Network Working Group S. Shenker
Request for Comments: 2216 J. Wroclawski
Category: Informational Xerox PARC/MIT LCS
September 1997
Network Element Service Specification Template
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
This document defines a framework for specifying services provided by
network elements, and available to applications, in an internetwork
which offers multiple qualities of service. The document first
provides some necessary context -- including relevant definitions and
suggested data formats -- and then specifies a "template" which
service specification documents should follow. The specification
template includes per-element requirements such as the service's
packet handling behavior, parameters required and made available by
the service, traffic specification and policing requirements, and
traffic ordering relationships. It also includes evaluation criteria
for elements providing the service, and examples of how the service
might be implemented (by network elements) and used (by
applications).
Introduction
This document defines the framework used to specify the functionality
of internetwork system components which support the the ability to
provide multiple, dynamically selectable qualities of service to
applications using an internetwork. The behavior of individual
routers and subnetworks is captured as a set of "services", some or
all of which may be offered by each element. The concatenation of
these services along the end-to-end data paths used by an application
provides overall quality of service control.
The definition of a service states what is required of a router (or,
more generally, any network element; a router, switch, subnet, etc.)
which supports a particular service. The service definition also
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specifies parameters used to invoke the service, the relationship
between those parameters and the service delivered, and the end-to-
end behavior obtained by concatenating several instances of the
service.
Each service definition also specifies the interface between that
service and the environment. This includes the parameters needed to
invoke the service, informational parameters which the service must
make available for use by setup, routing, and management mechanisms,
and information which should be carried between end-nodes and network
elements by those mechanisms in order to achieve the desired end-to-
end behavior. However, a service definition does not describe the
specific protocols or mechanisms used to establish state in the
network elements for flows that use the described service.
Services defined following the guidelines of this document are
intended for use both within the global Internet and private IP
networks. In certain cases a concatenation of network element
services may be used to provide a range of end-to-end behaviors, some
more suited to a decentralized internet and some more appropriate for
a tightly managed private network. This document points out places
where such distinction may be appropriate.
This document is comprised of three parts. The first defines some
terms used both in this document and in the various service
specification documents. The second discusses data formats and
representations. The third portion of the document describes the
various components of the service specification template.
Definitions
The following terms are used throughout this document. Service
specification documents should employ the same terms to express these
concepts.
o Quality of Service (QoS)
In the context of this document, quality of service refers to the
nature of the packet delivery service provided, as described by
parameters such as achieved bandwidth, packet delay, and packet loss
rates. Traditionally, the Internet has offered a single quality of
service, best-effort delivery, with available bandwidth and delay
characteristics dependent on instantaneous load. Control over the
quality of service seen by applications is exercised by adequate
provisioning of the network infrastructure. In contrast, a network
with dynamically controllable quality of service allows individual
application sessions to request network packet delivery
characteristics according to their perceived needs, and may provide
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different qualities of service to different applications. It should
be understood that there is a range of useful possibilities between
the two endpoints of providing no dynamic QoS control at all and
providing extremely precise and accurate control of QoS parameters.
o Network Element
A "Network Element" (or the equivalent shorter form "Element"), is
any component of an internetwork which directly handles data packets
and thus is potentially capable of exercising QoS control over data
flowing through it. Network elements include routers, subnetworks,
and end-node operating systems. A QoS-capable network element is one
which offers one or more of the services defined according to the
rules given in this document. Note that this definition, by itself,
preclude QoS-capable network elements that meet performance goals
purely through adequate provisioning rather than active admission and
traffic control mechanisms. A "QoS-aware" network element is one
which supports the interfaces (described below) required by the
service definitions. Thus, a QoS-aware network element need not
actually offer any of the services defined according to the format of
this document; it merely needs to know how to deny service requests.
o Flow
For the purposes of this document a flow is a set of packets
traversing a network element all of which are covered by the same
request for control of quality of service. At a given network element
a flow may consist of the packets from a single application session,
or it may be an aggregation comprising the combined data traffic from
a number of application sessions.
NOTE: this definition of a flow is different from that used in
IPv6, where a flow is defined as those packets with the same
source address and FlowID.
