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rfc:rfc1193

Network Working Group D. Ferrari Request for Comments: 1193 UC Berkeley

                                                         November 1990
      CLIENT REQUIREMENTS FOR REAL-TIME COMMUNICATION SERVICES

Status of this Memo

 This memo describes client requirements for real-time communication
 services.  This memo provides information for the Internet community,
 and requests discussion and suggestions for improvements.  It does
 not specify any standard.  Distribution of this memo is unlimited.

Abstract

 A real-time communication service provides its clients with the
 ability to specify their performance requirements and to obtain
 guarantees about the satisfaction of those requirements.  In this
 paper, we propose a set of performance specifications that seem
 appropriate for such services; they include various types of delay
 bounds, throughput bounds, and reliability bounds.  We also describe
 other requirements and desirable properties from a client's
 viewpoint, and the ways in which each requirement is to be translated
 to make it suitable for lower levels in the protocol hierarchy.
 Finally, we present some examples of requirements specification, and
 discuss some of the possible objections to our approach.
 This research has been supported in part by AT&T Bell Laboratories,
 the University of California under a MICRO grant, and the
 International Computer Science Institute.  The views and conclusions
 in this document are those of the author and should not be
 interpreted as representing official policies, either expressed or
 implied, of any of the sponsoring organizations.

1. Introduction

 We call real-time a computer communication service whose clients are
 allowed to specify their performance requirements and to obtain
 guarantees about the fulfillment of those requirements.
 Three terms in this definition need further discussion and
 clarification: clients, performance, and guarantees.
 Network architecture usually consists, at least from a logical
 viewpoint, of a stack of protocol layers. In the context of such an
 architecture, the notions of client and server apply to a number of

Ferrari [Page 1] RFC 1193 Requirements for Real-Time Services November 1990

 different pairs of entities: every layer (with the support of the
 underlying layers) provides a service to the layer immediately above
 it and is a client of its underlying layers.  In this paper, our
 considerations generally apply to any client-server pair.  However,
 most of them particularly refer to human clients (users, programmers)
 and to the ways in which such clients express their communication and
 processing needs to the system (e.g., interactive commands,
 application programs).  This type of client is especially important,
 since client needs at lower layers can be regarded as translations of
 the needs expressed by human clients at the top of the hierarchy.
 When the client is human, the server consists of the entire
 (distributed) system, including the hosts, their operating systems,
 and the networks interconnecting them.
 As for the generic term, performance, we will give it a fairly broad
 meaning.  It will include not only delay and throughput, the two main
 network performance indices, but also reliability of message
 delivery.  Real-time communication is concerned with those aspects of
 quality of service that have to do with performance in this broad
 sense.
 The term guarantee in this paper has a rather strong legal flavor.
 When a server guarantees a given level of performance for the
 communications of a client, it commits itself to providing that
 performance and to paying appropriate penalties if the actual
 performance turns out to be insufficient.  On the other hand, the
 client will have to obey certain rules, and will not be entitled to
 the requested performance guarantees unless those rules are
 scrupulously obeyed.  In other words, client and server have to enter
 into a contract specifying their respective rights and duties, the
 benefits that will accrue, the conditions under which those benefits
 will materialize, and the penalties they will incur for not keeping
 their mutual promises.  We believe that a legal viewpoint is to be
 adopted if serious progress in the delivery of communication services
 (not only the real-time ones) is desired.  Utility services, as well
 as other kinds of service, are provided under legally binding
 contracts, and a mature computer communication utility cannot fail to
 do the same.  In the field of real-time communication, such a
 contract will by definition include performance guarantees.
 Real-time services may be offered in any kind of network or
 internetwork. Some of their predictable applications are:
    (a)  digital continuous-media (motion video, audio)
         communication: lower bounds on throughput and upper bounds
         on delay or delay variability or both are needed to ensure
         any desired level of output quality; in the interactive case,
         both the values of delay and delay variabilities have to be

Ferrari [Page 2] RFC 1193 Requirements for Real-Time Services November 1990

         bounded; some limited message losses are often tolerable in
         the cases of video and voice (whenever very high quality is
         not required), but usually not in the case of sound;
    (b)  transmission of urgent messages in real-time distributed
         systems: delay bounds are the important guarantees to be
         provided in these applications; losses should ideally be
         impossible;
    (c)  urgent electronic-mail messages and, more in general,
         urgent datagrams: again, delay is the obvious index to be
         bounded in this case, but small probabilities of losses can
         often be tolerated;
    (d)  transfers of large files: minimum throughput bounds are
         usually more important than delay bounds in this
         application; also, all pieces of a file must be delivered
         with probability 1;
    (e)  fast request-reply communication: e.g., data base queries,
         information retrieval requests, remote procedure calls; this
         is another case in which delay (more precisely, round-trip
         delay) is the index of primary interest; reliability
         requirements are generally not very stringent.
 We conjecture that, when networks start offering well-designed and
 reasonably-priced real-time services, the use of such services will
 grow beyond the expectations of most observers.  This will occur
 primarily because new performance needs will be induced by the
 availability of guaranteed-performance options.  As the history of
 transportation and communication has repeatedly shown, faster
 services bring about major increases of the shipments that are
 perceived as urgent.  The phenomenon will be more conspicuous
 whenever the quality of service provided to non-real-time clients
 will deteriorate.  It is clear from this comment that we assume that
 real-time services will coexist within the same networks and
 internetworks with non-real-time communications.  Indeed, postulating
 a world in which the two types of service are segregated rather than
 integrated would be unrealistic, as it would go against the clear
 trend towards the eventual integration of all information services.
 For the same reason, the traffic in the network is assumed to be
 heterogeneous, i.e., to consist of a variety of types of messages,
 representing a variety of information media and their combinations,
 with a wide spectrum of burstiness values (from uncompressed
 continuous fixed-rate streams to very short and erratic bursts of
 information).
 This paper discusses the client requirements and characteristics of a

Ferrari [Page 3] RFC 1193 Requirements for Real-Time Services November 1990

 real-time communication service.  Server requirements and design
 principles will be the subject of a subsequent paper.  Section 2
 contains some considerations about the ways in which the clients
 specify their requirements, and those in which a server should reply
 to requests for real-time services.  Performance requirements are
 presented in Section 3; other properties that clients may need or
 desire are described in Section 4.  Section 5 deals with the problem
 of translating the requirements of a human client or an application
 for the equivalent lower-level ones.  In Section 6, we briefly
 present four examples of client requirement specifications, and in
 Section 7 we discuss some of the objections that can be raised
 against our approach.

