NAI: The Network Access Identifier

Network Working Group                                         B. Aboba
Request for Comments: 2486                                   Microsoft
Category: Standards Track                                   M. Beadles
                                            WorldCom Advanced Networks
                                                          January 1999


                     The Network Access Identifier

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 Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1.  Abstract

   In order to enhance the interoperability of roaming and tunneling
   services, it is desirable to have a standardized method for
   identifying users.  This document proposes syntax for the Network
   Access Identifier (NAI), the userID submitted by the client during
   PPP authentication. It is expected that this will be of interest for
   support of roaming as well as tunneling.  "Roaming capability" 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.

2.  Introduction

   Considerable interest has arisen recently in a set of features that
   fit within the general category of "roaming capability" for dialup
   Internet users.  Interested parties have included:

        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.

        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.

        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 and tunneling
   services, it is desirable to have a standardized method for
   identifying users.  This document proposes syntax for the Network
   Access Identifier (NAI).  Examples of implementations that use the
   NAI, and descriptions of its semantics, can be found in [1].

2.1.  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.

   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).

2.2.  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 [9].

2.3.  Purpose

   As described in [1], there are now 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.

2.4.  Notes for Implementors

   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
   conforms to the BNF described in [5], the BNF of the realm portion
   allows the realm to begin with a digit, which is not permitted by the
   BNF described in [4]. This change was made to reflect current
   practice; although not permitted by the BNF described in [4], FQDNs
   such as 3com.com are commonly used, and accepted by current software.

   Please note that NAS vendors may need to modify their devices so as
   to support the NAI as described in this document. Devices handling
   NAIs MUST support an NAI length of at least 72 octets.

3.  Formal definition of the NAI

   The grammar for the NAI is given below, described in ABNF as
   documented in [7].  The grammar for the username is taken from [5],
   and the grammar for the realm is an updated version of [4].

   nai         = username / ( username "@" realm )
   username    = dot-string

   realm       = realm "." label

   label       = let-dig * (ldh-str)

   ldh-str     = *( Alpha / Digit / "-" ) let-dig

   dot-string  = string / ( dot-string "." string )

   string      = char / ( string char )

   char        = c / ( "\" x )

   let-dig     = Alpha / Digit

   Alpha       = %x41-5A / %x61-7A   ; A-Z / a-z

   Digit       = %x30-39  ;0-9

   c           = < any one of the 128 ASCII characters, but
                  not any special or SP >

   x           = %x00-7F
                 ; all 127 ASCII characters, no exception

   SP          = %x20 ; Space character

   special     = "<" / ">" / "(" / ")" / "[" / "]" / "\" / "."
                  / "," / ";" / ":" / "@" / %x22  / Ctl

   Ctl         = %x00-1F / %x7F
                 ; the control characters (ASCII codes 0 through 31
                 ; inclusive and 127)

   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
        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

4.  References

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

   [2]  Rigney C., Rubens A., Simpson W. and S. Willens, "Remote
        Authentication Dial In User Service (RADIUS)", RFC 2138, April
        1997.

   [3]  Rigney C., "RADIUS Accounting", RFC 2139, April 1997.

   [4]  Mockapetris, P., "Domain Names  -  Implementation  and
        Specification", STD 13, RFC 1035, November 1987.

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

   [6]  Gulbrandsen A. and P. Vixie, "A DNS RR for specifying the
        location of services (DNS SRV)", RFC 2052, October 1996.

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

   [8]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

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

5.  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
   [2] and [3].  In order to prevent snooping of the user name,
   protocols may use confidentiality services provided by IPSEC,
   described in [8].

6.  IANA Considerations

   This document defines a new namespace that will need to be
   administered, namely the NAI realm namespace. In order to to avoid
   creating any new administrative procedures, administration of the NAI
   realm namespace will piggyback 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 [1] 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 [6].  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.

7.  Acknowledgements

   Thanks to Glen Zorn of Microsoft for many useful discussions  of
   this problem space.

8.  Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: 425-936-6605
   EMail: bernarda@microsoft.com


   Mark A. Beadles
   WorldCom Advanced Networks
   5000 Britton Rd.
   Hilliard, OH 43026

   Phone: 614-723-1941
   EMail: mbeadles@wcom.net

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   Copyright (C) The Internet Society (1999).  All Rights Reserved.

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