EAP identity hints: Identity Selection Hints for the Extensible Authentication Protocol (EAP)

Network Working Group                                         F. Adrangi
Request for Comments: 4284                                      V. Lortz
Category: Informational                                            Intel
                                                                 F. Bari
                                                       Cingular Wireless
                                                               P. Eronen
                                                                   Nokia
                                                            January 2006


                     Identity Selection Hints for
              the Extensible Authentication Protocol (EAP)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

IESG Note:

   EAP Identity Selection was developed by 3GPP.  Documentation is
   provided as information to the Internet community.  The EAP WG has
   verified that this specification is compatible with EAP as defined in
   RFC 3748.  Required 3GPP client behavior is described in 3GPP TS
   24.234.

Abstract

   The Extensible Authentication Protocol (EAP) is defined in RFC 3748.
   This document defines a mechanism that allows an access network to
   provide identity selection hints to an EAP peer -- the end of the
   link that responds to the authenticator.  The purpose is to assist
   the EAP peer in selecting an appropriate Network Access Identifier
   (NAI).  This is useful in situations where the peer does not receive
   a lower-layer indication of what network it is connecting to, or when
   there is no direct roaming relationship between the access network
   and the peer's home network.  In the latter 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 ....................................................2
      1.1. Relationship with Other Specifications .....................3
      1.2. Applicability ..............................................3
      1.3. Terminology ................................................4
   2. Implementation Requirements .....................................4
      2.1. Packet Format ..............................................5
   3. Security Considerations .........................................6
   4. Acknowledgements ................................................7
   5. Appendix - Delivery Options .....................................8
   6. References .....................................................12
      6.1. Normative References ......................................12
      6.2. Informative References ....................................12

1.  Introduction

   The Extensible Authentication Protocol (EAP) is defined in [RFC3748].
   An EAP peer (hereafter, also referred to as the peer) may have
   multiple credentials.  Where the lower layer does not provide an
   indication of which network it is connecting to, or where its home
   network may have roaming relationships with several mediating
   networks, the peer may be uncertain of which Network Access
   Identifier (NAI) to include in an EAP-Response/Identity.

   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.

   This mechanism may assist the peer in selecting a credential and
   associated NAI, or in formatting the NAI [RFC4282] to facilitate
   routing of Authentication, Authorization, and Accounting (AAA)
   messages to the home AAA server.  If there are several mediating
   networks available, the peer can 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 [RFC4282] to assist in proper AAA
   routing.  The exact client behavior is described by standard bodies
   using this specification such as 3GPP [TS-24.234].

   Section 2 describes the required behavior of implementations,
   including the format for identity hints.

1.1.  Relationship with Other Specifications

   This document specifies behavior of Remote Authentication Dial-In
   User Service (RADIUS) proxies that handle EAP messages.  This
   includes the specification of the behavior of proxies in response to
   an unknown realm within the User-Name(1) attribute of an
   Access-Request containing one or more EAP-Message attributes.  This
   document, if used in a scenario requiring NAI "decoration" as
   specified in [RFC4282], assumes a source-routing model for
   determination of the roaming relationship path, and therefore affects
   the behavior of RADIUS proxies in roaming situations.

1.2.  Applicability

   Identity hints are useful in situations where the peer cannot
   determine which credentials to use, or where there may be multiple
   alternative routes by which an access network can reach a home
   network.  This can occur when access networks support multiple
   roaming consortiums but do not have a full list of the home networks
   reachable through them.

   In such scenarios, identity hints (e.g., a list of roaming partners
   of the access network) can be provided to enable the EAP peer to
   influence route selection, using the NAI [RFC4282] to specify the
   desired source route.  The immediate application of the proposed
   mechanism is in 3GPP systems interworking with WLANs [TS-23.234] and
   [TS-24.234].

   The number of hints that can be provided by this mechanism is limited
   by the EAP MTU.  For example, assuming 20 octets per hint and an EAP
   MTU of 1096, a maximum of 50 roaming partners can be advertised.
   Scaling limitations imposed by the EAP MTU should be taken into
   account when deploying this solution.

   Since this mechanism relies on information provided in the
   EAP-Request/Identity packet, it is necessary for the peer to select a
   point of attachment prior to obtaining identity hints.  Where there
   are multiple points of attachment available, the mechanism defined in
   this specification does not allow the peer to utilize the identity
   hints in making a decision about which point of attachment to select.
   In roaming situations, this can require the peer to try multiple
   points of attachment before it finds a compatible one, increasing
   handoff latency.