2. Client Requests and Server Replies

 No real-time service can be provided if the client does not specify,
 together with the requirements, the characteristics of the expected
 input traffic.  Describing input traffic and all the various
 requirements entails much work on the part of a client.  Gathering
 the necessary information and inputting it may be very time-
 consuming.  A well-designed real-time communication service will
 minimize the effort to be spent by a client.
 Sensible default values, the possibility of partial or incremental
 specifications (e.g., by editing preexisting specifications), and a
 number of standard descriptions should be provided.  These
 descriptions will include characterizations of inputs (e.g., those of
 a video stream for multimedia conferencing, an HDTV stream, a hi-fi
 audio stream, a file transfer stream, and so on) and standard sets of
 requirements.  With these aids, it might be possible for a human
 client to specify his or her request by a short phrase, perhaps
 followed by a few characters representing options or changes to the
 standard or default values.
 Since requests for real-time services may be denied because of a
 mismatch between the client's demands and the resources available to
 the server, the client will appreciate being informed about the
 reasons for any rejection, so that the request can be modified and
 resubmitted, or postponed, or cancelled altogether [Herr89].  The
 information provided by the server to a human client should be
 meaningful, useful, and non-redundant.  The reason for rejection
 should be understandable by the client (who should be assumed not to
 know any of the details of the operating system, of the protocols or
 of the network) and should be accompanied by data that will be useful
 to the client in deciding what to do as well as how the request ought
 to be modified to make it successful.  If, for example, a bound
 specified by the client cannot be guaranteed by the server under its
 current load, the information returned to the client should include

Ferrari [Page 4] RFC 1193 Requirements for Real-Time Services November 1990

 the minimum or maximum value of the bound that the server could
 guarantee; the client will thus be able to decide whether that bound
 would be acceptable (possibly with some other modifications as well)
 or not, and act accordingly.
 When the client is not a human being but an application or a process,
 the type of a server's replies should be very different from that
 just described [Herr89]; another standard interface, the one between
 an application and a real-time service, must therefore be defined,
 possibly in multiple, application-specific versions.
 Clients will also be interested in the pricing policies implemented
 by the server: these should be fair (or at least perceived to be
 fair) and easy to understand. The client should be able easily to
 estimate charges for given performance guarantees as a function of
 distance, time of day, and other variables, or to obtain these
 estimates from the server as a free off-line service.

3. Performance Requirements

 A client can specify a service requirement using the general form
                             pred = TRUE,
 where some of the variables in predicate pred can be controlled or
 influenced by the server.
 A simple and popular form of performance requirement is that
 involving a bound.  A deterministic bound can be specified as
                (var <= bound) = TRUE, or var <= bound,
 where variable var is server-controlled, while bound is client-
 specified.  The bounds in these expressions are upper bounds; if  <
 is replaced by  > , they become lower bounds.
 When the variable in the latter expression above is a probability, we
 have a statistical bound, and bound in that case is a probability
 bound; if the predicate is a deterministic bound, we have:
               Prob (var <= bound) >= probability-bound.
 In this requirement, the variable has an upper bound, and the
 probability a lower bound.  Note that deterministic bounds can be
 viewed as statistical bounds that are satisfied with probability 1.
 A form of bound very similar to the statistical one is the fractional
 bound:

Ferrari [Page 5] RFC 1193 Requirements for Real-Time Services November 1990

                        Ca (var <= bound) >= b,
 where variable var has a value for each message in a stream, and Ca
 is a function that counts the number of times var satisfies the bound
 for any a consecutive messages in the stream; this number Ca must
 satisfy bound b.  Obviously, a fractional bound is realizable only if
 b <= a .  Fractional bounds will not be explicitly mentioned in the
 sequel, but they can be used in lieu of statistical bounds, and have
 over these bounds the avantages of easy verifiability and higher
 practical interest.
 In this section, we restrict our attention to those requirements that
 are likely to be the most useful to real-time clients.

3.1 Delay requirements

 Depending on the application, clients may wish to specify their delay
 requirements in different ways [Gait90].  The delays involved will
 usually be those of the application-oriented messages known to the
 client; for instance, the delay between the beginning of the client-
 level transmission of a video frame, file, or urgent datagram and the
 end of the client-level reception of the same frame, file, or urgent
 datagram.  (In those cases, e.g., in some distributed real-time
 systems, where message deadlines are assigned instead of message
 delays, we can always compute the latter from knowledge of the former
 and of the sending times, thereby reducing ourselves again to a delay
 bound requirement.)  Also, they will be the delays of those messages
 that are successfully delivered to the destination; the fraction of
 messages that are not, to which the delay bounds will not apply, will
 be bounded by reliability specifications.  Note that clients will
 express delay bounds by making implicit reference to their own
 clocks; the design of a real-time service for a large network will
 have to consider the impact on bounds enforcement of non-synchronized
 clocks [Verm90].  Some of the forms in which a delay requirement may
 be specified are
 (i)  deterministic delay bound:
                        Di <= Dmax  for all i,
 the client is delivered to the destination client-level entity, and
 Dmax is the delay upper bound specified by the client.  In our
 descriptions we assume, without loss of generality, that the client
 requesting a real-time service is the sending client, and that the
 destination (which could be a remote agent of the client or another
 user) is a third party with respect to the establishment of the
 particular communication being considered (In our descriptions we
 assume, without loss of generality, that the client requesting a

