[Search] [txt|pdf|bibtex] [Tracker] [WG] [Email] [Nits]

Versions: 00 01 02 03 04 05 06 rfc2381                                  
INTERNET-DRAFT                                        Marty Borden,
                                                      Bay Networks,
                                                      Mark W. Garrett,
                                                      Bellcore.
                                                      September, 1996.


   Interoperation of Controlled-Load and Guaranteed-Service with ATM
                <draft-ietf-issll-atm-mapping-00.txt>


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
   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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


Abstract

   Service mappings are an important aspect of effective interoperation
   between Internet Integrated Services and ATM networks.  Both Internet
   and ATM technologies have well-defined service architectures.  In
   each, a small number of services are identified, with behavioral
   descriptions and related parameters to quantify network traffic and
   Quality of Service (QoS).

   This draft provides mappings between the services of each technology,
   in order to facilitate effective end-to-end Quality of Service in the
   case where ATM subnetwork technology occurs in the path between
   Internet end systems.  Specifically, it identifies the types of ATM
   Virtual Circuits (VCs) which can be utilized for each of the two
   currently defined IP services.  A detailed discussion is given of the
   accompanying parameters and options, and the interactions between the
   two models.  Some of the text may be considered preliminary
   discussion and is expected to be refined as this draft evolves into a



Borden, Garrett           Expires March, 1997                   [Page 1]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   proper specification.

1.0 Introduction

   We consider the problem of providing IP Integrated Services [RFC1633]
   with an ATM subnetwork.  We assume the use of the rsvp protocol
   [RSVP] for IP-level resource reservation.  In the ATM network, we
   consider ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [UNI3.0,
   UNI3.1, UNI4.0].  The latter uses the more complete service model of
   The ATM Forum's TM 4.0 specification [TM40, ATMsvc].

   This is a complex problem with many facets.  In this draft, we focus
   on the service types, parameters and signalling elements needed for
   service interoperation.  The resulting service mappings can be used
   to provide effective end-to-end Quality of Service (QoS) for IP
   traffic that traverses ATM networks.

   The IP services considered are Guaranteed Service [GS] and Controlled
   Load Service [CLS].  We also treat the default Best Effort Service
   (BE) in parallel with these.  Our recommendations for BE are intended
   to be consistent with RFC 1755 [RFC1755], which defines how ATM VCs
   can be used in support of normal BE IP service.  The ATM services we
   consider are:

       CBR           Constant Bit Rate
       rtVBR         Real-time Variable Bit Rate
       nrtVBR        Non-real-time Variable Bit Rate
       UBR           Unspecified Bit Rate
       ABR           Available Bit Rate

   In the case of UNI 3.0 and 3.1 signaling, where these service are not
   all clearly distinguishable, we identify equivalent services where
   possible.

   The service mappings which follow most naturally from their
   definitions are as follows:

       Guaranteed Service    ->     CBR or rtVBR
       Controlled Load       ->     nrtVBR or ABR (with a minimum cell rate)
       Best Effort           ->     UBR or ABR

   However, for completeness we provide detailed mappings for all
   service combinations and identify how each meets or fails to meet the
   requirements of the higher level IP services.  A number of details,
   such as treatment of packets in excess of the flow traffic
   descriptor, make service mapping a complicated subject, which cannot
   be expressed briefly and accurately at the same time.




Borden, Garrett           Expires March, 1997                   [Page 2]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   The remainder of this introduction provides a general discussion of
   the system configuration and other assumptions.  Section 2 considers
   the relevant ATM protocol elements and their effects as related to
   Guaranteed, Controlled Load and Best Effort services (the latter
   being the default "service").  Section 3 discusses a number of
   important features of the IP services and how they can be handled on
   an ATM subnetwork.  Section 4 gives detailed VC setup parameters for
   Guaranteed Service, and considers the effect of using each of the ATM
   service categories.  Section 5 provides a similar treatment for
   Controlled Load Service.  Section 6 considers Best Effort service.

   This document is only a part of the total solution to providing the
   interworking of IP integrated services with ATM subnetworks.  We do
   not consider the important issues of when ATM VCs should be created
   or destroyed, how they should be used or coordinated, or of how
   routing -- QoS sensitive or not -- interacts with the use of VCs,
   especially in the case of multicast (or point-to-multipoint) flows.

1.1 General System Architecture

   The network architecture we consider is illustrated in Figure 1,
   below.  An IP-attached host may send unicast datagrams to another
   host, or may use an IP multicast address to send packets to all of
   the hosts which have "joined" the multicast "tree".  In either case,
   any destination host may use RSVP to establish resource reservation
   in routers along the internet path for the data flow.

   An ATM network lies in the path (chosen by the IP routing), and
   consists of one or many ATM switches.  It uses VCs to provide both
   resources and QoS within the ATM cloud.  These connections are set
   up, added to (in the case of multipoint trees), torn down, and
   controlled by the edge devices, which act as both IP routers and ATM
   interfaces, capable of initiating and managing VCs across the ATM
   user-to-network (UNI) interface.  The edge devices are assumed to be
   fully functional in both the IP int-serv/RSVP protocols and the ATM
   UNI protocols, as well as translating between them.


                                  ATM Cloud
                              ------------------
        H ---\               (                  )          /------- H
        H ---- R -- R -- E --( ATM SW -- ATM SW ) -- E -- R -- R -- H
        H ---/      |        (                  )         \
                    |         ------------------           ------- H
        H ----------R

              Figure 1:  Network Architecture with hosts (H),
                         Routers (R) and Edge Devices (E).



Borden, Garrett           Expires March, 1997                   [Page 3]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996




   The edge devices may be considered part of the IP internet or part of
   the ATM cloud, or both.  This is not an issue since they must provide
   capabilities of both environments.  The edge devices have normal RSVP
   capability to process RSVP messages, reserve resources, and maintain
   soft state (in the control path), and to classify and schedule
   packets (in the data path).  They also have the normal ATM
   capabilities to initiate connections by signaling, and to accept or
   refuse connections signaled to them.  They police and schedule cells
   going into the ATM cloud.  An IP-level reservation (RESV message)
   triggers the edge device to translate the RSVP service requirements
   into ATM VC (UNI) semantics.

