Internet Engineering Task Force      M. Borden, E. Crawley, J. Krawczyk
INTERNET-DRAFT                                             Bay Networks
draft-crawley-rsvp-over-atm-00                                 F. Baker
                                                          Cisco Systems
                                                              S. Berson
                                                      February 22, 1996

            Issues for RSVP and Integrated Services over ATM

Status of this Memo

   This document is an Internet Draft.  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
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   Please check the I-D abstract listing contained in each Internet
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   The RSVP and Integrated Services working groups have been working
   towards the goal of an "Integrated Services Internet" where
   applications can request a Quality of Service (QoS) from the network
   in order to meet their end-to- end service requirements.
   Simultaneously, the IP-over-ATM working group has been specifying the
   requirements and interactions between IP and Asynchronous Transfer
   Mode (ATM) technology.  The internals and interoperability of ATM
   devices are specified by the ATM Forum and its working groups.  One
   of the features of ATM technology is the ability to request a virtual
   circuit (VC) with a specified QoS.  Obviously, it would be useful for
   the work of the Integrated Services Internet to be able to take
   advantage of the QoS abilities of ATM.

   This document outlines the issues related to running RSVP and
   Integrated Service models over ATM and presents an initial attempt at
   dividing up the issues between the various working groups and
   organizations.  This is intended as a starting point for the
   discussions in the joint IP-ATM, RSVP, and Int-Serv meeting to be
   held at the Los Angeles IETF meeting in March, 1996.

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1.   Introduction

   It is only natural that the RSVP protocol [1] and the Integrated
   Services (IntServ) models [6,7] would like to utilize the QoS
   properties of ATM as a subnetwork layer. [2] discusses providing RSVP
   services over ATM and brings out some of the open issues.
   Contributions to the ATM Forum MPOA Working Group [e.g. 10, 11, 12]
   have also investigated some of the issues related to running RSVP
   over ATM.  However, no clear set of directions and plan of action has
   been determined.  Further, there are several groups in the IETF and
   the ATM Forum that have expressed interest in the issues of RSVP and
   IntServ over ATM.  While it is very important that the issues
   regarding RSVP and the IntServ models over ATM be addressed, it is
   important not to duplicate effort between groups.  It is also
   important to avoid considering models and objectives that cannot be
   demonstrated to be solvable in other switched networks.

   The purpose of this document is to briefly describe the issues as
   they are currently known and suggest which working groups should
   address them.  It is not the purpose of this document to describe
   RSVP, the IntServ models, or IP over ATM.  The reader should be
   familiar with the concepts of these protocols and models.

2.  Issues Regarding the Operation of RSVP and IntServ over ATM

   The issues related to RSVP and IntServ over ATM fall into several
   general classes:
   - How to make RSVP run over ATM now and in the future
   - When to set up a virtual circuit (VC) for a specific Quality of
     Service (QoS) related to RSVP
   - How to map the IntServ models to ATM QoS models
   - How to know that an ATM network is providing the QoS necessary for
     a flow
   - How to handle the many-to-many connectionless features of IP
     multicast and RSVP in the one-to-many connection-oriented world of

2.1  Modes/Models for RSVP and IntServ over ATM

   [3] Discusses several different models for running IP over ATM
   networks.  Any one of these models would work as long as the RSVP
   control packets (IP protocol 46) and data packets can follow the same
   IP path through the network.  It is important that the RSVP PATH
   messages follow the same IP path as the data such that appropriate
   PATH state may be installed in the routers along the path.  For an
   ATM subnetwork, this means the ingress and egress points must be the
   same in both directions for the RSVP control and data messages.
   Within each of these models, there are decisions about using
   different types of data distribution in ATM as well as different
   connection initiation.  The following sections look at some of the
   different ways QoS connections can be set up for RSVP.

