Network Working Group F. Adrangi
Internet-Draft Intel
Expires: July 5, 2005 A. Lior
Bridgewater Systems
J. Korhonen
Teliasonera
J. Loughney
Nokia
January 2005
Chargeable User Identity
draft-ietf-radext-chargeable-user-id-02
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a new RADIUS attribute,
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Chargeable-User-Identity. This attribute can be used by a home
network to identify a user for the purpose of roaming transactions
that occur outside of the home network.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Chargeable-User-Identity (CUI) Attribute . . . . . . . . . 5
3. Attribute Table . . . . . . . . . . . . . . . . . . . . . . . 7
4. Diameter Consideration . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5.1 CUI RADIUS Attribute . . . . . . . . . . . . . . . . . . . 7
5.2 Error-Cause Attribute . . . . . . . . . . . . . . . . . . 8
6. Security considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1 Normative references . . . . . . . . . . . . . . . . . . . 8
8.2 Informative references . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction
Some authentication methods, including EAP-PEAP, EAP-TTLS, EAP-SIM
and EAP-AKA, can hide the true identity of the user from RADIUS
servers outside of the user's home network. In these methods, the
User-Name(1) attribute contains an anonymous identity (e.g.,
@example.com) sufficient to route the RADIUS packets to the home
network but otherwise insufficient to identify the user. While this
mechanism is good practice in some circumstances, there are problems
if local and intermediate networks require a user identity.
This document introduces an attribute that serves as an alias or
handle (hereafter, it is called Chargeable-User-Identity) to the real
user's identity. Chargeable-User-Identity can be used outside the
home network in scenarios that traditionaly relied on User-Name(1) to
correlate a session to a user.
For example, local or intermediate networks may limit the number of
simultaneous sessions for specific users; they may require a
Chargeable-User-Identity in order to demonstrate willingness to pay
or otherwise limit the potential for fraud.
This implies that an authenticated and unique identity provided by
the home network should be able to be conveyed to all parties
involved in the roaming transaction for correlating the
authentication and accounting packets.
Providing a unique identity, Chargeable-User-Identity (CUI), to
intermediaries, is necessary to fulfill certain business needs. This
should not undermine the anonymity of the user. The mechanism
provided by this draft allows the home operator to meet these
business requirements by providing a temporary identity representing
the subscriber and at the same time protecting the anonymity of the
subscriber.
When the home network assigns a value to the CUI, it asserts that
this value represents a user in the home network. The assertion
should be temporary. Long enough to be useful for the external
applications and not too long such that it can be used to identify
the user.
Several organizations, including WISPr, GSMA, 3GPP, Wi-Fi Alliance,
IRAP, have been studying mechanisms to provide roaming services,
using RADIUS. One missing element is a mechanism for providing the
current deployments with the capacity to deploy, bill and oversee WPA
networks against fraud.
The CUI attribute is intended to close operational loopholes in
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RADIUS specifications that have impacted roaming solutions
negatively, especially when tunneled protocols with multiple
identities, such as PEAP or TTLS, are used. Use of the CUI is geared
to multi-identity EAP authentications which are, for the most part,
recent deployments. A chargeable identity reflecting the user
profile authenticated by the home network is needed in such roaming
scenarios.
1.1 Motivation
Some other mechanisms have been proposed in place of the CUI
attribute. These mechanisms are insufficient or cause other
problems. It has been suggested that standard RADIUS Class(25) or
User-Name(1) attributes could be used to indicate the CUI. However,
in a complex global roaming environment where there could be one or
more intermediaries between the NAS and the home RADIUS server, the
use of aforementioned attributes could lead to problems as described
below.
- On the use of RADIUS Class(25) attribute:
[RFC2865] states: "This Attribute is available to be sent by the
server to the client in an Access-Accept and SHOULD be sent
unmodified by the client to the accounting server as part of the
Accounting-Request packet if accounting is supported. The client
MUST NOT interpret the attribute locally." So RADIUS clients or
intermediaries MUST NOT interpret the Class(25) attribute, which
precludes determining whether it contains a CUI. Additionally,
there could be multiple class attributes in a RADIUS packet, and
since the contents of Class(25) attribute is not to be interpreted
by clients, this makes it hard to the entities outside home
network to determine which one contains the CUI.
