Internet Draft
Internet Engineering Task Force Y. Bernet
Diffserv Working Group Microsoft
INTERNET-DRAFT S. Blake
Expires January 2001 Ericsson
draft-ietf-diffserv-model-04.txt D. Grossman
Motorola
A. Smith
July 2000
An Informal Management 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
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft
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This document is a product of the IETF's Differentiated Services Working
Group. Comments should be addressed to WG's mailing list at
diffserv@ietf.org. The charter for Differentiated Services may be found
at http://www.ietf.org/html.charters/diffserv-charter.html Copyright (C)
The Internet Society (2000). All Rights Reserved.
Distribution of this memo is unlimited.
Abstract
This memo proposes an informal management model of Differentiated
Services (Diffserv) routers for use in their management and
configuration. This model defines functional datapath elements (e.g.
classifiers, meters, actions (e.g. marking, absolute dropping, counting,
multiplexing), algorithmic droppers, queues and schedulers. It describes
possible configuration parameters for these elements and how they might
be interconnected to realize the range of traffic conditioning and per-
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hop behavior (PHB) functionalities described in the Diffserv
Architecture [DSARCH].
The model is intended to be abstract and capable of representing the
configuration parameters important to Diffserv functionality for a
variety of specific router implementations. It is not intended as a
guide to system implementation nor as a formal modelling description.
This model serves as the rationale for the design of an SNMP MIB [DSMIB]
and for other configuration interfaces (e.g. [DSPIB]) and, possibly,
more detailed formal models (e.g. [QOSDEVMOD]): these should all be
consistent with this model.
1. Introduction
Differentiated Services (Diffserv) [DSARCH] is a set of technologies
which allow network service providers to offer services with different
kinds of network quality-of-service (QoS) objectives 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 memo is to define
such a model.
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The basic forwarding functionality of a Diffserv router is defined in
other specifications; e.g., [DSARCH, DSFIELD, AF-PHB, EF-PHB].
This document is not intended in any way to constrain or to dictate the
implementation alternatives of Diffserv routers. 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 memo,
configuration and management tools will more readily be able to
configure and manage networks incorporating Diffserv routers of assorted
origins.
o Section 3 starts by describing the basic high-level blocks of a
Diffserv router. It explains the concepts used in the model,
including the hierarchical management model for these blocks which
uses low-level functional datapath elements such as Classifiers,
Actions, Queues.
o Section 4 describes Classifier elements.
o Section 5 discusses Meter elements.
o Section 6 discusses Action elements.
o Section 7 discusses the basic queueing elements of Algorithmic
Droppers, Queues and Schedulers and their functional behaviors
(e.g. traffic shaping).
o Section 8 shows how the low-level elements can be combined to build
modules called Traffic Conditioning Blocks (TCBs) which are useful
for management purposes.
o Section 9 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.
Absolute A functional datapath element which simply discards all
Dropper packets arriving at its input.
Algorithmic A functional datapath element which selectively discards
Dropper packets that arrive at its input, based on a discarding
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algorithm. It has one data input and one output.
Classifier A functional datapath element which consists of filters
which select matching and non-matching packets and
forwards them along a particular datapath within the
router. A classifier splits a single incoming traffic
stream into multiple outgoing ones.
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
statistics.
Filter A set of wildcard, prefix, masked, range and/or exact
match conditions on the content of a packet's
headers or other data, and/or on implicit or derived
attributes associated with the packet. A filter is
said to match only if each condition is satisfied.
Functional A basic building block of the conceptual router.
Datapath Typical elements are Classifiers, Meters, Actions,
Element Algorithmic Droppers, Queues and Schedulers.
Multiplexer A multiplexor.
(Mux)
Multiplexor 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.
Policing The process of comparing the arrival of data packets
against a temporal profile and forwarding, delaying
or dropping them so as to make the output stream
conformant to the profile. Policing is modelled
here as the combination of either a meter or a
scheduler with either an absolute dropper or an
algorithmic dropper.
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
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discarding them.
Scheduling An algorithm which determines which queue of a set
algorithm of queues 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-
work-conserving.
Shaping The process of delaying packets within a traffic stream
to cause it to conform to some defined temporal 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 functional datapath elements interconnected in
Block (TCB) such a way as to perform a specific set of traffic
conditioning functions on an incoming traffic stream.
A TCB can be thought of as an entity with one
input and one or more outputs and a set of control
parameters.
Work- A property of a scheduling algorithm such that it
conserving services a packet, if one is available, at every
transmission opportunity."
3. Conceptual Model
This section introduces a block diagram of a Diffserv router and
describes the various components illustrated in Figure 1. 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 Actions of Marking, Absolute Dropping, Counting and
Multiplexing.
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o Queueing elements, including capabilities of algorithmic
dropping and scheduling.
o Certain combinations of the above functional datapath elements
into higher-level blocks known as Traffic Conditioning Blocks
(TCBs).
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.
Figure 1 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
behavior of a specific router's switching fabric/backplane is not
modeled by the routing core; these should be modeled using the
functional datapath 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
functional datapath elements (e.g. classifiers, queueing elements) 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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+---------------+
| 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 +------------+
msgs
Figure 1: Diffserv Router Major Functional Blocks
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 rules and goals governing the Diffserv behaviour of a
router are presumed to be installed by 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
successfully implemented by the router. External reporting of such
implementation capabilities are considered out of scope for this
document.
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
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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
[E2E].
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.
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,
metering, action and queueing functions which might be instantiated on
each interface's ingress and egress component.
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, possibly just
a "match everything" rule.
- 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].
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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
- 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
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 datapath elements may be
modelled on both the ingress and egress components of an interface.
