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Internet Engineering Task Force                   Integrated Services WG
INTERNET-DRAFT                           F. Baker/ R. Guerin/ D. Kandlur
draft-ietf-intserv-commit-rate-svc-00.txt                  CISCO/IBM/IBM
                                                              June, 1996
                                                        Expires: 9/15/96



           Specification of Committed Rate Quality of Service


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   This document is a product of the Integrated Services working group
   of the Internet Engineering Task Force.  Comments are solicited and
   should be addressed to the working group's mailing list at int-
   serv@isi.edu and/or the author(s).


Abstract

   This document describes the network element behavior required to
   deliver a Committed Rate service in the Internet.  The Committed Rate
   service provides applications with a firm commitment from the
   network, that at a minimum the transmission rate they requested is
   available to them at each network element on their path.  The
   commitment of a given transmission rate by a network element is not
   associated with a specific delay guarantee, but requires that network
   elements perform admission control to avoid over-allocation of
   resources.





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Introduction

   This document defines the requirements for network elements that
   support a Committed Rate service.  This memo is one of a series of
   documents that specify the network element behavior required to
   support various qualities of service in IP internetworks.  Services
   described in these documents are useful both in the global Internet
   and private IP networks.

   This document is based on the service specification template given in
   [1]. Please refer to that document for definitions and additional
   information about the specification of qualities of service within
   the IP protocol family.


End-to-End Behavior

   The end-to-end behavior provided to an application by a series of
   network elements that conform to the service described in this
   document is a committed transmission rate that, when used by a
   policed flow, ensures that its packets are transmitted with no or
   minimal queueing losses through the network (assuming no failure of
   network components or changes in routing during the life of the
   flow).  In addition, while this service does not provide any specific
   delay guarantees, the provision of a committed transmission rate at
   each network element should ensure that packets do not experience
   delays at a network element that are significantly in excess of what
   they would experienced from a dedicated transmission facility
   operating at the committed rate.

   To ensure that this service is provided, clients requesting Committed
   Rate service provide the network elements with both the transmission
   rate they want to have guaranteed and information on their traffic
   characteristics.  Traffic characteristics are specified in the TSpec
   of the flow, which is defined in the section on Invocation
   Information below.  In return, the network elements perform admission
   control to allocate enough resources (bandwidth and buffer) to ensure
   that over some reasonable time interval, a flow with packets waiting
   to be transmitted sees a transmission rate at least equal to its
   requested transmission rate and only experiences packet losses very
   rarely.


Motivation

   The Committed Rate service is intended to offer applications a
   service that provides them with the guarantee that the network will
   commit a certain amount of bandwidth to them in an attempt to emulate



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   a dedicated circuit of at least that amount of bandwidth.  This
   service is intended for applications that require a given amount of
   bandwidth in order to operate properly.  This bandwidth can be
   related to an intrinsic rate at which the applications generates
   data, e.g., the average transmission rate of a video codec, or chosen
   so as to allow the transmission of a certain amount of data within a
   reasonable time, e.g., a function of the size of the data object to
   be transmitted and how fast it needs to be received.

   The rate guarantees provided by the Committed Rate service are in a
   sense similar to those provided by the Guaranteed Service [2].
   However, a key difference is that they are not coupled to the same
   rigorous delay guarantees provided by the Guaranteed Service.  This
   decoupling simplifies the invocation of the service and its support
   at intermediate network elements as the service is only a function of
   local resources at each node and, therefore, independent of the end-
   to-end characteristics of the path itself.  In addition, the
   relaxation of the delay guarantees to be provided, can allow a higher
   utilization of network resources, e.g., bandwidth.  However, note
   that this greater simplicity and higher efficiency come at a cost,
   namely


      - The lack of hard delay guarantees.  This is because the
      commitment of a transmission rate at a network element can be
      provided through a range of mechanisms, that correspond to
      different delay behaviors (see the section on Network Element Data
      Handling Requirements for additional details).  Specifically,
      depending on the characteristics of the implementation used to
      support the Committed Rate service at network elements, the worst
      case delay experienced by packets receiving this service could be
      much higher than under Guaranteed service, i.e., the delay bounds
      are relaxed.