Mechanisms used to associate a request for quality of service control
with the packets covered by that request are beyond the scope of this
document.
o Service
The phrase "service" or "QoS Control Service" describes a named,
coordinated set of QoS control capabilities provided by a single
network element. The definition of a service includes a
specification of the functions to be performed by the network
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element, the information required by the element to perform these
functions, and the information made available by the element to other
elements of the system. A service is conceptually implemented within
the "service module" contained within the network element.
NOTE: The above defines a precise meaning for the word "service".
Service is a word which has a variety of meanings throughout the
networking community; the definition of "service" given here
refers specifically to the actions and responses of a single
network element such as a router or subnet. This contrasts with
the more end-to-end oriented definition of the same word seen in
some other networking contexts.
o Behavior
A "behavior" is the QoS-related end-to-end performance seen by an
application session. This behavior is the end result of composing the
services offered by each network element along the path of the
application's data flow.
When each network element along a data flow path offers the same
service, it is frequently possible to explain the resulting end-to-
end behavior in a straightforward fashion. The behavior of a data
flow path comprised of elements using different services is more
complicated, and may in fact be undefined. A future version of this
document may impose additional requirements on the service
specification relating to multi-service concatenation.
o Characterization
A characterization is a computed approximation of the actual end-to-
end behavior which would be seen by a flow requesting specific QoS
services from the network. By providing additional information to
the end-nodes before a flow is established, characterizations assist
the end-nodes in choosing the services to be requested from the
network.
o Characterization Parameters
Characterizations are computed from a set of characterization
parameters provided by each network element on the flow's path, and a
composition function which computes the end-to-end characterization
from those parameters. The composition function may in practice be
executed in a distributed fashion by the setup or routing protocol,
or the characterization parameters may be gathered to a single point
and the characterization computed at that point.
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Several characterizations may be computed for a single candidate data
flow. Conversely, a service may provide no characterizations, and
under some conditions no characterizations may be available to the
end-nodes requesting QoS services.
o Composition Function
A composition function accepts characterization parameters as input
and computes a characterization, as described above.
o Traffic Specification (TSpec)
A Traffic Specification, or TSpec, is a description of the traffic
pattern for which service is being requested. In general, the TSpec
forms one side of a "contract" between the data flow and the service.
Once a service request is accepted, the service module has agreed to
provide a specific QoS as long as the flow's data traffic continues
to be accurately described by the TSpec.
As examples, this specification might take the form of a token bucket
filter (defined below) or an upper bound on the peak rate. Note that
the traffic specification specifies the flow's *allowed* traffic
pattern, not the flows *actual* traffic pattern. The behavior of a
service when a flow's actual traffic does not conform to the traffic
specification must be defined by the service (see "Policing" below).
o Service Request Specification (RSpec)
A Service Request Specification, or RSpec, is a specification of the
quality of service a flow wishes to request from a network element.
The contents of a service request specification is highly specific to
a particular service. As examples, these specifications might contain
information about bandwidth allocated to the flow, maximum delays, or
packet loss rates.
o Setup Protocol
A setup protocol is used to carry QoS-related information from the
end-nodes requesting QoS control to network elements which must
exercise that control, and to install and maintain to required QoS
control state in those network elements. A setup protocol may also
be used to collect QoS-related information from interior network
elements along an application's data flow path for ultimate delivery
to end nodes. Examples of protocols which perform setup functions are
RSVP [RFC 2205], ST-II [RFC 1819], and Q.2931.
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Note that other mechanisms, such as network management protocols, may
also perform this function. The phrase "setup protocol"
conventionally refers to a protocol with this function as its primary
purpose.
o Token Bucket
A Token Bucket is a particular form of traffic specification
consisting of a "token rate" r and a "bucket size" b. Essentially,
the r parameter specifies the continually sustainable data rate,
while the b parameter specifies the extent to which the data rate can
exceed the sustainable level for short periods of time. More
specifically, the traffic must obey the rule that over all time
periods, the amount of data sent cannot exceed rT+b, where T is the
length of the time period.
