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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: September 23, 2005                                        Intel
                                                                 F. Bari
                                                       Cingular Wireless
                                                               P. Eronen
                                                                   Nokia
                                                          March 22, 2005


 Identity selection hints for Extensible Authentication Protocol (EAP)
                 draft-adrangi-eap-network-discovery-11

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
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 23, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The Extensible Authentication Protocol (EAP) is defined in RFC 3748.
   This document defines a mechanism that allows an access network to



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   provide identity selection hints to an EAP peer.  The purpose is to
   assist the EAP peer in selecting an appropriate Network Access
   Identifier (NAI) when there is no direct roaming relationship between
   the access network and the peer's home network.  In this case,
   authentication is typically accomplished via a mediating network such
   as a roaming consortium or broker.

   The mechanism defined in this document is limited in its scalability.
   It is intended for access networks that have a small to moderate
   number of direct roaming partners.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Applicability  . . . . . . . . . . . . . . . . . . . . . .  3
     1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Implementation requirements  . . . . . . . . . . . . . . . . .  4
     2.1   Packet format  . . . . . . . . . . . . . . . . . . . . . .  5
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Security considerations  . . . . . . . . . . . . . . . . . . .  6
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Appendix - Delivery Options  . . . . . . . . . . . . . . . . .  7
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1   Normative references . . . . . . . . . . . . . . . . . . . 10
     7.2   Informative references . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13
























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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.  In some cases, the
   peer may be uncertain which Network Access Identity (NAI) to include
   in an EAP-Response/Identity.

   The Extensible Authentication Protocol (EAP) is defined in [RFC3748].
   This document defines a mechanism that allows the access network to
   provide an EAP peer with identity selection hints, including
   information about its roaming relationships.  This information is
   sent to the peer in an EAP-Request/Identity message by appending it
   after the displayable message and a NUL character.

   One possible application for this mechanism is to help an EAP peer
   perform NAI decoration [rfc2486bis] to facilitate routing of AAA
   messages to the home AAA server.  If there are several possible
   mediating networks, the peer can use this method to influence which
   one is used.

   Exactly how the selection is made by the peer depends largely on the
   peer's local policy and configuration, and is outside the scope of
   this document.  For example, the peer could decide to use one of its
   other identities, decide to switch to another access network, or
   attempt to reformat its NAI [rfc2486bis]  to assist in proper AAA
   routing.  The exact client behaviour is described by standard bodies
   using this specification such as 3GPP [TS 24.234].

   Section 2 describes the required behavior of implementations of this
   Specification, including the packet format for structuring and
   presenting identity hint information to an EAP peer.

1.1  Applicability

   The identity hints are typically useful only when there's too much
   ambiguity for an access network to determine how to route the AAA
   packet.  This can happen, for instance, when  access networks have
   contracts with multiple roaming consortiums but do not have a full
   list of home networks reachable through them.  In such scenarios, a
   limited number of identity hints (e.g., a list of roaming partners of
   the access network) can be provided by the mechanism to enable the
   EAP peer to influence routing of AAA packets.

   The immediate application of the proposed mechanism is in 3GPP
   systems interworking with WLANs [TS 23.234] and [TS 24.234].  The
   roaming partner information provided via this mechanism is limited by
   the link layer MTU size.  For example, assuming an average of 20



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   octets per roaming partner / home network information and the link
   layer MTU size of 1096, the approximate number of roaming partners
   that can be advertised would be 50.  The scalability limitation
   imposed by the link layer MTU size should be taken into consideration
   when deploying this solution.

   This document is also related to the general network discovery and
   selection problem described in [netsel-problem].  The proposed
   mechanism described in this document solves only a part of the
   problem in [netsel-problem].  IEEE 802.11 is also looking into more
   comprehensive and long-term solutions for network discovery and
   selection.

1.2  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].

   NAI             Network Address Identifier [rfc2486bis].

   Decorated NAI   An NAI with additional information for influencing
                   AAA routing.  Please refer to section 2.7 of
                   [rfc2486bis] for its construction.