      - The lack of a priori estimates of the end-to-end delay to be
      expected.  This is because rate guarantees are local to each
      network element, and hence do not provide any end-to-end delay
      characterization for the path on which the flow is routed.

      - Weaker loss guarantees as the lack of characterization of the
      behavior of individual network elements also means, that accurate
      sizing of buffer requirements to ensure lossless operation cannot
      be provided.

   The Committed Rate service is also different from the Controlled-Load
   service [3] in that it allows policing at the edge of the network and
   reshaping at intermediated network elements.  Note that the emulation
   of a dedicated circuit at the requested committed rate, can amount to



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   reshaping the flow of packets to this rate.  In addition, contrary to
   the Controlled-Load service that specifies that the transit delay
   experienced by most packets should be close to the minimum transit
   delay, the Committed Rate service only guarantees that the transit
   delay of any packet should not significantly exceed what it would
   have experienced over a dedicated circuit at the committed rate.

   The Committed Rate service can, therefore, be viewed as an
   intermediate service level in between the Guaranteed Service and the
   Controlled Load service.  It provides weaker service guarantees than
   the Guaranteed Service, but imposes fewer constraints on network
   elements.  This may facilitate deployment in an heterogeneous
   environment, where not all network elements may be capable of
   satisfying the requirements of the Guaranteed Service.  Similarly,
   the provision of a fixed rate service guarantee may be less flexible
   than the Controlled-Load service for adaptive applications, but may
   simplify the call admission and scheduling functions at network
   elements.  In particular, the buffer and bandwidth allocation
   functions may benefit from the stricter traffic (TSpec)
   specification.

Network Element Data Handling Requirements

   The network element must ensure that the service approximates a
   dedicated circuit of rate at least equal to the requested rate R.
   This means that the network element must perform call admission to
   ensure the availability of sufficient bandwidth to accommodate a
   flow's request for a transmission rate R.  This will typically mean
   allocating an amount of link bandwidth at least equal to R.  However,
   note that this service specification does not require that a network
   element provide an application with a transmission rate greater than
   R even when there is excess bandwidth available, i.e., reshaping of
   the traffic to the rate R is allowed.  More generally, approximating
   a dedicated rate circuit of rate at least R only implies, that if for
   a period of time T an application has data packets waiting to be
   transmitted at a network element, i.e., it is backlogged, the amount
   of data it is able to send during T should approach RT as T grows
   large.  The smaller the value of T for which this is achieved, the
   better the approximation of a dedicated circuit at rate R.

   Specifically, the difference between the service provided at a
   network element and a dedicated rate circuit is a function of the
   scheduler used at the service element and also reflects the impact of
   the packetized nature of transmission units.  Many recently proposed
   schedulers, e.g., Weighted Fair Queueing (WFQ) [4], Virtual Clock
   [5], Rate Controlled Service Disciplines (RCSD) [6,7], Latency Rate
   Servers [8], etc., can provide reasonably good approximations for
   such a service, i.e., typically with a value for T of the order of



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   L/R, where L is the size of the packet to be transmitted. On the
   other hand, other simpler schedulers such as FIFO, static priorities,
   frame based schedulers [9], may only provide relatively coarse
   approximations.  While this service specification does not mandate
   the use of a particular type of scheduler, the nature of its service
   definition, i.e., the close approximation of a dedicated rate
   circuit, means that the use of schedulers that perform well in
   regards to this measure is recommended.

   In addition, the manner in which a network element approximates a
   dedicated rate circuit will impact the amount of buffering it needs
   to provide to ensure minimum losses for a compliant flow.  For
   example, a network element for which the scheduler can delay
   transmission of packets from a given flow for a time period T, may
   need to buffer up to b+rT amount of data for that flow, where b
   corresponds to the token bucket depth advertised in the TSpec of the
   flow and r is the associated token bucket rate (see next section on
   Invocation Information for details).  It is, therefore, expected that
   each network element allocates, when accepting a new flow, not only
   enough bandwidth to accommodate the requested rate, but also
   sufficient buffer space to provide compliant flows with minimal
   losses.  Note that it is possible for a network element to trade
   bandwidth for buffer space, i.e., allocate to each flow more
   bandwidth than it requests, so as to ensure a low enough load to keep
   buffer requirements low.  The necessary amount of buffer space is a
   quantity to be engineered for each network element.  For example,
   edge network elements may need to allocate b or more to each flow
   (this depends in part on how much larger R is than r), while core
   network elements may be able to take advantage of statistical
   multiplexing to allocate much less.

