Internet Engineering Task Force                                Y. Bernet
Diffserv Working Group                                         Microsoft
INTERNET-DRAFT                                                  A. Smith
Expires: April 2000                                     Extreme Networks
                                                                S. Blake
                                                            October 1999

                A Conceptual Model for Diffserv Routers


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This document is a product of the Diffserv working group.  Comments
   on this draft should be directed to the Diffserv mailing list

   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   This draft proposes a conceptual model of Differentiated Services
   (Diffserv) routers for use in their management and configuration.
   This model defines the general functional datapath elements
   (classifiers, meters, markers, droppers, monitors, mirrors, muxes,
   queues), their possible configuration parameters, and how they might
   be interconnected to realize the range of classification, traffic
   conditioning, and per-hop behavior (PHB) functionalities described in
   [DSARCH].  The model is intended to be abstract and capable of
   representing the configuration parameters important to Diffserv
   functionality for a variety of specific router implementations.  It

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   is not intended as a guide to hardware implementation.

   This model should serve as a rationale for the design of a Diffserv
   MIB [DSMIB], as well for various configuration interfaces (such as
   [PIB]).  Since these documents are all evolving simultaneously there
   are discrepancies between their current revisions; this should be
   resolved in a future revision of this draft.

Table of Contents

   1.  Introduction .................................................  3
   2.  Glossary  ....................................................  4
   3.  Conceptual Model .............................................  6
     3.1  Elements of a Diffserv Router .............................  6
       3.1.1  Datapath ..............................................  7
       3.1.2  Configuration and Management Interface ................  8
       3.1.3  Optional RSVP Module ..................................  8
     3.2  Hierarchical Model of Diffserv Components .................  8
   4.  Classifiers .................................................. 10
     4.1  Definition ................................................ 10
       4.1.1  Filters ............................................... 11
       4.1.2  Overlapping Filters ................................... 12
       4.1.3  Filter Groups ......................................... 12
     4.2  Examples .................................................. 12
       4.2.1  Behavior Aggregate (BA) Classifier .................... 12
       4.2.2  Multi-Field (MF) Classifier ........................... 13
       4.2.3  IEEE802 MAC Address Classifier ........................ 13
       4.2.4  Free-form Classifier .................................. 14
       4.2.5  Other Possible Classifiers ............................ 14
     4.3  MPLS ...................................................... 15
   5.  Meters ....................................................... 15
     5.1  Definition ................................................ 15
     5.2  Examples .................................................. 16
       5.2.1  Average Rate Meter .................................... 16
       5.2.2  Exponentially Weighted Moving Average (EWMA) Meter .... 17
       5.2.3  Two-Parameter Token Bucket Meter ...................... 17
       5.2.4  Multi-Stage Token Bucket Meter ........................ 18
       5.2.5  Null Meter ............................................ 19
   6.  Action Elements .............................................. 19
     6.1  Marker .................................................... 19
     6.2  Dropper ................................................... 20
     6.3  Shaper .................................................... 20
     6.4  Mirroring Element ......................................... 20
     6.5  Multiplexor ............................................... 20
     6.6  Enqueueing Element ........................................ 20
     6.7  Monitor ................................................... 21
     6.8  Null Action ............................................... 21
   7.  Queues ....................................................... 21
     7.1  Queue Sets and Scheduling ................................. 21
     7.2  Shaping ................................................... 23
   8.  Traffic Conditioning Blocks (TCBs) ........................... 23
     8.1  An Example TCB ............................................ 24

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     8.2  An Example TCB to Support Multiple Customers .............. 27
     8.3  TCBs Supporting Microflow-based Services .................. 28
   9.  Open Issues .................................................. 31
  10.  Security Considerations ...................................... 31
  11.  Acknowledgments .............................................. 31
  12.  References ................................................... 32
  Appendix A.  Simple Token Bucket Definition ....................... 33

1. Introduction

   Differentiated Services (Diffserv) [DSARCH] is a set of technologies
   which allow network service providers to offer differing levels of
   network quality-of-service (QoS) to different customers and their
   traffic streams.  The premise of Diffserv networks is that routers
   within the core of the network handle packets in different traffic
   streams by forwarding them using different per-hop behaviors (PHBs).
   The PHB to be applied is indicated by a Diffserv codepoint (DSCP) in
   the IP header of each packet [DSFIELD].   Note that this document
   uses the terminology defined in [DSARCH, DSTERMS] and in Sec. 2.

   The advantage of such a scheme is that many traffic streams can be
   aggregated to one of a small number of behavior aggregates (BA)
   which are each forwarded using the same PHB at the router, thereby
   simplifying the processing and associated storage.  In addition,
   there is no signaling, other than what is carried in the DSCP of
   each packet, and no other related processing that is required in the
   core of the Diffserv network since QoS is invoked on a packet-by-
   packet basis.

   The Diffserv architecture enables a variety of possible services
   which could be deployed in a network.  These services are reflected
   to customers at the edges of the Diffserv network in the form of a
   Service Level Specification (SLS) [DSTERMS].  The ability to provide
   these services depends on the availability of cohesive management and
   configuration tools that can be used to provision and monitor a set
   of Diffserv routers in a coordinated manner.  To facilitate the
   development of such configuration and management tools it is helpful
   to define a conceptual model of a Diffserv router that abstracts
   away implementation details of particular Diffserv routers from the
   parameters of interest for configuration and management.  The purpose
   of this draft is to define such a model.

   The basic forwarding functionality of a Diffserv router is defined in
   other specifications; e.g., [DSARCH, DSFIELD, AF-PHB, EF-PHB].

   This document is not intended in any way to constrain or to dictate
   the implementation alternatives of Diffserv routers.  We expect that
   router vendors will demonstrate a great deal of variability in their
   implementations.  To the extent that vendors are able to model their
   implementations using the abstractions described in this draft,
   configuration and management tools will more readily be able to
   configure and manage networks incorporating Diffserv routers of

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   various implementations.
   In Sec. 3 we start by describing the basic high-level functional
   elements of a Diffserv router and then describe the various
   components.  We then focus on the Diffserv-specific components of
   the router and describe a hierarchical management model for these.

   In Sec. 4 we describe classification elements and in Sec. 5, we
   discuss the meter elements.

   In Sec. 6 we discuss action elements.  In Sec. 7 we discuss the
   basic queueing elements and their functional behaviors (e.g.,

   In Sec. 8, we show how the basic classification, meter, action, and
   queueing elements can be combined to build modules called Traffic
   Conditioning Blocks (TCBs).

   In Sec. 9 we discuss open issues with this document and in Sec. 10 we
   discuss security concerns.

   Appendix A discusses token bucket implementation details.

2.  Glossary

   Some of the terms used in this draft are defined in [DSARCH] and in
   [DSTERMS].  We define a few of them here again only to provide
   additional detail.

   Buffer        An algorithm used to determine whether an arriving
   management    packet should be stored in a queue, or discarded.  This
   algorithm     decision is usually a function of the instantaneous or
                 average queue occupancy, but also may be a function of
                 the aggregate queue occupancy in a queue set, or of
                 other parameters.

   Classifier    A functional datapath element which consists of filters
                 which select packets based on the content of packet
                 headers or other packet data, and/or on implicit or
                 derived attributes associated with the packet, and
                 forwards the packet along a particular datapath within
                 the router.  A classifier splits a single incoming
                 traffic stream into multiple outgoing ones.