Token buckets are further discussed in [PARTRIDGE].
o Token Bucket Filter
A Token Bucket Filter is a filtering or policing function which
differentiates those packets in a traffic flow which conform to a
particular token bucket specification from those packets which do
not. The specific treatment accorded nonconforming packets is not
specified in this definition; common actions are relegating the
packet to best effort service, discarding the packet, or marking the
packet in some fashion.
o Admission Control
Admission control is the process of deciding whether a newly arriving
request for service from a network element can be granted. This
action must be performed by any service which wishes to offer
absolute quantitative bounds on overall performance. It is not
necessary for services which provide only relative statements about
performance, such as the Internet's current best-effort service. The
precise criteria for making the admission control decision are a
specific to each particular service.
o Policing
Policing is the set of actions triggered when a flow's actual data
traffic characteristics exceed the expected values given in the
flow's traffic specification. Services which require policing
functions to operate correctly must specify both the action to be
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taken when such discrepancies occur and the locations in the network
where discrepancies are to be detected. Examples of such actions
might include relegating the packet to best effort service, dropping
packets, reshaping the traffic, or marking non-conforming traffic in
some fashion.
o Interfaces
The service module conceptually interacts with other portions of the
network element through a number of interfaces. The service
specification document should clearly define the specific data,
including formats, which moves across each conceptual interface, and
ensure that the mapping between conceptual interfaces and the
specific mechanisms of the service being defined are clear.
Data Format and Representation
A service module will import and export a variety of data according
to the specific requirements of the services the network element
supports. Each service definition MUST specify the format of each
such data item in an abstract manner. The information specified must
be sufficient for the designer of a setup protocol to correctly
select an appropriate concrete (packet) format for variables
containing this data. At minimum, the following information must be
given:
- Type: whether the quantity is an enumeration, a numerical value,
etc.
- Range: for numerical quantities, the minimum and maximum values
the quantity must be able to represent. For enumerated quantities,
an estimate of the maximum number of items which may need be
enumerated in the future, even if many of the values are currently
unused.
- Precision: the precision with which a numerical quantity must be
represented, and whether that precision is absolute (calling for an
integer quantity) or a percentage of the value (allowing for a
floating point quantity).
The service definition SHOULD additionally specify a preferred
concrete format for each data field, in the usual packet-layout
format used in current Internet Standard documents or in some other
accepted specification format. If the service definition contains
these concrete definitions, they should be sufficiently complete and
detailed to allow the service definition to be incorporated by
reference into the specifications for setup protocols and other users
of the specified data.
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NOTE: The wording above is intended to encourage the use of common
data formats by all protocols carrying data related to a specific
service, while not mandating this common format or infringing on
the freedom of protocol specification designers to define data
representations using alternative mechanisms such as ASN.1 or XDR.
Service and Data Element Naming
End-nodes, network elements, setup protocols, and management entities
within an integrated services internetwork need to exchange
information about services, service invocation parameters,
characterization parameters, and the intermediate variables and end
results of composition functions. To support this requirement, a
single uniform namespace is established for services and their
parameters.
The namespace is a two-level hierarchy:
..
Each of these elements is a integer numerical quantity.
is an integer in the range 1 to 254. The number space
is broken into three regions.
Service number 1 is used to indicate that the associated parameter is
generic", and is not associated with a specific service. This use of
generic parameters is described more fully in [RFC 2215].
The range from 2 to 127 used to name services defined by the IETF.
Procedures for allocating service numbers in this region will be
established by the IETF INT-SERV WG and the IANA. Services designed
for public use should obtain a number from this space. The minimum
requirement for doing so is a published RFC following the format
described in this note.
Service numbers in the region above 127 are reserved for experimental
or private services. Service designers may allocate numbers from this
space at random for local experimental use. A policy for global but
temporary allocation of these numbers may be established in the
future if necessary.
The value 0 is left unused to allow the direct mapping of parameter
names to MIB object names, as described below.
The value 255 is reserved to facilitate future expansion of the
service number space, if required.
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is a number in the range 1 to 254, allocated on a
per-service basis. Within this range, the values 1 to 127 are
reserved for assignment to parameters with a common, shared meaning
across all services. These parameters are defined in [RFC 2215].
Numbers for parameters specific to a service are assigned from the
range 128-254 by the author of the service specification document.
The value 0 is left unused to allow the direct mapping of parameter
names to MIB object names, as described below.
The value 255 is reserved to facilitate future expansion of the
parameter number space, if required.
In addition to their uses within the integrated services framework,
these . pairs should be used as
last two levels of the MIB name when the corresponding values are
made available to network management protocols.
