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rfc:rfc2881

Network Working Group D. Mitton Request for Comments: 2881 Nortel Networks Category: Informational M. Beadles

                                                         SmartPipes Inc.
                                                               July 2000
   Network Access Server Requirements Next Generation (NASREQNG)
                             NAS Model

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 (2000).  All Rights Reserved.

Abstract

 This document describes the terminology and gives a model of typical
 Network Access Server (NAS).  The purpose of this effort is to set
 the reference space for describing and evaluating NAS service
 protocols, such as RADIUS (RFCs 2865, 2866) [1], [2] and follow-on
 efforts like AAA Working Group, and the Diameter protocol [3].  These
 are protocols for carrying user service information for
 authentication, authorization, accounting, and auditing, between a
 Network Access Server which desires to authenticate its incoming
 calls and a shared authentication server.

Table of Contents

 1. INTRODUCTION...................................................2
  1.1 Scope of this Document ......................................2
  1.2 Specific Terminology ........................................3
 2. NETWORK ACCESS SYSTEM EQUIPMENT ASSUMPTIONS....................3
 3. NAS SERVICES...................................................4
 4. AUTHENTICATION, AUTHORIZATION AND ACCOUNTING (AAA) SERVERS.....5
 5. TYPICAL NAS OPERATION SEQUENCE:................................5
  5.1 Characteristics of Systems and Sessions: ....................6
  5.2 Separation of NAS and AAA server functions ..................7
  5.3 Network Management and Administrative features ..............7
 6. AUTHENTICATION METHODS.........................................8
 7. SESSION AUTHORIZATION INFORMATION..............................8
 8. IP NETWORK INTERACTION.........................................9
 9. A NAS MODEL...................................................10

Mitton & Beadles Informational [Page 1] RFC 2881 NASreq NAS Model July 2000

  9.1 A Reference Model of a NAS .................................10
  9.2 Terminology ................................................11
  9.3 Analysis ...................................................13
   9.3.1 Authentication and Security .............................13
   9.3.2 Authorization and Policy ................................14
   9.3.3 Accounting and Auditing .................................14
   9.3.4 Resource Management .....................................14
   9.3.5 Virtual Private Networks (VPN's) ........................14
   9.3.6 Service Quality .........................................15
   9.3.7 Roaming .................................................15
 10. SECURITY CONSIDERATIONS......................................15
 11. REFERENCES ..................................................16
 12. ACKNOWLEDGMENTS..............................................17
 13. AUTHORS' ADDRESSES ..........................................17
 14. APPENDIX - ACRONYMS AND GLOSSARY:............................18
 15. FULL COPYRIGHT STATEMENT.....................................20

1. Introduction

 A Network Access Server is the initial entry point to a network for
 the majority of users of network services.  It is the first device in
 the network to provide services to an end user, and acts as a gateway
 for all further services.  As such, its importance to users and
 service providers alike is paramount.  However, the concept of a
 Network Access Server has grown up over the years without being
 formally defined or analyzed [4].

1.1 Scope of this Document

 There are several tradeoffs taken in this document.  The purpose of
 this document is to describe a model for evaluating NAS service
 protocols.  It will give examples of typical NAS hardware and
 software features, but these are not to be taken as hard limitations
 of the model, but merely illustrative of the points of discussion.
 An important goal of the model is to offer a framework that allows
 further development and expansion of capabilities in NAS
 implementation.
 As with most IETF projects, the focus is on standardizing the
 protocol interaction between the components of the system.  The
 documents produced will not address the following areas:
  1. AAA server back-end implementation is abstracted and not

prescribed. The actual organization of the data in the server, its

   internal interfaces, and capabilities are left to the
   implementation.

Mitton & Beadles Informational [Page 2] RFC 2881 NASreq NAS Model July 2000

  1. NAS front-end call technology is not assumed to be static.

Alternate and new technology will be accommodated. The resultant

   protocol specifications must be flexible in design to allow for new
   technologies and services to be added with minimal impact on
   existing implementations.

1.2 Specific Terminology

 The following terms are used in this document in this manner:  A
 "Call" - the initiation of a network service request to the NAS.
 This can mean the arrival of a telephone call via a dial-in or
 switched telephone network connection, or the creation of a tunnel to
 a tunnel server which becomes a virtual NAS.  A "Session" - is the
 NAS provided service to a specific authorized user entity.

