Network Working Group                                        M. Garrett
Request for Comments: 2381                                     Bellcore
Category: Standards Track                                     M. Borden
                                                           Bay Networks
                                                            August 1998

Interoperation of Controlled-Load Service
and Guaranteed Service with ATM

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Abstract

   This document provides guidelines for mapping service classes, and
   traffic management features and parameters between Internet and ATM
   technologies.  The service mappings are useful for providing
   effective interoperation and end-to-end Quality of Service for IP
   Integrated Services networks containing ATM subnetworks.

   The discussion and specifications given here support the IP
   integrated services protocols for Guaranteed Service (GS),
   Controlled-Load Service (CLS) and the ATM Forum UNI specification,
   versions 3.0, 3.1 and 4.0.  Some discussion of IP best effort service
   over ATM is also included.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].  (Note,
   in many cases the use of "MUST" or "REQUIRED" reflects our
   interpretation of the requirements of a related standard, e.g., ATM
   Forum UNI 4.0, rsvp, etc.)

1.0 Introduction ....................................................  3
    1.1 General System Architecture .................................  4
    1.2 Related Documents ...........................................  7
2.0 Major Protocol Features for Traffic Management and QoS ..........  8
    2.1 Service Category and Bearer Capability ......................  8
        2.1.1 Service Categories for Guaranteed Service .............  9
        2.1.2 Service Categories for Controlled Load ................ 10
        2.1.3 Service Categories for Best Effort .................... 11
    2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions . 11
    2.3 ATM Adaptation Layer ........................................ 13
    2.4 Broadband Low Layer Information ............................. 13
    2.5 Traffic Descriptors ......................................... 13
        2.5.1 Translating Traffic Descriptors for Guaranteed Service. 15
        2.5.2 Translating Traffic Descriptors for Controlled Load
              Service  .............................................. 18
        2.5.3 Translating Traffic Descriptors for Best Effort Service 19
    2.6 QoS Classes and Parameters .................................. 19
    2.7 Additional Parameters -- Frame Discard Mode ................. 22
3.0 Additional IP-Integrated Services Protocol Features ............. 22
    3.1 Path Characterization Parameters for IP Integrated Services . 22
    3.2 Handling of Excess Traffic .................................. 24
    3.3 Use of Guaranteed Service Adspec Parameters and Slack Term .. 25
4.0 Miscellaneous Items ............................................. 26
    4.1 Units Conversion ............................................ 26
5.0 Summary of ATM VC Setup Parameters for Guaranteed Service ....... 27
    5.1 Encoding GS Using Real-Time VBR ............................. 28
    5.2 Encoding GS Using CBR ....................................... 29
    5.3 Encoding GS Using Non-Real-Time VBR ......................... 30
    5.4 Encoding GS Using ABR ....................................... 30
    5.5 Encoding GS Using UBR ....................................... 30
    5.6 Encoding GS Using UNI 3.0 and UNI 3.1. ...................... 31
6.0 Summary of ATM VC Setup Parameters for Controlled Load Service .. 32
    6.1 Encoding CLS Using Non-Real-Time VBR ........................ 32
    6.2 Encoding CLS Using ABR ...................................... 33
    6.3 Encoding CLS Using CBR ...................................... 35
    6.4 Encoding CLS Using Real-Time VBR ............................ 35
    6.5 Encoding CLS Using UBR ...................................... 35
    6.6 Encoding CLS Using UNI 3.0 and UNI 3.1. ..................... 35
7.0 Summary of ATM VC Setup Parameters for Best Effort Service ...... 36
    7.1 Encoding Best Effort Service Using UBR ...................... 37
8.0 Security Considerations ......................................... 38
9.0 Acknowledgements ................................................ 38
Appendix 1  Abbreviations ........................................... 39
References .......................................................... 40
Authors' Addresses .................................................. 42
Full Copyright Statement ............................................ 43

1.0 Introduction

   We consider the problem of providing IP Integrated Services [2] with
   an ATM subnetwork.  This document is intended to be consistent with
   the rsvp protocol [3] for IP-level resource reservation, although it
   applies also in the general case where GS and CLS services are
   supported through other mechanisms.  In the ATM network, we consider
   ATM Forum UNI Signaling, versions 3.0, 3.1 and 4.0 [4, 5, 6].  The
   latter uses the more complete service model of the ATM Forum's TM 4.0
   specification [7, 8].

   This is a complex problem with many facets.  In this document, 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) [9] and
   Controlled Load Service (CLS) [10].  We also discuss the default Best
   Effort Service (BE) in parallel with these.  Our recommendations for
   BE are intended to be consistent with RFC 1755 [11], and [12], which
   define 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.x signalling, where these service are not all
   clearly distinguishable, we identify the appropriate available
   services.

   We recommend the following service mappings, since they follow most
   naturally from the service definitions:

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

   For completeness, however, we provide detailed mappings for all
   service combinations in Sections 5, 6, 7 and identify how each meets
   or fails to meet the requirements of the higher level IP services.
   The reason for not restricting mappings to the most obvious or
   natural ones is that we cannot predict how widely available all of
   these services will be as ATM deployment evolves.  A number of

   differences in service definitions, such as the treatment of packets
   in excess of the flow traffic descriptor, make service mapping a
   relatively complicated subject.

   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 the corresponding features of
   Guaranteed, Controlled Load and Best Effort services (the latter
   being the default "service").  Section 3 discusses a number of
   remaining features of the IP services and how they can be handled on
   an ATM subnetwork.  Section 4 addresses the conversion of traffic
   descriptors to account for ATM-layer overheads.  Section 5 gives the
   detailed VC setup parameters for Guaranteed Service, and considers
   the effect of using each of the ATM service categories.  Section 6
   provides a similar treatment for Controlled Load Service.  Section 7
   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.  The
   important issue of VC management, including flow aggregation, is
   considered in [13, 14, 15].  We do not consider how routing, QoS
   sensitive or not, interacts with the use of ATM VCs.  We expect that
   a considerable degree of implementation latitude will exist, even
   within the guidelines presented here.  Many aspects of interworking
   between IP and ATM will depend on economic factors, and will not be
   subject to standardization.

1.1 General System Architecture

   We assume that the reader has a general working knowledge of IP, rsvp
   and ATM protocols.  The network architecture we consider is
   illustrated in Figure 1.  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, a destination host may then 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 more ATM switches.  It uses SVCs 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-( X  --  X  --  X )-E -- R -- R -- H
        H ----/     |      (                 )       \
                    |       -----------------         \------ H
        H ----------R

            Figure 1:  Network Architecture with Hosts (H),
                       Routers (R), Edge Devices (E) and ATM
                       Switches (X).

   When considering the edge devices with respect to traffic flowing
   from source to destination, the upstream edge device is called the
   "ingress" point and the downstream device the "egress" point.  The
   edge devices may be considered part of the IP internet or part of the
   ATM cloud, or both.  They process RSVP messages, reserve resources,
   and maintain soft state (in the control path), and classify and
   schedule packets (in the data path).  They also initiate ATM
   connections by signalling, and accept or refuse connections signalled
   to them.  They police and schedule cells going into the ATM cloud.
   The service mapping function occurs when 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 initiates a new VC or joins 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.  The topic of VC management is
   considered at length in other ISSLL documents [13, 14, 15].

   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.

   IP                                                ATM
                         ____________________
                        |        IWF         |
                        |                    |
   admission and   <--> | service mapping    | <-->  ATM
   policy control       | VC management      |       signalling &
                        | address resolution |       admission
                        |....................|       control
                        |                    |
   classification,      |ATM Adaptation 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 present on both the
   IP and ATM sides.  However it may be possible in an integrated
   implementation to combine such functions.

   The service mapping and VC management functions can be highly
   interdependent.  For example: (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.
   (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 desired characteristics of those
   VCs.  See [13, 14, 15] for further details on VC management.

   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 can then identify the appropriate VC
   parameters as if a new VC were to be created for this service.  The
   VC management function can then use this information consistent with
   its own policy, which determines whether the resulting action uses
   new or existing VCs.

1.2 Related Documents

   Earlier ATM Forum documents combined signalling, traffic management
   and other areas into a single document, e.g., UNI 3.0 [4] and UNI 3.1
   [5].  The 3.1 release was used to correct errors and fix alignment
   with the ITU.  While UNI 3.0 and 3.1 are incompatible in terms of
   actual codepoints, the semantics are generally the same.  Therefore,
   we will often refer to UNI 3.x to mean either version of the ATM
   protocol.

   After 3.1, the ATM Forum released documents separately for each
   technical working group.  The UNI Signalling 4.0 [6] and Traffic
   Management 4.0 [7] documents represent a consistent overall ATM
   protocol, and we will sometime refer to the combination as TM/UNI
   4.0.

   Within the IETF, related material includes the work of the rsvp [3],
   int-serv [2, 9, 10, 16, 17] and ion working groups [11, 12].  Rsvp
   defines the resource reservation protocol (which is analogous to
   signalling in ATM). Int-serv defines the behavior and semantics of
   particular services (analogous to the Traffic Management working
   group in the ATM Forum).  Ion defines interworking of IP and ATM for
   traditional Best Effort service, and generally covers issues related
   to IP/ATM routing and addressing.

