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Versions: 00                                                            
INTERNET DRAFT                                   Nancy Greene
Category: Informational             Nortel (Northern Telecom)
Title: <draft-greene-nasreq-00.txt>           Fernando Cuervo
Date: March 1998                    Nortel (Northern Telecom)
Expires: September 1998

               Best Current Practice for Modem Outsourcing

Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
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This document describes an architecture and the protocol used with
respect to a Network Access Server (NAS), when modems are outsourced
from the data network operator to the carrier network operator. At the
heart of modem outsourcing there are several key areas, namely, varied
mechanisms for authentication, authorization based on network wide state
and policy for resource sharing, accounting/auditing and other
management functions.

1.0 Introduction

Presently, dial-up connections over the public telephone network are
used for on-demand connection to the Internet or corporate networks.  An
ISP may wish to outsource its modems to the telephone or carrier network
operator.  In this case, the carrier network provides connections and
modems while the data network operator (e.g., an ISP or a corporate
network) is responsible for other functions such as subscriber
authentication, or accounting.  Data network operators benefit by
replacing remote access hardware with a virtual modem pool service
provided by a carrier, traffic is forwarded from the resources that make
up the virtual modem pool over broadband connections to one or more “ISP
gateways”, or Home Gateways. The virtual modem pool provides the ISP
with independence from the signaling used at the NAS (for example, PRI
or SS7).

2.0 Modem outsourcing requirements

Modem outsourcing and mass deployment of dial-up access, need
capabilities that are beyond the functionality provided by RADIUS. When
NAS boxes are placed in the carrier network operator domain many new
factors are introduced in the way NAS boxes operate:

1-Resource control is more complex since network resources can be shared
to  optimize  cost  (e.g., modem pools  may  be  dynamically shared
between ISPs).

2-Resources such as tunnels may be authorized, set up and controlled in
many ways (e.g., according to ISP, or tunnel type).

3-Access becomes a carrier’s responsibility. The  carrier may need to
manage resources for different access networks.

Increased flexibility is introduced when the NAS is placed in the
carrier network. In the case of modem outsourcing,   several distinct
configurations can be defined depending on the following factors:

1-Where the point of authentication is (e.g., carrier network operator
domain or ISP).

2-The level and distribution of authorization (for example, before and
after end-user authentication, or just after. Note that RADIUS uses an
end-user based authentication-authorization model. However, in the
shared environment that results from modem outsourcing, authorization
functions in the carrier network operator domain must often be based on
the attributes of both the end-user and the ISP.

3-Whether signaling is physically co-located with the connection it
establishes (e.g., front-end PRI signaling), or whether it is physically
separate from the connection (e.g., back-end SS7 signaling).

4-Control and management relationships between carrier and ISP network
elements, e.g. ISP Home Gateways, NAS Controller/AAA Servers in the
carrier network, AAA Servers/Proxies.

These factors place requirements on the protocol that are above and
beyond the scope of RADIUS. The protocol described in section 6.0, DSM-
CC, includes functions for system configuration and resource control
that provide the flexibility required to properly address these

3.0 Terminology

AAA Server function
   This function provides the NAS with Authentication, Authorization,
Accounting and/or other management functions. It may be located in the
ISP, in the carrier network, or both.

AAA Proxy function
   It is a proxy to a AAA Server.

Network Access Server (NAS) Control function
   A NAS Control function allocates and deallocates resources according
to some resource policy.  A NAS Control function  may control many NAS.
It may share a server platform with  AAA server functions and/or proxies
to other AAA Servers. It may be located at an ISP, but is more likely
found in a carrier network, for example, allowing NAS to be shared among

NAS Controller/AAA Server/Proxy (NCAP)
   This is a server platform that hosts the NAS Control function. This
platform may also host AAA server functions and/or proxies  to other AAA
Servers. It is typically deployed in the carrier network domain, for
example, allowing NAS to be shared among ISPs. In some situations it may
be deployed in the data network domain. The AAA functions may be RADIUS
based or other.

   The subject of the authentication/authorization.

Data Network Operator
   An ISP or corporation, sometimes referred to as the wholesale-

Carrier Network Operator
   Provider of access and transport services between the end-user and a
data network.

Network Access Server (NAS)
   The Network Access Server (NAS) is the device that provides resources
for users to access the data network. A NAS provides physical
terminations of user access connections, and modems. A NAS includes a
client that uses the functions of a NAS control server.

ISP (Home) Gateway
    Network interworking platform between the Carrier Network and Data
Network domains.