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2.1.1     UNI 3.x

   In the User Network Interface (UNI) 3.0 and 3.1 specifications, [8,9]
   both permanent and switched virtual circuits (PVC and SVC) may be
   established with a specified service category (CBR, VBR, and UBR) and
   specific traffic descriptors in point-to-point and
   point-to-multipoint configurations.  Additional QoS parameters are
   not available in UNI 3.x and those that are available are
   vendor-specific.  Consequently, the level of QoS control available in
   standard UNI 3.x networks is somewhat limited.  However, using these
   building blocks, it is possible to use RSVP and the IntServ models.   Permanent Virtual Circuits (PVCs)

   PVCs emulate dedicated point-to-point lines in a network, so the
   operation of RSVP can be identical to the operation over any
   point-to-point network.  The QoS of the PVC must be consistent and
   equivalent to the type of traffic and service model used.  The
   devices on either end of the PVC have to provide traffic control
   services in order to multiplex multiple flows over the same PVC.
   With PVCs, there is no issue of when or how long it takes to set up
   VCs, since they are made in advance but the resources of the PVC are
   limited to what has been pre-allocated.  PVCs that are not fully
   utilized can tie up ATM network resources that could be used for
   SVCs.   Switched Virtual Circuits (SVCs)

   SVCs allow paths in the ATM network to be set up "on demand".  This
   allows flexibility in the use of RSVP over ATM along with some
   complexity.  Parallel VCs can be set up to allow best-effort and
   better service class paths through the network.  The cost and time to
   set up SVCs can impact their use.  For example, it may be better to
   initially route QoS traffic over existing VCs until an SVC with the
   desired QoS has been set up.  Scaling issues can come into play if a
   single VC is used per RSVP flow.  An RSVP flow is a data flow from
   one or more sources to one or more receivers as defined by an RSVP
   filter specification.  The number of VCs in any ATM device is limited
   so the number of RSVP flows that can be handled by a device would be
   strictly limited to the number of VCs available, if we assume one VC
   per flow.   Point to MultiPoint

   In order to provide QoS for IP multicast, an important feature of
   RSVP, data flows must be distributed to multiple destinations from a
   given source.  Point-to-multipoint VCs provide such a mechanism.  It
   is important to map the actions of IP multicasting and RSVP
   (e.g. IGMP JOIN/LEAVE and RSVP RESV/RESV TEAR) to add party and drop
   party functions for ATM.  Point-to-multipoint VCs as defined in UNI
   3.x have a single service class for all destinations.  This is

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   contrary to the RSVP "heterogeneous receiver" concept.  It is
   possible to set up a different VC to each receiver requesting a
   different QoS, but this again could run into scaling problems.

   RSVP sends messages both up and down the multicast distribution tree.
   In the case of a large ATM cloud, this could result in a RSVP message
   implosion at an RSVP traffic source with many receivers.   Multicast Servers

   IP-over-ATM has the concept of a multicast server or reflector that
   can accept cells from multiple senders and send them via a
   point-to-multipoint VC to a set of receivers.  This moves the VC
   scaling issues noted previously for point-to-multipoint VCs to the
   multicast server.  Additionally, the multicast server will need to
   know how to interpret RSVP packets so it will be able to provide VCs
   of the appropriate QoS for the flows.

2.1.2     ATM Forum Release 4.0

   Release 4.0 adds a Leaf Initiated Join (LIJ) capability and
   standardizes additional QoS parameters for finer control of the
   characteristics of a VC.  LIJ allows an ATM end point to join into an
   existing point-to-multipoint VC without necessarily contacting the
   source of the VC.  This will reduce the burden on the ATM source
   point for setting up new branches and more closely matches the
   receiver-based model of RSVP and IP multicast.  However, many of the
   same scaling issues exist and the new branches added to a point-to-
   multipoint VC would use the same QoS as existing branches.

   The ATM Forum Traffic Management (TM) specification 4.0 greatly
   enhances the flexibility of choosing QoS in an ATM network.  Rather
   than QoS only being defined by a service category, it is possible to
   request individual traffic parameters.  This will aid enormously in
   matching QoS models between ATM and IntServ.  In addition, TM 4.0
   defines new classes, such as VBR-rt and ABR that will be helpful in
   new service models.

2.1.3     Hop-by-Hop vs. Short Cut

   If the ATM "cloud" is made up a number of logical IP subnets (LISs),
   then it is possible to use "short cuts" from a node on one LIS
   directly to a node on another LIS, avoiding router hops between the
   LISs. NHRP [4], is one mechanism for determining the ATM address of
   the egress point on the ATM network given a destination IP address.
   It is a topic for further study to determine if significant benefit
   is achieved from short cut routes vs. the extra state required.