   A large number of ATM signalling details are covered in RFC 1755
   [10]; e.g., differences between UNI 3.0 and UNI 3.1, encapsulation,
   frame-relay interworking, etc.  These considerations extend to IP
   over ATM with QoS as well.  The description given in this document of
   IP Best Effort service (i.e. the default behavior) over ATM is
   intended to be consistent with RFC 1755 and it's extension for UNI
   4.0 [11], and those documents are to be considered definitive.  For
   non-best-effort services, certain IP/ATM features will diverge from
   the following RFC 1755.  We have attempted to note such differences
   explicitly.  (For example, best effort VCs may be taken down on
   timeout by either edge device, while QoS VCs are only removed by the
   upstream edge device when the corresponding rsvp reservation is
   deleted.)

   Another related document is RFC 1821 [17], which represents an early
   discussion of issues involved with interoperating IP and ATM
   protocols for integrated services and QoS.

2.0 Major Protocol Features for Traffic Management and QoS

   The ATM Call Setup is sent by the ingress 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 Class and/or Parameters
        Additional Parameters of TM/UNI 4.0

   In this section, we discuss each of these items as they relate to
   creating ATM VCs suitable for GS, CLS and BE services.  We do not
   discuss routing and addressing at all, since they are (at least
   presently) independent of QoS.  Following the section on service
   categories, we discuss tagging and conformance definitions for IP and
   ATM.  These do not appear explicitly as set-up parameters in the
   above list, since they are implied by the policing method used.

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.

   These elements indicate the general properties 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
   (e.g., ABR).

   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.

   A third capability, BCOB-X, can be used as a substitute for the above
   two capabilities, with its dependent parameters (traffic type and
   timing requirement) set appropriately.  The distinction between the
   BCOB-X formulation and the "equivalent" (for our purposes) BCOB-A and
   BCOB-C constructs is whether the ATM network is to provide pure cell
   relay service or interwork with other technologies (with
   interoperable signalling), such as narrowband ISDN.  Where this

   distinction is applicable, the appropriate code SHOULD be used (see
   [5] and related ITU specs, e.g., I.371).

   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, although it is called "best effort" in
   UNI 3.x.  In either case it is indicated by the "best effort"
   indication flag (and the QoS Class set to 0).

   The Service Category in TM/UNI 4.0 is encoded into the same signalled
   Information Element (IE) as the Bearer Capability in UNI 3.x, for the
   purpose of backward compatibilty.  Thus, we use the convention of
   referring to Service Category (CBR, rtVBR, nrtVBR, UBR, ABR) for
   TM/UNI 4.0 (where the bearer capability is implicit).  When we refer
   to the Bearer Capability explicitly (BCOB-A, BCOB-C, BCOB-X), we are
   describing a UNI 3.x signalling message.

   In principle, it is possible to support any service through the use
   of BCOB-A/CBR.  This is because the CBR service is equivalent to
   having a "pipe" of a specified bandwidth.  However, it may be
   significantly more efficient to use the other ATM services where
   applicable and available [17].

2.1.1 Service Categories for Guaranteed Service

   There are two possible mappings for GS:

        CBR (BCOB-A)
        rtVBR

   Real-time support is REQUIRED for GS.  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 CBR or rtVBR is appropriate.  The use of rtVBR may
   encourage recovery of allocated bandwidth left unused by a source.
   It also accommodates more bursty sources with a larger token bucket
   burst parameter, and permits the use of tagging for excess traffic
   (see Section 2.2).

   Neither the BCOB-C Bearer Class, nor nrtVBR, UBR, ABR are good
   matches for the GS service.  These provide no delay estimates and
   cannot guarantee consistently low delay for every packet.

   For BCOB-A or CBR, specification of a peak cell rate (PCR) is
   REQUIRED by ATM standards.  In these cases, PCR is the nominal
   clearing rate with a nominal jitter toleration (bucket size), CDVT.

   When rtVBR is specifed, two rates, PCR and SCR are REQUIRED (by ATM
   standards).  This models bursty traffic with specified peak and
   sustainable rates.  The corresponding ATM token bucket depth values
   are CDVT, and CDVT+BT, respectively.

2.1.2 Service Categories for Controlled Load

   There are three possible good mappings for CLS:

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

   Note that under UNI 3.x, there are equivalent services to CBR and
   nrtVBR, but not ABR.  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 we expect to
   be ubiquitously supported in ATM networks.  However unless this is
   the only choice available, it would probably be wasteful of network
   resources.

   The nrtVBR/BCOB-C category is perhaps the best match, since it
   provides for allocation of bandwidth and buffers with an additional
   peak rate indication, similar to the CLS TSpec.  Excess traffic can
   be handled by CLP bit tagging with VBR.

   The ABR category with a positive MCR aligns with the CLS idea of
   "best effort with a floor."  The ATM network agrees to forward cells
   with a rate of at least MCR, which MUST be directly converted from
   the token bucket rate of the receiver TSpec.  The bucket size
   parameter measures approximately the amount of buffer necessary at
   the IWF.  This buffer serves to absorb the bursts allowed by the
   token bucket, since they cannot be passed directly into an ABR VC.

   The rtVBR category can be used, although the edge device MUST then
   determine values for CTD and CDV.  Since there are no corresponding
   IP-level parameters, their values are set as a matter of local
   policy.

   The UBR category does not provide enough capability for Controlled
   Load.  The point of CLS is to allow an allocation of resources.  This
   is facilitated by the token bucket traffic descriptor, which is
   unavailable with UBR.

2.1.3 Service Categories for Best Effort

   All of the service categories have 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 indication set).  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
   ingress edge device's best effort connectivity to the egress edge
   router.  It would be normal for traffic from many source/destination
   pairs to be aggregated on this connection; indeed, since Best Effort
   is the default IP behavior, the individual flows are not normally
   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.  Both CBR and nrt-VBR provide
   explicit bandwidth allocation which may be useful for billing
   purposes.  In the case of UBR, the network operator SHOULD allocate
   bandwidth for the overall service through the admission control
   function, although such allocation is not done explicitly per VC.

   An ABR connection could similarly be used to support Best Effort
   traffic.  Indeed, the support of data communications protocols such
   as TCP/IP is the explicit purpose for which ABR was designed.  It is
   conceivable that a separate ABR connection would be made for each IP
   flow, although the normal case would probably have all IP Best Effort
   traffic with a common egress router sharing a single ABR connection.

   The rt-VBR service category may be considered less suitable, simply
   because both the real-time delay constraint and the use of SCR/BT add
   unnecessary complexity.

   See specifications from the IETF ion working group [10, 11] for
   related work on support of Best Effort service with ATM.

2.2 Cell Loss Priority Bit, Tagging and Conformance Definitions

   Each ATM cell header carries a Cell Loss Priority (CLP) bit.  Cells
   with CLP=1 are said to be "tagged" or "marked" and have lower
   priority.  This tagging may be done by the source, to indicate
   relative priority within the VC, or by a switch, to indicate traffic
   in violation of policing parameters.  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.
   The conformance definition also specifies rules for tagging traffic
   in excess of the {SCR, MBS} GCRA traffic descriptor.  (Note, the term
   "compliance" in ATM is used to describe the behavior of a connection,
   as opposed to "conformance", which applies to a single cell.)

   The network may tag cells that are non-conforming, rather than
   dropping them if the VC set-up requests tagging and the network
   supports the tagging option.  When tagging is used and congestion
   occurs, a switch MUST attempt to discard tagged cells in preference
   to discarding CLP=0 cells.  However, the mechanism for doing this is
   completely implementation specific.  The behavior that best meets the
   requirements of IP Integrated Services is where tagged cells are
   treated as "best effort" in the sense that they are transported when
   bandwidth is available, queued when buffers are available, and
   dropped when resources are overcommitted.  ATM standards, however, do
   not explicitly specify treatment of tagged traffic.  Providers of GS
   and CLS service with ATM subnetworks SHOULD ascertain the actual
   behavior of ATM implementation with respect to tagged cells.

   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" the cells
   comprising non-conformant packets.  The term "best effort" can be
   interpreted in two ways.  The first is as a service class that, for
   example, may be implemented as a separate queue.  The other sense is
   more generic, meaning that the network makes a best effort to
   transport the traffic.  A reasonable interpretation of this is that a
   network with no contending traffic would transport the packet, while
   a very congested network would drop the packet.  A mechanism that
   tags best effort packets with lower loss priority (such as with the
   ATM CLP bit) would drop some of these packets, but would not reorder
   the remaining ones with respect to the conforming portion of the
   flow.  The "best effort" mechanism for excess traffic does not
   necessarily have to be the same as that for best effort "service", as
   long as it fits this generic sense of best effort.

   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 rate PCR
   to the aggregate CLP=0+1 cells, and another GCRA with rate SCR to the
   CLP=0 cells.  This conformance definition SHOULD always be used with
   a VBR service supporting 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; rather, it may be used by the

   network to indicate congestion.

   In TM/UNI 4.0 tagging is not a feature of the conformance definitions
   for the CBR or ABR service categories.  (Since conformance
   definitions are generally network specific, some implementations CBR
   or ABR may, in fact, use tagging in some way.)  Wherever an ATM
   network does support tagging, in the sense of transporting CLP=1
   cells on a "best effort" basis, it is a useful and preferable
   mechanism for handling excess traffic.

   It is always better for the IWF to tag cells when it can anticipate
   that the ATM network would do so.  This is because the IWF knows the
   IP packet boundaries and can tag all of the cells corresponding to a
   packet.  If left to the ATM layer UPC, the network would inevitably
   drop some of the cells of a packet while carrying others, which would
   then be dropped by the receiver.  Therefore, the IWF, knowing the VC
   GCRA parameters, SHOULD always anticipate the cells which will be
   tagged by the ATM UPC and tag all of the cells uniformly across each
   affected packet.  See Section 3.2 for further discussion of excess
   traffic.