   Enqueueing    The process of executing a buffer management algorithm
                 to determine whether an arriving packet should be
                 stored in a queue.

   Filter        A set of (wildcard/prefix/masked/range/exact)
                 conditions on the components of a packet's
                 classification key.  A filter is said to match only if
                 each condition is satisfied.

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   Mirroring     A functional datapath element which makes one or more
   element       copies of a packet and forwards them on distinct
                 datapaths; for example to a monitoring port.

   Monitor       A functional datapath element which increments an octet
                 and a packet counter for every packet which passes
                 through it.  Used for collecting statistics.

   Multiplexer   A functional datapath element that merges multiple
   (Mux)         traffic streams (datapaths) into a single traffic
                 stream (datapath).

   Non-work      A property of a scheduling algorithm such that it does
   conserving    not necessarily service a packet if available at every
                 transmission opportunity.

   Queue         A storage location for packets awaiting transmission or
                 processing by the next functional element in the data-
                 path.  The queues represented in this model are
                 abstract elements that may be implemented by multiple
                 physical queues in series and/or in parallel in a
                 specific implementation.  Note that we assume that a
                 queue is serviced such as to preserve the required
                 ordering constraint for each Ordering Aggregate (OA)
                 it queues [DSTERMS].  This can be achieved by a FIFO
                 (first in, first out) service policy or by other means
                 (e.g., multiple FIFOs exclusively servicing particular

   Queue set     A set of queues which are serviced by a scheduling
                 algorithm and which may share a buffer management

   Scheduling    An algorithm which determines which queue of a queue
   algorithm     set to service next.  This may be based on the relative
                 priority of the queues, or on a weighted fair bandwidth
                 sharing policy, or some other policy.  A scheduling
                 algorithm may be either work-conserving or non-work-

   Shaping       The process of delaying packets within a traffic stream
                 to cause it to conform to some defined traffic profile.
                 Shaping can be implemented using a queue serviced by a
                 non-work conserving scheduling algorithm.

   Traffic       A logical datapath element consisting of a number of
   Conditioning  other functional datapath elements interconnected in
   Block (TCB)   such a way as to perform a specific set of traffic
                 conditioning functions on an incoming traffic stream.
                 A TCB can be thought of as a "black box" with a single
                 input and output.

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   Work          A property of a scheduling algorithm such that it
   conserving    services a packet if available at every transmission

3.  Conceptual Model

   In this section we introduce a block diagram of a Diffserv router and
   describe the various components illustrated.  Note that a Diffserv
   core router is assumed to include only a subset of these components:
   the model we present here is intended to cover the case of both
   Diffserv edge and core routers.

3.1  Elements of a Diffserv Router

   The conceptual model we define includes abstract definitions for the

   o  The basic traffic classification components.

   o  The basic traffic conditioning components.

   o  Certain combinations of traffic classification and conditioning

   o  Queueing components.

   The components and combinations of components described in this
   document form building blocks that need to be manageable by Diffserv
   configuration and management tools.  One of the goals of this
   document is to show how a model of a Diffserv device can be built
   using these component blocks.  This model is in the form of a
   connected directed acyclic graph (DAG) of functional datapath
   elements that describes the traffic conditioning and queueing
   behaviors that any particular packet will experience when forwarded
   to the Diffserv router.

   The following diagram illustrates the major functional blocks of a
   Diffserv router:

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               |  Diffserv     |
        Mgmt   | configuration |
      <----+-->| & management  |------------------+
      SNMP,|   |  interface    |                  |
      COPS |   +---------------+                  |
      etc. |        |                             |
           |        |                             |
           |        v                             v
           |   +-------------+   +---------+   +-------------+
      data |   | ingress i/f |   |         |   | egress i/f  |
      -------->|   class.,   |-->| routing |-->|   class.,   |---->
           |   |     TC,     |   |  core   |   |     TC,     |
           |   |   queueing  |   |         |   |   queueing  |
           |   +-------------+   +---------+   +-------------+
           |        ^                             ^
           |        |                             |
           |        |                             |
           |   +------------+                     |
           +-->|    RSVP    |                     |
      -------->| (optional) |---------------------+
        RSVP   +------------+

      Figure 1:  Diffserv Router Major Functional Blocks

3.1.1  Datapath

   An ingress interface, routing core, and egress interface are
   illustrated at the center of the diagram.  In actual router
   implementations, there may be an arbitrary number of ingress and
   egress interfaces interconnected by the routing core.  The routing
   core element serves as an abstraction of a router's normal routing
   and switching functionality.  The routing core moves packets between
   interfaces according to policies outside the scope of Diffserv.  The
   actual queueing delay and packet loss behavior of a specific router's
   switching fabric/backplane is not modeled by the routing core; these
   should be modeled using the functional elements described later.  The
   routing core should be thought of as an infinite bandwidth, zero-
   delay backplane connecting ingress and egress interfaces.

   The components of interest on the ingress/egress interfaces are the
   traffic classifiers, traffic conditioning (TC) components, and the
   queueing components that support Diffserv traffic conditioning and
   per-hop behaviors [DSARCH].  These are the fundamental components
   comprising a Diffserv router and will be the focal point of our
   conceptual model.

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3.1.2  Configuration and Management Interface

   Diffserv operating parameters are monitored and provisioned through
   this interface.  Monitored parameters include statistics regarding
   traffic carried at various Diffserv service levels.  These statistics
   may be important for accounting purposes and/or for tracking
   compliance to traffic conditioning specifications (TCSs) [DSTERMS]
   negotiated with customers.  Provisioned parameters are primarily
   classification rules, TC and PHB configuration parameters.  The
   network administrator interacts with the Diffserv configuration and
   management interface via one or more management protocols, such as
   SNMP or COPS, or through other router configuration tools such as
   serial terminal or telnet consoles.

3.1.3 Optional RSVP Module

   Diffserv routers may snoop or participate in either per-microflow or
   per-flow-aggregate signaling of QoS requirements [E2E].  The example
   discussed here uses the RSVP protocol.  Snooping of RSVP messages may
   be used, for example, to learn how to classify traffic without
   actually participating as a RSVP protocol peer.  Diffserv routers may
   reject or admit RSVP reservation requests to provide a means of
   admission control to Diffserv-based services or they may use these
   requests to trigger provisioning changes for a flow-aggregation in
   the Diffserv network.  A flow-aggregation in this context might be
   equivalent to a Diffserv BA or it may be more fine-grained, relying
   on a MF classifier [DSARCH].  Note that the conceptual model of such
   a router starts to look the same as a Integrated Services (intserv)
   router in its component makeup [E2E].

   Note that a RSVP component of a Diffserv router, if present, might
   be active only in the control plane and not in the data plane.  In
   this scenario, RSVP is used strictly as a signaling protocol.  The
   data plane of such a Diffserv router can still act purely on Diffserv
   DSCPs and PHBs in handling data traffic.

