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Versions: 01                                                            
Internet Engineering Task Force                   Integrated Services WG
INTERNET-DRAFT               S. Shenker/C. Partridge/B. Davie/L. Breslau
draft-ietf-intserv-predictive-svc-01.txt        Xerox/BBN/Bellcore/Xerox
                                                                  ? 1995
                                                         Expires: ?/?/96



             Specification of Predictive 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 memo describes the network element behavior required to deliver
   Predictive service in the Internet.  Predictive service is a real-
   time service that provides low packet loss and a fairly reliable
   delay bound.  This service is intended for applications that are
   tolerant of occasional late arriving packets, but require substantial
   and quantified levels of delay control from the network.  Predictive
   service is very similar to Controlled Delay service, and the two
   specifications have a fair amount of shared language.  The main
   salient different between the two services is that Predictive service
   offers a delay bound and Controlled Delay does not. If no
   characterizations are provided, then Predictive service is, from an
   application's perspective, almost indistinguishable from Controlled



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   Delay; the delay bounds are of little use if the endpoints are not
   aware of them.  Thus, the distinction between Predictive and
   Controlled Delay is important only in contexts where
   characterizations are made available to endpoints.  This
   specification follows the service specification template described in
   [1].

Introduction

   This document defines the requirements for network elements that
   support Predictive 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.

   This memo describes the specification for Predictive service in the
   Internet.  Predictive service is a real-time service that provides a
   fairly reliable delay bound.  That is, the large majority of packets
   are delivered within the delay bound.  This is in contrast to
   Guaranteed service [2], which provides an absolute bound on packet
   delay, and Controlled Delay service [3], which provides no
   quantitative assurance about end-to-end delays.  Predictive service
   is intended for use by applications that require an upper bound on
   end-to-end delay, but that can be tolerant of occasional violations
   of that bound.

   This document is one of a series of documents specifying network
   element behavior in IP internetworks that provide multiple qualities
   of service to their clients.  Services described in these documents
   are useful both in the global Internet and private IP networks.

   This document follows 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 by a series of network elements that
   conform to this document provides three levels of delay control.
   Each service level is associated with a fairly reliable delay bound,
   and almost all packets are delivered within this delay bound.



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   Moreover, all three levels of predictive service will have average
   delays that are no worse than best effort service, and the maximal
   delays should be significantly better than best effort service when
   there is significant load on the network.  Packet losses are rare as
   long as the offered traffic conforms to the specified traffic
   characterization  (see Invocation Information).  This
   characterization of the end-to-end behavior assumes that there are no
   hard failures in the network elements or packet routing changes
   within the lifetime of an application using the service.
      NOTE: While the per-hop delay bounds are exported by the service
      module (see Exported Information below), the mechanisms needed to
      collect per-hop bounds and make these end-to-end bounds known to
      the applications are not described in this specification.  These
      functions, which can be provided by reservation setup protocols,
      routing protocols or by other network management functions, are
      outside the scope of this document.

   The delay bounds are not absolute firm.  Some packets may arrive
   after their delay bound, or they may be lost in transit.  At the same
   time, packets may often arrive well before the bound provided by the
   service.  No attempt to control jitter, beyond providing an upper
   bound on delay, is required by network elements implementing this
   service.  It is expected that most packets will experience delays
   well below the actual delay bound and that only the tail of the delay
   distribution will approach (or occasionally exceed) the bound.
   Consequently, the average delay will also be well below the delay
   bound.

   This service is designed for use by playback applications that desire
   a bound on end-to-end delay.  Such applications may or may not be
   delay adaptive.  The delay bound is useful for those applications
   that do not wish to adapt their playback point or that require an
   upper bound on end-to-end delay.  Note that the delay bound provided
   along an end-to-end path should be stable.  That is, it should not
   change as long as the end-to-end path does not change.

   This service is subject to admission control.