2.3 ATM Adaptation Layer

   The AAL type 5 encoding SHOULD be used, as specified in RFC 1483 and
   RFC 1755.  For AAL-5, specification of the maximum SDU size in both
   the forward and reverse directions is REQUIRED.  Both GS and CLS
   specify a maximum packet size, M, as part of the TSpec and this value
   SHOULD be used (corrected for AAL headers) as the maximum SDU in each
   direction for unicast connections, and for unidirectional point-to-
   multipoint connections.  When multiple flows are aggregated into a
   single VC, the M parameters of the receiver TSpecs are merged
   according to rules given in the GS and CLS specs.

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 LLC/SNAP encapsulation [18] SHOULD be supported as
   the default, but "null" or "VC encapsulation" MAY also be allowed.
   Implementations SHOULD follow RFC 1577 [19] and RFC 1755 [10] for
   BLLI usage.

2.5 Traffic Descriptors

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

   and MBS form a leaky bucket pair (rate, depth), while the bucket
   depth parameter for PCR is CDVT.  Note that CDVT is not signalled
   explicitly, but is determined by the network operator, and can be
   viewed as a measure of the jitter imposed by the network.

   Since CDVT is generally presumed to be small (equivalent to a few
   cells of token bucket depth), and cannot be set independently for
   each connection, it cannot be used to account for the burstiness
   permitted by b of the IP-layer TSpec.  Additional buffering may be
   needed at the IWF to account for the depth of the token bucket.

   The ATM Burst Tolerance (BT) is equivalent to MBS (see TM 4.0 [6] for
   the exact equation).  They are both expressions of the bucket depth
   parameter associated with SCR.  The units of BT are time while the
   units of MBS are cells.  Since both SCR and MBS are signalled, they
   can be computed directly from the IP layer traffic description.  The
   specific manner in which resources are allocated from the traffic
   description is implementation specific.  Note that when translating
   the traffic parameters, the segmentation overhead and minimum policed
   unit need to be taken into account (see Section 4.1 below).

   In ATM UNI Signalling 4.0 there are the notions of Alternative
   Traffic Descriptors and Minimal Traffic Descriptors.  Alternative
   Traffic Descriptors enumerate other acceptable choices for traffic
   descriptors and are not considered here.  Minimal Traffic Descriptors
   are used in "negotiation," which refers to the specific way in which
   an ATM connection is set up.  To illustrate, roughly, taking PCR as
   an example: A minimal PCR and a requested PCR are signalled, the
   requested PCR being the usual item signalled, and the minimal PCR
   being the absolute minimum that the source edge device will accept.
   When both minimal and requested parameters are present, the
   intermediate switches along the path may reduce the requested PCR to
   a "comfortable" level.  This choice is part of admission control, and
   is therefore implementation specific.  If at any point the requested
   PCR falls below the minimal PCR then the call is cleared.  Minimal
   Traffic Descriptors can be used to present an acceptable range for
   parameters and ensure a higher likelihood of call admission.  In
   general, our discussion of connection parameters assumes the values
   resulting from successful connection setup.

   The Best Effort indicator (used only with UBR) and Tagging indicators
   (see Section 2.2) are also part of the signalled information element
   (IE) containing the traffic descriptor.  In the UNI 4.0 traffic
   descriptor IE there is an additional parameter, the Frame Discard
   indicator, which is discussed below in Section 2.7.

2.5.1 Translating Traffic Descriptors for Guaranteed Service

   For Guaranteed Service the source TSpec contains peak rate, rate and
   and bucket depth parameters, p_s, r_s, b_s.  The receiver TSpec
   contains corresponding parameters p_r, r_r, b_r.  The (receiver)
   RSpec also has a rate, R.  The two different TSpec rates are intended
   to support receiver heterogeneity, in the sense that receivers can
   accept different rates representing different subsets of the sender's
   traffic.  Whenever rates from different receivers differ, the values
   MUST always be merged appropriately before being mapping into ATM
   parameters.

   Note that when the sender and receiver TSpec rates r_s, r_r differ,
   there is no mechanism specified (in either rsvp or the int-serv
   specs) for indicating which subset of the traffic is to be
   transported.  Implementation of this feature is therefore completely
   network specific.  The policing and scheduling mechanisms may simply
   be parameterized with the (lower) receiver rate, resulting in the
   random loss of traffic sufficient to make up the difference in rates.

   The receiver TSpec rate describes the traffic for which resources are
   to be reserved, and may be used for policing, while the RSpec rate
   (which cannot be smaller) is used (perhaps in an implementation
   specific way) to modify the allocated service bandwidth in order to
   reduce the delay.

   When mapping Guaranteed Service onto a rtVBR VC, the ATM traffic
   descriptor parameters (PCR, SCR, MBS) can be set cannonically as:

        PCR = p_r
        SCR = R
        MBS = b_r.

   There are a number of conditions that may lead to different choices.
   The following discussion is not intended to set hard requirements,
   but to provide some interpretation and guidance on the bounds of
   possible parameter mappings.  The ingress edge device generally
   includes a buffer preceding the ATM network interface.  This buffer
   can be used to absorb bursts that fall within the IP-level TSpec, but
   not within the ATM traffic descriptor.  The minimal REQUIREMENT for
   guaranteed service is that the delay in this buffer MUST NOT exceed
   b/R, and the delays within the ATM network MUST be accurately
   accounted for in the values of Adspec parameters C and D advertised
   by the ingress router (see Section 3.3 below).

   If either an edge device buffer of size b_r exists or the ATM maximum
   burst size (MBS) parameter is at least b_r, then the various rate
   parameters will generally exhibit the following relationship:

        r_r <= SCR <= R <= PCR <= APB <= line rate

        r_r <=       p_r       <= APB

   APB refers to the General Characterization Parameter,
   AVAILABLE_PATH_BANDWIDTH, which is negotiated in the Adspec portion
   of the PATH message.  APB reflects the narrowest bottleneck rate
   along the path, and so is always no larger than the local line rate.
   The receiver SHOULD choose a peak rate no greater than APB for the
   reservation to be accepted, although the source peak rate, p_s, could
   be higher, as the source does not know the value of APB.  There is no
   advantage to allocating any rate above APB of course, so it is an
   upper bound for all the other parameters.

   We might normally expect to find R <= p_r, as would be necessary for
   the simple mapping of PCR = p_r, SCR = R given above.  However, a
   receiver is free to choose R > p_r to lower the GS delay [8].  In
   this case, PCR cannot be set below R, because a burst of size b
   arriving into the buffer MUST be cleared at rate R to keep the first
   component of GS delay down to b/R.  So here we will have PCR = R.  It
   may seem that PCR = p_r would be sufficient to avoid buffer overflow,
   since data is generated at the source at a rate bounded by p_r.
   However, setting PCR < R, can result in the delay bound advertised by
   C and D not being met.  Also, traffic is always subject to jitter in
   the network, and the arrival rate at a network element can exceed p_r
   for short periods of time.

   In the case R <= p_r, we may still choose PCR such that R <= PCR <
   p_r.  The edge device buffer is then necessary (and sufficient) to
   absorb the bursts (limited to size b_r + C_sum + R D_sum) which
   arrive faster than they depart.  For example, it may be the case that
   the cost of the ATM VC depends on PCR, while the cost of the Internet
   service reservation is not strongly dependent on the IP-level peak
   rate.  The user may then have an incentive to set p_r to APB, while
   the operator of the IP/ATM edge router has an incentive to reduce PCR
   as much as possible.  This may be a realistic concern, since the
   charging models of IP and ATM are historically different as far as
   usage sensitivity, and the value of p_r, if set close to APB, could
   be many times the nominal GS allocated rate of R.  Thus, we can set
   PCR to R, with a buffer of size b_r + C_sum + R D_sum, with no loss
   of traffic, and no violation of the GS delay bound.

   A more subtle, and perhaps controversial case is where we set SCR to
   a value below R.  The major feature of the GS service is to allow a
   receiver to specify the allocated rate R to be larger than the rate
   r_r sufficient to transport the traffic, in order to lower the
   queueing delay (roughly) from b/r + C_TOT/r + D_TOT to b/R + C_TOT/R
   + D_TOT.  To effectively allocate bandwidth R to the flow, we set SCR

   to match R.  (Note it is unnecessary in any case to set SCR above R,
   so the relation, SCR <= R, is still true.)  It is possible to set SCR
   to a value as low as r_r, without violating the delay bounds or
   overflowing the edge device buffer.  With PCR = R, a burst of size b
   will be buffered and sent into the ATM network at rate R, so the last
   byte suffers delay only b/R.  Any further traffic will be limited to
   rate r_r, which is SCR, so with the arriving and departing rates
   matched, its delay will also be no more than b/R.

   While this scenario meets the GS service requirements, the penalty
   for allocating SCR = r_r rather than R is that the delay in the ATM
   network will have a component of MBS/SCR, which will be b/r rather
   than b/R, contained in the D term advertised for the ATM sub-network
   (see further discussion in Section 3.3 below).  It is also true that
   allocating r instead of R in a portion of the path is rather against
   the spirit of GS.  As mentioned above, this mapping may however be
   useful in practice in the case where pricing in the ATM network leads
   to different incentives in the tradeoff between delay and bandwidth
   than those of the user who buys IP integrated services.

   Another point of view on parameter mapping suggests that SCR may
   merely reflect the traffic description, hence SCR = r_r, while the
   service requirement is expressed in the QoS parameter as CDV = b/R.
   Thus the ATM network may internally allocate bandwidth R, but it is
   free to use other methods as well to achieve the delay constraint.
   Mechanisms such as statistical flow/connection aggregation may be
   implemented in the ATM network and hidden from the user (or parameter
   mapping module in the edge router) which sees only the interface
   implemented in the signalled parameters.

