[ RFC Index | RFC Search | Usenet FAQs | Web FAQs | Documents | Cities ]
Alternate Formats: rfc3670.txt | rfc3670.txt.pdf
RFC 3670 - Information Model for Describing Network Device QoS Datapath Mechanisms
|
 |
RFC3670 - Information Model for Describing Network Device QoS D
Network Working Group B. Moore
Request for Comments: 3670 IBM Corporation
Category: Standards Track D. Durham
Intel
J. Strassner
INTELLIDEN, Inc.
A. Westerinen
Cisco Systems
W. Weiss
Ellacoya
January 2004
Information Model for Describing
Network Device QoS Datapath Mechanisms
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The purpose of this document is to define an information model to
describe the quality of service (QoS) mechanisms inherent in
different network devices, including hosts. Broadly speaking, these
mechanisms describe the properties common to selecting and
conditioning traffic through the forwarding path (datapath) of a
network device. This selection and conditioning of traffic in the
datapath spans both major QoS architectures: Differentiated Services
and Integrated Services.
This document should be used with the QoS Policy Information Model
(QPIM) to model how policies can be defined to manage and configure
the QoS mechanisms (i.e., the classification, marking, metering,
dropping, queuing, and scheduling functionality) of devices.
Together, these two documents describe how to write QoS policy rules
to configure and manage the QoS mechanisms present in the datapaths
of devices.
This document, as well as QPIM, are information models. That is,
they represent information independent of a binding to a specific
type of repository.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Policy Management Conceptual Model . . . . . . . . . . . 6
1.2. Purpose and Relation to Other Policy Work. . . . . . . . 7
1.3. Typical Examples of Policy Usage . . . . . . . . . . . . 7
2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Common Needs Of DiffServ and IntServ . . . . . . . . . . 8
2.2. Specific Needs Of DiffServ . . . . . . . . . . . . . . . 9
2.3. Specific Needs Of IntServ. . . . . . . . . . . . . . . . 9
3. Methodology. . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Level of Abstraction for Expressing QoS Policies . . . . 10
3.2. Specifying Policy Parameters . . . . . . . . . . . . . . 11
3.3. Specifying Policy Services . . . . . . . . . . . . . . . 12
3.4. Level of Abstraction for Defining QoS Attributes and
Classes. . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5. Characterization of QoS Properties . . . . . . . . . . . 14
3.6. QoS Information Model Derivation . . . . . . . . . . . . 15
3.7. Attribute Representation . . . . . . . . . . . . . . . . 16
3.8. Mental Model . . . . . . . . . . . . . . . . . . . . . . 17
3.8.1. The QoSService Class . . . . . . . . . . . . . . 17
3.8.2. The ConditioningService Class. . . . . . . . . . 18
3.8.3. Preserving QoS Information from Ingress to
Egress . . . . . . . . . . . . . . . . . . . . . 19
3.9. Classifiers, FilterLists, and Filter Entries . . . . . . 21
3.10. Modeling of Droppers . . . . . . . . . . . . . . . . . . 23
3.10.1. Configuring Head and Tail Droppers . . . . . . . 23
3.10.2. Configuring RED Droppers . . . . . . . . . . . . 24
3.11. Modeling of Queues and Schedulers. . . . . . . . . . . . 25
3.11.1. Simple Hierarchical Scheduler. . . . . . . . . . 25
3.11.2. Complex Hierarchical Scheduler . . . . . . . . . 27
3.11.3. Excess Capacity Scheduler. . . . . . . . . . . . 29
3.11.4. Hierarchical CBQ Scheduler . . . . . . . . . . . 31
4. The Class Hierarchy. . . . . . . . . . . . . . . . . . . . . . 33
4.1. Associations and Aggregations. . . . . . . . . . . . . . 33
4.2. The Structure of the Class Hierarchies . . . . . . . . . 34
4.3. Class Definitions. . . . . . . . . . . . . . . . . . . . 38
4.3.1. The Abstract Class ManagedElement. . . . . . . . 38
4.3.2. The Abstract Class ManagedSystemElement. . . . . 39
4.3.3. The Abstract Class LogicalElement. . . . . . . . 39
4.3.4. The Abstract Class Service . . . . . . . . . . . 39
4.3.5. The Class ConditioningService. . . . . . . . . . 39
4.3.6. The Class ClassifierService. . . . . . . . . . . 40
4.3.7. The Class ClassifierElement. . . . . . . . . . . 41
4.3.8. The Class MeterService . . . . . . . . . . . . . 42
4.3.9. The Class AverageRateMeterService. . . . . . . . 44
4.3.10. The Class EWMAMeterService . . . . . . . . . . . 44
4.3.11. The Class TokenBucketMeterService. . . . . . . . 46
4.3.12. The Class MarkerService. . . . . . . . . . . . . 47
4.3.13. The Class PreambleMarkerService. . . . . . . . . 47
4.3.14. The Class ToSMarkerService . . . . . . . . . . . 48
4.3.15. The Class DSCPMarkerService. . . . . . . . . . . 49
4.3.16. The Class 8021QMarkerService . . . . . . . . . . 49
4.3.17. The Class DropperService . . . . . . . . . . . . 50
4.3.18. The Class HeadTailDropperService . . . . . . . . 52
4.3.19. The Class REDDropperService. . . . . . . . . . . 52
4.3.20. The Class QueuingService . . . . . . . . . . . . 54
4.3.21. The Class PacketSchedulingService. . . . . . . . 55
4.3.22. The Class NonWorkConservingSchedulingService . . 56
4.3.23. The Class QoSService . . . . . . . . . . . . . . 57
4.3.24. The Class DiffServService. . . . . . . . . . . . 58
4.3.25. The Class AFService. . . . . . . . . . . . . . . 59
4.3.26. The Class FlowService. . . . . . . . . . . . . . 60
4.3.27. The Class DropThresholdCalculationService. . . . 60
4.3.28. The Abstract Class FilterEntryBase . . . . . . . 61
4.3.29. The Class IPHeaderFilter . . . . . . . . . . . . 62
4.3.30. The Class 8021Filter . . . . . . . . . . . . . . 62
4.3.31. The Class PreambleFilter . . . . . . . . . . . . 62
4.3.32. The Class FilterList . . . . . . . . . . . . . . 63
4.3.33. The Abstract Class ServiceAccessPoint. . . . . . 63
4.3.34. The Class ProtocolEndpoint . . . . . . . . . . . 63
4.3.35. The Abstract Class Collection. . . . . . . . . . 65
4.3.36. The Abstract Class CollectionOfMSEs. . . . . . . 65
4.3.37. The Class BufferPool . . . . . . . . . . . . . . 65
4.3.38. The Abstract Class SchedulingElement . . . . . . 65
4.3.39. The Class AllocationSchedulingElement. . . . . . 66
4.3.40. The Class WRRSchedulingElement . . . . . . . . . 67
4.3.41. The Class PrioritySchedulingElement. . . . . . . 69
4.3.42. The Class BoundedPrioritySchedulingElement . . . 70
4.4. Association Definitions. . . . . . . . . . . . . . . . . 70
4.4.1. The Abstract Association Dependency. . . . . . . 71
4.4.2. The Association ServiceSAPDependency . . . . . . 71
4.4.3. The Association
IngressConditioningServiceOnEndpoint . . . . . . 71
4.4.4. The Association
EgressConditioningServiceOnEndpoint. . . . . . . 72
4.4.5. The Association HeadTailDropQueueBinding . . . . 72
4.4.6. The Association CalculationBasedOnQueue. . . . . 73
4.4.7. The Association ProvidesServiceToElement . . . . 74
4.4.8. The Association ServiceServiceDependency . . . . 74
4.4.9. The Association CalculationServiceForDropper . . 75
4.4.10. The Association QueueAllocation. . . . . . . . . 75
4.4.11. The Association ClassifierElementUsesFilterList. 76
4.4.12. The Association AFRelatedServices. . . . . . . . 77
4.4.13. The Association NextService. . . . . . . . . . . 78
4.4.14. The Association
NextServiceAfterClassifierElement. . . . . . . . 79
4.4.15. The Association NextScheduler. . . . . . . . . . 80
4.4.16. The Association FailNextScheduler. . . . . . . . 81
4.4.17. The Association NextServiceAfterMeter. . . . . . 82
4.4.18. The Association QueueToSchedule. . . . . . . . . 83
4.4.19. The Association SchedulingServiceToSchedule. . . 84
4.4.20. The Aggregation MemberOfCollection . . . . . . . 85
4.4.21. The Aggregation CollectedBufferPool. . . . . . . 85
4.4.22. The Abstract Aggregation Component . . . . . . . 86
4.4.23. The Aggregation ServiceComponent . . . . . . . . 86
4.4.24. The Aggregation QoSSubService. . . . . . . . . . 86
4.4.25. The Aggregation QoSConditioningSubService. . . . 87
4.4.26. The Aggregation
ClassifierElementInClassifierService . . . . . . 88
4.4.27. The Aggregation EntriesInFilterList. . . . . . . 89
4.4.28. The Aggregation ElementInSchedulingService . . . 90
5. Intellectual Property Statement. . . . . . . . . . . . . . . . 91
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 91
7. Security Considerations. . . . . . . . . . . . . . . . . . . . 91
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 92
8.1. Normative References. . . . . . . . . . . . . . . . . . . 92
8.2. Informative References . . . . . . . . . . . . . . . . . 92
9. Appendix A: Naming Instances in a Native CIM Implementation . 94
9.1. Naming Instances of the Classes Derived from Service. . . 94
9.2. Naming Instances of Subclasses of FilterEntryBase . . . . 94
9.3. Naming Instances of ProtocolEndpoint. . . . . . . . . . . 94
9.4. Naming Instances of BufferPool. . . . . . . . . . . . . . 95
9.4.1. The Property CollectionID. . . . . . . . . . . . 95
9.4.2. The Property CreationClassName . . . . . . . . . 95
9.5. Naming Instances of SchedulingElement . . . . . . . . . . 95
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 96
11. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 97
1. Introduction
The purpose of this document is to define an information model to
describe the quality of service (QoS) mechanisms inherent in
different network devices, including hosts. Broadly speaking, these
mechanisms describe the attributes common to selecting and
conditioning traffic through the forwarding path (datapath) of a
network device. This selection and conditioning of traffic in the
datapath spans both major QoS architectures: Differentiated Services
(see [R2475]) and Integrated Services (see [R1633]).
This document is intended to be used with the QoS Policy Information
Model [QPIM] to model how policies can be defined to manage and
configure the QoS mechanisms (i.e., the classification, marking,
metering, dropping, queuing, and scheduling functionality) of
devices. Together, these two documents describe how to write QoS
policy rules to configure and manage the QoS mechanisms present in
the datapaths of devices.
This document, as well as [QPIM], are information models. That is,
they represent information independent of a binding to a specific
type of repository. A separate document could be written to provide
a mapping of the data contained in this document to a form suitable
for implementation in a directory that uses (L)DAP as its access
protocol. Similarly, a document could be written to provide a
mapping of the data in [QPIM] to a directory. Together, these four
documents (information models and directory schema mappings) would
then describe how to write QoS policy rules that can be used to store
information in directories to configure device QoS mechanisms.
The approach taken in this document defines a common set of classes
that can be used to model QoS in a device datapath. Vendors can then
map these classes, either directly or using an intervening format
like a COP-PR PIB, to their own device-specific implementations.
Note that the admission control element of Integrated Services is not
included in the scope of this model.
The design of the class, association, and aggregation hierarchies
described in this document is influenced by the Network QoS submodel
defined by the Distributed Management Task Force (DMTF) - see [CIM].
