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Network Working Group                                         F. Adrangi
Internet-Draft                                                  V. Lortz
Expires: February 6, 2005                                          Intel
                                                                 F. Bari
                                                           AT&T Wireless
                                                               P. Eronen
                                                                   Nokia
                                                               M. Watson
                                                                  Nortel
                                                          August 8, 2004


 Identity selection hints for Extensible Authentication Protocol (EAP)
                 draft-adrangi-eap-network-discovery-02

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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   RFC 3668.

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Copyright Notice

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

Abstract

   This document defines a mechanism that allows an access network to



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   provide identity selection hints to an EAP client.  The purpose is to
   help the client in selecting the most appropriate identity and NAI
   decoration to use.  This solution is especially useful in roaming
   scenarios where the access network does not have a direct
   relationship with the client's home network, but instead a mediating
   network, such as a roaming consortium or broker, is used.

1.  Introduction

   In many roaming situations, an access network can have several
   roaming relationship, either with several home networks, or mediating
   networks such as roaming consortiums and brokers, or both.  A client
   can also have several sets of credentials, and its home network may
   have roaming relationships with several mediating networks.

   This document defines a mechanism that allows the access network to
   provide identity selection hints, and more specifically information
   about its roaming relationships, to an EAP client.  This information
   is sent to the client in an EAP Identity Request message by appending
   it after the displayable message and a NUL character.

   Exactly how the identity hint information is used by the client
   depends largely on the client's local policy and configuration, and
   is outside the scope of this document.

   One possible application for this mechanism is to help in selecting
   what kind of NAI decoration [1] must be applied to allow proper
   routing of AAA messages to the home AAA server.  If there are several
   possible mediating networks, the client can choose which one to use.
   However, exactly how the selection is made is beyond the scope of
   this document.  See [6] for more detailed discussion about this
   problem space.

1.1  Applicability

   Although the proposed solution here is discussed in the context of
   public IEEE 802.11 access networks, it is applicable to other access
   networks that use EAP [2] for authentication and use the NAI format
   [1] for identifying users.

   This document assumes that the AAA protocol in use is RADIUS [8].
   Diameter [7] could also be used instead of RADIUS without introducing
   significant architectural differences.

1.2  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in [3].

   Network Access Identifier (NAI)
      An identifier that represents a client or user identity.  The
      basic structure of a NAI is user@realm, where the realm part of
      the NAI indicates the domain responsible for interpretation and
      resolution of the user name.  See [1] for more details on NAI
      format.


2.  Packet format

   Identity hint information is placed after the displayable string and
   a NUL character in the EAP Identity Request.  The following ABNF [4]
   defines a "NAIRealms" attribute for presenting the identity hint
   information.  The attribute's value consists of a set of realm names
   separated by a semicolon.

      identity-request-data = [ displayable-string ]
                              [ %x00 "NAIRealms=" realm-list  ]
      displayable-string    = *OCTET
      realm-list            = realm /
                              ( realm-list ";" realm )

   The "OCTET" rule is defined in [4] and the "realm" rule is defined in
   [1].

   A sample hex dump of an EAP Identity Request packet is shown below.

      01                        ; Code: Request
      00                        ; Identifier: 0
      00 43                     ; Length: 67 octets
      01                        ; Type: Identity
      48 65 6c 6c 6f 21 00 4e   ; "Hello\0NAIRealms=example.com;mnc014.
      41 49 52 65 61 6c 6d 73   ; mcc310.3gppnetwork.org"
      3d 69 73 70 2e 65 78 61
      6d 70 6c 65 2e 63 6f 6d
      3b 6d 6e 63 30 31 34 2e
      6d 63 63 33 31 30 2e 33
      67 70 70 6e 65 74 77 6f
      72 6b 2e 6f 72 67

   EAP does not support fragmentation for Identity Request messages, so
   the size of identity hint information is limited by the link MTU.
   The exact limit depends on the lower layer in question, but it is at
   least 1020 octets.