Note that it is not mandatory that each of these functional datapath
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
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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-level configuration management perspective, the following
hierarchy exists:
At the lowest level considered here, are individual functional
datapath elements, each with their own configuration parameters and
management counters and flags.
At the next level, the network administrator manages groupings of
these functional datapath elements interconnected in a DAG. These
functional datapath elements are organized in self-contained 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 preceding TCB.
A TCB can be thought of as a "black box" with one input and one or
more outputs (in the data path). Each interface (ingress or egress)
may have a different TCB configuration.
At the topmost level considered here, the network administrator
manages interfaces. Each interface consists of an ingress component
and an egress component. Each component may contain one or more
TCBs.
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 using different filters
are described in the following sections. Figure 3 illustrates a
classifier, where the outputs connect to succeeding functional datapath
elements.
The simplest possible Classifier element is one that matches all packets
that are applied at its input. In this case, the Classifier element is
just a no-op and may be omitted.
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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
route.
The following classifier separates traffic into one of three output
streams based on three filters:
Filter Matched Output Stream
-------------- ---------------
Filter1 A
Filter2 B
Filter3 (no match) C
Where Filters1 and Filter2 are defined to be the following BA filters
([DSARCH], Section 4.2.1 ):
Filter DSCP
------ ------
1 101010
2 111111
3 ****** (wildcard)
4.1.1. Filters
A filter consists of a set of conditions on the component values of a
packet's classification key (the header values, contents, and attributes
relevant for classification). In the BA classifier example above, the
classification key consists of one packet header field, the DSCP, and
both Filter1 and Filter2 specify exact-match conditions on the value of
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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 172.31.8.1/32 172.31.3.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:
Filter5:
Type: Masked-DSCP
Value: 111000
Mask: 111000
Filter6:
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
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manager which precedence is being used. Such precedence mechanisms must
be supported in any translation of this model into specific syntax for
configuration and management protocols.
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
precedence.
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.
This model of classifier operation makes the assumption that all filters
of the same precedence be applied simultaneously. Whilst convenient from
a modelling point-of-view, this may or may not be how the classifier is
actually implemented - this assumption is not intended to dictate how
the implementation actually handles this, merely to clearly define the
required end result.
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.
The following defines a possible BA filter:
Filter8:
Type: BA
Value: 111000
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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:
Filter9:
Type: IPv4-6-tuple
IPv4DestAddrValue: 0.0.0.0
IPv4DestAddrMask: 0.0.0.0
IPv4SrcAddrValue: 172.31.8.0
IPv4SrcAddrMask: 255.255.255.0
IPv4DSCP: 28
IPv4Protocol: 6
IPv4DestL4PortMin: 0
IPv4DestL4PortMax: 65535
IPv4SrcL4PortMin: 20
IPv4SrcL4PortMax: 20
A similar type of classifier can be defined for IPv6.
4.2.3. Free-form Classifier
A Free-form classifier is made up of a set of user definable arbitrary
filters each made up of {bit-field size, offset (from head of packet),
mask}:
Classifier2:
Filter12: OutputA
Filter13: OutputB
Default: OutputC
Filter12:
Type: FreeForm
SizeBits: 3 (bits)
Offset: 16 (bytes)
Value: 100 (binary)
Mask: 101 (binary)
Filter13:
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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:
Classifier3:
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 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 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, by counting
conforming and/or non-conforming traffic, it might also be used for
collecting data for out-of-band management functions such as billing
applications.
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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
element.
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. Figure 4 illustrates a meter with 3 levels of
conformance.
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 datapath element following a meter output, but
care must be taken not to inadvertently configure a datapath that
results in packet reordering that is not consistent with PHB
requirements.
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.
unmetered metered
traffic traffic
+---------+
| |--------> conformance A
--------->| meter |--------> conformance B
| |--------> conformance C
+---------+
Figure 4. A Generic Meter
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5.1. Token-Bucket Model
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". Both token buckets and leaky
buckets are, by definition, theoretical relationships between some
defined burst_size, rate and interval:
rate = burst_size/interval
Thus, a token bucket or leaky bucket might specify an information rate
of 1.2 Mbps with a burst size of 1500 bytes. In this case, the token
rate is 1,200,000 bits per second, the token burst is 12,000 bits and
the token interval is 10 milliseconds. The specification says that
conforming traffic will in the worst case come in 100 bursts per second
of 1500 bytes and at a rate not to exceed 1.2 Mbps.
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 (in
the example, one bit every 0.83 microseconds) but, in fact, departs in
multi-bit units (packets) at a rate approximating that. In the example,
it might send one 1500 byte packet every 10 ms, or one 500 byte packet
every 3.3 ms. 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.
Implementations generally seek as constant a transmission rate as
achievable. In theory, a 10 Mbps shaped transmission stream from an
algorithmic implementation and a stream which is running at 10 Mbps
because its bottleneck link has been a 10 Mbps Ethernet link should be
indistinguishable. Depending on configuration, the approximation to
theoretical smoothness may vary by moving as much as an MTU from one
token interval to another. Traffic may also be jostled by traffic
competing for the same transmission resources.
A token bucket measures the arrival rate of traffic from another system,
which may have originally been shaped using a leaky bucket shaper or its
equivalent, and determines whether it (still) conforms to the
specification. Multi-rate token buckets (token buckets with both a peak
and a mean rate, and sometimes more rates) are commonly used, such as
described in [SRTCM] and [TRTCM]. In this case, absolute smoothness is
not expected, but conformance to one or more of the specified rates is
expected.
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Simplistically, a data stream is said to conform to a simple token
bucket parameterised by a {rate, burst_size} if the system receives in
any time interval, t, at most, an amount of data not exceeding (rate *
t) + burst_size.
