Network Working Group                                           B. Aboba
Internet-Draft                                                 Microsoft
Expires: March 31, April 19, 2005                                       M. Beadles
                                                                J. Arkko
                                                               P. Eronen
                                                      September 30,
                                                        October 19, 2004

                     The Network Access Identifier

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   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 network authentication.  "Roaming" may
   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 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 . . . . . . . . . . . . .  7
         2.5   Compatibility with E-Mail Usernames  . . . . . . . . .  7
         2.6   Compatibility with DNS . . . . . . . . . . . . . . . .  7
         2.7   Realm Construction . . . . . . . . . . . . . . . . . .  8
         2.8   Examples . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       5.1   Normative References . . . . . . . . . . . . . . . . . . 12
       5.2   Informative References . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
   A.  Changes from RFC 2486  . . . . . . . . . . . . . . . . . . . . 14
   B.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
       Intellectual Property and Copyright Statements . . . . . . . . 16

1.  Introduction

   Considerable interest exists for a set of features that fit within
   the general category of "roaming capability" for network access,
   including dialup Internet users, VPN usage, wireless LAN
   authentication, and other applications.  Interested parties have
   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

   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 [7].

   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 user identity submitted
      by the client during network access 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 user identity submitted in an application layer
   Network Access Server

      The Network Access Server (NAS) is the device that clients connect
      to 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).  In IEEE 802.11, it is referred to as an
      Access Point.
   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 [2].

1.3  Purpose

   As described in [7], there are a number of providers offering network
   access services, 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 network 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.

2.  NAI Definition

2.1  Formal Syntax

   The grammar for the NAI is given below, described in ABNF as
   documented in [3].  The grammar for the username is based on [6], and
   the grammar for the realm is an updated version of [1].

   nai         =  username
   nai         =/ "@" realm
   nai         =/ username "@" realm

   username    =  dot-string
   dot-string  =  string
   dot-string  =/ dot-string "." string
   string      =  char
   string      =/ string char
   char        =  c
   char        =/ "\" x

   c           =  %x21    ; '!'              allowed
                          ; '"'              not allowed
   c           =/ %x23    ; '#'              allowed
   c           =/ %x24    ; '$'              allowed
   c           =/ %x25    ; '%'              allowed
   c           =/ %x26    ; '&'              allowed
   c           =/ %x27    ; '''              allowed
                          ; '(', ')'         not allowed
   c           =/ %x2a    ; '*'              allowed
   c           =/ %x2b    ; '+'              allowed
                          ; ','              not allowed
   c           =/ %x2d    ; '-'              allowed
                          ; '.'              not allowed
   c           =/ %x2f    ; '/'              allowed
   c           =/ %x30-39 ; '0'-'9'          allowed
                          ; ';', ':', '<'    not allowed
   c           =/ %x3d    ; '='              allowed
                          ; '>'              not allowed
   c           =/ %x3f    ; '?'              allowed
                          ; '@'              not allowed
   c           =/ %x41-5a ; 'A'-'Z'          allowed
                          ; '[', '\', ']'    not allowed
   c           =/ %x5e    ; '^'              allowed
   c           =/ %x5f    ; '_'              allowed
   c           =/ %x60    ; '`'              allowed
   c           =/ %x61-7a ; 'a'-'z'          allowed
   c           =/ %x7b    ; '{'              allowed
   c           =/ %x7c    ; '|'              allowed
   c           =/ %x7d    ; '}'              allowed
   c           =/ %x7e    ; '~'              allowed
                          ; DEL              not allowed
   c           =/ %x80-ff ; UTF-8            allowed (not in RFC 2486)
               ; c must also satisfy rules in Section 2.4
   x           =  %x00-7F  %x00-FF ; all 128 ASCII characters, no exception exception;
                          ; as well as all UTF-8 characters (this
                          ; was not allowed in RFC 2486)

   realm       =  1*( label "." ) label
   label       =  let-dig * (ldh-str)
   ldh-str     =  *( alpha / digit / "-" ) let-dig
   let-dig     =  alpha / digit
   alpha       =  %x41-5A  ; 'A'-'Z'
   alpha       =/ %x61-7A  ; 'a'-'z'
   digit       =  %x30-39  ; '0'-'9'

2.2  NAI Length Considerations

   Devices handling NAIs MUST support an NAI length of at least 72
   octets.  Support for an NAI length of 253 octets is RECOMMENDED.
   However, the following implementation issues should be considered:
   o  NAIs are often transported in the User-Name attribute of RADIUS.
      Unfortunately, RFC 2865 [9] Section 5.1 states that "the ability
      to handle at least 63 octets is recommended." As a result, it may
      not be possible to transfer NAIs beyond 63 octets through all
      devices.  In addition, since only a single User-Name attribute may
      be included in a RADIUS message and the maximum attribute length
      is 253 octets, RADIUS is unable to support NAI lengths beyond 253
   o  NAIs can also be transported in the User-Name attribute of
      Diameter [12], which supports content lengths up to 2^24 - 9
      octets.  As a result, NAIs processed only by Diameter nodes can be
      very long.  Unfortunately, an 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 a part of the
   authoritative domain (in the sense of Section 4) 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 together 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, the realm
   portion is typically required in order for the authentication
   exchange to be routed to the appropriate server.

