Internet Engineering Task Force                                Y. Bernet
Diffserv Working Group                                         Microsoft
INTERNET-DRAFT                                                  A. Smith
Expires November 2000                                   Extreme Networks
draft-ietf-diffserv-model-03.txt                                S. Blake
                                                             D. Grossman
                                                                May 2000
                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-Drafts.

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

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 IETF's Differentiated Services Working
Group. Comments should be addressed to WG's mailing list at The charter for Differentiated Services may be found
at Copyright (C)
The Internet Society (2000). All Rights Reserved.

Distribution of this memo is unlimited.


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, multiplexors, 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

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intended to be abstract and capable of representing the configuration
parameters important to Diffserv functionality for a variety of specific
router implementations. It is not intended as a guide to hardware

This model serves as the rationale for the design of an SNMP MIB [DSMIB]
and for other configuration interfaces (e.g.  [DSPIB]) and more detailed
models (e.g. [QOSDEVMOD]): these should all be based upon and consistent
with this model.

1.  Introduction

Differentiated Services (Diffserv) [DSARCH] is a set of technologies
which allow network service providers to offer different kinds 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 Section 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].

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This document is not intended in any way to constrain or to dictate the
implementation alternatives of Diffserv routers. It is expected that
router implementers will demonstrate a great deal of variability in
their implementations. To the extent that implementers 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 assorted

o    Section 3 starts by describing the basic high-level functional
     elements of a Diffserv router and then describe the various
     components, then focussing on the Diffserv-specific components of
     the router and a hierarchical management model for these

o    Section 4 describes classification elements.

o    Section 5 discusses meter elements.

o    Section 6 discusses action elements.

o    Section 7 discusses the basic queueing elements and their
     functional behaviors (e.g. shaping).

o    Section 8 shows how the basic classification, meter, action and
     queueing elements can be combined to build modules called Traffic
     Conditioning Blocks (TCBs).

o    Section 9 discusses open issues with this document

o    Section 10 discusses security concerns.

2.  Glossary

This memo uses terminology which is defined in [DSARCH] and in
[DSTERMS].  Some of the terms defined there are defined again here in
order to provide additional detail, along with some new terms specific
to this document.

   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.

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   Counter       A functional datapath element which updates a packet
                 counter and also an octet counter for every
                 packet that passes through it. Used for collecting

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

   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
   conserving    services packets no sooner than a scheduled departure
                 time, even if this means leaving packets in a FIFO
                 while the link is idle.

   Queueing      A combination of functional datapath elements
   Block         that modulates the transmission of packets belonging
                 to a traffic streams and determines their
                 ordering, possibly storing them temporarily or
                 discarding them.

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

   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 entity consisting of a number of
   Conditioning  other functional datapath entities 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 an entity with one
                 input and one output and a set of control parameters.

   Work-         A property of a scheduling algorithm such that it
   conserving    servicess a packet, if one is available, at every
                 transmission opportunity."

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3.  Conceptual Model

This section introduces a block diagram of a Diffserv router and
describes the various components illustrated. Note that a Diffserv core
router is assumed to include only a subset of these components: the
model presented here is intended to cover the case of both Diffserv edge
and core routers.

3.1.  Elements of a Diffserv Router

The conceptual model includes abstract definitions for the following:

   o    Traffic Classification elements.

   o    Metering functions.

   o    Traffic Conditioning (TC) actions of Marking, Absolute Dropping,
        Counting and Multiplexing.

   o    Queueing elements, including capabilities of algorithmic

   o    Certain combinations of traffic classification, traffic
        conditioning and queueing elements.

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:

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

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               | Diffserv      |
        Mgmt   | configuration |
      <----+-->| & management  |------------------+
      SNMP,|   | interface     |                  |
      COPS |   +---------------+                  |
      etc. |        |                             |
           |        |                             |
           |        v                             v
           |   +-------------+                 +-------------+
           |   | ingress i/f |   +---------+   | egress i/f  |
     --------->|  classify,  |-->| routing |-->|  classify,  |---->
     data  |   |  meter,     |   |  core   |   |  meter      |data out
      in   |   |  action,    |   +---------+   |  action,    |
           |   |  queueing   |                 |  queueing   |
           |   +-------------+                 +-------------+
           |        ^                             ^
           |        |                             |
           |        |                             |
           |   +------------+                     |
           +-->| QOS agent  |                     |
      -------->| (optional) |---------------------+
        QOS    | (e.g. RSVP)|
        cntl   +------------+
              Figure 1:  Diffserv Router Major Functional Blocks

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.

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

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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.

Specific policy objectives are presumed to be installed by or retrieved
from policy management mechanisms. However, diffserv routers are subject
to implementation decisions which form a meta- policy that scopes the
kinds of policies which can be created.

3.1.3.  Optional QoS Agent Module

Diffserv routers may snoop or participate in either per-microflow or
per-flow-aggregate signaling of QoS requirements [E2E] e.g.  using 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 implements the Integrated Services
Model as described in [INTSERV], applying the control plane controls to
the data classified and conditioned in the data plane, as desribed in

Note that a QoS Agent 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 could be used merely to signal reservation state without
installing any actual reservations in the data plane of the Diffserv
router: the data plane could still act purely on Diffserv DSCPs and
provide PHBs for handling data traffic without the normal per-microflow
handling expected to support some Intserv services.

3.2.  Hierarchical Model of Diffserv Components

This document focuses on the Diffserv-specific components of the router:
classification, traffic conditioning and queueing functions.  Figure 2
shows a high-level view of ingress and egress interfaces of a router.
The 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.

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In principle, if one were to construct a network entirely out of two-
port routers (in appropriate places connected by LANs or similar media),
then it would be necessary for each router to perform four QoS control
functions in the datapath on traffic in each direction:

-    Classify each message according to some set of rules.

-    If necessary, determine whether the data stream the message is part
     of is within or outside its rate by metering the stream.

-    Perform a set of resulting actions, including applying a drop
     policy appropriate to the classification and queue in question and
     perhaps additionally marking the traffic with a Differentiated
     Services Code Point (DSCP) as defined in [DSCP].

-    Enqueue the traffic for output in the appropriate queue, which may
     either shape the traffic or simply forward it with some minimum
     rate or maximum latency.

