Internet Engineering Task Force Integrated Services WG
INTERNET-DRAFT S. Shenker/J. Wroclawski
draft-ietf-intserv-svc-template-01.txt Xerox PARC/MIT LCS
June, 1995
Expires: 12/25/95
Network Element Service Specification Template
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
This draft is a product of the Integrated Services Working Group of
the Internet Engineering Task Force. Comments are solicited and
should be addressed to the working group's mailing list at int-
serv@isi.edu and/or the author(s).
Abstract
This document defines a format for specifying services provided by
network elements, and available to applications, in a network
which offers multiple classes of service. The document provides
necessary context, including definitions, data formats, and
interfaces; 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
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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 format used to specify the functionality of
internetwork system components related to "Integrated Services"; the
ability to provide multiple, dynamically selectable qualities of
service to applications using the 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, as specified by this document, 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 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.
The service definition does not describe the protocols or mechanisms
used to establish state in the network elements for flows that use
the described service, but may specify information which must be
carried between end-nodes and network elements by those mechanisms.
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.
Definitions
The following terms are used throughout this document. Service
specification documents should employ the same terms to express these
concepts.
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o Quality of Service (QoS)
In the context of this document, quality of service refers to the
suitability of a packet delivery service for the needs of a
particular application, as defined 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 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-aware" network element is one
which offers one or more of the services defined according to the
format of this document. Note that this definition does not by itself
preclude QoS-aware network elements which meet performance goals
purely through adequate provisioning, rather than active control
mechanisms.
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.
Mechanisms used to associate a request for quality of service control
with the packets covered by that request are beyond the scope of this
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document.
o Service
The word "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 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 a "service module"
contained within the network element. A network element may and
generally will contain more than one service module and hence offer
more than one service.
NOTE: The above defines a precise meaning for the word "service";
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
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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.
Several characterizations may be computed for a single candidate data
flow. Conversely, characterizations are not mandatory, 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 specification of the traffic
pattern which a flow expects to exhibit. 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 -expected- 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).
Traffic specifications are most frequently created by the originator
of the data flow.
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o Service Request Specification (RSpec)
A Service Request Specification, or RSpec, is a specification of the
quality of service a flow desires from a network element. The form 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. Service request specifications may originate from either the
sender or receiver(s) of a data flow.
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 [xxx], ST-II [xxx] and Q.2931 [xxx].
o Token Bucket
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.
Token buckets are further discussed in [xxx].
o Token Bucket Filter
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 discarding of the packet or marking the packet in
some fashion.
NOTE: The definition of token bucket in this document does not
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allow for token bucket filters with a "backing buffer" which
queues packets that do not immediately pass through the filter.
The intent is to cleanly distinguish the bufferless case from the
buffer case through the use of different names; however this draft
does not yet provide a definition for the second case.
o Admission Control
Admission control is the process of deciding whether a newly arriving
request for service from a network element can be granted, or must be
refused due to scarcity of resources. 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 the locations in the
network where discrepancies are to be detected and the action to be
taken when such discrepancies occur. Examples of such actions might
include dropping packets, reshaping the traffic, or marking non-
conforming traffic for later discard if necessary.
Data Format and Representation
Each service module will import and export a variety of data
according to its specific requirements. The 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
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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. 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.
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.
Interfaces
The service module conceptually interacts with other portions of the
network element through a number of interfaces. These interfaces are
documented in [xxx; the non-existent new IS - Overview RFC]. The
service specification document should clearly define each 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.
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
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document.
o Components
The body of a service specification document incorporates the
following sections:
- Motivation [required]
- Network Element Data Handling Requirements [required]
- Invocation Information [required]
- Exported Information [required]
- Policing [required]
- Ordering and Merging [required]
- Resulting Service [required]
- Guidelines for Implementors
- Evaluation Criteria [required]
- Examples of Implementation
- Examples of Use
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.
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
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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 the
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 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. These two components of the
invocation information should be specified separately and
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independently, as they will often be generated and transported 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 portions 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. A service which defines
characterization parameters also specifies the characterizations they
are used to generate and the composition functions used to generate
the characterizations.