   Note that this discussion considers an edge device buffer size of
   b_r.  In practice, it may be necessary for the AAL/segmentation
   module to buffer M bytes in converting packets to cells.  Also an
   additional amount of buffer equal to C_sum + R D_sum is generally
   necessary to absorb jitter imposed by the upstream network [8].

   With ATM, it is possible to have little or no buffer in the edge
   router, because the ATM VC can be set to accept bursts at peak rate.
   This may be unusual, since the edge router normally has enough buffer
   to absorb bursts according to the TSpec token bucket parameters.  We
   consider two cases.  First, if PCR >= p_r, then MBS can be set to b_r
   and no buffering is necessary to absorb non-excessive bursts.  The
   extra buffering needed to absorb jitter can also be transferred to
   MBS.  This effectively moves the buffering across the UNI into the
   ATM network.

   For completeness, we consider an edge router with no burst-absorbing
   buffers and an MBS parameter of approximately zero.  In this case it
   is sufficient to set the rate parameters to PCR = SCR = max (R, p_r).
   This amounts to peak-rate allocation of bandwidth, which will not
   usually be very cost effective.  This case may be relevant where the
   IP routers and ATM switches differ substantially in their buffering
   designs.  IP-level users may typically specify much larger burst
   parameters than can be handled in the ATM subnet.  Peak-rate
   bandwidth allocation provides a means to work around this problem.
   It is also true that intermediate tradeoffs can be formulated, where
   the burst-absorbing buffer is less than b bytes, and SCR is set above
   R and below p_r.  Note that jitter-absorbing buffers (C_sum + R
   D_sum) can not be avoided, generally, by increasing ATM rates, unless
   SCR is set to exceed the physical line rate(s) into the edge device
   for the flow.

   For GS over CBR, the value of PCR may be mapped to the RSpec rate R,
   if the edge device has a buffer of size b_r + C_sum + R D_sum.  With
   little or no burst buffering, the requirements resemble the zero-
   buffer case above, and we have PCR = max (R, p_r).  Additional
   buffers sufficient to absorb network jitter, given by C_sum, D_sum,
   MUST always be provided in the edge router, or in the ATM network via
   MBS.

2.5.2 Translating Traffic Descriptors for Controlled Load Service

   The Controlled Load service TSpec has a peak rate, p, a "token
   bucket" rate, r, and a corresponding token bucket depth parameter, b.
   The receiver TSpec values are used to determine resource allocation,
   and a simple mapping for the nrtVBR service category is given by,

        PCR = p_r
        SCR = r_r
        MBS = b_r.

   The discussions in the preceding section on using edge device buffers
   to reduce PCR and/or MBS apply generally to the CLS over nrtVBR case
   as well.  Extra buffers to accommodate jitter accumulated (beyond the
   b_r burst size allowed at the source) MUST be provided.  For CLS,
   there are no Adspec parameters C and D, so the dimensioning of such
   buffers is an implementation design issue.

   For ABR VCs, the TSpec rate r_r is used to set the minimum cell rate
   (MCR) parameter.  Since there is no corresponding signalled bucket
   depth parameter, the edge device SHOULD have a buffer of at least b_r
   bytes, plus additional buffers to absorb jitter.  With ABR, the ATM
   network can quickly throttle the actual transfer rate down to MCR, so
   a source transmitting above that rate can experience high loss at the

   ingress edge device when the ATM network becomes congested.

   For CBR, the TSpec rate r_r sets a lower bound on PCR, and again, the
   available buffering in the edge device SHOULD be adequate to
   accommodate possible bursts of b_r.

   The REQUIREMENT for CLS that network delays approximate "best-effort
   service under unloaded conditions", is interpreted here to mean that
   it would be sufficient to allocate bandwidth resources so that the
   last byte of a burst of size b_r sees a delay approximately b_r/r_r.
   For example, a network element with no cross-traffic, a work
   conserving scheduler and an output link rate of r_L, might provide
   delays in the range from M/r_L to b_r/r_L, that are much lower than
   b_r/r_r.  While the access to the full link bandwidth (r_L), as
   reflected in this example, is a more literal interpretation of delay
   "under unloaded conditions" for a shared link, an ATM VC may only
   have access to bandwidth equal to its nominal allocation (some
   implementation specific function of SCR and PCR).

2.5.3 Translating Traffic Descriptors for Best Effort Service

   For Best Effort service, there is no traffic description.  The UBR
   service category allows negotiation of PCR simply to allow the source
   to discover the smallest physical bottleneck along the path.  The
   ingress edge router may set PCR to the ATM line rate, and then when
   the VC setup is complete, the returned value indicates an upper bound
   on throughput.  For UBR service, resources may be allocated for the
   overall service (i.e., not per-VC) using the (implementation
   specific) admission control features of the ATM switches.

   Often a service provider will statically configure large VCs with a
   certain bandwidth allocation to handle all best effort traffic
   between two edge routers.  ABR, CBR or nrtVBR VCs are appropriate for
   this design, with traffic parameters set to comfortably accommodate
   the expected traffic load.  See IETF ION specifications for IP over
   ATM signalling [10, 11].

2.6 QoS Classes and Parameters

   In UNI 3.x the quality of service is indicated by a single parameter
   called "QoS Class," which is essentially an index to a network
   specific table of values for the actual QoS parameters.  In TM/UNI
   4.0 three QoS parameters may be individually signalled, and the
   signalled values override those implied by the QoS Class, which is
   still present.  These parameters are the Cell Loss Ratio (CLR), Cell
   Transfer Delay (CTD), and Cell Delay Variation (CDV) [6].

   A network provider may choose to associate other parameters, such as
   Severely Errored Cell Block Ratio, with a QoS Class definition, but
   these cannot be signalled individually.  The ATM Forum UNI 3.0, 3.1
   and TM 4.0 specs, following prior ITU specs, give vague qualitative
   definitions for QoS Classes 1 to 4.  (QoS Class 0 is well-defined as
   "no QoS parameters defined".)  Since our mapping is based on these
   specifications, we generally follow this guidance by setting the QoS
   Class value to 0 for UBR and ABR (as REQUIRED), 1 for CBR and rtVBR
   and 3 for nrtVBR.  Note that the QoS Class follows the ATM service
   category, and not the IP service, to avoid combination that are
   unlikely to be supported.  For example, if only nrtVBR is available
   for GS, then choosing QoS Class = 1 would probably result in
   connection failure.  The QoS Class MUST NOT be interpreted as a way
   to add real-time behavior to an inherently non-real-time service.

   The ITU has included a standard set of parameter values for a (small)
   number of QoS Classes in the latest version of Recommendation I.356
   [21].  Network providers may choose to define further network-
   specific QoS Classes in addition to these.  Note that the QoS class
   definitions in the new I.356 version might not align with the model
   we follow from the ATM Forum UNI specs.  Apart from these
   definitions, there is no consistent agreement on QoS Class
   definitions among providers in practice.

   The ATM QoS parameters have no explicitly signalled IP layer
   counterparts.  The values that are signalled in the ATM network are
   determined by the IP service definitions and knowledge of the
   underlying ATM network characteristics, as explained below.

   The ingress edge router SHOULD keep a table of QoS information for
   the set of egress routers that it may establish VCs with.  This table
   may be simplified by using default values, but it will probably be
   good practice to maintain a table of current data for the most
   popular egress points.  An edge device that initiates VC setup
   generally needs to have some way to propose initial value for CDV and
   CTD, even if they are changed by negotiation; so by positing such a
   table, we are not creating any new design burden.  Cached information
   can be updated when VCs are successfully established, and to the
   extent that IP-layer reservations can wait for VCs to complete, the
   values can be refined through iterated negotiation.

   Both GS and CLS REQUIRE that losses of packets due to congestion be
   minimized, so that the loss rate is approximately the same as for an
   unloaded network.  The characteristic loss behavior of the physical
   medium not due to congestion (e.g., bit errors or fading on wireless
   channels) determines the order of the permitted packet loss rate.
   The ingress edge device MUST choose a value of CLR that provides the
   appropriate IP-level packet loss rate.  The CLR value may be uniform

   over all egress points in the ATM network, or may differ, e.g., when
   wireless or satellite ATM links are in some paths.  The determination
   of CLR MUST account for the effects of packet size distribution and
   ATM Frame Discard mode (which can change the effective packet loss
   rate by orders of magnitude [22]).

   The ingress router will also tabulate values for the Minimum Path
   Latency (MPL) and estimated queueing delays (D_ATM) for each egress
   point.  The latter will be used as part of the Adspec "D" parameter
   for GS, but its use here applies to CLS as well (when the VC setup
   includes delay parameters).  MPL represents all constant (non-
   congestion related) delays, including propagation delay.  D_ATM
   accounts for the variable component of delays in the ATM network.
   (It may depend on non-signalled parameters such as CDVT.)  Given
   these quantities, a new VC can be set up with delay-related QoS
   parameters given by

        CDV = D_ATM
        CTD = D_ATM + MPL.

   (CDV and CTD may be adjusted (increased) by the slack term in GS, see
   Section 3.3 below.)

