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Network Working Group                                         F. Adrangi
Internet-Draft                                                  V. Lortz
Expires: April 22, 2005                                            Intel
                                                                 F. Bari
                                                           AT&T Wireless
                                                               P. Eronen
                                                                   Nokia
                                                               M. Watson
                                                                  Nortel
                                                        October 22, 2004


 Identity selection hints for Extensible Authentication Protocol (EAP)
                 draft-adrangi-eap-network-discovery-05

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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   RFC 3668.

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   This Internet-Draft will expire on April 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

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 is especially useful when the access network
   does not have a direct roaming relationship with the client's home
   network, so that a mediating network, such as a roaming consortium or
   broker, is used.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Implementation requirements  . . . . . . . . . . . . . . . . .  3
     2.1   Packet format  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Security considerations  . . . . . . . . . . . . . . . . . . .  5
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Appendix (informative) - Delivery Options  . . . . . . . . . .  6
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.1   Normative references . . . . . . . . . . . . . . . . . . . .  9
   7.2   Informative references . . . . . . . . . . . . . . . . . . . 10
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 12





























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1.  Introduction

   An EAP peer (hereafter, also referred to as the peer) can have
   several sets of credentials, and its home network may have roaming
   relationships with several mediating networks.  As a result, the peer
   may be unclear about the appropriate Network Access Identity (NAI) to
   include in an EAP-Identity/Response.

   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.

   In many roaming situations, an access network can have several
   roaming relationships, either with several home networks, or
   mediating networks such as roaming consortiums and brokers, or both.
   One possible application for this mechanism is to help in selecting
   what kind of NAI decoration [rfc2486bis] 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 [netsel-problem] for more detailed discussion
   about this problem space.

   Section 2 describes the required behavior of implementations of this
   specification, as well as 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2.  Implementation requirements

   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.



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   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
   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.

   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.

2.1  Packet format

   The Identity hint information is placed after the displayable string
   and a NUL character in the EAP Identity Request.  The following ABNF
   [RFC2234] 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 [RFC2234] and the "realm" rule is
   defined in [rfc2486bis].

   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



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      67 70 70 6e 65 74 77 6f
      72 6b 2e 6f 72 67

   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.

   Unauthenticated hints may result in peers inadvertently revealing
   other or additional identities than they intended to, leading to a
   privacy vulnerability.  Note that in EAP, the identity the peer wants
   to use is in general carried in a cleartext message, so this is only
   a variation of an existing vulnerability.  Method-specific identity
   protection is one of the ways that this vulnerability can be
   addressed.

   Similarly, in a situation where the peer has multiple identities to
   choose from, an unauthenticated hint can lead to a situation where an
   attacker convinces the peer to choose an identifier that is bound to
   the weakest EAP method.  To guard against this vulnerability, the use
   of as strong EAP methods as possible is recommended.  Note that this
   vulnerability is similar to an existing vulnerability where link
   layers advertise network names (such as 802.11 SSIDs) without
   authenticating these advertisements either at all or only at the end
   of the authentication process.

   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



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   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 [RFC3748] for authentication and use the NAI format
   [rfc2486bis] for identifying users.  Also, the options assume that
   the AAA protocol in use is RADIUS [RFC2865].  Diameter [RFC3588]
   could also be used instead of RADIUS 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 acts only on the RADIUS UserName(1) attribute
   and does not have to parse the EAP-Message attribute.

   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.













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     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
   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 local RADIUS proxy/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 [RFC3579] within a
   RADIUS Access-Request.  The access network RADIUS server can then
   send the EAP Identity/Request containing the identity hint
   information.















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     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.

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

   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 proxy/server.  If the RADIUS proxy/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.









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     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 --------------------->|


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

7.  References

7.1  Normative references

   [rfc2486bis]
              Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The
              Network Access Identifier",



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              draft-arkko-roamops-rfc2486bis-02 (work in progress), July
              2004.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
              3748, June 2004.

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

   [RFC2234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, November 1997.

7.2  Informative references

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

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

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

   [RFC2865]  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









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   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


   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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