2. Network Access System Equipment Assumptions

 A typical hardware-based NAS is implemented in a constrained system.
 It is important that the NAS protocols don't assume unlimited
 resources on the part of the platform.  The following are typical
 constraints:
  1. A computer system of minimal to moderate performance

(example processors: Intel 386 or 486, Motorola 68000)

  1. A moderate amount, but not large RAM (typically varies with

supported # of ports 1MB to 8MB)

  1. Some small amount of non-volatile memory, and/or way to be

configured out-of-band

  1. No assumption of a local file system or disk storage
 A NAS system may consist of a system of interconnected specialized
 processor system units.  Typically they may be circuit boards (or
 blades) that are arrayed in a card cage (or chassis) and referred to
 by their position (i.e., slot number).  The bus interconnection
 methods are typically proprietary and will not be addressed here.
 A NAS is sometimes referred to as a Remote Access Server (RAS) as it
 typically allows remote access to a network.  However, a more general
 picture is that of an "Edge Server", where the NAS sits on the edge
 of an IP network of some type, and allows dynamic access to it.
 Such systems typically have;
  1. At least one LAN or high performance network interface (e.g.,

Ethernet, ATM, FR)

Mitton & Beadles Informational [Page 3] RFC 2881 NASreq NAS Model July 2000

  1. At least one, but typically many, serial interface ports, which

could be;

  1. serial RS232 ports direct wired or wired to a modem, or
  2. have integral hardware or software modems (V.22bis,V.32, V.34,

X2, Kflex, V.90, etc.)

  1. have direct connections to telephone network digital WAN lines

(ISDN, T1, T3, NFAS, or SS7)

  1. an aggregation of xDSL connections or PPPoe sessions [5].
 However, systems may perform some of the functions of a NAS, but not
 have these kinds of hardware characteristics.  An example would be a
 industry personal computer server system, that has several modem line
 connections.  These lines will be managed like a dedicated NAS, but
 the system itself is a general file server.  Likewise, with the
 development of tunneling protocols (L2F [6], ATMP [7], L2TP [8]),
 tunnel server systems must behave like a "virtual" NAS, where the
 calls come from the network tunneled sessions and not hardware ports
 ([11], [9], [10]).

3. NAS Services

 The core of what a NAS provides, are dynamic network services.  What
 distinguishes a NAS from a typical routing system, is that these
 services are provided on a per-user basis, based on an authentication
 and the service is accounted for.  This accounting may lead to
 policies and controls to limit appropriate usage to levels based on
 the availability of network bandwidth, or service agreements between
 the user and the provider.
 Typical services include:
  1. dial-up or direct access serial line access; Ability to access the

network using a the public telephone network.

  1. network access (SLIP, PPP, IPX, NETBEUI, ARAP); The NAS allows the

caller to access the network directly.

  1. asynchronous terminal services (Telnet, Rlogin, LAT, others); The

NAS implements the network protocol on behalf of the caller, and

   presents a terminal interface.
 - dial-out connections; Ability to cause the NAS to initiate a
   connection over the public telephone network, typically based on the
   arrival of traffic to a specific network system.
 - callback (NAS generates call to caller); Ability to cause the NAS to
   reverse or initiate a network connection based on the arrival of a
   dial-in call.
 - tunneling (from access connection to remote server); The NAS
   transports the callers network packets over a network to a remote
   server using an encapsulation protocol. (L2TP [8], RADIUS support
   [11])

Mitton & Beadles Informational [Page 4] RFC 2881 NASreq NAS Model July 2000

4. Authentication, Authorization and Accounting (AAA) Servers

 Because of the need to authenticate and account, and for practical
 reasons of implementation, NAS systems have come to depend on
 external server systems to implement authentication databases and
 accounting recording.
 By separating these functions from the NAS equipment, they can be
 implemented in general purpose computer systems, that may provide
 better suited long term storage media, and more sophisticated
 database software infrastructures.  Not to mention that a centralized
 server can allow the coordinated administration of many NAS systems
 as appropriate (for example a single server may service an entire POP
 consisting of multiple NAS systems).
 For ease of management, there is a strong desire to piggyback NAS
 authentication information with other authentication databases, so
 that authentication information can be managed for several services
 (such as OS shell login, or Web Server access) from the same
 provider, without creating separate passwords and accounts for the
 user.
 Session activity information is stored and processed to produce
 accounting usage records.  This is typically done with a long term
 (nightly, weekly or monthly) batch type process.
 However, as network operations grow in sophistication, there are
 requirements to provide real-time monitoring of port and user status,
 so that the state information can be used to implement policy
 decisions, monitor user trends, and the ability to possibly terminate
 access for administrative reasons.  Typically only the NAS knows the
 true dynamic state of a session.