If the network is now built out of N-port routers, the expected behavior
of the network should be identical. Therefore, this model must provide
for essentially the same set of functions on the ingress as on the
egress port of the router. Some interfaces will be "edge" interfaces and
some will be "interior" to the Differentiated Services domain. The one
point of difference between an ingress and an egress interface is that

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

      Figure 2. Traffic Conditioning and Queueing Elements

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all traffic on an egress interface is queued, while traffic on an
ingress interface will typically be queued only for shaping purposes, if
at all.  Therefore, equivalent functional elements are modelled on both
the ingress and egress components of an interface.

Note that it is not mandatory that each of these functional elements be
implemented on both ingress and egress components; equally, the model
allows that multiple sets of these elements may be placed in series
and/or in parallel at ingress or at egress. The arrangement of elements
is dependent on the service requirements on a particular interface on a
particular router. By modelling these elements on both ingress and
egress components, it is not implied 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. Furthermore, 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 selection.

>From a device-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 elements.

     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
     Section 8). One or more TCBs may be instantiated on each ingress or
     egress component; they may be connected in series and/or in
     parallel configurations on the multiple outputs of a classifier.
     The TCB is defined optionally to include classification and
     queueing elements so as to allow for flexible functionality. A TCB
     can be thought of as a "black box" with one input and one output in
     the data path. Each interface (ingress or egress) may have
     different TCB configurations.

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

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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 --> OutputA
      ------->| classifier |--> match Filter2 --> OutputB
              |            |--> no match      --> OutputC

      Figure 3. An Example Classifier

Note that we allow a multiplexor (see Section 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 example of a packet attribute could be an integer representing
the BGP community string associated with the packet's best-matching

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], Section 4.2.1 ):

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      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 matches.

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".

MF classification of fragmented packets is impossible if the filter uses
transport-layer port numbers e.g. TCP port numbers. MTU-discovery is
therefore a prerequisite for proper operation of a Diffserv network that
uses such classifiers.

4.1.2.  Overlapping Filters

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

      Type:   Masked-DSCP
      Value:  111000

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      Mask:   111000

      Type:   Masked-DSCP
      Value:  000111 (binary)
      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.

As another example, one might want first to disallow certain
applications from using the network at all, or to classify some
individual traffic streams that are not Diffserv-marked. Traffic that is
not classified by those tests might then be inspected for a DSCP. The
word "then" implies sequence and this must be specified by means of

An unambiguous classifier requires that every possible classification
key match at least one filter (possibly the wildcard default) and that
any ambiguity between overlapping filters be resolved by precedence.
Therefore, the classifiers on any given interface must be "complete" and
will often include an "everything else" filter as the lowest precedence
element in order for the result of classification to be deterministic.
Note that this completeness is only required of the first classifier
that incoming traffic will meet as it enters an interface - subsequent
classifiers on an interface only need to handle the traffic that it is
known that they will receive.

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

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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
(possibly including the DSCP). A common type of MF classifier is a 6-
tuple classifier that classifies based on six fields from the IP and TCP
or UDP headers (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
      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.  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),

      Filter12:    OutputA
      Filter13:    OutputB
      Default:     OutputC

      Type:        FreeForm

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      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.

4.2.4.  Other Possible Classifiers

Classification may also be performed based on information at the
datalink layer below IP (e.g. VLAN or datalink-layer priority) or
perhaps on the ingress or egress IP, logical or physical interface
identifier.  (e.g. the incoming channel number on a channelized
interface).  A classifier that filters based on IEEE 802.1p Priority and
on 802.1Q VLAN-ID might be represented as:

      Filter14 AND Filter15:  OutputA
      Default:                OutputB

      Filter14:                        -- priority 4 or 5
      Type:        Ieee8021pPriority
      Value:       100 (binary)
      Mask:        110 (binary)

      Filter15:                        -- VLAN 2304
      Type:        Ieee8021QVlan
      Value:       100100000000 (binary)
      Mask:        111111111111 (binary)

Such classifiers may be subject of other standards or may be enterprise-
specific but are not discussed further here.

5.  Meters

Metering is is defined in [DSARCH].  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

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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 applications.

Meters are logically 1:N (fan-out) devices (although a multiplexor 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 slightly 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 signals to
action elements to dynamically modulate their behavior based on the
conformance of the packet.

The following diagram illustrates a meter with 3 levels of conformance:

      unmetered              metered
      traffic                traffic
                |         |--------> conformance A
      --------->|  meter  |--------> conformance B
                |         |--------> conformance C

      Figure 4. A Generic 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 may be used to trigger different queueing,
marking or dropping treatment later on in the processing. 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.

A meter, according to this model, measures the rate at which packets
making up a stream of traffic pass it, compares the rate to some set of
thresholds and produces some number (two or more) potential results: a
given packet is said to "conform" to the meter if, at the time that the
packet is being looked at, the stream appears to be within the meter's
limit rate.

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The concept of conformance to a meter bears comment. The concept applied
in several rate-control architectures, including ATM, Frame Relay,
Integrated Services and Differentiated Services, is variously described
as a "leaky bucket" or a "token bucket".

A leaky bucket algorithm is primarily used for traffic shaping (handled
under Queues and Schedulers in this model): traffic theoretically
departs from a device at a rate of one bit every so many time units but,
in fact, departs in multi-bit units (packets) at a rate approximating
that. It is also possible to build multi-rate leaky buckets, in which
traffic departs from the switch at varying rates depending on recent
activity or inactivity.

A simple token bucket is usually used in a Meter to measure the behavior
of a peer's leaky bucket, for verification purposes. It is, by
definition, a relationship between some defined burst_size, rate and

      interval = burst_size/rate
      rate = burst_size/interval

Multi-rate token buckets (token buckets with both a peak and a mean rate
and sometimes more rates) are commonly used. In this case, the burst
size for the baseline traffic is conventionally referred to as the
"committed burst" and the time interval is as specified by

      interval = committed_burst/mean_rate

but additional burst sizes (each an increment over its predecessor) are
defined, which are conventionally referred to as "excess" burst sizes.
The peak rate therefore equals the sum of the burst sizes for any given

A data stream is said to conform to a simple token bucket if the switch
receives at most the "burst_size" of data in any time interval of length
"interval". In the multi-rate case, the traffic is said to conform at a
given level to the token bucket at if its rate does not exceed the sum
of the relevant burst sizes in any given interval. Received traffic that
arrives pre-classified as one of the "excess" rates (e.g. AF12 or AF13
traffic for a device implementing the AF1x PHB) is only compared to the
relevant excess buckets.