   It is interesting (and perhaps unfortunate) to note that in a typical
   GS/rtVBR service, the delay bound advertised can contain two
   components of b/R instead of one.  Consider the simple case where SCR
   = R is the rate allocated to the flow in both IP routers and ATM
   switches along the path, and the buffer allocation is MBS = b.
   Parekh's theory [23], which is the basis of the GS delay formula [8]
   states that the b/R delay term occurs only once, because once a burst
   of size b has been served by a congested node at rate R, the packets
   will not arrive at a subsequent node as a single burst.  However, we
   can't tell a priori if this bottleneck will occur in the ATM network
   or elsewhere in the IP network, so the declaration of CDV SHOULD
   account for it (i.e., CDV >= b/R).  Once CDV is set, the ATM network
   can impose this delay, whether or not the traffic arrives in a burst.
   Since the delay b/R can also occur elsewhere, it cannot be removed
   from the first term of the GS delay formula.  The ATM b/R delay
   component appears in the third term of the GS delay formula, D_tot.
   See Section 3.3 below for more on GS Adspec parameters.  This effect
   may be mitigated when the ATM network employs more efficient
   statistical resource allocation schemes.

2.7 Additional Parameters -- Frame Discard Mode

   TM/UNI 4.0 allows the user to choose a mode where the ATM network is
   aware, for the purpose of congestion management, of PDUs larger than
   an ATM cell (i.e., AAL PDUs that correspond in our context to IP
   packets).  This facilitates implementation of algorithms such as
   partial packet discard, where a dropped cell causes subsequent cells
   in the same AAL-5 PDU to be dropped as well.  Several other
   applicable buffer management schemes have been proposed [22, 24].

   Frame discard can improve the efficiency and performance of end-to-
   end protocols such as TCP, since the remaining cells of a damaged PDU
   are generally useless to the receiver.  For IP over ATM, Frame
   Discard MUST always be indicated, if available.

3.0 Additional IP-Integrated Services Protocol Features

3.1 Path Characterization Parameters for IP Integrated Services with ATM

   This section discusses the setting of General Characterization
   Parameters (GCPs) at an ATM egress edge router.  GCPs are signalled
   from IP source to IP destination, and modified by intermediate nodes
   using the Adspec portion of PATH messages in rsvp.  The GS-specific
   Adspec parameters are discussed below in Section 3.3.  These
   parameters are denoted as <x,y> where x is the service and y is the
   parameter number.  Service number 1 indicates default or general
   parameter values.  Please refer to [25] for definitions and details.

   The IS break bit <1,2> MUST, of course, be left alone by
   implementations following these guidelines (as they are presumably
   IS-aware).  Similarly, the router MUST always increment IS_HOPS
   <1,4>.  The GS and CLS service-specific break bits, <2,2> and <5,2>
   respectively, MUST be set if the support of the service is
   inadequate.  In general GS is adequately supported by CBR (BCOB-A)
   and rtVBR service categories, and not adequately supported by UBR,
   ABR and nrtVBR because delays are not controlled.  CLS may be
   adequately supported by all service categories except UBR (or Best
   Effort in UNI 3.x).  See Sections 5, 6 for further discussion.

   For GS, the ATM network MUST meet the delay performance advertised
   through the Adspec parameters, MPL, C, and D.  If it cannot
   predictably meet these requirements, the GS break bit MUST be set.
   Similarly both break bits MUST be set if reservations are honored,
   but sufficient resources to avoid congestion loss are not allocated
   in practice.  If the service break bits are not set, then the
   corresponding service hop counters, <2,4>, <5,4>, MUST be
   incremented.

   The Available Path Bandwidth (APB) parameters <x,6> indicate the
   minimum physical bottleneck rate along the path.  This may be
   discoverable in an ATM network as the negotiated PCR value for any
   UBR VC along the same path.  This value MUST be corrected for AAL,
   ATM and physical-layer headers, as necessary, to reflect the
   effective IP datagram bandwidth.  With ATM, it is possible that there
   is some policy limitation on the value of PCR, below the physical
   link bottleneck.  In this case, the advertised value of APB (in
   general, and for each service if the values of APB signalled are
   service specific) MUST reflect this limit, since excess traffic
   beyond this rate will be dropped.  (Note that there is no tagging of
   traffic in excess of PCR for TM/UNI 4.0.)  These values SHOULD
   generally be cached by the ingress router for the set of egress
   routers with which it typically needs to establish VCs.  The APB
   parameters are only adjusted down, to reflect the minimum as the
   composed value.

   In the case of a multipoint VC, several parameters can be different
   for each egress point, e.g., because the characteristics of the
   physical links of the VC branches differ.  When this occurs, the IWF
   at the egress routers MUST correct these values in PATH messages as
   they exit the ATM network.  (We use the word "correct" because the
   ingress router SHOULD set the parameters to a value that is
   appropriate for the largest number of branches, or a value that would
   do the least harm if the egress routers failed to correct such
   parameters for each branch.)  This is the only case where the egress
   router needs to operate on rsvp control messages.  (A similar
   correction MUST be implemented for any non-rsvp set-up mechanism).
   The parameters for which such correction is REQUIRED are the
   Available Path Bandwidth (APB), the Minimum Path Latency (MPL), the
   Path MTU (although for ATM/AAL-5 this may typically be constant), and
   the ATM-specific components of the GS Adspec parameters C_ATM and
   D_ATM.

   The ingress router table SHOULD store values for the ATM-network MPL
   <x,7> for the various egress points.  The composed values <x,8> are
   formed by addition and forwarded along the path.  In the cases where
   ATM routing chooses different paths, depending on the service
   category, for VCs to a given egress point, the table will generally
   reflect different values for each service.  If the ATM network is
   very large and complex, it may become difficult to predict the routes
   that VCs will take once they are set up.  This could be a significant
   source of misconfiguration, resulting in discrepancies between GS
   delay advertisements and actual results.  The RSpec Slack term may be
   useful in mitigating this problem.

   AAL-5 will support any message size up to 65,535 bytes, so setting
   the AAL SDU to the receiver TSpec M parameter value (plus 8 bytes for

   the LLC/SNAP header) will generally not be an issue.  In the PATH
   Adspec, however, the PATH_MTU parameter <x,10> for each service
   SHOULD be set to 9188 bytes, which is the default MTU for AAL-5 [19].

3.2 Handling of Excess Traffic

   For IP Integrated Services, network elements will transport traffic
   in excess of the TSpec parameters whenever physical resources
   (bandwidth, buffers and processing) are available.  (In CLS a
   "network element MUST attempt to forward the excess traffic on a
   best-effort basis" under certain conditions; and in GS a traffic
   policers "SHOULD relegate non-conforming datagrams to best effort".)
   While excess traffic SHOULD be supported on a best effort basis, it
   MUST NOT interfere with the QoS (delay and loss) of conforming CLS
   and GS traffic, nor with normal service of non-reserved best effort
   traffic.

   There are several solutions with ATM: the most attractive is to use a
   VBR service category (with an appropriate conformance definition) and
   tag excess traffic as low priority using the CLP bit.  This avoids
   reordering of the flow, but necessitates careful design of the egress
   router scheduler.  To avoid reordering, the excess traffic can be
   queued with conforming traffic.  A threshold SHOULD be used to ensure
   that conforming traffic is not unnecessarily delayed by the excess.
   Also, for GS, the extra delay that would be incurred due to excess
   traffic in the queue ahead of conforming packets would have to be
   accurately reflected in the delay advertisement.  Note that the
   ingress router SHOULD tag all cells of each non-conforming packet,
   rather than letting the ATM network apply tagging due to ATM-level
   non-conformance.

   There is no requirement in ATM standards that tagged cells, marked
   either by the user or by policing, be transported if possible.
   Therefore, the operator of an edge router supporting IP-IS SHOULD
   ascertain the actual behavior of the ATM equipment in the path, which
   may span multiple administrative domains in the ATM network.  If
   tagged cells are simply dropped at some point, regardless of load,
   then the operator may consider setting the break bit, at least for
   CLS service.

   The other solutions generally involve a separate VC to carry the
   excess.  A distinct VC can be set up for each VC supporting a GS or
   CLS flow, or, if many flows are aggregated into a single QoS VC, then
   another VC can handle the excess traffic for that set of flows.  A VC
   can be set up to handle all excess traffic from the ingress router to
   the egress point.  Since the QoS of the excess traffic is not
   particularly constrained, the design is quite flexible.  However,
   using a separate VC may cause misordering of packets within a flow.

   The service category for the excess-traffic VC may typically be UBR
   or ABR, although one could use CBR or nrtVBR if the excess traffic
   were predictable enough to know what rate to allocate.  (This
   wouldn't normally be expected for excess traffic, though.)

   Whether a separate VC is used may be influenced by the design of the
   router scheduler.  The CLS spec suggests two possible
   implementations: one where excess traffic shares the Best Effort
   class scheduler allocation, but at lower priority than other best
   effort traffic.  The other, where a separate allocation is made.  The
   first would allow excess traffic to use the same VC as normal best
   effort traffic, and the second would suggest a separate VC.

   TM/UNI 4.0. does not support tagging of traffic in excess of PCR.
   Although UNI 3.x does have a separate PCR parameter for CLP=0 cells
   only, we do not recommend using this feature for reasons of
   interoperability with TM/UNI 4.0 equipment.  This restricts CBR VCs
   to use solutions other than tagging.  The value of PCR can be set
   higher than necessary for conformant traffic, in an effort to support
   excess traffic on the same VC.  In some cases this may be a viable
   solution, such as when there is little additional cost imposed for a
   high PCR.  If PCR can be set as high as APB, then the excess traffic
   is fully accommodated.

