RFC 1193 (rfc1193) - Page 2 of 24

Client requirements for real-time communication services

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



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