<ed: the following paragraphs may need fixing when we can all agree on a
stricter vs. looser definition: for now we assume strict schedulers and
lenient meters.>

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The fact that data is organized into variable length packets introduces
some uncertainty in this conformance decision. When used in a Scheduler,
a leaky bucket releases a packet only when all of its bits would have
been allowed: it does not borrow from future capacity. When used in a
Meter, a token bucket accepts a packet if any of its bits would have
been accepted and "borrows" any excess capacity required from that
allotted to equivalently classified traffic in a previous or subsequent
interval. Note that [SRTCM] and [TRTCM] insist on stricter behaviour
from a meter than the model here insists on.

Multiple classes of traffic, as identified by the classifier table, may
be presented to the same meter. Imagine, for example, that it is desired
to drop all traffic that uses any DSCP that has not been publicly
defined.  A classifier entry might exist for each such DSCP, shunting it
to an "accepts everything" meter and dropping all traffic that conforms
to only that meter.

It is necessary to identify what is to be done with packets that conform
to the meter and with packets that do not. It is also necessary for the
meter to be arbitrarily extensible as some PHBs require the successive
application of an arbitrary number of meters.  The approach taken in
this model is to have each meter indicate what action is to be taken for
conforming traffic and what meter is to be used for traffic which fails
to conform. With the definition of a special type of meter to which all
traffic conforms, this has the necessary flexibility.

Note that this definition of a simple token bucket meter requires that
the minimal bucket size be at least the MTU of the incoming link and it
should also be initialised with sufficient tokens to allow for at least
one MTU-sized packet to conform if it arrives at time zero.

5.1.  Examples

The following are some examples of possible meters.

5.1.1.  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
      Profile:             Profile1
      ConformingOutput:    Queue1

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      NonConformingOutput: Counter1

      Type:                AverageRate
      AverageRate:         120 kbps
      Delta:               100 msec

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 - 100 msecs. So long as an arriving packet does not
push the count over 12 kbits in the last 100 msec then the packet would
be deemed conforming. Any packet that pushes the count over 12 kbits
would be deemed non-conforming. Thus, this meter deems packets to
correspond to one of two conformance levels: conforming or non-
conforming and sends them on for the appropriate subsequent treatment.

5.1.2.  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
something like the following:

      Type:                ExpWeightedMovingAvg
      Profile:             Profile2
      ConformingOutput:    Queue1
      NonConformingOutput: AbsoluteDropper1

      Type:                ExpWeightedMovingAvg
      AverageRate:         25 kbps

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      Delta:               10 usec
      Gain:                1/16

5.1.3.  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 and a burst size. TB meters compare the arrival rate of
packets to the average rate specified by the TB profile.  Logically,
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 any tokens are available in the bucket at the time of
packet arrival: up to L bytes may then be borrowed from future token
allocations. Packets are allowed to exceed the average rate in bursts up
to the burst size. Packets which arrive to find a bucket with no 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. Note that this is a
"lenient" meter that allows some borrowing, as discussed above.

A two-parameter TB meter might appear as follows:

      Type:                SimpleTokenBucket
      Profile:             Profile3
      ConformingOutput:    Queue1
      NonConformingOutput: AbsoluteDropper1

      Type:                SimpleTokenBucket
      AverageRate:         200 kbps
      BurstSize:           100 kbytes

5.1.4.  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.

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

      Type:                TwoRateTokenBucket
      ProfileA:            Profile4
      ConformingOutputA:   Queue1
      ProfileB:            Profile5
      ConformingOutputB:   Marker1
      NonConformingOutput: AbsoluteDropper1

      Type:                SimpleTokenBucket
      AverageRate:         100 kbps
      BurstSize:           20 kbytes

      Type:                SimpleTokenBucket
      AverageRate:         100 kbps
      BurstSize:           100 kbytes

5.1.5.  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.

      Type:                NullMeter
      Output:              Queue1

6.  Action Elements

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

-    Marking

-    Absolute Dropping

-    Multiplexing

-    Counting

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-    Null action - do nothing

The corresponding action elements are described in the following

Diffserv nodes may apply shaping, policing and/or marking to traffic
streams that exceed the bounds of their TCS in order to prevent a
traffic stream from seizing more than its share of resources from a
Diffserv network. Shaping, sometimes considered as a TC action, is
treated as a part of the queueing module in this model, as is the use of
Algorithmic Dropping techniques - see section 7.  Policing is modelled
as the combination of either a meter or a scheduler with either an
absolute dropper or an algorithmic dropper.  These elements will discard
packets which exceed the TCS.  Marking is performed by a marker, which
(in this context) alters the DSCP, and thus the PHB, of the packet to
give it a lower-grade treatment at subsequent Diffserv nodes.

6.1.  Marker

Markers are 1:1 elements which set a codepoint (e.g. the DSCP in an IP
header). Markers may also act on unmarked packets (e.g. those 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 mark set in a packet will determine its
subsequent treatment in downstream nodes of a network and possibly also
in subsequent processing stages within this router.

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

      Type:                DSCPMarker
      Mark:                010010

6.2.  Absolute Dropper

Absolute droppers simply discard packets. There are no parameters for
these droppers. Because this dropper is a terminating point of the
datapath and have no outputs, it is probably desirable to forward the
packet through a counter action first for instrumentation purposes.

      Type:                AbsoluteDropper

Absolute droppers are not the only elements than can cause a packet to
be discarded: another element is an Algorithmic Dropper element (see
Section 6.6). However, since this element's behavior is closely tied the

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state of one or more queues, we choose to distinguish it as a separate
functional element.

6.3.  Multiplexer

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

      Type:                Multiplexer
      Output:              Queue2

6.4.  Counter

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. Counters
are 1:1 functional elements which update a counter by L and a packet
counter by 1 every time a L-byte sized packet passes through them.
Counters can be used to count packets about to be be dropped by a
dropper or a queueing element.

      Type:                Counter
      Output:              Queue1

6.5.  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.

      Type:                Null
      Output:              Queue1

7.  Queueing Blocks

Queueing blocks modulate the transmission of packets belonging to the
different traffic streams and determine their ordering, possibly storing
them temporarily or discarding them. Packets are usually stored either
because there is a resource constraint (e.g., available bandwidth) which
prevents immediate forwarding, or because the queueing block is being

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used to alter the temporal properties of a traffic stream (i.e.
shaping). Packets are discarded either because of buffering limitations,
because a buffer threshold is exceeded (including when shaping is
performed), as a feedback control signal to reactive control protocols
such as TCP, because a meter exceeds a configured rate (i.e. policing).