3.3 Use of Guaranteed Service Adspec Parameters and Slack Term

   The Adspec is used by the Guaranteed Service to allow a receiver to
   calculate the worst-case delay associated with a GS flow.  Three
   quantities, C, D, and MPL, are accumulated (by simple addition of
   components corresponding to each network element) in the PATH message
   from source to receiver.  The resulting delay values can be different
   for each unique receiver.  The maximum delay is computed as

        delay <=  b_r/R + C_TOT/R + D_TOT + MPL

   The Minimum Path Latency (MPL) includes propagation delay, while
   b_r/R accounts for bursts due to the source and C and D include other
   queueing, scheduling and serialization delays.  (We neglect the
   effect of maximum packet size and peak rate here; see the GS
   specification [8] for a more detailed equation.)  The service rate
   requested by the receiver, R, can be greater than the TSpec rate,
   r_r, resulting in lower delay.  The burst size, b_r, is the leaky
   bucket parameter from the receiver TSpec.

   The values of C and D that a router advertises depend on both the
   router packet scheduler and the characteristics of the subnet
   attached to the router.  Each router (or the source host) takes
   responsibility for its downstream subnet in its advertisement.  For

   example, if the subnet is a simple point-to-point link, the subnet-
   specific parts of C and D need to account for the link transmission
   rate and MTU.  An ATM subnet is generally more complex.

   For this discussion, we consider only the ATM subnet-specific
   components, denoted C_ATM and D_ATM.  The ATM network can be
   represented as a "pure delay" element, where the variable queueing
   delay, given by CVD is captured in D_ATM, and C_ATM is set to zero.
   It is possible to use C_ATM only when the ATM service rate equals R.
   This may be the case, for example with a CBR VC with PCR = R.

   Usually it will be simpler to just advertise the total delay
   variation (CDV) in D_ATM.

   As discussed in Section 2.6, the edge router keeps a table with
   values of MPL and D_ATM for each egress router it needs to share VCs
   with.  The value of D_ATM contributes to the D parameter advertised
   by the edge router, and SHOULD accurately reflect the CDV that the
   router will get in a VC when it is set up.  Factors that affect CDV,
   such as statistical multiplexing in the ATM network, SHOULD be taken
   into account when compiling data for the router's table.  In case of
   uncertainty, D_ATM can be set to an upper bound.  When an RESV
   message arrives, causing a VC to be set up, the requested values for
   CTD and CDV can be relaxed using the slack term in the receiver
   RSpec:

        CTD = D_ATM + MPL + S_ATM
        CDV = D_ATM + S_ATM.

   The term S_ATM is the portion of the slack term applied to the ATM
   portion of the path.  Recall that the slack term [8] is positive when
   the receiver can afford more delay than that computed from the
   Adspec.  The ATM edge device may take part (or all) of the slack
   term, S.  The distribution of delay slack among the nodes and subnets
   is network specific.

   Note that with multipoint VCs the egress edge router may need to
   correct advertised values of C and D.  See discussion in Section 3.1.

4.0 Miscellaneous Items

4.1 Units Conversion

   All rates and token bucket depth parameters that are mapped from IP-
   level parameters to ATM parameters MUST be corrected for the effects
   of added headers and the segmentation of packets into cells.  At the
   IP layer, token bucket depths and rates are measured in bytes and
   bytes/sec, respectively, whereas for ATM, they are measured in cells

   and cells/sec.

   Each IP Packet is wrapped into an AAL-5 PDU, having a number of
   additional header bytes (8 for LLC/SNAP and perhaps others, e.g. 12
   for MPOA, etc.), and an 8-byte AAL-5 trailer.  The AAL-5 PDU is then
   segmented into multiple ATM cells, each having a 5-byte cell header
   followed by a 48-byte cell payload.  The number of cells used to
   carry an IP packet with

        B = number of IP-packet Bytes,
        H = number of AAL-5 header bytes (LLC/SNAP etc.)
        C = number of cells,

   is roughly

        C = B/48,

   and more precisely

        C = floor[(H + B + 8 + 47)/48]

   where floor[] is rounds down to the nearest integer.  The '8'
   accounts for the AAL-5 trailer and the '47' accounts for the last
   cell which may be only partially filled.

5.0 Summary of ATM VC Setup Parameters for Guaranteed Service

   This section describes how to create ATM VCs appropriately matched
   for Guaranteed Service. The key points 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 REQUIRED to be
   in agreement with the definition of GS.  For this reason, we prefer
   an rtVBR service in which tagging is supported.  Another good match
   is to use CBR with special handling of any non-conforming traffic,
   e.g., through another VC, since a CBR VC will not accommodate traffic
   in excess of PCR.

   Note, these encodings assume point to multipoint connections, where
   the backward channel is not used.  If the IP session is unicast only,
   then a point-to-point VC may be used and the IWF may make use of the
   backward channel, with QoS parameters set appropriately for the
   service provided.

   We provide a mapping for all combinations of IP service and ATM
   service category, and comments indicating whether or not each
   combination meets the requirements of the IP-IS service.

5.1 Encoding GS Using Real-Time VBR (ATM Forum TM/UNI 4.0)

   RtVBR with conformance definition VBR.3 [6] MEETS the requirements of
   GS.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                     Note 1
     Backward PCR CLP=0+1            0
     Forward SCR CLP=0                                       Note 1
     Backward SCR CLP=0              0
     Forward MBS (CLP=0)                                     Note 1
     Backward MBS (CLP=0)            0
     BE indicator                    NOT included
     Forward Frame Discard bit       1
     Backward Frame Discard bit      1
     Tagging Forward bit             1 (Tagging requested)
     Tagging Backward bit            1 (Tagging requested)

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)              Note 2
     ATM Transfer Capability         9  (Real time VBR)       Note 3
     Susceptible to Clipping         00 (Not Susceptible)
     User Plane Configuration        01 (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 4
       ISO/IEC TR 9577 IPI            204

   QoS Class
     QoS Class Forward               1                       Note 5
     QoS Class Backward              1                       Note 5

   Extended QoS Parameters                                   Note 6
     Acceptable Forward CDV
     Acceptable Forward CLR
     Forward Max CTD

   Note 1:  See discussion in Section 2.5.1.

   Note 2:  Value 3 (BCOB-C) can also be used.
            If Bearer Class C is chosen the ATC field MUST be absent.
   Note 3:  The ATC value 19 is not used.  The value 19 implies that the
            CLR objective applies to the aggregate  CLP=0+1 stream and
            that does not give desirable treatment of excess traffic.
   Note 4:  For QoS VCs supporting GS or CLS, the layer 3 protocol
            SHOULD be specified.  For BE VCs, it can be left
            unspecified, allowing the VC to be shared by multiple
            protocols, following RFC 1755.
   Note 5:  Cf ITU Rec. I.356 [21] for new QoS Class definitions.
   Note 6:  See discussion in Section 2.6.

5.2 Encoding GS Using CBR (ATM Forum TM/UNI 4.0)

   A CBR VC MEETS the requirements of GS.  The main advantage of this is
   that CBR is widely supported; the disadvantage is that data flows
   might not fill the pipe (utilization loss) and there is no tagging
   option available.  Excess traffic MUST be handled using a separate
   VC.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                     Note 1
     Backward PCR CLP=0+1            0
     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 2
     ATM Transfer Capability         5  (CBR)                Note 3
     Susceptible to Clipping         00 (Not Susceptible)
     User Plane Configuration        01 (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 4
      ISO/IEC TR 9577 IPI            204

   QoS Class
     QoS Class Forward               1                       Note 5
     QoS Class Backward              1                       Note 5

   Extended QoS Parameters                                   Note 6
     Acceptable Forward CDV
     Acceptable Forward CLR
     Forward Max CTD

   Note 1:  See discussion in Section 2.5.1.
   Note 2:  Value 1 (BCOB-A) can also be used.
            If Bearer Class A is chosen the ATC field MUST be absent.
   Note 3:  The ATC value 7 is not used.  The value 7 implies CLR
            objective applies to the aggregate  CLP=0+1 stream and that
            does not give desirable treatment of excess traffic.
   Note 4:  For QoS VCs supporting GS or CLS, the layer 3 protocol
            SHOULD be specified.  For BE VCs, it can be left
            unspecified, allowing the VC to be shared by multiple
            protocols, following RFC 1755.
   Note 5:  Cf ITU Rec. I.356 [21] for new QoS Class definitions.
   Note 6:  See discussion in Section 2.6.

5.3 Encoding GS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)

   NrtVBR does not provide delay guarantees and is NOT RECOMMENDED for
   GS.  If GS/nrtVBR is used and network utilization is low, the delay
   may be `reasonable', but will not be controlled.  The encoding of GS
   with nrtVBR is the same as that for CLS using nrtVBR.  See Section
   6.1 below.

5.4 Encoding GS Using ABR (ATM Forum TM/UNI 4.0)

   GS using ABR is a very unlikely combination, and DOES NOT meet the
   service requirements of GS.  The objective of the ABR service is to
   provide "low" loss rates.  The delay objectives for ABR SHOULD be
   expected to be very loose.  If ABR were used for GS, the VC
   parameters would follow as for CLS over ABR.  See Section 6.2.

5.5 Encoding GS Using UBR (ATM Forum TM/UNI 4.0)

   The UBR service is the lowest common denominator of the services.  It
   cannot provide delay or loss guarantees, and therefore DOES NOT meet
   the requirements of GS.  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 Forward PCR would be determined from the peak rate of the
   receiver TSpec.  See Section 7.1.

5.6 Encoding GS Using ATM Forum UNI 3.0/3.1 Specifications

   It is not recommended to support GS using UNI 3.x VBR mode because
   the BCOB-C Bearer Class does not represent real-time behavior.  Also,
   Appendix F of the UNI 3.1 specification precludes the specification
   of traffic type "VBR" with the timing requirement "End to End timing
   Required" in conjunction with Bearer Class X.

