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RFC 3169 - Criteria for Evaluating Network Access Server Protoco

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Network Working Group                                         M. Beadles
Request for Comments: 3169                              SmartPipes, Inc.
Category: Informational                                        D. Mitton
                                                         Nortel Networks
                                                          September 2001

        Criteria for Evaluating Network Access Server Protocols

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   This document defines requirements for protocols used by Network
   Access Servers (NAS).

1.  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [KEYWORDS].

2.  Introduction

   This document defines requirements for protocols used by Network
   Access Servers (NAS).  Protocols used by NAS's may be divided into
   four spaces:  Access protocols, Network protocols, AAA protocols, and
   Device Management protocols.  The primary focus of this document is
   on AAA protocols.

   The reference model of a NAS used by this document, and the analysis
   of the functions of a NAS which led to the development of these
   requirements, may be found in [NAS-MODEL].

3.  Access Protocol Requirements

   There are three basic types of access protocols used by NAS's.  First
   are the traditional telephony-based access protocols, which interface
   to the NAS via a modem or terminal adapter or similar device.  These
   protocols typically support asynchronous or synchronous PPP [PPP]

   carried over a telephony protocol.  Second are broadband pseudo-
   telephony access protocols, which are carried over xDSL or cable
   modems, for example.  These protocols typically support an
   encapsulation method such as PPP over Ethernet [PPPOE].  Finally are
   the virtual access protocols used by NAS's that terminate tunnels.
   One example of this type of protocol is L2TP [L2TP].

   It is a central assumption of the NAS model used here that a NAS
   accepts multiple point-to-point links via one of the above access
   protocols.  Therefore, at a minimum, any NAS access protocol MUST be
   able to carry PPP.  The exception to this requirement is for NAS's
   that support legacy text login methods such as telnet [TELNET],
   rlogin, or LAT.  Only these access protocols are exempt from the
   requirement to support PPP.

4.  Network Protocol Requirements

   The network protocols supported by a NAS depend entirely on the kind
   of network to which a NAS is providing access.  This document does
   not impose any additional requirements on network protocols beyond
   the protocol specifications themselves.  For example, if a NAS that
   serves a routed network includes internet routing functionality, then
   that NAS must adhere to [ROUTING-REQUIREMENTS], but there are no
   additional protocol requirements imposed by virtue of the device
   being a NAS.

5.  AAA Protocol Requirements

5.1.  General protocol characteristics

   There are certain general characteristics that any AAA protocol used
   by NAS's must meet.  Note that the transport requirements for
   authentication/authorization are not necessarily the same as those
   for accounting/auditing.  An AAA protocol suite MAY use the same
   transport and protocol for both functions, but this is not strictly

5.1.1.  Transport requirements  Transport independence

   The design of the AAA protocol MUST be transport independent.
   Existing infrastructures use UDP-based protocols [RADIUS], gateways
   to new protocols must be practical to encourage migration.  The
   design MUST comply with congestion control recommendations in RFC
   2914 [CONGEST].  Scalability

   Very large scale NAS's that serve up to thousands of simultaneous
   sessions are now being deployed.  And a single server system may
   service a large number of ports.  This means that, in the extreme,
   there may be an almost constant exchange of many small packets
   between the NASes and the AAA server.  An AAA protocol transport
   SHOULD support being optimized for a long-term exchange of small
   packets in a stream between a pair of hosts.

   The protocol MUST be designed to support a large number of ports,
   clients, and concurrent sessions.  Examples of poor design would
   include message identifiers which values are so small that queues and
   reception windows wrap under load, unique session identifier ranges
   that are so small that they wrap within the lifetime of potential
   long sessions, counter values that cannot accommodate reasonable
   current and future bandwidth usage, and computational processes with
   high overhead that must be performed frequently.  Support for Multiple AAA Servers and Failure Recovery

   In order to operationally support large loads, load balancing and
   fail-over to multiple AAA servers will be required.  The AAA protocol
   MUST provide for NAS's to balance individual AAA requests between two
   or more AAA servers.  The load balancing mechanism SHOULD be built in
   to the AAA protocol itself.