For the multi-rate token bucket case, the data stream is said to conform
if, for each of the rates, the stream conforms to the token-bucket
profile appropriate for traffic of that class. For example, 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" token bucket profile.
When used as a leaky bucket shaper, this definition interacts with clock
granularity in ways one might not expect. A leaky bucket releases a
packet only when all of its bits would have been allowed: it does not
borrow from future capacity. If the clock is very fine grain, on the
order of the bit rate or faster, this is not an issue. But if the clock
is relatively slow (and millisecond or multi-millisecond clocks are not
unusual in networking equipment), this can introduce jitter to the
shaped stream.
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. Ideally, when
used in a Meter, a token bucket accepts a packet only if all of its bits
would have been accepted and does not borrow excess capacity required
from future capacity. This is consistent with [SRTCM] and [TRTCM]. In
real-world deployment, where MTUs are often larger than the burst size
offered by a link-layer network service provider and TCP is more
commonly ACK-paced than shaped using a leaky bucket, the loose model
offers a solution to the problems that arise. For a more detailed look
at the practical issues, see Appendix A.
5.2. Examples
The following are some examples of possible meters.
5.2.1. Average Rate Meter
An example of a very simple meter is an average rate meter. This type of
meter measures the average rate at which packets are submitted to it
over a specified averaging time.
An average rate profile may take the following form:
Meter1:
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Type: AverageRate
Profile: Profile1
ConformingOutput: Queue1
NonConformingOutput: Counter1
Profile1:
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.
5.2.2. Exponential Weighted Moving Average (EWMA) Meter
The EWMA form of meter is easy to implement in hardware and can be
parameterized as follows:
avg_rate(t) = (1 - Gain) * avg_rate(t') + Gain * rate(t)
t = t' + Delta
For a packet arriving at time t:
if (avg_rate(t) > AverageRate)
non-conforming
else
conforming
"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:
Meter2:
Type: ExpWeightedMovingAvg
Profile: Profile2
ConformingOutput: Queue1
NonConformingOutput: AbsoluteDropper1
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Profile2:
Type: ExpWeightedMovingAvg
AverageRate: 25 kbps
Delta: 10 usec
Gain: 1/16
5.2.3. Two-Parameter Token Bucket Meter
A more sophisticated meter might measure conformance to a token bucket
(TB) profile. A TB profile generally has two parameters, an average
token rate 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:
Meter3:
Type: SimpleTokenBucket
Profile: Profile3
ConformingOutput: Queue1
NonConformingOutput: AbsoluteDropper1
Profile3:
Type: SimpleTokenBucket
AverageRate: 200 kbps
BurstSize: 100 kbytes
5.2.4. Multi-Stage Token Bucket Meter
More complicated TB meters might define two burst sizes and three
conformance levels. Packets found to exceed the larger burst size are
deemed non-conforming. Packets found to exceed the smaller burst size
are deemed partially conforming. Packets exceeding neither are deemed
conforming. Token bucket meters designed for Diffserv networks are
described in more detail in [SRTCM, TRTCM, GTC]; in some of these
references three levels of conformance are discussed in terms of colors,
with green representing conforming, yellow representing partially
conforming and red representing non- conforming. Often these multi-
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conformance level meters can be implemented using an appropriate
configuration of multiple two- parameter TB meters.
A profile for a multi-stage TB meter with three levels of conformance
might look as follows:
Meter4:
Type: TwoRateTokenBucket
ProfileA: Profile4
ConformingOutputA: Queue1
ProfileB: Profile5
ConformingOutputB: Marker1
NonConformingOutput: AbsoluteDropper1
Profile4:
Type: SimpleTokenBucket
AverageRate: 100 kbps
BurstSize: 20 kbytes
Profile5:
Type: SimpleTokenBucket
AverageRate: 100 kbps
BurstSize: 100 kbytes
5.2.5. Null Meter
A null meter has only one output: always conforming, and no associated
temporal profile. Such a meter is useful to define in the event that the
configuration or management interface does not have the flexibility to
omit a meter in a datapath segment.
Meter5:
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 that can then be applied
include:
- Marking
- Absolute Dropping
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- Multiplexing
- Counting
- Null action - do nothing
The corresponding action elements are described in the following
sections.
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 either a concatenation of a Meter with an Absolute Dropper or as a
concatenation of an Algorithmic Dropper with a Scheduler. These elements
will discard packets which exceed the TCS. Marking is performed by a
Marker Action, 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.
Marker1:
Type: DSCPMarker
Mark: 010010
6.2. Absolute Dropper
Absolute Droppers simply discard packets. There are no parameters for
these droppers. Because this Algorithmic Dropper is a terminating point
of the datapath and has no outputs, it is probably desirable to forward
the packet through a Counter Action first for instrumentation purposes.
AbsoluteDropper1:
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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 7.1.3). However, since this element's behavior is closely tied
the state of one or more queues, we choose to distinguish it as a
separate functional datapath element.
6.3. Multiplexor
It is occasionally necessary to multiplex traffic streams into a
functional datapath element with a single input. A M:1 (fan-in)
multiplexor is a simple logical device for merging traffic streams. It
is parameterized by its number of incoming ports.
Mux1:
Type: Multiplexor
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 datapath 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 an
Absolute Dropper or to count packets arriving at or departing from some
other functional element.
Counter1:
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.
Null1:
Type: Null
Output: Queue1
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7. Queueing Elements
Queueing elements 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
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 profile (i.e.
policing).
The queueing elements in this model represent a logical abstraction of a
queueing system, which is used to configure PHB-related parameters. 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 element 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 systems 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: Queues, Schedulers and Algorithmic Droppers,
respectively. These elements may be connected together as part of a TCB
containing one or more Queues, zero or more Algorithmic Droppers and one
or more Schedulers.