   A CBR VC MEETS the requirements of GS.  The following table specifies
   the support of GS using CBR.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0                                       Note 2
     Backward PCR CLP=0              0
     Forward PCR CLP=0+1                                     Note 2
     Backward PCR CLP=0+1            0
     BE indicator                    NOT included
     Tagging Forward bit             1 (Tagging requested)
     Tagging Backward bit            1 (Tagging requested)

   Broadband Bearer Capability
     Bearer Class                    16  (BCOB-X)            Note 3
     Traffic Type                    001 (Constant Bit Rate)
     Timing Requirements             01  (Timing Required)
     Susceptible to Clipping         00  (Not Susceptible)
     User Plane Configuration        01  (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 4
      ISO/IEC TR 9577 IPI            204

   QoS Class                                                 Note 5
     QoS Class Forward               1
     QoS Class Backward              1

   Note 1:  Only included for UNI 3.0.
   Note 2:  See discussion in Section 2.5.1.  PCR CLP=0 SHOULD be set

            identical to PCR CLP=0+1.  Although this could potentially
            allow a CBR VC to carry excess traffic as tagged cells, it
            is not recommended since it is not supported in UNI 4.0
   Note 3:  Value 1 (BCOB-A) can also be used. If BCOB-A is used Traffic
            Type and Timing Requirements fields are not included.
   Note 4:  For QoS VCs supporting GS or CLS, the layer 3 protocol
            SHOULD be specified.  For BE VCs, it can be left
            unspecified, allowing the VC to be shared by multiple
            protocols, following RFC 1755.
   Note 5:  QoS Parameters are implied by the QoS Class.

6.0 Summary of ATM VC Setup Parameters for Controlled Load Service

   This section describes how to create ATM VCs appropriately matched
   for Controlled Load Service.  CLS traffic is partly delay tolerant
   and has variable rate.  NrtVBR and ABR (TM/UNI 4.0 only) are the best
   choices for supporting CLS.

   Note, these encodings assume point to multipoint connections where
   the backward channel is not used.  If the IP session is unicast only,
   then a point-to-point VC may be used and the IWF may make use of the
   backward channel, with QoS parameters set appropriately for the
   service provided.

   We provide a mapping for all combinations of IP service and ATM
   service category, and comments indicating whether or not each
   combination meets the requirements of the IP-IS service.

6.1 Encoding CLS Using Non-Real-Time VBR (ATM Forum TM/UNI 4.0)

   NrtVBR MEETS the requirements for CLS.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                     Note 1
     Backward PCR CLP=0+1            0
     Forward SCR CLP=0                                       Note 1
     Backward SCR CLP=0              0
     Forward MBS (CLP=0)                                     Note 1
     Backward MBS (CLP=0)            0
     BE indicator                    NOT included
     Forward Frame Discard bit       1
     Backward Frame Discard bit      1

     Tagging Forward bit             1 (Tagging requested)
     Tagging Backward bit            1 (Tagging requested)

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)             Note 2
     ATM Transfer Capability         10 (Non-real time VBR)  Note 3
     Susceptible to Clipping         00 (Not Susceptible)
     User Plane Configuration        01 (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 4
      ISO/IEC TR 9577 IPI            204

   QoS Class
     QoS Class Forward               3                       Note 5
     QoS Class Backward              3                       Note 5

   Extended QoS Parameters                                   Note 6
     Acceptable Forward CDV
     Acceptable Forward CLR
     Forward Max CTD

   Note 1:  See discussion in Section 2.5.2.
   Note 2:  Value 3 (BCOB-C) can also be used.
            If Bearer Class C is used, the ATC field MUST be absent.
   Note 3:  The ATC value 11 is not used.  The value 11 implies CLR
            objective applies to the aggregate  CLP=0+1 stream and
            that does not give desirable treatment of excess traffic.
   Note 4:  For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
            be specified.  For BE VCs, it can be left unspecified, allowing
            the VC to be shared by multiple protocols, following RFC 1755.
   Note 5:  Cf ITU Rec. I.356 [21] for new QoS Class definitions.
   Note 6:  See discussion in Section 2.6.

6.2 Encoding CLS Using ABR (ATM Forum TM/UNI 4.0)

   ABR MEETS the requirements for CLS when MCR is set to the CLS TSpec
   rate.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes

     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                      Note 1
     Backward PCR CLP=0+1            0
     Forward MCR CLP=0+1                                      Note 1
     Backward MCR CLP=0+1            0
     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 2
     ATM Transfer Capability         12  (ABR)
     Susceptible to Clipping         00  (Not Susceptible)
     User Plane Configuration        00  (Point-to-Point)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 3
      ISO/IEC TR 9577 IPI            204

   QoS Class
     QoS Class Forward               0                       Note 4
     QoS Class Backward              0                       Note 4

   Extended QoS Parameters                                   Note 5
     Acceptable Forward CDV
     Acceptable Forward CLR
     Forward Max CTD

   ABR Setup Parameters                                      Note 6
   ABR Additional Parameters                                 Note 6

   Note 1:  See discussion in Section 2.5.2.
   Note 2:  Value 3 (BCOB-C) can also be used.
            If Bearer Class C is chosen the ATC field MUST be absent.
   Note 3:  For QoS VCs supporting GS or CLS, the layer 3 protocol
            SHOULD be specified.  For BE VCs, it can be left
            unspecified, allowing the VC to be shared by multiple
            protocols, following RFC 1755.
   Note 4:  Cf ITU Rec. I.356 [21] for new QoS Class definitions.
   Note 5:  See discussion in Section 2.6.

   Note 6:  The ABR-specific parameters are beyond the scope of this
            document.  These generally depend on local implementation
            and not on values mapped from IP level service parameters
            (except for MCR).  See [6, 11] for further information.

6.3 Encoding CLS Using CBR (ATM Forum TM/UNI 4.0)

   Although CBR does not explicitly take into account the variable rate
   of source data, it may be convenient to use ATM connectivity between
   edge routers to provide a simple "pipe" service, as a leased line
   replacement.  Since no tagging option is available with CBR, excess
   traffic MUST be handled using a separate VC.  Under this condition,
   CBR MEETS the requirements of CLS.

   To use CBR for CLS, the same encoding for GS over CBR (Section 5.2)
   would be used.  See discussion in Section 2.5.2.

6.4 Encoding CLS Using Real-Time VBR (ATM Forum TM/UNI 4.0)

   The encoding of CLS using rtVBR implies a hard limit on the end-to-
   end delay in the ATM network.  This creates more complexity in the VC
   setup than the CLS service requires, and is therefore not a preferred
   combination, although it DOES MEET the requirements of CLS.

   If rtVBR is used to encode CLS, then the encoding is essentially the
   same as that for GS.  See discussions in Section 5.1 and Section
   2.5.2.

6.5 Encoding CLS Using UBR (ATM Forum TM/UNI 4.0)

   This encoding gives no QoS guarantees and DOES NOT MEET the
   requirements of CLS.  If used, it is coded in the same way as for BE
   over UBR (Section 7.1), except that the PCR would be determined from
   the peak rate of the receiver TSpec.

6.6 Encoding CLS Using ATM Forum UNI 3.0/3.1 Specifications

   This encoding is equivalent to the nrtVBR service category.  It MEETS
   the requirements of CLS.

   AAL
     Type                            5
     Forward CPCS-SDU Size           parameter M of rcvr TSpec + 8 Bytes
     Backward CPCS-SDU Size          parameter M of rcvr TSpec + 8 Bytes
     Mode                            1 (Message mode)        Note 1
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                     Note 2
     Backward PCR CLP=0+1            0
     Forward SCR CLP=0                                       Note 2
     Backward SCR CLP=0              0
     Forward MBS (CLP=0)                                     Note 2
     Backward MBS (CLP=0)            0
     BE indicator                    NOT included
     Tagging Forward bit             1 (Tagging requested)
     Tagging Backward bit            1 (Tagging requested)

   Broadband Bearer Capability
     Bearer Class                    16  (BCOB-X)            Note 3
     Traffic Type                    010 (Variable Bit Rate)
     Timing Requirements             00  (No Indication)
     Susceptible to Clipping         00  (Not Susceptible)
     User Plane Configuration        01  (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)
     User Information Layer 3
       Protocol                      11 (ISO/IEC TR 9577)    Note 4
      ISO/IEC TR 9577 IPI            204

   QoS Class
     QoS Class Forward               3                       Note 5
     QoS Class Backward              3                       Note 5

   Note 1:  Only included for UNI 3.0.
   Note 2:  See discussion in Section 2.5.2.
   Note 3:  Value 3 (BCOB-C) can also be used. If BCOB-C is used Traffic
            Type and Timing Requirements fields are not included.
   Note 4:  For QoS VCs supporting GS or CLS, the layer 3 protocol
            SHOULD be specified.  For BE VCs, it can be left
            unspecified, allowing the VC to be shared by multiple
            protocols, following RFC 1755.
   Note 5:  Cf ITU Rec. I.356 [21] for new QoS Class definitions.  QoS
            Parameters are implied by the QoS Class.

7.0 Summary of ATM VC Setup Parameters for Best Effort Service

   This section is provided for completeness only.  The IETF ION working
   group documents on ATM signalling support for IP over ATM [10, 11]
   provide definitive specifications for Best Effort IP service over
   ATM.

