rfc2881.txt

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RFC 2881                    NASreq NAS Model                   July 2000


        -  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)

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.



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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:

     - 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 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.



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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:

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




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RFC 2881                    NASreq NAS Model                   July 2000


        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.




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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.












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