These hierarchies are not derived from the Policy Core Information
Model [PCIM]. This is because the modeling of the QoS mechanisms of
a device is separate and distinct from the modeling of policies that
manage those mechanisms. Hence, there is a need to separate QoS
mechanisms (this document) from their control (specified using the
generic policy document [PCIM] augmented by the QoS Policy document
[QPIM]).
While it is not a policy model per se, this document does have a
dependency on the Policy Core Information Model Extensions document
[PCIME]. The device-level packet filtering, through which a
Classifier splits a traffic stream into multiple streams, is based on
the FilterEntryBase and FilterList classes defined in [PCIME].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[R2119].
1.1. Policy Management Conceptual Model
The Policy Core Information Model [PCIM] describes a general
methodology for constructing policy rules. PCIM Extensions [PCIME]
updates and extends the original PCIM. A policy rule aggregates a
set of policy conditions and an ordered set of policy actions. The
semantics of a policy rule are such that if the set of conditions
evaluates to TRUE, then the set of actions are executed.
Policy conditions and actions have two principal components: operands
and operators. Operands can be constants or variables. To specify a
policy, it is necessary to specify:
o the operands to be examined (also known as state variables);
o the operands to be changed (also known as configuration
variables);
o the relationships between these two sets of operands.
Operands can be specified at a high-level, such as Joe (a user) or
Gold (a service). Operands can also be specified at a much finer
level of detail, one that is much closer to the operation of the
device. Examples of the latter include an IP Address or a queue's
bandwidth allocation. Implicit in the use of operands is the binding
of legal values or ranges of values to an operand. For example, the
value of an IP address cannot be an integer. The concepts of
operands and their ranges are defined in [PCIME].
The second component of policy conditions and actions is a set of
operators. Operators can express both relationships (greater than,
member of a set, Boolean OR, etc.) and assignments. Together,
operators and operands can express a variety of conditions and
actions, such as:
If Bob is an Engineer...
If the source IP address is in the Marketing Subnet...
Set Joe's IP address to 192.0.2.100
Limit the bandwidth of application x to 10 Mb
We recognize that the definition of operator semantics is critical to
the definition of policies. However, the definition of these
operators is beyond the scope of this document. Rather, this
document (with [QPIM]) takes the first steps in identifying and
standardizing a set of properties (operands) for use in defining
policies for Differentiated and Integrated Services.
1.2. Purpose and Relation to Other Policy Work
This model establishes a canonical model of the QoS mechanisms of a
network device (e.g., a router, switch, or host) that is independent
of any specific type of network device. This enables traffic
conditioning to be described using a common set of abstractions,
modeled as a set of services and sub-services.
When the concepts of this document are used in conjunction with the
concepts of [QPIM], one is able to define policies that bind the
services in a network to the needs of applications using that
network. In other words, the business requirements of an
organization can be reflected in one set of policies, and those
policies can be translated to a lower-level set of policies that
control and manage the configuration and operation of network
devices.
1.3. Typical Examples of Policy Usage
Policies could be implemented as low-level rules using the
information model described in this specification. For example, in a
low-level policy, a condition could be represented as an evaluation
of a specific attribute from this model. Therefore, a condition such
as "If filter = HTTP" would be interpreted as a test determining
whether any HTTP filters have been defined for the device. A high-
level policy, such as "If protocol = HTTP, then mark with
Differentiated Services Code Point (DSCP) 24," would be expressed as
a series of actions in a low-level policy using the classes and
attributes described below:
1. Create HTTP filter
2. Create DSCP marker with the value of 24
3. Bind the HTTP filter to the DSCP marker
Note that unlike "mark with DSCP 24," these low-level actions are not
performed on a packet as it passes through the device. Rather, they
are configuration actions performed on the device itself, to make it
ready to perform the correct action(s) on the correct packet(s). The
act of moving from a high-level policy rule to the correct set of
low-level device configuration actions is an example of what
[POLTERM] characterizes as "policy translation" or "policy
conversion".
2. Approach
QoS activities in the IETF have mainly focused in two areas,
Integrated Services (IntServ) and Differentiated Services (DiffServ)
(see [POLTERM], [R1633] and [R2475]). This document focuses on the
specification of QoS properties and classes for modeling the datapath
where packet traffic is conditioned. However, the framework defined
by the classes in this document has been designed with the needs of
the admission control portion of IntServ in mind as well.
2.1. Common Needs Of DiffServ and IntServ
First, let us consider IntServ. IntServ has two principal
components. One component is embedded in the datapath of the
networking device. Its functions include the classification and
policing of individual flows, and scheduling admitted packets for the
outbound link. The other component of IntServ is admission control,
which focuses on the management of the signaling protocol (e.g., the
PATH and RESV messages of RSVP). This component processes
reservation requests, manages bandwidth, outsources decision making
to policy servers, and interacts with the Routing Table manager.
We will consider RSVP when defining the structure of this information
model. As this document focuses on the datapath, elements of RSVP
applicable to the datapath will be considered in the structure of the
classes. The complete IntServ device model will, as we have
indicated earlier, be addressed in a subsequent document.
This document models a small subset of the QoS policy problem, in
hopes of constructing a methodology that can be adapted for other
aspects of QoS in particular, and of policy construction in general.
The focus in this document is on QoS for devices that implement
traffic conditioning in the datapath.
DiffServ operates exclusively in the datapath. It has all of the
same components of the IntServ datapath, with two major differences.
First, DiffServ classifies packets based solely on their DSCP field,
whereas IntServ examines a subset of a standard flow's addressing 5-
tuple. The exception to this rule occurs in a router or host at the
boundary of a DiffServ domain. A device in this position may examine
a packet's DSCP, its addressing 5-tuple, other fields in the packet,
or even information wholly outside the packet, in determining the
DSCP value with which to mark the packet prior to its transfer into
the DiffServ domain. However, routers in the interior of a DiffServ
domain will only need to classify based on the DSCP field.
The second difference between IntServ and DiffServ is that the
signaling protocol used in IntServ (e.g., RSVP) affects the
configuration of the datapath in a more dynamic fashion. This is
because each newly admitted RSVP reservation requires a
reconfiguration of the datapath. In contrast, DiffServ requires far
fewer changes to the datapath after the Per Hop Behaviors (PHBs) have
been configured.
The approach advocated in this document for the creation of policies
that control the various QoS mechanisms of networking devices is to
first identify the attributes with which policies are to be
constructed. These attributes are the parameters used in expressions
that are necessary to construct policies. There is also a parallel
desire to define the operators, relations, and precedence constructs
necessary to construct the conditions and actions that constitute
these policies. However, these efforts are beyond the scope of this
document.
2.2. Specific Needs Of DiffServ
DiffServ-specific rules focus on two particular areas: the core and
the edges of the network. As explained in the DiffServ Architecture
document [R2475], devices at the edge of the network classify traffic
into different traffic streams. The core of the network then
forwards traffic from different streams by using a set of Per Hop
Behaviors (PHBs). A DSCP identifies each PHB. The DSCP is part of
the IP header of each packet (as described in [R2474]). This enables
multiple traffic streams to be aggregated into a small number of
aggregated traffic streams, where each aggregate traffic stream is
identified by a particular DSCP, and forwarded using a particular
PHB.
The attributes used to manipulate QoS capabilities in the core of the
network primarily address the behavioral characteristics of each
supported PHB. At the edges of the DiffServ network, the additional
complexities of flow classification, policing, RSVP mappings,
remarkings, and other factors have to be considered. Additional
modeling will be required in this area. However, first, the
standards for edges of the DiffServ network need more detail - to
allow the edges to be incorporated into the policy model.
2.3. Specific Needs Of IntServ
This document focuses exclusively on the forwarding aspects of
network QoS. Therefore, while the forwarding aspects of IntServ are
considered, the management of IntServ is not considered. This topic
will be addressed in a future document.
3. Methodology
There is a clear need to define attributes and behavior that together
define how traffic should be conditioned. This document defines a
set of classes and relationships that represent the QoS mechanisms
used to condition traffic; [QPIM] is used to define policies to
control the QoS mechanisms defined in this document.
However, some very basic issues need to be considered when combining
these documents. Considering these issues should help in
constructing a schema for managing the operation and configuration of
network QoS mechanisms through the use of QoS policies.
3.1. Level of Abstraction for Expressing QoS Policies
The first issue requiring consideration is the level of abstraction
at which QoS policies should be expressed. If we consider policies
as a set of rules used to react to events and manipulate attributes
or generate new events, we realize that policy represents a continuum
of specifications that relate business goals and rules to the
conditioning of traffic done by a device or a set of devices. An
example of a business level policy might be: from 1:00 pm PST to 7:00
am EST, sell off 40% of the network capacity on the open market. In
contrast, a device-specific policy might be: if the queue depth grows
at a geometric rate over a specified duration, trigger a potential
link failure event.
A general model for this continuum is shown in Figure 1 below.
+---------------------+
| High-Level Business | Not directly related to device
| Policies | operation and configuration details
+---------------------+
|
|
+---------V-----------+
| Device-Independent | Translate high-level policies to
| Policies | generic device operational and
+---------------------+ configuration information
|
|
+---------V-----------+
| Device-Dependent | Translate generic device information
| Policies | to specify how particular devices
+---------------------+ should operate and be configured
Figure 1. The Policy Continuum
High-level business policies are used to express the requirements of
the different applications, and prioritize which applications get
"better" treatment when the network is congested. The goal, then, is
to use policies to relate the operational and configuration needs of
a device directly to the business rules that the network
administrator is trying to implement in the network that the device
belongs to.
Device-independent policies translate business policies into a set of
generalized operational and configuration policies that are
independent of any specific device, but dependent on a particular set
of QoS mechanisms, such as random early detection (RED) dropping or
weighted round robin scheduling. Not only does this enable different
types of devices (routers, switches, hosts, etc.) to be controlled by
QoS policies, it also enables devices made by different vendors that
use the same types of QoS mechanisms to be controlled. This enables
these different devices to each supply the correct relative
conditioning to the same type of traffic.
In contrast, device-dependent policies translate device-independent
policies into ones that are specific for a given device. The reason
that a distinction is made between device-independent and device-
dependent policies is that in a given network, many different devices
having many different capabilities need to be controlled together.
Device-independent policies provide a common layer of abstraction for
managing multiple devices of different capabilities, while device-
dependent policies implement the specific conditioning that is
required. This document provides a common set of abstractions for
representing QoS mechanisms in a device-independent way.
This document is focused on the device-independent representation of
QoS mechanisms. QoS mechanisms are modeled in sufficient detail to
provide a common device-independent representation of QoS policies.
They can also be used to provide a basis for specialization, enabling
each vendor to derive a set of vendor-specific classes that represent
how traffic conditioning is done for that vendor's set of devices.
3.2. Specifying Policy Parameters
Policies are a function of parameters (attributes) and operators
(boolean, arithmetic, relational, etc.). Therefore, both need to be
defined as part of the same policy in order to correctly condition
the traffic. If the parameters of the policy are specified too
narrowly, they will reflect the individual implementations of QoS in
each device. As there is currently little consensus in the industry
on what the correct implementation model for QoS is, most defined
attributes would only be applicable to the unique characteristics of
a few individual devices. Moreover, standardizing all of these
potential implementation alternatives would be a never-ending task as
new implementations continued to appear on the market.
On the other hand, if the parameters of the policy are specified too
broadly, it is impossible to develop meaningful policies. For
example, if we concentrate on the so-called Olympic set of policies,
a business policy like "Bob gets Gold Service," is clearly
meaningless to the large majority of existing devices. This is
because the device has no way of determining who Bob is, or what QoS
mechanisms should be configured in what way to provide Gold service.
Furthermore, Gold service may represent a single service, or it may
identify a set of services that are related to each other. In the
latter case, these services may have different conditioning
characteristics.
This document defines a set of parameters that fit into a canonical
model for modeling the elements in the forwarding path of a device
implementing QoS traffic conditioning. By defining this model in a
device-independent way, the needed parameters can be appropriately
abstracted.
