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Versions: 00 01 02 rfc2881                                 Informational
Network Access Server Requirements                         David Mitton
Internet Draft                                          Nortel Networks
Expires November 2000                                      Mark Beadles
                                                        SmartPipes Inc.
                                                               May 2000


     Network Access Server Requirements Next Generation (NASREQNG)
                               NAS Model
                   draft-ietf-nasreq-nasmodel-02.txt



Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This document is a product of the Network-Access-Server Requirements
Next Generation (NASREQNG) Working Group of the Internet Engineering
Task Force (IETF).  Comments should be submitted to the mailing list
nasreq@tdmx.rutgers.edu.


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 (RFC 2138, 2139)[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.
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Table of Contents

1. INTRODUCTION.......................................................3

 1.1 Scope of this Document ..........................................3
 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: ........................7
 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........................................................9

 9.1 A Reference Model of a NAS .....................................11
 9.2 Terminology ....................................................12
 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. ACKNOWLEDGMENTS..................................................17

12. AUTHOR'S INFORMATION:............................................17

13. FULL COPYRIGHT STATEMENT.........................................18

14. APPENDIX - ACRONYMS AND GLOSSARY:................................18










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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:

- 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.
- 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:



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- A computer system of minimal to moderate performance
  (example processors: Intel 386 or 486, Motorola 68000)
- A moderate amount, but not large RAM (typically varies with supported
  # of ports 1MB to 8MB)
- Some small amount of non-volatile memory, and/or way to be configured
  out-of-band
- 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;
- At least one LAN or high performance network interface (e.g.
  Ethernet, ATM, FR)
- At least one, but typically many, serial interface ports, which could
  be;
  -  serial RS232 ports direct wired or wired to a modem, or
  -  have integral hardware or software modems (V.22bis,V.32, V.34, X2,
     Kflex, V.90, etc.)
  -  have direct connections to telephone network digital WAN lines
     (ISDN, T1, T3, NFAS, or SS7)
  -  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:


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- dial-up or direct access serial line access; Ability to access the
  network using a the public telephone network.
- network access (SLIP, PPP, IPX, NETBEUI, ARAP); The NAS allows the
  caller to access the network directly.
- 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])


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:


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The following details a typical NAS operational sequence:
     - Call arrival on port or network
       -  Port:
          - auto-detect (or not) type of call
          - CLI/SLIP: prompt for username and password (if security
            set)
          - PPP: engage LCP, Authentication
          - Request authentication from AAA server
          - if okay, proceed to service
          - may challenge
          - may ask for password change/update
       -  Network:
          - activate internal protocol server (telnet, ftp)
          - engage protocol's authentication technique
          - confirm authentication information with AAA server

     - Call Management Services
       -  Information from the telephone system or gateway controller
          arrives indicating that a call has been received
       -  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.

     - Dial-out
       -  packet destination matches outbound route pre-configured
       -  find profile information to setup call
       -  Request information from AAA server for call details

     - VPN/Tunneling (compulsory)
       -  authentication server identifies user as remote
       -  tunnel protocol is invoked to a remote server
       -  authentication information may be forwarded to remote AAA
          server
       -  if successful, the local link is given a remote identity

     - Multi-link aggregation
       -  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

     - Hardwired (non-interactive) services
       -  permanent WAN connections (Frame Relay or PSVCs)
       -  permanent serial connections (printers)



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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.

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:

     - Server provides authentication services; checks passwords
       (static or dynamic)
     - Server databases may be organized in any way (only protocol
       specified)
     - 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

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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.

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:
     - banners
     - custom prompts
     - menus
     - CLI macros - which could be used for: shortcuts, compound
       commands, restrictive scripts


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     For network users:
     - addresses, and routes
     - callback instructions
     - packet and activity filters
     - network server addresses
     - 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:

     - IP Address management
     - Concurrent login limitations
     - Tunnel usage limitations
     - Real-time account expirations
     - 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.



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

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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.



























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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.

                            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      |    \_           _/
   +---------------+      \_       _/
                            \_   _/
                              \_/













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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:

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.