3.2  Hierarchical Model of Diffserv Components

   We focus on the Diffserv specific functional components of the
   router: the classification, traffic conditioning, and queueing
   functionality.  The diagram below is based on the larger block
   diagram shown above:

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             Interface A                        Interface B
          +-------------+     +---------+     +-------------+
          | ingress i/f |     |         |     | egress i/f  |
          |   class.,   |     |         |     |   class.,   |
      --->|   meter,    |---->|         |---->|   meter,    |--->
          |   action,   |     |         |     |   action,   |
          |   queueing  |     |         |     |   queueing  |
          +-------------+     | routing |     +-------------+
                              |  core   |
          +-------------+     |         |     +-------------+
          | egress i/f  |     |         |     | ingress i/f |
          |   class.,   |     |         |     |   class.,   |
      <---|   meter,    |<----|         |<----|   meter,    |<---
          |   action,   |     |         |     |   action,   |
          |   queueing  |     +---------+     |   queueing  |
          +-------------+                     +-------------+

      Figure 2.  Traffic Conditioning and Queueing Elements

   This diagram illustrates two Diffserv router interfaces, each having
   an ingress and an egress component.  It shows classification, meter,
   action, and queueing elements which might be instantiated on each
   interface's ingress and egress component.  The TC functionality is
   implemented by a combination of classification, action, meter, and
   queueing elements.  We show equivalent functional elements on both
   the ingress and egress components of an interface because we expect
   an N-port router to display the same Diffserv capabilities as a
   network of 2-port routers interconnected by LAN media [DSMIB].  Note
   that it is not mandatory that each of these functional elements be
   implemented on both ingress and egress components; it is dependent on
   the service requirements on a particular interface on a particular
   router.  Further, we wish to point out that by showing these elements
   on both ingress and egress components we do not mean to imply that
   they must be implemented in this way in a specific router.  For
   example, a router may implement all shaping and PHB queueing on the
   interface egress component, or may instead implement it only on the
   ingress component.  Further, the classification needed to map a
   packet to an egress component queue (if present) need not be
   implemented on the egress component but instead may be implemented on
   the ingress component, with the packet passed through the routing
   core with in-band control information to allow for egress queue

   From a configuration and management perspective, the following
   hierarchy exists:

   At the top level, the network administrator manages interfaces.  Each
   interface consists of an ingress component and an egress component.
   Each component may contain classifier, action, meter, and queueing

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   At the next level, the network administrator manages groups of
   functional elements interconnected in a DAG.  These elements are
   organized in self-contained Traffic Conditioning Blocks (TCBs) which
   are used to implement some desired network policy (see Sec. 8).  One
   or more TCBs may be instantiated on each ingress or egress component,
   may be connected in series, and/or may be connected in a
   parallel configuration on the multiple outputs of a classifier.
   We define the TCB to optionally include classification and queueing
   elements so as to allow for rich functionality.  A TCB can be thought
   of as a "black box" with a single input and a single output (on the
   main data path).  TCBs can be constructed out of a DAG of other TCBs,
   recursively.  We do not assume the same TCB configuration on every
   interface (ingress or egress).

   At the lowest level are individual functional elements, each with
   their own configuration parameters and management counters and flags.

4.  Classifiers

4.1  Definition

   Classification is performed by a classifier element.  Classifiers are
   1:N (fan-out) devices: they take a single traffic stream as input and
   generate N logically separate traffic streams as output.  Classifiers
   are parameterized by filters and output streams.  Packets from the
   input stream are sorted into various output streams by filters which
   match the contents of the packet or possibly match other attributes
   associated with the packet.  Various types of classifiers are
   described in the following sections.

   We use the following diagram to illustrate a classifier, where the
   outputs connect to succeeding functional elements:

      unclassified              classified
      traffic                   traffic
              |            |--> match Filter1 --> output A
      ------->| classifier |--> match Filter2 --> output B
              |            |--> no match      --> output C

      Figure 3.  An Example Classifier

   Note that we allow a mux (see Sec. 6.5) before the classifier to
   allow input from multiple traffic streams.  For example, if multiple
   ingress sub-interfaces feed through a single classifier then the
   interface number can be considered by the classifier as a packet
   attribute and be included in the packet's classification key.  This
   optimization may be important for scalability in the management
   plane.  Another possible packet attribute could be an integer

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   representing the BGP community string associated with the packet's
   best-matching route.

   The following classifier separates traffic into one of three output
   streams based on three filters:

      Filter Matched        Output Stream
      --------------       ---------------
      Filter1                    A
      Filter2                    B
      Filter3 (no match)         C

   Where Filters1 and Filter2 are defined to be the following BA filters
   ([DSARCH], see Sec. 4.2.1 ):

      Filter        DSCP
      ------       ------
        1           101010
        2           111111
        3           ****** (wildcard)

4.1.1  Filters

   A filter consists of a set of conditions on the component values of
   a packet's classification key (the header values, contents, and
   attributes relevant for classification).  In the BA classifier
   example above, the classification key consists of one packet header
   field, the DSCP, and both Filter1 and Filter2 specify exact-match
   conditions on the value of the DSCP.  Filter3 is a wildcard default
   filter which matches every packet, but which is only selected in the
   event that no other more specific filter matches.

   In general there are a set of possible component conditions including
   exact, prefix, range, masked, and wildcard matches.  Note that ranges
   can be represented (with less efficiency) as a set of prefixes and
   that prefix matches are just a special case of both masked and range

   In the case of a MF classifier [DSARCH], the classification key
   consists of a number of packet header fields.  The filter may
   specify a different condition for each key component, as illustrated
   in the example below for a IPv4/TCP classifier:

      Filter   IP Src Addr    IP Dest Addr   TCP SrcPort TCP DestPort
      ------   -------------  -------------  -----------  ------------
      Filter4  172.31.3.X/24       X          5003

   In this example, the fourth octet of the destination IPv4 address
   and the source TCP port are wildcard or "don't cares".

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4.1.2  Overlapping Filters

   Note that it is easy to define sets of overlapping filters in a
   classifier.  For example:

      Filter5:              Filter6:
      Type:   Masked-DSCP   Type:   Masked-DSCP
      Value:  111000        Value:  000111 (binary)
      Mask:   111000        Mask:   000111 (binary)

   A packet containing DSCP = 111111 cannot be uniquely classified by
   this pair of filters and so a precedence must be established between
   Filter5 and Filter6 in order to break the tie.  This precedence must
   be established either (a) by a manager which knows that the router
   can accomplish this particular ordering; e.g., by means of reported
   capabilities or (b) by the router along with a mechanism to report
   to a manager which precedence is being used.  These ordering
   mechanisms must be supported by the configuration and management
   protocols although further discussion of this is outside the scope of
   this document.