Ferrari [Page 6] RFC 1193 Requirements for Real-Time Services November 1990

 real-time service is the sending client, and that the destination
 (which could be a remote agent of the client or another user) is a
 third party with respect to the establishment of the particular
 communication being considered.);
 (ii)  statistical delay bound:
                     Prob ( Di <= Dmax ) >= Zmin,
    where Di and Dmax are defined as above, and Zmin is the lower
    bound of the probability of successful and timely delivery;
 (iii)  deterministic delay-jitter bound:
                 Ji = | Di - D | <= Jmax   for  all i,
    where D is the ideal, or target delay, Ji is the delay jitter of
    the i-th message delivered to the destination, and Jmax is the
    upper jitter bound to be specified by the client together with D;
    note that an equivalent form of this requirement consists of
    assigning a deterministic upper bound D + Jmax and a deterministic
    lower bound D - Jmax to the delays Di [Herr90];
 (iv)  statistical delay-jitter bound:
                 Prob (Ji <= Jmax) >= Umin, for all i,
    where  Umin  is the lower bound of the probability that Ji  be
    within its limits.
 Other forms of delay bound include bounds on average delay, delay
 variance, and functions of the sequence number of each message, for
 example, Dmax(i) for the deterministic case.  There may be
 applications in which one of these will be the preferred form, but,
 since we have not found any so far, we believe that the four types of
 bounds listed as (i)-(iv) above will cover the great majority of the
 practical cases.

3.2 Throughput requirements

 The actual throughput of an information transfer from a source to a
 destination is bounded above by the rate at which the source sends
 messages into the system.  Throughput may be lower than this rate
 because of the possibility of unsuccessful delivery or message loss.
 It is also bounded above by the maximum throughput, which is a
 function of, among other things, network load.  As the source
 increases its input rate, the actual throughput will grow up to a
 limit and then stop.  Clients concerned with the throughput of their

Ferrari [Page 7] RFC 1193 Requirements for Real-Time Services November 1990

 transfers will want to make sure that saturation is never reached, or
 is reached only with a suitably small probability and for acceptably
 short intervals.  Also, if the bandwidth allocated to a transfer is
 not constant, but varies dynamically on demand to accommodate, at
 least to some extent, peak requests, clients will be interested in
 adding an average throughput requirement, which should include
 information about the length of the interval over which the average
 must be computed [Ferr89a].
 Thus, reasonable forms for throughput requirements appear to be the
 following:
 (i)  deterministic throughput bound:
                        Ti >= Tmin, for all i,
    where Ti is the throughput actually provided by the server, and
    Tmin is the lower bound of throughput specified by the client,
    that is, the minimum throughput the server must offer to the
    client;
 (ii)  statistical throughput bound:
                      Prob (Ti >= Tmin) >= Vmin,
    where Ti and Tmin are defined as above, and Vmin is the lower
    bound of the probability that the server will provide a throughput
    greater than the lower bound;
 (iii) average throughput bound:
                              T >= Tave,
    where T is the average throughput provided by the server, Tave is
    its lower bound specified by the client, and both variables are
    averaged over an interval of duration I specified by the client;
    the above inequality must obviously hold for all intervals of
    duration I, i.e., even for that over which T is minimum.
 One clear difference between delay bounds and throughput bounds is
 that, while the server is responsible for delays, the actual
 throughputs of a non-saturated system are dictated by the input
 rates, which are determined primarily by the clients (though they may
 be influenced by the server through flow-control mechanisms).

Ferrari [Page 8] RFC 1193 Requirements for Real-Time Services November 1990

3.3 Reliability requirements

 The usefulness of error control via acknowledgments and
 retransmission in real-time applications is doubtful, especially in
 those environments where message losses are usually higher, i.e., in
 wide-area networks: the additional delays caused by acknowledgment
 and retransmission, and out-of-sequence delivery are likely to be
 intolerable in applications with stringent delay bounds, such as
 those having to do with continuous media.  Fortunately, the loss of
 some of the messages (e.g., video frames, voice packets) is often
 tolerable in these applications, but that of sound packets is
 generally intolerable.  In other cases, however, completeness of
 information delivery is essential (e.g., in file transfer
 applications), and traditional retransmission schemes will probably
 have to be employed.
 A message may be incorrect when delivered or may be lost in the
 network, i.e., not delivered at all.  Network unreliability (due, for
 example, to noise) is usually the cause of the former problem; buffer
 overflow (due to congestion) or node or link failure are those of the
 latter.  The client is not interested in this distinction: for the
 client, the message is lost in both cases.  Thus, the simplest form
 in which a reliability bound may be expressed and also, we believe,
 the one that will be most popular, is
            Prob (message is correctly delivered) >= Wmin,
 where Wmin is the lower bound of the probability of correct delivery,
 to be specified by the client.  The probability of message loss will
 obviously be bounded above by 1 - Wmin.  This is a statistical bound,
 but, as noted in Section 3, a deterministic reliability bound results
 if we set Wmin = 1.
 In those applications in which any message delivered with a delay
 greater than Dmax must be discarded, the fraction of messages usable
 by the destination will be bounded below by Wmin Zmin.  The client
 may actually specify the value of this product, and let the server
 decide the individual values of the two bounds, possibly subject to a
 client-assigned constraint, e.g., that the price of the service to
 the client be minimum.
 If the value of Wmin is greater than the system's reliability (the
 probability that a delivered message is correct), then there is no
 buffer space allocation in the hosts, interfaces, switches and
 routers or gateways that will allow the client-specified Wmin to be
 guaranteed.  In this case, the server uses error correcting codes, or
 (if the application permits) retransmission, or duplicate messages,
 or (if the sequencing problem discussed in Section 4.1 can be solved

Ferrari [Page 9] RFC 1193 Requirements for Real-Time Services November 1990

 satisfactorily or is not a problem) multiple physical channels for
 the same logical channel, or has to refuse the request.

