Network Working Group F. Adrangi
Internet-Draft V. Lortz
Expires: April 22, 2005 Intel
F. Bari
AT&T Wireless
P. Eronen
Nokia
M. Watson
Nortel
October 22, 2004
Identity selection hints for Extensible Authentication Protocol (EAP)
draft-adrangi-eap-network-discovery-05
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines a mechanism that allows an access network to
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provide identity selection hints to an EAP client. The purpose is to
help the client in selecting the most appropriate identity and NAI
decoration to use. This is especially useful when the access network
does not have a direct roaming relationship with the client's home
network, so that a mediating network, such as a roaming consortium or
broker, is used.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Implementation requirements . . . . . . . . . . . . . . . . . 3
2.1 Packet format . . . . . . . . . . . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security considerations . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
6. Appendix (informative) - Delivery Options . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Normative references . . . . . . . . . . . . . . . . . . . . 9
7.2 Informative references . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . 12
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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-Identity/Response.
This document defines a mechanism that allows the access network to
provide identity selection hints, and more specifically information
about its roaming relationships, to an EAP peer. This information is
sent to the peer in an EAP Identity/Request message by appending it
after the displayable message and a NUL character.
Exactly how the identity hint information is used by the peer depends
largely on the peer's local policy and configuration, and is outside
the scope of this document.
In many roaming situations, an access network can have several
roaming relationships, either with several home networks, or
mediating networks such as roaming consortiums and brokers, or both.
One possible application for this mechanism is to help 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. However, exactly how the selection is made is beyond the scope
of this document. 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 in
section 6 describes the delivery options that can be implemented by
an access network to deliver identity hint information to an EAP
peer.
1.1 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].
2. Implementation requirements
An EAP peer implementing this specification MUST be able to receive
an identity hint in an initial EAP Identity/Request, or in a
subsequent EAP Identity/Request.
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The EAP authenticator MAY send an identity hint to the peer in the
initial EAP Identity/Request. If the identity hint is not sent
initially (such as when the authenticator does not support this
specification), then if the EAP server receives an EAP
Identity/Response with an unacceptable NAI Realm, EAP servers
implementing this specification SHOULD reply with an EAP
Identity/Request containing an identity hint.
If after the EAP server sends an EAP Identity/Request containing an
identity hint, the peer responds with an EAP Identity/Response
containing an unacceptable NAI Realm, then the EAP server MAY respond
immediately with an EAP Failure packet, or it MAY first send an
EAP-Notification providing information on the reason for the failure.
EAP does not support fragmentation for Identity/Request messages, so
the size of identity hint information is limited by the link MTU.
The exact limit depends on the lower layer in question, but it is at
least 1020 octets.
2.1 Packet format
The Identity hint information is placed after the displayable string
and a NUL character in the EAP Identity Request. The following ABNF
[RFC2234] defines a "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 "NAIRealms=" realm-list ]
displayable-string = *OCTET
realm-list = realm /
( realm-list ";" realm )
The "OCTET" rule is defined in [RFC2234] and the "realm" rule is
defined in [rfc2486bis].
A sample hex dump of an EAP Identity Request 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
Some existing systems are known to use EAP Identity/Request messages
to send proprietary information to the peer. This proprietary
information is considered to be part of the displayable-string in the
ABNF shown above. In other words, the NUL character followed by the
NAIRealms list MUST be placed at the end.
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 Identity Request
before the user authenticates to the network, and before the network
is authenticated to the user. This information can be modified by an
attacker. Therefore, it MUST be considered an unauthenticated hint.
Unauthenticated hints may result in peers inadvertently revealing
other or additional identities than they intended to, leading to a
privacy vulnerability. Note that in EAP, the identity the peer wants
to use is in general carried in a cleartext message, so this is only
a variation of an existing vulnerability. Method-specific identity
protection is one of the ways that this vulnerability can be
addressed.