The queueing block in this model is a logical abstraction of a queueing
system, which is used to configure PHB-related parameters.  There is no
conformance to this model. The model can be used to represent a broad
variety of possible implementations. However, it need not necessarily
map one-to-one with physical queueing systems in a specific router
implementation. Implementors should map the configurable parameters of
the implementation's queueing systems to these queueing block parameters
as appropriate to achieve equivalent behaviors.

7.1.  Queueing Model

Queueing is a function a which lends itself to innovation. It must be
modelled to allow a broad range of possible implementations to be
represented using common structures and parameters. This model uses
functional decomposition as a tool to permit the needed lattitude.

Queueing sytems, such as the queueing block defined in this model,
perform three distinct, but related, functions:  they store packets,
they modulate the departure of packets belonging to various traffic
streams and they selectively discard packets. This model decomposes the
queueing block into the component elements that perform each of these
functions. These elements which may be connected together either
dynamically or statically to  construct queueing blocks. A queueing
block is thus composed of of one or more FIFOs, one or more Schedulers
and zero or more Algorithmic Droppers.

     <ed: should this be *one* or more? There are valid cases that do
     not require a dropper but they are exceptional.>

Note that the term FIFO has multiple different common usages: it is
sometimes taken to mean, among other things, a data structure that
permits items to be removed only in the order in which they were
inserted or a service discipline which is non- reordering.

7.1.1.  FIFO

In this model, a FIFO element is a data structure which at any time may
contain zero or more packets. It may have one or more thresholds
associated with it. A FIFO has one or more inputs and exactly one
output. It must support an enqueue operation to add a packet to the tail
of the queue, and a dequeue operation to remove a packet from the head

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of the queue. Packets must be dequeued in the order in which they were
enqueued. A FIFO has a current depth, which indicates the number of
packets that it contains at a particular time. FIFOs in this model are
modelled without inherent limits on their depth - obviously this does
not reflect the reality of implementations: FIFO size limits are
modelled here by an algorithmic dropper associated with the FIFO,
typically at its input. It is quite likely that, every FIFO will be
preceded by an algorithmic dropper.  One exception might be the case
where the packet stream has already been policed to a profile that can
never exceed the scheduler bandwidth available at the FIFO's output -
this would not need an algorithmic dropper at the input to the FIFO.

This representation of a FIFO allows for one common type of depth limit,
one that results from a FIFO supplied from a limited pool of buffers,
shared between multiple FIFOs.

     <ed: should we instead model a FIFO as having a single input and
     use a "multiplexer" at its input if it needs to collect from
     multiple input sources?>

Typically, the FIFO element of this model will be implemented as a FIFO
data structure. However, this does not preclude implementations which
are not strictly FIFO, in that they also support operations that remove
or examine packets (e.g., for use by discarders) other than at the head
or tail. However, such operations MUST NOT have the effect of reordering
packets belonging to the same microflow.

In an implementation, packets are presumably stored in one or more
buffers. Buffers are allocated from one or more free buffer pools. If
there are multiple instances of a FIFO, their packet buffers may or may
not be allocated out of the same free buffer pool. Free buffer pools may
also have one or more threshold associated with them, which may affect
discarding and/or scheduling. Other than this, buffering mechanisms are
implementation specific and not part of this model.

A FIFO might be represented using the following parameters:

     Type:       FIFO
     Output:     Scheduler1

Note that a FIFO must provide triggers and/or current state information
to other elements upstream and downstream from it: in particular, it is
likely that the current depth will need to be used by Algorithmic
Dropper elements placed before or after the FIFO. It will also likely
need to provide an implicit "I have packets for you" signal to
downstream Scheduler elements.

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7.1.2.  Scheduler

A scheduler is an element which gates the departure of each packet that
arrives at one of its inputs, based on a service discipline. It has one
or more input and exactly one output. Each input has an upstream element
to which it is connected, and a set of parameters that affects the
scheduling of packets received at that input.

The service discipline (also known as a scheduling algorithm) is an
algorithm which might take any of the following as its input(s):

a)   static parameters such as relative priority associated with each of
     the scheduler's inputs.

b)   absolute token bucket parameters for maximum or minimum rates
     associated with each of the scheduler's inputs.

c)   parameters, such as packet length or DSCP, associated with the
     packet currently present at its input.

d)   absolute time and/or local state.

Possible service disciplines fall into a number of categories, including
(but not limited to) first come, first served (FCFS), strict priority,
weighted fair bandwidth sharing (e.g., WFQ, WRR, etc.), rate-limited
strict priority and rate-based. Service disciplines can be further
distinguished by whether they are work-conserving or non-work-conserving
(see Glossary). Non-work-conserving schedulers can be used to shape
traffic streams to match some profile by delaying packets that might be
deemed non-conforming by some downstream node: a packet is delayed until
such time as it would conform to a downstream meter using the same

[DSARCH] defines PHBs without specifying required scheduling algorithms.
However, PHBs such as  the class selectors [DSFIELD], EF [EF-PHB] and AF
[AF-PHB] have descriptions or configuration parameters which strongly
suggest the sort of scheduling discipline needed to implement them. This
memo discusses a minimal set of queue parameters to enable realization
of these per- hop behaviors. It does not attempt to specify an all-
embracing set of parameters to cover all possible implementation models.
A mimimal set includes:

a)   a minimum service rate profile which allows rate guarantees for
     each traffic stream as required by EF and AF without specifying the
     details of how excess bandwidth between these traffic streams is
     shared. Additional parameters to control this behavior should be
     made available, but are dependent on the particular scheduling

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     algorithm implemented.

b)   a service priority, used only after the minimum rate profiles of
     all inputs have been satisfied, to decide how to allocate any
     remaining bandwidth.

c)   a maximum service rate profile, for use only with a non-work-
     conserving service discipline.

For an implementation of the EF PHB using a strict priority scheduling
algorithm that assumes that the aggregate EF rate has been appropriately
bounded to avoid starvation, the minimum rate profile would be reported
as zero and the maximum service rate would be reported as line rate.
Such an implementation, with multiple priority classes, could also be
used for the Diffserv class selectors [DSFIELD].

Alternatively, setting the service priority values for each input to the
scheduler to the same value enables the scheduler to satisfy the minimum
service rates for each input, so long as the sum of all minimum service
rates is less than or equal to the line rate.