   However, if the PATH and RESV messages must follow the same set of IP
   hops, then short cut paths in both directions must be established.

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   This is not a problem for point-to- point VCs, but it is a problem
   for point-to-multipoint connections since they are unidirectional.
   This topic requires further study and guidelines.

2.1.4     Future Models

   ATM is constantly evolving.  If we assume that RSVP and IntServ
   applications are going to be wide-spread, it makes sense to consider
   changes to ATM that would improve the operation of RSVP and IntServ
   over ATM.  Similarly, the RSVP protocol and IntServ models will
   continue to evolve and changes that affect them should also be
   considered.   Heterogeneous Point-to-MultiPoint

   The IntServ models and RSVP support the idea of "heterogeneous
   receivers"; i.e., not all receivers of a particular multicast flow
   are required to ask for the same QoS from the network.

   The most important case of this is where some receivers ask for a
   specific QoS while others receive the flow in a best- effort manner.
   In some cases where there are multiple senders on a
   shared-reservation flow (e.g., an audio conference), an individual
   receiver only needs to reserve enough resources to receive one sender
   at a time.  However, other receivers may elect to reserve more
   resources, perhaps to allow for some amount of "over- speaking" or in
   order to record the conference (post processing during playback can
   separate the senders by their source addresses).

   In order to prevent denial-of-service attacks via reservations, the
   service models do not allow the service elements to simply drop
   non-conforming packets.  Guaranteed Delay [6] service requires that
   the flow must be reshaped before running it through the policing
   function.  Both Guaranteed and Controlled Load [7] assign
   non-conformant packets to best-effort status (which may result in
   packet drops if there is congestion).

   Emulating these behaviors over an ATM network is problematic and
   needs to be studied.  If a single maximum QoS is used over a
   point-to-multipoint VC, resources could be wasted if cells are sent
   over certain links where the reassembled packets will eventually be
   dropped.  In addition, the "maximum QoS" may actually be a
   degradation in service to the best-effort branches.

   The term "variegated VC" has been coined to describe a point-
   to-multipoint VC that allows a different QoS on each branch.  This
   approach seems to match the spirit of the Integrated Service and RSVP
   models, but some thought has to be put into the cell drop strategy
   when traversing from a "bigger" branch to a "smaller" one.  The
   "best-effort for non- conforming packets" behavior must be retained.
   Early Packet Discard (EPD) schemes must be used so that all the cells

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   for a given packet can be discarded at the same time rather than
   discarding only a few cells from several packets making all the
   packets useless to the receivers.   Lightweight Signalling

   Q.2931 signalling is very complete and carries with it a significant
   burden for signalling in all possible public and private connections.
   It might be worth investigating a lighter weight signalling mechanism
   for faster connection setup in private networks.   QoS Renegotiation

   Another change that would help RSVP over ATM is the ability to
   request a different QoS for an active VC.  This would eliminate the
   need to setup and tear down VCs as the QoS changed.  RSVP allows
   receivers to change their reservations and senders to change their
   traffic descriptors dynamically.  This, along with the merging of
   reservations can create a situation where the QoS needs of a VC can
   change.  Allowing changes to the QoS of an existing VC would allow
   these features to work without creating a new VC.   Group Addressing

   The model of one-to-many communications provided by point-to-
   multipoint VCs does not really match the many-to-many communications
   provided by IP multicasting.  It would be nice to have a scaleable
   mapping from IP multicast addresses to an ATM "group address" to
   address this problem.

2.1.5     QoS Routing

   RSVP is explicitly not a routing protocol.  However, since it conveys
   QoS information, it may prove to be a valuable input to a routing
   protocol that could make path determinations based on QoS and network
   load information.  In other words, instead of asking for just the IP
   next hop for a given destination address, it might be worthwhile for
   RSVP to provide information on the QoS needs of the flow if routing
   has the ability to use this information in order to determine a
   route.  The work in this area is just starting and there are numerous
   issues to consider.