   An unambiguous classifier requires that every possible classification
   key match at least one filter (including the wildcard default), and
   that any ambiguity between overlapping filters be resolved by

4.1.3  Filter Groups

   Filters may be logically combined.  For example, consider the
   following DestMacAddress filter:

      Type:        DestMacAddress
      Value:       01-02-03-04-05-06
      Mask:        FF-FF-FF-FF-FF-FF

   Classifier0 could then be declared as:

      Filter1 and Filter7:         output A
      Filter2 and Filter7:         output B
      Default (wildcard) filter:   output C

4.2  Examples

4.2.1  Behaviour Aggregate (BA) Classifier

   The simplest Diffserv classifier is a behavior aggregate (BA)
   classifier [DSARCH].  A BA classifier uses only the Diffserv
   codepoint (DSCP) in a packet's IP header to determine the logical
   output stream to which the packet should be directed.  We allow only
   an exact-match condition on this field because the assigned DSCP

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   values have no structure, and therefore no subset of DSCP bits are

   The following defines a possible BA filter:

      Type:   BA
      Value:  111000

4.2.2  Multi-Field (MF) Classifier

   Another type of classifier is a multi-field (MF) classifier [DSARCH].
   This classifies packets based on one or more fields in the packet
   header (including the DSCP).  A common type of MF classifier is a 6-
   tuple classifier that classifies based on six IP header fields
   (destination address, source address, IP protocol, source port,
   destination port, and DSCP).  MF classifiers may classify on other
   fields such as MAC addresses, VLAN tags, link-layer traffic class
   fields or other higher-layer protocol fields.

   The following defines a possible MF filter:

      Type:              IPv4-6-tuple
      IPv4DestAddrValue: 0
      IPv4DSCP:          28
      IPv4Protocol:      6
      IPv4DestL4PortMin: 0
      IPv4DestL4PortMax: 65535
      IPv4SrcL4PortMin:  20
      IPv4SrcL4PortMax:  20

   A similar type of classifier can be defined for IPv6.

4.2.3 IEEE802 MAC Address Classifier

   A MacAddress filter is parameterized by a 6-byte {value, mask} pair
   for either source or destination MAC address.  For example, the
   following classifier sends packets matching either DA =
   01-02-03-04-05-06 or SA = 00-E0-2B-XX-XX-XX to output A:

      Filter10:     output A
      Filter11:     output A
      Default:      output B

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      Type:        DestMacAddress
      Value:       01-02-03-04-05-06 (hex)
      Mask:        FF-FF-FF-FF-FF-FF (hex)

      Type:        SrcMacAddress
      DestValue:   00-E0-2B-00-00-00 (hex)
      DestMask:    FF-FF-FF-00-00-00 (hex)

4.2.4  Free-form Classifier

   A Free-form classifier is made up of a set of user definable
   arbitrary filters each made up of {bit-field size, offset (from head
   of packet), mask}:

      Filter12:    output A
      Filter13:     output B
      Default:     output C

      Type:        FreeForm
      SizeBits:    3 (bits)
      Offset:      16 (bytes)
      Value:       100 (binary)
      Mask:        101 (binary)

      Type:        FreeForm
      SizeBits:    12 (bits)
      Offset:      16 (bytes)
      Value:       100100000000 (binary)
      Mask:        111111111111 (binary)

   Free-form filters can be combined into filter groups to form very
   powerful filters.

4.2.5  Other Possible Classifiers

      Filter14:     output A
      Filter15:     output B
      Default:      output C

      Type:        IEEEPriority
      Value:       100 (binary)
      Mask:        101 (binary)

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      Type:        IEEEVLAN
      Value:       100100000000 (binary)
      Mask:        111111111111 (binary)

   Classification may be performed based on implicit information
   associated with a packet (e.g. the incoming channel number on a
   channelized interface) or on information derived from a different
   non-Diffserv classification operation (e.g. the outgoing interface
   determined by the route lookup operation).  Other vendor-specific
   filter formats are possible.  We do not discuss these further here.

4.3  MPLS

   It is possible for an MPLS label-switched router (LSR) to function as
   a Diffserv router [MPLSDS].  In this case the IP header is not
   visible for inspection and all header classification must be
   performed on the MPLS label, and in the event of shim encapsulation,
   on the 3-bit EXP field in addition.  In general a MPLS classification
   filter may specify either wildcard- or exact-match conditions for
   either field (but not both wildcard at once).  The distinction to be
   drawn here is that MPLS labels are dynamically established and torn
   down.  An EXP-only classifier may be statically configured but a
   label or label + EXP classifier must be established dynamically along
   with the LSP.  In all other respects (except marking) the labeled
   packet can be treated identically to an unlabeled packet.

5.  Meters

5.1  Definition

   Metering is the function of monitoring the arrival times of packets
   of a traffic stream and determining the level of conformance of each
   packet to a pre-established traffic profile.  Diffserv network
   providers may choose to offer services to customers based on a
   temporal (i.e., rate) profile within which the customer submits
   traffic for the service.  In this event, a meter might be used to
   trigger real-time traffic conditioning actions (e.g., marking) by
   routing a non-conforming packet through an appropriate next-stage
   action element.  Alternatively, it might also be used for out-of-band
   management functions like statistics monitoring for billing

   Meters are logically 1:N (fan-out) devices (although a mux can be
   used in front of a meter).  Meters are parameterized by a temporal
   profile and by conformance levels, each of which is associated with
   a meter's output.  Each output can be connected to another functional

   Note that this model of a meter differs from that described in
   [DSARCH].  In that description the meter is not a datapath element
   but is instead used to monitor the traffic stream and send control

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   signals to action elements to dynamically modulate their behavior
   based on the conformance of the packet.  We find the description here
   more powerful.

   We use the following diagram to illustrate a meter with 3 levels of

      unmetered              metered
      traffic                traffic

                |         |--------> conformanceA
      --------->|  meter  |--------> conformanceB
                |         |--------> conformanceC

      Figure 4.  An Example Meter

   In some Diffserv examples, three levels of conformance are discussed
   in terms of colors, with green representing conforming, yellow
   representing partially conforming, and red representing non-
   conforming [AF-PHB].  These different conformance levels are used to
   trigger different buffer management actions.  Other example meters
   use a binary notion of conformance; in the general case N levels of
   conformance can be supported.  In general there is no constraint on
   the type of functional element following a meter output, but care
   must be taken not to inadvertently configure a datapath that results
   in packet reordering within an OA.

5.2  Examples

   The following is a non-exhaustive list of possible meters.

5.2.1  Average Rate Meter

   An example of a very simple meter is an average rate meter.  This
   type of meter measures the average rate at which packets are
   submitted to it over a specified averaging time.

   An average rate profile may take the following form:

      Type:                AverageRate
      Profile1:            output A
      NonConforming:       output B

      Type:                AverageRate
      AverageRate:         120 KBps
      Delta:               1.0 msec

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   A meter measuring against this profile would continually maintain a
   count that indicates the total number of packets arriving between
   time T (now) and time T - 1.0 msecs.  So long as an arriving packet
   does not push the count over 120 bytes, the packet would be deemed
   conforming.  Any packet that pushes the count over 120 would be
   deemed non-conforming.  Thus, this meter deems packets to correspond
   to one of two conformance levels: conforming or non-conforming.