3.3. Specifying Policy Services
Administrators want the flexibility to be able to define traffic
conditioning without having to have a low-level understanding of the
different QoS mechanisms that implement that conditioning.
Furthermore, administrators want the flexibility to group different
services together, describing a higher-level concept such as "Gold
Service". This higher-level service could be viewed as providing the
processing to deliver "Gold" quality of service.
These two goals dictate the need for the following set of
abstractions:
o a flexible way to describe a service
o must be able to group different services that may use different
technologies (e.g., DiffServ and IEEE 802.1Q) together
o must be able to define a set of sub-services that together make up
a higher-level service
o must be able to associate a service and the set of QoS mechanisms
that are used to condition traffic for that service
o must be able to define policies that manage the QoS mechanisms
used to implement a service.
This document addresses this set of problems by defining a set of
classes and associations that can represent abstract concepts like
"Gold Service," and bind each of these abstract services to a
specific set of QoS mechanisms that implement the conditioning that
they require. Furthermore, this document defines the concept of
"sub-services," to enable Gold Service to be defined either as a
single service or as a set of services that together should be
treated as an atomic entity.
Given these abstractions, policies (as defined in [QPIM]) can be
written to control the QoS mechanisms and services defined in this
document.
3.4. Level of Abstraction for Defining QoS Attributes and Classes
This document defines a set of classes and properties to support
policies that configure device QoS mechanisms. This document
concentrates on the representation of services in the datapath that
support both DiffServ (for aggregate traffic conditioning) and
IntServ (for flow-based traffic conditioning). Classes and
properties for modeling IntServ admission control services may be
defined in a future document.
The classes and properties in this document are designed to be used
in conjunction with the QoS policy classes and properties defined in
[QPIM]. For example, to preserve the delay characteristics committed
to an end-user, a network administrator may wish to create policies
that monitor the queue depths in a device, and adjust resource
allocations when delay budgets are at risk (perhaps as a result of a
network topology change). The classes and properties in this
document define the specific services and mechanisms required to
implement those services. The classes and properties defined in
[QPIM] provide the overall structure of the policy that manages and
configures this service.
This combination of low-level specification (using this document) and
high-level structuring (using [QPIM]) of network services enables
network administrators to define new services required of the
network, that are directly related to business goals, while ensuring
that such services can be managed. However, this goal (of creating
and managing service-oriented policies) can only be realized if
policies can be constructed that are capable of supporting diverse
implementations of QoS. The solution is to model the QoS
capabilities of devices at the behavioral level. This means that for
traffic conditioning services realized in the datapath, the model
must support the following characteristics:
o modeling of a generic network service that has QoS capabilities
o modeling of how the traffic conditioning itself is defined
o modeling of how statistics are gathered to monitor QoS traffic
conditioning services - this facet of the model will be added in a
future document.
This document models a network service, and associates it with one or
more QoS mechanisms that are used to implement that service. It also
models in a canonical form the various components that are used to
condition traffic, such that standard as well as custom traffic
conditioning services may be described.
3.5. Characterization of QoS Properties
The QoS properties and classes will be described in more detail in
Section 4. However, we should consider the basic characteristics of
these properties, to understand the methodology for representing
them.
There are essentially two types of properties, state and
configuration. Configuration properties describe the desired state
of a device, and include properties and classes for representing
desired or proposed thresholds, bandwidth allocations, and how to
classify traffic. State properties describe the actual state of the
device. These include properties to represent the current
operational values of the attributes in devices configured via the
configuration properties, as well as properties that represent state
(queue depths, excess capacity consumption, loss rates, and so
forth).
In order to be correlated and used together, these two types of
properties must be modeled using a common information model. The
possibility of modeling state properties and their corresponding
configuration settings is accomplished using the same classes in this
model - although individual instances of the classes would have to be
appropriately named or placed in different containers to distinguish
current state values from desired configuration settings.
State information is addressed in a very limited fashion by QDDIM.
Currently, only CurrentQueueDepth is proposed as an attribute on
QueuingService. The majority of the model is related to
configuration. Given this fact, it is assumed that this model is a
direct memory map into a device. All manipulation of model classes
and properties directly affects the state of the device. If it is
desired to also use these classes to represent desired configuration,
that is left to the discretion of the implementor.
It is acknowledged that additional properties are needed to
completely model current state. However, many of the properties
defined in this document represent exactly the state variables that
will be configured by the configuration properties. Thus, the
definition of the configuration properties has an exact
correspondence with the state properties, and can be used in modeling
both actual (state) and desired/proposed configuration.
3.6. QoS Information Model Derivation
The question of context also leads to another question: how does the
information specified in the core and QoS policy models ([PCIM],
[PCIME], and [QPIM], respectively) integrate with the information
defined in this document? To put it another way, where should
device-independent concepts that lead to device-specific QoS
attributes be derived from?
Past thinking was that QoS was part of the policy model. This view
is not completely accurate, and it leads to confusion. QoS is a set
of services that can be controlled using policy. These services are
represented as device mechanisms. An important point here is that
QoS services, as well as other types of services (e.g., security),
are provided by the mechanisms inherent in a given device. This
means that not all devices are indeed created equal. For example,
although two devices may have the same type of mechanism (e.g., a
queue), one may be a simple implementation (i.e., a FIFO queue)
whereas one may be much more complex and robust (e.g., class-based
weighted fair queuing (CBWFQ)). However, both of these devices can
be used to deliver QoS services, and both need to be controlled by
policy. Thus, a device-independent policy can instruct the devices
to queue certain traffic, and a device-specific policy can be used to
control the queuing in each device.
Furthermore, policy is used to control these mechanisms, not to
represent them. For example, QoS services are implemented with
classifiers, meters, markers, droppers, queues, and schedulers.
Similarly, security is also a characteristic of devices, as
authentication and encryption capabilities represent services that
networked devices perform (irrespective of interactions with policy
servers). These security services may use some of the same
mechanisms that are used by QoS services, such as the concepts of
filters. However, they will mostly require different mechanisms than
the ones used by QoS, even though both sets of services are
implemented in the same devices.
Thus, the similarity between the QoS model and models for other
services is not so much that they contain a few common mechanisms.
Rather, they model how a device implements their respective services.
As such, the modeling of QoS should be part of a networking device
schema rather than a policy schema. This allows the networking
device schema to concentrate on modeling device mechanisms, and the
policy schema to focus on the semantics of representing the policy
itself (conditions, actions, operators, etc.). While this document
concentrates on defining an information model to represent QoS
services in a device datapath, the ultimate goal is to be able to
apply policies that control these services in network devices.
Furthermore, these two schemata (device and policy) must be tightly
integrated in order to enable policy to control QoS services.
3.7. Attribute Representation
The last issue to be considered is the question of how attributes are
represented. If QoS attributes are represented as absolute numbers
(e.g., Class AF2 gets 2 Mbs of bandwidth), it is more difficult to
make them uniform across multiple ports in a device or across
multiple devices, because of the broad variation in link capacities.
However, expressing attributes in relative or proportional terms
(e.g., Class AF2 gets 5% of the total link bandwidth) makes it more
difficult to express certain types of conditions and actions, such
as:
(If ConsumedBandwidth = AssignedBandwidth Then ...)
There are really three approaches to addressing this problem:
o Multiple properties can be defined to express the same value in
various forms. This idea has been rejected because of the
difficulty in keeping these different properties synchronized
(e.g., when one property changes, the others all have to be
updated).
o Multi-modal properties can be defined to express the same value,
in different terms, based on the access or assignment mode. This
option was rejected because it significantly complicates the model
and is impossible to express in current directory access protocols
(e.g., (L)DAP).
o Properties can be expressed as "absolutes", but the operators in
the policy schema would need to be more sophisticated. Thus, to
represent a percentage, division and multiplication operators are
required (e.g., Class AF2 gets .05 * the total link bandwidth).
This is the approach that has been taken in this document.
3.8. Mental Model
The mental model for constructing this schema is based on the work
done in the Differentiated Services working group. This schema is
based on information provided in the current versions of the DiffServ
Informal Management Model [DSMODEL], the DiffServ MIB [DSMIB], the
PIB [PIB], as well as on information in the set of RFCs that
constitute the basic definition of DiffServ itself ([R2475], [R2474],
[R2597], and [R3246]). In addition, a common set of terminology is
available in [POLTERM].
This model is built around two fundamental class hierarchies that are
bound together using a set of associations. The two class
hierarchies derive from the QoSService and ConditioningService base
classes. A set of associations relate lower-level QoSService
subclasses to higher-level QoS services, relate different types of
conditioning services together in processing a traffic class, and
relate a set of conditioning services to a specific QoS service.
This combination of associations enables us to view the device as
providing a set of services that can be configured, in a modular
building block fashion, to construct application-specific services.
Thus, this document can be used to model existing and future standard
as well as application-specific network QoS services.
3.8.1. The QoSService Class
The first of the classes defined here, QoSService, is used to
represent higher-level network services that require special
conditioning of their traffic. An instance of QoSService (or one of
its subclasses) is used to bring together a group of conditioning
services that, from the perspective of the system manager, are all
used to deliver a common service. Thus, the set of classifiers,
markers, and related conditioning services that provide premium
service to the "selected" set of user traffic may be grouped together
into a premium QoS service.
QoSService has a set of subclasses that represent different
approaches to delivering IP services. The currently defined set of
subclasses are a FlowService for flow-oriented QoS delivery and a
DiffServService for DiffServ aggregate-oriented QoS service delivery.
The QoS services can be related to each other as peers, or they can
be implemented as subservient services to each other. The
QoSSubService aggregation indicates that one or more QoSService
objects are subservient to a particular QoSService object. For
example, this enables us to define Gold Service as a combination of
two DiffServ services, one for high quality traffic treatment, and
one for servicing the rest of the traffic. Each of these
DiffServService objects would be associated with a set of
classifiers, markers, etc, such that the high quality traffic would
get EF marking and appropriate queuing.
The DiffServService class itself has an AFService subclass. This
subclass is used to represent the specific notion that several
related markings within the AF PHB Group work together to provide a
single service. When other DiffServ PHB Groups are defined that use
more than one code point, these will be likely candidates for
additional DiffServService subclasses.
Technology-specific mappings of these services, representing the
specific use of PHB marking or 802.1Q marking, are captured within
the ConditioningService hierarchy, rather than in the subclasses of
QoSService.
These concepts are depicted in Figure 2. Note that both of the
associations are aggregations: a QoSService object aggregates both
the set of QoSService objects subservient to it, and the set of
ConditioningService objects that realize it. See Section 4 for class
and association definitions.
/\______
0..1 \/ |
+--------------+ | QoSSubService +---------------+
| |0..n | | |
| QoSService |----- | Conditioning |
| | | Service |
| | | |
| |0..n 0..n| |
| | /\______________________| |
| | \/ QoSConditioning | |
+--------------+ SubService +---------------+
Figure 2. QoSService and its Aggregations
3.8.2. The ConditioningService Class
The goal of the ConditioningService classes is to describe the
sequence of traffic conditioning that is applied to a given traffic
stream on the ingress interface through which it enters a device, and
then on the egress interface through which it leaves the device.
This is done using a set of classes and relationships. The routing
decision in the device core, which selects which egress interface a
particular packet will use, is not represented in this model.
A single base class, ConditioningService, is the superclass for a set
of subclasses representing the mechanisms that condition traffic.
These subclasses define device-independent conditioning primitives
(including classifiers, meters, markers, droppers, queues, and
schedulers) that together implement the conditioning of traffic on an
interface. This model abstracts these services into a common set of
modular building blocks that can be used, regardless of device
implementation, to model the traffic conditioning internal to a
device.
