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Network Working Group B. Aboba Request for Comments: 2477 G. Zorn Category: Informational Microsoft Corporation

                                                         January 1999
             Criteria for Evaluating Roaming Protocols

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

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

1. Abstract

 This document describes requirements for the provisioning of "roaming
 capability" for dialup Internet users.  "Roaming capability" is
 defined as the ability to use multiple Internet service providers
 (ISPs), while maintaining a formal, customer-vendor relationship with
 only one.

2. Introduction

 Operational roaming services are currently providing worldwide
 roaming capabilities, and these services continue to grow in
 popularity [1].  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 services over a
    wider area.
    National ISPs wishing to combine their operations with those of
    one or more ISPs in another nation to provide greater coverage 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 can
    include Internet access as well as secure access to corporate
    intranets via a Virtual Private Network (VPN).

Aboba & Zorn Informational [Page 1] RFC 2477 Evaluating Roaming Protocols January 1999

 This document provides an architectural framework for the
 provisioning of roaming capabilities, as well as describing the
 requirements that must be met by elements of the architecture.

2.1. 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 [4].
 Please note that the requirements specified in this document are to
 be used in evaluating protocol submissions.  As such, the
 requirements language refers to capabilities of these protocols; the
 protocol documents will specify whether these features are required,
 recommended, or optional for use in roaming.  For example, requiring
 that a protocol support confidentiality is NOT the same thing as
 requiring that all protocol traffic be encrypted.
 A protocol submission is not compliant if it fails to satisfy one or
 more of the must or must not requirements for the capabilities that
 it implements.  A protocol submission that satisfies all the must,
 must not, should and should not requirements for its capabilities is
 said to be "unconditionally compliant"; one that satisfies all the
 must and must not requirements but not all the should or should not
 requirements for its protocols is said to be "conditionally

2.2. Terminology

 This document frequently uses the following terms:
 phone book
    This is a database or document containing data pertaining to
    dialup access, including phone numbers and any associated
 phone book server
    This is a server that maintains the latest version of the phone
    book.  Clients communicate with phone book servers in order to
    keep their phone books up to date.
 Network Access Server
    The Network Access Server (NAS) is the device that clients dial in
    order to get access to the network.
 Authentication server
    This is a server which provides for authentication/authorization
    within the roaming architecture.

Aboba & Zorn Informational [Page 2] RFC 2477 Evaluating Roaming Protocols January 1999

 Accounting server
    This is a server which provides for accounting within the roaming
 Authentication proxy
    Authentication proxies may be deployed within the roaming
    architecture for several purposes, including authentication
    forwarding, policy implementation, shared secret management, and
    attribute editing.  To the NAS, the authentication proxy appears
    to act as an authentication server; to the authentication server,
    the proxy appears to act as an authentication client.
 Accounting proxy
    Accounting proxies may be deployed within the roaming architecture
    for several purposes, including accounting forwarding, reliability
    improvement, auditing, and "pseudo-transactional" capability.  To
    the NAS, the accounting proxy appears to act as an accounting
    server; to the accounting server, the proxy appears to act as an
    accounting client.
 Network Access Identifier
    In order to provide for the routing of authentication and
    accounting packets, user name MAY contain structure.  This
    structure provides a means by which the authentication or
    accounting proxies will locate the authentication or accounting
    server that is to receive the request.

3. Architectural framework

 The roaming architecture consists of three major subsystems:
    Phone book Subsystem
    Authentication Subsystem
    Accounting Subsystem
 The phone book subsystem is concerned with the maintenance and
 updating of the user phone book.  The phone book provides the user
 with information on the location and phone numbers of Points of
 Presence (POPs) that are roaming enabled.  The function of the
 authentication subsystem is to provide authorized users with access
 to the POPs in the phonebook, and to deny access to unauthorized
 users.  The goal of the accounting subsystem is to provide
 information on the resources utilized during the user's session.

3.1. Phone Book Subsystem

 The phone book subsystem provides for the following:

Aboba & Zorn Informational [Page 3] RFC 2477 Evaluating Roaming Protocols January 1999

    Phone number presentation
    Phone number exchange
    Phone book compilation
    Phone book update
 Phone number presentation
    Phone number presentation involves the display of available phone
    numbers to the user, and culminates in the choosing of a number.
    Since the user interface and sequence of events involved in phone
    number presentation is a function of the connection management
    software being used, it is likely that individual vendors will
    take different approaches to the problem.  These differences can
    include variances in the format of the client phone books, varying
    approaches to presentation, etc.  There is no inherent problem
    with this. As a result, phone number presentation need not be
 Phone number exchange
    Phone number exchange involves propagation of phone number changes
    between providers in a roaming association.  Current roaming
    implementations do not provide for complete automation of the
    phone number exchange process [1].  As a result, phone number
    exchange need not be standardized at this time.
 Phone book compilation
    Once an ISP's phone book server has received its updates it needs
    to compile a new phone book and propagate this phone book to all
    the phone book servers operated by that ISP.  Given that the
    compilation process does not affect protocol interoperability, it
    need not be standardized.
 Phone book update
    Once the phone book is compiled, it needs to be propagated to
    users.  Standardization of the phone book update process allows
    for providers to update user phone books, independent of their
    client software or operating system.

