Network Access Identifier(NAI) - RFC2486

RFC2486 - 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
@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

9. Full Copyright Statement

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

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