   The AAA protocol MUST be able to detect a failure of the transport
   protocol to deliver a message or messages within a known and
   controllable time period, so it can engage retransmission or server
   fail-over processes.  The reliability and robustness of
   authentication requests MUST be predictable and configurable.

   The AAA protocol design MUST NOT introduce a single point of failure
   during the AAA process.  The AAA protocol MUST allow any sessions
   between a NAS and a given AAA server to fail-over to a secondary
   server without loss of state information.  This fail-over mechanism
   SHOULD be built in to the AAA protocol itself.  Support for Multiple Administrative Domains

   NAS's operated by one authority provide network access services for
   clients operated by another authority, to network destinations
   operated by yet another authority.  This type of arrangement is of
   growing importance; for example, dial roaming is now a nearly
   ubiquitous service.  Therefore, the AAA protocol MUST support AAA

   services that travel between multiple domains of authority.  The AAA
   protocol MUST NOT use a model that assumes a single domain of

   The AAA protocol MUST NOT dictate particular business models for the
   relationship between the administrative domains.  The AAA protocol
   MUST support proxy, and in addition SHOULD support other multi-domain
   relationships such as brokering and referral.

   The AAA protocol MUST also meet the protocol requirements specified

5.1.2.  Attribute-Value Protocol Model

   Years of operational experience with AAA protocols and NAS's has
   proven that the Attribute-Value protocol model is an optimal
   representation of AAA data.  The protocol SHOULD use an Attribute-
   Value representation for AAA data.  This document will assume such a
   model.  Even if the AAA protocol does not use this as an on-the-wire
   data representation, Attribute-Value can serve as abstraction for
   discussing AAA information.

   Experience has also shown that attribute space tends to run out
   quickly.  In order to provide room for expansion in the attribute
   space, the AAA protocol MUST support a minimum of 64K Attributes (16
   bits), each with a minimum length of 64K (16 bits).  Attribute Data Types

   The AAA protocol MUST support simple attribute data types, including
   integer, enumeration, text string, IP address, and date/time.  The
   AAA protocol MUST also provide some support for complex structured
   data types.  Wherever IP addresses are carried within the AAA
   protocol, the protocol MUST support both IPv4 and IPv6 [IPV6]
   addresses.  Wherever text information is carried within the AAA
   protocol, the protocol MUST comply with the IETF Policy on Character
   Sets and Languages [RFC 2277].  Minimum Set of Attributes

   At a minimum, the AAA protocol MUST support, or be easily extended to
   support, the set of attributes supported by RADIUS [RADIUS] and
   RADIUS Accounting [RADIUS-ACCOUNTING].  If the base AAA protocol does
   not support this complete set of attributes, then an extension to
   that protocol MUST be defined which supports this set.  Attribute Extensibility

   NAS and AAA development is always progressing.  In order to prevent
   the AAA protocol from being a limiting factor in NAS and AAA Server
   development, the AAA protocol MUST provide a built-in extensibility
   mechanism, which MUST include a means for adding new standard
   attribute extensions.  This MUST include a method for registering or
   requesting extensions through IANA, so that long-term working group
   involvement is not required to create new attribute types.  Ideally,
   the AAA protocol SHOULD separate specification of the transport from
   specification of the attributes.

   The AAA protocol MUST include a means for individual vendors to add
   value through vendor-specific attributes and SHOULD include support
   for vendor-specific data types.

5.1.3.  Security Requirements  Mutual Authentication

   It is poor security practice for a NAS to communicate with an AAA
   server that is not trusted, and vice versa.  The AAA protocol MUST
   provide mutual authentication between AAA server and NAS.  Shared Secrets

   At a minimum, the AAA protocol SHOULD support use of a secret shared
   pairwise between each NAS and AAA server to mutually verify identity.
   This is intended for small-scale deployments.  The protocol MAY
   provide stronger mutual security techniques.  Public Key Security

   AAA server/NAS identity verification based solely on shared secrets
   can be difficult to deploy properly at large scale, and it can be
   tempting for NAS operators to use a single shared secret (that rarely
   changes) across all NAS's.  This can lead to an easy compromise of
   the secret.  Therefore, the AAA protocol MUST also support mutual
   verification of identity using a public-key infrastructure that
   supports expiration and revocation of keys.  Encryption of Attributes