5. Typical NAS Operation Sequence:

 The following details a typical NAS operational sequence:
  1. Call arrival on port or network
    1. Port:
      1. auto-detect (or not) type of call
      2. CLI/SLIP: prompt for username and password (if security

set)

  1. PPP: engage LCP, Authentication
  2. Request authentication from AAA server
  3. if okay, proceed to service
  4. may challenge
  5. may ask for password change/update

Mitton & Beadles Informational [Page 5] RFC 2881 NASreq NAS Model July 2000

  1. Network:
    1. activate internal protocol server (telnet, ftp)
    2. engage protocol's authentication technique
    3. confirm authentication information with AAA server
  1. Call Management Services
    1. Information from the telephone system or gateway controller

arrives indicating that a call has been received

  1. The AAA server is consulted using the information supplied by

the telephone system (typically Called or Calling number

         information)
      -  The server indicates whether to respond to the call by
         answering it, or by returning a busy to the caller.
      -  The server may also need to allocate a port to receive a
         call, and route it accordingly.
  1. Dial-out
    1. packet destination matches outbound route pre-configured
    2. find profile information to setup call
    3. Request information from AAA server for call details
  1. VPN/Tunneling (compulsory)
    1. authentication server identifies user as remote
    2. tunnel protocol is invoked to a remote server
    3. authentication information may be forwarded to remote AAA

server

  1. if successful, the local link is given a remote identity
  1. Multi-link aggregation
    1. after a new call is authenticated by the AAA server, if MP

options are present, then other bundles with the same

         identifying information is searched for
      -  bundle searches are performed across multiple systems
      -  join calls that match authentication and originator
         identities as one network addressable data source with a
         single network IP address
  1. Hardwired (non-interactive) services
    1. permanent WAN connections (Frame Relay or PSVCs)
    2. permanent serial connections (printers)

5.1 Characteristics of Systems and Sessions:

 Sessions must have a user identifier and authenticator to complete
 the authentication process. Accounting starts from time of call or
 service, though finer details are allowed. At the end of service, the
 call may be disconnected or allow re-authentication for additional
 services.

Mitton & Beadles Informational [Page 6] RFC 2881 NASreq NAS Model July 2000

 Some systems allow decisions on call handling to be made based on
 telephone system information provided before the call is answered
 (e.g., caller id or destination number). In such systems, calls may
 be busied-out or non-answered if system resources are not ready or
 available.
 Authorization to run services are supplied and applied after
 authentication. A NAS may abort call if session authorization
 information disagrees with call characteristics. Some system
 resources may be controlled by server driven policies
 Accounting messages are sent to the accounting server when service
 begins, and ends, and possibly periodically during service delivery.
 Accounting is not necessarily a real-time service, the NAS may be
 queue and batch send event records.

5.2 Separation of NAS and AAA server functions

 As a distributed system, there is a separation of roles between the
 NAS and the Server:
  1. Server provides authentication services; checks passwords

(static or dynamic)

  1. Server databases may be organized in any way (only protocol

specified)

  1. Server may use external systems to authenticate (including OS

user databases, token cards, one-time-lists, proxy or other

     means)
   - Server provides authorization information to NAS
   - The process of providing a service may lead to requests for
     additional information
   - Service authorization may require real-time enforcement
     (services may be based on Time of Day, or variable cost
     debits)
   - Session accounting information is tallied by the NAS and
     reported to server

5.3 Network Management and Administrative features

 The NAS system is presumed to have a method of configuration that
 allows it to know it's identity and network parameters at boot time.
 Likewise, this configuration information is typically managed using
 the standard management protocols (e.g., SNMP).  This would include
 the configuration of the parameters necessary to contact the AAA
 server itself.  The purpose of the AAA server is not to provide
 network management for the NAS, but to authorize and characterize the
 individual services for the users.  Therefore any feature that can be
 user specific is open to supply from the AAA server.