Motivation

   Predictive service is designed for playback applications that desire
   a reserved rate with low packet loss and a maximum bound on end-to-
   end packet delay, and that are tolerant of occasional dropped or late
   packets.  The presence of delay bounds serves two functions.  First,
   they provide some characterization of the service so that a non-
   service-adaptive application (that is, an application that does not
   want to continually change its service request in response to current
   conditions) can know beforehand the maximum delays its packets will



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   experience in a given service class.  These bounds will allow such
   applications to choose an appropriate service class.  Second, such
   delay bounds can help applications that are not interested in
   adapting to current delays set their playback point.  For many
   noninteractive "playback" applications, fidelity is of more
   importance than reducing the playback delay; the delay bound allows
   the application to achieve high fidelity by having a stable playback
   point with a very few late packets.

   Some real-time applications may want a service providing end-to-end
   delay bounds.  However, they may be willing to forgo the absolute
   bound on delay provided by Guaranteed service [2].  By relaxing the
   service commitment from a firm to a fairly reliable delay bound,
   network elements will in many environments be able to accommodate
   more flows using Predictive service while meeting the service
   requirement.  Thus, Predictive service relaxes the service commitment
   in favor of higher utilization, when compared to Guaranteed service.

   At the same time, these applications may require a higher level of
   assurance, in the form of a quantitative delay bound, than Controlled
   Delay service [3] provides.  The use of Predictive service, rather
   than Controlled Delay, may also allow applications to avoid adapting
   their service requests to changing network performance.

   In order to accommodate the requirements of different applications,
   Predictive service provides multiple levels of service.  Applications
   can choose the level of service providing the most appropriate delay
   bound.

   For additional discussion of Predictive service, see [4,6].

   Associated with this service are characterization parameters which
   describe the delay bound and the current delays experienced in the
   three services levels.  If the characterizations are provided to the
   endpoints, these will provide some hint about the likely end-to-end
   delays that might result from requesting a particular level of
   service, as well as providing information about the end-to-end delay
   bound.  This is intended to aid applications in choosing the
   appropriate service level.  The delay bound information can also be
   used by applications not wishing to adapt to current delays.

   Predictive service is very similar to controlled delay service.  The
   only salient difference is the predictive service provides a fairly
   reliable delay bound, whereas controlled delay does not have any
   quantified service assurance.  Note that if no characterizations are
   provided, then this service is, from an application's perspective,
   almost indistinguishable from controlled delay; the delay bounds are
   of little use if the endpoints are not aware of them.  Thus, the



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   distinction between predictive and controlled delay is important only
   in contexts where characterizations are made available to endpoints.




Network Element Data Handling Requirements

   The network element must ensure that packet delays are below a
   specified delay bound.  There can be occasional violations of the
   delay bound, but these violations should be very rare. Similarly,
   Predictive service must maintain a very low level of packet loss.
   Although packets may be lost or experience delays in excess of the
   delay bound, any substantial loss or delay bound violations
   represents a "failure" of the admission control algorithm.  However,
   vendors may employ admission control algorithms with different levels
   of conservativeness, resulting in very different levels of delay
   violations and/or loss (delay bound violations might, for instance,
   vary from 1 in 10^4 to 1 in 10^8).

   This service must use admission control.  Overprovisioning alone is
   not sufficient to deliver predictive service;  the network element
   must be able to turn flows away if accepting them would cause the
   network element to experience queueing delays in excess of the delay
   bound.

   There are three different logical levels of predictive service.  A
   network element may internally implement fewer actual levels of
   service, but must map them into three levels at the predictive
   service invocation interface.  Each level of service is associated
   with a delay bound, with level 1 having the smallest delay and level
   3 the largest.  If the network element implements different levels of
   service internally, the delay bounds of the different service levels
   should differ substantially.  The actual choice of delays is left to
   the network element, and it is expected that different network
   elements will select different delay bounds for the same level of
   service.

   All three levels of service should be given better service, i.e.,
   more tightly controlled delay, than best effort traffic.  The average
   delays experienced by packets receiving different levels of
   predictive service and best-effort service may not differ
   significantly.  However, the tails of the delay distributions, i.e.,
   the maximum packet delays seen, for the levels of Predictive service
   that are implemented and for best-effort service should be
   significantly different when the network has substantial load.