The different conditioning mechanisms need to be related to each
other to describe how traffic is conditioned. Several important
variations of how these services are related together exist:
o A particular ingress or egress interface may not require all the
types of ConditioningServices.
o Multiple instances of the same mechanism may be required on an
ingress or egress interface.
o There is no set order of application for the ConditioningServices
on an ingress or egress interface.
Therefore, this model does not dictate a fixed ordering among the
subclasses of ConditioningService, or identify a subclass of
ConditioningService that must appear first or last among the
ConditioningServices on an ingress or egress interface. Instead,
this model ties together the various ConditioningService instances on
an ingress or egress interface using the NextService,
NextServiceAfterMeter, and NextServiceAfterConditioningElement
associations. There are also separate associations, called
IngressConditioningServiceOnEndpoint and
EgressConditioningServiceOnEndpoint, which, respectively, tie an
ingress interface to its first ConditioningService, and tie an egress
interface to its last ConditioningService(s).
3.8.3. Preserving QoS Information from Ingress to Egress
There is one important way in which the QDDIM model diverges from the
[DSMODEL]. In [DSMODEL], traffic passes through a network device in
three stages:
o It comes in on an ingress interface, where it may receive QoS
conditioning.
o It traverses the routing core, where logic outside the scope of
QoS determines which egress interface it will use to leave the
device.
o It may receive further QoS conditioning on the selected egress
interface, and then it leaves the device.
In this model, no information about the QoS conditioning that a
packet receives on the ingress interface is communicated with the
packet across the routing core to the egress interface.
The QDDIM model relaxes this restriction, to allow information about
the treatment that a packet received on an ingress interface to be
communicated along with the packet to the egress interface. (This
relaxation adds a capability that is present in many network
devices.) QDDIM represents this information transfer in terms of a
packet preamble, which is how many devices implement it. But
implementations are free to use other mechanisms to achieve the same
result.
+---------+
| Meter-A |
a | | b d
--->| In-|---PM-1--->
| | c e
| Out-|---PM-2--->
+---------+
Figure 3: Meter Followed by Two Preamble Markers
Figure 3 shows an example in which meter results are captured in a
packet preamble. The arrows labeled with single letters represent
instances of either the NextService association (a, d, and e), or of
its peer association NextServiceAfterMeter (b and c). PreambleMarker
PM-1 adds to the packet preamble an indication that the packet exited
Meter A as conforming traffic. Similarly, PreambleMarker PM-2 adds to
the preambles of packets that come through it indications that they
exited Meter A as nonconforming traffic. A PreambleMarker appends
its information to whatever is already present in a packet preamble,
as opposed to overwriting what is already there.
To foster interoperability, the basic format of the information
captured by a PreambleMarker is specified. (Implementations, of
course, are free to represent this information in a different way
internally - this is just how it is represented in the model.) The
information is represented by an ordered, multi-valued string
property FilterItemList, where each individual value of the property
is of the form "<type>,<value>". When a PreambleMarker "appends" its
information to the information that was already present in a packet
preamble, it does so by adding additional items of the indicated
format to the end of the list.
QDDIM provides a limited set of <type>'s that a PreambleMarker may
use:
o ConformingFromMeter: the value is the name of the meter.
o PartConformingFromMeter: the value is the name of the meter.
o NonConformingFromMeter: the value is the name of the meter.
o VlanId: the value is the virtual LAN identifier (VLAN ID).
Implementations may recognize other <type>'s in addition to these.
If collisions of implementation-specific <type>'s become a problem,
it is possible that <type>'s may become an IANA-administered range in
a future revision of this document.
To make use of the information that a PreambleMarker stores in a
packet preamble, a specific subclass PreambleFilter of
FilterEntryBase is defined, to match on the "<type>,<value>" strings.
To simplify the case where there's just a single level of metering in
a device, but different individual meters on each ingress interface,
PreambleFilter allows a wildcard "any" for the <value> part of the
three meter-related filters. With this wildcard, an administrator
can specify a Classifier to select all packets that were found to be
conforming (or partially conforming, or non-conforming) by their
respective meters, without having to name each meter individually in
a separate ClassifierElement.
Once a meter result has been stored in a packet preamble, it is
available for any subsequent Classifier to use. So while the
motivation for this capability has been described in terms of
preserving QoS conditioning information from an ingress interface to
an egress interface, a prior meter result may also be used for
classifying packets later in the datapath on the same interface where
the meter resides.
3.9. Classifiers, FilterLists, and Filter Entries
This document uses a number of classes to model the classifiers
defined in [DSMODEL]: ClassifierService, ClassifierElement,
FilterList, FilterEntryBase, and various subclasses of
FilterEntryBase. There are also two associations involved:
ClassifierElementUsesFilterList and EntriesInFilterList. The QDDIM
model makes no use of CIM's FilterEntry class.
In [DSMODEL], a single traffic stream coming into a classifier is
split into multiple traffic streams leaving it, based on which of an
ordered set of filters each packet in the incoming stream matches. A
filter matches either a field in the packet itself, or possibly other
attributes associated with the packet. In the case of a multi-field
(MF) classifier, packets are assigned to output streams based on the
contents of multiple fields in the packet header. For example, an MF
classifier might assign packets to an output stream based on their
complete IP-addressing 5-tuple.
To optimize the representation of MF classifiers, subclasses of
FilterEntryBase are introduced, which allow multiple related packet
header fields to be represented in a single object. These subclasses
are IPHeaderFilter and 8021Filter. With IPHeaderFilter, for example,
criteria for selecting packets based on all five of the IP 5-tuple
header fields and the DiffServ DSCP can be represented by a
FilterList containing one IPHeaderFilter object. Because these two
classes have applications beyond those considered in this document,
they, as well as the abstract class FilterEntryBase, are defined in
the more general document [PCIME] rather than here.
The FilterList object is always needed, even if it contains only one
filter entry (that is, one FilterEntryBase subclass) object. This is
because a ClassifierElement can only be associated with a Filter
List, as opposed to an individual FilterEntry. FilterList is also
defined in [PCIME].
The EntriesInFilterList aggregation (also defined in [PCIME]) has a
property EntrySequence, which in the past (in CIM) could be used to
specify an evaluation order on the filter entries in a FilterList.
Now, however, the EntrySequence property supports only a single
value: '0'. This value indicates that the FilterEntries are ANDed
together to determine whether a packet matches the MF selector that
the FilterList represents.
A ClassifierElement specifies the starting point for a specific
policy or data path. Each ClassifierElement uses the
NextServiceAfterClassifierElement association to determine the next
conditioning service to apply for packets to.
A ClassifierService defines a grouping of ClassifierElements. There
are certain instances where a ClassifierService actually specifies an
aggregation of ClassifierServices. One practical case would be where
each ClassifierService specifies a group of policies associated with
a particular application and another ClassifierService groups the
application-specific ClassifierService instances. In this particular
case, the application-specific ClassifierService instances are
specified once, but unique combinations of these ClassifierServices
are specified, as needed, using other ClassifierService instances.
ClassifierService instances grouping other ClassifierService
instances may not specify a FilterList using the
ClassifierElementUsesFilterList association. This special use of
ClassifierService serves just as a Classifier collecting function.
3.10. Modeling of Droppers
In [DSMODEL], a distinction is made between absolute droppers and
algorithmic droppers. In QDDIM, both of these types of droppers are
modeled with the DropperService class, or with one of its subclasses.
In both cases, the queue from which the dropper drops packets is tied
to the dropper by an instance of the NextService association. The
dropper always plays the PrecedingService role in these associations,
and the queue always plays the FollowingService role. There is
always exactly one queue from which a dropper drops packets.
Since an absolute dropper drops all packets in its queue, it needs no
configuration beyond a NextService tie to that queue. For an
algorithmic dropper, however, further configuration is needed:
o a specific drop algorithm;
o parameters for the algorithm (for example, token bucket size);
o the source(s) of input(s) to the algorithm;
o possibly per-input parameters for the algorithm.
The first two of these items are represented by properties of the
DropperService class, or properties of one of its subclasses. The
last two, however, involve additional classes and associations.
3.10.1. Configuring Head and Tail Droppers
The HeadTailDropQueueBinding is the association that identifies the
inputs for the algorithm executed by a tail dropper. This
association is not used for a head dropper, because a head dropper
always has exactly one input to its drop algorithm, and this input is
always the queue from which it drops packets. For a tail dropper,
this association is defined to have a many-to-many cardinality.
There are, however, two distinct cases:
One dropper bound to many queues: This represents the case where the
drop algorithm for the dropper involves inputs from more than one
queue. The dropper still drops from only one queue, the one to which
it is tied by a NextService association. But the drop decision may
be influenced by the state of several queues. For the classes
HeadTailDropper and HeadTailDropQueueBinding, the rule for combining
the multiple inputs is simple addition: if the sum of the lengths of
the monitored queues exceeds the dropper's QueueThreshold value, then
packets are dropped. This rule for combining inputs may, however, be
overridden by a different rule in subclasses of one or both of these
classes.
One queue bound to many droppers: This represents the case where the
state of one queue (which is typically also the queue from which
packets are dropped) provides an input to multiple droppers' drop
algorithms. A use case here is a classifier that splits a traffic
stream into, say, four parts, representing four classes of traffic.
Each of the parts goes through a separate HeadTailDropper, then
they're re-merged onto the same queue. The net is a single queue
containing packets of four traffic types, with, say, the following
drop thresholds:
o Class 1 - 90% full
o Class 2 - 80% full
o Class 3 - 70% full
o Class 4 - 50% full
Here the percentages represent the overall state of the queue. With
this configuration, when the queue in question becomes 50% full,
Class 4 packets will be dropped rather than joining the queue, when
it becomes 70% full, Class 3 and 4 packets will be dropped, etc.
The two cases described here can also occur together, if a dropper
receives inputs from multiple queues, one or more of which are also
providing inputs to other droppers.
3.10.2. Configuring RED Droppers
Like a tail dropper, a RED dropper, represented by an instance of the
REDDropperService class, may take as its inputs the states of
multiple queues. In this case, however, there is an additional step:
each of these inputs may be smoothed before the RED dropper uses it,
and the smoothing process itself must be parameterized. Consequently,
in addition to REDDropperService and QueuingService, a third class,
DropThresholdCalculationService, is introduced, to represent the
per-queue parameterization of this smoothing process.
The following instance diagram illustrates how these classes work
with each other:
RDSvc-A
| | |
+-----+ | +-----+
| | |
DTCS-1 DTCS-2 DTCS-3
| | |
Q-1 Q-2 Q-3
Figure 4. Inputs for a RED Dropper
So REDDropperService-A (RDSvc-A) is using inputs from three queues to
make its drop decision. (As always, RDSvc-A is linked to the queue
from which it drops packets via the NextService association.) For
each of these three queues, there is a
(DropThresholdCalculationService) DTCS instance that represents the
smoothing weight and time interval to use when looking at that queue.
Thus each DTCS instance is tied to exactly one queue, although a
single queue may be examined (with different weight and time values)
by multiple DTCS instances. Also, a DTCS instance and the queue
behind it can be thought of as a "unit of reusability". So a single
DTCS can be referred to by multiple RDSvc's.
Unless it is overridden by a different rule in a subclass of
REDDropperService, the rule that a RED dropper uses to combine the
smoothed inputs from the DTCS's to create a value to use in making
its drop decision is simple addition.
3.11. Modeling of Queues and Schedulers
In order to appreciate the rationale behind this rather complex model
for scheduling, we must consider the rather complex nature of
schedulers, as well as the extreme variations in algorithms and
implementations. Although these variations are broad, we have
identified four examples that serve to test the model and justify its
complexity.