3.2. Authentication Subsystem

 The authentication subsystem provides for the following:
    Connection management
    NAS Configuration/Authorization
    Address Assignment/Routing

Aboba & Zorn Informational [Page 4] RFC 2477 Evaluating Roaming Protocols January 1999

 Connection management
    In order to be able to use the POPs of the local provider, it is
    first necessary to bring up a connection.
    Authentication consists of two parts: the claim of identity (or
    identification) and the proof of the claim (or verification).  As
    part of the authentication process, users identify themselves to
    the Network Access Server (NAS) in a manner that allows the
    authentication request to be routed its home destination.
    Authentication is typically required prior to allowing access to
    the network.  CHAP [8] and PAP [9] are the two authentication
    protocols most commonly used within the PPP [10] framework today.
    Some groups of users are requiring different forms of proof of
    identity (e.g., token or smart cards, Kerberos credentials, etc.)
    for special purposes (such as acquiring access to corporate
    intranets).  The Extensible Authentication Protocol (EAP) [7] was
    created in order to provide a general mechanism for support of
    these methods.
 NAS configuration/authorization
    In order to set up the session, authorization parameters need to
    be sent to from the home authentication server to the local ISP's
 Address assignment/routing
    If it is desired that the user be able to communicate with the
    rest of the Internet, then the session will be assigned a routable
    IP address by the NAS.
    In the process of authenticating and authorizing the user session,
    it may be desirable to provide protection against a variety of
    security threats.

3.3. Accounting Subsystem

 The function of the accounting subsystem is to enable the
 participants in the roaming consortium to keep track of what
 resources are used during a session. Relevant information includes
 how long the user was connected to the service, connection speed,
 port type, etc.

Aboba & Zorn Informational [Page 5] RFC 2477 Evaluating Roaming Protocols January 1999

4. Roaming Requirements

4.1. Phonebook requirements

4.1.1. Phone book update protocol

 The update protocol MUST allow for updating of clients on a range of
 platforms and operating systems. Therefore the update mechanism MUST
 NOT impose any operating system-specific requirements.
 The client MUST be able to determine the authenticity of the server
 sending the phone book update.  The server MAY also be able to
 authenticate the client.
 The update protocol MUST provide for updating of the phone book from
 an arbitrary previous version to the latest available version.
 Integrity Checking
 The client MUST be able to determine the integrity of the received
 update before applying it, and MUST be able to determine the
 integrity of the newly produced phone book after updating it.
 Light weight transfers
 Since the client may be a low-end machine or internet appliance, the
 update protocol MUST be lightweight.
 Language support
 The phone book update mechanism MUST support the ability to request
 that the phone book be transmitted in a particular language and
 character set.  For example, if the customer has a Russian language
 software package, then the propagation and update protocols MUST
 provide a mechanism for the user to request a Russian language phone

4.1.2. Phone book format

 Phone number attributes
 The phone book format MUST support phone number attributes commonly
 used by Internet service providers.  These attributes are required in
 order to provide users with information on the capabilities of the
 available phone numbers.
 Provider attributes
 In addition to providing information relating to a given phone
 number, the phone book MUST provide information on the individual

Aboba & Zorn Informational [Page 6] RFC 2477 Evaluating Roaming Protocols January 1999

 roaming consortium members.  These attributes are required in order
 to provide users with information about the individual providers in
 the roaming consortium.
 Service attributes
 In addition to providing information relating to a given phone
 number, and service provider, the phone book MUST provide information
 relevant to configuration of the service.  These attributes are
 necessary to provide the client with information relating to the
 operation of the service.
 Since it will frequently be necessary to add phone book attributes,
 the phone book format MUST support the addition of phone number,
 provider and service attributes without modification to the update
 protocol.  Registration of new phone book attributes will be handled
 by IANA.  The attribute space MUST be sufficiently large to
 accomodate growth.
 Since phone book will typically be frequently updated, the phone book
 format MUST be compact so as to minimize the bandwidth used in
 updating it.

4.2. Authentication requirements

4.2.1. Connection Management

 Given the current popularity and near ubiquity of PPP, a roaming
 standard MUST provide support for PPP and IP. A roaming standard MAY
 provide support for other framing protocols such as SLIP.  However,
 SLIP support is expected to prove difficult since SLIP does not
 support negotiation of connection parameters and lacks support for
 protocols other than IP.
 A roaming standard MAY provide support for non-IP protocols (e.g.,
 IPX or AppleTalk) since these may be useful for the provision of
 corporate intranet access via the Internet.  Since it is intended
 that the client will begin PPP negotiation immediately on connection,
 support for scripting SHOULD NOT be part of a roaming standard.

4.2.2. Identification

 A roaming standard MUST provide a standardized format for the userID
 and realm presented to the NAS.