   This document is 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.

1.3.  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 Access Identifier [RFC4282].

   Decorated NAI   An NAI specifying a source route.  See [RFC4282]
                   Section 2.7 for more information.

   NAI Realm       Realm portion of an NAI [RFC4282].

2.  Implementation Requirements

   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 the EAP peer may select the wrong NAI.  If the
   local AAA proxy does not have a default route configured, then it may
   find that the User-Name(1) attribute in the request contains a realm
   for which there is no corresponding route.

   As noted in [RFC2607], Section 5.1:

   "Proxies are frequently used to implement policy in roaming
   situations.  Proxies implementing policy MAY reply directly to
   Access-Requests without forwarding the request.  When replying
   directly to an Access-Request, the proxy MUST reply either with an
   Access-Reject or an Access-Challenge packet.  A proxy MUST NOT reply
   directly with an Access-Accept."

   Where no route is found, existing AAA proxies will typically send an
   Access-Reject.  However, where the request contains an EAP-Message
   attribute, AAA proxies implementing this specification should instead
   reply with a challenge including an identity hint.

   For example, if a RADIUS proxy receives an Access-Request with an
   EAP-Message attribute and a User-Name(1) attribute containing an
   unknown realm, it SHOULD reply with an Access-Challenge with an
   EAP-Message attribute encapsulating an EAP-Request/Identity packet
   containing an identity hint, rather than an Access-Reject.  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 sends an identity hint, then
   a local AAA proxy/server implementing this specification MUST
   eventually send an Access-Reject containing an EAP-Failure.  Prior to
   doing so, it MAY send an Access-Challenge containing an
   EAP-Notification, in order to provide an explanation for the failure.
   In order to determine whether an identity hint had been previously
   sent, the State(24) attribute defined in [RFC2865] can be used.

   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
   [RFC4234] 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 [RFC4234] and the "realm"
   rule is defined in [RFC4282].

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

   01                        ; Code: Request
   00                        ; Identifier: 0
   00 3f                     ; Length: 63 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 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 an NAIRealms list in addition to
   proprietary information.  The proprietary information can be placed
   before or after the NAIRealms list.  To extract the 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.  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.  That is, the peer must treat the hint
   as an unreliable indication

   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
   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 a network is accessible.  Therefore, the consent of the
   network being advertised in the hints is required before such hints
   can be sent.

4.  Acknowledgements

   The authors would especially like to thank Jari Arkko, Bernard Aboba,
   and Glen Zorn for their help in scoping the problem, and for
   reviewing the document in progress and 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.

5.  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 [RFC4282] 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
   the 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 NAIRealm is used for forwarding).

   The RADIUS proxy/server acts only on the RADIUS User-Name(1)
   attribute and does not have to parse the EAP-Message attribute.

   Option 1: Initial EAP-Request/Identity from the 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.

   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 the 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.

   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.

   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) |                    |
   |                   |------------------->|                    |
   |                   |                    |                    |
   ======================Failure due to unknown realm=============
   |                   |                    |                    |
   |                   | 7a. Access-Reject  |                    |
   |                   |    (EAP-Failure)   |                    |
   |                   |<-------------------|                    |
   |    7b. EAP        |                    |                    |
   |     Failure       |                    |                    |
   |<------------------|                    |                    |
   ================================================================
   |                   |                    |                    |
   |                   |                    | 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.

6.  References

6.1.  Normative References

   [RFC4282]        Aboba, B., Beadles, M., Arkko, J., and P. Eronen,
                    "The Network Access Identifier", RFC 4282, December
                    2005.

   [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.

   [RFC4234]        Crocker, D. and P. Overell, "Augmented BNF for
                    Syntax Specifications: ABNF", RFC 4234, October
                    2005.

   [RFC2607]        Aboba, B. and J. Vollbrecht, "Proxy Chaining and
                    Policy Implementation in Roaming", RFC 2607, June
                    1999.

6.2.  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", Work in Progress, October 2005.

   [TS-23.234]      3GPP TS 23.234 V6.6.0, "3GPP System to Wireless
                    Local Area Network (WLAN) interworking; System
                    description (Release 6)", September 2005.

   [TS-24.234]      3GPP TS 24.234 V6.4.0, "3GPP System to Wireless
                    Local Area Network (WLAN) interworking; User
                    Equipment (UE) to network protocols; Stage 3
                    (Release 6)", September 2005.

   [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


   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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