For example, a non-work-conserving scheduler, allocating spare bandwidth
equally between all its inputs, might be represented using the following

     Type:           Scheduler2Input

     MaxRateProfile: Profile1
     MinRateProfile: Profile2
     Priority:       none

     MaxRateProfile: Profile3
     MinRateProfile: Profile4
     Priority:       none

A work-conserving scheduler might be represented using the following

     Type:           Scheduler3Input

     MaxRateProfile: WorkConserving
     MinRateProfile: Profile5

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     Priority:       1

     MaxRateProfile: WorkConserving
     MinRateProfile: Profile6
     Priority:       2

     MaxRateProfile: WorkConserving
     MinRateProfile: none
     Priority:       3

7.1.3.  Algorithmic Dropper

An Algorithmic Dropper is an element which selectively discards packets
that arrive at its input, based on a discarding algorithm. It has one
data input and one output. In this model (but not necessarily in a real
implementation), a packet enters the dropper at its input and either its
buffer is returned to a free buffer pool or the packet exits the dropper
at the output.

Alternatively, an Algorithmic Dropper may invoke operations on a FIFO
which selectively removes a packet, then return its buffer to the free
buffer pool, based on a discarding algorithm. In this case, the
operation is modelled as a side-effect on the FIFO upon which it
operates, rather than as having a discrete input and output.  These two
treatments are equivalent and we choose the former here.

The Algorithmic Dropper is modelled as having a single input. However,
it is likely that packets which were classified differently by a
Classifier in this TCB will end up passing through the same dropper. The
dropper's algorithm may need to apply different calculations based on
characteristics of the incoming packet e.g. its DSCP. So there is a
need, in implementations of this model, to be able to relate information
about which classifier element was matched by a packet from a Classifier
to an Algorithmic Dropper.  This is modelled here as a reverse pointer
from one of the drop probability calculation algorithms inside the
dropper to the classifier element that selects this algorithm.

There are many formulations of a model that could represent this
linkage, other than the one described above: one way would have been to
have multiple "inputs" fed from the preceding elements, leading
eventually to the classifier elements that matched the packet. Another
formulation might have been for the Classifier to (logically) include
some sort of "classification identifier" along with the packet along its
path, for use by any subsequent element. Yet another could have been to
include a classifier inside the dropper, in order for it to pick out the

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drop algorithm to be applied. All of these other approaches were deemed
to be more clumsy or less useful than the approach taken here.

An Algorithmic Dropper, shown in Figure 5, has one or more triggers that
cause it to make a decision whether or not to drop one (or possibly more
than one) packet. A trigger may be internal (the arrival of a packet at
the input to the dropper) or it may be external (resulting from one or
more state changes at another element, such as a FIFO depth exceeding a
threshold or a scheduling event). It is likely that an instantaneous
FIFO depth will need to be smoothed over some averaging interval. Some
dropping algorithms may require several trigger inputs feeding back from
events elsewhere in the system e.g. smoothing functions that calculate
averages over more than one time interval.  Smoothing functions are
outside the scope of this document and are not modelled here, we merely
indicate where they might be added in the model.

A trigger may be a boolean combination of events (e.g. a FIFO depth
exceeding a threshold OR a buffer pool depth falling below a threshold).

The dropping algorithm makes a decision on whether to forward or to
discard a packet. It takes as its parameters some set of dynamic
parameters (e.g. averaged or instantaneous FIFO depth) and some set of
static parameters (e.g. thresholds) and possibly parameters associated

           +------------------+      +-----------+
           | +-------+        |  n   |smoothing  |
           | |trigger|<----------/---|function(s)|
           | |calc.  |        |      |(optional) |
           | +-------+        |      +-----------+
           |     |            |          ^
           |     v            |          |Depth
  Input    | +-------+ no     |      ------------+   to Scheduler
  ---------->|discard|-------------->    |x|x|x|x|------->
           | |   ?   |        |      ------------+
           | +-------+        |           FIFO
           |    |yes          |
           |  | | |           |
           |  | v | count +   |
           |  +---+ bit-bucket|

      Figure 5. Algorithmic Dropper + Queue

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with the packet (e.g. its PHB, as determined by a classifier, which will
determine on which of the droppers inputs trhe packet arrives). It may
also have internal state and is likely to keep counters regarding the
dropped packets (there is no appropriate place here to include a Counter
Action element).

RED, RED-on-In-and-Out (RIO) and Drop-on-threshold are examples of
dropping algorithms. Tail-dropping and head-dropping are effected by the
location of the dropper relative to the FIFO.

Note that, although an Algorithmic Dropper may require knowledge of data
fields in a packet, as discovered by a Classifier in the same TCB, it
may not modify the packet (i.e. it is not a marker).

     <ed: have rearranged this example so as not to include a Classifier
     in the Dropper - this leads to needing either multiple inputs or an
     implicit classification stage to separate the in- and out-of-
     profile traffic. We have chosen the former representation.>

A dropper which uses a RIO algorithm might be represented using the
following parameters:

      Type:                   AlgorithmicDropper
      Discipline:             RIO
      Trigger:                Internal
      Output:                 Fifo1

      InputA: (in profile)
      MinThresh:              Fifo1.Depth > 20 kbyte
      MaxThresh:              Fifo1.Depth > 30 kbyte

      InputB: (out of profile)
      MinThresh:              Fifo1.Depth > 10 kbyte
      MaxThresh:              Fifo1.Depth > 20 kbyte

      SampleWeight            .002
      MaxDropProb             1%

Another form of dropper, a threshold-dropper, might be represented using
the following parameters:

      Type:                   AlgorithmicDropper
      Discipline:             Drop-on-threshold
      Trigger:                Fifo2.Depth > 20 kbyte
      Output:                 Fifo1

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Yet another dropper which drops all out-of-profile packets whenever the
FIFO threshold exceeds a certain depth (this dropper is not part of the
larger TCB example) might be represented with the following parameters:

      Type:                   AlgorithmicDropper2Input
      Discipline:             Drop-out-packets-on-threshold
      Output:                 Fifo3

      InputA: (in profile)
      Trigger:                none
      InputB: (out of profile)
      Trigger:                Fifo3.Depth > 100 kbyte

     <ed: this models the dropper without using an embedded Classifier
     which seems a cleaner model than embedding a classifier here>

7.1.4.  Constructing queueing blocks from the elements

A queueing block is constructed by concatenation of these elements so as
to meet the meta-policy objectives of the implementation, subject to the
grammar rules specified in this section.