2.2  Reliance on Multicast Routing

   RSVP was designed to support both unicast and IP multicast
   applications.  This means that RSVP needs to work closely with
   multicast and unicast routing.  Unicast routing over ATM has been
   addressed in RFC1577[13] and RFC1755[14].  MARS[5] provides multicast

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   address resolution for IP over ATM networks, an important part of the
   solution for multicast routing.  Further work is needed for full
   interoperability of IP multicast routing with ATM.

2.3  Aggregation of Flows

   Some of the scaling issues noted in previous sections can be
   addressed by aggregating several RSVP flows over a single VC if the
   destinations of the VC match for all the flows being aggregated.
   However, this causes some complexity in the management of VCs and in
   the scheduling of packets within each VC at the originating point of
   the VC.  Note that he rescheduling of flows within a VC is not
   possible in the switches in the ATM network. Additionally, virtual
   paths (VPs) could be used for aggregating multiple VCs.  These topics
   need to be investigated and some guidelines need to be provided.

2.4  Mapping QoS Parameters

   The mapping of QoS parameters from the IntServ models to the ATM
   service classes is an important issue in making RSVP and IntServ work
   over ATM.  The issue is that while some guidelines can be developed
   for mapping the parameters of a given service model to the traffic
   descriptors of an ATM traffic class, implementation variables,
   policy, and cost factors can make strict standards problematic.

2.5  Directly Connected ATM Hosts

   It is obvious that the needs of hosts that are directly connected to
   ATM networks must be considered for RSVP and IntServ over ATM.
   Functionality for RSVP over ATM must not assume that an ATM host has
   all the functionality of a router, but such things as MARS and NHRP
   clients might be worthwhile features.

2.6  API Issues

   While it has not been much of a concern to the IETF, there are groups
   considering how applications can specify QoS parameters to the
   network in a manner that makes sense to application programmers.  It
   makes sense that APIs specify parameters in a manner that is
   independent of the layer 2 network.  It is also important for the
   application to be able to renegotiate and change its requested QoS in
   a manner that is consistent with the RSVP and IntServ models.

2.7  Accounting and Policy Issues

   Since RSVP and IntServ create classes of preferential service, some
   form of administrative control or cost allocation is needed to
   control abuse.  There are certain types of policies specific to ATM

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   and IP over ATM that need to be studied to determine how they
   interoperate with IP policies being developed.  Typical IP policies
   would be that only certain users are allowed to make reservations.
   This policy would translate well to IP over ATM.  There may be a need
   for policies specific to IP over ATM.  For example, since signalling
   costs are high relative to IP, an IP over ATM specific policy might
   restrict the ability to change the prevailing QoS in a VC.  If VCs
   are relatively scarce, there also might be specific accounting costs
   in creating a new VC.  The work so far has been preliminary, and much
   work remains to be done.

3.   Responsibilities for Resolving Issues

   The following sections list the working groups involved in some of
   the issues listed in the previous section.  Along with each group is
   a list of issues that the authors believe each group should address
   for RSVP and IntServ over ATM.  The goal is to not have multiple
   working groups working on the same problem(s).  This list is a
   starting point for discussions.  However, it is hoped that the
   discussions will be brief such that the working groups can get down
   to business quickly.

3.1  IETF Working Groups

   The IETF has several working groups that can be involved in RSVP and
   IntServ over ATM.  IETF working groups are chartered to solve
   specific problems and some of the issues noted in the following may
   not be specifically in their current charter so it may require
   amendment of the charter or creation of a new or sub working group.

3.1.1     IP over ATM (IPATM)

   This group has been most involved in getting IP to work over ATM and
   is responsible for issues relating to both IP hosts and IP routers.
   They have spent time on encapsulations and multicast address
   resolution and have been the focal point for liaison activities
   between the ATM Forum and the IETF.  They should continue to be the
   focal point for liaison activities as they relate to RSVP and IntServ
   over ATM and should address specific issues related to the operation
   of the IP protocols, including RSVP, over ATM.

   As the focal point for liaison activities, the IPATM WG can encourage
   contributions for the ATM Forum as needed.

