Network Working Group                                          M. Seaman
Request for Comments: 2815                                       Telseon
Category: Standards Track                                       A. Smith
                                                        Extreme Networks
                                                              E. Crawley
                                                     Unisphere Solutions
                                                           J. Wroclawski
                                                                 MIT LCS
                                                                May 2000

Integrated Service Mappings on IEEE 802 Networks

Status of this Memo

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

Copyright Notice

Abstract

   This document describes mappings of IETF Integrated Services over
   LANs built from IEEE 802 network segments which may be interconnected
   by IEEE 802.1D MAC Bridges (switches).  It describes parameter
   mappings for supporting Controlled Load and Guaranteed Service using
   the inherent capabilities of relevant IEEE 802 technologies and, in
   particular, 802.1D-1998 queuing features in switches.

   These mappings are one component of the Integrated Services over IEEE
   802 LANs framework.

   1 Introduction ............................................... 2
   2 Flow Identification and Traffic Class Selection ............ 3
   3 Choosing a flow's IEEE 802 user_priority class ............. 5
   3.1 Context of admission control and delay bounds ............ 6
   3.2 Default service mappings ................................. 7
   3.3 Discussion ............................................... 9
   4 Computation of integrated services characterization parameters
        by IEEE 802 devices .....................................10
   4.1 General characterization parameters ......................10
   4.2 Parameters to implement Guaranteed Service ...............11
   4.3 Parameters to implement Controlled Load ..................11
   4.4 Parameters to implement Best Effort ......................12
   5 Merging of RSVP/SBM objects ................................12
   6 Applicability of these service mappings ....................13
   7 References .................................................14
   8 Security Considerations ....................................15
   9 Acknowledgments ............................................15
   10 Authors' Addresses ........................................16
   11 Full Copyright Statement ..................................17

1. Introduction

   The IEEE 802.1 Interworking Task Group has developed a set of
   enhancements to the basic MAC Service provided in Bridged Local Area
   Networks (a.k.a. "switched LANs"). As a supplement to the original
   IEEE MAC Bridges standard, IEEE 802.1D-1990 [802.1D-ORIG], the
   updated IEEE 802.1D-1998 [802.1D] proposes differential traffic class
   queuing in switches. The IEEE 802.1Q specification [802.1Q] extends
   the capabilities of Ethernet/802.3 media to carry a traffic class
   indicator, or "user_priority" field, within data frames.

   The availability of this differential traffic queuing, together with
   additional mechanisms to provide admission control and signaling,
   allows IEEE 802 networks to support a close approximation of the IETF
   Integrated Services capabilities [CL][GS]. This document describes
   methods for mapping the service classes and parameters of the IETF
   model into IEEE 802.1D network parameters.  A companion document
   [SBM] describes a signaling protocol for use with these mappings.  It
   is recommended that readers be familiar with the overall framework in
   which these mappings and signaling protocol are expected to be used;
   this framework is described fully in [IS802FRAME].

   Within this document, Section 2 describes the method by which end
   systems and routers bordering the IEEE Layer-2 cloud learn what
   traffic class should be used for each data flow's packets.  Section 3
   describes the approach recommended to map IP-level traffic flows to

   IEEE traffic classes within the Layer 2 network.  Section 4 describes
   the computation of Characterization Parameters by the layer 2
   network.  The remaining sections discuss some particular issues with
   the use of the RSVP/SBM signaling protocols, and describe the
   applicability of all of the above to different layer 2 network
   topologies.

2. Flow Identification and Traffic Class Selection

   One model for supporting integrated services over specific link
   layers treats layer-2 devices very much as a special case of routers.
   In this model, switches and other devices along the data path make
   packet handling decisions based on the RSVP flow and filter
   specifications, and use these specifications to classify the
   corresponding data packets. The specifications could either be used
   directly, or could be used indirectly by mapping each RSVP session
   onto a layer-2 construct such as an ATM virtual circuit.

   This approach is inappropriate for use in the IEEE 802 environment.
   Filtering to the per-flow level becomes expensive with increasing
   switch speed; devices with such filtering capabilities are likely to
   have a very similar implementation complexity to IP routers, and may
   not make use of simpler mechanisms such as 802.1D user priority.