- On the use of RADIUS User-Name(1) attribute:
The User-Name(1) attribute included in the Access-Request may be
used for the purpose of routing the Access-Request packet, and in
the process may be rewritten by intermediaries. As a result, a
RADIUS server receiving an Access-Request packet relayed by a
proxy cannot assume that the User-Name(1) attribute remained
unmodified.
On the other hand, rewriting of a User-Name(1) attribute sent
within an Access-Accept packet occurs more rarely, since a
Proxy-State(33) attribute can be used to route the Access-Accept
packet without parsing the User-Name(1) attribute. As a result, a
RADIUS server cannot assume that a proxy stripping routing
information from a User-Name(1) attribute within an Access-Request
will add this information to a User-Name(1) attribute included
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within an Access-Accept. The result is that when a User-Name(1)
attribute is sent in an Access-Accept it is possible that the
Access-Request and Accounting-Request packets will follow
different paths. Where this outcome is undesirable, the RADIUS
client should use the original User-Name(1) in accounting packets.
Therfore, another mechanism is required to convey a CUI within an
Access-Accept packet to the RADIUS client, so that the CUI can be
included in the accounting packets.
The CUI attribute provides a solution to the above problem and avoids
overloading the use of current RADIUS attributes (e.g., User-Name(1)
re-write). The CUI is the correct standards-based approach to fixing
the problems which have arisen with multiple-identity RADIUS
authorization and accounting methods. It does not solve all related
problems, but does provide networks the ability to bill and oversee
WPA networks against fraud.
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].
3GPP - Third Generation Partnership Program
AAA - Authentication, Authorization and Accounting
CUI - Chargeable-User-Identity
GSMA - GSM Association
IRAP - International Roaming Access Protocols Program
NAS - Network Access Server
PEAP - Protected Extensible Authentication Protocol
TTLS - Tunneled Transport Layer Security
WISPr - Wireless ISP Roaming
WPA - Wi-Fi Protected Access
2. Operation
This document assumes that the RADIUS protocol operates as specified
in [RFC2865], [RFC2866], dynamic authorization as specified in
[RFC3576], and the Diameter protocol as specified in [RFC3588].
2.1 Chargeable-User-Identity (CUI) Attribute
The CUI attribute serves as an alias to the user's real identity,
representing a chargeable identity as defined and provided by the
home network as a supplemental or alternative information to
User-Name(1). Typically the CUI represents the identity of the
actual user but it may also indicate other chargeable identities such
as a group of users. RADIUS clients (proxy or NAS) outside the home
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network MUST NOT modify the CUI attribute.
The RADIUS server (a RADIUS proxy, home RADIUS server) may include
the CUI attribute in the Access-Accept packet destined to a roaming
partner. The CUI support by RADIUS infrastructure is driven by the
business requirements between roaming entities. Therefore whether a
RADIUS server/proxy or client accepts or rejects the presence or lack
of presence of the CUI attribute is a matter of business policy.
If an Access-Accept packet without the CUI attribute was received by
a RADIUS client (NAS or Proxy) that requires the presence of the CUI
attribute, then the Access-Accept packet MAY be treated as an
Access-Reject packet based on local policies.
If the CUI was included in the Access-Accept packet, RADIUS client
(Proxy or NAS) that supports the CUI attribute MUST ensure that the
CUI attribute appears in the RADIUS Accounting-Request (Start,
Interim, and Stop).
RFC 2865 includes the following statements about behaviors of RADIUS
client and server with respect to unsupported attributes:
- "A RADIUS client MAY ignore Attributes with an unknown Type."
- "A RADIUS server MAY ignore Attributes with an unknown Type."
Therefore, RADIUS client or server that does not support the CUI
attribute MAY ignore this attribute.
If RADIUS client (Proxy or NAS) requires the presence of the CUI
attribute in the Access-Accept, it MUST indicate its requirement by
including the CUI attribute in the Access-Request packet with a value
set to the nul character (hereafter, it is also referred to as a nul
CUI).