5.2.2  Exponential Weighted Moving Average (EWMA) Meter

   The EWMA form of meter is easy to implement in hardware and can be
   parameterized as follows:

      avg_rate(t) = (1 - Gain) * avg_rate(t') +  Gain * rate(t)
      t = t' + Delta

   For a packet arriving at time t:

      if (avg_rate(t) > AverageRate)

   Gain controls the time constant (e.g. frequency response) of what is
   essentially a simple IIR low-pass filter.  rate(t) measures the
   number of incoming bytes in a small fixed sampling interval, Delta.
   Any packet that arrives and pushes the average rate over a predefined
   rate AverageRate is deemed non-conforming.  An EWMA meter profile
   might look as follows:

      Type:                ExpWeightedMovingAvg
      Profile2:            output A
      NonConforming:       output B

      Type:                ExpWeightedMovingAvg
      AverageRate:         25 KBps
      Delta:               10.0 usec
      Gain:                1/16

5.2.3  Two-Parameter Token Bucket Meter

   A more sophisticated meter might measure conformance to a token
   bucket (TB) profile.  A TB profile generally has two parameters, an
   average token rate, a burst size.  TB meters compare the arrival
   rate of packets to the average rate specified by the TB profile.
   Logically, byte tokens accumulate in a bucket at the average rate,
   up to a maximum credit which is the burst size.  Packets of length
   L bytes are considered conforming if L tokens are available in the
   bucket at the time of packet arrival.  Packets are allowed to
   exceed the average rate in bursts up to the burst size.  Packets

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   which arrive to find a bucket with insufficient tokens in it are
   deemed non-conforming.  A two-parameter TB meter has exactly two
   possible conformance levels (conforming, non-conforming).  TB
   implementation details are discussed in Appendix A.

   A two-parameter RB meter profile might look as follows:

      Type:                SimpleTokenBucket
      Profile3:            output A
      NonConforming:       output B

      Type:                SimpleTokenBucket
      AverageRate:         100 KBps
      BurstSize:           100 KB

5.2.4  Multi-Stage Token Bucket Meter

   More complicated TB meters might define two burst sizes and three
   conformance levels.  Packets found to exceed the larger burst size
   are deemed non-conforming.  Packets found to exceed the smaller
   burst size are deemed partially conforming.  Packets exceeding
   neither are deemed conforming.  Token bucket meters designed for
   Diffserv networks are described in more detail in [SRTCM, TRTCM,
   GTC]; in some of these references three levels of conformance are
   discussed in terms of colors, with green representing conforming,
   yellow representing partially conforming and red representing non-
   conforming.  Often these multi-conformance level meters can be
   implemented using an appropriate configuration of multiple two-
   parameter TB meters.

   A profile for a multi-stage TB meter with three levels of conformance
   might look as follows:

      Type:                MultiTokenBucket
      Profile4:            output A
      Profile5:            output B
      NonConforming:       output C

      Type:                SimpleTokenBucket
      AverageRate:         100 KBps
      BurstSize:           20 KB

      Type:                SimpleTokenBucket
      AverageRate:         100 KBps
      BurstSize:           100 KB

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5.2.5  Null Meter

   A null meter has only one output: always conforming, and no
   associated temporal profile.  Such a meter is useful to define in the
   event that the configuration or management interface does not have
   the flexibility to omit a meter in a datapath segment.

6.  Action Elements

   Classifiers and meters are fan-out elements which are generally used
   to determine the appropriate action to apply to a packet.  The set of
   possible actions include:

   1) Marking
   2) Dropping
   2) Shaping
   3) Mirroring
   4) Monitoring

   The corresponding action elements are described in the following

   Policing is a general term for the process of preventing a traffic
   stream from seizing more than its share of resources from a Diffserv
   network.  Each of the first three actions described above may be used
   to police traffic.  Markers do so by re-marking non-conforming
   packets to a DSCP value that is entitled to fewer network resources.
   Shapers and droppers do so by limiting the rate at which a particular
   traffic stream is submitted to the network.

6.1  Marker

   Markers are 1:1 elements which set the DSCP in an IP header (in
   the case of unlabeled packets).  Markers may act on unmarked packets
   (submitted with DSCP of zero) or may re-mark previously marked
   packets.  In particular, the model supports the application of
   marking based on a preceding classifier match.  The DSCP set in a
   packet will determine its subsequent treatment in downstream nodes
   of a network, and possible in subsequent processing stages within the
   router (depending on configuration).

   Markers are normally parameterized by a single parameter: the 6-bit
   DSCP to be marked in the packet header.

      Type:                Marker
      Mark:                010010

   In the case of a MPLS labeled packet, the marker is parameterized
   by a 3-bit EXP value to be marked in the MPLS shim header.

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6.2  Dropper

   Droppers simply discard packets. There are no parameters for
   droppers.  Because a dropper is a terminating point of the datapath,
   it may be desirable to forward the packet through a monitor first
   for instrumentation purposes.

   Droppers are not the only elements than can cause a packet to be
   discarded.  The other element is an enqueueing element (see Sec.
   6.6).  However, since the enqueueing element's behavior is closely
   tied the state of one or more queues, we choose to distinguish them
   as separate functional elements.

6.3  Shaper

   Shapers are used to shape traffic streams to a certain temporal
   profile.  For example, a shaper can be used to smooth traffic
   arriving in bursts.  In [DSARCH] a shaper is described as a
   queueing element controlled by a meter which defines its temporal
   profile.  This model of a shaper differs substantially from typical
   shaper implementations.  Further, with the inclusion of queueing
   elements in the model a separate shaping element becomes confusing.
   Therefore, the function of a shaper is embedded in a queue and is
   covered in Sec. 7.

6.4  Mirroring Element

   It is occasionally desirable to mirror data traffic on one or more
   additional interfaces for data collection purposes.  A mirroring
   element is a 1:N (fan-out) element.  However, each and every packet
   follows each output path simultaneously.  A mirroring element is
   parameterized by the number of outputs it supports.

6.5  Mux

   It is occasionally necessary to multiplex traffic streams into a 1:1
   or 1:N action element or classifier.  A M:1 (fan-in) mux is a simple
   logical device for merging traffic streams.  It is parameterized by
   its number of incoming ports.

6.6  Enqueueing Element

   Queueing elements (discussed in Sec. 7) require an action element to
   execute the appropriate buffer management algorithm and store or
   discard a packet.  This is performed by an enqueueing element, which
   is an M:1 (fan-in) element.  An enqueueing element executes the
   buffer management algorithm appropriate for the queue it is feeding.
   This may include a deterministic discard behavior if the queue size
   exceeds a threshold, it may include a random discard behavior that
   is a function of the average queue size [AF-PHB], or it may include
   a more complex policy which is a function of the state of several
   queues in a queue set (see Sec. 7).  The particular parameters to

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   apply to a packet may depend on the particular input port the element
   receives it on; this allows packets which are classified into
   different colors to follow different datapaths and be processed
   appropriately at the enqueueing element.

   The configuration parameters for an enqueueing element will depend on
   the details of the algorithm it is executing.  For an algorithm such
   as the one recommended in [AF-PHB], the parameters would include
   separate RED min_th, max_th, and max_p parameters per-element input

   An enqueueing element must maintain octet/packet counters for both
   the forwarded and discarded packets received at each element input
   port.  Counters should be provided to distinguish between losses due
   to the normal operation of the algorithm (e.g., random drop) and
   those due to resource exhaustion (e.g., tail drop) [DSMIB].

6.7  Monitor

   One passive action is to account for the fact that a data packet was
   processed.  The statistics that result might be used later for
   customer billing, service verification, or network engineering
   purposes.  Monitors are 1:1 functional elements which increment an
   octet counter by L and a packet counter by 1 every time a L-byte
   sized packet passes through it.  Monitors can also be used to count
   packets on the verge of being dropped by a dropper.