4. Other Required or Desirable Properties

 In this section, we briefly describe client requirements that cannot
 be easily expressed as bounds on, but are related to, communication
 performance.  These include sequencing, absence of duplications,
 failure recovery, and service setup time. We are not concerned here
 with features that may be very important but have a functionality
 (e.g., multicast capabilities) or security (e.g., client
 authentication) rather than a performance flavor. Requirements in
 these areas will generally have appreciable effects also on
 performance; we do not discuss them only because of space
 limitations.
 For a given application, some of these properties may be required,
 some others only desirable.  Also, some may be best represented as
 Boolean variables (present or absent), some others as continuous or
 multi-valued discrete variables, others yet as partially qualitative
 specifications.

4.1 Sequencing

 For applications involving message streams (rather than single
 datagrams), it may be necessary or desirable that messages be
 delivered in sequence, even though the sequence may not be complete.
 If the lower-level servers are not all capable of delivering messages
 sequentially, a resequencing operation may have to be performed at
 some higher level in the hierarchy.  In those cases in which
 reliability requirements make retransmission necessary, resequencing
 may delay delivery of a large number of messages by relatively long
 times.  An adequate amount of buffer space will have to be provided
 for this purpose at the level of the resequencer in the protocol
 hierarchy.
 If sequencing is not guaranteed by all servers at all levels, the
 application may be able to tolerate out-of-sequence messages as long
 as their number is small, or if the delay bound is so large that very
 few out-of-sequence messages have to be discarded because they are
 too late.  The client could be allowed to specify a bound on the
 probability that a message be delivered out of sequence, or to bundle
 out-of-sequence losses with the other types of message loss described
 by Wmin.  The client would specify the value of Wmin (or Wmin Zmin),
 and the server would have to decide how much probability to allow for
 buffer overflow, how much for network error, and how much for
 imperfect sequencing, taking into account the stringency of the delay
 bounds.

Ferrari [Page 10] RFC 1193 Requirements for Real-Time Services November 1990

 On the other hand, with fixed-route connections and appropriate
 queueing and scheduling in the hosts and in the network, it is often
 not too hard to ensure sequenced delivery at the various layers,
 hence also at the top.

4.2 Absence of duplications

 Most of the discussion of sequencing applies also to duplication of
 messages.  It is, however, easier and faster to eliminate
 duplications than to resequence, as long as some layer keeps track of
 the sequence numbers of the messages already received.  The
 specification of a bound may be needed only if duplications become
 very frequent, but this would be a symptom of serious network
 malfunction, and should not be dealt with in the same way as we
 handle delays or message losses.  These observations do not apply, of
 course, to the case of intentional duplication for higher
 reliability.

4.3 Failure recovery

 The contract between client and server of a real-time service will
 have to specify what will happen in the event of a server failure.
 Ideally, from the client's viewpoint, failures should be perfectly
 masked, and service should be completely fault-tolerant.  As we have
 already mentioned, however, it is usually unrealistic to expect that
 performance guarantees can be honored even in presence of failures.
 A little less unrealistic is to assume that service can resume a
 short time after a failure has disrupted it.  In general, clients may
 not only wish to know what will happen if a failure occurs, but also
 have a guaranteed upper bound on the likelihood of such an
 occurrence:
                        Prob (failure) <= Fmax.
 Different applications have different failure recovery requirements.
 Urgent datagrams or urgent message streams in most real-time
 distributed systems will probably not benefit much from recovery,
 unless it can be made so fast that hard deadlines may still be
 satisfied, at least in some cases.  In the case of video or audio
 transmission, timely resumption of service will normally be very
 useful or even necessary; thus, clients may need to be given
 guarantees about the upper bounds of mean or maximum time to repair;
 this may also be the case of other applications in which the
 deadlines are not so stringent, or where the main emphasis is on
 throughput and/or reliability rather than on delay.
 In communications over multi-node routes and/or long distances, the
 network itself may contain several messages for each source-

Ferrari [Page 11] RFC 1193 Requirements for Real-Time Services November 1990

 destination pair at the time a failure occurs.  The recovery scheme
 will have to solve the problems of failure notification (to all the
 system's components involved, and possibly also to the clients) and
 disposition of messages in transit.  The solutions adopted may make
 duplicate elimination necessary even in contexts in which no
 duplicates are ever created in the absence of failures.

4.4 Service setup time

 Real-time services must be requested before they can be used to
 communicate [Ferr89b].  Some clients may be interested in long-term
 arrangements which are set up soon after the signing of a contract
 and are kept in existence for long times (days, months, years).
 Others, typically for economical reasons, may wish to be allowed to
 request services dynamically and to avoid paying for them even when
 not in use.  The extreme case of short-term service is that in which
 the client wants to send one urgent datagram, but this is probably
 best handled by a service broker ("the datagraph office") using a
 permanent setup shared by many (or all) urgent datagrams.  In most
 other cases, a request for a short-term or medium-term service must
 be processed by the server before the client is allowed to receive
 that service (i.e., to send messages).  Certain applications will
 need the setup time to be short or, in any case, bounded: the maximum
 time the client will have to wait for a (positive or negative) reply
 to a request may have to be guaranteed by the server in the contract.