Specification Document Format
The following portion of this document describes the layout and
contents of a service specification. Each service specification
document MUST contain the sections marked [required] below, in the
order listed. Each document SHOULD contain each of the remaining
sections in the list below, unless there is a compelling argument
that the presence of the section is not beneficial. Additional
material, including references, should be included at the end of the
document.
Some of these sections are normative, in that they describe specific
requirements to which conformant implementations must adhere. Other
sections are informational in nature, in that they describe necessary
context and technical considerations important to the implementor of
a service. The sections, and their nature (required or optional, and
informational or normative) are listed below:
o Components
The body of a service specification document incorporates the
following sections:
- End-to-End Behavior [required] [informational]
- Motivation [required] [informational]
- Network Element Data Handling Requirements [required] [normative]
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- Invocation Information [required] [normative]
- Exported Information [required] [normative]
- Policing [required] [normative]
- Ordering and Merging [required] [normative]
- Guidelines for Implementors [optional] [informational]
- Evaluation Criteria [required] [informational]
- Examples of Implementation [optional] [informational]
- Examples of Use [optional] [informational]
o End-to-end Behavior
This is a description of the behavior that results if all network
elements along the path offer the same service, invoked with a
defined set of parameters.
In private networks it will generally be the case that the required
end-to-end behavior is obtained by concatenation of network elements
utilizing the same service and making significant use of
characterizations.
In the global Internet, this will not always be true. End-to-end
behaviors will frequently be obtained through a concatenation of
network elements supporting different services, including in some
cases elements which exercise no QoS control at all. Mechanisms to
characterize end-to-end behavior in this circumstance are not fully
established at this time. Future versions of this document may impose
additional requirements on service specifications to facilitate
inter-service composition.
This section is for informational purposes only.
o Motivation
This section discusses why this service is being defined. It
describes what kinds of applications might make use of this service,
and why this service might be more appropriate for those applications
than other possible choices. This section is for informational
purposes only.
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o Network Element Data Handling Requirements
This section contains a description of the QoS properties seen by
data packets processed by a network element using this service. The
description must clearly explain what variables are controlled, the
degree of control exercised, and what aspects of the service's
handling model are fixed or assumed. Examples of degree of control
information include "this property must be mathematically assured"
and "this property should be met under most conditions". An example
of a stated assumption is "this service is assumed to have extremely
low packet loss; delay targets must be met using admission control
rather than by discarding packets when overloaded".
Requirements on packet handling SHOULD, when at all possible, be
expressed as performance requirements rather than by specifying a a
particular packet scheduling algorithm. The performance requirements
might, for example, be a specification of the maximal packet delays
or the minimal bandwidth share given to a flow.
This section also specifies actions which the packet handling path is
required to take to actively provide feedback to end-nodes about
conditions at the network element. Such actions might include
explicitly generated congestion feedback, indicated either as bits
set in the header of data packets or separate control messages sent.
When writing this section of the service specification document, the
authors' goal should be to specify the required behavior as precisely
as necessary while still leaving adequate room for the implementation
and architectural tradeoffs appropriate to different circumstances
and classes of network elements. Successfully achieving this balance
may require some care.
o Invocation Information
This section describes the set of parameters required by a service
module to invoke the service, and a description of how the parameter
values are used by the service module. For example, a hypothetical
"bounded delay" service might be described as accepting a request
indicating a delay target for the network element and the set of
packets subject to that delay target, and processing packets in the
given set with a delay of the target value or less.
Necessary invocation information for most services can be broken into
two parts, the Traffic Specification (TSpec) and the Service Request
Specification (RSpec). The TSpec gives characteristics of the data
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traffic to be handled, while the Rspec specifies the properties
desired from the service. For example, a service offering a
mathematical bound on delay might accept a TSpec giving the traffic
flow's bandwidth and burstiness specified as a Token Bucket, and an
RSpec giving the maximum tolerable queueing delay.
A service accepting an invocation request may be thought of as
entering into a "contract" to provide the service described by the
RSpec as long as the flow's traffic continues to be described by the
TSpec. If the flow's traffic pattern falls outside the bounds of the
TSpec, the QoS provided to the flow may change. The precise nature of
this change is also described by the service specification (see
"Policing" below).
