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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 4284

Network Working Group                                         F. Adrangi
Internet-Draft                                                  V. Lortz
Expires: February 12, 2005                                         Intel
                                                                 F. Bari
                                                           AT&T Wireless
                                                               P. Eronen
                                                               M. Watson
                                                         August 14, 2004

 Identity selection hints for Extensible Authentication Protocol (EAP)

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
   author represents that any applicable patent or other IPR claims of
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   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   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 when the
   access network does not have a direct roaming relationship with the
   client's home network, so 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.  An EAP
   peer (hereafter, also referred to as peer) 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 peer.  This information is
   sent to the peer 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 peer depends
   largely on the peer'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 peer 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.

   An EAP peer implementing this specification MUST be able to receive
   an identity hint in an initial EAP Identity/Request, or in a
   subsequent EAP Identity/Request.

   The EAP authenticator MAY send an identity hint to the peer in the
   initial EAP Identity/Request.  If the identity hint is not sent
   initially (such as when the authenticator does not support this
   specification), then if the EAP server receives an EAP Identity/
   Response with an unacceptable NAI Realm, EAP servers implementing
   this specification SHOULD reply with an EAP Identity/Request
   containing an identity hint.

   If after the EAP server sends an EAP Identity/Request containing an
   identity hint, the peer responds with an EAP Identity/Response

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   containing an unacceptable NAI Realm, then the EAP server MAY respond
   immediately with an EAP Failure packet, or it MAY first send an
   EAP-Notification providing information on the reason for the failure.

   Section 2 describes the packet format for structuring and presenting
   identity hint information to an EAP peer.  The appendix in section 6
   describes the delivery options that can be implemented by an access
   network to deliver identity hint information to an EAP peer.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [3].

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

   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

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   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 EAP Identity/Request messages
   to send proprietary information to the peer.  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.  IANA Considerations

   This document does not define any new namespaces to be managed by
   IANA, and does not require any assignments in existing namespaces.

4.  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, it MUST be considered an unauthenticated hint
   that does not override any local policies at the peer.

   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 AAA server to
   verify that the mediating network used was actually the same one that
   the peer had requested.

5.  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.

6.  Appendix (informative) - Delivery Options

   Although the delivery options are described in the context of IEEE
   802.11 access networks, they are applicable to other access networks
   that use EAP [2] for authentication and use the NAI format [1] for
   identifying users.  Also, the options assume that the AAA protocol in
   use is RADIUS [8].  Diameter [7] could also be used instead of RADIUS

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   without introducing significant architectural differences.

   The main difference amongst the options is which entity in the access
   network creates the EAP Identity/ Request.  For example, the role of
   EAP server may be played by the EAP authenticator (where an initial
   EAP Request/Identity is sent with an identity hint), or a RADIUS
   proxy/server (where the NAI Realm is used for forwarding).

   When an Identity hint is sent by a RADIUS proxy/server, a RADIUS
   State (24) attribute can be used to help the RADIUS proxy/server
   determine if an identity hint had previously been sent by it to the
   EAP peer.

   The RADIUS proxy/server is EAP aware.  It acts only on the RADIUS
   UserName(1) attribute and does not have to parse the EAP-Message

   Option 1: Initial EAP Identity/Request from access point

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

     EAP          Access Point        local RADIUS           home RADIUS
     Peer                               proxy/server            server
     |     1. EAP        |                    |                    |
     | Identity/Request  |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     2. EAP        |                    |                    |
     | Identity/Response |                    |                    |
     |------------------>|                    |                    |
     |                   | 3. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   | Identity/Response) |                    |
     |                   |------------------->|                    |
     |                   |                    | 4.Access-Request   |
     |                   |                    |      (EAP          |
     |                   |                    | Identity/Response) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<-------------------EAP conversation ----------------------->|

   Current access points do not support this mechanism, so other options

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   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.

   Option 2: Initial EAP Identity/Request from local RADIUS proxy/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 peer 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

     EAP          Access Point          local RADIUS           home RADIUS
     Peer                                proxy/server            server
     |                   | 1. Access-Request  |                    |
     |                   |    (EAP-Start)     |                    |
     |                   |------------------->|                    |
     |                   | 2.Access-Challenge |                    |
     |                   |       (EAP         |                    |
     |                   |  Identity/Request  |                    |
     |                   |   with NAIRealms)  |                    |
     |                   |<-------------------|                    |
     |     3. EAP        |                    |                    |
     | Identity/Request  |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     4. EAP        |                    |                    |
     | Identity/Response |                    |                    |
     |------------------>|                    |                    |
     |                   | 5. Access-Request  |                    |
     |                   |       (EAP         |                    |
     |                   | Identity/Response) |                    |
     |                   |------------------->|                    |
     |                   |                    | 6. Access-Request  |
     |                   |                    |        (EAP        |
     |                   |                    | Identity Response) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<------------------- EAP conversation ---------------------->|

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

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   Option 3: Subsequent EAP-Identity/Request from local RADIUS proxy/

   In the third option, the access point sends the initial EAP Identity/
   Request without any hint information.  The peer 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 peer, it sends a second EAP Identity Request
   containing the identity hint information.

     EAP            Access Point       local RADIUS           home RADIUS
     Peer                              Proxy/Server             server
     |                   |                    |                    |
     |     1. EAP        |                    |                    |
     | Identity Request  |                    |                    |
     | (w/o NAIRealms)   |                    |                    |
     |<------------------|                    |                    |
     |     2. EAP        |                    |                    |
     | Identity Response |                    |                    |
     |------------------>|                    |                    |
     |                   | 3. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   | Identity Response) |                    |
     |                   |------------------->|                    |
     |                   | 4.Access-Challenge |                    |
     |                   |      (EAP          |                    |
     |                   |  Identity Request  |                    |
     |                   |  with NAIRealms)   |                    |
     |                   |<-------------------|                    |
     |     5. EAP        |                    |                    |
     | Identity Request  |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     6. EAP        |                    |                    |
     | Identity Response |                    |                    |
     |------------------>|                    |                    |
     |                   | 7. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   | Identity Response) |                    |
     |                   |------------------->|                    |
     |                   |                    | 8. Access-Request  |
     |                   |                    |       (EAP         |
     |                   |                    | Identity Response) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<-------------------- EAP conversation --------------------->|

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

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
        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

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Authors' Addresses

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

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

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

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

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

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

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

   EMail: pasi.eronen@nokia.com

   Mark Watson
   Nortel Networks
   2221 Lakeside Blvd
   Richardson, TX  75082

   EMail: mwatson@nortel.com

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