Network Working Group F. Adrangi

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.

Adrangi, et al. Informational [Page 1]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 2]

RFC 4284 Identity Selection Hints for EAP January 2006

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

Adrangi, et al. Informational [Page 3]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 4]

RFC 4284 Identity Selection Hints for EAP January 2006

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].

Adrangi, et al. Informational [Page 5]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 6]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 7]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 8]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 9]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 10]

RFC 4284 Identity Selection Hints for EAP January 2006

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

Adrangi, et al. Informational [Page 11]

RFC 4284 Identity Selection Hints for EAP January 2006

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.

Adrangi, et al. Informational [Page 12]

RFC 4284 Identity Selection Hints for EAP January 2006

[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: [email protected]

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

Phone: +1 503-264-3253
EMail: [email protected]

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

Phone: +1 425-580-5526
EMail: [email protected]

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

EMail: [email protected]

Adrangi, et al. Informational [Page 13]

RFC 4284 Identity Selection Hints for EAP January 2006

Full Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].

Acknowledgement

Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).

Adrangi, et al. Informational [Page 14]