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Network Working Group                                         S. ShenkerRequest for Comments: 2216                                 J. WroclawskiCategory: Informational                               Xerox PARC/MIT LCS                                                          September 1997Network Element Service Specification TemplateStatus 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 alsoShenker & 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 provideShenker & 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 networkShenker & 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 beShenker & 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:     - 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.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:     - End-to-End Behavior [required] [informational]     - Motivation [required]  [informational]     - Network Element Data Handling Requirements [required] [normative]Shenker & Wroclawski         Informational                      [Page 9]

RFC 2216            Network Element Service Template      September 1997     - 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.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 dataShenker & 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 andShenker & 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     - 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.Shenker & Wroclawski         Informational                     [Page 15]

RFC 2216            Network Element Service Template      September 1997     - 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:     - 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.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. TheShenker & 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+",RFC1819, 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.eduShenker & Wroclawski         Informational                     [Page 22]