The remainder of this section discusses FIFO Queues: typically, the
Queue 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.
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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 Queue
In this model, a FIFO Queue 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 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.
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:
Queue1:
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
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downstream Scheduler elements.
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
profile.
[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
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shared. Additional parameters to control this behavior should be
made available, but are dependent on the particular scheduling
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
parameters:
Scheduler1:
Type: Scheduler2Input
Input1:
MaxRateProfile: Profile1
MinRateProfile: Profile2
Priority: none
Input2:
MaxRateProfile: Profile3
MinRateProfile: Profile4
Priority: none
A work-conserving scheduler might be represented using the following
parameters:
Scheduler2:
Type: Scheduler3Input
Input1:
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MaxRateProfile: WorkConserving
MinRateProfile: Profile5
Priority: 1
Input2:
MaxRateProfile: WorkConserving
MinRateProfile: Profile6
Priority: 2
Input3:
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 ould be modelled as a side-effect on the FIFO upon which it
operated, 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. It is
possible 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. In the rare cases where this is required,
the chosen model is to insert another Classifier element at this point
in the flow and for it to feed into multiple Algorithmic Dropper
elements, each one implementing a drop calculation that is independent
of any classification keys of the packet: this will likely require the
creation of a new TCB to contain the Classifier and the Algorithmic
Dropper elements.
There are many other formulations of a model that could represent this
linkage, other than the one described above: one formulation would have
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been to have a pointer from one of the drop probability calculation
algorithms inside the dropper to the original Classifier element that
selects this algorithm. Another way would have been to have multiple
"inputs" to the Algorithmic Dropper element fed from the preceding
elements, leading eventually back to the Classifier elements that
matched the packet. Yet 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. And yet another could have been to include a
classifier inside the dropper, in order for it to pick out the 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
with the packet (e.g. its PHB, as determined by a classifier, which will
determine on which of the droppers inputs the 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).
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+--------------------------------------+
| +------------+ +-----------+ |Algorithmic
| | smoothing | n |trigger & | |Dropper
| | function(s)|---/--->|discard | |
| | (optional) | |calc. | |
| +------------+ +-----------+ |
| ^ TailDrop| |HeadDrop |
+------------|-------------|-|---------+
| | |
+---|-------------+ |
| | |
v |Depth v
Input ----------------------+ to Scheduler
-----------------------------> |x|x|x|x|x|x|x|------------------->
----------------------+
FIFO |
|
| | |
| v | bit-bucket
+---+
Figure 5. Algorithmic Dropper + Queue
An Algorithmic Dropper which uses a RIO algorithm might be represented
using the following parameters:
AlgorithmicDropper1:
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%
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Another form of Algorithmic Dropper, a threshold-dropper, might be
represented using the following parameters:
AlgorithmicDropper2:
Type: AlgorithmicDropper
Discipline: Drop-on-threshold
Trigger: Fifo2.Depth > 20 kbyte
Output: Fifo1
Yet another Algorithmic Dropper which drops all out-of-profile packets
whenever the FIFO threshold exceeds a certain depth (this Algorithmic
Dropper is not part of the larger TCB example) might be represented with
the following parameters:
AlgorithmicDropper3:
Type: AlgorithmicDropper2Input
Discipline: Drop-out-packets-on-threshold
Output: Fifo3
InputA: (in profile)
Trigger: none
InputB: (out of profile)
Trigger: Fifo3.Depth > 100 kbyte
7.1.4. Constructing queueing blocks from the elements
A queueing block is constructed by concatenation of these functional
datapath elements. 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 Queue, zero or more
Algorithmic Droppers and at least one Scheduler. The following inter-
connections are allowed:
1) The input of a Queue may be the input of the queueing block or it
may be connected to the output of an Algorithmic Dropper or to an
output of a Scheduler.
2) Each input of a Scheduler may be connected to the output of a
Queue, to the output of an Algorithmic Dropper or to the output of
another Scheduler.
3) The input of an Algorithmic Dropper must be the input of the
queueing block.
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4) The output of the queueing block may be the output of a Queue, an
Algorithmic Dropper or a Scheduler.
Note, in particular, that Schedulers may operate in series such that a
packet at the head of a Queue feeding the concatenated Schedulers is
serviced only after all of the scheduling criteria are met. For example,
a Queue 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 Queue and/or a dropper between
the two Schedulers.
7.2. Sharing load among traffic streams using queueing
Queues are used, in Differentiated Services, for a number of purposes.
In essence, they are simply places to store traffic until it is
transmitted. However, when several queues are used together in a
queueing system, they can also achieve effects beyond that for given
traffic streams. They can be used to limit variation in delay or impose
a maximum rate (shaping), to permit several streams to share a link in a
semi-predictable fashion (load sharing), or to move variation in delay
from some streams to other streams.
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.
7.2.1. Load Sharing
Load sharing is the traditional use of queues. It was theoretically
explored in a paper by Floyd [FLOYD] in 1993, but has been in use in
communications systems since the 1970's.
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[DSARCH] discusses load sharing as dividing an interface among traffic
classes predictably or applying a minimum rate to each of a set of
traffic classes, which might be measured as an absolute lower bound on
the rate a traffic stream achieves, or a fraction of the rate an
interface offers. It is generally implemented as some form of weighted
round robin among a set of FIFO queues or WFQ system. This has
interesting side-effects.
A key effect sought is to ensure that the mean rate the traffic in a
stream experiences is never lower than some threshold when there is at
least that much traffic to send. When there is less traffic than this,
the queue tends to be starved for traffic, meaning that the queuing
system will not delay its traffic by very much. When there is
significantly more and the queue fills, packets in this class will be
delayed significantly more than traffic in other classes that are under-
using their available capacity. This form of queuing system therefore
tends to move delay and variation in delay from under-used classes of
traffic to heavier users, as well as managing the rates of the traffic
streams.