   Predictive service does not require any control of delay jitter



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   (variation in network element transit delay between different packets
   in the flow) beyond the limit imposed by the per-service level delay
   bound.  Network element implementors who find it advantageous to do
   so may use resource scheduling algorithms that exercise some jitter
   control. See the guidelines for implementors section for more
   discussion of this issue.

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



Invocation Information

   The Predictive service is invoked by specifying the traffic (TSpec)
   and the desired service (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
   their RSpec (i.e., their new TSpec/RSpec is strictly smaller than the
   old TSpec/RSpec according to the ordering rules described in the
   section on Ordering below) should never be denied service.

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

   The token bucket has a bucket depth, b, and a bucket rate, r.  Both b
   and r must be positive.  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).  Clearly,
   particularly for large bandwidths, only the first few digits are
   significant and so the use of floating point representations,
   accurate to at least 0.1% is encouraged.

   The bucket depth, b, is also measured in bytes and can range from 1
   byte to 250 gigabytes.  Again, floating point representations
   accurate to at least 0.1% are encouraged.

   The range of values is intentionally large to allow for the future
   bandwidths.  The range is not intended to imply that a network
   element must support the entire range.

   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



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   packet that will conform to the traffic specification; it is also
   measured in bytes.  The flow must be rejected 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 RSpec is a service level.  The service level is specified by one
   of the integers 1, 2, or 3.  Implementations should internally choose
   representations that leave a range of at least 256 service levels
   undefined, for possible extension in the future.

   The TSpec can be represented by 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).

   The RSpec may be represented as an unsigned 16-bit integer carried in
   network byte order.

   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.


Exported Information

   Each predictive service module must export the following information.
   All of the data elements described below are characterization
   parameters.

   For each logical level of service, the network element exports the
   delay bound as well as three measurements of delay (thus making
   twelve quantities in total).  Each of the measured characterization
   parameters is based on the maximal packet transit delay experienced
   over some set of previous time intervals of length T; these delays do
   not include discarded packets.  The three time intervals T are 1
   second, 60 seconds, and 3600 seconds.  The exported parameters are
   averages over some set of these previous time intervals.

   There is no requirement that these characterization parameters be
   based on exact measurements.  In particular, these delay measurements
   can be based on estimates of packet delays or aggregate measurements
   of queue loading.  This looseness is intended to avoid placing undue
   burdens on network element designs in which obtaining precise delay
   measurements is difficult.

   These delay parameters (both the measured values and the bound) have



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   an additive composition rule.  For each parameter the composition
   function computes the sum, enabling a setup protocol to deliver the
   cumulative sum along the path to the end nodes.

   The characterization parameters are measured in units of one
   microsecond.  An individual element can advertise a delay value
   between 1 and 2**28 (somewhat over two minutes) and the total delay
   added across all elements can range as high as 2**32-1.  Should the
   sum of the values of individual network elements along a path exceed
   2**32-1, the end-to-end advertised value should be 2**32-1.

   Note that while the delay measurements are expressed in microseconds,
   a network element is free to measure delays more loosely.  The
   minimum requirement is that the element estimate its delay accurately
   to the nearest 100 microseconds.  Elements that can measure more
   accurately are encouraged to do so.

      NOTE:  Measuring delays in milliseconds is not acceptable, as it
      may lead to composed delay values with unacceptably large errors
      along paths that are several hops long.

   The characterization parameters may be represented as a sequence of
   twelve 32-bit unsigned integers in network byte order.  The first
   four integers are the parameters for the delay bound and for the
   measurement values for T=1, T=60 and T=3600 for level 1. The next
   four integers are the parameters for the delay bound and for the
   measurement values for T=1, T=60 and T=3600 for level 2.  The last
   four integers are the parameters for the delay bound and for the
   measurement values for T=1, T=60 and T=3600 for level 3.

   The following values are assigned from the characterization parameter
   name space.

   Predictive service is service_name 3.

