Subject: draft-ietf-intserv-ctrl-load-svc-00.txt

From: John Wroclawski <[email-masked]>
Date: Sun, 26 Nov 1995 00:04:24 -0500

Internet Engineering Task Force                   Integrated Services WG
INTERNET-DRAFT                                             J. Wroclawski
draft-ietf-intserv-ctrl-load-svc-00.txt                          MIT LCS
                                                          November, 1995
                                                           Expires: 5/96

      Specification of the Controlled-Load Network Element 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
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   This draft is a product of the Integrated Services Working Group of
   the Internet Engineering Task Force.  Comments are solicited and
   should be addressed to the working group's mailing list at int-
   [email-masked] and/or the author(s).

Abstract

      This memo specifies the network element behavior required to
      deliver Controlled-Load service in the Internet.  The controlled-
      load service provides the client data flow with a quality of
      service closely approximating the QoS that same flow would receive
      from an unloaded network element, but uses capacity (admission)
      control to assure that this service is received even when the
      network element is overloaded.

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Introduction

   This document defines the requirements for network elements that
   support the Controlled-Load 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 conforming to this specification tightly
   approximates the behavior visible to applications receiving best-
   effort service *under unloaded conditions* from the same series of
   network elements.  Assuming the network is functioning correctly,
   these applications may assume that:

     - A very high percentage of transmitted packets will be
     successfully delivered by the network to the receiving end-nodes.
     (The percentage of packets not successfully delivered must closely
     approximate the basic packet error rate of the transmission
     medium).

     - The transit delay experienced by a very high percentage of the
     delivered packets will not greatly exceed the minimum transmit
     delay experienced by any successfully delivered packet (the
     "speed-of-light delay").

      NOTE: the term "unloaded" above is used in the sense of "not
      heavily loaded or congested" rather than in the sense of "no other
      network traffic whatsoever".

   To ensure that these conditions are met, clients requesting

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   controlled-load service provide the intermediate network elements
   with a estimation of the data traffic they will generate; the TSpec.
   In return, the service ensures that network element resources
   adequate to process traffic falling within this descriptive envelope
   will be available to the client. Should the client's traffic
   generation properties fall outside of the region described by the
   TSpec parameters, the QoS provided to the client may exhibit
   characteristics indicative of overload, including large numbers of
   delayed or dropped packets. The service definition does not require
   that the precise characteristics of this overload behavior match
   those which would be received by a best-effort data flow traversing
   the same path under overloaded conditions.

Motivation

   The controlled load service is intended to support a broad class of
   applications which have been developed for use in today's Internet,
   but are highly sensitive to overloaded conditions.  Important
   examples of this class are the "adaptive real-time applications"
   currently offered by a number of vendors and researchers. These
   applications have been shown to work well on unloaded nets, but
   poorly on much of todays overloaded Internet. A service which mimics
   unloaded nets serves these applications well.

   The controlled-load service is intentionally minimal, in that there
   are no optional functions or capabilities in the specification. The
   service offers only a single function but system and application
   designers can assume that all implementations will be indentical in
   this respect.

   Internally, the controlled-load service is suited to a wide range of
   implementation techniques; including evolving scheduling and
   admission control algorithms which allow sophisticated
   implementations to be highly efficient in the use of network
   resources. It is equally amenable to extremely simple implementation
   in circumstances where maximum utilization of network resources is
   not the only concern.

Network Element Data Handling Requirements

   Each network element accepting a request for controlled-load service
   must ensure that adequate bandwidth and packet processing resources
   are available to handle the requested level of traffic, as given by
   the requestor's TSpec. This must be accomplished through active

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   admission control. All resources important to the operation of the
   network element must be considered when admitting a request. Common
   examples of such resources include link bandwidth, router or switch
   port buffer space, and computational capacity of the packet
   forwarding engine.

   The controlled-load service does not accept or make use of specific
   target values for control parameters such as delay or loss. Instead,
   acceptance of a request for controlled-load service is defined to
   imply a commitment by the network element to provide the requestor
   with service closely equivalent to that provided to uncontrolled
   (best-effort) traffic under unloaded conditions. This definition may
   be taken to include:

     - Little or no average packet queueing delay over all timescales
     significantly larger than the "burst time". The burst time is
     defined as the time required for the flow's maximum size data burst
     to be transmitted at the flow's requested transmission rate, where
     the burst size and rate are given by the flow's TSpec, as described
     below.

