Network Working Group F. Adrangi
Internet-Draft V. Lortz
Expires: February 12, 2006 Intel
F. Bari
Cingular Wireless
P. Eronen
Nokia
August 11, 2005
Identity selection hints for Extensible Authentication Protocol (EAP)
draft-adrangi-eap-network-discovery-14
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The Extensible Authentication Protocol (EAP) is defined in [RFC3748].
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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Relationship with other specifications . . . . . . . . . . 3
1.2 Applicability . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Implementation requirements . . . . . . . . . . . . . . . . . 5
2.1 Packet format . . . . . . . . . . . . . . . . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4. Security considerations . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Appendix - Delivery Options . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1 Normative references . . . . . . . . . . . . . . . . . . . 12
7.2 Informative references . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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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 which Network Access Identity
(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 [rfc2486bis] to facilitate
routing of 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 [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,
including the format for identity hints.
1.1 Relationship with other specifications
This document specifies behavior of 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 EAP-Message attributes. This
document, if used in a scenario requiring NAI "decoration" specified
in [rfc2486bis], 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
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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 [RFC2486bis] 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 point 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 Address Identifier [rfc2486bis].
Decorated NAI An NAI specifying a source route. See [rfc2486bis]
section 2.7 for more information.
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NAI Realm Realm portion of an NAI [rfc2486bis].
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.
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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.
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.
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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. 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 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
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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.
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) |
| | |------------------->|
| | | |
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|<-------------------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.
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 ---------------------->|
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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.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
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.
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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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