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rfc:rfc2381

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

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

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.)

Garrett & Borden Standards Track [Page 1] RFC 2381 Interoperation of CLS and GS with ATM August 1998

Table of Contents

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

Garrett & Borden Standards Track [Page 2] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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

Garrett & Borden Standards Track [Page 3] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 4] RFC 2381 Interoperation of CLS and GS with ATM August 1998

                               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.

Garrett & Borden Standards Track [Page 5] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 6] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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.

Garrett & Borden Standards Track [Page 7] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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

Garrett & Borden Standards Track [Page 8] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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).

Garrett & Borden Standards Track [Page 9] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 10] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 11] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 12] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 13] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 14] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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:

Garrett & Borden Standards Track [Page 15] RFC 2381 Interoperation of CLS and GS with ATM August 1998

      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

Garrett & Borden Standards Track [Page 16] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 17] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 18] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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].

Garrett & Borden Standards Track [Page 19] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 20] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 21] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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.

Garrett & Borden Standards Track [Page 22] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 23] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 24] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 25] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 26] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 27] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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.

Garrett & Borden Standards Track [Page 28] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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

Garrett & Borden Standards Track [Page 29] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 30] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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

Garrett & Borden Standards Track [Page 31] RFC 2381 Interoperation of CLS and GS with ATM August 1998

          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

Garrett & Borden Standards Track [Page 32] RFC 2381 Interoperation of CLS and GS with ATM August 1998

   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

Garrett & Borden Standards Track [Page 33] RFC 2381 Interoperation of CLS and GS with ATM August 1998

   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.

Garrett & Borden Standards Track [Page 34] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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)

Garrett & Borden Standards Track [Page 35] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 36] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 37] RFC 2381 Interoperation of CLS and GS with ATM August 1998

          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.

Garrett & Borden Standards Track [Page 38] RFC 2381 Interoperation of CLS and GS with ATM August 1998

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

Garrett & Borden Standards Track [Page 39] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 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.

Garrett & Borden Standards Track [Page 40] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 [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.

Garrett & Borden Standards Track [Page 41] RFC 2381 Interoperation of CLS and GS with ATM August 1998

 [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

Garrett & Borden Standards Track [Page 42] RFC 2381 Interoperation of CLS and GS with ATM August 1998

Full Copyright Statement

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Garrett & Borden Standards Track [Page 43]

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