   The Integrated Services over IEEE 802 LANs framework [IS802FRAME] and
   this document use an "aggregated flow" approach based on use of
   layer-2 traffic classes. In this model, each arriving flow is
   assigned to one of the available classes for the duration of the flow
   and traverses the 802 cloud in this class.  Traffic flows requiring
   similar service are grouped together into a single class, while the
   system's admission control and class selection rules ensure that the
   service requirements for flows in each of the classes are met.  In
   many situations this is a viable intermediate point between no QoS
   control and full router-type integrated services. The approach can
   work effectively even with switches implementing only the simplest
   differential traffic classification capability specified in the
   802.1D model.  In the aggregated flow model, traffic arriving at the
   boundary of a layer-2 cloud is tagged by the boundary device (end
   host or border router) with an appropriate traffic class, represented
   as an 802.1D "user_priority" value. Two fundamental questions are
   "who determines the correspondence between IP-level traffic flows and
   link-level classes?" and  "how is this correspondence conveyed to the
   boundary devices that must mark the data frames?"

   One approach to answering these questions would be for the meanings
   of the classes to be universally defined. This document would then
   standardize the meanings of a set of classes; e.g., 1 = best effort,
   2 = 100 ms peak delay target, 3 = 10 ms peak delay target, 4 = 1 ms

   peak delay target, etc. The meanings of these universally defined
   classes could then be encoded directly in end stations, and the
   flow-to-class mappings computed directly in these devices.

   This universal definition approach would be simple to implement, but
   is too rigid to map the wide range of possible user requirements onto
   the limited number of available 802.1D classes. The model described
   in [IS802FRAME] uses a more flexible mapping: clients ask "the
   network" which user_priority traffic class to use for a given traffic
   flow, as categorized by its flow-spec and layer-2 endpoints. The
   network provides a value back to the requester that is appropriate
   considering the current network topology, load conditions, other
   admitted flows, etc.  The task of configuring switches with this
   mapping (e.g., through network management, a switch-switch protocol
   or via some network-wide QoS-mapping directory service) is an order
   of magnitude less complex than performing the same function in end
   stations. Also, when new services (or other network reconfigurations)
   are added to such a network, the network elements will typically be
   the ones to be upgraded with new queuing algorithms etc. and can be
   provided with new mappings at this time.

   In the current model it is assumed that all data packets of a flow
   are assigned to the same traffic class for the duration of the flow:
   the characteristics of the MAC service, as defined by Clause 6 of
   [802.1D], then ensure the ordering of the data packets of the flow
   between adjacent Layer 3 routers. This is usually desirable to avoid
   potential re-ordering problems as discussed in [IS802FRAME] and [CL].
   Note that there are some scenarios where it might be desirable to
   send conforming data traffic in one traffic class and non-conforming
   traffic for the same flow in a different, lower traffic class: such a
   division into separate traffic classes is for future study.  When a
   new session or "flow" requiring QoS support is created, a client must
   ask "the network" which traffic class (IEEE 802 user_priority) to use
   for a given traffic flow, so that it can label the packets of the
   flow as it places them into the network.  A request/response protocol
   is needed between client and network to return this information. The
   request can be piggy-backed onto an admission control request and the
   response can be piggy-backed onto an admission control
   acknowledgment. This "one pass" assignment has the benefit of
   completing the admission control transaction in a timely way and
   reducing the exposure to changing conditions that could occur if
   clients cached the knowledge for extensive periods. A set of
   extensions to the RSVP protocol for communicating this information
   have been defined [SBM].

   The network (i.e., the first network element encountered downstream
   from the client) must then answer the following questions:

     1. Which of the available traffic classes would be appropriate for
        this flow?

        In general, a newly arriving flow might be assigned to a number
        of classes. For example, if 10ms of delay is acceptable, the
        flow could potentially be assigned to either a 10ms delay class
        or a 1ms delay class. This packing problem is quite difficult to
        solve if the target parameters of the classes are allowed to
        change dynamically as flows arrive and depart.  It is quite
        simple if the target parameters of each class is held fixed, and
        the class table is simply searched to find a class appropriate
        for the arriving flow.  This document adopts the latter
        approach.