   A range of VC management policies are possible, which determine
   whether a flow should initiate a new VC or join an existing one.  VCs
   are managed according to a combination of standards and local policy
   rules, which are specific to either the implementation (equipment) or
   the operator (network service provider).  Point-to-multipoint
   connections within the ATM cloud can be used to support general IP
   multicast flows.  In ATM, a point to multipoint connection can be
   controlled by the source (or root) node, or a leaf initiated join
   (LIJ) feature in ATM may be used.  (Note, a longer section on VC
   management will be written, either in as part of this draft or
   another one from the issll working group at some point.

   Figure 2 shows the functions of an edge device, summarizing the work
   not part of IP or ATM abstractly as an InterWorking Function (IWF),
   and segregating the control and data planes.  (Note: for expositional
   convenience, policy control and other control functions are included
   as part of the admission control in the diagram.)




















Borden, Garrett           Expires March, 1997                   [Page 4]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996




         IP                                                ATM
                               ____________________
                              |        IWF         |
                              |                    |
         admission       <--> | service mapping    | <-->  ATM
         control              | VC management      |       signalling &
                              | address resolution |       admission
                              |....................|       control
                              |                    |
         classification/      | ATM Adaption Layer |       cell
         policing &      <--> | Segmentation and   | <-->  scheduling/
         scheduling           |  Reassembly        |       shaping
                              | Buffering          |
                               ____________________

                 Figure 2: Edge Device Functions showing the IWF



   In the logical view of Figure 2, some functions, such as scheduling,
   are shown separately, since these functions are required of both the
   IP and ATM sides.  However it may be possible in an integrated
   implementation to combine such functions.

   It is not possible to completely separate the service mapping and VC
   management functions.  Several illustrative examples come to mind:
   (i) Multiple integrated-services flows may be aggregated to use one
   point-to-multipoint VC.  In this case, we assume the IP flows are of
   the same service type and their parameters have been merged
   appropriately.  (ii) The VC management function may choose to
   allocate extra resources in anticipation of further reservations or
   based on an empiric of changing TSpecs.  In this case we can assume
   that the additional resources are still specifiable in the form of a
   TSpec, which can be mapped using the same algorithm.  (iii) There
   must exist a path for best effort flows and for sending the rsvp
   control messages.  How this interacts with the establishment of VCs
   for QoS traffic may alter the characteristics required of those VCs.

   Therefore, in discussing the service-mapping problem, we will assume
   that the VC management function of the IWF can always express its
   result in terms of an IP-level service with some QoS and TSpec.  The
   service mapping algorithm, which is the subject of this draft, can
   then identify the appropriate VC parameters, whether the resulting
   action is initiation of a new VC, the addition/deletion of a leaf to
   an existing multipoint tree, or the modification of an existing VC to
   one of another description.



Borden, Garrett           Expires March, 1997                   [Page 5]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


1.2 Related documents

   Earlier ATM Forum documents were called UNI 3.0 and UNI 3.1.  The 3.1
   release was used to correct errors and fix alignment with the ITU.
   Unfortunately UNI 3.0 and 3.1 are incompatible.  However this is in
   terms of actual codepoints, not semantics.  Therefore, descriptions
   of parameter values can generally be used for both.

   After 3.1, the ATM Forum decided to release documents separately for
   each technical subcommittee.  The Traffic Management and Signalling
   4.0 documents are available publically at ftp.atmforum.com/pub.  We
   refer to the combination of traffic management and signalling as
   TM/UNI 4.0, although specific references may be made to the TM 4.0
   specification or the UNI SIG 4.0 specification.

   Within the IETF area, related material includes:

       RSVP functional specification,
       Guaranteed Service specification,
       Controlled Load service specification,
       Int-serv data encoding specification,
       RFC 1577,
       RFC 1755,
       RFC 1821,
       draft-crawley-rsvp-over-atm,
       draft-birman-ipatm-rsvpatm,
       draft-onvural-srinivasan-rsvp-atm.


1.3 Abbreviations


       ABR           Available Bit Rate
       BCOB          Broadband Connection-Oriented Bearer Capability
       BCOB-{A,C,X}  Bearer Class A, C, or X
       BE            Best Effort
       BT            Burst Tolerance
       CBR           Constant Bit Rate
       CDV           Cell Delay Variation
       CDVT          Cell Delay Variation Tolerance
       CLS           Controlled Load Service
       CLP           Cell Loss Priority (bit)
       CLR           Cell Loss Ratio
       CTD           Cell Transfer Delay
       GS            Guaranteed Service
       IWF           Interworking Function
       MBS           Maximum Burst Size
       MCR           Minimum Cell Rate



Borden, Garrett           Expires March, 1997                   [Page 6]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


       PCR           Peak Cell Rate
       SCR           Sustained Cell Rate
       UBR           Unspecified Bit Rate
       VBR           Variable Bit Rate
       nrtVBR        Non-real-time VBR
       rtVBR         Real-time VBR

2.0 Discussion of Relevant ATM Protocol Features


   In this section, we discuss each of the items that must be specified
   in the setup of an ATM VC.  For each of these we discuss which
   specified items and values may be most appropriate for each of the
   integrated services.

   The ATM Call Setup is sent by the edge device to the ATM network to
   establish end-to-end [ATM] service.   This setup contains the
   following information.

       Service Category/Broadband Bearer Capability
       AAL Parameters
       Broadband Low Layer Information
       Calling and Called Party Addressing Information
       Traffic Descriptors
       QoS Parameters
       Additional Parameters of TM/UNI 4.0

   We will discuss each of these, except addressing information, as they
   relate to the translation of GS and CLS to ATM services.  Following
   the discussion of the service categories, we discuss the tagging and
   conformance definitions for IP and ATM, since the policing method is
   implicit in the call setup.  We then continue with mappings of the
   other parameters and information elements.