   Some existing systems are known to use Identity Request messages to



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   send proprietary information to the client.  This proprietary
   information is considered to be part of the displayable-string in the
   ABNF shown above.  In other words, the NUL character followed by the
   NAIRealms list MUST be placed at the end.

3.  Delivery mechanisms

   This section describes three different options for delivering the
   identity hint information to the client.  The main difference is
   which entity in the access network creates the EAP Identity Request:
   this could be either the access point or a RADIUS server.

   Access network operators can choose to deploy any of the options.
   Client implementation MUST support all three options.

3.1  Option 1: Initial request from access point

   In typical IEEE 802.11 wireless LANs, the initial EAP Identity
   Request is sent by the access point.  In the simplest case, the
   identity hint information is simply included in this request, as
   shown below.

   Wireless               AP              AN RADIUS         next RADIUS
    client                                  server              server
     |                    |                    |                    |
     | 1. Association     |                    |                    |
     |------------------->|                    |                    |
     |     2. EAP Id.Req. |                    |                    |
     |        (NAIRealms) |                    |                    |
     |<-------------------|                    |                    |
     | 3. EAP Id.Resp.    |                    |                    |
     |------------------->|                    |                    |
     |                    | 4. Access-Request  |                    |
     |                    |    (EAP Id.Resp.)  |                    |
     |                    |------------------->|                    |
     |                    |                    | 5. Access-Request  |
     |                    |                    |    (EAP Id.Resp.)  |
     |                    |                    |------------------->|
     |                    |                    |                    |
     |                   <-- EAP conversation -->                   |

   Current access points do not support this mechanism, so other options
   may be preferable.  This option can also require configuring the
   identity hint information in a potentially large number of access
   points, which may be problematic if the information changes often.






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3.2  Option 2: Initial request from access network RADIUS server

   This is similar to Option 1, but the initial EAP Identity Request is
   created by the access network RADIUS server instead of the access
   point.  Once a client associates with an access network AP using IEEE
   802.11 procedures, the AP sends an EAP-Start message [5] within a
   RADIUS Access-Request.  The access network RADIUS server can then
   send the EAP Identity Request containing the identity hint
   information.

   Wireless               AP              AN RADIUS         next RADIUS
    client                                  server              server
     |                    |                    |                    |
     | 1. Association     |                    |                    |
     |------------------->|                    |                    |
     |                    | 2. Access-Request  |                    |
     |                    |    (EAP-Start)     |                    |
     |                    |------------------->|                    |
     |                    | 3.Access-Challenge |                    |
     |                    |  (EAP Id.Req. with |                    |
     |                    |         NAIRealms) |                    |
     |                    |<-------------------|                    |
     |     4. EAP Id.Req. |                    |                    |
     |        (NAIRealms) |                    |                    |
     |<-------------------|                    |                    |
     | 5. EAP Id.Resp.    |                    |                    |
     |------------------->|                    |                    |
     |                    | 6. Access-Request  |                    |
     |                    |    (EAP Id.Resp.)  |                    |
     |                    |------------------->|                    |
     |                    |                    | 7. Access-Request  |
     |                    |                    |    (EAP Id.Resp.)  |
     |                    |                    |------------------->|
     |                    |                    |                    |
     |                   <-- EAP conversation -->                   |

   This option can work with current access points if they support the
   EAP-Start message.

3.3  Option 3: Subsequent request from access network RADIUS server

   In the third option, the access point sends the initial EAP Idntity
   Request without any hint information.  The client then responds with
   an Identity Response, which is forwarded to the local RADIUS server.
   If the RADIUS server cannot route the message based on the identity
   provided by the client, it sends a second EAP Identity Request
   containing the identity hint information.