     2. Of the appropriate traffic classes, which if any have enough
        capacity available to accept the new flow?

        This is the admission control problem. It is necessary to
        compare the level of traffic currently assigned to each class
        with the available level of network resources (bandwidth,
        buffers, etc), to ensure that adding the new flow to the class
        will not cause the class's performance to go below its target
        values. This problem is compounded because in a priority queuing
        system adding traffic to a higher-priority class can affect the
        performance of lower-priority classes. The admission control
        algorithm for a system using the default 802 priority behavior
        must be reasonably sophisticated to provide acceptable results.

   If an acceptable class is found, the network returns the chosen
   user_priority value to the client.

   Note that the client may be an end station, a router at the edge of
   the layer 2 network, or a first switch acting as a proxy for a device
   that does not participate in these protocols for whatever reason.
   Note also that a device e.g., a server or router may choose to
   implement both the "client" as well as the "network" portion of this
   model so that it can select its own user_priority values. Such an
   implementation would generally be discouraged unless the device has a
   close tie-in with the network topology and resource allocation
   policies. It may, however, work acceptably in cases where there is
   known over-provisioning of resources.

3. Choosing a flow's IEEE 802 user_priority class

   This section describes the method by which IP-level flows are mapped
   into appropriate IEEE user_priority classes. The IP-level services
   considered are Best Effort, Controlled Load, and Guaranteed Service.

   The major issue is that admission control requests and application
   requirements are specified in terms of a multidimensional vector of
   parameters e.g., bandwidth, delay, jitter, service class.  This
   multidimensional space must be mapped onto a set of traffic classes
   whose default behavior in L2 switches is unidimensional (i.e., strict
   priority default queuing). This priority queuing alone can provide
   only relative ordering between traffic classes. It can neither
   enforce an absolute (quantifiable) delay bound for a traffic class,
   nor can it discriminate amongst Int-Serv flows within the aggregate
   in a traffic class. Therefore, it cannot provide the absolute control
   of packet loss and delay required for individual Int-Serv flows.

   To provide absolute control of loss and delay three things must
   occur:

   (1) The amount of bandwidth available to the QoS-controlled flows
       must be known, and the number of flows admitted to the network
       (allowed to use the bandwidth) must be limited.

   (2) A traffic scheduling mechanism is needed to give preferential
       service to flows with lower delay targets.

   (3) Some mechanism must ensure that best-effort flows and QoS
       controlled flows that are exceeding their Tspecs do not damage
       the quality of service delivered to in-Tspec QoS controlled
       flows. This mechanism could be part of the traffic scheduler, or
       it could be a separate policing mechanism.

   For IEEE 802 networks, the first function (admission control) is
   provided by a Subnet Bandwidth Manager, as discussed below. We use
   the link-level user_priority mechanism at each switch and bridge to
   implement the second function (preferential service to flows with
   lower delay targets). Because a simple priority scheduler cannot
   provide policing (function three), policing for IEEE networks is
   generally implemented at the edge of the network by a layer-3 device.
   When this policing is performed only at the edges of the network it
   is of necessity approximate. This issue is discussed further in
   [IS802FRAME].

3.1. Context of admission control and delay bounds

   As described above, it is the combination of priority-based
   scheduling and admission control that creates quantified delay
   bounds. Thus, any attempt to quantify the delay bounds expected by a
   given traffic class has to made in the context of the admission
   control elements. Section 6 of the framework [IS802FRAME] provides
   for two different models of admission control - centralized or
   distributed Bandwidth Allocators.

   It is important to note that in this approach it is the admission
   control algorithm that determines which of the Int-Serv services is
   being offered. Given a set of priority classes with delay targets, a
   relatively simple admission control algorithm can place flows into
   classes so that the bandwidth and delay behavior experienced by each
   flow corresponds to the requirements of the Controlled-Load service,
   but cannot offer the higher assurance of the Guaranteed service. To
   offer the Guaranteed service, the admission control algorithm must be
   much more stringent in its allocation of resources, and must also
   compute the C and D error terms required of this service.

   A delay bound can only be realized at the admission control element
   itself so any delay numbers attached to a traffic class represent the
   delay that a single element can allow for.  That element may
   represent a whole L2 domain or just a single L2 segment.