   Some attributes are more operationally sensitive than others.  Also,
   in a multi-domain scenario, attributes may be inserted by servers
   from different administrative domains.  Therefore, the AAA protocol

   MUST support selective encryption of attributes on an attribute-by-
   attribute basis, even within the same message.  This requirement
   applies equally to Authentication, Authorization, and Accounting

5.2.  Authentication and User Security Requirements

5.2.1.  Authentication protocol requirements

   End users who are requesting network access through a NAS will
   present various types of credentials.  It is the purpose of the AAA
   protocol to transport these credentials between the NAS and the AAA
   server.  Bi-directional Authentication

   The AAA protocol MUST support transport of credentials from the AAA
   server to the NAS, between the User and the NAS, and between the NAS
   and the AAA server.  Periodic Re-Authentication

   The AAA protocol MUST support re-authentication at any time during
   the course of a session, initiated from either the NAS or the AAA
   server.  This is a requirement of CHAP [CHAP].  Multi-phase Authentication

   The AAA protocol MUST be able to support multi-phase authentication
   methods, including but not limited to support for:

      -  Text prompting from the NAS to the user

      -  A series of binary challenges and responses of arbitrary length

      -  An authentication failure reason to be transmitted from the NAS
         to the user

      -  Callback to a pre-determined phone number  Extensible Authentication Types

   Security protocol development is going on constantly as new threats
   are identified and better cracking methods are developed.  Today's
   secure authentication methods may be proven insecure tomorrow.  The
   AAA protocol MUST provide some support for addition of new
   authentication credential types.

5.2.2.  Authentication Attribute Requirements

   In addition to the minimum attribute set, the AAA protocol must
   support and define attributes that provide the following functions:  PPP Authentication protocols

   Many authentication protocols are defined within the framework of
   PPP.  The AAA protocol MUST be able to act as an intermediary
   protocol between the authenticate and the authenticator for the
   following authentication protocols:

      -  PPP Password Authentication Protocol [PPP]

      -  PPP Challenge Handshake Authentication Protocol [CHAP]

      -  PPP Extensible Authentication Protocol [EAP]  User Identification

   The following are common types of credentials used for user
   identification.  The AAA protocol MUST be able to carry the following
   types of identity credentials:

      -  A user name in the form of a Network Access Identifier [NAI].

      -  An Extensible Authentication Protocol [EAP] Identity Request
         Type packet.

      -  Telephony dialing information such as Dialed Number
         Identification Service (DNIS) and Caller ID.

   If a particular type of authentication credential is not needed for a
   particular user session, the AAA protocol MUST NOT require that dummy
   credentials be filled in.  That is, the AAA protocol MUST support
   authorization by identification or assertion only.  Authentication Credentials

   The following are common types of credentials used for
   authentication.  The AAA protocol MUST be able to carry the following
   types of authenticating credentials at a minimum:

      -  A secret or password.

      -  A response to a challenge presented by the NAS to the user

      -  A one-time password

      -  An X.509 digital certificate [X.509]

      -  A Kerberos v5 ticket [KERBEROS]

5.2.3.  Authentication Protocol Security Requirements  End-to-End Hiding of Credentials

   Where passwords are used as authentication credentials, the AAA
   protocol MUST provide a secure means of hiding the password from
   intermediates in the AAA conversation.  Where challenge/response
   mechanisms are used, the AAA protocol MUST also prevent against
   replay attacks.

5.3.  Authorization, Policy, and Resource management

5.3.1.  Authorization Protocol Requirements

   In all cases, the protocol MUST specify that authorization data sent
   from the NAS to the AAA server is to be regarded as information or
   "hints", and not directives.  The AAA protocol MUST be designed so
   that the AAA server makes all final authorization decisions and does
   not depend on a certain state being expected by the NAS.  Dynamic Authorization

   The AAA protocol MUST support dynamic re-authorization at any time
   during a user session.  This re-authorization may be initiated in
   either direction.  This dynamic re-authorization capability MUST
   include the capability to request a NAS to disconnect a user on
   demand.  Resource Management

   Resource Management MUST be supported on demand by the NAS or AAA
   Server at any time during the course of a user session.  This would
   be the ability for the NAS to allocate and deallocate shared
   resources from a AAA server servicing multiple NASes.  These
   resources may include, but are not limited to; IP addresses,
   concurrent usage limits, port usage limits, and tunnel limits.  This
   capability should have error detection and synchronization features
   that will recover state after network and system failures.  This may
   be accomplished by session information timeouts and explicit interim
   status and disconnect messages.  There should not be any dependencies
   on the Accounting message stream, as per current practices.