The RSPec and TSpec components of the invocation information should
be specified separately and independently, as they will often be
generated by different elements of the internetwork
All quantitative information specifications in this section should
follow the guidelines given in the Data Formats section of this
document, above.
o Exported Information and Characterization Parameters
This section describes information which must be collected and
exported by the service module. Exported information is available to
other modules of the network element, and by extension to setup
protocols, routing protocols, network management tools, and the like.
Information exported by service modules may be used in several ways.
For example, quantities such as the amount of link bandwidth
dedicated to the service and the set of data flows currently
receiving the service are appropriate pieces of information to make
available as network management variables.
A service definition may identify a particular subset of the
information exported by a service module as characterization
parameters. These characterization parameters may be used to compute
or estimate the end-to-end behavior of a data flow traversing a
concatenation of network service elements. They may also be used to
characterize portions of the path for use by network elements (e.g.,
in computing the buffer necessary, an element may need to know
something about the service characteristics of the upstream portion
of the path). A service which defines characterization parameters
also specifies the characterizations they are used to generate and
the composition functions used to generate the characterizations.
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NOTE: Characterization parameters are identified as such by virtue
of being the inputs to a service's defined composition functions.
Because characterization parameters are part of a service's
overall exported data set, they are also available to other
functions, such as network management. The discussion below
relates solely to their use as characterization parameters, and is
not intended to limit other uses.
Characterization parameters may be relatively static quantities, such
as the bandwidth available on a specific link, or relatively dynamic
quantities, such as a running estimation of current packet delay.
Support for a service's defined characterization parameters is
mandatory. Any network element offering this service must be able to
measure, compute, or, if allowed by the specification, estimate the
service's characterization parameters. Service designers are
encouraged to understand the implications of specifying
characterization parameters for a service, particularly with respect
to not unduly restricting the choice of hardware and software
architectures used to implement the network element.
Characterization parameters are used by composing the values exported
by each network element along a data flow's path according to a
composition rule. For each parameter or set of parameters used to
develop a characterization, the service specification must specify
the composition rule to be used. These composition rules should
result in characterizations that are independent of the order in
which the element are composed; commutativity and associativity are
sufficient but not necessary conditions for this.
Characterization parameters are available through a general
interface, and are provided in response to a request from some other
module, such as a setup protocol or the routing protocol. The
question of exactly how, or if, a specific protocol (e.g., RSVP) uses
characterization parameters to generate characterizations is
described in the specification of that specific protocol.
The correct use of characterization parameters supplied by service
modules is a function of the setup, routing, or management protocol
controlling the module. There is no absolute guarantee that
characterizations will be available to end-nodes desiring to use a
QoS control service. Service designers targeting services for the
global Internet may wish to ensure that a service is useful even in
the absence of characterizations, and to exhibit such uses in the
"Examples" sections of the service description document.
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Conversely, the availability of characterizations may be mandatory in
certain circumstances, particularly for private IP networks providing
tightly controlled qualities of service for specific applications.
Service designers targeting this environment should particularly
ensure that the service provides adequate characterization parameters
and composition functions to meet the needs of target audiences. It
may be appropriate to specify the same basic service with additional
characterizations for meeting specific requirements beyond those of
the global Internet.
Some useful "general" characterization parameters and corresponding
composition rules are not associated with any specific service.
These include the speed-of-light latency of communication links and
available link bandwidth. These general characterization parameters
are defined in [RFC 2215].
Although every conformant implementation of a service is required to
provide that service's characterization parameters, it is still
possible that the desired characterization parameters will not be
available for composition at all network elements in a path. This
situation may arise when different network element services are used
at different points in the end-to-end path, as may be required in a
heterogeneous internetworking environment. For this reason,
characterization parameters and composition function results
conceptually include a "validity flag". A network element which is
unable to provide the characterization parameter must set this flag,
and otherwise leave parameter or composed value unchanged. Once set,
the flag is preserved by the composition function, and serves as an
indicator of the validity of the data when the final composed result
is delivered to its destination.
Protocols which transport characterization parameters and composition
data must define and support a concrete representation for this
validity flag, as well as for the characterization parameters
themselves.
NOTE: This service specification template does not allow a service
definition to *require* that a setup or invocation mechanism used
with the service perform any function other than transport of
invocation parameters to the network elements and signalling of
errors generated by the network elements to the end nodes. A notable
example of this is that service specification documents may not
require or assume that characterizations defined in the specification
are actually computed or presented to the end nodes.