3.1.2     Integrated Services (INTSERV)

   IntServ has been developing service models for the integrated
   services internet.  These models turn into flow specification formats
   for RSVP.  It is important for IntServ to examine methods by which
   these flow requirements can be achieved over ATM, using the ATM

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   notions of QoS.  There is typically not a unique method of doing so,
   and it is expected that the working group would be developing
   guidelines and not standardized rules.

   In addition to describing the parameters of the flow there is the
   problem of managing the flows.  This needs to be studied at the same
   time.  The issues involve policies for aggregating flows over a
   single VC, when new VCs need to be established, and managing
   point-to-multipoint "variegated" VCs.  Since most of these decisions
   are local, the need not be standardized but guidelines may be

   As new application requirements are generated, the IntServ WG will
   need to develop new service models, flow specification formats, and
   methods to achieve the services over ATM.

3.1.3     ReSerVation Protocol (RSVP)

   The RSVP WG has been focused on developing version 1 of the RSVP
   protocol in a media and routing independent manner.  They should
   continue addressing further issues with RSVP such as security,
   authentication, and access control.  Related to ATM, the RSVP WG
   should consider the ramifications of RSVP in Non-Broadcast
   MultiAccess (NBMA) networks such as ATM and publish guidelines for
   RSVP over such nets.

3.1.4     InterDomain Multicast Routing (IDMR)

   The IDMR WG has been concerned with scaleable multicast routing
   protocols.  Obviously, there are going to be issues for multicast
   routing in NBMA networks and IDMR must consider the implications for
   the protocols being developed.  QoS multicast routing is also
   something that IDMR should consider as it relates to RSVP and ATM.

3.1.5     Routing Over Large Clouds (ROLC)

   As an NMBA network that can span a "large cloud," ATM can make use of
   the NHRP protocol being developed in the ROLC WG.  The WG has been
   very aware of the needs of IP over ATM in the development of NHRP and
   should continue to do so.  As mentioned earlier, the benefit of short
   cut routing needs to be weighed against the state and cost of
   establishing direct VCs between nodes on different ATM LISs.  The
   benefit vs.  the scaling issues must be considered for RSVP and

3.2  ATM Forum

   The ATM Forum exists for its members and all of its activities must
   be approved by the members.  There are several working groups that

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   have interest in RSVP and IntServ over ATM.  There was a
   well-attended Birds of a Feather (BOF) session for RSVP and IntServ
   over ATM as part of the MPOA WG at the February 1996 ATM Forum
   meeting in Beverly Hills, CA.

3.2.1     Signalling (SIG)

   Obviously, any changes to ATM signalling to better support RSVP and
   IntServ will have to be approved by the Signalling WG.  Such features
   include variegated point-to-multipoint VCs and renegotiated QoS for
   existing VCs.  These are not trivial problems and are expected to
   take some time, but contributions from members are expected in this
   area soon.

   The Signalling WG should also be involved in making a "group
   addressing" scheme to provide many-to-many communications in a
   scaleable manner, much like the current IP multicast model.

3.2.2     MultiProtocol Over ATM (MPOA)

   The MPOA WG is addressing the issues of running multiple layer 3
   protocols in an ATM environment.  The features of ATM such as QoS and
   short cut routing are expected to be utilized.  The features of RSVP
   that affect the MPOA environment should be considered by MPOA but
   there will be considerable overlap with the work of the IETF RSVP WG.
   In these cases, the RSVP WG should address the issues.

   MPOA is also the focal point for the liaisons between the ATM Forum
   and the IETF and should continue to do so with feedback from other
   ATM Forum and IETF working groups feeding into the liaisons.

3.2.3     Traffic Management (TM)

   The TM working group has recently been concerned with divorcing
   strict traffic classes from negotiated QoS parameters, and the
   introduction of the Available Bit Rate (ABR) class.  The group is at
   the point where new work items are being defined, but it is plain
   that future work in the areas of clarification of QoS objectives and
   point-to- multipoint QoS is warranted.  These are areas in which the
   developing IntServ ideas can have influence. In addition, it would be
   helpful for the TM group to provide examples of how IntServ QoS
   objectives can be obtained with TM models.