6.8  Null Action

   A null action has one input and one output.  The element performs no
   action on the packet.  Such an element is useful to define in the
   event that the configuration or management interface does not have
   the flexibility to omit an action element in a datapath segment.

7.  Queues

7.1  Queue Sets and Scheduling

   Queues are used to store packets prior to transmission or prior to
   forwarding to the next functional element.  Packets are usually
   stored either because there is a resource constraint (e.g., available
   bandwidth) which prevents immediate forwarding, or because the queue
   is being used to alter the temporal properties of a traffic stream
   (shaping).  Queues may be organized into queue sets, which are
   serviced using a common scheduling algorithm (although each queue may
   be individually parameterized).  Queue sets can be treated as
   functional elements and organized hierarchically in queue supersets,
   using an n-th order scheduling algorithm.  Such a queue set may be
   used to implement the entire range of PHBs on an egress interface,
   for instance.

   Possible queue scheduling algorithms fall into a number of

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   categories, including strict priority, weighted fair bandwidth
   sharing (e.g., WFQ, WRR, etc.), rate-limited strict priority, etc.
   Scheduling algorithms can be further distinguished by whether they
   are work conserving or non-work conserving.  A work conserving
   algorithm will always transmit an available packet at every
   transmission opportunity, while a non-work conserving algorithm will
   not.  Non-work conserving schedulers can be used to shape traffic
   streams by delaying packets that would be deemed non-conforming by
   some traffic profile.  The packet is delayed until such time that it
   would conform to a meter using the same profile.

   [DSARCH] defines PHBs without specifying required queueing
   algorithms.  However, PHBs such as EF [EF-PHB] and AF [AF-PHB] have
   configuration parameters which strongly suggest the sort of queue
   scheduling algorithm needed to implement them.  We have selected a
   minimal set of queue parameters to enable realization of these per-
   hop behaviors.  These include a minimum service rate and a strict
   service priority along with an optional maximum service rate profile
   (depending on whether the queue is meant to be non-work conserving).
   The minimum service rate allows throughput guarantees for each queue
   as required by EF and AF without specifying the details of how excess
   bandwidth between these queues is shared (additional parameters to
   control this behavior should be made available, but are dependent on
   the particular scheduling algorithm implemented).  The strict service
   priority is useful for implementing EF on some links (assuming that
   the aggregate EF rate has been appropriately bounded to avoid
   starvation).  Setting the service priority of each queue in a queue
   set to the same value enables the scheduler to satisfy the minimum
   service rate for each queue.  Queue sets can be serviced like
   individual queues in a queue superset using the same scheduling

   It should be noted that the queues in this model are logical
   abstractions used to configure PHB-related parameters.  They are not
   expected to map one-to-one with physical queues in a specific router
   implementation.  An implementor should map the configurable
   parameters of the physical queues to these queue parameters as
   appropriate to achieve equivalent behaviors.

   Other queue parameters such as maximum capacity are assumed to be
   mapped to the buffer management algorithm used by the enqueueing
   element feeding the queue.

   A queue set might be represented using the following parameters:

      Type:        QueueSet
      MaxProfile:  WorkConserving
      MinGuarRate: 20 MBps
      Interface:   ifIndex

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      Type:        Queue
      QueueSet:    QueueSet1
      MaxProfile:  Profile1
      MinGuarRate: 2 MBps
      Priority:    3

      Type:        Queue
      QueueSet:    QueueSet1
      MaxProfile:  WorkConserving
      MinGuarRate: 8 MBps
      Priority:    3

7.2  Shaping

   Shapers are often used to pre-condition traffic such that packets
   are deemed conforming by subsequent meters, e.g., in downstream
   Diffserv nodes.  Shapers may also be used to isolate certain traffic
   streams from the effects of other traffic streams of the same BA.

   A shaper action element is implemented in this model by using a non-
   work conserving queue.  Shapers operate by delaying packets that
   would be deemed non-conforming by a meter configured to the shaper's
   maximum service rate profile.  The packet is delayed until such
   time that it would become conforming.

   Profile definitions are identical in format to those described for
   meters.  The use of a meter algorithm to control shaping is further
   discussed in Appendix A.  Average, EWMA, and TB profiles are all
   feasible for shaping.  Because a shaper is implemented as a queue it
   can also utilize a variety of buffer management algorithms
   (implemented in a enqueueing element).

   A shaping queue might be represented using the following parameters:

      Type:        Queue
      QueueSet:    QueueSet1
      MaxProfile:  Profile1
      MinGuarRate: 2 MBps
      Priority:    3

      Type:                SimpleTokenBucket
      AverageRate:         3 MBps
      BurstSize:           8 KB

8.  Traffic Conditioning Blocks (TCBs)

   The classifiers, meters, action elements, and queueing elements
   described above can be combined into traffic conditioning blocks

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   (TCBs).  The TCB is an abstraction of a functional element that may
   be used to facilitate the definition of specific traffic conditioning

   One of the simplest possible TCBs would consist of the following

   1.  Classifier stage
   2.  Enqueueing stage
   3.  Queueing stage

   Note that a classifier is a 1:N element, while an enqueueing stage is
   a N:1 element and a queue is a 1:1 element.  If the classifier split
   traffic across multiple enqueueing elements then the queueing stage
   may consist of a hierarchy of queue sets, all resulting in a 1:1
   abstract element.

   A more general TCB might consists of the following four stages:

   1. Classifier stage
   2. Metering stage
   3. Action stage
   4. Queueing stage

   where each stage may consist of a set of parallel datapaths
   consisting of pipelined elements.

   TCBs are constructed by connecting elements corresponding to these
   stages in any sensible order.  It is possible to omit stages, to
   include null elements, or to concatenate multiple stages of the same
   type.  TCB outputs may drive additional TCBs (on either the ingress
   or egress interfaces).   Classifiers and meters are fan-out elements,
   muxes and enqueueing elements are fan-in elements.

8.1  An Example TCB

   The following diagram illustrates an example TCB:

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                                       +------------> to Queue A
                              +-----+  |              (not shown)
                              |     |--+
                           +->|     |
                           |  |     |--+  +-----+    +-----+
                           |  +-----+  |  |     |    |     |
                           |   meter   +->|     |--->|     |
                           |              |     |    |     |
                           |              +-----+    +-----+
                           |              monitor    dropper
     submitted +-----+     |  +-----+     +-----+
     traffic   |  A  |-----+  |     |     |     |
           --->|  B  |------->|     |---->|     |---> to Queue B
               |  C  |-----+  |     |     |     |     (not shown)
               |  X  |--+  |  +-----+     +-----+
               +-----+  |  |   marker     shaper
                 BA     |  |              queue
              classifier|  |
                        |  |
                        |  |
                        |  |
                        |  |
                        |  |  +-----+                +-----+
                        |  |  |     |--------------->|     |  to Queue C
                        |  +->|     |                |     |->
                        |     |     |--+  +-----+ +->|     | (not shown)
                        |     +-----+  |  |     | |  +-----+
                        |      meter   +->|     |-+    mux
                        |                 |     |
                        |                 +-----+
                        |                 marker
                        +---------------------------> to Queue D
                                                      (not shown)
      Figure 5:  An Example Traffic Conditioning Block

   This sample TCB might be suitable for an ingress interface at a
   customer/provider boundary.  A SLS is presumed to have been
   negotiated between the customer and the provider which specifies the
   handling of the customer's traffic by the provider's network.  The
   agreement might be of the following form:

      DSCP         PHB       Profile       Non-Conforming Packets
      ----         ---       -------       ----------------------
      001001       PHB1      Profile1      Discard
      001100       PHB2      Profile2      Wait in shaper queue
      001101       PHB3      Profile3      Re-mark to DSCP 001000

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   It is implicit in this agreement that conforming packets are given
   the PHB originally indicated by the packets' DSCP field.  It
   specifies that the customer may submit packets marked for DSCP
   001001 which will get PHB1 treatment so long as they remain
   conforming to Profile1 and will be discarded if they exceed this
   profile.  Similar contract rules are applied for 001100 and 001101

   In this example, the classification stage consists of a single BA
   classifier.  The BA classifier is used to separate traffic based on
   the Diffserv service level requested by the customer (as indicated
   by the DSCP in each submitted packet's IP header).  We illustrate
   three DSCP filter values: A, B and C.  The 'X' in the BA classifier
   is the default wildcard filter that matches every packet.

