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

   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

   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

   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.

   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.

   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

   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.

      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.

   <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:

     - End-to-End Behavior [required] [informational]

     - Motivation [required]  [informational]

     - Network Element Data Handling Requirements [required] [normative]

     - 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.

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

   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.

      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.

   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

   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.

     - 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.

   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.

   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

     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.

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.

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.

   [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