2.1 Service Category and Bearer Capability


   The highest level of abstraction distinguishing features of ATM VCs
   is in the service category or bearer capability.  Service categories
   were introduced in TM/UNI 4.0; previously the bearer capability was
   used to discriminate at this level.

   In each version of the ATM specifications, these indicate the general
   properties required of a VC: whether there is a real-time delay
   constraint, whether the traffic is constant or variable rate, the
   applicable traffic and QoS description parameters and (implicitly)
   the complexity of some supporting switch mechanisms.



Borden, Garrett           Expires March, 1997                   [Page 7]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   For UNI 3.0 and UNI 3.1, there are only two distinct options for
   bearer capabilities (in our context):

       BCOB-A:  constant rate, timing required, unicast/multipoint;
       BCOB-C:  variable rate, timing not required, unicast/multipoint.

   There is a third capability, BCOB-X, but in the case of AAL5 (which
   we require -- see below) it can be used interchangeably and
   consistently with the above two capabilities.

   In TM/UNI 4.0 the service categories are:

       Constant Bit Rate (CBR)
       Real-time Variable Bit Rate (rtVBR)
       Non-real-time Variable Bit Rate (nrtVBR)
       Unspecified Bit Rate (UBR)
       Available Bit Rate (ABR)

   The first two of these are real-time services, so that rtVBR is new
   to TM/UNI 4.0.  The ABR service is also new to TM/UNI 4.0.  UBR
   exists in all specifications, except perhaps in name, through the
   ``best effort'' indication flag and/or the QoS Class 0.

   The encoding used in 4.0 is consistent with the earlier versions.
   For example, the Service Category is indicated solely by the
   combination of the Bearer Capabilty and the Best Effort indication
   flag.

   In principle, it is possible to support any forseeable service
   through the use of BCOB-A/CBR.  This is because the CBR service is
   equivalent to having a ``pipe'' with specified bandwidth/timing.
   However, it may be desirable to make better use of the ATM network's
   resources by using other, less demanding, services when available.
   (See RFC 1821 for a discussion of this.)


   2.1.1 Service Categories for Guaranteed Service

   There are two possible mappings for GS:

       CBR (BCOB-A)
       rtVBR

   GS requires real-time support, that is, timing is required.  Thus in
   UNI 3.x, the bearer class BCOB-A (or an equivalent BCOB-X
   formulation) must be used.  In TM/UNI 4.0 either of CBR or rtVBR is
   appropriate, the latter allowing the network to possibly take
   advantage of the statistical multiplexing gain of variable rate flows



Borden, Garrett           Expires March, 1997                   [Page 8]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   and to use tagging (see section 2.2).

   Neither the BCOB-C bearer class, nor nrtVBR, UBR, ABR are matches for
   the GS service.  These provide no delay estimates and one cannot
   expect low, predictable, or consistent delays.

   Specification of BCOB-A or CBR requires specification of a PCR.  The
   PCR should be specified as the the token bucket rate parameter, with
   appropriate conversion from bytes to cells (accounting for overhead),
   of the GS TSpec.  For both of these, the network provides a nominal
   clearing rate of PCR with jitter toleration (bucket size) CDVT,
   specified in a network specific manner (see below).

   Specification of rtVBR requires the specification of two rates, SCR
   and PCR.  This models bursty traffic with specified peak and average
   rates.  With rtVBR, it is appropriate to map the PCR to the line rate
   of incoming traffic and the SCR to the GS TSpec bucket rate.  The ATM
   bucket sizes are CDVT, in a network specific manner, and CDVT+BT,
   respectively for the PCR and SCR parameters (see below).


   2.1.2 Service Categories for Controlled Load

   There are three possible mappings for CLS:

       CBR (BCOB-A)
       ABR
       nrtVBR (BCOB-C)

   Note that under UNI 3.x, only the first and third choices are
   applicable.   The first, with a CBR/BCOB-A connection, provides a
   higher level of QoS than is necessary, but it may be convenient to
   simply allocate a fixed-rate ``pipe'', which should be ubiquitously
   supported in ATM networks.  However unless this is the only choice
   available, this will probably be wasteful of network resources.

   The ABR category with a positive MCR aligns with the CLS idea of
   ``best effort with floor.''  The ATM network agrees to forward cells
   with a rate of at least MCR, which should be directly converted from
   the token bucket rate of the TSpec.  The bucket size parameter
   measures approximately the amount of buffer required at the IWF.

   The nrtVBR/BCOB-C category can also be used. It does introduce some
   unaligned complexity in the conformance definition (see section 2.2)
   by the use of two leaky buckets.  The CLS rate parameter would
   correspond to the SCR, while the PCR should be set to the line rate,
   as for Guaranteed Service.




Borden, Garrett           Expires March, 1997                   [Page 9]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   The remaining service categories are inappropriate for CLS.  The
   rtVBR category adds complexity without providing useful features:
   there is no need for tightly constrained delays, and the double-rate
   traffic description is not needed.  The UBR category does not provide
   enough capability for Controlled Load.  The point of CLS is to allow
   an allocation of resources, which is facilitated by the token bucket
   traffic descriptor, and is unavailable in UBR.


   2.1.3 Service Categories for Best Effort

   Any of the service categories has the capability to carry Best Effort
   service, but the natural service category is UBR (or, in UNI 3.x,
   BCOB-C or BCOB-X, with the best effort indicator flag).  A CBR or
   rtVBR clearly could be used, and since the service is not real-time,
   a nrtVBR connection could also be used.  In these cases the rate
   parameter used reflects a bandwidth allocation in support of the edge
   device's best effort connectivity to the far edge router.  It would
   be normal for many flows to be aggregated on this connection; indeed,
   since Best Effort is the default IP behavior, the individual flows
   are not necessarily identified or accounted for.  CBR may be a
   preferred solution in the case where best effort traffic is
   sufficiently highly aggregated that a simple fixed-rate pipe is
   efficient.  An ABR connection could similarly be used to support Best
   Effort traffic.  This is the purpose for which ABR was specifically
   designed.  It is conceivable that a separate ABR connection would be
   made for different IP flows, although the normal case would probably
   have all IP Best Effort traffic with a common exit router sharing a
   single ABR connection.