Mitton & Beadles Informational [Page 7] RFC 2881 NASreq NAS Model July 2000

 The system may have other operational services that are used to run
 and control the NAS.  Some users that have _Administrative_
 privileges may have access to system configuration tools, or services
 that affect the operation and configuration of the system (e.g.,
 loading boot images, internal file system access, etc..)  Access to
 these facilities may also be authenticated by the AAA server
 (provided it is configured and reachable!) and levels of access
 authorization may be provided.

6. Authentication Methods

 A NAS system typically supports a number of authentication systems.
 For async terminal users, these may be a simple as a prompt and
 input.  For network datalink users, such as PPP, several different
 authentication methods will be supported (PAP, CHAP [12], MS-CHAP
 [13]).  Some of these may actually be protocols in and of themselves
 (EAP [14] [15], and Kerberos).
 Additionally, the content of the authentication exchanges may not be
 straightforward.  Hard token cards, such as the Safeword and SecurId,
 systems may generate one-time passphrases that must be validated
 against a proprietary server.  In the case of multi-link support, it
 may be necessary to remember a session token or certificate for the
 later authentication of additional links.
 In the cases of VPN and compulsory tunneling services, typically a
 Network Access Identifier (RFC 2486 [16]) is presented by the user.
 This NAI is parsed into a destination network identifier either by
 the NAS or by the AAA server.  The authentication information will
 typically not be validated locally, but by a AAA service at the
 remote end of the tunnel service.

7. Session Authorization Information

 Once a user has been authenticated, there are a number of individual
 bits of information that the network management may wish to configure
 and authorize for the given user or class of users.
 Typical examples include:
      For async terminal users:
  1. banners
  2. custom prompts
  3. menus
  4. CLI macros - which could be used for: shortcuts, compound

commands, restrictive scripts

Mitton & Beadles Informational [Page 8] RFC 2881 NASreq NAS Model July 2000

      For network users:
  1. addresses, and routes
  2. callback instructions
  3. packet and activity filters
  4. network server addresses
  5. host server addresses
 Some services may require dynamic allocation of resources.
 Information about the resources required may not be known during the
 authentication phase, it may come up later. (e.g., IP Addresses for
 multi-link bundles) It's also possible that the authorization will
 change over the time of the session. To provide these there has to be
 a division of responsibility between the NAS and the AAA server, or a
 cooperation using a stateful service.
 Such services include:
  1. IP Address management
  2. Concurrent login limitations
  3. Tunnel usage limitations
  4. Real-time account expirations
  5. Call management policies
 In the process of resolving resource information, it may be required
 that a certain level of service be supplied, and if not available,
 the request refused, or corrective action taken.

8. IP Network Interaction

 As the NAS participates in the IP network, it interacts with the
 routing mechanisms of the network itself.  These interactions may
 also be controlled on a per-user/session basis.
 For example, some input streams may be directed to specific hosts
 other than the default gateway for the destination subnet.  In order
 to control services within the network provider's infrastructure,
 some types of packets may be discarded (filtered) before entering the
 network.  These filters could be applied based on examination of
 destination address and port number.  Anti-spoofing packet controls
 may be applied to disallow traffic sourced from addresses other than
 what was assigned to the port.
 A NAS may also be an edge router system, and apply Quality of Service
 (QoS) policies to the packets.  This makes it a QOS Policy
 Enforcement Point [19], [17].  It may learn QOS and other network
 policies for the user via the AAA service.