   A metering stage is next in the upper and lower branches.  There is a
   separate meter for each set of packets corresponding to DSCPs A and
   C.  Each meter uses a specific profile as specified in the TCS for
   the corresponding Diffserv service level.  The meters in this
   example indicate one of two conforming levels, conforming or
   non-conforming.  The middle branch has a marker which re-marks all
   packets received with DSCP B.

   Following the metering stage is the action stage in the upper and
   lower branches.  Packets submitted for DSCP A that are deemed non-
   conforming and are counted and discarded.  Packets that are
   conforming are passed on to Queue A.  Packets submitted for DSCP C
   that are deemed non-conforming are re-marked, and then conforming and
   non-conforming packets are muxed together before being forwarded to
   Queue C.  Packets submitted for DSCP B are shaped to Profile2 before
   being forwarded to Queue B.

   The interconnections of the TCB elements illustrated in Fig. 5 can be
   represented as follows:


      Output A --> Meter1
      Output B --> Marker1
      Output C --> Meter2
      Output X --> QueueD

      Output A --> QueueA
      Output B --> Monitor1

      Output A --> Dropper1

      Output A --> Shaper1

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      Output A --> Queue B

      Output A --> Mux1
      Output B --> Marker2

      Output A --> Mux1

      Output A --> Queue C

8.2  An Example TCB to Support Multiple Customers

   The TCB described above can be installed on an ingress interface to
   implement a provider/customer TCS if the interface is dedicated to
   the customer.  However, if a single interface is shared between
   multiple customers, then the TCB above will not suffice, since it
   does not differentiate among traffic from different customers.  Its
   classification stage uses only BA classifiers.

   The TCB is readily extended to support the case of multiple customers
   per interface, as follows.  First, we define a TCB for each customer
   to reflect the TCS with that customer.  TCB1, defined above is the
   TCB for customer 1.  We add definitions for TCB2 and for TCB3 which
   reflect the agreements with customers 2 and 3 respectively.

   Finally, we add a classifier which provides a front end to separate
   the traffic from the three different customers.  This forms a new
   TCB which incorporates TCB1, TCB2, and TCB3, and can be illustrated
   as follows:

      submitted +-----+
      traffic   |  A  |--------> TCB1
            --->|  B  |--------> TCB2
                |  C  |--------> TCB3
                |  X  |--------> Dropper4

      Figure 6: An Example of a Multi-Customer TCB

   A formal representation of this multi-customer TCB might be:

      (as defined above)

      (similar to TCB1, perhaps with different numeric parameters)

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      (similar to TCB1, perhaps with different numeric parameters)

      (the total TCB)

      Output A --> TCB1
      Output B --> TCB2
      Output C --> TCB3
      Output X --> Dropper4

   Where Classifier2 is defined as follows:

      Filter1:     Output A
      Filter2:     Output B
      Filter3:     Output C
      No Match:    Output X

   and the filters, based on each customer's source MAC address, are
   defined as follows:

      Type:        MacAddress
      SrcValue:    01-02-03-04-05-06 (source MAC address of customer 1)
      SrcMask:     FF-FF-FF-FF-FF-FF
      DestValue:   00-00-00-00-00-00
      DestMask:    00-00-00-00-00-00

      (similar to Filter1 but with customer 2's source MAC address as

      (similar to Filter1 but with customer 3's source MAC address as

   In this example, Classifier4 separates traffic submitted from
   different customers based on the source MAC address in submitted
   packets.  Those packets with recognized source MAC addresses are
   passed to the TCB implementing the TCS with the corresponding
   customer.  Those packets with unrecognized source MAC addresses are
   passed to a dropper.

   TCB4 has a classification stage and an action element stage, which
   consists of either a dropper or another TCB.

8.3 TCBs Supporting Microflow-based Services

   The TCB illustrated above describes a configuration that might be
   suitable for enforcing a SLS at a router's ingress.  It assumes that

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   the customer marks its own traffic for the appropriate service level.
   It then limits the rate of aggregate traffic submitted at each
   service level, thereby protecting the resources of the Diffserv
   network.  It does not provide any isolation between the customer's
   individual microflows (other than from separated queueing).

   Next we present a TCB configuration that offers additional
   functionality to the customer.  It recognizes individual customer
   microflows and marks each one independently.  It also isolates the
   customer's individual microflows from each other in order to prevent
   a single microflow from seizing an unfair share of the resources
   available to the customer at a certain service level.  This is
   illustrated in Figure 7 below:

                     +-----+   +-----+
                     |     |   |     |---------------+
                  +->|     |-->|     |     +-----+   |
        +-----+   |  |     |   |     |---->|     |   |
        |     |----  +-----+   +-----+     +-----+   |
      ->|     |----  marker     meter      dropper   |   +-----+   to
        |     |-+ |  +-----+   +-----+               +-->|     |
        +-----+ | |  |     |   |     |------------------>|     |--->
          MF    | +->|     |-->|     |     +-----+   +-->|     |
        class.  |    |     |   |     |---->|     |   |   +-----+  TCB2
                |    +-----+   +-----+     +-----+   |    mux
                |    marker     meter      dropper   |
                |    +-----+   +-----+               |
                |    |     |   |     |---------------+
                |--->|     |-->|     |     +-----+
                |    |     |   |     |---->|     |
                |    +-----+   +-----+     +-----+
                |    marker     meter      dropper
                |       .         .     .
                V       V         V     V

      Figure 7: An Example of a Marking and Traffic Isolation TCB

   Traffic is first directed to a MF classifier which classifies traffic
   based on miscellaneous classification criteria, to a granularity
   sufficient to identify individual customer microflows.  Each
   microflow can then be marked for a specific DSCP (in this particular
   example we assume that one of two different DSCPs is marked).  The
   metering stage limits the contribution of each of the customer's
   microflows to the service level for which it was marked.  Packets
   exceeding the allowable limit for the microflow are dropped.

   The TCB could be formally specified as follows:

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      Classifier1: (MF)
      Output A --> Marker1
      Output B --> Marker2
      Output C --> Marker3
      . . .