2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions


   An ATM header carries the Cell Loss Priority (CLP) bit.  Cells with
   bit CLP=1 are said to have been tagged and have lower priority. This
   tagging may have been done by the source or an upstream switch.
   Options involving the use of tagging are decided at call setup time.

   A Conformance Definition is a rule that determines whether a cell is
   conforming to the traffic descriptor of the VC.  The conformance
   definition is given in terms of a Generic Cell Rate Algorithm (GCRA),
   also known as a "leaky bucket" algorithm, for CBR and VBR services.
   (UBR and ABR have network-specific conformance definitions.  Note,
   the term "compliance" in ATM is used to describe the behavior of a
   connection.)

   The network may tag cells which are non-conforming, rather than



Borden, Garrett           Expires March, 1997                  [Page 10]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   dropping them only if the VC is set up to request tagging and the
   network supports the tagging option.  When congestion occurs, a
   switch must attempt to discard tagged cells in preference to the
   discarding of CLP=0 cells.  However, the mechanism for doing this is
   completely implementation specific.   Tagged cells are treated with a
   behavior which is Best Effort in the sense that they are transported
   when bandwidth is available, queued when buffers are available, and
   dropped when the resources are overcommitted.

   Since GS and CLS services require excess traffic to be treated as
   Best Effort, the tagging option should always be chosen (if
   supported) in the VC setup as a means of ``downgrading''
   nonconformant cells.  However, we wish to point out that the term
   ``best effort'' seems to be used with two distinguishable meanings in
   the int-serv specs.  The first interpretation is that of a service
   class that, in some typical scheduler implementations, would
   correspond to a separate queue.  Placing excess traffic in best
   effort in this sense would be giving it lower delay priority.  The
   other sense is more generic, meaning that the network would make a
   best effort to transport the traffic.  A reasonable expectation is
   that a network with no contending traffic would transport the packet,
   while a very congested network would drop the packet.  A packet that
   could be tagged with lower loss priority (such as the ATM CLP bit)
   would be more likely to be dropped, but would not normally be
   transported out of order with respect to the conforming portion of
   the flow.  Such a mechanism would agree with the latter definition of
   best effort, but not the former.

   In TM/UNI 4.0 tagging does not apply to the CBR or ABR services.
   However, there are three conformance definitions of VBR service (for
   both rtVBR and nrtVBR) to consider.  In VBR, only the conformance
   definition VBR.3 supports tagging and applies the GCRA with PCR to
   the aggregate CLP=0+1 cells, and another GCRA with SCR to the CLP=0
   cells.  Thus this conformance definition should always be used in
   support of IP integrated services.  For UBR service, conformance
   definition UBR.2 supports the use of tagging, but a CLP=1 cell does
   not imply non-conformance; it may be a hint of network congestion.

   Once an ATM connection is established, the use of the conformance
   definition and resulting policing action is mandatory.  Since the
   conformance algorithm operates on cells, when mapping rates and
   bucket sizes from IP services to corresponding ATM parameters, a
   correction needs to be made (at call setup time) for the ATM
   segmentation overhead.  Unfortunately this overhead, as a ratio,
   depends on packet length, with the overhead largest for small
   packets.  Thus the appropriate correction could be based on minimum
   packet size, expected packet size, or otherwise in a network specific
   manner, determined at the edge device IWF.



Borden, Garrett           Expires March, 1997                  [Page 11]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


2.3 ATM Adaptation Layer


   The AAL type 5 encoding must be used, as specified in RFC 1483 and
   RFC 1755. AAL5 requires specification of the maximum SDU size in both
   the forward and reverse directions. Both GS and CLS specify a maximum
   packet size as part of the TSpec and this value shall be used as the
   maximum SDU in each direction for unicast connections, but only in
   one direction for point-to-multipoint connections, which are
   unidirectional.  When more than one flow aggregated into a single VC,
   the TSpecs are merged to yield the largest packet size.  In no case
   can this exceed 65535 (or, of course, the MTU of the link).


2.4 Broadband Low Layer Information


   The B-LLI Information Element is transferred transparently by the ATM
   network between the edge devices and is used to specify the
   encapsulation method.  Multiple B-LLI IEs may be sent as part of
   negotiation.  The default encapsulation LLC/SNAP must be supported as
   specified in RFC 1577 and RFC 1755.  Additional encapsulations are
   discussed in RFC 1755 and we refer to the discussion there.


2.5 Traffic Descriptor


   The ATM traffic descriptor always contains specification of a peak
   cell rate (PCR) (in each direction). For variable rate services it
   also contains specification of a sustainable cell rate (SCR) and
   maximum burst size (MBS).

   The Best Effort indicators and Tagging indicators are also part of
   the traffic descriptors in the signalling sense.  In UNI SIG 4.0
   there is an additional parameter, the Frame Discard indicator in the
   traffic descriptor.  The latter is used to indicate the request that
   if a cell is to be dropped, then all subsequent cells of a frame be
   dropped up to the End of Message (EOM) cell (AAL 5); see section 2.7.