Mitton & Beadles Informational [Page 9] RFC 2881 NASreq NAS Model July 2000

9. A NAS Model

 So far we have looked at examples of things that NASes do.  The
 following attempts to define a NAS model that captures the
 fundamentals of NAS structure to better categorize how it interacts
 with other network components.
 A Network Access Server is a device which sits on the edge of a
 network, and provides access to services on that network in a
 controlled fashion, based on the identity of the user of the network
 services in question and on the policy of the provider of these
 services.  For the purposes of this document, a Network Access Server
 is defined primarily as a device which accepts multiple point-to-
 point [18] links on one set of interfaces, providing access to a
 routed network or networks on another set of interfaces.
 Note that there are many things that a Network Access Server is not.
 A NAS is not simply a router, although it will typically include
 routing functionality in it's interface to the network.  A NAS is not
 necessarily a dial access server, although dial access is one common
 means of network access, and brings its own particular set of
 requirements to NAS's.
 A NAS is the first device in the IP network to provide services to an
 end user, and acts as a gateway for all further services.  It is the
 point at which users are authenticated, access policy is enforced,
 network services are authorized, network usage is audited, and
 resource consumption is tracked.  That is, a NAS often acts as the
 policy enforcement point for network AAAA (authentication,
 authorization, accounting, and auditing) services.  A NAS is
 typically the first place in a network where security measures and
 policy may be implemented.

9.1 A Reference Model of a NAS

 For reference in the following discussion, a diagram of a NAS, its
 dependencies, and its interfaces is given below.  This diagram is
 intended as an abstraction of a NAS as a reference model, and is not
 intended to represent any particular NAS implementation.

Mitton & Beadles Informational [Page 10] RFC 2881 NASreq NAS Model July 2000

                             Users
                           v v v v v v v
                           | | PSTN  | |
                           | |  or   | |
                           |encapsulated
                        +-----------------+
                        |    (Modems)     |
                        +-----------------+
                           | | | | | | |
                 +--+----------------------------+
                 |  |                            |
                 |N |     Client Interface       |
                 |  |                            |
                 |A +----------Routing ----------+
                 |  |                            |
                 |S |    Network Interface       |
                 |  |                            |
                 +--+----------------------------+
                         /      |     \
                        /       |      \
                       /        |       \
                      /         |        \
    POLICY MANAGEMENT/          |         \  DEVICE MANAGEMENT
    +---------------+           |          +-------------------+
    | Authentication|         _/^\_        |Device Provisioning|
    +---------------+       _/     \_      +-------------------+
    | Authorization |     _/         \_    |Device Monitoring  |
    +---------------+   _/             \_  +-------------------+
    | Accounting    |  /       The       \
    +---------------+  \_   Network(s)  _/
    | Auditing      |    \_           _/
    +---------------+      \_       _/
                             \_   _/
                               \_/

9.2 Terminology

 Following is a description of the modules and interfaces in the
 reference model for a NAS given above:
 Client Interfaces - A NAS has one or more client interfaces, which
    provide the interface to the end users who are requesting network
    access.  Users may connect to these client interfaces via modems
    over a PSTN, or via tunnels over a data network.  Two broad
    classes of NAS's may be defined, based on the nature of the
    incoming client interfaces, as follows. Note that a single NAS
    device may serve in both classes:

Mitton & Beadles Informational [Page 11] RFC 2881 NASreq NAS Model July 2000

 Dial Access Servers - A Dial Access Server is a NAS whose client
    interfaces consist of modems, either local or remote, which are
    attached to a PSTN.
 Tunnel Servers - A Tunnel Server is a NAS whose client interfaces
    consists of tunneling endpoints in a protocol such as L2TP
 Network Interfaces - A NAS has one or more network interfaces, which
    connect to the networks to which access is being granted.
 Routing - If the network to which access is being granted is a routed
    network, then a NAS will typically include routing functionality.
 Policy Management Interface - A NAS provides an interface which
    allows access to network services to be managed on a per-user
    basis. This interface may be a configuration file, a graphical
    user interface, an API, or a protocol such as RADIUS, Diameter, or
    COPS [19].  This interface provides a mechanism for granular
    resource management and policy enforcement.
 Authentication - Authentication refers to the confirmation that a
    user who is requesting services is a valid user of the network
    services requested.  Authentication is accomplished via the
    presentation of an identity and credentials.  Examples of types of
    credentials are passwords, one-time tokens, digital certificates,
    and phone numbers (calling/called).
 Authorization - Authorization refers to the granting of specific
    types of service (including "no service") to a user, based on
    their authentication, what services they are requesting, and the
    current system state.  Authorization may be based on restrictions,
    for example time-of-day restrictions, or physical location
    restrictions, or restrictions against multiple logins by the same
    user.  Authorization determines the nature of the service which is
    granted to a user.  Examples of types of service include, but are
    not limited to: IP address filtering, address assignment, route
    assignment, QoS/differential services, bandwidth control/traffic
    management, compulsory tunneling to a specific endpoint, and
    encryption.
 Accounting - Accounting refers to the tracking of the consumption of
    NAS resources by users. This information may be used for
    management, planning, billing, or other purposes.  Real-time
    accounting refers to accounting information that is delivered
    concurrently with the consumption of the resources.  Batch
    accounting refers to accounting information that is saved until it