   With either admission control model, the delay bound has no scope
   outside of a L2 domain. The only requirement is that it be understood
   by all Bandwidth Allocators in the L2 domain and, for example, be
   exported as C and D terms to L3 devices implementing the Guaranteed
   Service.  Thus, the end-to-end delay experienced by a flow can only
   be characterized by summing along the path using the usual RSVP
   mechanisms.

3.2. Default service mappings

   Table 1 presents the default mapping from delay targets to IEEE 802.1
   user_priority classes. However, these mappings must be viewed as
   defaults, and must be changeable.

   In order to simplify the task of changing mappings, this mapping
   table is held by *switches* (and routers if desired) but generally
   not by end-station hosts.  It is a read-write table. The values
   proposed below are defaults and can be overridden by management
   control so long as all switches agree to some extent (the required
   level of agreement requires further analysis).

   In future networks this mapping table might be adjusted dynamically
   and without human intervention. It is possible that some form of
   network-wide lookup service could be implemented that serviced
   requests from clients e.g., traffic_class = getQoSbyName("H.323
   video") and notified switches of what traffic categories they were
   likely to encounter and how to allocate those requests into traffic
   classes.  Alternatively, the network's admission control mechanisms
   might directly adjust the mapping table to maximize the utilization
   of network resources. Such mechanisms are for further study.

   The delay bounds numbers proposed in Table 1 are for per-Bandwidth
   Allocator element delay targets and are derived from a subjective
   analysis of the needs of typical delay-sensitive applications e.g.,
   voice, video. See Annex H of [802.1D] for further discussion of the
   selection of these values. Although these values appear to address
   the needs of current video and voice technology, it should be noted
   that there is no requirement to adhere to these values and no
   dependence of IEEE 802.1 on these values.

            user_priority  Service

                 0         Default, assumed to be Best Effort
                 1         reserved, "less than" Best Effort
                 2         reserved
                 3         reserved
                 4         Delay Sensitive, no bound
                 5         Delay Sensitive, 100ms bound
                 6         Delay Sensitive, 10ms bound
                 7         Network Control

             Table 1 - Example user_priority to service mappings

      Note: These mappings are believed to be useful defaults but
      further implementation and usage experience is required. The
      mappings may be refined in future editions of this document.

   With this example set of mappings, delay-sensitive, admission
   controlled traffic flows are mapped to user_priority values in
   ascending order of their delay bound requirement. Note that the
   bounds are targets only - see [IS802FRAME] for a discussion of the
   effects of other non-conformant flows on delay bounds of other flows.
   Only by applying admission control to higher-priority classes can any
   promises be made to lower-priority classes.

   This set of mappings also leaves several classes as reserved for
   future definition.

      Note: this mapping does not dictate what mechanisms or algorithms
      a network element (e.g., an Ethernet switch) must perform to
      implement these mappings: this is an implementation choice and
      does not matter so long as the requirements for the particular
      service model are met.

      Note: these mappings apply primarily to networks constructed from
      devices that implement the priority-scheduling behavior defined as
      the default in 802.1D. Some devices may implement more complex
      scheduling behaviors not based only on priority. In that
      circumstance these mappings might still be used, but other, more

specialized mappings may be more appropriate.

3.3. Discussion

   The recommendation of classes 4, 5 and 6 for Delay Sensitive,
   Admission Controlled flows is somewhat arbitrary; any classes with
   priorities greater than that assigned to Best Effort can be used.
   Those proposed here have the advantage that, for transit through
   802.1D switches with only two-level strict priority queuing, all
   delay-sensitive traffic gets "high priority" treatment (the 802.1D
   default split is 0-3 and 4-7 for a device with 2 queues).

   The choice of the delay bound targets is tuned to an average expected
   application mix, and might be retuned by a network manager facing a
   widely different mix of user needs. The choice is potentially very
   significant: wise choice can lead to a much more efficient allocation
   of resources as well as greater (though still not very good)
   isolation between flows.