   In ATM UNI SIG 4.0 there are also the notions of Alternative Traffic
   Descriptors and Minimal Traffic Descriptors.  Alternative Traffic
   Descriptors enumerate other acceptable choices for traffic
   descriptors and do not seem to be relevant here.  Minimal Traffic
   Descriptors are used in ``negotiation,'' a term which when
   interpreted colloquially will lead to confusion.  Very roughly it
   works like this, e.g., for PCR. A minimal PCR and a requested PCR are
   signalled, the requested PCR being the usual item signalled, and the



Borden, Garrett           Expires March, 1997                  [Page 12]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   minimal PCR being the absolute minimum that the source edge device
   will accept. When sensing the existence of both minimal and requested
   parameters, intermediate switches along the path may reduce the
   requested PCR to a comfortable level.  If at any point the requested
   PCR falls below the minimal PCR then the call is cleared. This is a
   very rough sketch, but we do see potential to make use of Minimal
   Traffic Descriptors in future versions of this draft in order to
   present an acceptable range for parameters and have higher liklihood
   of call admission.  Minimal Traffic Descriptors are not explored
   further in this version of the draft.

   The Traffic Management viewpoint, which we examine next, is more
   concerned with the value of the PCR, SCR and MBS parameters after
   call setup.

   PCR and CDVT are used in the CBR and VBR conformance definitions as
   parameters for a leaky bucket.  However CDVT is not signalled and is
   determined by the network operator as a measure of the ``clumping''
   done by the network.  This makes it difficult to map any leaky bucket
   description of a TSpec to the PCR-CDVT leaky bucket.  Additional
   buffering will be needed at the IWF to account for the depth of the
   bucket.

   The SCR and MBS are used with the VBR services.  They are used in an
   implementation specific manner to allocate resources.  The Burst
   Tolerance (BT) is derived from MBS (see TM 4.0) to be used in a
   second SCR-BT leaky bucket.  Since both parameters are available to
   be signalled, this leaky bucket has the potential to be used in the
   same way as the integrated services bucket.  Note that the
   segmentation overhead and minimum policed unit need to be taken into
   account when translating the bucket parameters.

   For Guaranteed Service there is a bucket rate, r and a service rate,
   R.  The bucket rate describes the traffic, and can be used for
   policing, while the service rate (which cannot be smaller) is the
   allocated service rate.  When mapping Guaranteed Service onto a rtVBR
   VC, the mapping is straightforward.  The bucket rate maps to the SCR
   and the peak rate maps to PCR.  The bucket depth parameter maps to
   MBS.  The minimum policed unit may need to be taken into account when
   translating the leaky bucket parameters.  Note that due to cell
   segmentation, the ATM traffic parameters will increase due to the
   additional headers.  The minimum packet size can be used to identify
   the worst case situation.

   For GS over CBR, the bucket rate can be mapped to the PCR parameter.
   As noted above, the edge device may need to ensure that adequate
   buffering exists at the ATM network ingress to accommodate the TSpec
   bucket depth.  If the available buffering is not sufficient, then a



Borden, Garrett           Expires March, 1997                  [Page 13]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   VC may have to be set up using the IP peak rate parameter mapping to
   PCR.  It is probably inadvisable to try to set the PCR to a value
   between the bucket rate and the peak rate, since such a value would
   depend on assumptions about the statistical properties of the source.

   Controlled Load service has a single bucket rate and corresponding
   depth parameter.  The minimum policed unit and maximum packet size
   play the same roles in mapping parameters as for Guaranteed Service.
   When using nrtVBR, the bucket rate and depth map to SCR and MBS,
   while the PCR parameter can be set to the line rate as a worst case
   value.  For ABR VCs, the bucket rate would be used to set the minimum
   cell rate (MCR) parameter.  The bucket depth parameter does not map
   directly to a signalled ATM parameter, but the edge device should
   check that the buffering at the ATM ingress is sufficient to account
   for the size of bursts allowed by that parameter.  Finally for CBR,
   the bucket rate sets the PCR, and again, the available buffering in
   the edge device must be adequate to accommodate possible bursts.

   For Best Effort service, there is no traffic description.  The UBR
   service category allows negotiation of PCR, simply to identify to the
   source the smallest physical bottleneck along the path.


2.6 QoS Classes and Parameters


   In TM/UNI 4.0 the three QoS parameters may be individually signalled.
   These parameters are the Cell Loss Ratio (CLR), Cell Transfer Delay
   (CTD), and Cell Delay Variation (CDV).  In UNI 3.x the setup message
   includes only the QoS Class, which is essentially an index to a
   network specific table of values for these three parameters.  A
   network provider may choose to associate other parameters, such as
   Severely Errored Cell Block Ratio, but these are less well understood
   and accepted compared to the basic loss, delay and jitter parameters
   mentioned here.  The ITU may include a standard set of parameter
   values for a number (probably four) of QoS classes.  In that case,
   the network provider could define further network-specific QoS
   classes in addition.  The problem of agreement between network
   providers as to the definition of QoS classes is completely
   unaddressed to date.  We will adopt a convention expressed in UNI
   3.x, that assumes that QoS class 1 is appropriate for low-delay,
   low-loss CBR connections, and QoS class 3 is appropriate for variable
   rate connections with loss and delay roughly appropriate for non-
   real-time data applications.

   Since no IP layer counterparts to these ATM QoS parameters exist in
   any of the IP services, they must be set by policy of the edge
   device.  The QoS classes can be chosen relatively easily.  QoS class



Borden, Garrett           Expires March, 1997                  [Page 14]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   1 should be used with Guaranteed Service and QoS class 3 should be
   used with Controlled Load Service.  Best Effort Service always gets
   QoS class 0, which is unspecified QoS by definition.  There are two
   issues which amount to the same thing: First, the choice of
   individually signalled parameter values (under TM/UNI 4.0) for GS and
   CLS is the edge device policy.  The second issue is choosing
   parameter values for the two QoS classes, which is the ATM network
   policy.  If the same network operator controls both, then these
   problems are identical; if not, an agreement to make the values
   identical would be extremely desirable.

   Note that we have mapped QoS class 1 and 3 onto Guaranteed and
   Controlled Load service respectively.  This is regardless of what
   service category is used.  So when running CLS over a CBR pipe, it
   would not be inappropriate to use QoS class 3.  This leaves the delay
   unspecified (or much looser than with QoS 1).  These comments should
   be taken as preliminary, as these issues are far from clear, and
   industry consensus should be sought.