Mitton & Beadles Informational [Page 12] RFC 2881 NASreq NAS Model July 2000

    is delivered at a later time.  Typical information that is
    gathered in accounting is the identity of the user, the nature of
    the service delivered, when the service began, and when it ended.
 Auditing - Auditing refers to the tracking of activity by users.  As
    opposed to accounting, where the purpose is to track consumption
    of resources, the purpose of auditing is to determine the nature
    of a user's network activity.  Examples of auditing information
    include the identity of the user, the nature of the services used,
    what hosts were accessed when, what protocols were used, etc.
 AAAA Server - An AAAA Server is a server or servers that provide
    authentication, authorization, accounting, and auditing services.
    These may be co-located with the NAS, or more typically, are
    located on a separate server and communicate with the NAS's User
    Management Interface via an AAAA protocol.  The four AAAA
    functions may be located on a single server, or may be broken up
    among multiple servers.
 Device Management Interface - A NAS is a network device which is
    owned, operated, and managed by some entity.  This interface
    provides a means for this entity to operate and manage the NAS.
    This interface may be a configuration file, a graphical user
    interface, an API, or a protocol such as SNMP [20].
 Device Monitoring - Device monitoring refers to the tracking of
    status, activity, and usage of the NAS as a network device.
 Device Provisioning - Device provisioning refers to the
    configurations, settings, and control of the NAS as a network
    device.

9.3 Analysis

 Following is an analysis of the functions of a NAS using the
 reference model above:

9.3.1 Authentication and Security

 NAS's serve as the first point of authentication for network users,
 providing security to user sessions.  This security is typically
 performed by checking credentials such as a PPP PAP user
 name/password pair or a PPP CHAP user name and challenge/response,
 but may be extended to authentication via telephone number
 information, digital certificates, or biometrics.  NAS's also may
 authenticate themselves to users.  Since a NAS may be shared among
 multiple administrative entities, authentication may actually be
 performed via a back-end proxy, referral, or brokering process.

Mitton & Beadles Informational [Page 13] RFC 2881 NASreq NAS Model July 2000

 In addition to user security, NAS's may themselves be operated as
 secure devices.  This may include secure methods of management and
 monitoring, use of IP Security [21] and even participation in a
 Public Key Infrastructure.

9.3.2 Authorization and Policy

 NAS's are the first point of authorization for usage of network
 resources, and NAS's serve as policy enforcement points for the
 services that they deliver to users.  NAS's may provision these
 services to users in a statically or dynamically configured fashion.
 Resource management can be performed at a NAS by granting specific
 types of service based on the current network state.  In the case of
 shared operation, NAS policy may be determined based on the policy of
 multiple end systems.

9.3.3 Accounting and Auditing

 Since NAS services are consumable resources, usage information must
 often be collected for the purposes of soft policy management,
 reporting, planning, and accounting.  A dynamic, real-time view of
 NAS usage is often required for network auditing purposes.  Since a
 NAS may be shared among multiple administrative entities, usage
 information must often be delivered to multiple endpoints.
 Accounting is performed using such protocols as RADIUS [2].

9.3.4 Resource Management

 NAS's deliver resources to users, often in a dynamic fashion.
 Examples of the types of resources doled out by NAS's are IP
 addresses, network names and name server identities, tunnels, and
 PSTN resources such as phone lines and numbers.  Note that NAS's may
 be operated in a outsourcing model, where multiple entities are
 competing for the same resources.

9.3.5 Virtual Private Networks (VPN's)

 NAS's often participate in VPN's, and may serve as the means by which
 VPN's are implemented.  Examples of the use of NAS's in VPN's are:
 Dial Access Servers that build compulsory tunnels, Dial Access
 Servers that provide services to voluntary tunnelers, and Tunnel
 Servers that provide tunnel termination services.  NAS's may
 simultaneously provide VPN and public network services to different
 users, based on policy and user identity.