   Placing Network Control traffic at class 7 is necessary to protect
   important traffic such as route updates and network management.
   Unfortunately, placing this traffic higher in the user_priority
   ordering causes it to have a direct effect on the ability of devices
   to provide assurances to QoS controlled application traffic.
   Therefore, an estimate of the amount of Network Control traffic must
   be made by any device that is performing admission control (e.g.,
   SBMs). This would be in terms of the parameters that are normally
   taken into account by the admission control algorithm. This estimate
   should be used in the admission control decisions for the lower
   classes (the estimate is likely to be a configuration parameter of
   SBMs).

   A traffic class such as class 1 for "less than best effort" might be
   useful for devices that wish to dynamically "penalty tag" all of the
   data of flows that are presently exceeding their allocation or Tspec.
   This provides a way to isolate flows that are exceeding their service
   limits from flows that are not, to avoid reducing the QoS delivered
   to flows that are within their contract. Data from such tagged flows
   might also be preferentially discarded by an overloaded downstream
   device.

   A somewhat simpler approach would be to tag only the portion of a
   flow's packets that actually exceed the Tspec at any given instant as
   low priority. However, it is often considered to be a bad idea to
   treat flows in this way as it will likely cause significant re-
   ordering of the flow's packets, which is not desirable. Note that the
   default 802.1D treatment of user_priorities 1 and 2 is "less than"
   the default class 0.

4. Computation of integrated services characterization parameters by
IEEE 802 devices

   The integrated service model requires that each network element that
   supports integrated services compute and make available certain
   "characterization parameters" describing the element's behavior.
   These parameters may be either generally applicable or specific to a
   particular QoS control service.  These parameters may be computed by
   calculation, measurement, or estimation. When a network element
   cannot compute its own parameters (for example, a simple link), we
   assume that the device sending onto or receiving data from the link
   will compute the link's parameters as well as it's own.  The accuracy
   of calculation of these parameters may not be very critical; in some
   cases loose estimates are all that is required to provide a useful
   service. This is important in the IEEE 802 case, where it will be
   virtually impossible to compute parameters accurately for certain
   topologies and switch technologies.  Indeed, it is an assumption of
   the use of this model by relatively simple switches (see [IS802FRAME]
   for a discussion of the different types of switch functionality that
   might be expected) that they merely provide values to describe the
   device and admit flows conservatively.  The discussion below presents
   a general outline for the computation of these parameters, and points
   out some cases where the parameters must be computed accurately.
   Further specification of how to export these parameters is for
   further study.

4.1. General characterization parameters

   There are some general parameters [GENCHAR] that a device will need
   to use and/or supply for all service types:

   *  Ingress link

   *  Egress links and their MTUs, framing overheads and minimum packet
      sizes (see media-specific information presented above).

   *  Available path bandwidth: updated hop-by-hop by any device along
      the path of the flow.

   *  Minimum latency

   Of these parameters, the MTU and minimum packet size information must
   be reported accurately. Also, the "break bits" must be set correctly,
   both the overall bit that indicates the existence of QoS control
   support and the individual bits that specify support for a particular
   scheduling service. The available bandwidth should be reported as
   accurately as possible, but very loose estimates are acceptable. The
   minimum latency parameter should be determined and reported as

   accurately as possible if the element offers Guaranteed service, but
   may be loosely estimated or reported as zero if the element offers
   only Controlled-Load service.

4.2. Parameters to implement Guaranteed Service

   A network element supporting the Guaranteed Service [GS] must be able
   to determine the following parameters:

   *  Constant delay bound through this device (in addition to any value
      provided by "minimum latency" above) and up to the receiver at the
      next network element for the packets of this flow if it were to be
      admitted.  This includes any access latency bound to the outgoing
      link as well as propagation delay across that link. This value is
      advertised as the 'C' parameter of the Guaranteed Service.

   *  Rate-proportional delay bound through this device and up to the
      receiver at the next network element for the packets of this flow
      if it were to be admitted.  This value is advertised as the 'D'
      parameter of the Guaranteed Service.

   *  Receive resources that would need to be associated with this flow
      (e.g., buffering, bandwidth) if it were to be admitted and not
      suffer packet loss if it kept within its supplied Tspec/Rspec.
      These values are used by the admission control algorithm to decide
      whether a new flow can be accepted by the device.