   NAI Realm       Realm portion of an NAI [rfc2486bis].

2.  Implementation requirements

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

   The EAP authenticator MAY send an identity hint to the peer in the
   initial EAP-Request/Identity.  If the identity hint is not sent
   initially (such as when the authenticator does not support this
   specification), then if the local EAP-aware AAA proxy/server
   implementing this specification receives an AAA Request packet with
   an unknown realm, it SHOULD reply with an EAP-Request/Identity
   containing an identity hint.  For example, in case of RADIUS, if the
   EAP-aware RADIUS proxy/server [RFC3579] receives an Access-Request
   packet with an unknown realm in the UserName(1) attribute, then it
   can reply with an EAP-Request/Identity containing an identity hint
   within an Access-Challenge packet.  See "option 3" in the appendix
   for the message flow diagram.

   If the peer responds with an EAP-Response/Identity containing an
   unknown realm after the local AAA proxy/server sends an identity



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   hint, then the local AAA proxy/server MAY respond immediately with an
   EAP Failure packet.  Alternatively, it MAY first send an
   EAP-Notification providing the reason for the failure.

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

   As noted in [RFC3748], Section 3.1, the minimum EAP MTU size is 1020
   octets.  EAP does not support fragmentation of EAP-Request/Identity
   messages, so the maximum length of the identity hint information is
   limited by the link MTU.

2.1  Packet format

   The Identity hint information is placed after the displayable string
   and a NUL character in the EAP-Request/Identity.  The following ABNF
   [RFC2234] defines an 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" [ Network-Info ]

      displayable-string    = *CHAR

      Network-Info     =   "NAIRealms=" realm-list
      Network-Info     =/  1*OCTET ",NAIRealms=" realm-list
      Network-Info     =/  "NAIRealms=" realm-list "," 1*OCTET
      Network-Info     =/  1*OCTET ",NAIRealms=" realm-list "," 1*OCTET

      realm-list            = realm /
                              ( realm-list ";" realm )

   The "OCTET" and "CHAR" rules are defined in [RFC2234] and the "realm"
   rule is defined in [rfc2486bis].

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












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      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

   The Network-Info can contain a NAIRealms list in addition to
   proprietary information.  The proprietary information can be placed
   before or after NAIRealms list.  To extract NAIRealms list, an
   implementation can either find the "NAIRealms=" immediately after the
   NUL or seek forward to find ",NAIRealms" somewhere in the string.
   The realms data ends either at the first "," or at the end of the
   string, whichever comes first.

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-Request/Identity
   before the authentication conversation begins.  Therefore, it can be
   modified by an attacker.  The NAIRealms attribute therefore MUST be
   treated as a hint by the peer.

   Unauthenticated hints may result in peers inadvertently revealing
   additional identities, thus compromising privacy.  Since the
   EAP-Response/Identity is sent in the clear, this vulnerability
   already exists.  This vulnerability can be addressed via
   method-specific identity exchanges.

   Similarly, in a situation where the peer has multiple identities to
   choose from, an attacker can use a forged hint to convince the peer
   to choose an identity bound to a weak EAP method.  Requiring the use
   of strong EAP methods can protect against this.  A similar issue
   already exists with respect to unprotected link layer advertisements
   such as 802.11 SSIDs.

   If the identity hint is used to select a mediating network, existing
   EAP methods may not provide a way for the home AAA server to verify



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   that the mediating network selected by the peer was actually used.

   Any information revealed either from the network or client sides
   before authentication has occurred can be seen as a security risk.
   For instance, revealing the existence of a network that uses a weak
   authentication method can make it easier for attackers to discover
   that such network is accessible.  Therefore, the consent of the
   network being advertised in the hints is required before such hints
   can be sent.

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, Mark Watson, and Avi Lior for
   their support, feedback and guidance during the various stages of
   this work.

6.  Appendix - Delivery Options

   Although the delivery options are described in the context of IEEE
   802.11 access networks, they are also applicable to other access
   networks that use EAP [RFC3748] for authentication and use the NAI
   format [rfc2486bis] for identifying users.

