordering, which makes it hard to the entities outside
      home network to determine which one contains the CUI.

      - On use of RADIUS User-Name(1)

      The home network could use User-Name(1) in the Access-Accept
      message to convey the CUI to intermediaries and the NAS.  However,
      as the Access-Accept packet is routed to the NAS, the User-Name(1)
      attribute could be (completely) rewritten by an intermediary and
      therefore the NAS or other intermediaries along the way will not
      have access to the CUI.  Furthermore, the NAS may use the original
      value of the User-Name(1) attribute (the one sent in the
      Access-Request packet) in the Accounting-Request packets to ensure
      the billing follows the same path as authentication packets.

   The CUI attribute provides a solution to the above problem and avoids
   overloading the use of current RADIUS attributes (e.g., User-Name(1)
   re-write).  CUI is the correct standards-based approach to fixing the
   problems which have arisen with multiple-identity RADIUS
   authorization and accounting methods.  It does not solve all related
   problems, but does provide networks the ability to bill and oversee
   WPA networks against fraud.  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 to such that it
   can be used to identify the user.

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1.2  Terminology

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

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

2.  Operation

   This document assumes that the RADIUS protocol operates as specified
   in [RFC2865], [RFC2866], and the Diameter protocol as specified in

2.1  Chargeable User Identity (CUI) Attribute

   This attribute serves as an alias to the user's identity.  It is
   assigned by the home RADIUS server and MAY be sent in Access-Accept
   message.  The NAS or the access network AAA server MUST include  this
   attribute  in  the  Accounting  Requests  (Start, Interim, and Stop)
   messages if it was included in the Access Accept message and
   supported by the NAS.  Entities (e.g., NASes, proxies) outside the
   home network MUST NOT modify the CUI attribute.  Servers which do not
   understand the CUI attribute SHOULD silently discard the attribute.

   The NAS MAY include the CUI attribute with a null character for its
   data field in the Access-Request message to indicate its support for
   this attribute to the home RADIUS server.  In cases where the home
   RADIUS server cannot determine the NAS support for the CUI, if the
   home RADIUS server requires the NAS support for CUI for any reason
   (e.g., for billing or charging purposes), the home RADIUS server MUST
   reject the request by sending an Access-Reject message including an
   Error-Cause attribute [RFC3576] with value (to-be-defined) (decimal),
   "CUI-Support-Undetermined".  Otherwise, if the authentication is
   successful, the home RADIUS server MUST send both the User-Name (1)
   attribute and the CUI attribute, with the understanding that if the
   NAS supports the CUI attribute the CUI attribute will override the
   identity portion the User-Name (1) attribute.  That is, the
   User-Name(1) attribute will be used for routing and the CUI attribute

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   will be used for identity purposes.

   If the RADIUS server includes this attribute in an Access-Accept
   message it MAY also use this attribute as one of the identity
   attributes in a Disconnect Message and Change of Authorization
   message defined by [RFC3576].

   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: TBD for Chargeable User Identity.

   Length: >= 3


      The string identifies the CUI of the end-user and is of type
      UTF8String.  It consists two parts separated by a colon, ':'.  The
      first part determines the CUI type and the  second  part  is  the
      actual  Chargeable User Identity value.  The CUI type is coded as
      two octet strings representing a hexadecimal number.  The CUI
      value must be at least one octet.  In cases where the attribute is
      used to indicate the NAS support for the CUI, the string value
      contains a null character.

      The following User-Identity types have been defined:

      00 - E.164 number
         The  identifier  is  in  international  E.164  format (e.g.
         MSISDN, according to the ITU-T E.164 numbering plan as defined
         in [E164] and [CE164]).

      01 - IMSI
         The is in international IMSI format according to the ITU-T
         E.212 numbering plan as defined in [E212] and [CE212]).

      02 - SIP URI
         The identifier is in the form of a SIP URI as defined in

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      03 - NAI
         The identifier is in the form of a Network Access Identifier as
         defined in [rfc2486bis].

      04 - Opaque string
         Opaque string is a value that is assigned to the user by the
         home network in an unspecified format, where the home network
         asserts that this value represents a particular user.

      05 - reserved

      The length of time for which the CUI is valid is outside of the
      scope of this specification.  It is assumed to be deployment
      related.  It should typically be long enough to serve some
      business needs and short enough such that it minimizes the chance
      of revealing the true identity of the user (either directly or

      Below are examples of CUI strings with NAI and E.164 Charging


      The real user identity SHOULD NOT be revealed through this
      attribute.  However, the value of this attribute is determined by
      the service provider.

   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       TBD  Chargeable User ID

   [Note 1] If the Access-Accept contains CUI then the NAS MUST include
   the CUI in Accounting Requests (Start, Interim and Stop) packets.

   Change of Authorization and Disconnect-Request
   Request      ACK      NAK   #    Attribute
      0-1       0        0     TBD  Chargeable User

   [Note 2] Where CUI attribute is included in Disconnect-Request or
   CoA-Request messages, it is used for session identification purposes
   only.  This attribute MUST NOT be used for purposes other than
   identification (e.g.  within CoA-Request messages to request
   authorization changes).

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3.  Diameter RADIUS Interoperability

   In deployments with both RADIUS and Diameter interworking, a
   translation agent will be deployed and operate in accordance to the
   NASREQ specification.  The Diameter Credit-Control Application's
   specifies a similar concept, the Subscription-ID AVP [DiameterCC].

4.  IANA Considerations

   This document instructs IANA to assign a new RADIUS attribute number
   for the CUI attribute.

5.  Security considerations

   The CUI attribute must be protected against Man-in-the-Middle
   attacks.  The CUI appears in Access-Accept and Accounting Requests
   packets and is protected by the mechanisms that are defined for
   RADIUS [RFC2865] and [RFC2866].  Therefore there are no additional
   security considerations beyond those already identified in [RFC2865]
   and [RFC2866].

   Message-Authenticator(80) and Event-Timestamp can be used to further
   protect against Man-in-the-middle attacks.

   In this document, entities outside the home network are required not
   to modify the value of this attribute, however there are no
   provisions for protecting against or detecting that a RADIUS Proxy
   has modified the attribute.

   As the CUI contains an identity that can be used for authorizing and
   accounting of services, this attribute must be protected against

6.  Acknowledgements

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

7.  References

7.1  Normative references

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

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

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

              Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The
              Network Access Identifier",
              draft-arkko-roamops-rfc2486bis-02 (work in progress), July

   [E164]     "The International Public Telecommunication Numbering
              Plan", , May 1997.

   [CE164]    "List of ITU-T Recommendation E.164 assigned country
              codes", , June 2000.

   [E212]     "The international identification plan for mobile
              terminals and mobile users", , November 1998.

   [CE212]    "List of mobile country or geographical area codes", ,
              February 1999.

   [RFC3261]  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.

7.2  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.

              Hakala, H., Koskinen, j., Stura, M. and J. Loughney, "The
              Network Access Identifier",
              draft-ietf-aaa-diameter-cc-06.txt  (work in progress),
              July 2004.

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Authors' Addresses

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

   Phone: +1 503-712-1791

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

   Phone: +1 613-591-9104

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

   Phone: +358405344455

   John Loughney
   Itamerenkatu 11-13
   FIN-00180,   Helsinki

   Phone: +358504836342

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