Network Working Group F. Adrangi
Internet-Draft V. Lortz
Expires: February 12, 2005 Intel
F. Bari
AT&T Wireless
P. Eronen
Nokia
M. Watson
Nortel
August 14, 2004
Identity selection hints for Extensible Authentication Protocol (EAP)
draft-adrangi-eap-network-discovery-03
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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 solution is especially useful when the
access network does not have a direct roaming relationship with the
client's home network, so a mediating network, such as a roaming
consortium or broker, is used.
1. Introduction
In many roaming situations, an access network can have several
roaming relationship, either with several home networks, or mediating
networks such as roaming consortiums and brokers, or both. An EAP
peer (hereafter, also referred to as peer) can also have several sets
of credentials, and its home network may have roaming relationships
with several mediating networks.
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.
One possible application for this mechanism is to help in selecting
what kind of NAI decoration [1] 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 [6] for more detailed discussion about this
problem space.
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.
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
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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.
Section 2 describes 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 [3].
2. Packet format
Identity hint information is placed after the displayable string and
a NUL character in the EAP Identity Request. The following ABNF [4]
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 [4] and the "realm" rule is defined in
[1].
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
67 70 70 6e 65 74 77 6f
72 6b 2e 6f 72 67
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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.
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
that does not override any local policies at the peer.
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
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 [2] for authentication and use the NAI format [1] for
identifying users. Also, the options assume that the AAA protocol in
use is RADIUS [8]. Diameter [7] could also be used instead of RADIUS
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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 is EAP aware. It 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.
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
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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 access network RADIUS 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 [5] within a
RADIUS Access-Request. The access network RADIUS server can then
send the EAP Identity/Request 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 | |
| | 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.
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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 server. If
the RADIUS 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.
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 --------------------->|
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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
[1] Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The Network
Access Identifier", draft-arkko-roamops-rfc2486bis-02 (work in
progress), July 2004.
[2] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748,
June 2004.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
7.2 Informative references
[5] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial In
User Service) Support For Extensible Authentication Protocol
(EAP)", RFC 3579, September 2003.
[6] Arkko, J. and B. Aboba, "Network Discovery and Selection
Problem", draft-ietf-eap-netsel-problem-01 (work in progress),
July 2004.
[7] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[8] 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
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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