Elements of the same type may appear more than once in a queueing block,
either in parallel or in series. Typically, a queueing block will have
relatively many elements in parallel and few in series.  Iteration and
recursion are not supported constructs in this grammar. A queueing block
must have at least one FIFO, at least one dropper, and at least one
scheduler.  The following connections are allowed:

1)   The input of a FIFO may be the input of the queueing block or it
     may be connected to the output of a dropper or to an output of a

2)   Each input of a scheduler may be connected to the output of a FIFO,
     to the output of a dropper or to the output of another scheduler.

3)   The input of a dropper which has a discrete input and output may be
     the input of the queueing block or it may be connected to the
     output of a FIFO (e.g., head dropping).

4)   The output of the queueing block may be the output of a FIFO
     element, a discarding element or a scheduling element.

Note, in particular, that schedulers may operate in series such that a
packet at the head of a FIFO feeding the concatenated schedulers is
serviced only after all of the scheduling criteria are met. For example,

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a FIFO which carries EF traffic streams may be served first by a non-
work-conserving scheduler to shape the stream to a maximum rate, then by
a work-conserving scheduler to mix EF traffic streams with other traffic
streams. Alternatively, there might be a FIFO  and/or a dropper between
the two schedulers.

7.2.  Shaping

Traffic shaping is often used to condition traffic such that packets
arriving in a burst will be "smoothed" and deemed conforming by
subsequent downstream meters in this or other nodes. Shaping may also be
used to isolate certain traffic streams from the effects of other
traffic streams of the same BA.

In [DSARCH] a shaper is described as a queueing element controlled by a
meter which defines its temporal profile. However, this representation
of a shaper differs substantially from typical shaper implementations.

In this conceptual model, a shaper is realized by using a non-work-
conserving scheduler. Some implementations may elect to have queues
whose sole purpose is shaping, while others may integrate the shaping
function with other buffering, discarding and scheduling associated with
access to a resource. Shapers operate by delaying the departure of
packets that would be deemed non-conforming by a meter configured to the
shaper's maximum service rate profile. The packet is scheduled to depart
no sooner than such time that it would become conforming.

8.  Traffic Conditioning Blocks (TCBs)

The classifiers, meters, action elements and queueing elements described
above can be combined into traffic conditioning blocks (TCBs). The TCB
is an abstraction of a functional element that may be used to facilitate
the definition of specific traffic conditioning functionality.

A general TCB might consist of the following four stages:
  - Classification stage
  - Metering stage
  - Action stage
  - Queueing stage

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

Note that a classifier is a 1:N element, metering and actions are
typically 1:1 elements and queueing is a N:1 element. The whole TCB
should, however, result in a 1:1 abstract element.

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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

8.1.  An Example TCB

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     Treatment
   ----     ---   -------     ----------------------
   001001   EF    Profile4    Discard non-conforming.
   001100   AF11  Profile5    Shape to profile, tail-drop when full.
   001101   AF21  Profile3    Re-mark non-conforming to DSCP 001000,
                                 tail-drop when full.
   other    BE    none        Apply RED-like dropping.

This SLS specifies that the customer may submit packets marked for DSCP
001001 which will get EF treatment so long as they remain conforming to
Profile1 and will be discarded if they exceed this profile. The
discarded packets are counted in this example, perhaps for use by the
provider's sales department in convincing the customer to buy a larger
SLS.  Packets marked for DSCP 001100 will be shaped to Profile2 before
forwarding. Packets marked for DSCP 001101 will be metered to Profile3
with non-conforming packets "downgraded" by being re-marked with a DSCP
of 001000.  It is implicit in this agreement that conforming packets are
given the PHB originally indicated by the packets' DSCP field.

Figures 6 and 7 illustrates a TCB that might be used to handle this SLS
at an ingress interface at the customer/provider boundary.

The Classification stage of this example 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 a wildcard
filter that matches every packet not otherwise matched.

The paths for DSCP 001001 and 001101 then include a metering stage.
There is a separate meter for each set of packets corresponding to
classifier outputs 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 each indicate one of two conforming levels,

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                          |    A|---------------------------> to Queue1
                       +->|     |
                       |  |    B|--+  +-----+    +-----+
                       |  +-----+  |  |     |    |     |
                       |  Meter1   +->|     |--->|     |
                       |              |     |    |     |
                       |              +-----+    +-----+
                       |              Counter1   Absolute
 submitted +-----+     |                         Dropper1
 traffic   |    A|-----+
 --------->|    B|----------------------------------------> to Dropper1
           |    C|-----+
           |    X|--+  |
           +-----+  |  |  +-----+                +-----+
         Classifier1|  |  |    A|--------------->|A    |
            (BA)    |  +->|     |                |     |--> to Dropper2
                    |     |    B|--+  +-----+ +->|B    |
                    |     +-----+  |  |     | |  +-----+
                    |     Meter2   +->|     |-+    Mux1
                    |                 |     |
                    |                 +-----+
                    |                 Marker1
                    +-------------------------------------> to Dropper3

      Figure 6:  An Example Traffic Conditioning Block (Part 1)

conforming or non-conforming.

Following the Metering stage is the Action stage in the upper and lower
branches. Packets submitted for DSCP 001001 that are deemed non-
conforming are counted and discarded while packets that are conforming
are passed on to Dropper1/Queue1. Packets submitted for DSCP 001101 that
are deemed non-conforming are re-marked and then conforming and non-
conforming packets are multiplexed together before being passed on to
Dropper2/Queue3. Packets submitted for DSCP 001100 are passed straight
on to Queue2.

The Queueing stage is realised as follows, shown in figure 6.  The
conforming 001001 packets are passed directly to Queue1: there is no
way, with correct configuration of the scheduler for these to overflow
the depth of Queue1 so there is never a requirement for dropping.
Packets marked for 001100 must be passed through a tail-dropper,
Dropper1, which serves to limit the depth of the following queue,
Queue2: packets that arrive to a full queue will be discarded - this is
likely to be an error case: the customer is obviously not sticking to

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its agreed profile.  Similarly, packets from the 001101 stream are
passed to Dropper2 and Queue3.  Packets marked for all other DSCPs are
passed to Dropper3 which is a RED-like algorithmic dropper: based on
feedback of the current depth of Queue4, this dropper is likely to
discard enough packets from its input stream to keep the queue depth
under control.