   Links are not permitted to fragment packets as part of the Committed
   Rate service.  Packets larger than the MTU of the link must be
   policed as non-conformant which means that they will be policed
   according to the rules described in the Policing section below.

   Packet losses due to non-congestion-related causes, such as bit
   errors are not accounted for by this service.

Invocation Information

   The Committed Rate service is invoked by specifying the traffic
   (TSpec) of the flow and the desired Committed Rate (RSpec) to the
   network element.  A service request for an existing flow that has a
   new TSpec and/or RSpec should be treated as a new invocation, in the
   sense that admission control must be reapplied to the flow.  Flows
   that reduce their TSpec and/or RSpec (i.e., their new TSpec/RSpec is
   strictly smaller than the old TSpec/RSpec according to the ordering



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   rules described in the section on Ordering below) should never be
   denied service.

   The TSpec takes the form of a token bucket plus a peak rate (p), a
   minimum policed unit (m), and a maximum packet size (M).

   The token bucket has a bucket depth, b, and a bucket rate, r, which
   corresponds to the requested rate that the flow is requesting the
   network to commit.  Both b and r must be positive.  Note that it is
   necessary to habe b>=M.  The rate, r, is measured in bytes of IP
   datagrams per second, and can range from 1 byte per second to as
   large as 40 terabytes per second (or about what is believed to be the
   maximum theoretical bandwidth of a single strand of fiber).  Network
   elements MUST return an error for requests containing values outside
   this range. Network elements MUST return an error for any request
   containing a value within this range which cannot be supported by the
   element. In practice, only the first few digits of the r parameter
   are significant, so the use of floating point representations,
   accurate to at least 0.1%, is encouraged.

   The bucket depth, b, is measured in bytes and can range from 1 byte
   to 250 gigabytes. Network elements MUST return an error for requests
   containing values outside this range. Network elements MUST return an
   error for any request containing a value within this range which
   cannot be supported by the element. In practice, only the first few
   digits of the r parameter are significant, so the use of floating
   point representations, accurate to at least 0.1%, is encouraged.

   The range of values for these parameters is intentionally large to
   allow for future network and transmission technologies.  This range
   is not intended to imply that a network element must be capable of
   supporting the entire range of values.

   The peak rate, p, is measured in bytes of IP datagrams per second and
   has the same range and suggested representation as the bucket rate.
   The peak rate is the maximum rate at which the source and any
   reshaping points (reshaping points are defined below) may inject
   bursts of traffic into the network.  More precisely, it is the
   requirement that for all time periods the amount of data sent cannot
   exceed M+pT where M is the maximum packet size and T is the length of
   the time period.  Furthermore, p must be greater than or equal to the
   token bucket rate, r.  A peak rate value of 0 means the peak rate is
   not being used or is unknown.

   The minimum policed unit, m, is an integer measured in bytes.  All IP
   datagrams less than size m will be counted, when policed and tested
   for conformance to the TSpec, as being of size m. The maximum packet
   size, M, is the biggest packet that will conform to the traffic



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   specification; it is also measured in bytes.  A network element must
   reject a service request, if the requested maximum packet size is
   larger than the MTU of the link.  Both m and M must be positive, and
   m must be less than or equal to M.

   The RSpec consists of the desired service rate R.  The motivations
   for separating the specification of the rate R from the the token
   bucket rate in the TSpec, are to provide greater flexibility in the
   level of service a receiver can request and in simplifying support
   for shared reservations.  With shared reservations, a receiver can
   request a certain Committed Rate R from the network, that may not be
   directly related to the token bucket rates specified in the TSpec of
   the different flows that are to share the reservation.

   The preferred representation for the TSpec consists of three floating
   point numbers in single-precision IEEE floating point format followed
   by two 32-bit integers in network byte order.  The first value is the
   rate (r), the second value is the bucket size (b), the third is the
   peak rate (p), the fourth is the minimum policed unit (m), and the
   fifth is the maximum packet size (M).