That point notwithstanding, the practical usefulness of a specific
service may be highly dependent on the presence of some additional
behavior in the networked system, such as the computation and
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presentation of characterizations to end-nodes or the reliable
assurance that every network element in the path from sender to
receivers supports the given service. Service specification authors
are strongly encouraged to clearly explain the situation of their
service in this regard. Statements such as:
The characterizations defined by this service serve as useful
hints to the application. However, the service is specifically
intended to be useful even if characterizations are not available.
or
The usefulness of this service depends strongly on the delivery of
both characterizations and the knowledge that all network elements
on the path support the service. Requests for this service when
characterizations are not available are likely to lead to
incorrect or misleading results.
are appropriate. It may also be useful to consider this point in the
"Examples of Use" section described below.
NOTE: The possibility of modifying the overall architecture to
provide information about the invoking protocol in a service request,
and to allow a service to require that the invocation protocol
support specific additional functionality, is an area of active
study.
o Policing
This portion of the service description describes the nature of
policing used to enforce adherence to a flow's Traffic Specification.
The specification document must specify the following points
- Expected policing action. This is the action taken when packets
not conforming to the TSpec are detected. Example actions include
relegating nonconforming packets to best effort, immediately
dropping nonconforming packets, delaying these packets until they
once again "fit" into the TSpec, or "marking" nonconforming packets
in some way.
- Legality of alternative policing actions. The section must
specify whether actions not specifically mentioned in
specification's description of policing behavior are legal. For
example, a service description which specifies that nonconforming
packets are to be dropped should state whether an alternate action,
such as delaying these packets, is acceptable.
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- Location of policing actions in the internetwork. The description
of policing must specify where that policing is done. Possibilities
include "at the edges of the network only", "at every hop",
"heterogeneous branch points" (points where the branches of a
multicast tree converge and have different TSpecs reserved
downstream), and "source merge points" (points where multiple data
streams covered by a single resource reservation converge). The
specification should clearly state requirements about topology
information (for example "this is an edge node" or "this is a
source merge point") which must be available from the setup
protocol or another source.
In this section the specification should also specify the legality
of policing at additional points in the network, beyond those
listed above. This is important due to technical effects such as
are described in the next paragraph.
Applicable additional technical considerations. If policing of data
flows is required or legal at points other than the flow's first
entry into the network, the service definition should describe any
additional technical considerations which affect the design of such
policing. For example, many potential services will allow a data
flow to become more bursty as it progresses through the network. If
such a service allows policing at points other than the network
edge, the traffic specification describing the flow will have to be
modified from that given by the application to the network to
account for this growing burstiness. Otherwise, it is likely that
the flow will be overpoliced, with packets being penalized
unnecessarily.
o Ordering and Merging
Ordering and merging come into play when a network element receives
several invocation requests covering the same data flow. As examples,
this could occur if several receivers of a multicast data flow
requested QoS services for that flow using the RSVP setup protocol,
or if a flow was subject to both a statically installed permanent
invocation request and a dynamic request from a resource setup
protocol.
In this situation the service module must be able to answer questions
about the ordering between different invocation requests, and must be
able to generate a single new invocation request which meets the
semantics of the setup protocol and the requirements of all the
original requesters. Operationally, this is achieved by having the
invoking protocol ask the service module, given a set of invocation
requests I1...In, to compute a new request which results in the
desired behavior.
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RFC 2216 Network Element Service Template September 1997
Five operations must be defined in this section. These are:
- Ordering. The section must define an ordering relationship
between the service's TSpecs and RSpecs. This may be a partial
ordering, in that some TSpecs or RSpecs may be unordered with
respect to each other.
- Summation. This function computes an invocation request which
represents the sum of N input invocation requests. Typically this
function is used to compute the size of a service request adequate
for a shared reservation for N different flows. It is desirable but
not required that this function compute the "least possible sum".
- Minimum. This function computes the minimum of two TSpecs.
Typically this function is used to compute the TSpec for an actual
service invocation given a target TSpec for the service request and
a TSpec for the flow's actual traffic pattern. The minimum function
must compute the smallest TSpec adequate to describe the minimum of
the requested TSpec and the flow's actual traffic.
- RSVP-Merge function. This function computes the invocation
request used to request service at an RSVP [RFC 2205] merge point.