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   Wireless               AP              AN RADIUS         next RADIUS
    client                                  server              server
     |                    |                    |                    |
     | 1. Association     |                    |                    |
     |------------------->|                    |                    |
     |     2. EAP Id.Req. |                    |                    |
     |    (w/o NAIRealms) |                    |                    |
     |<-------------------|                    |                    |
     | 3. EAP Id.Resp.    |                    |                    |
     |------------------->|                    |                    |
     |                    | 4. Access-Request  |                    |
     |                    |    (EAP Id.Resp.)  |                    |
     |                    |------------------->|                    |
     |                    | 5.Access-Challenge |                    |
     |                    |  (EAP Id.Req. with |                    |
     |                    |         NAIRealms) |                    |
     |                    |<-------------------|                    |
     |     6. EAP Id.Req. |                    |                    |
     |        (NAIRealms) |                    |                    |
     |<-------------------|                    |                    |
     | 7. EAP Id.Resp.    |                    |                    |
     |------------------->|                    |                    |
     |                    | 8. Access-Request  |                    |
     |                    |    (EAP Id.Resp.)  |                    |
     |                    |------------------->|                    |
     |                    |                    | 9. Access-Request  |
     |                    |                    |    (EAP Id.Resp.)  |
     |                    |                    |------------------->|
     |                    |                    |                    |
     |                   <-- EAP conversation -->                   |

   When the initial RADIUS Access-Request is received, if the access
   network RADIUS proxy cannot route the RADIUS packet to the next AAA
   hop, it SHOULD send identity hint information to the client (via
   Access-Challenge encapsulating an EAP Identity Request).  When a
   subsequent RADIUS Access-Request is received, if the access network
   RADIUS proxy still cannot route the RADIUS packet to the next AAA
   hop, then it SHOULD discard the packet.  Optionally, the access
   network RADIUS server can also send an error notification (via
   Access-Challenge encapsulating an EAP Notification) with an
   appropriate error message.

   This option does not require changes to existing NASes, so it may be
   preferable in many environments.

4.  IANA Considerations

   This document does not define any new namespaces to be managed by



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   IANA, and does not require any assignments in existing namespaces.

5.  Security considerations

   Identity hint information is delivered inside an EAP Identity Request
   before the user authenticates to the network, and before the network
   is authenticated to the user.  This information can be modified by an
   attacker.  Therefore, is MUST be considered just an unauthenticated
   hint that does not override any local policies at the client.

   In case the identity hint information is used to select a mediating
   network for NAI decoration, it should be noted that at least with
   some EAP methods, there is no way for the home network RADIUS server
   to verify that the mediating network used was actually the same one
   that the client had requested.

6.  Acknowledgements

   The authors would specially like to thank Jari Arkko and Bernard
   Aboba for their help in scoping the problem, for reviewing the draft
   work in progress and for suggesting improvements to it.

   The authors would also like to acknowledge and thank Adrian Buckley,
   Blair Bullock, Jose Puthenkulam, Johanna Wild, Joe Salowey, Marco
   Spini, Simone Ruffino, Mark Grayson, and Avi Lior for their support,
   feedback and guidance during the various stages of this work.

7.  References

7.1  Normative references

   [1]  Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The Network
        Access Identifier", draft-arkko-roamops-rfc2486bis-02 (work in
        progress), July 2004.

   [2]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
        Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748,
        June 2004.

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

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

7.2  Informative references

   [5]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In



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        User Service) Support For Extensible Authentication Protocol
        (EAP)", RFC 3579, September 2003.

   [6]  Arkko, J. and B. Aboba, "Network Discovery and Selection
        Problem", draft-ietf-eap-netsel-problem-01 (work in progress),
        July 2004.

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

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


Authors' Addresses

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

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


   Victor Lortz
   Intel Corporation
   2111 N.E. 25th Avenue
   Hillsboro, OR  97124
   USA

   Phone: +1 503-264-3253
   EMail: victor.lortz@intel.com


   Farooq Bari
   AT&T Wireless
   7277 164th Avenue N.E.
   Redmond, WA  98052
   USA

   Phone: +1 425-580-5526
   EMail: farooq.bari@attws.com






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   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group
   Finland

   EMail: pasi.eronen@nokia.com


   Mark Watson
   Nortel Networks
   2221 Lakeside Blvd
   Richardson, TX  75082
   USA

   EMail: mwatson@nortel.com



































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