Extensible Authentication Protocol B. Aboba
Internet-Draft Microsoft
Expires: August 9, 2004 M. Beadles
WorldCom Advanced Networks
J. Arkko
Ericsson
P. Eronen
Nokia
February 9, 2004
The Network Access Identifier
draft-arkko-roamops-rfc2486bis-00
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
In order to provide roaming services, it is necessary to have a
standardized method for identifying users. This document defines the
syntax for the Network Access Identifier (NAI), the user identity
submitted by the client during, for instance, PPP and wireless LAN
authentication. "Roaming" may be loosely defined as the ability to
use any one of multiple Internet service providers (ISPs), while
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maintaining a formal, customer-vendor relationship with only one.
Examples of where roaming capabilities might be required include ISP
"confederations" and ISP-provided corporate network access support.
This document is a revised version of RFC 2486 which originally
defined NAIs. Enhancements include international character set and
privacy support, as well as a number of corrections to the original
RFC.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Requirements language . . . . . . . . . . . . . . . . . . 4
1.3 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. NAI Definition . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 5
2.2 NAI Length Considerations . . . . . . . . . . . . . . . . 6
2.3 Support for Username Privacy . . . . . . . . . . . . . . . 6
2.4 International Character Sets . . . . . . . . . . . . . . . 6
2.5 Compatibility with E-Mail Usernames . . . . . . . . . . . 7
2.6 Compatibility with DNS . . . . . . . . . . . . . . . . . . 7
2.7 Realm Construction . . . . . . . . . . . . . . . . . . . . 7
2.8 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 11
Informative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
A. Changes from RFC 2486 . . . . . . . . . . . . . . . . . . . . 14
B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 16
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1. Introduction
Considerable interest exists for a set of features that fit within
the general category of "roaming capability" for dialup Internet
users, wireless LAN authentication, and other applications.
Interested parties have included:
o Regional Internet Service Providers (ISPs) operating within a
particular state or province, looking to combine their efforts
with those of other regional providers to offer dialup service
over a wider area.
o National ISPs wishing to combine their operations with those of
one or more ISPs in another nation to offer more comprehensive
dialup service in a group of countries or on a continent.
o Wireless LAN hotspots providing service to one or more ISPs.
o Businesses desiring to offer their employees a comprehensive
package of dialup services on a global basis. Those services may
include Internet access as well as secure access to corporate
intranets via a Virtual Private Network (VPN), enabled by
tunneling protocols such as PPTP, L2F, L2TP, and IPSEC tunnel
mode.
In order to enhance the interoperability of roaming services, it is
necessary to have a standardized method for identifying users. This
document defines syntax for the Network Access Identifier (NAI).
Examples of implementations that use the NAI, and descriptions of its
semantics, can be found in [8].
This document is a revised version of RFC 2486 which originally
defined NAIs. Differences and enhancements compared to RFC 2486 are
listed in Appendix A.
1.1 Terminology
This document frequently uses the following terms:
Network Access Identifier
The Network Access Identifier (NAI) is the userID submitted by the
client during PPP authentication. In roaming, the purpose of the
NAI is to identify the user as well as to assist in the routing of
the authentication request. Please note that the NAI may not
necessarily be the same as the user's e-mail address or the userID
submitted in an application layer authentication.
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Network Access Server
The Network Access Server (NAS) is the device that clients dial in
order to get access to the network. In PPTP terminology this is
referred to as the PPTP Access Concentrator (PAC), and in L2TP
terminology, it is referred to as the L2TP Access Concentrator
(LAC).
Roaming Capability
Roaming capability can be loosely defined as the ability to use
any one of multiple Internet service providers (ISPs), while
maintaining a formal, customer-vendor relationship with only one.
Examples of cases where roaming capability might be required
include ISP "confederations" and ISP- provided corporate network
access support.
Tunneling Service
A tunneling service is any network service enabled by tunneling
protocols such as PPTP, L2F, L2TP, and IPSEC tunnel mode. One
example of a tunneling service is secure access to corporate
intranets via a Virtual Private Network (VPN).
1.2 Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [3].
1.3 Purpose
As described in [8], there are a number of services implementing
dialup roaming, and the number of Internet Service Providers involved
in roaming consortia is increasing rapidly.
In order to be able to offer roaming capability, one of the
requirements is to be able to identify the user's home authentication
server. For use in roaming, this function is accomplished via the
Network Access Identifier (NAI) submitted by the user to the NAS in
the initial PPP authentication. It is also expected that NASes will
use the NAI as part of the process of opening a new tunnel, in order
to determine the tunnel endpoint.