   The preferred representation for the RSpec rate, R, is also in
   single-precision IEEE floating point format.

   For all IEEE floating point values, the sign bit must be zero. (All
   values must be positive).  Exponents less than 127 (i.e., 0) are
   prohibited.  Exponents greater than 162 (i.e., positive 35) are
   discouraged.

   The Committed Rate service is assigned service_name 6.

   The Committed Rate traffic specification parameter (TSpec) is
   assigned parameter_name 1, as indicated in the listing of well-known
   parameter name assignments given in [1].


Exported Information


   The Committed Rate service has no required characterization
   parameters. Individual implementations may export appropriate
   implementation-specific measurement and monitoring information.


Policing and Reshaping


   Policing and reshaping are two related forms of traffic control, that



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   are meant to limit the amount of traffic that an application can
   inject into the network.  In either cases, the result is that only
   conformant packets are forwarded.  Conformance is determined as a
   function of the TSpec for the flow.  A flow is deemed conformant if
   the amount of data it sent during any given time period of duration T
   does not exceed M+min[pT, rT+b-M], where p is the peak rate, r and b
   are the token bucket parameters, and M is the maximum packet size for
   that flow. For the purposes of this accounting, links must count
   packets which are smaller than the minimal policing unit to be of
   size m.  Packets which arrive at an element and cause a violation of
   the M+min[pT, rT+b-M] bound are considered non-conformant.
   Additionally, packets bigger than the outgoing link MTU are
   considered non-conformant.  It is expected that such a situation will
   typically not arise, because flow setup mechanisms are expected to
   notify the sending application of the appropriate path MTU.

   Policing and reshaping differ in their treatment of non-conformant
   packets.  Policing performs a strict control on the traffic of a
   given flow by either discarding non-conformant packets, or possibly
   sending them as best effort packets.  Note that if and when a marking
   ability becomes available, non-conformant packets sent as best-effort
   packets SHOULD be ''marked'' as being non-compliant so that they can
   be treated as best effort packets at all subsequent network elements.
   In the context of the Committed Rate service, policing should ONLY be
   performed at the edge of the network, where it is used to ensure
   conformance of the user traffic with the TSpec it advertised.

   On the other hand, the strict traffic control implied by policing is
   NOT appropriate inside the network, since the perturbations caused by
   the queueing and scheduling delays at network elements will often
   turn an initially conformant flow into a non-conformant one.
   Instead, it is recommended that reshaping be used at intermediate
   network elements inside the network.  Reshaping amounts to delaying
   (buffering) non-conformant packets until they are compliant, rather
   than discard or send them as best-effort.  Reshaping, therefore,
   restores the traffic characteristics of a flow to conform to the
   specified token bucket and peak rate parameters used by the reshaper.
   (To avoid delaying unnecessarily the initial packets of a flow, the
   token bucket at a reshaper should be initialized full).

   The benefit of restoring a flow to its original envelope is that it
   limits the magnitude of the "distortions" that schedulers at network
   elements can introduce in the initial stream of packets from a flow.
   As discussed in the section on Network Element Data Handling
   Requirements, depending on how well a scheduler approximates a
   dedicated rate circuit, significant bunching up of packets can be
   introduced. This translates in turn into bigger buffer requirements
   at downstream network elements.  Reshaping the flow ensures that



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   downstream network elements are isolated from the bunching effects
   introduced by upstream schedulers.  However, note that in order to
   achieve these benefits, the reshapers must provide sufficient buffer
   space to hold packets until they can be released as compliant with
   the traffic envelope to which the flow is being reshaped.

   Contrary to the Guaranteed Service where the information exported by
   network elements allows the computation of an upper bound on the
   amount of buffer needed when reshaping traffic, in the context of the
   Committed Rate service this quantity can only be estimated.  The
   required amount to ensure minimum packet losses to Committed Rate
   flows is, therefore, a quantity to be engineered for each network
   element.

   If a packet arrives at a reshaper and finds the reshaping buffer
   full, the packet can either be discarded or accommodated by
   forwarding as best effort a packet of the flow.

      NOTE: As with policers, it should be possible to configure how
      reshapers handle packets that arrive to a full reshaping buffer.
      If such cases are to be handled by forwarding a packet as best
      effort, reshaping points may wish to forward a packet from the
      front of the reshaping queue, in order to minimize packet
      reordering problems at the receiver(s).