   This feature is primarily intended for NAS-local network resources.
   In a proxy or multi-domain environment, resource information should
   only be retained by the server doing the allocation, and perhaps it's
   backups.  Authorization resources in remote domains should use the
   dynamic authorization features to change and revoke authorization

5.3.2.  Authorization Attribute Requirements  Authorization Attribute Requirements - Access Restrictions

   The AAA protocol serves as a primary means of gathering data used for
   making Policy decisions for network access.  Therefore, the AAA
   protocol MUST allow network operators to make policy decisions based
   on the following parameters:

      -  Time/day restrictions.  The AAA protocol MUST be able to
         provide an unambiguous time stamp, NAS time zone indication,
         and date indication to the AAA server in the Authorization

      -  Location restrictions:  The AAA protocol MUST be able to
         provide an unambiguous location code that reflects the
         geographic location of the NAS.  Note that this is not the same
         type of thing as either the dialing or dialed station.

      -  Dialing restrictions:  The AAA protocol MUST be able to provide
         accurate dialed and dialing station indications.

      -  Concurrent login limitations:  The AAA protocol MUST allow an
         AAA Server to limit concurrent logins by a particular user or
         group of users.  This mechanism does not need to be explicitly
         built into the AAA protocol, but the AAA protocol must provide
         sufficient authorization  information for an AAA server to make
         that determination through an out-of-band mechanism.  Authorization Attribute Requirements - Authorization Profiles

   The AAA protocol is used to enforce policy at the NAS.  Essentially,
   on granting of access, a particular access profile is applied to the
   user's session.  The AAA protocol MUST at a minimum provide a means
   of applying profiles containing the following types of information:

      -  IP Address assignment: The AAA protocol MUST provide a means of
         assigning an IPv4 or IPv6 address to an incoming user.

      -  Protocol Filter application:  The AAA protocol MUST provide a
         means of applying IP protocol filters to user sessions.  Two
         different methods MUST be supported.

         First, the AAA protocol MUST provide a means of selecting a
         protocol filter by reference to an identifier, with the details
         of the filter action being specified out of band.  The AAA
         protocol SHOULD define this out-of-band reference mechanism.

         Second, the AAA protocol MUST provide a means of passing a
         protocol filter by value.  This means explicit passing of
         pass/block information by address range, TCP/UDP port number,
         and IP protocol number at a minimum.

      -  Compulsory Tunneling:  The AAA protocol MUST provide a means of
         directing a NAS to build a tunnel or tunnels to a specified
         end- point.  It MUST support creation of multiple simultaneous
         tunnels in a specified order.  The protocol MUST allow, at a
         minimum, specification of the tunnel endpoints, tunneling
         protocol type, underlying tunnel media type, and tunnel
         authentication credentials (if required by the tunnel type).
         The AAA protocol MUST support at least the creation of tunnels
         using the L2TP [L2TP], ESP [ESP], and AH [AH] protocols.  The
         protocol MUST provide means of adding new tunnel types as they
         are standardized.

      -  Routing:  The AAA protocol MUST provide a means of assigning a
         particular static route to an incoming user session.

      -  Expirations/timeouts:  The AAA protocol MUST provide a means of
         communication session expiration information to a NAS.  Types
         of expirations that MUST be supported are:  total session time,
         idle time, total bytes transmitted, and total bytes received.

      -  Quality of Service:  The AAA protocol MUST provide a means for
         supplying Quality of Service parameters to the NAS for
         individual user sessions.  Resource Management Requirements

   The AAA protocol is a means for network operators to perform
   management of network resources.  The AAA protocol MUST provide a
   means of collecting resource state information, and controlling
   resource allocation for the following types of network resources.