Mitton & Beadles Informational [Page 14] RFC 2881 NASreq NAS Model July 2000

9.3.6 Service Quality

 A NAS may delivery different qualities, types, or levels of service
 to different users based on policy and identity.  NAS's may perform
 bandwidth management, allow differential speeds or methods of access,
 or even participate in provisioned or signaled Quality of Service
 (QoS) networks.

9.3.7 Roaming

 NAS's are often operated in a shared or outsourced manner, or a NAS
 operator may enter into agreements with other service providers to
 grant access to users from these providers (roaming operations).
 NAS's often are operated as part of a global network.  All these
 imply that a NAS often provides services to users from multiple
 administrative domains simultaneously.  The features of NAS's may
 therefore be driven by requirements of roaming [22].

10. Security Considerations

 This document describes a model not a particular solution.
 As mentioned in section 9.3.1 and elsewhere, NAS'es are concerned
 about the security of several aspects of their operation, including:
  1. Providing sufficiently robust authentication techniques as

required by network policies,

  1. NAS authentication of configured authentication server(s),
  2. Server ability to authenticate configured clients,
  3. Hiding of the authentication information from network snooping

to protect from attacks and provide user privacy,

  1. Protecting the integrity of message exchanges from attacks

such as; replay, or man-in-the middle,

  1. Inability of other hosts to interfere with services authorized

to NAS, or gain unauthorized services,

  1. Inability of other hosts to probe or guess at authentication

information.

  1. Protection of NAS system configuration and administration from

unauthorized users

  1. Protection of the network from illegal packets sourced by

accessing connections

Mitton & Beadles Informational [Page 15] RFC 2881 NASreq NAS Model July 2000

11. References

 [1]  Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
      Authentication Dial In User Service (RADIUS)", RFC 2865, June
      2000.
 [2]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
 [3]  Calhoun, P., "Diameter Base Protocol", Work in Progress.
 [4]  Zorn, G., "Yet Another Authentication Protocol (YAAP)", Work in
      Progress.
 [5]  Mamakos, L., Lidl, K., Evarts, K., Carrel, D., Simone, D. and R.
      Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)",
      RFC 2516, February 1999.
 [6]  Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two
      Forwarding (Protocol) L2F", RFC 2341, May 1998.
 [7]  Hamzeh, K., "Ascend Tunnel Management Protocol - ATMP", RFC
      2107, February 1997.
 [8]  Valencia, A., Townsley, W., Rubens, A., Pall, G., Zorn, G., and
      B. Palter, "Layer Two Tunneling Protocol (L2TP)", RFC 2661,
      August 1999.
 [9]  Zorn, G., Leifer, D., Rubens, A., Shriver, J. and M. Holdrege,
      "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, June
      2000.
 [10] Zorn, G., Aboba, B. and D. Mitton, "RADIUS Accounting
      Modifications for Tunnel Protocol Support", RFC 2867, June 2000.
 [11] Aboba, B. and G. Zorn, "Implementation of PPTP/L2TP Compulsory
      Tunneling via RADIUS", RFC 2809, April 2000.
 [12] Simpson, W., "PPP Challenge Handshake Authentication Protocol
      (CHAP)", RFC 1994, August 1996.
 [13] Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433,
      March 1998.
 [14] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
      Protocol (EAP)", RFC 2284, March 1998.
 [15] Calhoun, et al., "Extensible Authentication Protocol Support in
      RADIUS", Work in Progress.

Mitton & Beadles Informational [Page 16] RFC 2881 NASreq NAS Model July 2000

 [16] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
      2486, January 1999.
 [17] Braden, R., Zhang, L., Berson, S., Herzog, S. and S. Jamin,
      "Resource ReSerVation Protocol (RSVP) Version 1 Functional
      Specification", RFC 2205, September 1997.
 [18] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
      51, RFC 1661, July 1994.
 [19] Boyle, J., Cohen, R., Durham, D., Herzog, S., Raja, R. and A.
      Sastry. "The COPS (Common Open Policy Service) Protocol", RFC
      2748, January 2000.
 [20] Case, J., Fedor, M., Schoffstall, M. and J. Davin. "A Simple
      Network Management Protocol (SNMP)", STD 15, RFC 1157, May 1990.
 [21] Atkinson, R. and S. Kent, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [22] Aboba, Zorn, "Dialup Roaming Requirements", Work in Progress.