3.11.1. Simple Hierarchical Scheduler
A simple, hierarchical scheduler has the following properties. First,
when a scheduling opportunity is given to a set of queues, a single,
viable queue is determined based on some scheduling criteria, such as
bandwidth or priority. The output of the scheduler is the input to
another scheduler that treats the first scheduler (and its queues) as
a single logical queue. Hence, if the first scheduler determined the
appropriate packet to release based on a priority assigned to each
queue, the second scheduler might specify a bandwidth
limit/allocation for the entire set of queues aggregated by the first
scheduler.
+----------+ NextService
|QueuingSvc+----------------------------------------------+
| Name=EF1 | |
| | QueueTo +--------------+ ElementSched |
| +------------+PrioritySched +---------------+ |
+----------+ Schedule |Element | Service | |
| Name=EF1-Pri | | v
| Priority=1 | +-----------+-+-+
+--------------+ |SchedulingSvc +
| Name=PriSched1+
+--------------+ +----------+--+-+
|PrioritySched | ElementSched | ^
+----------+ |Element +---------------+ |
|QueuingSvc| QueueTo | Name=AF1x-Pri| Service |
| Name=AF1x+------------+ Priority=2 | |
| | Schedule +--------------+ |
| | NextService |
| +----------------------------------------------+
+----------+
:
+---------------+ NextScheduler
|SchedulingSvc +--------------------------------------------+
| Name=PriSched1| |
+-------+-------+ +--------------------+ElementSchedSvc|
| SchedToSched |AllocationScheduling+--------+ |
+---------------+Element | | |
| Name=PriSched1-Band| | |
| Units=Bytes | | v
| Bandwidth=100 | +------+------+--+
+--------------------+ |SchedulingSvc |
| Name=BandSched1|
+--------------------+ +------+------+--+
|AllocationScheduling| | ^
+---------------+ |Element +--------+ |
|QueuingService | | Name=BE-Band |ElementSchedSvc|
| Name=BE |QueueTo+ Units=Bytes | |
| |-------+ Bandwidth=50 | |
| |Sched +--------------------+ |
| | NextService |
| +--------------------------------------------+
+---------------+
Figure 5. Example 1: Simple Hierarchical Scheduler
Figure 5 illustrates the example and how it would be instantiated
using the model. In the figure, NextService determines the first
scheduler after the queue. NextScheduler determines the
subsequent ordering of schedulers. In addition, the
ElementSchedulingService association determines the set of
scheduling parameters used by a specific scheduler. Scheduling
parameters can be bound either to queues or to schedulers. In
the case of the SchedulingElement EF1-Pri, the binding is to a
queue, so the QueueToSchedule association is used. In the case
of the SchedulingElement PriSched1-Band, the binding is to
another scheduler, so the SchedulerToSchedule association is
used. Note that due to space constraints of the document, the
SchedulingService PRISched1 is represented twice, to show how it
is connected to all the other objects.
3.11.2. Complex Hierarchical Scheduler
A complex, hierarchical scheduler has the same characteristics as
a simple scheduler, except that the criteria for the second
scheduler are determined on a per queue basis rather than on an
aggregate basis. One scenario might be a set of bounded priority
schedulers. In this case, each queue is assigned a relative
priority. However, each queue is also not allowed to exceed a
bandwidth allocation that is unique to that queue. In order to
support this scenario, the queue must be bound to two separate
schedulers. Figure 6 illustrates this situation, by describing
an EF queue and a best effort (BE) queue both pointing to a
priority scheduler via the NextService association. The
NextScheduler association between the priority scheduler and the
bandwidth scheduler in turn defines the ordering of the
scheduling hierarchy. Also note that each scheduler has a
distinct set of scheduling parameters that are bound back to each
queue. This demonstrates the need to support two or more
parameter sets on a per queue basis.
+----------------+
|QueuingService |
| Name=EF |
| |QueueTo +----------------+ElementSchedSvc
| +----------+AllocationSched +--------+
++---+-----------+Schedule |Element | |
| | | Name=BandEF | |
| |QueueTo | Units=Bytes | |
| |Schedule | Bandwidth=100 | |
| | +----------------+ +------+---------+
| | |SchedulingSvc |
| | +------------------+ | Name=BandSched |
| +------+PriorityScheduling| +------------+--++
| |Element | ^ |
| | Name=PriEF |ElementSchedSvc | |
| | Priority=1 +---------------------+ | |
| +------------------+ | | |
|NextService | | |
+-------------------------------------------------+ | | |
| | | |
NextService | | | |
+-----------------------------------------------+ | | | |
| | | | | |
| +------------------+ElementSchedSvc | | | | |
| |PriorityScheduling+--------+ | | | | |
| |Element | | | | | | |
| | Name=PriBE | | v v | | |
| +------+ Priority=2 | +---+--------+-+-+-+Next| |
| | +------------------+ |SchedulingService +----+ |
| | | Name=PriSched |Sched |
| | +------------------+ |
| |QueueTo |
| |Schedule +----------------+ |
| | |AllocationSched |ElementSchedSvc |
+----+---------+ |Element +-----------------+
|QueuingService|QueueTo | Name=BandBE |
| Name=BE +------------+ Units=Bytes |
| |Schedule | Bandwidth=50 |
| | +----------------+
+--------------+
Figure 6. Example 2: Complex Hierarchical Scheduler
3.11.3. Excess Capacity Scheduler
An excess capacity scheduler offers a similar requirement to support
two scheduling parameter sets per queue. However, in this scenario
the reasons are a little different. Suppose a set of queues have
each been assigned bandwidth limits to ensure that no traffic class
starves out another traffic class. The result may be that one or
more queues have exceeded their allocation while the queues that
deserve scheduling opportunities are empty.
The question then is how is the excess (idle) bandwidth allocated.
Conceivably, the scheduling criteria for excess capacity are
completely different from the criteria that determine allocations
under uniform load. This could be supported with a scheduling
hierarchy. However, the problem is that the criteria for using the
subsequent scheduler are different from those in the last two cases.
Specifically, the next scheduler should only be used if a scheduling
opportunity exists that was passed over by the prior scheduler.
When a scheduler chooses to forgo a scheduling decision, it is
behaving as a non-work conserving scheduler. Work conserving
schedulers, by definition, will always take advantage of a scheduling
opportunity, irrespective of which queue is being serviced and how
much bandwidth it has consumed in the past. This point leads to an
interesting insight. The semantics of a non-work conserving
scheduler are equivalent to those of a meter, in that if a packet is
in profile it is given the scheduling opportunity, and if it is out
of profile it does not get a scheduling opportunity. However, with
meters there are semantics that determine the next action behavior
when the packet is in profile and when the packet is out of profile.
Similarly, with the non-work conserving scheduler, there needs to be
a means for determining the next scheduler when a scheduler chooses
not to utilize a scheduling opportunity.
Figure 7 illustrates this last scenario. It appears very similar to
Figure 6, except that the binding between the allocation scheduler
and the WRR scheduler is using a FailNextScheduler association. This
association is explicitly indicating the fact that the only time the
WRR scheduler would be used is when there are non-empty queues that
the allocation scheduler rejected for scheduling consideration. Note
that Figure 7 is incomplete, in that typically there would be several
more queues that are bound to an allocation scheduler and a WRR
scheduler.
+------------+
|QueuingSvc |
| Name=EF |
| |
| |
++-+---------+
| |
| |QueueTo
| |Schedule +--------------+
| | |SchedulingSvc |
| | +------------------+ | Name=WRRSched|
| +------+AllocationSched | +----------+-+-+
| |Element | ^ |
| | Name=BandEF |ElementSchedSvc | |
| | Units=Bytes +--------------------+ | |
| | Bandwidth=100 | | | |
| +------------------+ | | |
|NextService | | |
+----------------------------------------------+ | | |
| | | |
NextService | | | |
+--------------------------------------------+ | | | |
| | | | | |
| +------------------+ElementSchedSvc | | | | |
| |AllocationSched +--------+ | | | | |
| |Element | | | | | | |
| | Name=BandwidthAF1| | | | | | |
| | Units=Bytes | | v v | | |
| +------+ Bandwidth=50 | +--+----------+-+-++FailNext| |
| | +------------------+ |SchedulingService +--------+ |
| |QueueTo | Name=BandSched |Scheduler |
| |Schedule +------------------+ |
| | |
| | +---------------------+ |
++-+-----------+ | WRRSchedulingElement| |
|QueuingService|QueueTo | Name=WRRBE +------------+
| Name=BE +-----------+ Weight=30 |ElementSchedSvc
+--------------+Schedule +---------------------+
Figure 7. Example 3: Excess Capacity Scheduler
3.11.4. Hierarchical CBQ Scheduler
A hierarchical class-based queuing (CBQ) scheduler is the fourth
scenario to be considered. In hierarchical CBQ, each queue is
allocated a specific bandwidth allocation. Queues are grouped
together into a logical scheduler. This logical scheduler in turn
has an aggregate bandwidth allocation that equals the sum of the
queues it is scheduling. In turn, logical schedulers can be
aggregated into higher-level logical schedulers. Changing
perspectives and looking top down, the top-most logical scheduler has
100% of the link capacity. This allocation is parceled out to
logical schedulers below it such that the sum of the allocations is
equal to 100%. These second tier schedulers may in turn parcel out
their allocation across a third tier of schedulers and so forth until
the lowest tier that parcels out their allocations to specific queues
representing relatively fine-grained classes of traffic. The unique
aspect of hierarchical CBQ is that when there is insufficient
bandwidth for a specific allocation, schedulers higher in the tree
are tested to see if another portion of the tree has capacity to
spare.
Figure 8 demonstrates this example with two tiers. The example is
split in half because of space constraints, resulting in the CBQTier1
scheduling service instance being represented twice. Note that the
total allocation at the top tier is 50 Mb. The voice allocation is
22 Mb. The remaining 23 Mb is split between FTP and Web. Hence, if
Web traffic is actually consuming 20 Mb (5 Mb in excess of the
allocation). If FTP is consuming 5 Mb, then it is possible for the
CBQTier1 scheduler to offer 3Mb of its allocation to Web traffic.
However, this is not enough, so the FailNextScheduler association
needs to be traversed to determine if there is any excess capacity
available from the voice class. If the voice class is only consuming
15 Mb of its 22 Mb allocation, there are sufficient resources to
allow the web traffic through. Note that FailNextScheduler is used
as the association. The reason is because the CBQTier1 scheduler in
fact failed to schedule a packet because of insufficient resources.
It is conceivable that a variant of hierarchical CBQ allows a
hierarchy for successful scheduling as well. Hence, both
associations are necessary.
Note that due to space constraints of the document, the
SchedulingService CBQTier1 is represented twice, to show how it is
connected to all the other objects.
+-----------+ NextService
|QueuingSvc +-------------------------------------------+
| Name=Web | |
| |QueueTo+----------------+ ElementSchedSvc |
| +-------+AllocationSched +----------------+ |
+-----------+Sched |Element | | |
| Name=Web-Alloc | | v
| Bandwidth=15 | +-----------+-+-+
+----------------+ |SchedulingSvc +
| Name=CBQTier1 +
+----------------+ +-----------+-+-+
|AllocationSched | ElementSchedSvc| ^
+-----------+ |Element +----------------+ |
|QueuingSvc |QueueTo| Name=FTP-Alloc | |
| Name=FTP +-------+ Bandwidth=8 | |
| |Sched +----------------+ |
| | NextService |
| +-------------------------------------------+
+-----------+
:
+---------------+ FailNextScheduler
|SchedulingSvc +---------------------------------------------+
| Name=CBQTier1 | |
+-------+-------+ +---------------------+ElementSchedSvc|
| SchedToSched |AllocationScheduling +--------+ |
+---------------+Element | | |
| Name=LowPri-Alloc | | |
| Bandwidth=23 | | v
+---------------------+ +-----+------+-+
|SchedulingSvc |
| Name=CBQTop |
+---------------------+ +----------+-+-+
|AllocationScheduling |ElementSchedSvc | ^
+------------+ |Element +----------------+ |
|QueuingSvc |QueueTo| Name=BE-Band | |
| Name=Voice +-------+ Bandwidth=22 | |
| |Sched +---------------------+ |
| | NextService |
| +------------------------------------------------+
+------------+
Figure 8. Example 4: Hierarchical CBQ Scheduler
4. The Class Hierarchy
The following sections present the class and association hierarchies
that together comprise the information model for modeling QoS
capabilities at the device level.