A side-effect of a WRR or WFQ implementation is that between any two
packets in a given traffic class, the scheduler may emit one or more
packets from each of the other classes in the queuing system. In cases
where average behavior is in view, this is perfectly acceptable. In
cases where traffic is very intolerant of jitter and there are a number
of competing classes, this may have undesirable consequences.
7.2.2. Traffic Priority
Traffic Prioritization is a special case of load sharing, wherein a
certain traffic class is deemed so jitter-intolerant that if it has
traffic present, that traffic must be sent at the earliest possible
time. By extension, several priorities might be defined, such that
traffic in each of several classes is given preferential service over
any traffic of a lower class. It is the obvious implementation of IP
Precedence as described in [RFC 791], of 802.1p traffic classes
[802.1D], and other similar technologies.
Priority is often abused in real networks; people tend to think that
traffic which has a business priority deserves this treatment, and talk
more about the business imperatives than the actual application
requirements. This can have severe consequences; networks have been
configured which placed business-critical traffic at a higher priority
than routing traffic, resulting in congestive collapse of the networks.
However, it has a legitimate use in services like EF, where it is
absolutely known, due to policing, that a traffic stream does not abuse
its rate, and the application is indeed jitter-intolerant enough to
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merit this type of handling.
8. Traffic Conditioning Blocks (TCBs)
The Classifier, Meter, Action, Algorithmic Dropper, Queue and Scheduler
functional datapath elements described above can be combined into
Traffic Conditioning Blocks (TCBs). A TCB is an abstraction of a set of
functional datapath elements that may be used to facilitate the
definition of specific traffic conditioning functionality e.g. it might
be likened to a template which can be replicated multiple times for
different traffic streams or different customers. It has no likely
physical representation in the implementation of the data path: it is
invented purely as an abstraction for use by management tools.
This model describes the configuration and management of a Diffserv
interface in terms of a TCB that contains, by definition, zero or more
Classifier, Meter, Action, Algorithmic Dropper, Queue and Scheduler
elements. These elements are arranged arbitrarily according to the
policy being expressed, but always in the order here. Traffic may be
classified; classified traffic may be metered; each stream of traffic
identified by a combination of classifiers and meters may have some set
of actions performed on it, followed by drop algorithms; packets of the
traffic stream may ultimately be stored into a queue and then be
scheduled out to the next TCB or physical interface. It is possible to
omit elements or include null elements of any type, or to concatenate
multiple functional datapath elements of the same type.
When the Diffserv treatment for a given packet needs to have those
building blocks repeated, this is performed by cascading multiple TCBs:
an output of one TCB may drive the input of a succeeding one. For
example, consider the case where traffic of a set of classes is shaped
to a set of rates, but the total output rate of the group of classes
must also be limited to a rate. One might imagine a set of network news
feeds, each with a certain maximum rate, and a policy that their
aggregate may not exceed some figure. This may be simply accomplished by
cascading two TCBs. The first classifies the traffic into its separate
feeds and queues each feed separately. The feeds (or a subset of them)
are now fed into a second TCB, which places all input (these news feeds)
into a single queue with a certain maximum rate. In implementation, one
could imagine this as the several literal queues, a CBQ or WFQ system
with an appropriate (and complex) weighting scheme, or a number of other
approaches. But they would have the same externally measurable effect on
the traffic as if they had been literally implemented with separate
TCBs.
A generalised TCB might consist of the following stages:
- Classification stage
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- Metering stage
- Action stage
- Algorithmic Dropping stage
- Queueing stage
- Scheduling stage
where each stage may consist of a set of parallel datapaths consisting
of pipelined elements.
A Classifier or a Meter is typically a 1:N element, an Action,
Algorithmic Dropper or Queue is typically a 1:1 element and a Scheduler
is a N:1 element. A complete TCB should, however, result in a 1:1 or 1:N
abstract element. Note that the fan-in or fan-out of an element is not
an important defining characteristic of this taxonomy.
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
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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)
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 path for DSCP 001100 proceeds directly to Dropper1 whilst the paths
for DSCP 001001 and 001101 include a metering stage. All other traffic
is passed directly on to Dropper3. 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 conformance levels: conforming or non-conforming.
Following the Metering stage is an Action stage in some of the branches.
Packets submitted for DSCP 001001 (Classifier output A) that are deemed
non- conforming by Meter1 are counted and discarded while packets that
are conforming are passed on to Queue1. Packets submitted for DSCP
001101 (Classifier output C) that are deemed non-conforming by Meter2
are re-marked and then both conforming and non-conforming packets are
multiplexed together before being passed on to Dropper2/Queue3.
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The Algorithmic Dropping, Queueing and Scheduling stages are realised as
follows, illustrated in figure 7. Note that the figure does not show any
of the implicit control linkages between elements that allow e.g. an
Algorithmic Dropper to sense the current state of a succeeding Queue.
Conforming DSCP 001001 packets from Meter1 are passed directly to
Queue1: there is no way, with a configuration of the following Scheduler
that patches the metering, for these packets to overflow the depth of
Queue1 so there is never a requirement for dropping at this point.
Packets marked for DSCP 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
its agreed profile. Similarly, all packets from the original DSCP
001101 stream (some may have been re-marked by this stage) 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 Queue elements are then serviced by a Scheduler element
Scheduler1: tis must be configured to give each of its inputs 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.