The function must a) compute an appropriate invocation request for
a set of downstream reservations being merged, and b) generate
appropriate reservation parameters to be passed upstream by RSVP.
This function is described further below and in [RFC 2210].
- Least Common Request function. This function computes an
invocation request sufficient to provide service at least
equivalent to any one of the original requests passed to the
function. This function differs from the RSVP-merge function in
that it simply computes an upper bound. It does not need to compute
new invocation parameters to be passed upstream by RSVP and cannot
utilize the second option discussed in "Notes on RSVP Merging"
below.
oo Notes on Ordering
Typically the ordering relation will be described separately for the
service's TSpec and RSpec. An invocation request is ordered with
respect to another if and only if both its TSpec and its RSpec are
similarly ordered with respect to each other.
For TSpecs, the basic ordering relation is well defined. TSpec A is
substitutable for TSpec B if and only any flow that is compliant with
TSpec B is also compliant with TSpec A. The service specification
must explain how to compare two TSpecs to determine whether this is
true.
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For RSpecs, the ordering relation is dependent on the service. RSpec
A is substitutable for RSpec B if the quality of service invoked by
RSpec A is at least as good as the quality of service invoked by
RSpec B. Since there is no precise mathematical description of
"goodness" of quality of service, these ordering relations must be
spelled out explicitly in the service description.
oo Notes on RSVP Merging
The purpose of the RSVP merging function is to compute an invocation
request which will provide service to the merged flow at least
equivalent to that which any of the original requests would obtain
for its corresponding unmerged flow. This equivalence may be obtained
in two ways
1) The merged request may be computed as an upper bound on the set
of original (unmerged) invocation requests. In this case, the
service offered by the merged request to any particular traffic
flow is identical to that offered by the largest unmerged request,
by definition.
2) The merged request may be computed as a value smaller than the
upper bound on the set of original requests, but the results passed
upstream may restrict the traffic sources to behavior which makes
the merged and unmerged requests behave identically.
Note that the merging rules for a particular service may apply either
option 1 or option 2 to the different components of a TSpec, as
appropriate. The decision is typically made as follows:
When a downstream service module instance can tolerate a flow which
exceeds the parameter, the upper bound should be used. For example,
if the service supports policing to protect itself against excess
traffic, the traffic rate supported by a merged reservation might
be an upper bound across the traffic rates supported by each
unmerged reservation. The effect of this will be to install the
merged reservation at the local node and to inform each traffic
source of the largest traffic rate protected by reservation along
any *one* distribution path from the source to a receiver.
When a downstream service module instance will not function
properly if the parameter is exceeded, the merged function should
select the least agressive value of the parameter to install and
pass upstream. In this case, the traffic sources will be informed
of a parameter value which is appropriate for *all* distribution
paths traversed by the traffic flow. For example, services which
can handle packets of only limited size can incorporate packet size
in the TSpec, and treat the parmeter as described in option 2. The
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RFC 2216 Network Element Service Template September 1997
effect of this will be to limit packet sizes in the flow to those
which can be handled by every instance of the service along the
flow's path.
This merging calculation must be performed by the service module
because it is specific to a particular service.
oo Notes on Calculating Upper Bounds
Both the RSVP-Merge function and the Least Common Request function
may make use of calculated upper bounds on TSpec and RSpec
parameters.
The calculated upper bound need not be a least upper bound, nor do
the various network elements along the path need to all use the same
choice of upper bound. Any selection of invocation parameters Iu is
compliant as long as it substitutable for each of the parameters
I1...In from which it is calculated. Intuitively, one set of
parameters is substitutable for another if the resulting quality of
service is at least as desirable to all applications. A precise
definition of this "substitutable for" function; the ordering
relation, must be specified in the service definition. (It may be
specified as the empty set, in which case merging of dissimilar
requests will not be allowed). If the ordering function specified in
this section gives a partial order (if it is possible for two RSpecs
or TSpecs to be unordered), then a separate upper bound computation
for the parmeter must be given as well.
oo Notes on Service Substitution
This portion of the service description may also note any
relationships with other services which are strictly ordered with
respect to the service being defined. Two services A and B are
strictly ordered if it is always possible to substitute service B for
the service A given a set of invocation parameters for service A.