These four queues are then serviced by a scheduling algorithm in
Scheduler1 which has been configured to give each of the queues an
appropriate priority and/or bandwidth share. Inputs A and C are given
guarantees of bandwidth, as appropriate for the contracted profiles.
Input B is given a limit on the bandwidth it can use i.e. a non-work-
conserving discipline in order to achieve the desired shaping of this
stream.  Input D is given no limits or guarantees but a lower priority
than the other queues, appropriate for its best-effort status.  Traffic
then exits the scheduler in a single orderly stream.

    from Meter1                     +-----+
    ------------------------------->|     |----+
                                    |     |    |
                                    +-----+    |
                                    Queue1     |
                                               |  +-----+
    from Classifier1 +-----+        +-----+    +->|A    |
    ---------------->|     |------->|     |------>|B    |------->
                     |     |        |     |  +--->|C    |  exiting
                     +-----+        +-----+  | +->|D    |  traffic
                     Dropper1       Queue2   | |  +-----+
                                             | |  Scheduler1
    from Mux1        +-----+        +-----+  | |
    ---------------->|     |------->|     |--+ |
                     |     |        |     |    |
                     +-----+        +-----+    |
                     Dropper2       Queue3     |
    from Classifier1 +-----+        +-----+    |
    ---------------->|     |------->|     |----+
                     |     |        |     |
                     +-----+        +-----+
                     Dropper3       Queue4

      Figure 7: An Example Traffic Conditioning Block (Part 2)

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The interconnections of the TCB elements illustrated in Figures 6 and 7
can be represented as follows:


      FilterA:             Meter1
      FilterB:             Dropper1
      FilterC:             Meter2
      Default:             Dropper3

      Type:                AverageRate
      Profile:             Profile1
      ConformingOutput:    Queue1
      NonConformingOutput: Counter1

      Output:              AbsoluteDropper1

      Type:                AverageRate
      Profile:             Profile3
      ConformingOutput:    Mux1.InputA
      NonConformingOutput: Marker1

      Type:                DSCPMarker
      Mark:                001000
      Output:              Mux1.InputB

      Output:              Dropper2

      Type:                AlgorithmicDropper
      Discipline:          Drop-on-threshold
      Trigger:             Queue2.Depth > 10kbyte
      Output:              Queue2

      Type:                AlgorithmicDropper
      Discipline:          Drop-on-threshold
      Trigger:             Queue3.Depth > 20kbyte
      Output:              Queue3


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      Type:                AlgorithmicDropper
      Discipline:          RED93
      Trigger:             Internal
      Output:              Queue3
      MinThresh:           Queue3.Depth > 20 kbyte
      MaxThresh:           Queue3.Depth > 40 kbyte
         <other RED parms too>

      Type:                FIFO
      Output:              Scheduler1.InputA

      Type:                FIFO
      Output:              Scheduler1.InputB

      Type:                FIFO
      Output:              Scheduler1.InputC

      Type:                FIFO
      Output:              Scheduler1.InputD

      Type:                Scheduler4Input
      MaxRateProfile:      none
      MinRateProfile:      Profile4
      Priority:            20
      MaxRateProfile:      Profile5
      MinRateProfile:      none
      Priority:            40
      MaxRateProfile:      none
      MinRateProfile:      Profile3
      Priority:            20
      MaxRateProfile:      none
      MinRateProfile:      none
      Priority:            10

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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, a TCB is defined for each customer to
reflect the TCS with that customer: TCB1, defined above is the TCB for
customer 1 and definitions are then added for TCB2 and for TCB3 which
reflect the agreements with customers 2 and 3 respectively.

Finally, a classifier is added to the front end to separate the traffic
from the three different customers. This forms a new TCB, TCB4, which
incorporates TCB1, TCB2, and TCB3 and is illustrated in Figure 8.

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


      Filter1:     to TCB1
      Filter2:     to TCB2
      Filter3:     to TCB3
      No Match:    AbsoluteDropper4

      (as defined above)


      submitted +-----+
      traffic   |    A|--------> TCB1
            --->|    B|--------> TCB2
                |    C|--------> TCB3
                |    X|--------> AbsoluteDropper4

      Figure 8: An Example of a Multi-Customer TCB

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      (similar to TCB1, perhaps with different numeric parameters)

      (similar to TCB1, perhaps with different numeric parameters)

      (the total TCB)

and the filters, based on each customer's source MAC address, could be
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 Classifier 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 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.

A more complex example might be a TCB configuration that offers
additional functionality to the customer. It recognizes individual

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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 9.

Suppose that the customer has an SLS which specifices 2 service levels,
to be identifed to the provider by DSCP A and DSCP B.  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 The metering elements limit 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

This TCB could be formally specified as follows:

                     +-----+   +-----+
    Classifier1      |     |   |     |---------------+
        (MF)      +->|     |-->|     |     +-----+   |
      +-----+     |  |     |   |     |---->|     |   |
      |    A|------  +-----+   +-----+     +-----+   |
  --->|    B|-----+  Marker1   Meter1      Absolute  |
      |    C|---+ |                        Dropper1  |   +-----+
      |    X|-+ | |  +-----+   +-----+               +-->|A    |
      +-----+ | | |  |     |   |     |------------------>|B    |--->
              | | +->|     |-->|     |     +-----+   +-->|C    | to TCB2
              | |    |     |   |     |---->|     |   |   +-----+
              | |    +-----+   +-----+     +-----+   |    Mux1
              | |    Marker2   Meter2      Absolute  |
              | |                          Dropper2  |
              | |    +-----+   +-----+               |
              | |    |     |   |     |---------------+
              | |--->|     |-->|     |     +-----+
              |      |     |   |     |---->|     |
              |      +-----+   +-----+     +-----+
              |      Marker3   Meter3      Absolute
              |                            Dropper3
              V etc.

      Figure 9: An Example of a Marking and Traffic Isolation TCB

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      Classifier1: (MF)
      FilterA:             Marker1
      FilterB:             Marker2
      FilterC:             Marker3

      Output:              Meter1

      Output:              Meter2

      Output:              Meter3

      ConformingOutput:    Mux1.InputA
      NonConformingOutput: AbsoluteDropper1

      ConformingOutput:    Mux1.InputB
      NonConformingOutput: AbsoluteDropper2

      ConformingOutput:    Mux1.InputC
      NonConformingOutput: AbsoluteDropper3


      Output:              to TCB2

Note that the detailed 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 which is illustrated in
Figure 10.