Ordering and Merging

   TSpec's are ordered according to the following rule: TSpec A is a
   substitute ("as good or better than") for TSpec B if

      (1) both the token bucket depth and rate for TSpec A are greater
      than or equal to those of TSpec B,

      (2) the minimum policed unit m is at least as small for TSpec A as
      it is for TSpec B,

      (3) the maximum packet size M is at least as large for TSpec A as
      it is for TSpec B,

      (4) the peak rate p is at least as large in TSpec A as it is in
      TSpec B.

   A merged TSpec may be calculated over a set of TSpec's by taking the
   largest token bucket rate, largest bucket size, largest peak rate,
   smallest minimal policed unit, and largest maximum packet size across
   all members of the set.  This use of the word "merging" is similar to
   that in the RSVP protocol; a merged TSpec is one which is adequate to



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   describe the traffic from any one of a number of flows.

   RSpec's are merged in a similar manner as the TSpec's, i.e., a set of
   RSpec's is merged onto a single RSpec by taking the largest rate R of
   all RSpec's in the set.

      NOTE:  In case of shared reservations, i.e., a single RSpec whose
      rate, R, is to be shared between a number of flows, it is
      important to chose the requested rate R so as to ensure stable
      operation.  Selection of the appropriate value is an application
      level decision, but two general cases may be considered.  In the
      first case, the token bucket rates advertized by the senders
      sharing the reservation correspond to their average rate while
      active, e.g., the average rate for a voice signal when the speaker
      is talking.  In such situations, the requested service rate R may
      be chosen to be significantly less than the sum of the token
      bucket rates of all the flows sharing the reservation.   For
      example, this would apply to an audio conference call where only
      one speaker will typically be active at any time.  In the second
      case, the token bucket rates advertized by the senders sharing the
      reservation correspond to their true long term average rate.  In
      that case it is important that the requested service rate R be
      chosen larger than the sum of the token bucket rates of all the
      flows sharing the reservation.


Guidelines for Implementors

   This section reviews two important implementation aspects of a
   Committed Rate service, that are closely related.

      (1) The approximation of a dedicated rate circuit,

      (2) The allocation of sufficient buffering to ensure minimal
      losses.

   As mentioned in the section on Network Element Data Handling
   Requirement, support for the Committed Rate service requires that the
   network element approximates as well as possible the behavior of a
   dedicated rate circuit.  This means, assuming a requested rate R,
   that whenever a flow is backlogged (has packets waiting to be
   transmitted) at a network element, it should ideally be able to
   transmit the packet at the head of its queue within at most L/R time
   units, where L is the size in bits of the packet.  The ability of a
   network element to achieve this behavior depends on the type of
   scheduler it uses. While simple schedulers such as FIFO or priority
   queues may be used, it is highly recommended that an implementation
   of Committed Rate rely on a scheduler capable of providing service



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   guarantees to individual connections.  As mentioned in the section
   below on Examples of Use, there are a number of available schedulers
   that provide such capabilities.

   In addition to choosing an appropriate scheduling algorithm, an
   implementation of the Committed Rate service at a network element
   also requires the use of an admission control algorithm.  Admission
   control is required to ensure that the network element resources,
   i.e., bandwidth and buffers, are not over-committed.  Admission
   control is to be performed based on the TSpec and RSpec specified by
   each flow requesting the Committed Rate service.  The RSpec
   identifies the amount of bandwidth that the flow is requesting, and
   which should, therefore, at a minimum be available on the
   corresponding outgoing link from the network element.  The exact
   amount of bandwidth to be allocated to the flow by the call admission
   algorithm depends on both the scheduler and the buffering scheme
   used. It is, therefore, a quantity to be engineered for each network
   element.

   The admission control algorithm is also responsible for ensuring that
   sufficient buffer space is available to accommodate a new request.
   This is a function of the TSpec of the flow, in particular the peak
   rate, p, and the token bucket depth, b, but in general depends on a
   number of factors:

      - The token bucket and peak rate parameters and the requested
      service rate R, i.e., how fast and for how long can the flow be
      sending at a rate exceeding the transmission rate R that has been
      allocated to it.