If a home RADIUS server that supports the CUI attribute receives an
Access-Request containing a CUI (set to nul or otherwise), it MUST
include the CUI attribute in the Access-Accept. Otherwise, if the
Access-Request does not contain a CUI, the home RADIUS server MUST
NOT include the CUI attribute in the Access-Accept.
A RADIUS server (a RADIUS proxy or the home RADIUS server) that
requires the presence of the CUI in the Accounting-Request packets
(Start, Stop, Interims) MAY respond with an Access-Reject packet if
it receives an Access-Request messsage from a RADIUS client, that
does not support the CUI attribute. The Access-Reject packet MUST
include Error-Cause attribute [RFC3576] with value (to-be-defined)
(decimal), "CUI-Support-Required".
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A summary of the RADIUS CUI Attribute is given below.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBD for Chargeable-User-Identity.
Length: >= 3
String:
The string identifies the CUI of the end-user and is of type
UTF8String. This string value is a reference to a particular
user. The format and the interpretation of the string value , and
the binding lifetime of the reference to the user is determined
based on business agreements. For example, the lifetime can be
set to one billing period. In cases where the attribute is used
to indicate the NAS support for the CUI, the string value contains
a nul character.
3. Attribute Table
The following table provides a guide to which attribute(s) may be
found in which kinds of packets, and in what quantity.
Request Accept Reject Challenge Accounting # Attribute
Request
0-1 0-1 0 0 0-1 TBD Chargeable-User-identity
[Note 1] If the Access-Accept contains CUI then the NAS MUST include
the CUI in Accounting Requests (Start, Interim and Stop) packets.
4. Diameter Consideration
Diameter needs to define an identical attribute with the same Type
value. The CUI should be available as part of the NASREQ
application.
5. IANA Considerations
5.1 CUI RADIUS Attribute
This document uses the RADIUS [RFC2865] namespace, see
"http://www.iana.org/assignments/radius-types". This document
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instructs IANA to assign a new RADIUS attribute number for the CUI
attribute.
CUI TBA
5.2 Error-Cause Attribute
This document instructs IANA to assign a new value for Error-Cause
attribute [RFC3576],
"CUI-Support-Required" TBA
6. Security considerations
It is strongly recommended that the CUI format used is such that the
real user identity is not revealed. Furthermore, where a reference
is used to a real user identity, the binding lifetime of that
reference to the real user be kept as short as possible.
The RADIUS entities (RADIUS proxies and clients)outside the home
netowrk MUST NOT modify the CUI. However, there is no way to detect
or prevent this.
If the NAS includes CUI in an Access-Request. A man in the middle
may remove the CUI attribute from the Access-Request. The result is
that the Access-Accept will not have a CUI which will cause the NAS
to reject the session resulting in a DOS attack. To prevent this
attack, the NAS SHOULD include Message-Authenticator(80) in the
Access-Request packets that contain a CUI.
7. Acknowledgements
The authors would like to thank Jari Arkko, Bernard Aboba, David
Nelson, Barney Wolff, Blair Bullock, Sami Ala-Luukko, Lothar Reith,
David Mariblanca, Eugene Chang, Greg Weber, and Mark Grayson, for
their feedback and guidance.
8. References
8.1 Normative references
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[rfc2486bis]
Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The
Network Access Identifier",
Internet-Draft draft-arkko-roamops-rfc2486bis-02, July
2004.
8.2 Informative references
[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 3576,
July 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
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
Avi Lior
Bridgewater Systems Corporation
303 Terry Fox Drive
Ottawa, Ontario K2K 3J1
Canada
Phone: +1 613-591-9104
Email: avi@bridgewatersystems.com
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Jouni Korhonen
Teliasonera Corporation
P.O.Box 970
FIN-00051, Sonera
Finland
Phone: +358405344455
Email: jouni.korhonen@teliasonera.com
John Loughney
Nokia
Itamerenkatu 11-13
FIN-00180, Helsinki
Finland
Phone: +358504836342
Email: john.loughney@nokia.com
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