5. Translating Requirements

 Performance specifications and other requirements are assigned at the
 top level, that of the human client or application, either explicitly
 or implicitly (see Section 2).  To be satisfied, these specifications
 need the support of all the underlying layers: we believe that a
 real-time service cannot be implemented on top of a server at some
 level that is unable to guarantee performance.  (Some of the other
 requirements can be satisfied even without this condition: for
 example, reliable delivery (when retransmission is acceptable) and
 sequencing.)  Upper-level requirements must be translated into
 lower-level ones, so that the implementation of the former will be
 adequately supported.  How should this be done?

5.1 Delay requirements

 The method for translating delay bounds macroscopically depends on
 the type of bound to be translated.  All methods have to deal with
 two problems: the effects of delays in the individual layers, and the
 effects of message fragmentation on the requirements.
 (i)  Deterministic delay bound.  A deterministic bound on the delay

Ferrari [Page 12] RFC 1193 Requirements for Real-Time Services November 1990

      encountered by a message in each layer (or group of layers) in
      the hosts will have to be estimated and enforced.
      The delay bound for a server at a given level will be obtained
      by subtracting the delay bounds of the layers above it in both
      the sending and the receiving host from the original global
      bound:
                    Dmax' = Dmax - SUMi {d(max,i)}.
    Message fragmentation can be handled by recalling that delay is
    defined as the difference between the instant of completion of the
    reception of a message and the instant when its shipment began.
    If x is the interfragment time (assumed constant for simplicity
    here) and f is the number of fragments in a message, we have
                          Dmax' = Dmax - x(f-1),
    where Dmax' is the fragment delay bound corresponding to the
    message delay bound Dmax, i.e., the delay of the first fragment.
 (ii)  Statistical delay bound.  The statistical case is more
       complicated.  If the bounds on the delay in each layer
       (or group of layers) are statistical, we may approach the
       problem of the messages delayed beyond the bound
       pessimistically, in which case we shall write
                  Zmin' = Zmin / (PRODi {z(min,i)}),
    where the index i spans the layers (or group of layers) above the
    given lower-level server, Zmin' is the probability bound to be
    enforced by that lower-level server, and d(max,i) and z(min,i) are
    the bounds for layer i.  (A layer has a sender side and a receiver
    side at the same level in the hierarchy.)  The expression for
    Zmin' is pessimistic because it assumes that a message delayed
    beyond its bound in a layer will not be able to meet the global
    bound Dmax.  (The expression above and the next one assume that
    the delays of a message in the layers are statistically
    independent of each other.  This assumption is usually not valid,
    but, in the light of the observations that follow the next
    expression, the error should be tolerable.)
    At the other extreme, we have the optimistic approach, which
    assumes that a message will not satisfy the global bound only if
    it is delayed beyond its local bound in each layer:
              Zmin' = 1 - (1 - Zmin)/(PRODi {1 - z(min,i)}).

Ferrari [Page 13] RFC 1193 Requirements for Real-Time Services November 1990

    The correct assumption will be somewhere in between the
    pessimistic and the optimistic ones.  However, in order to be able
    to guarantee the global bound, the system will have to choose the
    pessimistic approach, unless a better approximation to reality can
    be found.  An alternative that may turn out to be more convenient
    is the one of considering the bounds in the layers as
    deterministic, in which case Zmin' will equal Zmin, and the global
    bound will be statistical only because the network will guarantee
    a statistical bound.
    When estimating the effects of message fragmentation, the new
    bounds must refer to the fragment stream as though its components
    were independent of each other.  Assuming sequential delivery of
    fragments, a message is delayed beyond its bound if its last
    fragment is delayed beyond the fragment bound.  Our goal can be
    achieved by imposing the same probability bound on fragments as on
    messages [Verm90]. Thus,
                              Zmin' = Zmin.
    Note that both expressions for D prime sub max given in (i) above
    apply to the statistical delay bound case as well.
 (iii) Deterministic delay-jitter bound.  For the case of layer to
       layer translation, the discussion above yields:
                   Jmax' = Jmax - SUMi {j(max,i)} ,
    where j(max,i) is the deterministic jitter bound of the i-th layer
    above the given lower-level server.  When messages are fragmented,
    the delay jitter bound can be left unchanged:
                              Jmax' = Jmax .
    There would be reasons to reduce it in the case of message
    fragmentation only if the underlying server did not guarantee
    sequenced delivery, and if no resequencing of fragments were
    provided by the corresponding reassembly layer on the receiving
    side.
 (iv)  Statistical delay-jitter bound.  The interested reader will
       be able with little effort to derive the translation formulas
       for this case from the definition in Section 3.1 (iv)
       and from the discussion in (ii) and (iii) above.

Ferrari [Page 14] RFC 1193 Requirements for Real-Time Services November 1990

5.2 Throughput requirements

 Since all layers are in cascade, the throughput bounds would be the
 same for all of them if headers and sometimes trailers were not added
 at each layer for encapsulation or fragmentation. Thus, throughput
 bounds have to be increased as the request travels downward through
 the protocol hierarchy, and the server at each layer knows by how
 much, since it is responsible for these additions.

5.3 Reliability requirements

 If we assume, quite realistically, that the probability of message
 loss in a host is extremely small, then we do not have to change the
 value of Wmin when we change layers.
 The effects of message fragmentation are similar to those on
 statistical delay bounds, but in a given application a message may be
 lost even if only one of its fragments is lost.  Thus, we have
                      Wmin' = 1 - (1 - Wmin)/f ,
 where Wmin' is the lower bound of the correct delivery probability
 for the fragment stream, and f is the number of fragments per
 message.  The optimistic viewpoint, which is the one we adopted in
 Section 5.1 (ii), yields Wmin' = Wmin, and the observations made in
 that section about the true bound and about providing guarantees
 apply.