   The options assume that the AAA protocol in use is RADIUS [RFC2865].
   However, 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-Request/Identity.  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).

   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-Request/Identity from access point

   In typical IEEE 802.11 wireless LANs, the initial
   EAP-Request/Identity 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        |                    |                    |
     |  Request/Identity |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     2. EAP        |                    |                    |
     |  Response/Identity|                    |                    |
     |------------------>|                    |                    |
     |                   | 3. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   |  Response/Identity)|                    |
     |                   |------------------->|                    |
     |                   |                    | 4.Access-Request   |
     |                   |                    |      (EAP          |
     |                   |                    | Response/Identity) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<-------------------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-Request/Identity from local RADIUS proxy/server

   This is similar to Option 1, but the initial EAP-Request/Identity 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-Request/Identity 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         |                    |
     |                   |  Request/Identity  |                    |
     |                   |   with NAIRealms)  |                    |
     |                   |<-------------------|                    |
     |     3. EAP        |                    |                    |
     | Request/Identity  |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     4. EAP        |                    |                    |
     | Response/Identity |                    |                    |
     |------------------>|                    |                    |
     |                   | 5. Access-Request  |                    |
     |                   |       (EAP         |                    |
     |                   | Response/Identity) |                    |
     |                   |------------------->|                    |
     |                   |                    | 6. Access-Request  |
     |                   |                    |        (EAP        |
     |                   |                    | Response/Identity) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<------------------- EAP conversation ---------------------->|



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

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

   In the third option, the access point sends the initial
   EAP-Request/Identity without any hint information.  The peer then
   responds with an EAP-Response/Identity, 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-Request/Identity containing the identity hint information.









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     EAP            Access Point       local RADIUS           home RADIUS
     Peer                              Proxy/Server             server
     |                   |                    |                    |
     |     1. EAP        |                    |                    |
     | Request/Identity  |                    |                    |
     | (w/o NAIRealms)   |                    |                    |
     |<------------------|                    |                    |
     |     2. EAP        |                    |                    |
     | Response/Identity |                    |                    |
     |------------------>|                    |                    |
     |                   | 3. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   | Response/Identity) |                    |
     |                   |------------------->|                    |
     |                   | 4.Access-Challenge |                    |
     |                   |      (EAP          |                    |
     |                   |  Request/Identity  |                    |
     |                   |  with NAIRealms)   |                    |
     |                   |<-------------------|                    |
     |     5. EAP        |                    |                    |
     | Request/Identity  |                    |                    |
     |   (NAIRealms)     |                    |                    |
     |<------------------|                    |                    |
     |     6. EAP        |                    |                    |
     | Response/Identity |                    |                    |
     |------------------>|                    |                    |
     |                   | 7. Access-Request  |                    |
     |                   |      (EAP          |                    |
     |                   | Response/Identity) |                    |
     |                   |------------------->|                    |
     |                   |                    | 8. Access-Request  |
     |                   |                    |       (EAP         |
     |                   |                    | Response/Identity) |
     |                   |                    |------------------->|
     |                   |                    |                    |
     |<-------------------- 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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              Internet-Draft draft-ietf-radext-rfc2486bis-05, 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., Ed. 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", Internet-Draft draft-ietf-eap-netsel-problem-02,
              July 2004.

   [TS 23.234]
              "3GPP System to Wireless Local Area Network (WLAN)
              interworking. Stage 2. (www.3gpp.org)", Release 6
              3GPP/WLAN Stage 2 Specification TS 23.234.

   [TS 24.234]
              "3GPP System to Wireless Local Area Network (WLAN)
              interworking. Stage 3. (www.3gpp.org)", Release 6
              3GPP/WLAN Stage 2 Specification TS 24.234.

   [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.












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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
   Cingular Wireless
   7277 164th Avenue N.E.
   Redmond, WA  98052
   USA

   Phone: +1 425-580-5526
   Email: farooq.bari@cingular.com


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

   Email: pasi.eronen@nokia.com












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Acknowledgment

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   Internet Society.




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