Network Working Group F. Adrangi
Internet-Draft V. Lortz
Expires: September 2, 2005 Intel
F. Bari
Cingular Wireless
P. Eronen
Nokia
March 2005
Identity selection hints for Extensible Authentication Protocol (EAP)
draft-adrangi-eap-network-discovery-12
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1 Normative references . . . . . . . . . . . . . . . . . . . 11
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
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limited by the link layer MTU size. For example, assuming an average
of 20 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
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unknown realm after the local AAA proxy/server sends an identity
hint, then the local AAA proxy/server MUST respond with an EAP
Failure packet. The local AAA proxy/server MAY also send an EAP-
Notification message providing the reason for the failure prior to
the EAP Failure packet.
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, Bernard Aboba,
and Glen Zorn 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) | |
| |------------------->| |
| | | |
======================Failure due to unknown realm=============
| | | |
| | 7a.Access-Reject | |
| | (EAP-Failure) | |
| |<-------------------| |
| 7b. EAP | | |
| Failure | | |
|<------------------| | |
================================================================
| | | |
| | | 8. Access-Request |
| | | (EAP |
| | | Response/Identity) |
| | |------------------->|
| | | |
|<-------------------- EAP conversation --------------------->|
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This option does not require changes to existing NASes, so it may be
preferable in many environments.
7. References
7.1 Normative references
[rfc2486bis]
Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier",
draft-ietf-radext-rfc2486bis-05 (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., 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", draft-ietf-eap-netsel-problem-02 (work in
progress), 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,
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"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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