   The delay characterization parameters are parameter_number's one
   through twelve, in the order given above.  That is,

         parameter_name       definition
         1                    Service Level = 1, Delay Bound
         2                    Service Level = 1, Delay Measure, T = 1
         3                    Service Level = 1, Delay Measure, T = 60
         4                    Service Level = 1, Delay Measure, T = 3600
         5                    Service Level = 2, Delay Bound
         6                    Service Level = 2, Delay Measure, T = 1
         7                    Service Level = 2, Delay Measure, T = 60
         8                    Service Level = 2, Delay Measure, T = 3600
         9                    Service Level = 3, Delay Bound



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         10                   Service Level = 3, Delay Measure, T = 1
         11                   Service Level = 3, Delay Measure, T = 60
         12                   Service Level = 3, Delay Measure, T = 3600


   The end-to-end composed results are assigned parameter_names N+12,
   where N is the value of the per-hop name given above.

   No other exported data is required by this specification.


Policing


   Policing is done 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 is
   where 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.

   The token bucket parameters require that traffic must obey the rule
   that over all time periods, the amount of data sent cannot 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.
   Policing to conformance with this token bucket is done in two
   different ways. At all policing point, non conforming packets are
   treated as best-effort datagrams.  [If and when a marking ability
   becomes available, these nonconformant packets should be ``marked''
   as being noncompliant and then treated as best effort packets at all
   subsequent routers.]  Other actions, such as delaying packets until
   they are compliant, are not allowed.
      NOTE: This point is open to discussion. The requirement given
      above may be too strict; it may be better to permit some delaying
      of a packet if that delay would allow it to pass the policing
      function.  Intuitively, a plausible approach is to allow a delay
      of (roughly) up to the maximum queueing delay experienced by



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      completely conforming packets before declaring that a packet has
      failed to pass the policing function and dropping it. The merit of
      this approach, and the precise wording of the specification that
      describes it, require further study.

   A related issue is that at all network elements, packets bigger than
   the MTU of the link must be considered nonconformant and should be
   classified as best effort (and will then either be fragmented or
   dropped according to the element's handling of best effort traffic).
   [Again, if marking is available, these reclassified packets should be
   marked.]




Ordering

   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, 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.

   Service request specifications (RSpecs) are ordered by their
   numerical values (in inverse order); service level 1 is substitutable
   for service level 2, and service level 2 is substitutable for service
   level 3.

   In addition, predictive service is related to controlled delay
   service in the sense that a given level of predictive service is
   considered at least as good as the same level of controlled delay
   service.  That is, predictive level 1 is substitutable for controlled
   delay level 1, and so on.  See additional comments in the guidelines
   section.

Guidelines for Implementors

   It is expected that the service levels implemented at a particular
   element will offer significantly different levels of delay bounds.
   There seems little advantage in offering levels whose delay bounds



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   differ only slightly.  So, while a particular element may offer less
   than three levels of service, the levels of service it does offer
   should have notably different delay bounds.  For example, appropriate
   delay bounds for three levels of predictive service are 1, 10 and 100
   milliseconds.

   For each level of service, packet loss and violation of the delay
   bound are expected to be very rare. As a preliminary guideline, we
   suggest that over long term use (measured in hours or days), the
   aggregate rate of delay bound violation and packet loss should be
   less than 1 in 10,000 packets.  Violations of the delay bound are
   likely to be correlated.  On shorter time scales, delay bound
   violation rates should not exceed 1 in 1,000 during any 60 second
   interval.

   An additional service currently being considered is the controlled
   delay service described in [3].  It is expected that if an element
   offers both predictive service and controlled delay service, it
   should not implement both but should use the predictive service as a
   controlled delay service.  This is allowed since (1) the required
   behavior of predictive service meets all of the requirements of
   controlled delay service, (2) the invocations are compatible, and (3)
   the ordering relationships are such that a given level of predictive
   service is at least as good as the same level of controlled delay
   service.


 Evaluation Criteria

   Evaluating a network element's implementation of predictive service
   is somewhat difficult, since the quality of service depends on
   overall traffic load and the traffic pattern presented.  In this
   section we sketch out a methodology for testing a network element's
   predictive service.