5.4 Other requirements

 Of the requirements and desiderata discussed in Section 4, those that
 are specified as a Boolean value or a qualitative attribute do not
 have to be modified for lower-level servers unless they are satisfied
 in some layer above those servers (e.g., no sequencing is to be
 required below the level where a resequencer operates).  When they
 are represented by a bound (e.g., one on the setup time, as described
 in Section 4.4), then bounds for the layers above a lower-level
 server will have to be chosen to calculate the corresponding bound
 for that server.  The above discussions of the translation of
 performance requirements will, in most cases, provide the necessary
 techniques for doing these calculations.
 The requirement that the server give clear and useful replies to
 client requests (see Section 2) raises the interesting problem of
 reverse translation, that from lower-level to upper-level
 specifications.  However, at least in most cases, this does not seem
 to be a difficult problem: all the translation formulas we have
 written above are very easily invertible (in other words, it is

Ferrari [Page 15] RFC 1193 Requirements for Real-Time Services November 1990

 straightforward to express Dmax as a function of Dmax', Zmin as a
 function of Zmin', and so on).

6. Examples

 In this section we describe some examples of client requirements for
 real-time services.  Simplifying assumptions are introduced to
 decrease the amount of detail and increase clarity.  Our intent is to
 determine the usefulness of the set of requirements proposed above,
 and to investigate some of the problems that may arise in practical
 cases.  An assumption underlying all examples is that the network's
 transmission rate is 45 Mbits/s, and that the hosts can keep up with
 this rate when processing messages.

6.1 Interactive voice

 Let us assume that human clients are to specify the requirements for
 voice that is already digitized (at a 64 kbits/s rate) and packetized
 (packet size: 48 bytes, coinciding with the size of an ATM cell;
 packet transmission time: 8.53 microseconds ; packet interarrival
 time: 6 ms).  Since the communication is interactive, deterministic
 (and statistical) delay bounds play a very important role.  Jitter is
 also important, but does not dominate the other requirements as in
 non-interactive audio or video communication (see Section 6.2).  The
 minimum throughput offered by the system must correspond to the
 maximum input rate, i.e., 64 kbits/s; in fact, because of header
 overhead (5 control bytes for every 48 data bytes), total guaranteed
 throughput should be greater than 70.66 kbits/s, i.e., 8,834 bytes/s.
 (Since the client may not know the overhead introduced by the system,
 the system may have to compute this value from the one given by the
 client, which in this case would be 8 kbytes/s.)  The minimum average
 throughput over an interval as long as 100 s is 44% of Tmin, due to
 the silence periods [Brad64].
 Voice transmission can tolerate limited packet losses without making
 the speech unintelligible at the receiving end.  We assume that a
 maximum loss of two packets out of 100 (each packet corresponding to
 6 ms of speech) can be tolerated even in the worst case, i.e., when
 the two packets are consecutive.  Since packets arriving after their
 absolute deadline are discarded if the delay bound is to be
 statistical, then this maximum loss rate must include losses due to
 lateness, i.e., 0.98 will have to be the value of Zmin Wmin rather
 than just that of Wmin.
 This is illustrated in the first column of Table Ia, which consists
 of two subcolumns: one is for the choice of a deterministic delay
 bound, the other one for that of a statistical delay bound and a
 combined bound on the probability of lateness or loss.  If in a row

Ferrari [Page 16] RFC 1193 Requirements for Real-Time Services November 1990

 there is a single entry, that entry is the same for both subcolumns.
 Note that the maximum setup time could be made much longer if
 connections had to be reserved in advance.
 Since voice is packetized at the client's level, we will not have to
 worry about the effects of fragmentation while translating the
 requirements into their lower-level correspondents.

6.2 Non-interactive video

 At the level of the client, the video message stream consists of 1
 Mbit frames, to be transmitted at the rate of 30 frames per second.
 Thus, the throughput bounds (both deterministic and average) are,
 taking into account the overhead of ATM cell headers, 4.14 Mbytes/s.
 As in the case of interactive voice, we have two alternatives for the
 specification of delay bounds: the first subcolumn is for the
 deterministic bound case, the second for that of a statistical bound
 on delays and a combined probability bound on lateness or loss; the
 latter bound is set to at most 10 frames out of 100, i.e., three out
 of 30.  However, the really important bound in this case is the one
 on delay jitter, set at 5 ms, which is roughly equal to half of the
 interval between two successive frames, and between 1/4 and 1/5 of
 the transmission time.  This dominance of the jitter bound is the
 reason why the other delay bounds are in parentheses.
 If we assume that video frames will have to be fragmented into cells
 at some lower level in the protocol hierarchy, then these
 requirements must be translated at that level into those shown in the
 first column of Table II.  The values of Dmax' have been calculated
 with x = 12.8 microseconds and f = 2605 fragments/frame.  The range
 of Wmin' and of (Zmin Wmin)' is quite wide, and achieving its higher
 value (a probability of 1) may turn out to be either very expensive
 or impossible.  We observe, however, that a frame in which a packet
 or more are missing or have been incorrectly received does not have
 to be discarded but can be played with gaps or patched with the old
 packets in lieu of the missing or corrupted ones.  Thus, it may be
 possible to consider an optimistic approach (e.g., Zmin' = Zmin,
 Wmin' = Wmin, (Zmin Wmin)' = Zmin Wmin ) as sufficiently safe.