      - The amount of statistical multiplexing that is expected at the
      network element, i.e., how likely is it that all flows
      simultaneously need the maximum possible amount of buffers.

      - The perturbations introduced by schedulers at upstream network
      elements since the last reshaping point, i.e., how much bunching
      of packets is likely to have been introduced.

   For example, assuming a scheduler that approximates reasonably
   closely the behavior of a dedicated rate circuit, e.g., a WFQ
   scheduler, a possible buffer allocation rule for a flow with given
   TSpec and committed rate R, is to ensure that the network element is
   able to buffer an amount of data of the order of b(p-R)/(p-r), where
   b is the token bucket depth, r the token bucket rate, and p the peak
   rate.  Depending on the amount of statistical multiplexing expected
   at the network element, this does NOT necessarily imply that this
   amount of buffers has to be dedicated to the flow, i.e., buffer
   allocation is not necessarily additive in the number of flows even if



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   this represents a simple and somewhat conservative rule.

   However, note that the amount of buffering needed for flow at a
   network element also depends, to some extent, on the behavior of
   upstream (previous) network elements.  For example, assuming that the
   scheduler at the previous network element did not approximate well
   the behavior of a dedicated rate circuit, e.g., could delay
   transmission of packets of a flow for a time period of duration T,
   the next (downstream) network element may then have to buffer the
   entire amount b+rT, if at the end of the period T this gets
   transmitted at at a speed much higher than the rate R.  Hence, as
   mentioned before, even though the Committed Rate service
   specification does not mandate a particular type of scheduler, it
   encourages the use of schedulers that approximates as closely as
   possible a dedicated rate circuit, so as to minimize buffering
   requirements at downstream network elements.


Evaluation Criteria

   The scheduling algorithm and admission control algorithm of the
   element must ensure that the requested committed rate is provided
   over some reasonably long time period, and that packets from a
   compliant flow are rarely lost.

   The closer a network element approximates the behavior of a dedicated
   circuit at the requested committed rate, the better it performs in
   supporting the Committed Rate service.

   This behavior can be evaluated by continuously sending packet into
   the network at the maximum possible rate allowed while remaining
   conformant, and by monitoring the delay experienced when traversing a
   series of network elements.  The lower the average delay and its
   variations, i.e., difference between the maximum and minimum values,
   as experienced by the packets, the higher the evaluation ratings for
   the service.  In addition, the smaller the value of the time period
   needed to transmit the amount of data corresponding to the Committed
   Rate, the higher the evaluation ratings for the service.

   This behavior should also be consistently provided to flows accepted
   by the admission control algorithm, independently of the load levels
   at network elements.  This should be tested by increasing the
   background best effort traffic on the network elements as well as by
   increasing, up to the maximum number allowed by the call admission
   algorithm of each network element, the number of Committed Rate flows
   being carried.  The smaller the worst case values for the delay
   experienced by Committed Rate service flows across the range of load
   conditions, the higher the evaluation ratings for the service.



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   Additionally, users may want to evaluate, when applicable, the
   behavior of the Committed Rate service at a network element, when
   provided jointly with some other services whose more rigorous service
   requirements may affect the level of service given to Committed Rate
   flows.  For example, this may apply to network elements that support
   both the Guaranteed Service and the Committed Rate service.
   Evaluation of this behavior can be achieved by loading the network
   element with a "test" Committed Rate flow and the maximum possible
   amount of Guaranteed Service traffic that the network element(s) can
   accept.  The delays experienced by the Committed Rate flow should
   then be compared to those experienced in the other configurations
   described above.  As before, the smaller the delay values, the higher
   the evaluation ratings for the service.

Examples of Implementation

   Several scheduling algorithms and implementations exist that allow a
   close approximation of a dedicated rate circuit. They include
   Weighted Fair Queueing (WFQ) [4], Virtual Clock [5], Rate Controlled
   Service Disciplines [6,7], etc.  Additional theoretical results
   positioning these results in the context of broader classes of
   algorithms can be found in [8,9].