Aboba & Zorn Informational [Page 7] RFC 2477 Evaluating Roaming Protocols January 1999

4.2.3. Verification of Identity

 Authentication types
    A roaming standard MUST support CHAP, and SHOULD support EAP.  Due
    to security concerns, PAP authentication SHOULD NOT be supported.
    A possible exception is where PAP is used to support a one time
    password or token.
    A roaming standard, once available, is likely to be widely
    deployed on the Internet.  A roaming standard MUST therefore
    provide sufficient scalability to allow for the formation of
    roaming associations with thousands of ISP members.
 RADIUS Support
    Given the current popularity and near ubiquity of RADIUS [2,3] as
    an authentication, authorization and accounting solution, a
    roaming standard MUST be able to incorporate RADIUS-enabled
    devices within the roaming architecture. It is expected that this
    will be accomplished by development of gateways between RADIUS and
    the roaming standard authentication, authorization, and accounting

4.2.4. NAS Configuration/Authorization

 In order to ensure compatibility with the NAS or the local network,
 authentication/authorization proxies often will add, delete, or
 modify attributes returned by the home authentication server. In
 addition, an authentication proxy will often carry out resource
 management and policy functions.  As a result, a roaming standard
 MUST support the ability of proxies to perform attribute editing and
 implement policy.

4.2.5. Address assignment/routing

 A roaming standard MUST support dynamic address assignment.  Static
 address assignment MAY be supported, most likely via layer 2 or layer
 3 tunneling.
 Layer 2 tunneling protocols
    Layer-2 tunneling protocols, such as PPTP, L2F, or L2TP, hold
    great promise for the implementation of Virtual Private Networks
    as a means for inexpensive access to remote networks.  Therefore
    proxy implementations MUST NOT preclude use of layer 2 tunneling.
 Layer 3 tunneling protocols
    Layer-3 tunneling protocols as embodied in Mobile IP [5], hold
    great promise for providing "live", transparent mobility on the

Aboba & Zorn Informational [Page 8] RFC 2477 Evaluating Roaming Protocols January 1999

    part of mobile nodes on the Internet.  Therefore, a roaming
    standard MUST NOT preclude the provisioning of Mobile IP Foreign
    Agents or other Mobile IP functionality on the part of service

4.2.6. Security

 Security analysis
    A roaming standard MUST include a thorough security analysis,
    including a description of security threats and countermeasures.
    This includes specification of mechanisms for fraud prevention and
 Hop by hop security
    A roaming standard MUST provide for hop-by-hop integrity
    protection and confidentiality.  This MAY be accomplished through
    support of network layer security (IPSEC) [6].
 End-to-end security
    As policy implementation and attribute editing are common in
    roaming systems, proxies may need to modify packets in transit
    between a local NAS and the home server. In order to permit
    authorized modifications while at the same time guarding against
    attacks by rogue proxies, it is necessary for a roaming standard
    to support data object security.  As a result, a roaming standard
    MUST provide end-to-end confidentiality and integrity protection
    on an attribute-by-attribute basis.  However, non-repudiation is
    NOT a requirement for a roaming standard.

4.3. Accounting requirements

 Real-time accounting
    In today's roaming implementations, real-time accounting is a
    practical necessity in order to support fraud detection and risk
    management.  As a result, a roaming standard MUST provide support
    for real-time accounting.
 Accounting record formats
    Today there is no proposed standard for NAS accounting, and there
    is wide variation in the protocols used by providers to
    communicate accounting information within their own organizations.
    Therefore, a roaming standard MUST prescribe a standardized format
    for accounting records.  For the sake of efficiency, the record
    format MUST be compact.
    A standard accounting record format MUST be able to encode metrics
    commonly used to determine the user's bill.  Since these metrics

Aboba & Zorn Informational [Page 9] RFC 2477 Evaluating Roaming Protocols January 1999

    change over time, the accounting record format MUST be extensible
    so as to be able to add future metrics as they come along.  The
    record format MUST support both standard metrics as well as
    vendor-specific metrics.

5. 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
 [3] Rigney, C., "RADIUS Accounting", RFC 2139, April 1997.
 [4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.
 [5] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
 [6] Kent, S. and R. Atkinson, "Security Architecture for the Internet
     Protocol", RFC 2401, November 1998.
 [7] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
     Protocol (EAP)", RFC 2284, March 1998.
 [8] Simpson, W., "PPP Challenge Handshake Authentication Protocol
     (CHAP)", RFC 1994, August 1996.
 [9] Lloyd, B. and Simpson, W., "PPP Authentication Protocols", RFC
     1334, October 1992.
 [10] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
      1661, July 1994.

6. Security Considerations

 This document, being a requirements document, does not have any
 security concerns.  The security requirements on protocols to be
 evaluated using this document are mainly described in section 5.2.

7. Acknowledgements

 Thanks to Pat Calhoun (, Butch Anton
 ( and John Vollbrecht ( for many useful
 discussions of this problem space.

Aboba & Zorn Informational [Page 10] RFC 2477 Evaluating Roaming Protocols January 1999

8. Authors' Addresses

 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: 425-936-6605
 Glen Zorn
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: 425-703-1559

Aboba & Zorn Informational [Page 11] RFC 2477 Evaluating Roaming Protocols January 1999

9. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Aboba & Zorn Informational [Page 12]

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