2.7 Additional Parameters -- Frame Discard Mode


   In TM/UNI 4.0 ATM allows the user to choose a mode where a dropped
   cell causes all cells up to the last remaining in the AAL5 PDU to be
   also dropped.  This improves efficiency and the behavior of end-to-
   end protocols such as TCP, since the remaining cells of a damaged PDU
   are useless to the receiver.  For IP over ATM, Frame Discard should
   always be used in both directions, if available, for all services.


3.0 Discussion of Miscellaneous Items


3.1 Units Conversion


   In the integrated services domain, buckets and rates are  measured in
   bytes and bytes/sec, respectively, whereas for ATM, they are measured
   in cells and cells/sec.

   Packets are segmented into 53 byte cells of which the first 5 bytes
   are header information.  For
         B = number of Bytes,
         C = number of cells,
   a rough approximation between the token bucket parameters (rate and
   bucket depth) is
         C = B/48.



Borden, Garrett           Expires March, 1997                  [Page 15]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   This is actually a lower bound on C and does not take into account
   the extra padding at the end of a partially filled cell, or the 8
   byte trailer in the last cell of an AAL5 encoding.  The actual
   relationship between the number of cells and bytes of one packet is
         C = 1 + int(B/48) + x,

         where x = 1 if B mod 48 > 41
                   0 otherwise.
   where int() is the rounding down operation.  The third term is  0 or
   1 and is 1 only when the remainder of B/48 is 41 or more.   (An
   additional cell is needed because the 41 bytes plus 8 byte trailer
   will not fit in a cell.)

   The above formula is not particularly amenable to engineering
   considerations.  By equating the number of bytes before and after
   segmentation we have
         48 C = B + 8 + A,
   where A is the additional padding used in the last 2 cells and has
   the range 0 <= A <= 47.  From this we obtain a number of  useful
   observations.

   For example, if one believes that the packet lengths are uniformly
   distributed mod 48, then on average, 48 C = B + 8 + 47/2, or C = B/48
   + .65625.

   We can also make use of the upper bound on A to state that 48 C <= B
   + 55.  This is true for any one packet.  Considering the number of
   bytes in a stream of P packets, we have
         48 C <= B + 55 P.
   The number of packets P may not be a readily available quantity.
   However, in terms of the minimum policed unit m, we know that P * m
   <= B.  Hence P <= B/m and 48 C <= B ( 1 + 55/m).  That is,
         C <= B/48 * (1 + 55/m).

4.0 Guaranteed Service over ATM


   This section describes how to create ATM VCs appropriately matched
   for Guaranteed Service. The key points differentiating among ATM
   choices are that real-time timing is required, that the data flow may
   have a variable rate, and that demotion of non-conforming traffic to
   best effort is desired.  For this reason, we prefer a rtVBR service
   in which tagging is supported.  Another good match is to use CBR with
   special handling of any non-conforming traffic.

   The encodings assume a point-to-multipoint connection.  For a unicast
   connection, the backward parameters would be equal to the forward
   parameters.



Borden, Garrett           Expires March, 1997                  [Page 16]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


4.1 Encoding GS as a real-time variable bit rate service


   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of TSpec
     Backward CPCS-SDU Size          0
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1             From TSpec peak rate
     Backward PCR CLP=0+1            0
     Forward SCR CLP=0               From TSpec token bucket rate
     Backward SCR CLP=0              0
     Forward MBS (CLP=0)             From TSpec bucket size param
     Backward MBS (CLP=0)            0
     BE indicator                    NOT included
     Forward Frame Discard bit       1                       Note 2
     Backward Frame Discard bit      1                       Note 2
     Tagging Forward bit             1 (Tagging requested)   Note 2
     Tagging Backward bit            0 (No Tagging)          Note 2

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)             Note 3
     ATM Transfer Capability         9                       Note 2
     Traffic Type                    010 (Variable Bit Rate)
     Timing Requirements             01 (Timing Required)
     Susceptible to Clipping         00 (Not susceptible)
     User Plane Configuration        01 (For pt-to-mpt)

   Broadband Low Layer Information
     Layer 2 protocol                12 (ISO 8802/2)
     Layer 3 protocol                204 (ISO/IEC TR 9577)

   QoS Class
     QoS Class Forward               1                       Note 4
     QoS Class Backward              1                       Note 4

   QoS Parameters
     Transit Delay                   100ms                   Notes 2,5
     Forward CLR (CLP=0)             1.0e-6                  Notes 2,5
     Backward CLR (CLP=0)            1.0e-6                  Notes 2,5
     Forward CDV                     30ms                    Notes 2,5
     Backward CDV                    30ms                    Notes 2,5


   Note 1:  Only included for UNI 3.0.



Borden, Garrett           Expires March, 1997                  [Page 17]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   Note 2:  Only included in TM/UNI 4.0.
   Note 3:  Value 1 (BCOB-A) can also be used.
   Note 4:  Optional in TM/UNI 4.0.
   Note 5:  Values chosen to initiate discussion.


4.2 Encoding GS as a constant bit rate service


   It is also possible to support GS using a CBR ``pipe.''   The
   advantage of this is that CBR is probably supported; the disadvantage
   is that data flows may not fill the pipe (utilization loss) and there
   is no tagging option available.