6.3 Real-time datagram

 A real-time datagram is, for instance, an alarm condition to be
 transmitted in an emergency from one machine to another (or a group
 of others) in a distributed real-time system.  The client
 requirements in this case are very simple: a deterministic bound is
 needed (we are assuming that this is a hard-real-time context), the
 reliability of delivery must be very high, and the service setup time
 should be very small.  The value of 0.98 for Wmin in Table Ib tries

Ferrari [Page 17] RFC 1193 Requirements for Real-Time Services November 1990

 to account for the inevitable network errors and to suggest that
 retransmission should not be used as might be necessary if we wanted
 to have Wmin = 1, because it would be too slow.  To increase
 reliability in this case, error correcting codes or spatial
 redundancy will have to be resorted to instead.
 Note that one method for obtaining a very small setup time consists
 of shipping such urgent datagrams on long-lasting connections
 previously created between the hosts involved and with the
 appropriate characteristics.  Note also that throughput requirements
 cannot be defined, since we are dealing with one small message only,
 which may not even have to be fragmented.  Guarantees on the other
 bounds will fully satisfy the needs of the client in this case.

6.4 File transfer

 Large files are to be copied from a disk to a remote disk.  We assume
 that the receiving disk's speed is greater than or equal to the
 sending disk's, and that the transfer could therefore proceed, in the
 absence of congestion, at the speed of the sending disk.  The message
 size equals the size of one track (11 Kbytes, including disk surface
 overhead such as intersector gaps), and the maximum input rate is
 5.28 Mbits/s.  Taking into account the ATM cell headers, this rate
 becomes 728 kbytes/s; this is the minimum peak throughput to be
 guaranteed by the system.  The minimum average throughput to be
 provided is smaller, due to head switching times and setup delays
 (seek times are even longer, hence need not be considered here): we
 set its value at 700 kbytes/s.
 Delay bounds are much less important in this example than in the
 previous ones; in Table Ib, we show deterministic and statistical
 bounds in parentheses.  Reliability must be eventually 1 to ensure
 the integrity of the file's copy.  This result will have to be
 obtained by error correction (which will increase the throughput
 requirements) or retransmission (which would break most delay bounds
 if they were selected on the basis of the first shipment only instead
 of the last one).
 The second column in Table II shows the results of translating these
 requirements to account for message fragmentation.  The values x =
 78.3 microseconds and f = 230 have been used to compute those of
 Dmax'.

7. Discussion

 In this section, we briefly discuss some of the objections that can
 be raised concerning our approach to real-time service requirements.
 Some of the objections are fundamental ones: they are at least as

Ferrari [Page 18] RFC 1193 Requirements for Real-Time Services November 1990

 related to the basic decisions to be made in the design of the server
 as they are to client requirements.
 Objection 1: Guarantees are not necessary.
 This is the most radical objection, as it stems from a basic
 disagreement with our definition of real-time service.  The problem,
 however, is not with definitions or terminologies: the really
 important question is whether a type of service such as the one we
 call "real-time" will be necessary or at least useful in future
 networks.  This objection is raised by the optimists, those who
 believe that network bandwidth will be so abundant that congestion
 will become a disease of the past, and that delays will therefore be
 small enough that the enforcement of legalistic guarantees will not
 be necessary.  The history of computers and communications, however,
 does not unfortunately support these arguments, while it supports
 those of the pessimists.  In a situation of limited resources
 (limited with respect to the existing demand for them), we believe
 that there is no serious solution of the real-time communication
 problem other than one based on a policy for the allocation of
 resources that rigorously guarantees the satisfaction of performance
 needs.  Even if the approaches to be adopted in practical networks
 will provide only approximate guarantees, it is important to devise
 methods that offer without exceptions precisely defined bounds.
 These methods can at the very least be used as reference approaches
 for comparison and evaluation.
 Objection 2: Real-time services are too expensive because reservation
 of resources is very wasteful.
 This may be true if resources are exclusively reserved; for example,
 physical circuits used for bursty traffic in a circuit-switched
 network.  There are, however, other ways of building real-time
 services, based on priority mechanisms and preemption rather than
 exclusive reservation of resources.  With these schemes, the real-
 time traffic always finds the resources it needs by preempting non-
 real-time traffic, as long as the real-time load is kept below a
 threshold.  The threshold corresponds to the point where the demand
 by real-time traffic for the bottleneck resource equals the amount of
 that resource in the system.  With this scheme, all resources not
 used by real-time traffic can be used at any time by local tasks and
 non-real-time traffic.  Congestion may affect the latter, but not
 real-time traffic.  Thus, the only limitation is that a network
 cannot carry unbounded amounts of real-time traffic, and must refuse
 any further requests when it has reached the saturation point.

Ferrari [Page 19] RFC 1193 Requirements for Real-Time Services November 1990

 Objection 3: Real-time services can be built on top of non-real-time
 servers.
 If one accepts our interpretation of the term "guarantee," one can
 easily see that performance guarantees cannot be provided by a
 higher-level server unless it can rely on real-time support by its
 underlying server.  Since this is true at all levels, we conclude
 that a real-time network service and similar services at all
 intermediate levels are needed to provide guaranteed performance to
 human clients and applications.
 Objection 4: Delay bounds are not necessary, throughput requirements
 suffice.
 Guaranteeing minimum throughput bounds does not automatically and in
 general result in any stringent upper bound on delay.  Delays in the
 hosts and nodes of a packet-switching network fluctuate because of
 bursty real-time message streams, starting and ending of traffic on
 individual connections (even those with continuous, constant-rate
 traffic), and the behavior of scheduling algorithms.  Even if delays
 did not fluctuate, but had a constant value, it would be possible for
 a given throughput bound to be satisfied with many different constant
 values for the delay of each message.  If delay bounds are wanted,
 they must be explicitly guaranteed and enforced.  (In a circuit-
 switching network, the circuit assigned to a connection has its own
 throughput and its own delay.  These values may be considered as
 explicitly guaranteed and enforced.)
 But are delay bounds wanted?  We believe they are in digital video
 and audio communication, especially in the form of delay jitter
 bounds, and they will be in other contexts as soon as a service which
 can bound delays is offered.
 Objection 5: Satisfaction of statistical bounds is impossible to
 verify.
 Strictly speaking, this objection is valid.  No matter how many
 packets on a connection have been delayed beyond their bound (or lost
 or delivered with errors), it is always in principle possible for the
 server to redress the situation in the future and meet the given
 statistical requirements.  A more sensible and verifiable bound would
 be a fractional one (see Section 3).  For instance, such a bound
 could be specified as follows: out of 100 consecutive packets, no
 less than 97 shall not be late.  In this case, the bound is no longer
 Zmin, a probability of 0.97, but is given by the two values B = 97
 and A = 100; it is not only their ratio that counts but also their
 individual values.