TCB2 could then be specified as follows:

      Classifier2: (BA)
      FilterA:               Meter5
      FilterB:               Meter6

      ConformingOutput:      Queue1

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                     |     |---------------> to Queue1
                  +->|     |     +-----+
        +-----+   |  |     |---->|     |
        |    A|---+  +-----+     +-----+
      ->|     |       Meter5     AbsoluteDropper4
        |    B|---+  +-----+
        +-----+   |  |     |---------------> to Queue2
      Classifier2 +->|     |     +-----+
         (BA)        |     |---->|     |
                     +-----+     +-----+
                      Meter6     AbsoluteDropper5

      Figure 10: Additional Example: TCB2

      NonConformingOutput:   AbsoluteDropper4

      ConformingOutput:      Queue2
      NonConformingOutput:   AbsoluteDropper5

8.4.  Cascaded TCBs

Nothing in this model prevents more complex scenarios in which one
microflow TCB precedes another (e.g. for TCBs implementing separate TCSs
for the source and for a set of destinations).

9.  Open Issues

<ed: this section to be deleted before WG last call and RFC publication.
The current stance of this draft is supplied in parentheses.

(1)  FIFOs are modelled here as having infinite depth: it is up to any
     preceding meter/dropper to make sure that they do not overflow - a
     hard stop on the depth would be modelled, for example, by preceding
     the FIFO with an Absolute Dropper. Is this appropriate? (Yes)

(2)  We must allow algorithmic droppers that apply different dropping
     behaviour to packets with different classifier matches, with these
     possibly fed through different meters and actions. Should we model
     the dropper as a single input element with implicit pointers back
     to the matching classifier that selects different dropper
     algorithms/treatments? Or as multiple droppers? Or as having
     multiple logical inputs? (single input, implicit pointers).

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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

One particular theft- or denial-of-service issue may arise where a
token-bucket meter, with an absolute dropper for non-conforming traffic,
is used in a TCB to police a stream to a given TCS: the definition of
the token-bucket meter in section 5 indicates that it should be lenient
in accepting a packet whenever any bits of the packet would have been
within the profile; the definition of the leaky-bucket scheduler is
conservative in that a packet is to be transmitted only if the whole
packet fits within the profile. This difference may be exploited by a
malicious scheduler either to obtain QoS treatment for more octets than
allowed in the TCS or to disrupt (perhaps only slightly) the QoS
guarantees promised to other traffic streams.

11.  Acknowledgments

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

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

12.  References

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

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

     K. Nichols, S. Blake, F. Baker, and D. Black, "Definition of the

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     Differentiated Services Field (DS Field) in the IPv4 and IPv6
     Headers", RFC 2474, December 1998.

     F. Baker, A. Smith, K. Chan, "Differentiated Services MIB",
     Internet Draft <draft-ietf-diffserv-mib-03.txt>, May 2000.

     M. Fine, K. McCloghrie, J. Seligson, K. Chan, S. Hahn, and A.
     Smith, "Quality of Service Policy Information Base", Internet Draft
     <draft-ietf-diffserv-pib-00.txt>, March 2000.

     D. Grossman, "New Terminology for Diffserv", Internet Draft <draft-
     ietf-diffserv-new-terms-02.txt>, November 1999.

     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-04.txt>, March 2000.

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

     L. Lin, J. Lo, and F. Ou, "A Generic Traffic Conditioner", Internet
     Draft <draft-lin-diffserv-gtc-01.txt>, August 1999.

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

     F. Reichmeyer,  D. Grossman, J. Strassner, M. Condell, "A Common
     Terminology for Policy Management", Internet Draft <draft-
     reichmeyer-polterm-terminology-00.txt>, March 2000

     J. Strassner, W. Weiss, D. Durham, A. Westerinen, "Information
     Model for Describing Network Device QoS Mechanisms", Internet Draft
     <draft-ietf-policy-qos-device-info-model-00.txt>, April 2000

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

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

13.  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
   920 Main Campus Drive, Suite 500
   Raleigh, NC  27606
   Phone:  +1 919 472 9913

   Daniel Grossman
   Motorola Inc.
   20 Cabot Blvd.
   Mansfield, MA  02048
   Phone:  +1 508 261 5312

Table of Contents

1 Introduction ....................................................    2
2 Glossary ........................................................    3
3 Conceptual Model ................................................    5
3.1 Elements of a Diffserv Router .................................    5
3.1.1 Datapath ....................................................    5
3.1.2 Configuration and Management Interface ......................    6
3.1.3 Optional QoS Agent Module ...................................    7
3.2 Hierarchical Model of Diffserv Components .....................    7

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4 Classifiers .....................................................   10
4.1 Definition ....................................................   10
4.1.1 Filters .....................................................   11
4.1.2 Overlapping Filters .........................................   11
4.2 Examples ......................................................   12
4.2.1 Behaviour Aggregate (BA) Classifier .........................   12
4.2.2 Multi-Field (MF) Classifier .................................   13
4.2.3 Free-form Classifier ........................................   13
4.2.4 Other Possible Classifiers ..................................   14
5 Meters ..........................................................   14
5.1 Examples ......................................................   17
5.1.1 Average Rate Meter ..........................................   17
5.1.2 Exponential Weighted Moving Average (EWMA) Meter ............   18
5.1.3 Two-Parameter Token Bucket Meter ............................   19
5.1.4 Multi-Stage Token Bucket Meter ..............................   19
5.1.5 Null Meter ..................................................   20
6 Action Elements .................................................   20
6.1 Marker ........................................................   21
6.2 Absolute Dropper ..............................................   21
6.3 Multiplexer ...................................................   22
6.4 Counter .......................................................   22
6.5 Null Action ...................................................   22
7 Queueing Blocks .................................................   22
7.1 Queueing Model ................................................   23
7.1.1 FIFO ........................................................   23
7.1.2 Scheduler ...................................................   25
7.1.3 Algorithmic Dropper .........................................   27
7.1.4 Constructing queueing blocks from the elements ..............   30
7.2 Shaping .......................................................   31
8 Traffic Conditioning Blocks (TCBs) ..............................   31
8.1 An Example TCB ................................................   32
8.2 An Example TCB to Support Multiple Customers ..................   37
8.3 TCBs Supporting Microflow-based Services ......................   38
8.4 Cascaded TCBs .................................................   41
9 Open Issues .....................................................   41
10 Security Considerations ........................................   42
11 Acknowledgments ................................................   42
12 References .....................................................   42
13 Authors' Addresses .............................................   44

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14.  Full Copyright

   Copyright (C) The Internet Society (2000). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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