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2. NAI Definition
2.1 Formal Syntax
The grammar for the NAI is given below, described in ABNF as
documented in [4]. The grammar for the username is based on [7], and
the grammar for the realm is an updated version of [1].
nai = username / ( username "@" realm ) / ( "@" realm" )
username = dot-istring
realm = [ realm "." ] ilabel
ilabel = let-dig * (ldh-str)
ldh-str = *( Alpha / Digit / "-" ) let-dig
dot-istring = istring / ( dot-istring "." istring )
istring = ichar / ( string ichar )
ichar = c / ( "\" x )
let-dig = Alpha / Digit
Alpha = %x41-5A / %x61-7A ; A-Z / a-z
Digit = %x30-39 ;0-9
c = < a character as specified in
Section 2.4
>
x = %x00-7F
; all 128 ASCII characters, no exception
SP = %x20 ; Space character
special = "<" / ">" / "(" / ")" / "[" / "]" / "\" / "."
/ "," / ";" / ":" / "@" / %x22 / Ctl
; %x22 is '"'
Ctl = %x00-1F / %x7F
; the control characters (ASCII codes 0 through 31
; inclusive and 127)
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2.2 NAI Length Considerations
Devices handling NAIs MUST support an NAI length of at least 253
octets. However, the following interoperability considerations
should be noted:
o RFC 2486 required the support of NAIs only up to the length of 72
octets. As a result, it can generally not be assumed that all
devices can support 253 octets.
o NAIs are often transported in the User-Name attribute of RADIUS
[10]. Unfortunately, RADIUS requires devices to support content
lengths of only 63 octets for this attribute. As a result, it may
not be possible to transfer NAIs beyond 63 octets through all
devices. In addition, due to its message structure, RADIUS is
unable to support content lengths beyond 253 octets
o NAIs can also be transported in the User-Name attribute of
Diameter [13], which supports content lengths up to 2^24 - 9
octets. As a result, NAIs processed only by Diameter nodes can be
very long. Unfortunately, a NAI transported over Diameter may
eventually be translated to RADIUS, in which case the above
limitations apply.
2.3 Support for Username Privacy
Interpretation of the "username" part of the NAI depends on the realm
in question. Therefore, the "username" part SHOULD be treated as
opaque data when processed by nodes that are not authoritative (in
some sense) for that realm.
Where privacy is a concern, NAIs MAY be provided in an abbreviated
form by omitting the username portion. This is possible only when
NAIs are used in connection with a separate authentication method
that can transfer the username in a secure manner.
For roaming purposes it is typically necessary to locate the
appropriate backend authentication server for the given NAI before
the authentication conversation can proceed. As a result, realm
portion is typically required in order for the authentication
exchange to be routed to the appropriate server.
2.4 International Character Sets
Characters of the username portion in a NAI MUST fulfill the
requirements specified in [6]. In addition, the use of the SP
character is prohibited as well in order to retain compatibility with
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the previous version of this RFC.
The realm name is an "IDN-unaware domain name slot" as defined in
[5]. That is, it can contain only ASCII characters. An
implementation MAY support internationalized domain names (IDNs)
using the ToASCII operation; see [5] for more information.
2.5 Compatibility with E-Mail Usernames
As proposed in this document, the Network Access Identifier is of the
form user@realm. Please note that while the user portion of the NAI
is based on the BNF described in [7], it has been extended for
internationalization support as well as for purposes of Section 2.7,
and is not necessarily compatible with the usernames used in e-mail.
Note also that the internationalization requirements for NAIs and
e-mail addresses are different, since the former need to be typed in
only by the user himself and his own operator, not by others.
2.6 Compatibility with DNS
The BNF of the realm portion allows the realm to begin with a digit,
which is not permitted by the BNF described in [1]. This change was
made to reflect current practice; although not permitted by the BNF
described in [1], FQDNs such as 3com.com are commonly used, and
accepted by current software.
2.7 Realm Construction
NAIs are used, among other purposes, for routing AAA transactions to
the user's home realm. Usually, the home realm appears in the realm
portion of the NAI, but in some cases a different realm can be used.
This may be useful, for instance, when the home realm is only
reachable via another mediating realm.
Such usage may prevent interoperability unless the parties involved
have a mutual agreement that the usage is allowed. In particular,
NAIs MUST NOT use a different realm than the home realm unless the
sender has explicit knowledge that (a) the specified other realm is
available and (b) the other realm supports such usage. The sender
may determine the fulfillment of these conditions through a database,
dynamic discovery, or other means not specified here. Note that the
first condition is affected by roaming, as the availability of the
other realm may depend on the user's location or the desired
application. The use of the home realm MUST be the default unless
otherwise configured.
Where these conditions are fulfilled, a NAI "user@homerealm" MAY be
represented as "homerealm!user@otherrealm". When receiving such NAI,
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the other realm MUST convert the format back to "user@homerealm" when
passing the NAI onwards, as well as apply necessary AAA routing for
the transaction.