   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of TSpec
     Backward CPCS-SDU Size          parameter M of TSpec
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR 0+1                 From TSpec token bucket rate
     Backward PCR 0+1                0
     BE indicator                    NOT included
     Forward Frame Discard bit       1                       Note 2
     Backward Frame Discard bit      1                       Note 2
     Tagging Forward bit             0 (No Tagging)          Note 2
     Tagging Backward bit            0 (No Tagging)          Note 2

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)             Note 3
     ATM Transfer Capability         7                       Note 2
     Traffic Type                    001 (Constant Bit Rate)
     Timing Requirements             01 (Timing Required)
     Susceptible to Clipping         00 (Not susceptible)
     User Plane Configuration        01 (For pt-to-mpt)

   Broadband Low Layer Information
     Layer 2 protocol                12 (ISO 8802/2)
     Layer 3 protocol                204 (ISO/IEC TR 9577)

   QoS Class
     QoS Class Forward               1                       Note 4
     QoS Class Backward              1                       Note 4

   QoS Parameters



Borden, Garrett           Expires March, 1997                  [Page 18]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


     Transit Delay                   100ms                   Notes 2,5
     Forward CLR (CLP=0)             1.0e-6                  Notes 2,5
     Backward CLR (CLP=0)            1.0e-6                  Notes 2,5
     Forward CDV                     30ms                    Notes 2,5
     Backward CDV                    30ms                    Notes 2,5



   Note 1:  Only included for UNI 3.0.
   Note 2:  Only included in TM/UNI 4.0.
   Note 3:  Value 1 (BCOB-A) can also be used.
   Note 4:  Optional in TM/UNI 4.0.
   Note 5:  Values chosen to initiate discussion.



4.3 Encoding GS as a non-real-time variable bit rate service


   The remaining ATM service categories, including nrtVBR, do not
   provide delay guarantees and cannot be recommended as the best fits.
   However in some circumstances, the best fits may not be available.

   If nrtVBR is used, no hard delay can be given.  However by using a
   variable rate service with low utilization, delay may be
   `reasonable', but not controlled.  The encoding of GS as nrtVBR is
   the same as that for CL using nrtVBR, except that the Forward PCR
   would be derived from the Tspec peak rate.  See section 5.2 below.


4.4 Encoding GS as an ABR service


   The authors feel that this is a very unlikely combination.  The
   objective of the ABR service is to provide `low' loss rates which,
   via flow control, can result in delays.  The introduction of delays
   is contrary to the point of GS.


4.5 Encoding GS as an UBR service


   The UBR service is the default lowest common denominator of the
   services.  It cannot provide delay or loss guarantees.  However if it
   is used for GS, it will be encoded in the same way as Best Effort
   over UBR, with the exception that the PCR would be determined from
   the peak rate of the Tspec.  See section 5.1.




Borden, Garrett           Expires March, 1997                  [Page 19]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


5.0 Controlled Load Service over ATM

   This section describes how to create ATM VCs appropriately matched
   for Controlled Load.  CL traffic is partly delay tolerant and of
   variable rate.  We see nrtVBR and ABR (for TM/UNI 4.0 only) as
   possible choices in supporting CL.

   Generally we prefer to use point-to-multipoint connections.  However
   this is not yet available in ABR. Other than in ABR, the encodings
   assume a point-to-multipoint connection.  For a unicast connection,
   the backward parameters would be equal to the forward parameters.



5.1 Encoding CL using an ABR service


   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of TSpec
     Backward CPCS-SDU Size          parameter M of TSpec
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1             From line rate
     Backward PCR CLP=0+1            From line rate
     Forward MCR CLP 0+1             From TSpec token bucket rate
     Backward MCR CLP 0+1            From TSpec token bucket rate
     BE indicator                    NOT included
     Forward Frame Discard bit       1
     Backward Frame Discard bit      1
     Tagging Forward bit             0 (Tagging not requested)
     Tagging Backward bit            0 (Tagging not requested)

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)              Note 3
     ATM Transfer Capability         12
     Traffic Type                    010 (Variable Bit Rate)
     Timing Requirements             10 (Timing Not Required)
     Susceptible to Clipping         00 (Not susceptible)
     User Plane Configuration        00 (For pt-to-pt)

   Broadband Low Layer Information
     Layer 2 protocol                12 (ISO 8802/2)
     Layer 3 protocol                204 (ISO/IEC TR 9577)

   QoS Class
     QoS Class Forward               3                       Note 4



Borden, Garrett           Expires March, 1997                  [Page 20]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


     QoS Class Backward              3                       Note 4

   ABR Setup Parameters              For Further Study
   ABR Additional Parameters         For Further Study


   Note 3:  Value 3 (BCOB-C) can also be used.
   Note 4:  Optional in TM/UNI 4.0.




5.2 Encoding CL using a non-real-time variable bit rate service


   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of TSpec
     Backward CPCS-SDU Size          0
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1             From line rate
     Backward PCR CLP=0+1            0
     Forward SCR CLP=0               From TSpec token bucket rate
     Backward SCR CLP=0              0
     Forward MBS (CLP=0)             From TSpec bucket size param
     Backward MBS (CLP=0)            0
     BE indicator                    NOT included
     Forward Frame Discard bit       1                       Note 2
     Backward Frame Discard bit      1                       Note 2
     Tagging Forward bit             1 (Tagging requested)   Note 2
     Tagging Backward bit            0 (No Tagging)          Note 2

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)             Note 3
     ATM Transfer Capability         Absent                  Note 2
     Traffic Type                    010 (Variable Bit Rate)
     Timing Requirements             10 (Timing Not Required)
     Susceptible to Clipping         00 (Not susceptible)
     User Plane Configuration        01 (For pt-to-mpt)

   Broadband Low Layer Information
     Layer 2 protocol                12 (ISO 8802/2)
     Layer 3 protocol                204 (ISO/IEC TR 9577)

   QoS Class



Borden, Garrett           Expires March, 1997                  [Page 21]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


     QoS Class Forward               3                       Note 4
     QoS Class Backward              3                       Note 4

   QoS Parameters
     Forward CLR (CLP=0)             1.0e-6                  Notes 2,5
     Backward CLR (CLP=0)            1.0e-6                  Notes 2,5


   Note 1:  Only included for UNI 3.0.
   Note 2:  Only included in TM/UNI 4.0.
   Note 3:  Value 3 (BCOB-C) can also be used.
   Note 4:  Optional in TM/UNI 4.0.
   Note 5:  Values chosen to initiate discussion.