     - A very low level of congestion loss. In this context, congestion
     loss includes packet losses due to shortage of any required
     processing resource, such as buffer space or link bandwidth.
     Although occasional congestion losses may occur, any substantial
     sustained loss represents a failure of the admission control
     algorithm.

      NOTE:

      Implementations of controlled-load service are not required to
      provide any control of short-term packet delay jitter beyond that
      described above. However, the use of packet scheduling algorithms
      that provide additional jitter control is not prohibited by this
      specification.

      Packet losses due to non-congestion-related causes, such as link
      errors, are not bounded by this service.

   A network element may employ statistical approaches to decide whether
   adequate capacity is available to accept a service request. For
   example, a network element processing a number of flows with long-

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   term characteristics predicted through measurement may be able to
   overallocate its resources to some extent without reducing the level
   of service delivered to the flows.

   A network element may employ any appropriate means to ensure that
   admitted flows receive appropriate service.

   Links are not permitted to fragment packets which receive the
   controlled-load service. Packets larger than the MTU of the link must
   be treated as nonconformant to the TSpec. This implies that they will
   be policed according to the rules described in the Policing section
   below.

   The controlled-load  service is invoked by specifying the data flow's
   desired traffic parameters (TSpec) to the network element. Requests
   placed for a new flow will be accepted if the network element has the
   capacity to forward the flow's packets as described above. Requests
   to change the TSpec for an existing flow should be treated as a new
   invocation, in the sense that admission control must be reapplied to
   the flow. Requests that reduce the TSpec for an existing flow (in the
   sense that the new TSpec is strictly smaller than the old TSpec
   according to the ordering rules given below) should never be denied
   service.

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

   The token bucket specification includes a bucket rate r and a bucket
   depth, b.  Both r and b must be positive.  The rate, r, is measured
   in bytes of IP datagrams per second. Values of this parameter may
   range from 1 byte per second to 40 terabytes per second. 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. Values of this parameter
   may 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 b parameter are
   significant, so the use of floating point representations, accurate
   to at least 0.1% is encouraged.

   The range of values allowed for these parameters is intentionally

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   large to allow for future network technologies. Any given network
   element is not expected to support the full range of values.

   The minimum policed unit, m, is an integer measured in bytes.  All IP
   datagrams less than size m will be counted against the token bucket
   as being of size m. The maximum packet size, M, is the biggest packet
   that will conform to the traffic specification; it is also measured
   in bytes.  Network elements 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 then or equal to M.

   The preferred concrete representation for the TSpec is two 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
   minimum policed unit (m), and the fourth is the maximum packet size
   (M).

Exported Information

   The controlled-load service is assigned service_name 5.

   The controlled-load service has no required characterization
   parameters. Specific implementations may export appropriate
   measurement and monitoring information.

Policing

   The controlled-load service is suitable for use with multicast as
   well as unicast data flows. This capability introduces some
   complexity into the policing requirements.

   Controlled-load traffic must be policed for conformance to its TSpec
   at every network element. The TSpec's token bucket parameters require
   that traffic must obey the rule that over all time periods, the
   amount of data sent does not exceed rT+b, where r and b are the token
   bucket parameters and T is the length of the time period.  For the
   purposes of this accounting, links must count packets that are
   smaller than the minimal policing unit to be of size m.  Packets that
   arrive at an element and cause a violation of the the rT+b bound are
   considered nonconformant.

   At all policing points, non-conforming packets are treated as BEST-
   EFFORT datagrams. (See the NOTEs below for further discussion of this

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   issue).

   If resources are available, it is desirable for the policing function
   at points within the interior of the network (but *not* at edge
   traffic entry points) to enforce slightly "relaxed" traffic
   parameters to accommodate packet bursts somewhat larger than the
   actual TSpec.

   Other actions, such as reshaping the traffic stream (delaying packets
   until they are compliant), are not allowed.

      NOTE: RESHAPING. The prohibition on delaying packets is one of
      many possible design choices.  It may be better to permit some
      delaying of a packet if that delay would allow it to pass the
      policing function.  (In other words, to reshape the traffic).  The
      challenge is to define a viable reshaping function.