      -  Network bandwidth usage per session, including multilink

      -  Access port usage, including concurrent usage and usage pools.

      -  Connect time.

      -  IP Addresses and pools.

      -  Compulsory tunnel limits.

5.3.3.  Authorization Protocol Security Requirements  Security of Compulsory Tunnel Credentials

   When an AAA protocol passes credentials that will be used to
   authenticate compulsory tunnels, the AAA protocol MUST provide a
   means of securing the credentials from end-to-end of the AAA
   conversation.  The AAA protocol MUST also provide protection against
   replay attacks in this situation.

5.4.  Accounting and Auditing Requirements

5.4.1.  Accounting Protocol Requirements  Guaranteed Delivery

   The accounting and auditing functions of the AAA protocol are used
   for network planning, resource management, policy decisions, and
   other functions that require accurate knowledge of the state of the
   NAS.  NAS operators need to be able to engineer their network usage
   measurement systems to a predictable level of accuracy.  Therefore,
   an AAA protocol MUST provide a means of guaranteed delivery of
   accounting information between the NAS and the AAA Server(s).  Real Time Accounting

   NAS operators often require a real time view onto the status of
   sessions served by a NAS.  Therefore, the AAA protocol MUST support
   real-time delivery of accounting and auditing information.  In this
   context, real time is defined as accounting information delivery
   beginning within one second of the triggering event.  Batch Accounting

   The AAA protocol SHOULD also support delivery of stored accounting
   and auditing information in batches (non-real time).  Accounting Time Stamps

   There may be delays associated with the delivery of accounting
   information.  The NAS operator will desire to know the time an event
   actually occurred, rather than simply the time when notification of
   the event was received.  Therefore, the AAA protocol MUST carry an
   unambiguous time stamp associated with each accounting event.  This
   time stamp MUST be unambiguous with regard to time zone.  Note that
   this assumes that the NAS has access to a reliable time source.  Accounting Events

   At a minimum, the AAA protocol MUST support delivery of accounting
   information triggered by the following events:

      -  Start of a user session

      -  End of a user session

      -  Expiration of a predetermined repeating time interval during a
         user session.  The AAA protocol MUST provide a means for the
         AAA server to request that a NAS use a certain interval
         accounting time.

      -  Dynamic re-authorization during a user session (e.g., new
         resources being delivered to the user)

      -  Dynamic re-authentication during a user session  On-Demand Accounting

   NAS operators need to maintain an accurate view onto the status of
   sessions served by a NAS, even through failure of an AAA server.
   Therefore, the AAA protocol MUST support a means of requesting
   current session state and accounting from the NAS on demand.

5.4.2.  Accounting Attribute Requirements

   At a minimum, the AAA protocol MUST support delivery of the following
   types of accounting/auditing data:

      -  All parameters used to authenticate a session.

      -  Details of the authorization profile that was applied to the

      -  The duration of the session.

      -  The cumulative number of bytes sent by the user during the

      -  The cumulative number of bytes received by the user during the

      -  The cumulative number of packets sent by the user during the

      -  The cumulative number of packets received by the user during
         the session.

      -  Details of the access protocol used during the session (port
         type, connect speeds, etc.)

5.4.3.  Accounting Protocol Security Requirements  Integrity and Confidentiality

   Note that accounting and auditing data are operationally sensitive
   information.  The AAA protocol MUST provide a means to assure end-
   to-end integrity of this data.  The AAA protocol SHOULD provide a
   means of assuring the end-to-end confidentiality of this data.  Auditibility

   Network operators use accounting data for network planning, resource
   management, and other business-critical functions that require
   confidence in the correctness of this data.  The AAA protocol SHOULD
   provide a mechanism to ensure that the source of accounting data
   cannot easily repudiate this data after transmission.

6.  Device Management Protocols

   This document does not specify any requirements for device management

7.  Acknowledgments

   Many of the requirements in this document first took form in Glen
   Zorn's, "Yet Another Authentication Protocol (YAAP)" document, for
   which grateful acknowledgment is made.

8.  Security Considerations

   See above for security requirements for the NAS AAA protocol.