5.3 Encoding CL using a real-time variable bit rate service


   The encoding of CL using rtVBR imposes a hard limit on the delay,
   which is specified as an end-to-end delay in the ATM network.  This
   is more stringent than the CL service specifies and may result in
   less utilization of the network.

   If rtVBR is used to encode CL, then the encoding is essentially the
   same as that for GS.  The exceptions are that the Forward PCR is
   derived from the line rate and probably a different value of the
   transit delay and CDV will be specified.  See section 3.1.


5.4 Encoding CL using a constant bit rate service


   The encoding of CL using CBR is more stringent than using rtVBR since
   it does not take into account the variable rate of the data.
   Consequently there may be even lower utilization of the network.

   To use CBR for CL, the same encoding as in section 3.2 would be used.
   However a different set of values of the QoS parameters will likely
   be used.


5.5 Encoding CL using a UBR service


   This encoding gives no QoS guarantees and would be done in the same
   way as for BE traffic.  See section 5.1.




Borden, Garrett           Expires March, 1997                  [Page 22]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


6.0 Best Effort Service over ATM


   This section describes how to create ATM VCs appropriately matched
   for Best Effort.  The BE service does not need a reservation of
   resources.


5.1 Best Effort Service using UBR

   AAL
     Type                            5
     Forward CPCS-SDU Size           MTU of link
     Backward CPCS-SDU Size          MTU of link
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1             From line rate
     Backward PCR CLP=0+1            0
     BE indicator                    included
     Forward Frame Discard bit       1                       Note 2
     Backward Frame Discard bit      1                       Note 2
     Tagging Forward bit             1 (Tagging requested)   Note 2
     Tagging Backward bit            0 (no tagging)          Note 2

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)
     Traffic Type                    010 (Variable Bit Rate)
     Timing Requirements             10 (Timing not required)
     Susceptible to Clipping         00 (Not susceptible)
     User Plane Configuration        01 (For pt-to-mpt)

   Broadband Low Layer Information
     Layer 2 protocol                12 (ISO 8802/2)
     Layer 3 protocol                204 (ISO/IEC TR 9577)

   QoS Class
     QoS Class Forward               0
     QoS Class Backward              0


   Note 1:  Only included for UNI 3.0.
   Note 2:  Only included in TM/UNI 4.0.







Borden, Garrett           Expires March, 1997                  [Page 23]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


1.  REFERENCES


   [RFC1633]
        R. Braden, D. Clark and S. Shenker, "Integrated Services in the
        Internet Architecture: an Overview", RFC 1633, June 1994.

   [RFC1755]
        M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman and A.
        Malis, "ATM Signlaing Support for IP over ATM", RFC 1755, Febru-
        ary 1995.

   [IPATM]
        M. Perez and A. Mankin, "ATM Signalling Support for IP over ATM
        - UNI 4.0 Update", Internet Draft, June 1996, <draft-ietf-ion-
        sig-uni4.0-oo.txt>

   [RFC1821]
        M. Borden, E. Crawley, B. Davie and S. Batsell, "Integration of
        Real-time Sevices in an IP-ATM Network Architecture", "IP
        Authentication Header", RFC 1821, August 1995.

   [RSVP]R. Braden, L. Zhang, S. Berson, S. Herzog and S. Jamin,
        "Resource ReSevVation Protocol (RSVP) - Version 1 Functional
        Specification", Internet Draft, May 1996, <draft-ietf-rsvp-
        spec-12.txt>

   [GS] S. Shenker, C. Partridge and R. Guerin, "Specification of
        Guaranteed Quality of Service", Internet Draft, August 1996,
        <draft-ietf-intserv-guaranteed-svc-06.txt>

   [CLS]J. Wroclawski, "Specification of the Controlled-Load Network
        Element Service", Internet Draft, August 1996, draft-ietf-
        intserv-ctrl-load-svc-03.txt

   [USE-RSVP-IS]
        J. Wroclawski, "The Use of RSVP with IETF Integrated Services",
        Internet Draft, August 1996, <draft-ietf-intserv-use-00.txt>

   [TEMPLATE]
        S. Shenker and J. Wroclawski, "Network Element Service Specifi-
        cation Template", Internet Draft, November 1995, <draft-ietf-
        intserv-svc-template-02.txt>

   [UNI3.0]
        The ATM Forum, "ATM User-Network Interface Specification, Ver-
        sion 3.0", Prentice Hall, Englewood Cliffs NJ, 1993.




Borden, Garrett           Expires March, 1997                  [Page 24]


INTERNET DRAFT     Interoperation of CLS and GS with ATM September, 1996


   [UNI3.1]
        The ATM Forum, "ATM User-Network Interface Specification, Ver-
        sion 3.1", Prentice Hall, Upper Saddle River NJ, 1995.

   [UNI4.0]
        The ATM Forum, "ATM User-Network Interface (UNI) Signalling
        Specification, Version 4.0", Prentice Hall, Upper Saddle River
        NJ, specification finalized July 1996; expected publication,
        late 1996; available at ftp://ftp.atmforum.com/pub.  The ATM
        Forum, "ATM Traffic Management Specification, Version 4.0",
        Prentice Hall, Upper Saddle River NJ; specification finalized
        April 1996; expected publication, late 1996; available at
        ftp://ftp.atmforum.com/pub.

   [ATMsvc]
        M. W. Garrett, "A Service Architecture for ATM: From Applica-
        tions to Scheduling", IEEE Network Mag., Vol. 10, No. 3, pp. 6-
        14, May 1996.


Acknowledgements


The authors would like to thank the members of the ISSLL working group
for their input. In particular, thanks to Jon Bennett of Fore Systems.


AUTHORS' ADDRESSES

Marty Borden                     Mark W. Garrett
Bay Networks                     Bellcore
3 Federal Street                 445 South Street
Billerica, MA 01821              Morristown, NJ 07960
USA                              USA

phone: +1 508 436-3903           phone: +1 201 829-4439
email: mborden@baynetworks.com   email: mwg@bellcore.com















Borden, Garrett           Expires March, 1997                  [Page 25]