      Marker1 --> Meter1
      Marker2 --> Meter2
      Marker3 --> Meter3

      Output A --> TCB2
      Output B --> ActionElement1 (dropper)

      Output A --> TCB2
      Output B --> ActionElement2 (dropper)

      Output A --> TCB2
      Output B --> ActionElement3 (dropper)

   The actual traffic element declarations are not shown here.

   Traffic is either dropped by TCB1 or emerges marked for one of two
   DSCPs.  This traffic is then passed to TCB2, illustrated below:

                     |     |--------------->
                  +->|     |     +-----+
        +-----+   |  |     |---->|     |
        |     |---+  +-----+     +-----+
      ->|     |       meter      dropper
        |     |---+  +-----+
        +-----+   |  |     |--------------->
          BA      +->|     |     +-----+
        classifier   |     |---->|     |
                     +-----+     +-----+
                      meter      dropper

      Figure 8: Additional Example TCB

   TCB2 would be formally specified as follows:

      Classifier2: (BA)
      Output A --> Meter10
      Output B --> Meter11

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      Output A --> PHBQueueA
      Output B --> Dropper10

      Output A --> PHBQueueB
      Output B --> Dropper11

9.  Open Issues

  o  There is a difference in interpretation of token bucket behavior
     between this document (Appendix A) and [DSMIB].  Specifically,
     [DSMIB] allows a packet to conform if any smaller packet would

  o  The meter in [SRTCM] cannot be precisely modeled using two
     two-parameter token buckets because its two buckets do not
     accumulate credits independently.  We intended to demonstrate how
     the [TRTCM] meter could be implemented but ran out of time.

  o  Are the queue parameters (scheduling and buffer management)
     parameters defined sufficient?

  o  Does Queue and Queue Set really belong in the model (and the MIB
     and PIB?), or should the model stick to the abstract PHB
     representation and leave the implementation details to the MIB and

  o  Should a classifier be part of a TCB?  We argue yes.  This allows a
     TCB to be a one input/one output black box element.

  o  Is the description of a shaper sufficient?  Is it overbroad?

10. Security Considerations

   Security vulnerabilities of Diffserv network operation are discussed
   in [DSARCH].  This document describes an abstract functional model of
   Diffserv router elements.  Certain denial-of-service attacks such as
   those resulting from resource starvation may be mitigated by
   appropriate configuration of these router elements; for example, by
   rate limiting certain traffic streams or by authenticating traffic
   marked for higher quality-of-service.

11.  Acknowledgments

   Concepts, terminology, and text have been borrowed liberally from
   [DSMIB] and [PIB].  We wish to thank the authors: Fred Baker,
   Michael Fine, Keith McCloghrie, John Seligson, Kwok Chan, and
   Scott Hahn, for their permission.

   This document has benefitted from the comments and suggestions of
   several participants of the Diffserv working group.

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12. References

   [DSARCH]   M. Carlson, W. Weiss, S. Blake, Z. Wang, D. Black, and
              E. Davies, "An Architecture for Differentiated Services",
              RFC 2475, December 1998

   [DSTERMS]  D. Grossman, "New Terminology for Diffserv", Internet
              Draft <draft-ietf-diffserv-new-terms-00.txt>, October

   [E2E]      Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang,
              M. Speer, K. Nichols, R. Braden, B. Davie, J. Wroclawski,
              and E. Felstaine, "Integrated Services Operation over
              Diffserv Networks", Internet Draft
              <draft-ietf-issll-diffserv-rsvp-02.txt>, September 1999.

   [DSFIELD]  K. Nichols, S. Blake, F. Baker, and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474, December

   [EF-PHB]   V. Jacobson,  K. Nichols, and K. Poduri, "An Expedited
              Forwarding PHB", RFC 2598, June 1999.

   [AF-PHB]   J. Heinanen, F. Baker, W. Weiss, and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597, June 1999.

   [DSMIB]    F. Baker, "Differentiated Services MIB", Internet Draft
              <draft-ietf-diffserv-mib-00.txt>, June 1999.

   [SRTCM]    J. Heinanen, and R. Guerin, "A Single Rate Three Color
              Marker", RFC 2697, September 1999.

   [PIB]      M. Fine, K. McCloghrie, J. Seligson, K. Chan, S. Hahn,
              and A. Smith, "Quality of Service Policy Information
              Base", Internet Draft <draft-mfine-cops-pib-01.txt>,
              June 1999.

   [TRTCM]    J. Heinanen, R. Guerin, "A Two Rate Three Color Marker",
              RFC 2698, September 1999.

   [GTC]      L. Lin, J. Lo, and F. Ou, "A Generic Traffic Conditioner",
              Internet Draft <draft-lin-diffserv-gtc-01.txt>, August

   [MPLSDS]   J. Heinanen, "Differentiated Services in MPLS Networks",
              Internet Draft <draft-heinanen-diffserv-mpls-00.txt>,
              June 1999.

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Appendix A.  Simple Token Bucket Definition

  [DSMIB] presents a fairly detailed exposition on the operation of
  two-parameter token buckets for metering.  However, the behavior
  described does not appear to be consistent with the behavior defined
  in [SRTCM] and [TRTCM].  Specifically, under the definition in
  [DSMIB], a packet is assumed to conform to the meter if any of its
  bytes would have been accepted, while in [SRTCM] and [TRTCM], a packet
  is assumed to conform only if sufficient tokens are available for
  every byte in the packet.  Further, a packet has no effect on the
  token occupancy if it does not conform (no tokens are decremented).

  The behavior defined in [SRTCM] and [TRTCM] is not mandatory for
  compliance, but we give here a mathematical definition of two-
  parameter token bucket operation which is consistent with these
  documents, and which can be used to define a shaping profile.

  Define a token bucket with bucket size BS, token accumulation rate
  R, and instantaneous token occupancy T(t).  Assume that T(0) = BS.

  Then after an arbitrary interval with no packet arrivals, T(t) will
  not change since the bucket is already full of tokens.  Assume a
  packet of size B bytes at time t'.  The bucket capacity T(t'-) = BS
  still.  Then, as long as B <= BS, the packet conforms to the meter,

     T(t') = BS - B.

  Assume an interval v = t - t' elapses before the next packet, of
  size C <= BS, arrives.  T(t-) is given by the following equation:

    T(t-) = max { BS, T(t') + v*R }

  (the packet has accumulated v*R tokens over the interval, up to a
  maximum of BS tokens).

  If T(t-) - C >= 0, the packet conforms and T(t) = T(t-) - C.
  Otherwise, the packet does not conform and T(t) = T(t-).

  This function can be used to define a shaping profile.  If a packet of
  size C arrives at time t, it will be eligible for transmission at time
  te given as follows (we still assume C <= BS):

     te = max { t, t" }


     t" = (C - T(t') + t'*R)/R.

  T(t") = C, the time when C credits have accumulated in the bucket,
  and when the packet would conform if the token bucket were a meter.
  te != t" only if t > t".

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Authors' Addresses

   Yoram Bernet
   One Microsoft Way
   Redmond, WA  98052
   Phone:  +1 425 936 9568

   Andrew Smith
   Extreme Networks
   3585 Monroe St.
   Santa Clara, CA  95051
   Phone:  +1 408 579 2821

   Steven Blake
   3000 Aerial Center Parkway, Suite 140
   Morrisville, NC  27560
   Phone:  +1 919 468 8466 x232

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