   The idea is that one chooses a particular traffic mix (for instance,
   three parts level 1, one part level 2, two parts level 3 and one part
   best-effort traffic) and loads the network element with progressively
   higher levels of this traffic mix (i.e., 40% of capacity, then 50% of
   capacity, on beyond 100% capacity).  For each load level, one
   measures the utilization, mean delays, packet loss rate, and delay
   bound violation rate for each level of service (including best
   effort).  Each test run at a particular load should involve enough
   traffic that is a reasonable predictor of the performance a long-
   lived application such as a video conference would experience (e.g.,
   an hour or more of traffic).

   This memo does not specify particular traffic mixes to test.



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   However, we expect in the future that as the nature of real-time
   Internet traffic is better understood, the traffic used in these
   tests will be chosen to reflect the current and future Internet load.

Examples of Implementation

   One implementation of predictive service would be to have a queueing
   mechanism with three priority levels, with level 1 packets being
   highest priority and level 3 packets being lowest priority. Maximum
   packet delays and link utilization would be measured for each class
   over some relatively short interval, such as 10,000 packet
   transmission times.  The admission control algorithm would use these
   measurements to determine whether or not to admit a new flow.
   Specifically, a new flow would be admitted if the network element
   expects to be able to meet the delay bounds of the packets in each
   service class after admitting a new flow.  For an example of an
   admission control algorithm for Predictive service, see [5].

   Note that the viability of measurement based admission control for
   predictive service depends on link bandwidth and traffic patterns.
   Specifically, with bursty traffic sources, sufficient multiplexing is
   needed for measurements of existing traffic to be good predictors of
   future traffic behavior.  In an environments where sufficient
   multiplexing is not possible, parameter based admission control may
   be necessary.

 Examples of Use

   We give two examples of use, both involving an interactive
   application.

   In the first example, we assume  that either the receiving
   application is ignoring characterizations or the setup protocol is
   not delivering the characterizations to the end-nodes.  We further
   assume that the application's data transmission units are
   timestamped.  The receiver, by inspecting the timestamps, can
   determine the end-to-end delays and determine if they are excessive.
   If so, then the application asks for a better level of service.  If
   the delays are well below the required level, the application can ask
   for a worse level of service.

   In the second example, we assume that characterizations are delivered
   to the receiving application.  The receiver chooses the worst service
   level whose characterization for the delay bound is less than the
   required level (once latencies are added in).






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 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, July 1995, <draft-ietf-intserv-
   guaranteed-svc-01.txt>

   [3] S. Shenker and C. Partridge and J. Wroclawski. "Specification of
   Controlled Delay Quality of Service",Internet Draft, June 1995,
   <draft-ietf-intserv-control-del-svc-01.txt>

   [4] R. Braden, D. Clark and S. Shenker, "Integrated Services in the
   Internet Architecture: an Overview", RFC 1633, June 1994.

   [5] S. Jamin, P. Danzig, S. Shenker and L. Zhang, "A Measurement-
   based Admission Control Algorithm for Integrated Services Packet
   Networks", Sigcomm '95, September 1995.

   [6] D. Clark, S. Shenker and L. Zhang, "Supporting Real-Time
   Applications in an Integrated Services Packet Network:  Architecture
   and Mechanism", Sigcomm '92, October 1992.

Security Considerations

   Security considerations are not discussed in this memo.

Authors' Address:

   Scott Shenker
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA  94304-1314
   shenker@parc.xerox.com
   415-812-4840
   415-812-4471 (FAX)

   Craig Partridge
   BBN
   2370 Amherst St
   Palo Alto CA 94306
   craig@bbn.com

   Bruce Davie
   Bellcore
   445 South St



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INTERNET-DRAFT  draft-ietf-intserv-predictive-svc-01.txt         ?, 1995


   Morristown, NJ, 07960
   bsd@bellcore.com

   Lee Breslau
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304-1314
   breslau@parc.xerox.com
   415-812-4402
   415-812-4471 (FAX)









































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