4.0 Modem Outsourcing Architectures

                         +----------+     +------------+
                         |NAS       |     |RADIUS (AAA)|
                         |Controller|     |Server      |
                         |          |     |            |
                         +----------+     +------------+
                                 ^                |
                                 |                |
                     +-----------+                |
                     |                            |
                     v                            |
              +-------+                     +-----------+
end-user --- >|       |                     | ISP (Home)|
              |NAS    | < --------------- > | Gateway   |
              |       |                     |           |
              +-------+                     +-----------+

Figure 1: Modem outsourcing architecture - scenario 1

In modem outsourcing there are currently two scenarios for establishing
a data session to an ISP. In the first scenario, authentication,
authorization and accounting is done by the ISP (Figure 1). PPP is
carried all the way to the ISP. Access to a tunnel may be subject to
authorization functions exercised by the NAS itself or an authorization
server (NAS Controller) in the carrier network operator domain. The
client in the NAS collects the authentication information from the user.
The information is then tunneled to a target network and its target
RADIUS (AAA) server.

                       +-----------+        +------------+
                       |NAS        |        |RADIUS (AAA)|
                       |Controller/|        |Server      |
                       |AAA Proxy  | < --- >|            |
                       +-----------+        +------------+
                             ^                      |
                             |                      |
                  +----------+                      |
                  |                                 |
                  v                                 |
              +-------+                     +-----------+
end-user --- >|       |                     | ISP (Home)|
              |NAS    | < --------------- > | Gateway   |
              |       |                     |           |
              +-------+                     +-----------+

Figure 2: Modem outsourcing architecture - scenario 2

In the second scenario (Figure 2), PPP is terminated at the NAS. When
this is the case, a client in the NAS must contact an appropriate server
for user authentication. If necessary, (normally for scalability
reasons,) a proxy may be used between the NAS and the ISP’s AAA Server.
In this scenario, end-user authorization functions are more naturally
integrated with the authentication steps, but it is likely that some
level of authorization would be exercised by NAS Controller/AAA Server
in the carrier network operator domain (e.g., based on attributes of the
target ISP). Accounting is fairly independent of the setup style, the
NAS collects resource and traffic information that can be relayed to the
ISP according to the specific requirements (i.e. main accounting source,
auditing, monitoring functions, etc.)

4.1 Properties of the NCAP-NAS architecture

Having a few NCAPs in the network for a large number of NAS boxes makes
the NAS systems scaleable. Thus, instead of an ISP’s AAA server needing
to be able to serve a large number of NAS, as the number of outsourced
modems  grows, it can deal with a lesser number of NCAPs in the network.
In modern large NAS systems (e.g., many NAS boxes, several ISPs, roaming
users, etc.) NAS boxes do not have the resources to store policy and
configuration information (let alone the complexity of maintaining all
these data). The NCAP is responsible for coordinating the administrative
functions, modem pool resource allocation and configuration policies.
The dependency between a NAS and a NCAP in the network varies according
to the NAS box capabilities for storing and enacting policy (resource
and administrative), and on the complexity of the interworking between
networking domains. The NCAP is also responsible for insulating the ISP
from specific aspects of NAS boxes (e.g., vintage, manufacturer, etc).

Additionally, as NAS boxes continue growing their port capacity the
NCAP-NAS protocol must be able to efficiently support the configuration
and control of a large number of resources and devices.

The interaction  between the NAS and the NCAP uses a subset of the
ISO/IEC DSM-CC User-Network protocol [DSM-CC], with extensions [DSM-CC
extensions]. This is done to support the additional flexibility that
modem outsourcing requires (See section 2.0.)  This protocol is outlined
in section 6.0.  Interaction between the NCAP in the network and an AAA
Server at an ISP may be based on the DSM-CC protocol with extensions, or
a RADIUS proxy. Ideally, all interaction between AAA servers can be
supported by the same protocol as the one between the NAS and its NCAP.

5.0  Requirements for a NAS <-> NCAP protocol

>From the discussion above, we can now determine some of the requirements
for a NAS <-> NCAP protocol. It must:

- allow separation of AAA (AAA -> A/A/A)

Separating the AAA allows different configurations. For example,
authorization may be handled by an NCAP in the network, while
authentication is always performed by the AAA Server at the ISP. Also,
accounting records may be kept by the ISP or by the network, or both.

- be a simple light-weight and symmetric protocol that allows NAS ->
Server and Server -> NAS requests.

An ISP may require information about NAS usage, or resources available.
This should be available on demand.

- support resource policy and configuration (e.g. tunnels).

The protocol should allow, for instance, tunneling attributes per user
to be stored at an ISP or in the network, to be requested by a NAS as
required for tunnel setup. NAS running independently of an NCAP is an
example of policy and configuration since the NAS must have this

- allows sharing of NAS resources between ISPs.