   *  Transmit resources that would need to be associated with this flow
      (e.g., buffering, bandwidth, constant- and rate-proportional delay
      bounds) if it were to be admitted. These values are used by the
      admission control algorithm to decide whether a new flow can be
      accepted by the device.

   The exported characterization parameters for this service should be
   reported as accurately as possible. If estimations or approximations
   are used, they should err in whatever direction causes the user to
   receive better performance than requested. For example, the C and D
   error terms should overestimate delay, rather than underestimate it.

4.3. Parameters to implement Controlled Load

   A network element implementing the Controlled Load service [CL] must
   be able to determine the following:

   *  Receive resources that would need to be associated with this flow
      (e.g., buffering) if it were to be admitted. These values are used
      by the admission control algorithm to decide whether a new flow
      can be accepted by the device.

   *  Transmit resources that would need to be associated with this flow
      (e.g., buffering) if it were to be admitted. These values are used
      by the admission control algorithm to decide whether a new flow
      can be accepted by the device.

   The Controlled Load service does not export any service-specific
   characterization parameters. Internal resource allocation estimates
   should ensure that the service quality remains high when considering
   the statistical aggregation of Controlled Load flows into 802 traffic
   classes.

4.4. Parameters to implement Best Effort

   For a network element that implements only best effort service there
   are no explicit parameters that need to be characterized. Note that
   an integrated services aware network element that implements only
   best effort service will set the "break bit" described in
   [RSVPINTSERV].

5. Merging of RSVP/SBM objects

   Where reservations that use the SBM protocol's TCLASS object [SBM]
   need to be merged, an algorithm needs to be defined that is
   consistent with the mappings to individual user_priority values in
   use in the Layer-2 cloud.  A merged reservation must receive at least
   as good a service as the best of the component reservations.

   There is no single merging rule that can prevent all of the following
   side-effects:

   *  If a merger were to demote the existing branch of the flow into a
      higher-delay traffic class then this is a denial of service to the
      existing flow which would likely receive worse service than
      before.

   *  If a merger were to promote the existing branch of the flow into a
      new, lower-delay, traffic class, this might then suffer either
      admission control failures or may cost more in some sense than the
      already-admitted flow. This can also be considered as a denial-
      of-service attack.

   *  Promotion of the new branch may lead to rejection of the request
      because it has been re-assigned to a traffic class that has not
      enough resources to accommodate it.

   Therefore, such a merger is declared to be illegal and the usual SBM
   admission control failure rules are applied. Traffic class selection
   is performed based on the TSpec information. When the first RESV for

   a flow arrives, a traffic class is chosen based on the request, an
   SBM TCLASS object is inserted into the message and admission control
   for that traffic class is done by the SBM. Reservation succeeds or
   fails as usual.

   When a second RESV for the same flow arrives at a different egress
   point of the Layer-2 cloud the process starts to repeat. Eventually
   the SBM-augmented RESV may hit a switch with an existing reservation
   in place for the flow i.e., an L2 branch point for the flow. If so,
   the traffic class chosen for the second reservation is checked
   against the first. If they are the same, the RESV requests are merged
   and passed on towards the sender(s).

   If the second TCLASS would have been different, an RSVP/SBM ResvErr
   error is returned to the Layer-3 device that launched the second RESV
   request into the Layer-2 cloud. This device will then pass on the
   ResvErr to the original requester according to RSVP rules. Detailed
   processing rules are specified in [SBM].

6. Applicability of these service mappings

   Switches using layer-2-only standards (e.g., 802.1D-1990, 802.1D-
   1998) need to inter-operate with routers and layer-3 switches. Wide
   deployment of such 802.1D-1998 switches will occur in a number of
   roles in the network: "desktop switches" provide dedicated 10/100
   Mbps links to end stations and high speed core switches often act as
   central campus switching points for layer-3 devices. Layer-2 devices
   will have to operate in all of the following scenarios:

   *  every device along a network path is layer-3 capable and intrusive
      into the full data stream

   *  only the edge devices are pure layer-2

   *  every alternate device lacks layer-3 functionality

   *  most devices lack layer-3 functionality except for some key
      control points such as router firewalls, for example.