The interconnections of the TCB elements illustrated in Figures 6 and 7
can be represented as follows:
TCB1:
Classifier1:
FilterA: Meter1
FilterB: Dropper1
FilterC: Meter2
Default: Dropper3
Meter1:
Type: AverageRate
Profile: Profile1
ConformingOutput: Queue1
NonConformingOutput: Counter1
Counter1:
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from Meter1 +-----+
------------------------------->| |----+
| | |
+-----+ |
Queue1 |
| +-----+
from Classifier1 +-----+ +-----+ +->|A |
---------------->| |------->| |------>|B |------->
| | | | +--->|C | exiting
+-----+ +-----+ | +->|D | traffic
AlgDropper1 Queue2 | | +-----+
| | Scheduler1
from Mux1 +-----+ +-----+ | |
---------------->| |------->| |--+ |
| | | | |
+-----+ +-----+ |
AlgDropper2 Queue3 |
|
from Classifier1 +-----+ +-----+ |
---------------->| |------->| |----+
| | | |
+-----+ +-----+
AlgDropper3 Queue4
Figure 7: An Example Traffic Conditioning Block (Part 2)
Output: AbsoluteDropper1
Meter2:
Type: AverageRate
Profile: Profile3
ConformingOutput: Mux1.InputA
NonConformingOutput: Marker1
Marker1:
Type: DSCPMarker
Mark: 001000
Output: Mux1.InputB
Mux1:
Output: Dropper2
AlgDropper1:
Type: AlgorithmicDropper
Discipline: Drop-on-threshold
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Trigger: Queue2.Depth > 10kbyte
Output: Queue2
AlgDropper2:
Type: AlgorithmicDropper
Discipline: Drop-on-threshold
Trigger: Queue3.Depth > 20kbyte
Output: Queue3
AlgDropper3:
Type: AlgorithmicDropper
Discipline: RED93
Trigger: Internal
Output: Queue3
MinThresh: Queue3.Depth > 20 kbyte
MaxThresh: Queue3.Depth > 40 kbyte
Queue1:
Type: FIFO
Output: Scheduler1.InputA
Queue2:
Type: FIFO
Output: Scheduler1.InputB
Queue3:
Type: FIFO
Output: Scheduler1.InputC
Queue4:
Type: FIFO
Output: Scheduler1.InputD
Scheduler1:
Type: Scheduler4Input
InputA:
MaxRateProfile: none
MinRateProfile: Profile4
Priority: 20
InputB:
MaxRateProfile: Profile5
MinRateProfile: none
Priority: 40
InputC:
MaxRateProfile: none
MinRateProfile: Profile3
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Priority: 20
InputD:
MaxRateProfile: none
MinRateProfile: none
Priority: 10
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 configuration is readily modified 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 elements are created for TCB2 and for TCB3
which reflect the agreements with customers 2 and 3 respectively. These
3 TCBs may or may not share the same elements and parameters.
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 is
illustrated in Figure 8.
A representation of this multi-customer TCB might be:
TCB4:
Classifier4:
submitted +-----+
traffic | A|--------> TCB1
--------->| B|--------> TCB2
| C|--------> TCB3
| X|------+ +-----+
+-----+ +-->| |
Classifier4 +-----+
AbsoluteDrop4
Figure 8: An Example of a Multi-Customer TCB
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Filter1: to TCB1
Filter2: to TCB2
Filter3: to TCB3
No Match: AbsoluteDropper4
TCB1:
(as defined above)
TCB2:
(similar to TCB1, perhaps with different
elements or numeric parameters)
TCB3:
(similar to TCB1, perhaps with different
elements or numeric parameters)
and the filters, based on each customer's source MAC address, could be
defined as follows:
Filter1:
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
Filter2:
(similar to Filter1 but with customer 2's source MAC address as
SrcValue)
Filter3:
(similar to Filter1 but with customer 3's source MAC address as
SrcValue)
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 performing
dropping of all unmatched traffic.
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
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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
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
+-----+ +-----+
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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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
dropped.
This TCB could be formally specified as follows:
TCB1:
Classifier1: (MF)
FilterA: Marker1
FilterB: Marker2
FilterC: Marker3
etc.
Marker1:
Output: Meter1
Marker2:
Output: Meter2
Marker3:
Output: Meter3
Meter1:
ConformingOutput: Mux1.InputA
NonConformingOutput: AbsoluteDropper1
Meter2:
ConformingOutput: Mux1.InputB
NonConformingOutput: AbsoluteDropper2
Meter3:
ConformingOutput: Mux1.InputC
NonConformingOutput: AbsoluteDropper3
etc.
Mux1:
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.
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+-----+
| |---------------> to Queue1
+->| | +-----+
+-----+ | | |---->| |
| A|---+ +-----+ +-----+
->| | Meter5 AbsoluteDropper4
| B|---+ +-----+
+-----+ | | |---------------> to Queue2
Classifier2 +->| | +-----+
(BA) | |---->| |
+-----+ +-----+
Meter6 AbsoluteDropper5
Figure 10: Additional Example: TCB2
TCB2 could then be specified as follows:
Classifier2: (BA)
FilterA: Meter5
FilterB: Meter6
Meter5:
ConformingOutput: Queue1
NonConformingOutput: AbsoluteDropper4
Meter6:
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. Security Considerations
Security vulnerabilities of Diffserv network operation are discussed in
[DSARCH]. This document describes an abstract functional model of
Diffserv router elements. Certain denial-of-service attacks such as
those resulting from resource starvation may be mitigated by appropriate
configuration of these router elements; for example, by rate limiting
certain traffic streams or by authenticating traffic marked for higher
quality-of-service.
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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.
10. 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.
11. References
[AF-PHB]
J. Heinanen, F. Baker, W. Weiss, and J. Wroclawski, "Assured
Forwarding PHB Group", RFC 2597, June 1999.