   Where an AAA architecture spans multiple domains of authority, AAA
   information may need to cross trust boundaries.  In this situation, a
   NAS might operate as a shared device that services multiple
   administrative domains.  Network operators are advised take this into
   consideration when deploying NAS's and AAA Servers.

9.  IANA Considerations

   This document does not directly specify any IANA considerations.
   However, the following recommendations are made:

   Future development and extension of an AAA protocol will be made much
   easier if new attributes and values can be requested or registered
   directly through IANA, rather than through an IETF Standardization

   The AAA protocol might use enumerated values for some attributes,
   which enumerate already-defined IANA types (such as protocol number).
   In these cases, the AAA protocol SHOULD use the IANA assigned numbers
   as the enumerated values.

10.  References

   [AH]                    Kent, S. and R. Atkinson, "IP Authentication
                           Header (AH)", RFC 2402, November 1998.

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

   [CONGEST]               Floyd, S., "Congestion Control Principles",
                           RFC 2914, Sept. 2000.

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

   [ESP]                   Kent, S. and R. Atkinson, "IP Encapsulating
                           Security Payload (ESP)", RFC 2406, November

   [KEYWORDS]              Bradner, S., "Key words for use in RFCs to
                           Indicate Requirement Levels", BCP 14, RFC
                           2119, March 1997.

   [KERBEROS]              Kohl, J. and C. Neuman, "The Kerberos Network
                           Authentication Service (V5)", RFC 1510,
                           September 1993.

   [IPV6]                  Deering, S. and R. Hinden, "Internet
                           Protocol, Version 6 (IPv6) Specification",
                           RFC 2460, December 1998.

   [L2TP]                  Townsley, W., Valencia, A., Rubens, A., Pall,
                           G., Zorn, G. and B. Plater, "Layer Two
                           Tunneling Protocol (L2TP)", RFC 2661, August

   [NAI]                   Aboba, B. and M. Beadles, "The Network Access
                           Identifier", RFC 2486, January 1999.

   [NAS-MODEL]             Mitton, D. and M. Beadles, "Network Access
                           Server Requirements Next Generation
                           (NASREQNG) NAS Model", RFC 2881, July 2000.

   [NAS-EXT]               Mitton, D., "Network Access Servers
                           Requirements: Extended RADIUS Practices", RFC
                           2882, July 2000.

   [PPP]                   Simpson, W., "The Point-to-Point Protocol
                           (PPP)", STD 51, RFC 1661, July 1994.

   [PPPOE]                 Mamakos, L., Lidl, K., Evarts, J., Carrel,
                           D., Simone, D. and R. Wheeler, "A Method for
                           Transmitting PPP Over Ethernet (PPPoE)", RFC
                           2516, February 1999.

   [ROUTING-REQUIREMENTS]  Baker, F., "Requirements for IP Version 4
                           Routers", RFC 1812, June 1995.

   [TELNET]                Postel, J. and J. Reynolds, "Telnet Protocol
                           Specification", STD 8, RFC 854, May 1983.

   [RFC 2277]              Alvestrand, H., "IETF Policy on Character
                           Sets and Languages", BCP 18, RFC 2277,
                           January 1998.

   [X.509]                 ITU-T Recommendation X.509 (1997 E):
                           Information Technology - Open Systems
                           Interconnection - The Directory:
                           Authentication Framework, June 1997.

   [RADIUS]                Rigney, C., Rubens. A., Simpson, W. and S.
                           Willens, "Remote Authentication Dial In User
                           Service (RADIUS)", RFC 2138, April 1997.

   [RADIUS-ACCOUNTING]     Rigney, C., "RADIUS Accounting", RFC 2139,
                           April 1997.

   [ROAMING-REQUIREMENTS]  Aboba, B. and G. Zorn, "Criteria for
                           Evaluating Roaming Protocols", RFC 2477,
                           January 1999.

11.  Authors' Addresses

   Mark Anthony Beadles
   SmartPipes, Inc.
   565 Metro Place South Suite 300
   Dublin, OH 43017

   Phone: 614-923-6200

   David Mitton
   Nortel Networks
   880 Technology Park Drive
   Billerica, MA 01821

   Phone: 978-288-4570
   EMail: dmitton@nortelnetworks.com

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