      Where int-serv flows pass through equipment which does not support
      Integrated Services or 802.1D traffic management and which places
      all packets through the same queuing and overload-dropping paths,
      it is obvious that some of a flow's desired service parameters
      become more difficult to support. In particular, the two
      integrated service classes studied here, Controlled Load and
      Guaranteed Service, both assume that flows will be policed and
      kept "insulated" from misbehaving other flows or from best effort
      traffic during their passage through the network. This cannot be

      done within an IEEE 802 network using devices with the default
      user_priority function; in this case policing must be approximated
      at the network edges.

      In addition, in order to provide a Guaranteed Service, *all*
      switching elements along the path must participate in special
      treatment for packets in such flows: where there is a "break" in
      guaranteed service, all bets are off. Thus, a network path that
      includes even a single switch transmitting onto a shared or half-
      duplex LAN segment is unlikely to be able to provide a very good
      approximation to Guaranteed Service. For Controlled Load service,
      the requirements on the switches and link types are less stringent
      although it is still necessary to provide differential queuing and
      buffering in switches for CL flows over best effort in order to
      approximate CL service. Note that users receive indication of such
      breaks in the path through the "break bits" described in y
      [RSVPINTSERV]. These bits must be correctly set when IEEE 802
      devices that cannot provide a specific service exist in a network.

      Other approaches might be to pass more information between
      switches about the capabilities of their neighbours and to route
      around non-QoS-capable switches: such methods are for further
      study. And of course the easiest solution of all is to upgrade
      links and switches to higher capacities.

7. References

   [802.1D-ORIG] "MAC Bridges", ISO/IEC 10038, ANSI/IEEE Std 802.1D-1993

   [802.1D]      "Information technology - Telecommunications and
                 information exchange between systems - Local and
                 metropolitan area networks - Common specifications -
                 Part 3: Media Access Control (MAC) Bridges:  Revision.
                 This is a revision of ISO/IEC 10038: 1993, 802.1j-1992
                 and 802.6k-1992. It incorporates P802.11c, P802.1p and
                 P802.12e."  ISO/IEC 15802-3:1998"

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

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

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

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

   [802.1Q]      ANSI/IEEE Standard 802.1Q-1998, "IEEE Standards for
                 Local and Metropolitan Area Networks: Virtual Bridged
                 Local Area Networks", 1998.

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

   [IS802FRAME]  Ghanwani, A., Pace, W., Srinivasan, V., Smith, A. and
                 M. Seaman, "A Framework for Providing Integrated
                 Services Over Shared and Switched LAN Technologies",
                 RFC 2816, May 2000.

   [SBM]         Yavatkar, R., Hoffman, D., Bernet, Y., Baker, F. and M.
                 Speer, "SBM (Subnet Bandwidth Manager): A Protocol for
                 Admission Control over IEEE 802-style Networks", RFC
                 2814, May 2000.

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

   [PROCESS]     Bradner, S., "The Internet Standards Process --
                 Revision 3", BCP 9, RFC 2026, October 1996.

8. Security Considerations

   Any use of QoS requires examination of security considerations
   because it leaves the possibility open for denial of service or theft
   of service attacks. This document introduces no new security issues
   on top of those discussed in the companion ISSLL documents
   [IS802FRAME] and [SBM].  Any use of these service mappings assumes
   that all requests for service are authenticated appropriately.

9. Acknowledgments

This document draws heavily on the work of the ISSLL WG of the IETF
and the IEEE P802.1 Interworking Task Group.

10. Authors' Addresses

   Mick Seaman
   Telseon
   480 S. California Ave
   Palo Alto, CA 94306
   USA

   Email: mick@telseon.com

   Andrew Smith
   Extreme Networks
   3585 Monroe St.
   Santa Clara, CA 95051
   USA

   Phone: +1 408 579 2821
   EMail: andrew@extremenetworks.com

   Eric Crawley
   Unisphere Solutions
   5 Carlisle Rd.
   Westford, MA 01886

   Phone: +1 978 692 1999
   Email: esc@unispheresolutions.com

   John Wroclawski
   MIT Laboratory for Computer Science
   545 Technology Sq.
   Cambridge, MA  02139
   USA

   Phone: +1 617 253 7885
   EMail: jtw@lcs.mit.edu

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