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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 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


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entities, authentication may actually be performed via a back-end
proxy, referral, or brokering process.

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.



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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:

     - Providing sufficiently robust authentication techniques as
       required by network policies,
     - NAS authentication of configured authentication server(s),
     - Server ability to authenticate configured clients,
     - Hiding of the authentication information from network snooping
       to protect from attacks and provide user privacy,
     - Protecting the integrity of message exchanges from attacks such
       as; replay, or man-in-the middle,
     - Inability of other hosts to interfere with services authorized
       to NAS, or gain unauthorized services,
     - Inability of other hosts to probe or guess at authentication
       information.
     - Protection of NAS system configuration and administration from
       unauthorized users
     - Protection of the network from illegal packets sourced by
       accessing connections











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References:

[1] C. Rigney, et.al. "Remote Authentication Dial In User Service
(RADIUS)" RFC 2138, April 1977.

[2] C. Rigney, et.al. "RADIUS Accounting", RFC 2139, April 1977.

[3] P. Calhoun "Diameter Base Protocol", draft-calhoun-diameter-07.txt,
November 1998.

[4] G. Zorn, "Yet Another Authentication Protocol (YAAP)", draft-zorn-
yaap-01.txt, 30 June 1996.

[5] L. Mamakos et al.  "A Method for Transmitting PPP Over Ethernet
(PPPoE)."  RFC 2516, UUNET Technologies, Inc., February 1999.

[6] A. Valencia, M. Littlewood, 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] A. Valencia, et.al. "Layer Two Tunneling Protocol (L2TP)", draft-
ietf-pppext-l2tp-12.txt, Oct 1998

[9] G. Zorn, D. Leifer, A. Rubens, J. Shriver, "RADIUS Attributes for
Tunnel Protocol Support", draft-ietf-radius-tunnel-auth-06.txt,
September 1998

[10] G. Zorn, D. Mitton, "RADIUS Accounting Modifications for Tunnel
Protocol Support",draft-ietf-radius-tunnel-acct-02.txt, September 1999

[11] Aboba, Zorn, "Implementation of PPTP/L2TP Compulsory Tunneling via
RADIUS", draft-ietf-radius-tunnel-imp-03.txt, July 1997.

[12] Simpson, W., "PPP  Challenge Handshake Authentication Protocol
(CHAP)", RFC 1994, August 1996

[13]G. Zorn, S. Cobb, Microsoft PPP CHAP Extensions, draft-ietf-pppext-
mschap-00.txt, March 1998.

[14] L. Blunk, J. Vollbrecht. "PPP Extensible Authentication Protocol
(EAP)." RFC 2284, March 1998.

[15] Calhoun, et.al. "Extensible Authentication Protocol Support in
RADIUS", draft-ietf-radius-eap-05.txt, May 1998.

[16] B. Aboba, M. Beadles, "The Network Access Identifier" RFC 2486,
Jan 1999.





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[17] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin, "Resource
ReSerVation Protocol (RSVP) Version 1 Functional Specification ", RFC
2205, September 1997.

[18] Simpson, Editor. "The Point-to-Point Protocol (PPP)", RFC 1661,
July 1994.

[19] Boyle, Cohen, Durham, Herzog, Raja, Sastry. "The COPS (Common Open
Policy Service) Protocol", draft-ietf-rap-cops-06.txt, February 1999.

[20] Case, Fedor, Schoffstall, and Davin. "A Simple Network Management
Protocol (SNMP)", RFC 1157, May 1990.

[21] Atkinson, Kent. "Security Architecture for the Internet Protocol",
draft-ietf-ipsec-arch-sec-07.txt, July 1998

[22] Aboba, Zorn, "Dialup Roaming Requirements", draft-ietf-roamops-
roamreq-05.txt, July 1997



11. Acknowledgments

This document is a synthesis of my earlier draft and Mark Beadles NAS
Reference Model draft (draft-beadles-nas-01.txt).


12. Author's Information:

     David Mitton
     Nortel Networks
     8 Federal St. BL8-05
     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







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13. Full Copyright Statement

Copyright (C) The Internet Society (May 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 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."


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



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     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
     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.


















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