[DSARCH]
M. Carlson, W. Weiss, S. Blake, Z. Wang, D. Black, and E. Davies,
"An Architecture for Differentiated Services", RFC 2475, December
1998
[DSFIELD]
K. Nichols, S. Blake, F. Baker, and D. Black, "Definition of the
Differentiated Services Field (DS Field) in the IPv4 and IPv6
Headers", RFC 2474, December 1998.
[DSMIB]
F. Baker, A. Smith, K. Chan, "Differentiated Services MIB",
Internet Draft , July 2000.
[DSPIB]
M. Fine, K. McCloghrie, J. Seligson, K. Chan, S. Hahn, and A.
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Smith, "Quality of Service Policy Information Base", Internet Draft
<draft-ietf-diffserv-pib-00.txt>, March 2000.
[DSTERMS]
D. Grossman, "New Terminology for Diffserv", Internet Draft , November 1999.
[E2E]
Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang, M. Speer, K.
Nichols, R. Braden, B. Davie, J. Wroclawski, and E. Felstaine,
"Integrated Services Operation over Diffserv Networks", Internet
Draft , March 2000.
[EF-PHB]
V. Jacobson, K. Nichols, and K. Poduri, "An Expedited Forwarding
PHB", RFC 2598, June 1999.
[FLOYD]
S. Floyd, "General Load Sharing", 1993.
[GTC]
L. Lin, J. Lo, and F. Ou, "A Generic Traffic Conditioner", Internet
Draft , August 1999.
[INTSERV]
R. Braden, D. Clark and S. Shenker, "Integrated Services in the
Internet Architecture: an Overview" RFC 1633, June 1994.
[POLTERM]
F. Reichmeyer, D. Grossman, J. Strassner, M. Condell, "A Common
Terminology for Policy Management", Internet Draft
, March 2000
[QOSDEVMOD]
J. Strassner, W. Weiss, D. Durham, A. Westerinen, "Information
Model for Describing Network Device QoS Mechanisms", Internet Draft
, April 2000
[QUEUEMGMT]
B. Braden et al., "Recommendations on Queue Management and
Congestion Avoidance in the Internet", RFC 2309, April 1998.
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[SRTCM]
J. Heinanen, and R. Guerin, "A Single Rate Three Color Marker", RFC
2697, September 1999.
[TRTCM]
J. Heinanen, R. Guerin, "A Two Rate Three Color Marker", RFC 2698,
September 1999.
[VIC]
McCanne, S. and Jacobson, V., "vic: A Flexible Framework for Packet
Video", ACM Multimedia '95, November 1995, San Francisco, CA, pp.
511-522.
[802.1D]
"Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area networks -
Common specifications - Part 3: Media Access Control (MAC) Bridges:
Revision. This is a revision of ISO/IEC 10038: 1993, 802.1j-1992
and 802.6k-1992. It incorporates P802.11c, P802.1p and P802.12e.",
ISO/IEC 15802-3: 1998.
12. Appendix A. Simple Token Bucket Discussion and Definition
Internet Protocol (IP) packets are of variable-length but theoretical
token buckets operate using fixed-length time intervals or pieces of
data. This leaves an implementor of a token bucket scheme with a
dilemma. When the amount of bandwidth tokens, TB, left in the token
bucket is positive but less than the size of the packet being operated
on, one of three things can be done:
(1) The whole size of the packet can be substracted from the bucket,
leaving it negative, remembering that the token bucket size must
be added to TB rather than simply setting it "full". This
potentially puts more than the token bucket size into this token
bucket interval and less into the next. It does, however, make
the average amount accepted per token bucket interval equal to
the token burst. This approach accepts traffic if any bit in the
packet would be accepted and borrows up to one MTU of capacity
from one or more subsequent intervals when necessary. Such a
token bucket implementation is said to be a "loose" token bucket.
(2) Alternatively, the amount can be left unchanged (and maybe an
attempt could be made to accept the packet under another
threshold in another bucket), remembering that the token bucket
size must be added to the TB variable rather than simply setting
it "full". This potentially puts less than the token bucket size
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into this token bucket interval and more into the next. Like the
first option, it makes the average amount accepted per token
bucket interval equal to the token burst. This approach accepts
traffic if every bit in the packet would be accepted and borrows
up to one MTU of capacity from one or more previous intervals
when necessary. Such a token bucket implementation is said to be
a "strict" (or perhaps "stricter") token bucket.
(3) The TB variable can be set to zero to account for the first part
of the packet and the remainder of the packet size can be taken
out of the next-colored bucket. This, of course, has another bug:
the same packet cannot have both conforming and nonconforming
components in the Diffserv architecture and so is not really
appropriate here.
Unfortunately, the thing that cannot be done is exactly to fit the token
burst specification with random sized packets: therefore token buckets
in a variable length packet environment always have a some variance from
theoretical reality. This has also been observed in the ATM Guaranteed
Frame Rate (GFR) service category specification and Frame Relay.
Some find the behavior of a "loose" token bucket unacceptable, as it is
significantly different than the token bucket description for ATM and
for Frame Relay. However, the "strict" token bucket approach has three
characteristics which are important to keep in mind:
(1) First, if the maximum token burst is smaller than the MTU, it is
possible that traffic never matches the specification. This may
be avoided by not allowing such a specification.
(2) Second, the strict token bucket specifications [SRTCM] and
[TRTCM], as specified, are subject to a persistent under-run.
