Network Working Group F. Adrangi
Internet-Draft Intel
Expires: June 9, 2005 A. Lior
Bridgewater Systems
J. Korhonen
Teliasonera
J. Loughney
Nokia
December 9, 2004
Chargeable User Identity
draft-ietf-radext-chargeable-user-id-00
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a new RADIUS attribute, Chargeable User
Identity. This attribute can be used by a home network to identity a
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user for the purpose of roaming transactions that occur outside of
the home network.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Chargeable User Identity (CUI) Attribute . . . . . . . . . 5
3. Diameter RADIUS Interoperability . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative references . . . . . . . . . . . . . . . . . . . 8
7.2 Informative references . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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 in order
to enforce usage policies.
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, called the 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 temporal identity
representing the subscriber and at the same time protecting the
anonymity of the subscriber.
1.1 Motivation
Several organizations, including WISPr, GSMA, 3GPP, Wi-Fi Alliance,
IRAP, have been studying mechanisms to provide roaming services,
using RADIUS. A mechanism for providing the current deployments with
the capacity to deploy, bill and oversee WPA networks against fraud.
The CUI attribute has been designed to close operational loopholes in
RADIUS specifications that have impacted roaming solutions
negatively, especially when tunneled protocols with multiple
identities, such as PEAP or TTLS, are used. A chargeable identity
reflecting the user profile authenticated by the home network is
needed in such roaming scenarios.
Existing RADIUS servers that do not understand the CUI attribute
SHOULD silently discard the attribute. Use of the CUI is geared to
multi-identity EAP authentications which are, for the most part,
recent deployments.
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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 Chargeable User
Identity. 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 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 for
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 with
unspecified ordering, which makes it hard to the entities outside
home network to determine which one contains the CUI.
- On use of RADIUS User-Name(1)
The home network could use User-Name(1) in the Access-Accept
message to convey the CUI to intermediaries and the NAS. However,
as the Access-Accept packet is routed to the NAS, the User-Name(1)
attribute could be (completely) rewritten by an intermediary and
therefore the NAS or other intermediaries along the way will not
have access to the CUI. Furthermore, the NAS may use the original
value of the User-Name(1) attribute (the one sent in the
Access-Request packet) in the Accounting-Request packets to ensure
the billing follows the same path as authentication 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). 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. 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 to such that it
can be used to identify the user.
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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], and the Diameter protocol as specified in
[RFC3588].
2.1 Chargeable User Identity (CUI) Attribute
This attribute serves as an alias to the user's identity. It is
assigned by the home RADIUS server and MAY be sent in Access-Accept
message. The NAS or the access network AAA server MUST include this
attribute in the Accounting Requests (Start, Interim, and Stop)
messages if it was included in the Access Accept message and
supported by the NAS. Entities (e.g., NASes, proxies) outside the
home network MUST NOT modify the CUI attribute. Servers which do not
understand the CUI attribute SHOULD silently discard the attribute.
The NAS MAY include the CUI attribute with a null character for its
data field in the Access-Request message to indicate its support for
this attribute to the home RADIUS server. In cases where the home
RADIUS server cannot determine the NAS support for the CUI, if the
home RADIUS server requires the NAS support for CUI for any reason
(e.g., for billing or charging purposes), the home RADIUS server MUST
reject the request by sending an Access-Reject message including an
Error-Cause attribute [RFC3576] with value (to-be-defined) (decimal),
"CUI-Support-Undetermined". Otherwise, if the authentication is
successful, the home RADIUS server MUST send both the User-Name (1)
attribute and the CUI attribute, with the understanding that if the
NAS supports the CUI attribute the CUI attribute will override the
identity portion the User-Name (1) attribute. That is, the
User-Name(1) attribute will be used for routing and the CUI attribute
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will be used for identity purposes.
If the RADIUS server includes this attribute in an Access-Accept
message it MAY also use this attribute as one of the identity
attributes in a Disconnect Message and Change of Authorization
message defined by [RFC3576].
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. It consists two parts separated by a colon, ':'. The
first part determines the CUI type and the second part is the
actual Chargeable User Identity value. The CUI type is coded as
two octet strings representing a hexadecimal number. The CUI
value must be at least one octet. In cases where the attribute is
used to indicate the NAS support for the CUI, the string value
contains a null character.
