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-09
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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). This is especially useful when the access network
does not have a direct roaming relationship with the peer's home
network, so that a mediating network, such as a roaming consortium or
broker, is used.
The mechanism defined in this document is primarily intended for
advertising connectivity of access network to a limited number of
roaming partners that find such advertisement of their presence
useful.
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 A (informative) - 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. As a result, the peer
may be unclear about the appropriate 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 identity selection hints, including information about its
roaming relationships, to an EAP peer. 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 in
selecting what kind of NAI decoration [rfc2486bis] must be applied to
allow proper routing of AAA messages to the home AAA server. If
there are several possible mediating networks, the peer can choose
which one to use.
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 another
identity it has, decide to switch to another access network, or
attempt to reformat its NAI [rfc2486bis] to assist in proper AAA
routing.
This document is also related to the general network discovery and
selection problem. See [netsel-problem] for more detailed discussion
about this problem space.
Section 2 describes the required behavior of implementations of this
specification, as well as the packet format for structuring and
presenting identity hint information to an EAP peer. The appendix A
describes the delivery options that can be implemented by an access
network to deliver identity hint information to an EAP peer.
1.1 Applicability
Basic roaming and AAA routing mechanisms are normally sufficient, and
the identity hints are typically useful only when there's too much
ambiquity to try all client identities and access network
combinations efficiently, or when the scale of the roaming
associations precludes full automatic connectivity from all access
networks to all home networks. 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.
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In the situations mentioned above, a limited number of identity hints
can be provided by the mechanism described in this document. Even in
this case, for security reasons it is required that the networks that
are listed in these hints consent to such advertisements.
The immediate application of the proposed mechanism is in 3GPP
systems inteworking 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
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.
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 AAA proxy/server implementing this
specification receives an EAP-Response/Identity with an unknown
realm, it SHOULD reply with an EAP-Request/Identity containing an
identity hint.
If after the local AAA proxy/server sends an EAP-Request/Identity
containing an identity hint, the peer responds with an
EAP-Response/Identity containing an unknown realm, then the local AAA
proxy/server MAY respond immediately with an EAP Failure packet, or
it MAY first send an EAP-Notification providing the reason for the
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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,
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 that 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.
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
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67 70 70 6e 65 74 77 6f
72 6b 2e 6f 72 67
The Network-Info can contain NAIRealms list in addition to
proprietary information. The proprietary information can be placed
before or after NAIRealms list. To extract NAIRealms list, an
implementation either finds the "NAIRealms=" immediately after the
NUL or seeks forward to find ",NAIRealms" somewhere in the string.
The realms data ends either at 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, and therefore 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, 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.
Where the identity hint is used to select a mediating network, with
existing EAP methods there may not be 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 network that uses a poor
authentication method can make it easier for attackers to discover
that such network can be accessed. As a result, the consent of the
network being advertised in the hints is required before such hints
can be sent.
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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 A (informative) - Delivery Options
Although the delivery options are described in the context of IEEE
802.11 access networks, they are applicable to other access networks
that use EAP [RFC3748] for authentication and use the NAI format
[rfc2486bis] for identifying users. Also, the options assume that
the AAA protocol in use is RADIUS [RFC2865]. 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-arkko-roamops-rfc2486bis-02, 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-01,
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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