4.1. Associations and Aggregations
Associations and aggregations are a means of representing
relationships between two (or theoretically more) objects.
Dependency, aggregation, and other relationships are modeled as
classes containing two (or more) object references. It should be
noted that aggregations represent either "whole-part" or "collection"
relationships. For example, aggregation can be used to represent the
containment relationship between a system and the components that
constitute the system.
Since associations and aggregations are classes, they can benefit
from all of the object-oriented features that other non-relationship
classes have. For example, they can contain properties and methods,
and inheritance can be used to refine their semantics such that they
represent more specialized types of their superclasses.
Note that an association (or an aggregation) object is treated as an
atomic unit (individual instance), even though it relates/collects/is
comprised of multiple objects. This is a defining feature of an
association (or an aggregation) - although the individual elements
that are related to other objects have their own identities, the
association (or aggregation) object that is constructed using these
objects has its own identity and name as well.
It is important to note that associations and aggregations form an
inheritance hierarchy that is separate from the class inheritance
hierarchy. Although associations and aggregations are typically bi-
directional, there is nothing that prevents higher order associations
or aggregations from being defined. However, such associations and
aggregations are inherently more complex to define, understand, and
use. In practice, associations and aggregations of orders higher
than binary are rarely used, because of their greatly increased
complexity and lack of generality. All of the associations and
aggregations defined in this model are binary.
Note also that by definition, associations and aggregations cannot be
unary.
Finally, note that associations and aggregations that are defined
between two classes do not affect the classes themselves. That is,
the addition or deletion of an association or an aggregation does not
affect the interfaces of the classes that it is connecting.
4.2. The Structure of the Class Hierarchies
The structure of the class, association, and aggregation class
inheritance hierarchies for managing the datapaths of QoS devices is
shown, respectively, in Figure 9, Figure 10, and Figure 11. The
notation (CIMCORE) identifies a class defined in the CIM Core model.
Please refer to [CIM] for the definitions of these classes.
Similarly, the notation [PCIME] identifies a class defined in the
Policy Core Information Model Extensions document. This model has
been influenced by [CIM], and is compatible with the Directory
Enabled Networks (DEN) effort.
+--ManagedElement (CIMCORE)
|
+--ManagedSystemElement (CIMCORE)
| |
| +--LogicalElement (CIMCORE)
| |
| +--Service (CIMCORE)
| | |
| | +--ConditioningService
| | | |
| | | +--ClassifierService
| | | | |
| | | | +--ClassifierElement
| | | |
| | | +--MeterService
| | | | |
| | | | +--AverageRateMeterService
| | | | |
| | | | +--EWMAMeterService
| | | | |
| | | | +--TokenBucketMeterService
| | | |
| | | +--MarkerService
| | | | |
| | | | +--PreambleMarkerService
| | | | |
| | | | +--TOSMarkerService
| | | | |
| | | | +--DSCPMarkerService
| | | | |
(continued from previous page;
the first four elements are repeated for convenience)
+--ManagedElement (CIMCORE)
|
+--ManagedSystemElement (CIMCORE)
| |
| +--LogicalElement (CIMCORE)
| |
| +--Service (CIMCORE)
| | | | +--8021QMarkerService
| | | |
| | | +--DropperService
| | | | |
| | | | +--HeadTailDropperService
| | | | |
| | | | +--RedDropperService
| | | |
| | | +--QueuingService
| | | |
| | | +--PacketSchedulingService
| | | |
| | | +--NonWorkConservingSchedulingService
| | |
| | +--QoSService
| | | |
| | | +--DiffServService
| | | | |
| | | | +--AFService
| | | |
| | | +--FlowService
| | |
| | +--DropThresholdCalculationService
| |
| +--FilterEntryBase [PCIME]
| | |
| | +--IPHeaderFilter [PCIME]
| | |
| | +--8021Filter [PCIME]
| | |
| | +--PreambleFilter
| |
| +--FilterList [PCIME]
| |
| +--ServiceAccessPoint (CIMCORE)
| |
| +--ProtocolEndpoint
(continued from previous page;
the first four elements are repeated for convenience)
+--ManagedElement (CIMCORE)
|
+--ManagedSystemElement (CIMCORE)
| |
| +--LogicalElement (CIMCORE)
| |
| +--Service (CIMCORE)
|
+--Collection (CIMCORE)
| |
| +--CollectionOfMSEs (CIMCORE)
| |
| +--BufferPool
|
+--SchedulingElement
|
+--AllocationSchedulingElement
|
+--WRRSchedulingElement
|
+--PrioritySchedulingElement
|
+--BoundedPrioritySchedulingElement
Figure 9. Class Inheritance Hierarchy
The inheritance hierarchy for the associations defined in this
document is shown in Figure 10.
+--Dependency (CIMCORE)
| |
| +--ServiceSAPDependency (CIMCORE)
| | |
| | +--IngressConditioningServiceOnEndpoint
| | |
| | +--EgressConditioningServiceOnEndpoint
| |
| +--HeadTailDropQueueBinding
| |
| +--CalculationBasedOnQueue
| |
| +--ProvidesServiceToElement (CIMCORE)
| | |
| | +--ServiceServiceDependency (CIMCORE)
| | |
| | +--CalculationServiceForDropper
| |
| +--QueueAllocation
| |
| +--ClassifierElementUsesFilterList
|
+--AFRelatedServices
|
+--NextService
| |
| +--NextServiceAfterClassifierElement
| |
| +--NextScheduler
| |
| +--FailNextScheduler
|
+--NextServiceAfterMeter
|
+--QueueToSchedule
|
+--SchedulingServiceToSchedule
Figure 10. Association Class Inheritance Hierarchy
The inheritance hierarchy for the aggregations defined in this
document is shown in Figure 11.
+--MemberOfCollection (CIMCORE)
| |
| +--CollectedBufferPool
|
+--Component (CIMCORE)
| |
| +--ServiceComponent (CIMCORE)
| | |
| | +--QoSSubService
| | |
| | +--QoSConditioningSubService
| | |
| | +--ClassifierElementInClassifierService
| |
| +--EntriesInFilterList [PCIME]
|
+--ElementInSchedulingService
Figure 11. Aggregation Class Inheritance Hierarchy
4.3. Class Definitions
This section presents the classes and properties that make up the
Information Model for describing QoS-related functionality in network
devices, including hosts. These definitions are derived from
definitions in the CIM Core model [CIM]. Only the QoS-related
classes are defined in this document. However, other classes drawn
from the CIM Core model, as well as from [PCIME], are described
briefly. The reader is encouraged to look at [CIM] and at [PCIME]
for further information. Associations and aggregations are defined
in Section 4.4.
4.3.1. The Abstract Class ManagedElement
This is an abstract class defined in the Core Model of CIM. It is
the root of the entire class inheritance hierarchy in CIM. Among the
associations that refer to it are two that are subclassed in this
document: Dependency and MemberOfCollection, which is an aggregation.
ManagedElement's properties are Caption and Description. Both are
free-form strings to describe an instantiated object. Please refer
to [CIM] for the full definition of this class.
4.3.2. The Abstract Class ManagedSystemElement
This is an abstract class defined in the Core Model of CIM; it is a
subclass of ManagedElement. ManagedSystemElement serves as the base
class for the PhysicalElement and LogicalElement class hierarchies.
LogicalElement, in turn, is the base class for a number of important
CIM hierarchies, including System. Any distinguishable component of
a System is a candidate for inclusion in this class hierarchy,
including physical components (e.g., chips and cards) and logical
components (e.g., software components, services, and other objects).
None of the associations in which this class participates is used
directly in the QoS device state model. However, the aggregation
Component, which relates one ManagedSystemElement to another, is the
base class for the two aggregations that form the core of the QoS
device state model: QoSSubService and QoSConditioningSubService.
Similarly, the association ProvidesServiceToElement, which relates a
ManagedSystemElement to a Service, is the base class for the model's
CalculationServiceForDropper association.
Please refer to [CIM] for the full definition of this class.
4.3.3. The Abstract Class LogicalElement
This is an abstract class defined in the Core Model of CIM. It is a
subclass of the ManagedSystemElement class, and is the base class for
all logical components of a managed System, such as Files, Processes,
or system capabilities in the form of Logical Devices and Services.
None of the associations in which this class participates is relevant
to the QoS device state model. Please refer to [CIM] for the full
definition of this class.
4.3.4. The Abstract Class Service
This is an abstract class defined in the Core Model of CIM. It is a
subclass of the LogicalElement class, and is the base class for all
objects that represent a "service" or functionality in a System. A
Service is a general-purpose object that is used to configure and
manage the implementation of functionality. As noted above in
section 4.3.2, this class participates in the
ProvidesServiceToElement association. Please refer to [CIM] for the
full definition of this class.
4.3.5. The Class ConditioningService
This is a concrete subclass of the CIM Core class Service; it
represents the ability to define how traffic is conditioned in the
data-forwarding path of a device. The subclasses of
ConditioningService define the particular types of conditioning that
are done. Six fundamental types of conditioning are defined in this
document. These are the services performed by a classifier, a meter,
a marker, a dropper, a queue, and a scheduler. Other, more
sophisticated types of conditioning may be defined in future
documents.
ConditioningService is a concrete class because at the time it was
defined in CIM, its superclass was concrete. While this class can be
instantiated, an instance of it would not accomplish anything,
because the nature of the conditioning, and the parameters that
control it, are specified only in the subclasses of
ConditioningService.
Two associations in which ConditioningService participates are
critical to its usage in QoS - QoSConditioningSubService and
NextService. QoSConditioningSubService aggregates
ConditioningServices into a particular QoS service (such as AF), to
describe the specific conditioning functionality that underlies that
QoS service in a particular device. NextService indicates the
subsequent conditioning service(s) for different traffic streams.
The class definition is as follows:
NAME ConditioningService
DESCRIPTION A concrete class to define how traffic
is conditioned in the data forwarding
path of a host or network device.
DERIVED FROM Service
TYPE Concrete
PROPERTIES (none)
4.3.6. The Class ClassifierService
The concept of a Classifier comes from [DSMODEL]. ClassifierService
is a concrete class that represents a logical entity in an ingress or
egress interface of a device, that takes a single input stream, and
sorts it into one or more output streams. The sorting is done by a
set of filters that select packets based on the packet contents, or
possibly based on other attributes associated with the packet. Each
output stream is the result of matching a particular filter.
The representation of classifiers in QDDIM is closely related to that
presented in [DSMIB] and [DSMODEL]. Rather than being linked
directly to its FilterLists, a classifier is modeled here as an
aggregation of ClassifierElements. Each of these ClassifierElements
is then linked to a single FilterList, by the association
ClassifierElementUsesFilterList.
A Classifier is modeled as a subclass of ConditioningService so that
it can be aggregated into a QoSService (using the
QoSConditioningSubService aggregation), and can use the NextService
association to identify the subsequent ConditioningService objects
for the different traffic streams.
ClassifierService is designed to allow hierarchical classification.
When hierarchical classification is used, a ClassifierElement may
point to another ClassifierService. When used for this purpose, the
ClassifierElement must not use the ClassifierElementUsesFilterList
association.