      Intuitively, a plausible approach is to allow a delay of (roughly)
      up to the maximum queueing delay experienced by completely
      conforming packets before declaring that a packet has failed to
      pass the policing function. The merit of this approach, and the
      precise wording of the specification that describes it, require
      further study.

      NOTE: INTERACTION WITH BEST-EFFORT TRAFFIC. Implementors of this
      service should clearly understand that in certain circumstances
      (routers acting as the "split points" of a multicast distribution
      tree supporting a shared reservation) large numbers of packets may
      fail the policing test *as a matter of normal operation*.
      According to the definition above, these packets should be
      processed as best-effort packets.

      If the network element's best-effort queueing algorithm does not
      distinguish between these packets and elastic best-effort traffic
      such as TCP flows, THESE PACKETS WILL "BACK OFF" THE ELASTIC
      TRAFFIC AND DOMINATE THE BEST-EFFORT BANDWIDTH USAGE. The
      integrated services framework does not currently address this
      issue. However, several possible solutions to the problem are
      known [RED, xFQ]. Network elements supporting the controlled load
      service should also implement some mechanism in their best-effort
      queueing path to discriminate between classes of best-effort
      traffic and provide elastic traffic with protection from inelastic
      best-effort flows.

      NOTE: EDGE POLICING. The text above specifys that the policing

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      function treats non-conformant packets as best-effort at all
      points. A possible alternative is to replace this with language
      reading:

        At points where traffic first enters the network (end-nodes),
        non-conforming packets are DROPPED. At these points, the
        reservation setup mechanism must ensure that the TSpec used is
        *no smaller* than the TSpec specified by the source for the
        traffic it is generating.

        At all other policing points, non-conforming packets are treated
        as BEST-EFFORT datagrams.

      The effect of this change is significant. Under the non-dropping
      model, it is possible for a source to vastly over-send its TSpec,
      with the excess packets being delivered if conditions permit. The
      service offered in this case has been described as "best-effort-
      with-floor"; essentially a best-effort delivery service with
      enough resources reserved for a certain minimum traffic level.

      Under the dropping model, the service loses its "best-effort-
      with-floor" characteristics, and becomes essentially a fixed-
      traffic-level service. In return, it offers significantly more
      protection against overload of the network resources and
      degradation of other flows' QoS.

      NOTE: ARCHITECTURAL OPTIONS. The text above specifies a functional
      and consistant model for policing of controlled-load data which
      can be implemented within the current IP protocols.

      In this model, it is necessary to police at every network element
      because the policing function does not actually drop traffic which
      exceeds the TSpec, but instead carries it as best-effort. Since
      there is no end-to-end mechanism in place to limit a controlled-
      load flow's traffic to the TSpec value, every network element must
      perform this function for itself. Since excess controlled-load
      traffic (traffic above the TSpec) is not dropped, every network
      element should also perform the best-effort service discrimination
      function described above.

      The alternative option of "marking" packets which have failed the
      policing test at some node is not available within the current IP
      protocol. If marking were available, it would be necessary to
      police only at certain points within the network. In this case,
      the relevant language above might be replaced with a paragraph

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      reading:

        Policing is performed at the edge of the network, at all
        heterogeneous source branch points and at all source merge
        points.  A heterogeneous source branch point is a spot where the
        multicast distribution tree from a source branches to multiple
        distinct paths, and the TSpec's of the reservations on the
        various outgoing links are not all the same.  Policing need only
        be done if the TSpec on the outgoing link is "less than" (in the
        sense described in the Ordering section) the TSpec reserved on
        the immediately upstream link.  A source merge point occurs when
        the multicast distribution trees from two different sources
        (sharing the same reservation) merge.  It is the responsibility
        of the invoker of the service (a setup protocol, local
        configuration tool, or similar mechanism) to identify points
        where policing is required.  Policing is allowed at points other
        than those mentioned above.

      Note that the best-effort traffic discrimination function
      described above must still be performed at every network element.
      In this case, the discrimination might be based in part on the
      mark bit.

   At all network elements, packets bigger than the outgoing link MTU
   must be considered nonconformant and classified as best effort (and
   will then either be fragmented or dropped according to the element's
   handling of best effort traffic). It is expected that this situation
   will not arise with any frequency, because flow setup mechanisms are
   expected to notify the sending application of the appropriate path
   MTU.