These accumulate burst capacity over time, up to the maximum
burst size. Suppose that the maximum burst size is exactly the
size of the packets being sent - which one might call the
"strictest" token bucket implementation. In such a case, when one
packet has been accepted, the token depth becomes zero, and
starts to accumulate. If the next packet is received any time
earlier than a token interval later, it will not be accepted. If
the next packet arrives exactly on time, it will be accepted and
the token depth again set to zero. If it arrives later, however,
the token depth will stop accumulating, as it is capped by the
maximum burst size, and tokens that would have accumulated
between the end of that token interval and the actual arrival of
the packet are lost. As a result, natural jitter in the network
conspires against the algorithm to reduce the actual acceptance
rate. Overcoming this error requires the maximum token bucket
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size to be significantly greater than the MTU.
(3) Third, operationally, a strict token bucket is reasonable for
traffic which has been shaped by a leaky bucket shaper or a
serial line. However, traffic in the Internet is rarely shaped in
that way. TCP applies no shaping to its traffic, but rather
depends on longer-range Ack Clocking behavior to help it
approximate a certain rate and explicitly sends traffic bursts
during slow start, retransmission and fast recovery. Video-on-IP
implementations such as [VIC] may have a leaky bucket shaper
available to them, but often do not, and simply enqueue the
output of their codec for transmission on the appropriate
interface. As a result, in each of these cases, a strict shaper
may reject traffic in the short term (single token interval)
which it would have accepted if it had a longer time in view and
which it needs to accept for the application to work properly. To
work around this, the token interval must approximate or exceed
the RTT of the session or sessions in question and the burst size
must accommodate the largest burst that the originator might
send.
The behavior defined in [SRTCM] and [TRTCM] is not mandatory for
compliance, but we give here a mathematical definition of two- parameter
token bucket operation which is consistent with those documents and
which can be used to define a shaping profile.
Define a token bucket with bucket size BS, token accumulation rate R and
instantaneous token occupancy T(t). Assume that T(0) = BS.
Then after an arbitrary interval with no packet arrivals, T(t) will not
change since the bucket is already full of tokens. Assume a packet of
size B bytes at time t'. The bucket capacity T(t'-) = BS still. Then, as
long as B <= BS, the packet conforms to the meter, and
T(t') = BS - B.
Assume an interval v = t - t' elapses before the next packet, of size C
<= BS, arrives. T(t-) is given by the following equation:
T(t-) = min { BS, T(t') + v*R }
maximum of BS tokens).
If T(t-) - C = 0, the packet conforms and T(t) = T(t-) - C. Otherwise,
the packet does not conform and T(t) = T(t-).
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This function can be used to define a shaping profile. If a packet of
size C arrives at time t, it will be eligible for transmission at time
te given as follows (we still assume C <= BS):
te = max { t, t" }
where t" = (C - T(t') + t'*R)/R, T(t") = C, the time when C credits have
accumulated in the bucket, and when the packet would conform if the
token bucket were a meter. te != t" only if t != t".
13. Authors' Addresses
Yoram Bernet
Microsoft
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 936 9568
E-mail: yoramb@microsoft.com
Andrew Smith
FAX: +1 414 345 1827
E-mail: ah_smith@pacbell.net
Steven Blake
Ericsson
920 Main Campus Drive, Suite 500
Raleigh, NC 27606
Phone: +1 919 472 9913
E-mail: slblake@torrentnet.com
Daniel Grossman
Motorola Inc.
20 Cabot Blvd.
Mansfield, MA 02048
Phone: +1 508 261 5312
E-mail: dan@dma.isg.mot.com
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 .................................................... 6
3.1.2 Configuration and Management Interface ...................... 6
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3.1.3 Optional QoS Agent Module ................................... 7
3.2 Hierarchical Model of Diffserv Components ..................... 8
4 Classifiers ..................................................... 10
4.1 Definition .................................................... 10
4.1.1 Filters ..................................................... 11
4.1.2 Overlapping Filters ......................................... 12
4.2 Examples ...................................................... 13
4.2.1 Behaviour Aggregate (BA) Classifier ......................... 13
4.2.2 Multi-Field (MF) Classifier ................................. 14
4.2.3 Free-form Classifier ........................................ 14
4.2.4 Other Possible Classifiers .................................. 15
5 Meters .......................................................... 15
5.1 Token-Bucket Model ............................................ 17
5.2 Examples ...................................................... 18
5.2.1 Average Rate Meter .......................................... 18
5.2.2 Exponential Weighted Moving Average (EWMA) Meter ............ 19
5.2.3 Two-Parameter Token Bucket Meter ............................ 20
5.2.4 Multi-Stage Token Bucket Meter .............................. 20
5.2.5 Null Meter .................................................. 21
6 Action Elements ................................................. 21
6.1 Marker ........................................................ 22
6.2 Absolute Dropper .............................................. 22
6.3 Multiplexor ................................................... 23
6.4 Counter ....................................................... 23
6.5 Null Action ................................................... 23
7 Queueing Elements ............................................... 24
7.1 Queueing Model ................................................ 24
7.1.1 FIFO Queue .................................................. 25
7.1.2 Scheduler ................................................... 26
7.1.3 Algorithmic Dropper ......................................... 28
7.1.4 Constructing queueing blocks from the elements .............. 31
7.2 Sharing load among traffic streams using queueing ............. 32
7.2.1 Load Sharing ................................................ 32
7.2.2 Traffic Priority ............................................ 33
8 Traffic Conditioning Blocks (TCBs) .............................. 34
8.1 An Example TCB ................................................ 35
8.2 An Example TCB to Support Multiple Customers .................. 40
8.3 TCBs Supporting Microflow-based Services ...................... 41
8.4 Cascaded TCBs ................................................. 44
9 Security Considerations ......................................... 44
10 Acknowledgments ................................................ 45
11 References ..................................................... 45
12 Appendix A. Simple Token Bucket Discussion and Definition ...... 47
13 Authors' Addresses ............................................. 50
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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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Bernet et al. Expires January 2001 [Page 52]