Ferrari [Page 20] RFC 1193 Requirements for Real-Time Services November 1990

8. Conclusion

 This paper has presented a specification of some of the requirements
 that human clients and applications may wish to impose on real-time
 communications.  Though those listed seem to be among the most useful
 and natural ones, no attempt has been made to be exhaustive and
 comprehensive.
 We have investigated delay bounds, throughput bounds, reliability
 bounds, and other requirements.  We have studied how the requirements
 should be translated from the client's level into forms suitable (and
 correct) for lower levels, described some examples of requirement
 specification, and discussed some of the objections that may be
 raised.
 The material in this paper covers only part of the first phase in the
 design of a real-time service: that during which the various
 requirements are assembled and examined to extract useful suggestions
 for the design of the server.  Server needs and design principles
 will be the subject of the subsequent paper mentioned several times
 above.

Acknowledgments

 Ralf Herrtwich and Dinesh Verma contributed ideas to, and corrected
 mistakes in, a previous version of the manuscript.  The author is
 deeply indebted to them for their help and for the many discussions
 he had with them on the topics dealt with in this paper.  The
 comments of Ramesh Govindan and Riccardo Gusella are also gratefully
 acknowledged.

References

 [Brad64]  Brady, P., "A Technique for Investigating On-Off Patterns
           of Speech", Bell Systems Technical Journal, Vol. 44,
           Pgs. 1-22, 1964.
 [Ferr89a] Ferrari, D., "Real-Time Communication in
           Packet-Switching Wide-Area Networks", Technical Report
           TR-89-022, International Computer Science Institute,
           Berkeley, May 1989.
 [Ferr89b] Ferrari D., and D. Verma, "A Scheme for Real-Time Channel
           Establishment in Wide-Area Networks", IEEE J. Selected
           Areas Communications SAC-8, April 1990.
 [Gait90]  Gaitonde, S., D. Jacobson, and A. Pohm, "Bounding Delay on
           a Multifarious Token Ring Network", Communications of the

Ferrari [Page 21] RFC 1193 Requirements for Real-Time Services November 1990

           ACM, Vol. 33, No. 1, Pgs. 20-28, January 1990.
 [Herr89]  Herrtwich R., and U. Brandenburg, "Accessing and
           Customizing Services in Distributed Systems", Technical
           Report TR-89-059, International Computer Science Institute,
           Berkeley, October 1989.
 [Herr90]  Herrtwich, R, personal communication, February 1990.
 [Verm90]  Verma, D., personal communication, February 1990.
                               Table Ia
                  Examples of Client Requirements
                         Interactive  Non-Interactive
                            Voice           Video

Delay Bounds deterministic:Dmax [ms] 200 - (1000) - statistical:Dmax [ms] - 200 - (1000)

          Zmin            -     (*)       -      (*)

jitter:Jmax [ms] 1 5

Throughput Bounds deterministic:Tmin [kby/s] 8.834 4140 average:Tave [kby/s] 3.933 4140

      I [s]                 100              100

Reliability Bound:Wmin 0.98 (*) (0.90) (*) Delay&Reliability:ZminWmin - 0.98 - 0.90

Sequencing yes yes Absence of Duplications yes yes Failure Recovery: max.repair time [s] 10 100 Max.Setup Time [s] 0.8 (o) 15 (o)


(*) To be chosen by the server (o) Could be much longer if advance reservations were required (+) Could be achieved by using a preexisting connection

Ferrari [Page 22] RFC 1193 Requirements for Real-Time Services November 1990

                               Table Ib
                  Examples of Client Requirements
                         Real-Time     File
                          Datagram   Transfer

Delay Bounds deterministic:Dmax [ms] 50 - (1500) statistical:Dmax [ms] - (1000) -

          Zmin                -    (0.95)   -

jitter:Jmax [ms] - -

Throughput Bounds deterministic:Tmin [kby/s] - 728 average:Tave [kby/s] - 700

      I [s]                   -          100

Reliability Bound:Wmin 0.98 1 Delay&Reliability:ZminWmin - -

Sequencing - yes Absence of Duplications yes yes Failure Recovery: max.repair time [s] - 100 Max.Setup Time [s] 0 (+) 5 (o)


(*) To be chosen by the server (o) Could be much longer if advance reservations were required (+) Could be achieved by using a preexisting connection

Ferrari [Page 23] RFC 1193 Requirements for Real-Time Services November 1990

                              Table II
                Translation of the Requirements in Table I
                         Non-Interactive            File
                              Video               Transfer

Delay Bounds deterministic:Dmax' [ms] (966) - - (1482) statistical:Dmax' [ms] - (966) (982) -

          Zmin'              -     (*)         (0.95)    -

jitter:Jmax' [ms] 5 -

Reliability Bound:Wmin' 0.90-1 (*) 1

Delay&Reliability:(ZminWmin)' - 0.90-1 -

_

(*) To be chosen by the server

Security Considerations

 Security considerations are not discussed in this memo.

Author's Address

 Domenico Ferrari
 University of California
 Computer Science Division
 EECS Department
 Berkeley, CA 94720
 Phone: (415) 642-3806
 EMail: ferrari@UCBVAX.BERKELEY.EDU

Ferrari [Page 24]

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