Ordering and Merging

   The controlled-load service TSpec is ordered according to the
   following rule: TSpec A is a substitute for ("as good or better
   than") TSpec B if and only if

     (1) both the token bucket depth and rate for TSpec A are greater

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     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, and

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

   A merged TSpec may be calculated over a set of TSpecs by taking the
   largest token bucket rate, largest bucket size, 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 that is adequate to describe the
   traffic from any one of a number of flows.

   The sum of n controlled-load service TSpecs is used when computing
   the TSpec for a shared reservation of n flows. It is computed by
   taking:

     - The minimum across all TSpecs of the minimum policed unit
     parameter m.

     - The maximum across all TSpecs of the maximum packet size
     parameter M.

     - The sum across all TSpecs of the token bucket rate parameter r.

     - The sum across all TSpecs of the token bucket size parameter b.

   The perfect minimum of two TSpecs is defined as a TSpec which would
   view as compliant any traffic flow that complied with both of the
   original TSpecs, but would reject any flow that was non-compliant
   with at least one of the original TSpecs. This perfect minimum can be
   computed only when the two original TSpecs are ordered, in the sense
   described above.

   A definition for computing the minimum of two unordered TSpecs is:

     - The minimum of the minimum policed units m.

     - The maximum of the maximum packet sizes M.

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     - The minimum of the token bucket rates r.

     - The maximum of the token bucket sizes b.

      NOTE: The proper definition the minimum TSpec function is a topic
      of current discussion. The definition above is provisional and
      subject to change.

Guidelines for Implementors

   The intention of this service specification is that network elements
   deliver a level of service closely approximating best-effort service
   under unloaded conditions. As with best-effort service under these
   conditions, it is not required that every single packet must be
   successfully delivered with zero queueing delay. Network elements
   providing controlled-load service are permitted to oversubscribe the
   available resources to some extent, in the sense that the bandwidth
   and buffer requirements indicated by summing the TSpec token buckets
   of all controlled-load flows may exceed the maximum capabilities of
   the network element. However, this oversubscription may only be done
   in cases where the element is quite sure that actual utilization is
   far less than the sum of the token buckets would suggest. The most
   conservative approach, rejection of new flows whenever the addition
   of their traffic would cause the sums of the token buckets to exceed
   the capacity of the network element, may be appropriate in other
   circumstances.

   Specific issues related to this subject are discussed in the
   "Evaluation Criteria" and "Examples of Implementation" sections
   below.

   Implementors are encouraged (but not required) to implement policing
   behavior (the behavior seen when a flow's actual traffic exceeds its
   TSpec) which closely approximates the behavior of well-designed
   best-effort services under overload. In particular, it is undesirable
   to employ queueing models which lead to heavily bi-modal delay
   distributions or large numbers of mis-ordered packet arrivals.

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Evaluation Criteria

   The basic requirement placed on an implementation of controlled-load
   service is that, under all conditions, it provide accepted data flows
   with service closely similar to the service that same flow would
   receive using best-effort service under unloaded conditions.

   This suggests a simple two-step evaluation strategy. Step one is to
   compare the service given best-effort traffic and controlled-load
   traffic under underloaded conditions.

     - Measure the packet loss rate and delay characteristics of a test
     flow using best-effort service and with no load on the network
     element.

     - Compare those measurements with measurements of the same flow
     receiving controlled-load service with no load on the network
     element.

     Closer measurements indicate higher evaluation ratings. A
     substantial difference in the delay characteristics, such as the
     smoothing which would be seen in an implementation which scheduled
     the controlled-load flow using a fixed, constant-bitrate algorithm,
     should result in a somewhat lower rating.

   Step two is to observe the change in service received by a
   controlled-load flow as the load increases.

     - Increase the background traffic load on the network element,
     while continuing to measuring the loss and delay characteristics of
     the controlled-load flow. Characteristics which remain essentially
     constant as the element is driven into overload indicate a high
     evaluation rating. Minor changes in the delay distribution indicate
     a somewhat lower rating. Significant increases in delay or loss
     indicate a poor evaluation rating.

   This simple model is not adequate to fully evaluate the performance
   of controlled-load service. Three additional variables affect the
   evaluation. The first is the short-term burstiness of the traffic

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   stream used to perform the tests outlined above. The second is the
   degree of long-term change in the controlled-load traffic within the
   bounds of its TSpec.  (Changes in this characteristic will have great
   effect on the effectiveness of certain admission control algorithms.)
   The third is the ratio of controlled-load traffic to other traffic at
   the network element (either best effort or other controlled
   services).