The class definition is as follows:
NAME ClassifierService
DESCRIPTION A concrete class describing how an input
traffic stream is sorted into multiple
output streams using one or more
filters.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES (none)
4.3.7. The Class ClassifierElement
The concept of a ClassifierElement comes from [DSMIB]. This concrete
class represents the linkage, within a single ClassifierService,
between a FilterList that specifies a set of criteria for selecting
packets from the stream of packets coming into the ClassifierService,
and the next ConditioningService to which the selected packets go
after they leave the ClassifierService. ClassifierElement has no
properties of its own. It is present to serve as the anchor for an
aggregation with its classifier, and for associations with its
FilterList and its next ConditioningService.
When a ClassifierElement is associated with a ClassifierService
through the NextServiceAfterClassifierElement association, the
ClassifierElement may not use the ClassifierElementUsesFilterList
association. Further, when a ClassifierElement is associated with a
ClassifierService as described above, the order of processing of the
associated ClassifierService is a function of the ClassifierOrder
property of the ClassifierElementInClassifierService aggregation.
For example, lets assume the following:
1. ClassifierService (C1) aggregates ClassifierElements (E1), (E2)
and (E3), with relative ClassifierOrder values of 1, 2, and 3.
2. ClassifierElements (E1) and (E3) associations to FilterLists (F1)
and (F3) respectively using the ClassifierElementUsesFilterList
association.
3. (E1) & (E3) are associated with Meters (M1) and (M3) through their
respective NextServiceAfterClassifierElement associations.
4. (E2) is associated with ClassifierService (C2) through its
NextServiceAfterClassifierElement association.
5. ClassifierService (C2) aggregates ClassifierElements (E4) and (E5)
with relative ClassifierOrder values of 1 and 2.
6. ClassifierElements (E4) and (E5) have associations to FilterLists
(F4) and (F5) respectively using the
ClassifierElementUsesFilterList association.
In this example, packet processing would match FilterLists in the
order of (F1), (F4), (F5), and (F3).
The class definition is as follows:
NAME ClassifierElement
DESCRIPTION A concrete class representing
the process by which a classifier
uses a filter to select packets
to forward to a specific next
conditioning service.
DERIVED FROM ClassifierService
TYPE Concrete
PROPERTIES (none)
4.3.8. The Class MeterService
This is a concrete class that represents the metering of network
traffic. Metering is the function of monitoring the arrival times of
packets of a traffic stream, and determining the level of conformance
of each packet with respect to a pre-established traffic profile. A
meter has the ability to invoke different ConditioningServices for
conforming and non-conforming traffic. Traffic leaving a meter may be
further conditioned (e.g., dropped or queued) by routing the packet
to another conditioning element. Please see [DSMODEL] for more
information on metering.
This class is the base class for defining different types of meters.
As such, it contains common properties that all meter subclasses
share. It is modeled as a ConditioningService so that it can be
aggregated into a QoSService (using the QoSConditioningSubService
association), to indicate that its functionality underlies that QoS
service. MeterService also participates in the NextServiceAfterMeter
association, to identify the subsequent ConditioningService objects
for conforming and non-conforming traffic.
The class definition is as follows:
NAME MeterService
DESCRIPTION A concrete class describing the
monitoring of traffic with respect to a
pre-established traffic profile.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES MeterType, OtherMeterType,
ConformanceLevels
Note: The MeterType property and the MeterService subclasses provide
similar information. The MeterType property is defined for query
purposes and for future expansion. It is possible that not all
MeterServices will require a subclass to define them. In these
cases, MeterService will be instantiated directly, and the MeterType
property will provide the only way of identifying the type of the
meter.
4.3.8.1. The Property MeterType
This property is an enumerated 16-bit unsigned integer that is used
to specify the particular type of meter represented by an instance of
MeterService. The following enumeration values are defined:
1 - Other
2 - Average Rate Meter
3 - Exponentially Weighted Moving Average Meter
4 - Token Bucket Meter
Note: if the value of MeterType is not one of these four values, it
SHOULD be interpreted as if it had the value '1' (Other).
4.3.8.2. The Property OtherMeterType
This is a string property that defines a vendor-specific description
of a type of meter. It is used when the value of the MeterType
property in the instance is equal to 1.
4.3.8.3. The Property ConformanceLevels
This property is a 16-bit unsigned integer. It indicates the number
of conformance levels supported by the meter. For example, when only
"in profile" versus "out of profile" metering is supported,
ConformanceLevels is equal to 2.
4.3.9. The Class AverageRateMeterService
This is a concrete subclass of MeterService that represents a simple
meter, called an Average Rate Meter. This type of meter measures the
average rate at which packets are submitted to it over a specified
time. Packets are defined as conformant if their average arrival
rate does not exceed the specified measuring rate of the meter. Any
packet that causes the specified measuring rate to be exceeded is
defined to be non-conforming. For more information, please see
[DSMODEL].
The class definition is as follows:
NAME AverageRateMeterService
DESCRIPTION A concrete class classifying traffic as
either conforming or non-conforming,
depending on whether the arrival of a
packet causes the average arrival rate
to exceed a pre-determined value.
DERIVED FROM MeterService
TYPE Concrete
PROPERTIES AverageRate, DeltaInterval
4.3.9.1. The Property AverageRate
This is an unsigned 32-bit integer that defines the rate used to
determine whether admitted packets are in conformance or not. The
value is specified in kilobits per second.
4.3.9.2. The Property DeltaInterval
This is an unsigned 64-bit integer that defines the time period over
which the average measurement should be taken. The value is
specified in microseconds.
4.3.10. The Class EWMAMeterService
This is a concrete subclass of the MeterService class that represents
an exponentially weighted moving average meter. This meter is a
simple low-pass filter that measures the rate of incoming packets
over a small, fixed sampling interval. Any admitted packet that
pushes the average rate over a pre-defined limit is defined to be
non-conforming. Please see [DSMODEL] for more information.
The class definition is as follows:
NAME EWMAMeterService
DESCRIPTION A concrete class classifying admitted
traffic as either conforming or non-
conforming, depending on whether the
arrival of a packet causes the average
arrival rate in a small fixed
sampling interval to exceed a
pre-determined value or not.
DERIVED FROM MeterService
TYPE Concrete
PROPERTIES AverageRate, DeltaInterval, Gain
4.3.10.1. The Property AverageRate
This property is an unsigned 32-bit integer that defines the average
rate against which the sampled arrival rate of packets should be
measured. Any packet that causes the sampled rate to exceed this
rate is deemed non-conforming. The value is specified in kilobits
per second.
4.3.10.2. The Property DeltaInterval
This property is an unsigned 64-bit integer that defines the sampling
interval used to measure the arrival rate. The calculated rate is
averaged over this interval and checked against the AverageRate
property. All packets whose computed average arrival rate is less
than the AverageRate are deemed conforming.
The value is specified in microseconds.
4.3.10.3. The Property Gain
This property is an unsigned 32-bit integer representing the
reciprocal of the time constant (e.g., frequency response) of what is
essentially a simple low-pass filter. For example, the value 64 for
this property represents a time constant value of 1/64.
4.3.11. The Class TokenBucketMeterService
This is a concrete subclass of the MeterService class that represents
the metering of network traffic using a token bucket meter. Two
types of token bucket meters are defined using this class - a simple,
two-parameter bucket meter, and a multi-stage meter.
A simple token bucket usually has two parameters, an average token
rate and a burst size, and has two conformance levels: "conforming"
and "non-conforming". This class also defines an excess burst size,
which enables the meter to have three conformance levels
("conforming", "partially conforming", and "non-conforming"). In
this case, packets that exceed the excess burst size are deemed non-
conforming, while packets that exceed the smaller burst size but are
less than the excess burst size are deemed partially conforming.
Operation of these meters is described in [DSMODEL].
The class definition is as follows:
NAME TokenBucketMeterService
DESCRIPTION A concrete class classifying admitted
traffic with respect to a token bucket.
Either two or three levels of
conformance can be defined.
DERIVED FROM MeterService
TYPE Concrete
PROPERTIES AverageRate, PeakRate,
BurstSize, ExcessBurstSize
4.3.11.1. The Property AverageRate
This property is an unsigned 32-bit integer that specifies the
committed rate of the meter. The value is expressed in kilobits per
second.
4.3.11.2. The Property PeakRate
This property is an unsigned 32-bit integer that specifies the peak
rate of the meter. The value is expressed in kilobits per second.
4.3.11.3. The Property BurstSize
This property is an unsigned 32-bit integer that specifies the
maximum number of tokens available for the committed rate (specified
by the AverageRate property). The value is expressed in kilobytes.
4.3.11.4. The Property ExcessBurstSize
This property is an unsigned 32-bit integer that specifies the
maximum number of tokens available for the peak rate (specified by
the PeakRate property). The value is expressed in kilobytes.
4.3.12. The Class MarkerService
This is a concrete class that represents the general process of
marking some field in a network packet with some value. Subclasses of
MarkerService identify particular fields to be marked, and introduce
properties to represent the values to be used in marking these
fields. Markers are usually invoked as a result of a preceding
classifier match. Operation of markers of various types is described
in [DSMODEL].
MarkerService is a concrete class because at the time it was defined
in CIM, its superclass was concrete. While this class can be
instantiated, an instance of it would not accomplish anything,
because both the field to be marked and the value to be used to mark
it are specified only in subclasses of MarkerService.
MarkerService is modeled as a ConditioningService so that it can be
aggregated into a QoSService (using the QoSConditioningSubService
association) to indicate that its functionality underlies that QoS
service. It participates in the NextService association to identify
the subsequent ConditioningService object that acts on traffic after
it has been marked by the marker.
The class definition is as follows:
NAME MarkerService
DESCRIPTION A concrete class representing the
general process of marking a selected
field in a packet with a specified
value. Packets are marked in order
to control the conditioning that
they will subsequently receive.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES (none)
4.3.13. The Class PreambleMarkerService
This is a concrete class that models the storing of traffic-
conditioning results in a packet preamble. See Section 3.8.3 for a
discussion of how, and why, QDDIM models the capability to store
these results in a packet preamble. An instance of
PreambleMarkerService appends to a packet preamble a two-part string
of the form "<type>,<value>". Section 3.8.3 provides a list of the
<type> strings defined by QDDIM. Implementations may support other
<type>'s in addition to these.
The class definition is as follows:
NAME PreambleMarkerService
DESCRIPTION A concrete class representing the saving
of traffic-conditioning results in a
packet preamble.
DERIVED FROM MarkerService
TYPE Concrete
PROPERTIES FilterItemList[ ]
4.3.13.1. The Multi-valued Property FilterItemList
This property is an ordered list of strings, where each string has
the format "<type>,<value>". See Section 3.8.3 for a list of
<type>'s defined in QDDIM, and the nature of the associated <value>
for each of these types.
4.3.14. The Class ToSMarkerService
This is a concrete class that represents the marking of the ToS field
in the IPv4 packet header [R791]. Following common practice, the
value to be written into the field is represented as an unsigned 8-
bit integer.
The class definition is as follows:
NAME ToSMarkerService
DESCRIPTION A concrete class representing the
process of marking the type of service
(ToS) field in the IPv4 packet header
with a specified value. Packets are
marked in order to control the
conditioning that they will subsequently
receive.
DERIVED FROM MarkerService
TYPE Concrete
PROPERTIES ToSValue
4.3.14.1. The Property ToSValue
This property is an unsigned 8-bit integer, representing a value to
be used for marking the type of service (ToS) field in the IPv4
packet header. The ToS field is defined to be a complete octet, so
the range for this property is 0..255. Some implementations,
however, require that the lowest-order bit in the ToS field always be
'0'. Such an implementation is consequently unable to support an odd
TosValue.