3.3  Other Organizations

3.3.1     IEEE

   IEEE 802.1p and 802.9 have proposed various services that may be
   useful in running the IntServ models over IEEE- specified

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   subnetworks.  To the extent that these have an impact on ATM, they
   should also be reflected in the layering of IntServ models on ATM.
   However, we recommend that IEEE considerations not further complicate
   these issues beyond liaison activities.

3.3.2     WinSock2 Forum

   WinSock2 is an API for Windows platforms that can specify QoS
   parameters independent of the network transport.  Obviously, the work
   of the groups above should be input to WinSock2 so the APIs match the
   services available.

4.   Summary

   In this paper, we have suggested a preliminary plan of action for
   addressing the issues in layering the Integrated Services models and
   RSVP onto an ATM subnetwork.  This problem is complicated by the fact
   that multiple IETF working groups and multiple organizations are
   involved and by the fact that ATM technology, the Integrated Services
   models, and RSVP are evolving over time.  Our objective is to see the
   work go forward openly and with a minimum of both duplicated effort
   and friction.

5.   References

   [1] R. Braden, L. Zhang, S. Berson, S. Herzog, S. Jamin.  Resource
   ReSerVation Protocol (RSVP) -- Version 1 Functional Specification.
   Internet Draft, draft-ietf-rsvp-spec-09. February 1996.

   [2] M. Borden, E. Crawley, B. Davie, S. Batsell. Integration of
   Real-time Services in an IP-ATM Network Architecture.  Request for
   Comments (Informational) RFC 1821, August 1995.

   [3] R. Cole, D. Shur, C. Villamizar. IP over ATM: A Framework
   Document.  Internet Draft, draft-ietf-ipatm- framework-doc-07,
   February 1996.

   [4] D. Katz, D. Piscitello, B. Cole, J. Luciani. NBMA Next Hop
   Resolution Protocol (NHRP).  Internet Draft,
   draft-ietf-rolc-nhrp-07.txt, December 1995.

   [5] G. Armitage, Support for Multicast over UNI 3.0/3.1 based ATM
   Networks. Internet Draft, draft-ietf-ipatm-ipmc-11.txt. February

   [6] S. Shenker, C. Partridge. Specification of Guaranteed Quality of
   Service. Internet Draft, draft-ietf-intserv- guaranteed-svc-03.txt,
   November 1995.

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   [7] J. Wroclawski. Specification of the Controlled-Load Network
   Element Service. Internet Draft,
   draft-ietf-intserv-ctrl-load-svc-01.txt, November 1995.

   [8] ATM Forum. ATM User-Network Interface Specification Version
   3.0. Prentice Hall, September 1993

   [9] ATM Forum. ATM User Network Interface (UNI) Specification Version
   3.1. Prentice Hall, June 1995.

   [10] A. Demirtjis, S. Berson, B. Edwards, M. P. Maher, B.  Braden,
   A. Mankin. RSVP and ATM Signalling, ATM Forum Contribution 96-0258,
   January 1996.

   [11] M. Borden, K. Faulkner, E. Stern. Support for RSVP in ATM
   Networks, ATM Forum Contribution 96-0039, February 1996.

   [12] F. Baker, A. Lin, Y. Rekhter, Support for RSVP and IP Integrated
   Services over ATM, ATM Forum Contribution 96- 0258, January 1996.

   [13] M. Laubach, Classical IP and ARP over ATM. Request for Comments
   (Proposed Standard) RFC1577, January 1994.

   [14] M. Perez, A. Mankin, E. Hoffman, G. Grossman, A. Malis, ATM
   Signalling Support for IP over ATM, Request for Comments (Proposed
   Standard) RFC1755, February 1995.

6.   Author's Address

   Marty Borden
   Eric S. Crawley
   John J. Krawczyk
   Bay Networks
   3 Federal Street
   Billerica, Ma 01821
   +1 508 670-8888

   Fred Baker
   Cisco Systems
   519 Lado Drive
   Santa Barbara, California 93111
   +1 805 681-0115

   Steve Berson
   USC Information Sciences Institute
   4676 Admiralty Way
   Marina del Rey, CA 90292
   +1 310 822-1511

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