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 soley 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
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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 that would most
likely come from either the setup protocol or the routing protocol.
The issue of exactly how, or if, a specific protocol (e.g., RSVP)
uses characterization parameters to generate characterizations should
be described in the specification of that specific protocol.
There is no requirement that setup and routing protocols use the
characterization parameters supplied by service modules, and there is
no 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.
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 fully 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.
It may be useful to define "generic" characterization parameters not
associated with any specific service. These might include the
speed-of-light latency of communication links (additive composition
rule) and available link bandwidth (minimum composition rule).
Eventually, these generic parameters should be defined in the
Integrated Services overview document, and incorporated by reference
in the Router Requirements document and the relevant service
specification documents.
Characterization parameters are named, so that protocols which
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transport and use these parameters can uniquely identify them. The
namespace is a two-level hierarchy: <service_name>.<parameter_name>;
each of these elements is a numerical quantity. This same namespace
is used to name the results of composition functions which are made
available to end-nodes. The service specification author may also
choose to name intermediate data which might be transported as part
of a distributed composition function computation. The reason for
assigning names to these data objects is to support a variety of
styles for calculating the composition function, and to ensure that
the end-nodes can clearly determine the meaning of data delivered to
them as part of a characterization.
- <Service Name> is a 16-bit number. The number space is broken
into two regions. The region below 32768 (high bit 0) is managed by
the IANA. 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 is a published RFC following the format
described in this note.
Numbers in the region above 32768 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 appropriate in the
future.
- <Parameter_name> is a 16-bit number assigned on a per-service
basis. Numbers are allocated by the author of the service
specification document.
[ Is 16 bits too big? Could be 8.8, or a non-byte split such as
6.10 ]
Service Number 0 (zero) is reserved for the "generic" parameters
described above. Parameter numbers within this space are initially
allocated by the INT-SERV WG, and may in the future be allocated by
IANA.
These <service_number>.<parameter_number> pairs should be used as
last two levels of the MIB name when the parameters are made
available to network management protocols.
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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
immediately dropping nonconforming packets, delaying these packets
until they once again "fit" into the TSpec, or "marking"
nonconforming packets in some way, to enable dropping them
preferentially if a later network element in the marked packet's
path should be overloaded.
- 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.
- 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", 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
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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 service 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
requirements of all the original requesters. This new request is a
"merged" request. It may be equal to one of the original requests, or
it may be a newly computed request. Operationally, this is achieved
by having the setup protocol ask the service module, given a set of
requests R1...Rn, to compute an merged request which which results in
service to the merged flow at least equivalent to that which any of
the original requests would obtain for its corresponding unmerged
flow. Hence, the merged invocation request represents an "upper
bound" on the set of original invocation requests. This upper bound
calculation must be performed by the service module because it is
specific to a particular service.
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 Ru is
compliant as long as it substitutable for each of the parameters
R1...Rn 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). A merging computation (upper bound
calculation) MAY be given in the document as well.
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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.
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.
This portion of the service description may also note any ordering
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 SHOULD
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.
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. This section should be as detailed as
possible. There are certain services where the end-to-end behavior
is a highly nontrivial result of the concatenation of per-hop
services; one purpose of this section is that such nontrivial results
are not left unrevealed to the reader of this document.
In private networks it will generally be the case that the required
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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.
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
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.
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are appropriate. It may also be useful to consider this point in
the "Examples of Use" section described below. ]
NOTE: The possibility of adopting alternative wording for this
document which allows a service to require that the invocation
protocol support specific additional functionality or return an
error is a topic open to discussion.
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
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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.
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.
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.
Security Considerations
Security considerations are not discussed in this memo.
Authors' Address:
Scott Shenker
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304-1314
shenker@parc.xerox.com
415-812-4840
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415-812-4471 (FAX)
John Wroclawski
MIT Laboratory for Computer Science
545 Technology Sq.
Cambridge, MA 02139
jtw@lcs.mit.edu
617-253-7885
617-253-2673 (FAX)
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