   The third variable should be specifically evaluated using the
   following procedure.

     With no controlled-load flows in place, overload the network
     element with best-effort traffic (as indicated by substantial
     packet loss and queueing delay).

     Execute requests for controlled-load service giving TSpecs with
     increasingly large rate and burst parameters. If the request is
     accepted, verify that traffic matching the TSpec is in fact handled
     with characteristics closely approximating the unloaded
     measurements taken above.

     Repeat these experiments to determine the range of traffic
     parameter (rate, burst size) values successfully handled by the
     network element. The useful range of each parameter must be
     determined for several settings of the other parameter, to map out
     a two-dimensional "region" of successfully handled TSpecs. When
     compared with network elements providing similar capabilities, this
     region indicates the relative ability of the elements to provide
     controlled-load service under high load. A larger region indicates
     a higher evaluation rating.

Examples of Implementation

   One possible implementation of controlled-load service is to provide
   a queueing mechanism with two priority levels; a high priority one
   for controlled-load and a lower priority one for best effort service.
   An admission control algorithm is used to limit the amount of traffic
   placed into the high-priority queue. This algorithm may be based
   either on the specified characteristics of the high-priority flows
   (using information provided by the TSpecs), or on the measured
   characteristics of the existing high-priority flows and the TSpec of
   the new request.

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   Another possible implementation of controlled-load service is based
   on the existing capabilities of network elements which support
   "traffic classes" based on mechanisms such as weighted fair queueing
   or class-based queueing [xxx]. In this case, it is sufficient to map
   data flows accepted for controlled-load service into an existing
   traffic class with adequate capacity to avoid overload. This
   requirement is enforced by an admission control algorithm which
   considers the characteristics of the traffic class, the
   characteristics of the traffic already admitted to the class, and the
   TSpec of the new flow requesting service. Again, the admission
   control algorithm may be based either on the TSpec-specified or the
   measured characteristics of the existing traffic.

   Admission control algorithms based on specified characteristics are
   likely be appropriate when the number of flows in the high-priority
   class is small, or the traffic characteristics of the flows appear
   highly variable. In these situations the measured behavior of the
   aggregate controlled-load traffic stream may not serve as an
   effective predictor of future traffic, leading a measurement-based
   admission control algorithm to produce incorrect results. Conversely,
   in situations where the past behavior of the aggregate controlled-
   load traffic *is* a good predictor of future behavior, a
   measurement-based admission control algorithm may allow more traffic
   to be admitted to the controlled-load service class with no
   degradation in performance. An implementation may choose to switch
   between these two approaches depending on the nature of the traffic
   stream at a given time.

Examples of Use

   The controlled-load service may be used by any application which can
   make use of best-effort service, but is best suited to those
   applications which can usefully characterize their traffic
   requirements.  Applications based on the transport of "continuous
   media" data, such as digitized audio or video, are an important
   example of this class.

   The controlled-load service is not isochronous and does not provide
   any explicit information about transmission delay. For this reason,
   applications with end-to-end timing requirements, including the
   continuous-media class mentioned above, provide an application-
   specific timing recovery mechanism, similar or identical to the
   mechanisms required when these applications use best-effort service.
   A protocol useful to applications requiring this capability is the
   IETF Real-Time Transport Protocol [2].

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   Load-sensitive applications may choose to request controlled-load
   service whenever they are run. Alternatively, these applications may
   monitor their own performance and request controlled-load service
   from the network only when best-effort service is not providing
   acceptable performance. The first strategy provides higher assurance
   that the level of quality delivered to the user will not change over
   the lifetime of an application session. The second strategy provides
   greated flexibility and offers cost savings in environments where
   levels of service above best-effort incur a charge.

Security Considerations

   Security considerations are not discussed in this memo.

References

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

   [2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson.  "RTP:
   A Transport Protocol for Real-Time Applications", Internet Draft,
   March 1995, <draft-ietf-avt-svc-rtp-07.txt>

Authors' Address:

   John Wroclawski
   MIT Laboratory for Computer Science
   545 Technology Sq.
   Cambridge, MA  02139
   [email-masked]
   617-253-7885
   617-253-2673 (FAX)

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