This is generally accomplished by allowing control of a NAS by an
intermediary such as a network operator (i.e. outsourcing).

6.0 DSM-CC Functionality

DSM-CC is a light-weight ISO standard protocol [DSM-CC]. It is a
request/response protocol that is usually implemented over UDP/IP. The
following NAS functionality is provided using its message set.

6.1  NAS Initialization

Used by the NAS to indicate that it is ready to respond to the NCAP, it
may indicate the “services” that it is ready to support. Basic
configuration information such as hardware and software versions may be
communicated to the NCAP. The response from the NCAP indicates whether
the management and control associations requested will take place.
Configuration information may be supplied at this point by the NCAP to
the NAS, for instance, several timers that govern the control
relationship between the NAS and the NCAP may be set at this point.

DSM-CC messages:
UN-Config*, * = <Request, Confirm>

6.2  NAS failure recovery

A failed NAS will try to reestablish a control association using the NAS
Initialization messages. The NCAP  will launch a NAS Audit to match
against the NAS state last known to the control server.

DSM-CC messages:
UN-Config*, * = <Request, Confirm>

6.3 NAS Control Server reset indication

The NCAP must reestablish the association with the NAS. Configuration
information may be exchanged, including the definition of a new NCAP.
This action must be followed by an update of the state changes of the
NAS and its resources that occurred while running without the NCAP.

DSM-CC messages:
UN-Config*, * = <Indication, Response>

6.4  Link Failure recovery

The NCAP or the NAS may reestablish the association. This must be
followed by an update of the state changes of the NAS and its resources
that occurred while running without the NCAP.

DSM-CC messages:
UN-Config*, * = <Request, Confirm, Indication, Response>

6.5  Resource Allocate/Release

DSM-CC Session messages are used to allocate NAS resources to end-users.
Session set-up messages may involve authentication or authorization
functionality. A session identifier is used to simplify the control and
management of resources used in a single association between an end-user
and an ISP. Session messages may be initiated by the NAS or the NCAP,
depending on the location of the native signaling and authentication
client (e.g., in the NAS for PRI or in the NCAP for SS7). Authentication
may be carried out in the NCAP, proxied to another server or tunneled to
the ISP. Authorization functions in the NCAP determine users rights to
access resources before and after authentication. Separate Add/Delete
resource messages are provided by DSM-CC, however, they are not
necessary for current NAS applications.

DSM-CC messages:
ClientSessionSetUp*, * = <Request, Confirm, Indication, Response>
ClientRelease*, * = <Request, Confirm, Indication, Response>

6.6  ISP Gateway (Home Gateway)

Coordinated action between the NAS, the NCAP and the ISP gateway is
necessary. Depending on the mode of operation, the state of a target ISP
may be known (e.g., via management) or inferred (e.g., via retries) by
either the NAS or the NCAP. When the ISP Gateway is unavailable, the NAS
and the NCAP must coordinate their actions for Session Set-Up and

DSM-CC messages:
ClientSessionSetUp*, * = <Request, Confirm, Indication, Response>
ClientRelease*, * = <Request, Confirm, Indication, Response>

6.7 Initiate accounting (for local PPP termination)

successful establishment of an end-to-end session is notified by the NAS
to the NCAP. The NAS signals the NCAP to indicate that it has
successfully connected the data session, and that it is proceeding to
forward packets to the ISP. This message is used to trigger generation
of accounting records and to convey additional call set-up information.

DSM-CC Messages:
ClientConnect*  * = <Request, Confirm>

6.8  NAS Audit

The NCAP requests status of a session or sessions from the NAS.

DSM-CC Messages:
ClientStatus*  * = <Request, Confirm>

7.0  Way Forward

It is proposed to use DSM-CC as a basis for a RADIUS replacement
protocol for modern NAS. DSM-CC would provide secure, bi-directional
functions for subscriber authentication, resource configuration, status
reports and subscriber management. Since RADIUS is  widely used for
authentication of dial-up users, DSM-CC  would be adapted for
compatibility with RADIUS.

8.0 Authors

    Fernando Cuervo
    Ottawa, ON, Canada.
    Phone: 613-763-4628
    EMail: cuervo@nortel.ca

    Nancy Greene
    Ottawa, ON, Canada
    Phone: 613-763-9789
    Email: ngreene@nortel.ca

9.0 References

[1] ISO/IEC 13818-6 Digital Storage Media - Command and Control, N3100,
July 1996

[2] ISO/IEC 14496-6 WD 2.0, Delivery Multimedia Integrated Framework V2,
ISO/IEC JTC1/SC29/WG11 N2059 MPEG 98, February 6/98, San Jose