   The best-matched ATM service category to IP Best Effort is UBR.  We
   provide the setup details for this case below.  The BE service does
   not involve reservation of resources.  ABR and nrtVBR are also well
   suited to BE service.  See discussion in Section 2.1.3.

   Note, VCs supporting best effort service are usually point to point,
   rather than point to multipoint, and the backward channels of VCs are
   used.  In cases where VCs are set up to support best effort multicast
   sessions, multipoint VCs can be used and the backward channels would
   be not have resources reserved.  Related situations include transport
   of excess traffic on IP-multicast QoS sessions, or to support the
   subset of multicast end systems that have not made rsvp reservations.
   See the discussion on VC management in [12].

7.1 Encoding Best Effort Service Using UBR (ATM Forum TM/UNI 4.0)

   AAL
     Type                            5
     Forward CPCS-SDU Size           9188 Bytes (default MTU for AAL-5)
     Backward CPCS-SDU Size          9188 Bytes (default MTU for AAL-5)
     SSCS Type                       0 (Null SSCS)

   Traffic Descriptor
     Forward PCR CLP=0+1                                         Note 1
     Backward PCR CLP=0+1            0
     BE indicator                    included
     Forward Frame Discard bit       1
     Backward Frame Discard bit      1
     Tagging Forward bit             1 (Tagging requested)
     Tagging Backward bit            1 (Tagging requested)

   Broadband Bearer Capability
     Bearer Class                    16 (BCOB-X)                 Note 2
     ATM Transfer Capability         10 (Non-real time VBR)
     Susceptible to Clipping         00 (Not Susceptible)
     User Plane Configuration        01 (Point-to-Multipoint)

   Broadband Low Layer Information
     User Information Layer 2
       Protocol                      12 (ISO 8802/2)             Note 3

   QoS Class
     QoS Class Forward               0
     QoS Class Backward              0

   Note 1:  See discussion in Section 2.5.3.
   Note 2:  Value 3 (BCOB-C) can also be used.

            If Bearer Class C is used, the ATC field MUST be absent
   Note 3:  For QoS VCs supporting GS or CLS, the layer 3 protocol SHOULD
            be specified.  For BE VCs, it can be left unspecified, allowing
            the VC to be shared by multiple protocols, following RFC 1755.

8.0 Security Considerations

   IP Integrated Services (including rsvp) and ATM are both complex
   resource reservation protocols, and SHOULD be expected to have
   complex feature interactions.

   Differences in IP and ATM billing styles could cause unforeseen
   problems since RESV messages can set up VCs.  For example, an end-
   user paying a flat rate for (non-rsvp aware) internet service may
   send an rsvp RESV message that encounters a (perhaps distant) ATM
   network with a usage-sensitive billing model.  Insufficient
   authentication could result in services being accidentally billed to
   an innocent third party, intentional theft of service, or malicious
   denial of service attacks where high volumes of reservations consume
   transport or processing resources at the edge devices.

   The difference in styles of handling excess traffic could result in
   denial of service attacks where the ATM network uses transport
   resources (bandwidth, buffers) or connection processing resources
   (switch processor cycles) in an attempt to accommodate excess traffic
   that was admitted by the internet service.

   Problems associated with translation of resource reservations at edge
   devices are probably more complex and susceptible to abuse when the
   IP-ATM edge is also an administrative boundary between service
   providers.  Note also that administrative boundaries can exist within
   the ATM cloud, i.e., the ingress and egress edge devices are operated
   by different service providers.

   Note, the ATM Forum Security Working Group is currently defining
   ATM-level security features such as data encryption and signalling
   authentication.  See also the security issues raised in the rsvp
   specification [3].

9.0 Acknowledgements

   The authors received much useful input from the members of the ISSLL
   working group.  In particular, thanks to Drew Perkins and Jon Bennett
   of Fore Systems, Roch Guerin of IBM, Susan Thomson and Sudha Ramesh
   of Bellcore.

Appendix 1  Abbreviations

   AAL           ATM Adaptation Layer
   ABR           Available Bit Rate
   APB           Available Path Bandwidth (int-serv GCP)
   ATC           ATM Transfer Capability
   ATM           Asynchronous Transfer Mode
   B-LLI         Broadband Low Layer Information
   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
   CLP           Cell Loss Priority (bit)
   CLR           Cell Loss Ratio
   CLS           Controlled Load Service
   CPCS          Common Part Convergence Sublayer
   CTD           Cell Transfer Delay
   EOM           End of Message
   GCP           General Characterization Parameter
   GCRA          Generic Cell Rate Algorithm
   GS            Guaranteed Service
   IE            Information Element
   IETF          Internet Engineering Task Force
   ION           IP Over Non-broadcast multiple access networks
   IP            Internet Protocol
   IPI           Initial Protocol Identifier
   IS            Integrated Services
   ISSLL         Integrated Services over Specific Link Layers
   ITU           International Telecommunication Union
   IWF           Interworking Function
   LIJ           Leaf Initiated Join
   LLC           Logical Link Control
   MBS           Maximum Burst Size
   MCR           Minimum Cell Rate
   MPL           Minimum Path Latency
   MTU           Maximum Transfer Unit
   nrtVBR        Non-real-time VBR
   PCR           Peak Cell Rate
   PDU           Protocol Data Unit
   PVC           Permanent Virtual Connection
   QoS           Quality of Service
   RESV          Reservation Message (of rsvp protocol)
   RFC           Request for Comments
   RSVP          Resource Reservation Protocol
   RSpec         Reservation Specification

   rtVBR         Real-time VBR
   SCR           Sustainable Cell Rate
   SDU           Service Data Unit
   SNAP          Subnetwork Attachment Point
   SSCS          Service-Specific Convergence Sub-layer
   SVC           Switched Virtual Connection
   TCP           Transport Control Protocol
   TM            Traffic Management
   TSpec         Traffic Specification
   UBR           Unspecified Bit Rate
   UNI           User-Network Interface
   UPC           Usage Parameter Control (ATM traffic policing function)
   VBR           Variable Bit Rate
   VC            (ATM) Virtual Connection

REFERENCES

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

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

   [3]  Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,
        "Resource ReSerVation Protocol (RSVP) - Version 1 Functional
        Specification", RFC 2205, September 1997.

   [4]  The ATM Forum, "ATM User-Network Interface Specification,
        Version 3.0", Prentice Hall, Englewood Cliffs NJ, 1993.

   [5]  The ATM Forum, "ATM User-Network Interface Specification,
        Version 3.1", Prentice Hall, Upper Saddle River NJ, 1995.

   [6]  The ATM Forum, "ATM User-Network Interface (UNI) Signalling
        Specification, Version 4.0", July 1996.  Available at
        ftp://ftp.atmforum.com/pub/approved-specs/af-sig-0061.000.ps.

   [7]  The ATM Forum, "ATM Traffic Management Specification, Version
        4.0", April 1996.  Available at
        ftp://ftp.atmforum.com/pub/approved-specs/af-tm-0056.000.ps.

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

   [9]  Shenker, S., Partridge, C., and R. Guerin, "Specification of
        Guaranteed Quality of Service", RFC 2212, September 1997.

   [10] Wroclawski, J., "Specification of the Controlled-Load Network
        Element Service", RFC 2211, September 1997.

   [11] Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D., and
        A. Malis, "ATM Signaling Support for IP over ATM", RFC 1755,
        February 1995.

   [12] Maher, M., "ATM Signalling Support for IP over ATM - UNI
        Signalling 4.0 Update", RFC 2331, April 1998.

   [13] Crawley, E., Berger, L., Berson, S., Baker, F., Borden, M., and
        J. Krawczyk, "A Framework for Integrated Services and RSVP over
        ATM", RFC 2382, August 1998.

   [14] Berger, L., "RSVP over ATM Implementation Requirements", RFC
        2380, August 1998.

   [15] Berger, L., "RSVP over ATM Implementation Guidelines", BCP 24,
        RFC 2379, August 1998.

   [16] Shenker, S., and J. Wroclawski, "Network Element Service
        Specification Template", RFC 2216, September 1997.

   [17] Wroclawski, J., "The Use of RSVP with IETF Integrated Services",
        RFC 2210, September 1997.

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

   [19] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
        Layer 5", RFC 1483, July 1993.

   [20] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, January
        1994.

   [21] ITU Recommendation I.356, "B-ISDN ATM layer cell transfer
        performance", International Telecommunication Union, Geneva,
        October 1996.

   [22] A. Romanow, S. Floyd, "Dynamics of TCP Traffic over ATM
        Networks", IEEE J. Sel. Areas in Commun., Vol. 13, No. 4, pp.
        633-41, May 1995.

   [23] A. K. Parekh, R. G. Gallager, "A Generalized Processor Sharing
        Approach to Flow Control in Integrated Services Networks: The
        Multiple Node Case", IEEE/ACM Trans. Networking, Vol. 2, No. 2,
        pp. 137-150, April 1994.

   [24] S. Floyd, V. Jacobson, "Link-sharing and Resource Management
        Models for Packet Networks", IEEE/ACM Trans. Networking, Vol. 3,
        No. 4, August 1995.

   [25] S. Shenker and J. Wroclawski, "General Characterization
        Parameters for Integrated Service Network Elements", RFC 2215,
        September 1997.

Authors' Addresses

   Mark W. Garrett
   Bellcore
   445 South Street
   Morristown, NJ 07960
   USA

   Phone: +1 201 829-4439
   EMail: mwg@bellcore.com

   Marty Borden
   Bay Networks
   42 Nagog Park
   Acton MA, 01720
   USA

   Phone: +1 508 266-1011
   EMail: mborden@baynetworks.com

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