Network Working Group F. Adrangi
Request for Comments: 4372 Intel
Category: Standards Track A. Lior
Chargeable User Identity
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
This document describes a new Remote Authentication Dial-In User
Service (RADIUS) attribute, 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 ....................................................2
1.1. Motivation .................................................3
1.2. Terminology ................................................4
2. Operation .......................................................5
2.1. Chargeable-User-Identity (CUI) Attribute ...................5
2.2. CUI Attribute ..............................................6
3. Attribute Table .................................................7
4. Diameter Consideration ..........................................7
5. IANA Considerations .............................................7
6. Security Considerations .........................................7
7. Acknowledgements ................................................8
8. References ......................................................8
8.1. Normative References .......................................8
8.2. Informative References .....................................8
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 surrogate identity to
bind the current session.
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 traditionally 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 a 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
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 document allows the home operator to meet these
business requirements by providing a temporary identity representing
the user and at the same time protecting the anonymity of the user.
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
Several organizations, including WISPr, GSMA, 3GPP, Wi-Fi Alliance,
and IRAP, have been studying mechanisms to provide roaming services,
using RADIUS. Missing elements include mechanisms for billing and
The CUI attribute is intended to close operational loopholes in
RADIUS specifications that have impacted roaming solutions
negatively. Use of the CUI is geared toward EAP methods supporting
privacy (such as PEAP and EAP-TTLS), which are, for the most part,
recent deployments. A chargeable identity reflecting the user
profile by the home network is needed in such roaming scenarios.
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 [RFC4282] 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 packet 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 for the entities outside the 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 packet
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
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
packet will add this information to a User-Name(1) attribute
included within an Access-Accept packet. The result is that when
a User-Name(1) attribute is sent in an Access-Accept packet, it is
possible that the Access-Request packet 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. Therefore, 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
The CUI attribute provides a solution to the above problems and
avoids overloading RADIUS User-Name(1) attribute or changing the
usage of existing RADIUS Class(25) attribute. The CUI therefore
provides a standard approach to billing and fraud prevention when EAP
methods supporting privacy are used. It does not solve all related
problems, but does provide for billing and fraud prevention.
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].
The following acronyms are used:
3GPP - Third Generation Partnership Project
AAA - Authentication, Authorization, and Accounting
AKA - Authentication and Key Agreement
CUI - Chargeable-User-Identity
GSMA - GSM Association
IRAP - International Roaming Access Protocols Program
NAS - Network Access Server
PEAP - Protected Extensible Authentication Protocol
SIM - Subscriber Identity Modules
TTLS - Tunneled Transport Layer Security
WISPr - Wireless ISP Roaming
WPA - Wi-Fi Protected Access
This document assumes that the RADIUS protocol operates as specified
in [RFC2865] and [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 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, a RADIUS
server supporting this specification may choose not to send the CUI
in response to an Access-Request packet from a given NAS, even if the
NAS has indicated that it supports CUI.
If an Access-Accept packet without the CUI attribute was received by
a RADIUS client that requested the CUI attribute, then the
Access-Accept packet MAY be treated as an Access-Reject.
If the CUI was included in an Access-Accept packet, RADIUS clients
supporting the CUI attribute MUST ensure that the CUI attribute
appears in the RADIUS Accounting-Request (Start, Interim, and Stop).
This requirement applies regardless of whether the RADIUS client
requested the CUI attribute.
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 clients or servers that do not support the CUI may
ignore the attribute.
A RADIUS client requesting the CUI attribute in an Access-Accept
packet MUST include within the Access-Request packet a CUI attribute.
For the initial authentication, the CUI attribute will include a
single NUL character (referred to as a nul CUI). And, during
re-authentication, the CUI attribute will include a previously
received CUI value (referred to as a non-nul CUI value) in the
Upon receiving a non-nul CUI value in an Access-Request, the home
RADIUS server MAY verify that the value of CUI matches the CUI from
the previous Access-Accept. If the verification fails, then the
RADIUS server SHOULD respond with an Access-Reject message.
If a home RADIUS server that supports the CUI attribute receives an
Access-Request packet containing a CUI (set to nul or otherwise), it
MUST include the CUI attribute in the Access-Accept packet.
Otherwise, if the Access-Request packet does not contain a CUI, the
home RADIUS server SHOULD NOT include the CUI attribute in the
Access-Accept packet. The Access-Request may be sent either in the
initial authentication or during re-authentication.
A NAS that requested the CUI during re-authentication by including
the CUI in the Access-Request will receive the CUI in the
Access-Accept. The NAS MUST include the value of that CUI in all
2.2. CUI Attribute
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: 89 for Chargeable-User-Identity.
Length: >= 3
The string identifies the CUI of the end-user. This string value
is a reference to a particular user. The format and content of
the string value are determined by the Home RADIUS server. 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. RADIUS entities other than the Home RADIUS
server MUST treat the CUI content as an opaque token, and SHOULD
NOT perform operations on its content other than a binary equality
comparison test, between two instances of CUI. 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
0-1 0-1 0 0 0-1 89 Chargeable-User-Identity
Note: If the Access-Accept packet contains CUI, then the NAS MUST
include the CUI in Accounting Requests (Start, Interim, and Stop)
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
This document uses the RADIUS [RFC2865] namespace; see
http://www.iana.org/assignments/radius-types. The IANA has assigned
a new RADIUS attribute number for the CUI attribute.
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, it is recommended that the binding
lifetime of that reference to the real user be kept as short as
The RADIUS entities (RADIUS proxies and clients) outside the home
network MUST NOT modify the CUI or insert a CUI in an Access-Accept.
However, there is no way to detect or prevent this.
Attempting theft of service, a man-in-the-middle may try to insert,
modify, or remove the CUI in the Access-Accept packets and Accounting
packets. However, RADIUS Access-Accept and Accounting packets
already provide integrity protection.
If the NAS includes CUI in an Access-Request packet, a
man-in-the-middle may remove it. This will cause the Access-Accept
packet to not include a CUI attribute, which may cause the NAS to
reject the session. To prevent such a denial of service (DoS)
attack, the NAS SHOULD include a Message-Authenticator(80) attribute
within Access-Request packets containing a CUI attribute.
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.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC4005] Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
"Diameter Network Access Server Application", RFC 4005,
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
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,
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
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