This ordering information may be used to allow network elements which
provide service B to respond to requests for service A, even if the
element does not provide service A directly. If the service
specification describes such an inter-service ordering, it MUST also
include a description of the invocation parameter mapping function
for that ordering.
Substitution of of one service for another in cases where they are
not strictly ordered is currently not supported. A future version of
this document may augment the service specification format to support
this capability.
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RFC 2216 Network Element Service Template September 1997
o Guidelines for Implementors
Many services may be defined in a manner which allows the range of
behavior of a compliant network element to be rather broad. This
section should provide some guidance as to what range of behaviors
the author of the service specification expects the community to
desire in their implementations. Because these guidelines depend on
such imprecise and undefinable notions at "typical loads", these
guidelines cannot be incorporated as part of a strict compliance
test. Instead, they are for informational purposes only.
o Evaluation Criteria
Specific functional behaviors required of an implementation for
conformance to a service specification is detailed in the previous
sections. However, the service specifications are intended to allow
a wide range of implementations, and these implementations will
differ in performance. This section describes tests that can be used
to evaluate a network element's implementation of a given service.
Implementors of service modules face a number of tradeoffs, and it is
unlikely that a single implementation would be considered "best"
under all circumstances. For instance, given the same service
specification, an implementation appropriate for a low-speed link
might target extremely high link utilization, while a different
implementation might attempt to reduce non-loaded packet forwarding
delay to the minimum at the expense of somewhat lower utilization of
the link. The intention of the tests specified in this section should
be to probe the tradeoffs made by the implementation designer, and to
provide metrics useful to guide the customer's choice of an
appropriate implementation for her needs.
The tests specified in this section should be designed to operate on
a single network element in isolation. This enables their use in a
comparative rating system for QoS-aware network elements. In
production networks, users will be more concerned with the end-to-end
behavior obtained, which will depend not just on the particular
network elements selected, but also on other factors such as the
setup protocol and the bandwidth of the links. Some user-relevant
performance factors are the rate of admission control rejections, the
range of services offered, and the packet delay and drop rates in the
various service classes. The form of any standardized end-to-end
metrics and measurement tools for integrated service internetworks is
not specified by this document or by service specification document
which follow the format given here.
This section is for informational purposes only.
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o Examples of Implementation
This section describes example instantiations of the service. Often
these will just be references to the literature, or brief sketches of
how the service could be implemented. The purposes of the section
are to to provide a more concrete sense of the service being
specified and to provide pointers and hints to aid the implementor.
However, the descriptions in this section are specifically not
intended to exclude other implementation strategies.
This section is for informational purposes only.
o Examples of Use
In order to provide more a more concrete sense of how this service
might be used, this section describes some example uses of the
service, for informational purposes only. The examples here are not
meant to be exhaustive, and do not exclude in any way other uses of
the service.
This section is for informational purposes only.
Security Considerations
Security considerations are not discussed in this memo.
References
[PARTRIDGE] C. Partridge, Gigabit Networking, Addison Wesley
Publishers (1994).
[RFC 2215] Shenker, S., and J. Wroclawski, "General Characterization
Parameters for Integrated Service Network Elements", RFC 2215,
September 1997.
[RFC 2205] Braden, R., Ed., et. al., "Resource Reservation Protocol
(RSVP) - Version 1 Functional Specification", RFC 2205, September
1997.
[RFC 2212] Shenker, S., Partridge, C., and R. Guerin, "Specification
of Guaranteed Quality of Service", RFC 2212, September 1997.
[RFC 2211] Wroclawski, J., "Specification of the Controlled Load
Quality of Service", RFC 2211, September 1997.
[RFC 1819] Delgrossi, L., and L. Berger, Editors, "Internet Stream
Protocol Version 2 (ST2) Protocol Specification - Version ST2+", RFC
1819, August 1995.
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RFC 2216 Network Element Service Template September 1997
[RFC 2210] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC 2210, September 1997.
Authors' Address:
Scott Shenker
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304-1314
Phone: 415-812-4840
Fax: 415-812-4471
EMail: shenker@parc.xerox.com
John Wroclawski
MIT Laboratory for Computer Science
545 Technology Sq.
Cambridge, MA 02139
Phone: 617-253-7885
Fax: 617-253-2673
EMail: jtw@lcs.mit.edu
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RFC, FYI, BCP