2.8 Examples
Examples of valid Network Access Identifiers include:
fred@3com.com
fred@foo-9.com
fred_smith@big-co.com
fred=?#$&*+-/^smith@bigco.com
fred@bigco.com
nancy@eng.bigu.edu
eng!nancy@bigu.edu
Examples of invalid Network Access Identifiers include:
fred@foo
fred@foo_9.com
@howard.edu
fred@bigco.com@smallco.com
eng:nancy@bigu.edu
eng;nancy@bigu.edu
<nancy>@bigu.edu
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3. Security Considerations
Since an NAI reveals the home affiliation of a user, it may assist an
attacker in further probing the username space. Typically this
problem is of most concern in protocols which transmit the user name
in clear-text across the Internet, such as in RADIUS, described in
[10] and [11]. In order to prevent snooping of the user name,
protocols may use confidentiality services provided by protocols
transporting them, such RADIUS protected by IPsec [12] or Diameter
protected by TLS [13].
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4. IANA Considerations
In order to to avoid creating any new administrative procedures,
administration of the NAI realm namespace piggybacks on the
administration of the DNS namespace.
NAI realm names are required to be unique and the rights to use a
given NAI realm for roaming purposes are obtained coincident with
acquiring the rights to use a particular fully qualified domain name
(FQDN). Those wishing to use an NAI realm name should first acquire
the rights to use the corresponding FQDN. Using an NAI realm without
ownership of the corresponding FQDN creates the possibility of
conflict and therefore is to be discouraged.
Note that the use of an FQDN as the realm name does not imply use of
the DNS for location of the authentication server or for
authentication routing. Since to date roaming has been implemented
on a relatively small scale, existing implementations typically
handle location of authentication servers within a domain and perform
authentication routing based on local knowledge expressed in proxy
configuration files. The implementations described in [8] have not
found a need for use of DNS for location of the authentication server
within a domain, although this can be accomplished via use of the DNS
SRV record, described in [2]. Similarly, existing implementations
have not found a need for dynamic routing protocols, or propagation
of global routing information. Note also that there is no
requirement that the NAI represent a valid email address.
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Normative References
[1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[2] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[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.
[5] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[6] Zeilenga, K., "SASLprep: Stringprep profile for user names and
passwords", draft-ietf-sasl-saslprep-04 (work in progress),
October 2003.
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Informative References
[7] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
August 1982.
[8] Aboba, B., Lu, J., Alsop, J., Ding, J. and W. Wang, "Review of
Roaming Implementations", RFC 2194, September 1997.
[9] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[10] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[11] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[12] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
In User Service) Support For Extensible Authentication Protocol
(EAP)", RFC 3579, September 2003.
[13] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
"Diameter Base Protocol", RFC 3588, September 2003.
[14] Arkko, J. and B. Aboba, "Network Discovery and Selection within
the EAP Framework", draft-ietf-eap-netsel-problem-00 (work in
progress), January 2004.
[15] Adrangi, F., "Network Discovery and Selection within the EAP
Framework",
draft-adrangi-eap-network-discovery-and-selection-00 (work in
progress), October 2003.
Authors' Addresses
Bernard Aboba
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
EMail: aboba@internaut.com
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Mark A. Beadles
WorldCom Advanced Networks
5000 Britton Rd.
Hilliard, OH 43026
USA
EMail: mbeadles@wcom.net
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
Pasi Eronen
Nokia Research Center
P.O. Box 407
FIN-00045 Nokia Group
Finland
EMail: pasi.eronen@nokia.com
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Appendix A. Changes from RFC 2486
This draft contains the following updates with respect to the
original NAI definition in RFC 2486:
o International character set support has been added for both
usernames and realms.
o Username privacy support has been added.
o A requirement to support NAI length of at least 253 octets has
been added, and compatibility considerations among NAI lengths in
this specification and various AAA protocols are discussed.
o The mediating network syntax and its implications have been fully
described and not given only as an example. Note that this syntax
is not intended to be a full solution to network discovery and
selection needs as defined in [14]. Rather, it is intended as a
clarification of RFC 2486. It could also be used as a component
in approaches such as [15].
o The realm BNF entry definition has been changed to avoid an error
(infinite recursion) in the original specification.
o The x and special BNF entries have been clarified.
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Appendix B. Acknowledgements
Thanks to Glen Zorn for many useful discussions of this problem
space, and for Farid Adrangi and others for suggesting mediating
network representation in NAIs. Jonathan Rosenberg reported the BNF
error. Dale Worley suggested clarifications of the x and special BNF
entries. Arne Norefors reported the length differences between RFC
2486 and RFC 2865.
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