2.4  International Character Sets

   This specification allows both international usernames and realms.
   International usernames are based on the use of Unicode characters,
   encoded as UTF-8 and processed with a certain algorithm to ensure a
   canonical representation.  The realm part internationalization is
   based on International Domain Name (IDN) [4].

   In order to ensure a canonical representation, characters of the
   username portion in an NAI MUST fulfill the requirements specified in
   [5].  In addition, the use of certain special characters (see grammar
   rule c) are prohibited as well in order to retain compatibility with
   the previous version of this RFC.

   The realm name is an "IDN-unaware domain name slot" as defined in
   [4].  That is, it can contain only ASCII characters.  An
   implementation MAY support internationalized domain names (IDNs)
   using the ToASCII operation; see [4] 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 [6], 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 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

   The use of the home realm MUST be the default unless otherwise

   Where these conditions are fulfilled, an NAI such as

   MAY be represented as in!

   In this case, the part before the (non-escaped) '!' MUST be a realm
   name as defined in the ABNF in Section 2.1.  When receiving such an
   NAI, the other realm MUST convert the format back to
   "" when passing the NAI forward, as well as
   applying appropriate AAA routing for the transaction.

   The conversion process may apply also recursively.  That is, after
   the conversion the result may still have one or more '!' characters
   in the username.  For instance, the NAI!!

   would first be converted in other1.example net to!

   and then at finally to

2.8  Examples

   Examples of valid Network Access Identifiers include:


   The last example uses an IDN converted into an ASCII representation.

   Examples of invalid Network Access Identifiers include:


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
   [9] and [10].  In order to prevent snooping of the user name,
   protocols may use confidentiality services provided by protocols
   transporting them, such RADIUS protected by IPsec [11] or Diameter
   protected by TLS [12].

   This specification adds the possibility of hiding the username part
   in the NAI, by omitting it.  As discussed in Section 2.3, this is
   possible only when NAIs are used together with a separate
   authentication method that can transfer the username in a secure
   manner.  In some cases application-specific privacy mechanism have
   also been used with NAIs.  For instance, some EAP methods apply a
   method-specific pseudonyms in the username part of the NAI.  While
   neither of these approaches can protect the realm part, their
   advantage over transport protection is that privacy of the username
   is protected even through intermediate nodes such as NASes.

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 require use
   of the DNS for location of the authentication server.  While Diameter
   [12] supports the use of DNS for location of authentication servers,
   existing RADIUS implementations typically use proxy configuration
   files in order to locate authentication servers within a domain and
   perform authentication routing.  The implementations described in [7]
   did not use DNS for location of the authentication server within a
   domain.  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 that the
   NAI represent a valid email address.

5.  References

5.1  Normative References

   [1]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [4]  Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
        Domain Names in Applications (IDNA)", RFC 3490, March 2003.

   [5]  Zeilenga, K., "SASLprep: Stringprep profile for user names and
        passwords", draft-ietf-sasl-saslprep-04 (work in progress),
        October 2003.

5.2  Informative References

   [6]   Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
         August 1982.

   [7]   Aboba, B., Lu, J., Alsop, J., Ding, J. and W. Wang, "Review of
         Roaming Implementations", RFC 2194, September 1997.

   [8]   Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
         2486, January 1999.

   [9]   Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
         Authentication Dial In User Service (RADIUS)", RFC 2865, June

   [10]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [11]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
         In User Service) Support For Extensible Authentication Protocol
         (EAP)", RFC 3579, September 2003.

   [12]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko,
         "Diameter Base Protocol", RFC 3588, September 2003.

   [13]  Arkko, J. and B. Aboba, "Network Discovery and Selection within
         the EAP Framework", draft-ietf-eap-netsel-problem-00 (work in
         progress), January 2004.

Authors' Addresses

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052


   Mark A. Beadles
   565 Metro Place South Suite 300
   Dublin  OH 43017


   Jari Arkko
   Jorvas  02420


   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group


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.  Note that this implies character codes 128
      - 255 may be used in the username portion, which may be
      unacceptable to nodes that only support RFC 2486.  Many devices
      already allow this behaviour, however.
   o  Username privacy support has been added.  Note that NAIs without a
      username (for privacy) may not be acceptable to RFC 2486 compliant
      nodes.  Many devices already allow this behaviour, however.
   o  A recommendation 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.  Note
      that long NAIs may not be acceptable to RFC 2486 compliant nodes.
   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 [13].  Rather, it is intended as a
      clarification of RFC 2486.

      However, as discussed in Section 2.7, this specification requires
      that this syntax be applied only when there is explicit knowledge
      that the peer system supports such syntax.
   o  The realm BNF entry definition has been changed to avoid an error
      (infinite recursion) in the original specification.
   o  Several clarifications and improvements have been incorporated to
      the ABNF specification for NAIs.

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.  Paul Hoffman helped with the international
   character set issues.  Kalle Tammela, Stefaan De Cnodder, Nagi
   Jonnala, Bert Wijnen, Blair Bullock, Yoshihiro Ohba, Ignacio Goyret,
   and Richard Perlman provided many useful comments on this draft.  The
   ABNF validator at was used to
   verify the syntactic correctness of the ABNF in Section 2.1.

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