4.3.15. The Class DSCPMarkerService
This is a concrete class that represents the marking of the
differentiated services codepoint (DSCP) within the DS field in the
IPv4 and IPv6 packet headers, as defined in [R2474]. Following common
practice, the value to be written into the field is represented as an
unsigned 8-bit integer.
The class definition is as follows:
NAME DSCPMarkerService
DESCRIPTION A concrete class representing the
process of marking the DSCP field
in a packet with a specified
value. Packets are marked in order
to control the conditioning that
they will subsequently receive.
DERIVED FROM MarkerService
TYPE Concrete
PROPERTIES DSCPValue
4.3.15.1. The Property DSCPValue
This property is an unsigned 8-bit integer, representing a value to
be used for marking the DSCP within the DS field in an IPv4 or IPv6
packet header. Since the DSCP consists of 6 bits, the values for
this property are limited to the range 0..63. When the DSCP is
marked, the remaining two bit in the DS field are left unchanged.
4.3.16. The Class 8021QMarkerService
This is a concrete class that represents the marking of the user
priority field defined in the IEEE 802.1Q specification [IEEE802Q].
Following common practice, the value to be written into the field is
represented as an unsigned 8-bit integer.
The class definition is as follows:
NAME 8021QMarkerService
DESCRIPTION A concrete class representing the
process of marking the Priority
field in an 802.1Q-compliant frame
with a specified value. Frames are
marked in order to control the
conditioning that they will
subsequently receive.
DERIVED FROM MarkerService
TYPE Concrete
PROPERTIES PriorityValue
4.3.16.1. The Property PriorityValue
This property is an unsigned 8-bit integer, representing a value to
be used for marking the Priority field in the 802.1Q header. Since
the Priority field consists of 3 bits, the values for this property
are limited to the range 0..7. When the Priority field is marked,
the remaining bits in its octet are left unchanged.
4.3.17. The Class DropperService
This is a concrete class that represents the ability to selectively
drop network traffic, or to invoke another ConditioningService for
further processing of traffic that is not dropped. This is the base
class for different types of droppers. Droppers are distinguished by
the algorithm that they use to drop traffic. Please see [DSMODEL]
for more information about the various types of droppers. Note that
this class encompasses both Absolute Droppers and Algorithmic
Droppers from [DSMODEL].
DropperService is modeled as a ConditioningService so that it can be
aggregated into a QoSService (using the QoSConditioningSubService
association) to indicate that its functionality underlies that QoS
service. It participates in the NextService association to identify
the subsequent ConditioningService object that acts on any remaining
traffic that is not dropped.
NextService has special semantics for droppers, in addition to the
general "what happens next" semantics that apply to all
ConditioningServices. The queue(s) from which a particular dropper
drops packets are identified by following chain(s) of NextService
associations "rightwards" from the dropper until they reach a queue.
The class definition is as follows:
NAME DropperService
DESCRIPTION A concrete base class describing the
common characteristics of droppers.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES DropperType, OtherDropperType, DropFrom
Note: The DropperType property and the DropperService subclasses
provide similar information. The DropperType property is defined for
query purposes, as well as for those cases where a subclass of
DropperService is not needed to model a particular type of dropper.
For example, the Absolute Dropper defined in [DSMODEL] is modeled as
an instance of the DropperService class with its DropperType set to
'4' ("Absolute Dropper").
4.3.17.1. The Property DropperType
This is an enumerated 16-bit unsigned integer that defines the type
of dropper. Values include:
1 - Other
2 - Random
3 - HeadTail
4 - Absolute Dropper
Note: if the value of DropperType is not one of these four values, it
SHOULD be interpreted as if it had the value '1' (Other).
4.3.17.2. The Property OtherDropperType
This string property is used in conjunction with the DropperType
property. When the value of DropperType is '1' (i.e., Other), then
the name of the type of dropper appears in this property.
4.3.17.3. The Property DropFrom
This is an unsigned 16-bit integer enumeration that indicates the
point in the associated queue from which packets should be dropped.
Defined enumeration values are:
o unknown(0)
o head(1)
o tail(2)
Note: if the value of DropFrom is '0' (unknown), or if it is not one
of the three values listed here, then packets MAY be dropped from any
location in the associated queue.
4.3.18. The Class HeadTailDropperService
This is a concrete class that represents the threshold information of
a head or tail dropper. The inherited property DropFrom indicates
whether a particular instance of this class represents a head dropper
or a tail dropper.
A head dropper always examines the same queue from which it drops
packets, and this queue is always related to the dropper as the
following service in the NextService association.
The class definition is as follows:
NAME HeadTailDropperService
DESCRIPTION A concrete class used to describe
a head or tail dropper.
DERIVED FROM DropperService
TYPE Concrete
PROPERTIES QueueThreshold
4.3.18.1. The Property QueueThreshold
This is an unsigned 32-bit integer that indicates the queue depth at
which traffic will be dropped. For a tail dropper, all newly
arriving traffic is dropped. For a head dropper, packets at the
front of the queue are dropped to make room for new packets, which
are added at the end. The value is expressed in bytes.
4.3.19. The Class REDDropperService
This is a concrete class that represents the ability to drop network
traffic using a Random Early Detection (RED) algorithm. This
algorithm is described in [RED]. The purpose of a RED algorithm is
to avoid congestion (as opposed to managing congestion). Instead of
waiting for the queues to fill up, and then dropping large numbers of
packets, RED works by monitoring the average queue depth. When the
queue depth exceeds a minimum threshold, packets are randomly
discarded. These discards cause TCP to slow its transmission rate
for those connections that experienced the packet discards. Other
TCP connections are not affected by these discards. Please see
[DSMODEL] for more information about a dropper.
A RED dropper always drops packets from a single queue, which is
related to the dropper as the following service in the NextService
association. The queue(s) examined by the drop algorithm are found
by following the CalculationServiceForDropper association to find the
dropper's DropThresholdCalculationService, and then following the
CalculationBasedOnQueue association(s) to find the queue(s) being
watched.
The class definition is as follows:
NAME REDDropperService
DESCRIPTION A concrete class used to describe
dropping using the RED algorithm (or
one of its variants).
DERIVED FROM DropperService
TYPE Concrete
PROPERTIES MinQueueThreshold, MaxQueueThreshold,
ThresholdUnits, StartProbability,
StopProbability
NOTE: In [DSMIB], there is a single diffServRandomDropTable, which
represents the general category of random dropping. (RED is one type
of random dropping, but there are also types of random dropping
distinct from RED.) The REDDropperService class corresponds to the
columns in the table that apply to the RED algorithm in particular.
4.3.19.1. The Property MinQueueThreshold
This is an unsigned 32-bit integer that defines the minimum average
queue depth at which packets are subject to being dropped. The units
are identified by the ThresholdUnits property. The slope of the drop
probability function is described by the Start/StopProbability
properties.
4.3.19.2. The Property MaxQueueThreshold
This is an unsigned 32-bit integer that defines the maximum average
queue length at which packets are subject to always being dropped,
regardless of the dropping algorithm and probabilities being used.
The units are identified by the ThresholdUnits property.
4.3.19.3. The Property ThresholdUnits
This is an unsigned 16-bit integer enumeration that identifies the
units for the MinQueueThreshold and MaxQueueThreshold properties.
Defined enumeration values are:
o bytes(1)
o packets(2)
Note: if the value of ThresholdUnits is not one of these two values,
it SHOULD be interpreted as if it had the value '1' (bytes).
4.3.19.4. The Property StartProbability
This is an unsigned 32-bit integer; in conjunction with the
StopProbability property, it defines the slope of the drop
probability function. This function governs the rate at which
packets are subject to being dropped, as a function of the queue
length.
This property expresses a drop probability in drops per thousand
packets. For example, the value 100 indicates a drop probability of
100 per 1000 packets, that is, 10%. Min and max values are 0 to
1000.
4.3.19.5. The Property StopProbability
This is an unsigned 32-bit integer; in conjunction with the
StartProbability property, it defines the slope of the drop
probability function. This function governs the rate at which
packets are subject to being dropped, as a function of the queue
length.
This property expresses a drop probability in drops per thousand
packets. For example, the value 100 indicates a drop probability of
100 per 1000 packets, that is, 10%. Min and max values are 0 to
1000.
4.3.20. The Class QueuingService
This is a concrete class that represents the ability to queue network
traffic, and to specify the characteristics for determining long-term
congestion. Please see [DSMODEL] for more information about queuing
functionality.
QueuingService is modeled as a ConditioningService so that it can be
aggregated into a QoSService (using the QoSConditioningSubService
association) to indicate that its functionality underlies that QoS
service.
The class definition is as follows:
NAME QueuingService
DESCRIPTION A concrete class describing the ability
to queue network traffic and to specify
the characteristics for determining
long-term congestion.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES CurrentQueueDepth, DepthUnits
4.3.20.1. The Property CurrentQueueDepth
This is an unsigned 32-bit integer, which functions as a (read-only)
gauge representing the current depth of this one queue. This value
may be important in diagnosing unexpected behavior by a
DropThresholdCalculationService.
4.3.20.2. The Property DepthUnits
This is an unsigned 16-bit integer enumeration that identifies the
units for the CurrentQueueDepth property. Defined enumeration values
are:
o bytes(1)
o packets(2)
Note: if the value of DepthUnits is not one of these two values, it
SHOULD be interpreted as if it had the value '1' (bytes). The
4.3.21. Class PacketSchedulingService
This is a concrete class that represents a scheduling service, which
is a process that determines when a queued packet should be removed
from a queue and sent to an output interface. Note that output
interfaces can be physical network interfaces or interfaces to
components internal to systems, such as crossbars or back planes. In
either case, if multiple queues are involved, schedulers are used to
provide access to the interface.
Each instance of a PacketSchedulingService describes a scheduler from
the perspective of the queues that it is servicing. Please see
[DSMODEL] for more information about a scheduler.
PacketSchedulingService is modeled as a ConditioningService so that
it can be aggregated into a QoSService (using the
QoSConditioningSubService association) to indicate that its
functionality underlies that QoS service. It participates in the
NextService association to identify the subsequent
ConditioningService object, if any, that acts on traffic after it has
been processed by the scheduler.
The class definition is as follows:
NAME PacketSchedulingService
DESCRIPTION A concrete class used to determine when
a packet should be removed from a
queue and sent to an output interface.
DERIVED FROM ConditioningService
TYPE Concrete
PROPERTIES SchedulerType, OtherSchedulerType
4.3.21.1. The Property SchedulerType
This property is an enumerated 16-bit unsigned integer, and defines
the type of scheduler. Values are:
1 - Other
2 - FIFO
3 - Priority
4 - Allocation
5 - Bounded Priority
6 - Weighted Round Robin Packet
Note: if the value of SchedulerType is not one of these six values,
it SHOULD be interpreted as if it had the value '2' (FIFO).
4.3.21.2. The Property OtherSchedulerType
This string property is used in conjunction with the SchedulerType
property. When the value of SchedulerType is 1 (i.e., Other), then
the type of scheduler is specified in this property.
4.3.22. The Class NonWorkConservingSchedulingService
This class does not add any properties beyond those it inherits from
its superclass, PacketSchedulingService. It does, however,
participate in one additional association, FailNextScheduler.
The class definition is as follows:
NAME NonWorkConservingSchedulingService
DESCRIPTION A concrete class representing a
scheduler that is capable of operating
in a non-work conserving manner.
DERIVED FROM PacketSchedulingService
TYPE Concrete
PROPERTIES (none)
4.3.23. The Class QoSService
This is a concrete class that represents the ability to conceptualize
a QoS service as a set of coordinated sub-services. This enables the
network administrator to map business rules to the network, and the
network designer to engineer the network such that it can provide
different functions for different traffic streams.
This class has two main purposes. First, it serves as a common base
class for defining the various sub-services needed to build higher-
level QoS services. Second, it serves as a way to consolidate the
relati