Examples of Use

   Consider an application that requires a specific guaranteed
   throughput in order to operate properly, but is reasonably tolerant
   in terms of the delay and delay variations it will experience, so
   that the delay guarantees of the Guaranteed service may not be
   warranted.  For example, this may consist of an application
   retrieving a large document including graphics and pictures from a
   web server.  This application wants a large enough rate to ensure
   that the document is retrieved reasonably fast, but does not require
   tight delay guarantees as the user will typically start browsing the
   initial material received and can, therefore, tolerate delay
   variations in receiving the remainder of the data.  Another example,
   is that of a transaction based application that requires the transfer
   of reasonably large amounts of data in sufficiently timely fashion to
   ensure an adequate response time.  Such an application will be
   satisfied by the provision of a large enough transmission rate, but
   again does not need very tight delay guarantees.


Security Considerations

   Security considerations are not discussed in this memo.





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Acknowledgments

   The authors would like to gratefully acknowledge the help of the
   INT-SERV working group and the many contributions to its mailing
   list.  In addition, they would like to acknowledge the Guaranteed
   Service specifications which served as a base for many of the aspects
   discussed in this draft.



References


   [1] S. Shenker and J. Wroclawski. "Network Element Service
   Specification Template," Internet Draft, June 1995, <draft-ietf-
   intserv-svc-template-01.txt>

   [2] S. Shenker and C. Partridge.  "Specification of Guaranteed
   Quality of Service," Internet Draft, November 1995, <draft-ietf-
   intserv-guaranteed-svc-03.txt>

   [3] J. Wroclawski. "Specification of The Controlled-Load Network
   Element Service," Internet Draft, November 1995, <draft-ietf-
   intserv-ctrl-load-svc-01.txt>

   [4] A. Demers, S. Keshav and S. Shenker, "Analysis and Simulation of
   a Fair Queueing Algorithm," in Internetworking: Research and
   Experience, Vol 1, No. 1., pp. 3-26.

   [5] L. Zhang, "Virtual Clock: A New Traffic Control Algorithm for
   Packet Switching Networks," in Proc. ACM SIGCOMM'90, pp. 19-29.

   [6] H. Zhang, and D. Ferrari, "Rate-Controlled Service Disciplines,"
   Journal of High Speed Networks, 3(4):389--412, 1994.

   [7] L. Georgiadis, R. Guerin, V. Peris, and K. N. Sivaraja,
   "Efficient Network QoS Provisioning Based on per Node Traffic
   Shaping," IEEE/ACM Transactions on Networking, 4(4), August 1996.

   [8] D. Stiliadis and A. Varma, "Latency-Rate Servers: A General Model
   for Analysis of Traffic Scheduling Algorithms," on Proc. INFOCOM'96,
   pp. 111-119.

   [9] P. Goyal, S.S. Lam and H.M. Vin, "Determining End-to-End Delay
   Bounds in Heterogeneous Networks," in Proc. 5th Intl. Workshop on
   Network and Operating System Support for Digital Audio and Video,
   April 1995.




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   [10] B. Braden, L. Zhang, S. Berson, S. Herzog, and J. Wroclawski.
   "Resource Reservation Protocol (RSVP) - Version 1 Functional
   Specification," Internet Draft, November 1995, <draft-ietf-rsvp-
   spec-08.txt>

   [11] J. Wroclawski. "Standard Data Encoding for Integrated Services
   Objects,"  Internet Draft, November 1995, <draft-ietf-intserv-data-
   encoding-01.txt>

   [12] S. Shenker. "Specification of General Characterization
   Parameters,"  Internet Draft, November 1995, <draft-ietf-intserv-
   charac-00.txt>


Authors' Address:


   Fred Baker
   Cisco Systems
   519 Lado Drive
   Santa Barbara, California 93111
   fred@cisco.com
   VOICE   +1 408 526-4257
   FAX     +1 805 681-0115

   Roch Guerin
   IBM T.J. Watson research Center
   P.O. Box 704
   Yorktown Heights, NY 10598
   guerin@watson.ibm.com
   VOICE   +1 914 784-7038
   FAX     +1 914 784-6318

   Dilip Kandlur
   IBM T.J. Watson research Center
   P.O. Box 704
   Yorktown Heights, NY 10598
   kandlur@watson.ibm.com
   VOICE   +1 914 784-7722
   FAX     +1 914 784-6625











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