Similarly, in a situation where the peer has multiple identities to
choose from, an unauthenticated hint can lead to a situation where an
attacker convinces the peer to choose an identifier that is bound to
the weakest EAP method. To guard against this vulnerability, the use
of as strong EAP methods as possible is recommended. Note that this
vulnerability is similar to an existing vulnerability where link
layers advertise network names (such as 802.11 SSIDs) without
authenticating these advertisements either at all or only at the end
of the authentication process.
In case the identity hint information is used to select a mediating
network for NAI decoration, it should be noted that at least with
some EAP methods, there is no way for the home network AAA server to
verify that the mediating network used was actually the same one that
the peer had requested.
5. Acknowledgements
The authors would specially like to thank Jari Arkko and Bernard
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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, and Avi Lior for their support,
feedback and guidance during the various stages of this work.
6. Appendix (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 Identity/ Request. 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).
When an Identity hint is sent by a RADIUS proxy/server, a RADIUS
State (24) attribute can be used to help the RADIUS proxy/server
determine if an identity hint had previously been sent by it to the
EAP peer.
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 Identity/Request from access point
In typical IEEE 802.11 wireless LANs, the initial EAP
Identity/Request 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 | | |
| Identity/Request | | |
| (NAIRealms) | | |
|<------------------| | |
| 2. EAP | | |
| Identity/Response | | |
|------------------>| | |
| | 3. Access-Request | |
| | (EAP | |
| | Identity/Response) | |
| |------------------->| |
| | | 4.Access-Request |
| | | (EAP |
| | | Identity/Response) |
| | |------------------->|
| | | |
|<-------------------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 Identity/Request from local RADIUS proxy/server
This is similar to Option 1, but the initial EAP Identity Request 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 Identity/Request 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 | |
| | Identity/Request | |
| | with NAIRealms) | |
| |<-------------------| |
| 3. EAP | | |
| Identity/Request | | |
| (NAIRealms) | | |
|<------------------| | |
| 4. EAP | | |
| Identity/Response | | |
|------------------>| | |
| | 5. Access-Request | |
| | (EAP | |
| | Identity/Response) | |
| |------------------->| |
| | | 6. Access-Request |
| | | (EAP |
| | | Identity Response) |
| | |------------------->|
| | | |
|<------------------- EAP conversation ---------------------->|
This option can work with current access points if they support the
EAP-Start message.
Option 3: Subsequent EAP-Identity/Request from local RADIUS
proxy/server
In the third option, the access point sends the initial EAP
Identity/Request without any hint information. The peer then
responds with an Identity Response, 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 Identity Request containing the identity hint information.
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EAP Access Point local RADIUS home RADIUS
Peer Proxy/Server server
| | | |
| 1. EAP | | |
| Identity Request | | |
| (w/o NAIRealms) | | |
|<------------------| | |
| 2. EAP | | |
| Identity Response | | |
|------------------>| | |
| | 3. Access-Request | |
| | (EAP | |
| | Identity Response) | |
| |------------------->| |
| | 4.Access-Challenge | |
| | (EAP | |
| | Identity Request | |
| | with NAIRealms) | |
| |<-------------------| |
| 5. EAP | | |
| Identity Request | | |
| (NAIRealms) | | |
|<------------------| | |
| 6. EAP | | |
| Identity Response | | |
|------------------>| | |
| | 7. Access-Request | |
| | (EAP | |
| | Identity Response) | |
| |------------------->| |
| | | 8. Access-Request |
| | | (EAP |
| | | Identity Response) |
| | |------------------->|
| | | |
|<-------------------- 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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draft-arkko-roamops-rfc2486bis-02 (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. 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-01 (work in
progress), July 2004.
[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
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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
AT&T Wireless
7277 164th Avenue N.E.
Redmond, WA 98052
USA
Phone: +1 425-580-5526
EMail: farooq.bari@attws.com
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
Mark Watson
Nortel Networks
2221 Lakeside Blvd
Richardson, TX 75082
USA
EMail: mwatson@nortel.com
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