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rfc:rfc2216

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

Shenker & Wroclawski Informational [Page 1] RFC 2216 Network Element Service Template September 1997

 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

Shenker & Wroclawski Informational [Page 2] RFC 2216 Network Element Service Template September 1997

 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

Shenker & Wroclawski Informational [Page 3] RFC 2216 Network Element Service Template September 1997

 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.

Shenker & Wroclawski Informational [Page 4] RFC 2216 Network Element Service Template September 1997

 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.

Shenker & Wroclawski Informational [Page 5] RFC 2216 Network Element Service Template September 1997

 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

Shenker & Wroclawski Informational [Page 6] RFC 2216 Network Element Service Template September 1997

 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:
  1. Type: whether the quantity is an enumeration, a numerical value,

etc.

  1. 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.
  1. 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.

Shenker & Wroclawski Informational [Page 7] RFC 2216 Network Element Service Template September 1997

    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:
   <service_name>.<parameter_name>.
 Each of these elements is a integer numerical quantity.
 <Service Name> 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.

Shenker & Wroclawski Informational [Page 8] RFC 2216 Network Element Service Template September 1997

 <Parameter_name> 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 <service_number>.<parameter_number> 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:
  1. End-to-End Behavior [required] [informational]
  1. Motivation [required] [informational]
  1. Network Element Data Handling Requirements [required] [normative]

Shenker & Wroclawski Informational [Page 9] RFC 2216 Network Element Service Template September 1997

  1. Invocation Information [required] [normative]
  1. Exported Information [required] [normative]
  1. Policing [required] [normative]
  1. Ordering and Merging [required] [normative]
  1. Guidelines for Implementors [optional] [informational]
  1. Evaluation Criteria [required] [informational]
  1. Examples of Implementation [optional] [informational]
  1. 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.

Shenker & Wroclawski Informational [Page 10] RFC 2216 Network Element Service Template September 1997

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

Shenker & Wroclawski Informational [Page 11] RFC 2216 Network Element Service Template September 1997

 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.

Shenker & Wroclawski Informational [Page 12] RFC 2216 Network Element Service Template September 1997

    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.

Shenker & Wroclawski Informational [Page 13] RFC 2216 Network Element Service Template September 1997

 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

Shenker & Wroclawski Informational [Page 14] RFC 2216 Network Element Service Template September 1997

 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
  1. 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.
  1. 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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  1. 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.

Shenker & Wroclawski Informational [Page 16] RFC 2216 Network Element Service Template September 1997

 Five operations must be defined in this section. These are:
  1. 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.
  1. 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".
  1. 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.
  1. 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].
  1. 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.

Shenker & Wroclawski Informational [Page 17] RFC 2216 Network Element Service Template September 1997

 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

Shenker & Wroclawski Informational [Page 18] 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.

Shenker & Wroclawski Informational [Page 19] 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.

Shenker & Wroclawski Informational [Page 20] RFC 2216 Network Element Service Template September 1997

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.

Shenker & Wroclawski Informational [Page 21] 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

Shenker & Wroclawski Informational [Page 22]

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