The following User-Identity types have been defined:
00 - E.164 number
The identifier is in international E.164 format (e.g.
MSISDN, according to the ITU-T E.164 numbering plan as defined
in [E164] and [CE164]).
01 - IMSI
The is in international IMSI format according to the ITU-T
E.212 numbering plan as defined in [E212] and [CE212]).
02 - SIP URI
The identifier is in the form of a SIP URI as defined in
[RFC3261].
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03 - NAI
The identifier is in the form of a Network Access Identifier as
defined in [rfc2486bis].
04 - Opaque string
Opaque string is a value that is assigned to the user by the
home network in an unspecified format, where the home network
asserts that this value represents a particular user.
05 - reserved
The length of time for which the CUI is valid is outside of the
scope of this specification. It is assumed to be deployment
related. It should typically be long enough to serve some
business needs and short enough such that it minimizes the chance
of revealing the true identity of the user (either directly or
indirectly).
Below are examples of CUI strings with NAI and E.164 Charging
Types:
"03:charging-id@realm.org"
"00:+4689761234"
"04:charging-id"
The real user identity SHOULD NOT be revealed through this
attribute. However, the value of this attribute is determined by
the service provider.
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 ID
[Note 1] If the Access-Accept contains CUI then the NAS MUST include
the CUI in Accounting Requests (Start, Interim and Stop) packets.
Change of Authorization and Disconnect-Request
Request ACK NAK # Attribute
0-1 0 0 TBD Chargeable User
[Note 2] Where CUI attribute is included in Disconnect-Request or
CoA-Request messages, it is used for session identification purposes
only. This attribute MUST NOT be used for purposes other than
identification (e.g. within CoA-Request messages to request
authorization changes).
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3. Diameter RADIUS Interoperability
In deployments with both RADIUS and Diameter interworking, a
translation agent will be deployed and operate in accordance to the
NASREQ specification. The Diameter Credit-Control Application's
specifies a similar concept, the Subscription-ID AVP [DiameterCC].
4. IANA Considerations
This document instructs IANA to assign a new RADIUS attribute number
for the CUI attribute.
5. Security considerations
The CUI attribute must be protected against Man-in-the-Middle
attacks. The CUI appears in Access-Accept and Accounting Requests
packets and is protected by the mechanisms that are defined for
RADIUS [RFC2865] and [RFC2866]. Therefore there are no additional
security considerations beyond those already identified in [RFC2865]
and [RFC2866].
Message-Authenticator(80) and Event-Timestamp can be used to further
protect against Man-in-the-middle attacks.
In this document, entities outside the home network are required not
to modify the value of this attribute, however there are no
provisions for protecting against or detecting that a RADIUS Proxy
has modified the attribute.
As the CUI contains an identity that can be used for authorizing and
accounting of services, this attribute must be protected against
snooping.
6. Acknowledgements
The authors would like to thank Jari Arkko, Bernard Aboba, David
Nelson, Blair Bullock, Sami Ala-Luukko, Lothar Reith, David
Mariblanca, Eugene Chang, Greg Weber, and Mark Grayson, for their
feedback and guidance.
7. References
7.1 Normative references
[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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[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[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",
draft-arkko-roamops-rfc2486bis-02 (work in progress), July
2004.
[E164] "The International Public Telecommunication Numbering
Plan", , May 1997.
[CE164] "List of ITU-T Recommendation E.164 assigned country
codes", , June 2000.
[E212] "The international identification plan for mobile
terminals and mobile users", , November 1998.
[CE212] "List of mobile country or geographical area codes", ,
February 1999.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M. and E. Schooler,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
7.2 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.
[DiameterCC]
Hakala, H., Koskinen, j., Stura, M. and J. Loughney, "The
Network Access Identifier",
draft-ietf-aaa-diameter-cc-06.txt (work in progress),
July 2004.
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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
Avi Lior
Bridgewater Systems Corporation
303 Terry Fox Drive
Ottawa, Ontario K2K 3J1
Canada
Phone: +1 613-591-9104
EMail: avi@bridgewatersystems.com
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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