12. Acknowledgments

 This document is a synthesis of my earlier draft and Mark Beadles'
 NAS Reference Model draft.

13. Authors' Addresses

 David Mitton
 Nortel Networks
 880 Technology Park Drive
 Billerica, MA 01821
 Phone: 978-288-4570
 EMail: dmitton@nortelnetworks.com
 Mark Beadles
 SmartPipes Inc.
 545 Metro Place South
 Suite 100
 Dublin, OH 43017
 Phone: 614-327-8046
 EMail: mbeadles@smartpipes.com

Mitton & Beadles Informational [Page 17] RFC 2881 NASreq NAS Model July 2000

14. Appendix - Acronyms and Glossary:

 AAA - Authentication, Authorization, Accounting, The three primary
 services required by a NAS server or protocol.
 NAS - Network Access Server, a system that provides access to a
 network.  In some cases also know as a RAS, Remote Access Server.
 CLI - Command Line Interface, an interface to a command line service
 for use with an common asynchronous terminal facility.
 SLIP - Serial Line Internet Protocol, an IP-only serial datalink,
 predecessor to PPP.
 PPP - Point-to-Point Protocol; a serial datalink level protocol that
 supports IP as well as other network protocols. PPP has three major
 states of operation: LCP - Link layer Control Protocol,
 Authentication, of which there are several types (PAP, CHAP, EAP),
 and NCP - Network layer Control Protocol, which negotiates the
 network layer parameters for each of the protocols in use.
 IPX - Novell's NetWare transport protocol
 NETBEUI - A Microsoft/IBM LAN protocol used by Microsoft file
 services and the NETBIOS applications programming interface.
 ARAP - AppleTalk Remote Access Protocol
 LAT - Local Area Transport; a Digital Equipment Corp. LAN protocol
 for terminal services.
 PPPoe - PPP over Ethernet; a protocol that forwards PPP frames on an
 LAN infrastructure.  Often used to aggregate PPP streams at a common
 server bank.
 VPN - Virtual Private Network; a term for networks that appear to be
 private to the user by the use of tunneling techniques.
 FR - Frame Relay, a synchronous WAN protocol and telephone network
 intraconnect service.
 PSVC - Permanent Switched Virtual Circuit - a service which delivers
 an virtual permanent circuit by a switched network.
 PSTN - Public Switched Telephone Network

Mitton & Beadles Informational [Page 18] RFC 2881 NASreq NAS Model July 2000

 ISDN - Integrated Services Digital Network, a telephone network
 facility for transmitting digital and analog information over a
 digital network connection.  A NAS may have the ability to receive
 the information from the telephone network in digital form.
 ISP - Internet Service Provider; a provider of Internet access (also
 Network Service Provider, NSP).
 BRI - Basic Rate Interface; a digital telephone interface.
 PRI - Primary Rate Interface; a digital telephone interface of 64K
 bits per second.
 T1 - A digital telephone interface which provides 24-36 channels of
 PRI data and one control channel (2.048 Mbps).
 T3 - A digital telephone interface which provides 28 T1 services.
 Signalling control for the entire connection is provided on a
 dedicated in-band channel.
 NFAS - Non-Facility Associated Signaling, a telephone network
 protocol/service for providing call information on a separate wire
 connection from the call itself.  Used with multiple T1 or T3
 connections.
 SS7 - A telephone network protocol for communicating call supervision
 information on a separate data network from the voice network.
 POP - Point Of Presence; a geographic location of equipment and
 interconnection to the network.  An ISP typically manages all
 equipment in a single POP in a similar manner.
 VSA - Vendor Specific Attributes; RADIUS attributes defined by
 vendors using the provision of attribute 26.

Mitton & Beadles Informational [Page 19] RFC 2881 NASreq NAS Model July 2000

15. Full Copyright Statement

 Copyright (C) The Internet Society (2000).  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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Mitton & Beadles Informational [Page 20]

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