RFC 983 (rfc983) - Page 2 of 27

ISO transport arrives on top of the TCP

RFC 983                                                       April 1986
ISO Transport Services on Top of the TCP


      a service-required definitions, which describes the services used
      by the layer and the interfaces it uses to access those services.

   Collectively, all of the entities in the network which co-operate to
   provide the service are known as the service-provider. Individually,
   each of these entities is known as a service-peer.

   Internally, a layer is defined by one definition:

      a protocol definition, which describes the rules which each
      service-peer uses when communicating with other service-peers.

   Putting all this together, the service-provider uses the protocol and
   services from the layer below to offer the its service to the layer
   above.  Protocol verification, for instance, deals with proving that
   this in fact happens (and is also a fertile field for many Ph.D.
   dissertations in computer science).

   The concept of layer-independence quite simply is:

      IF one preserves the services offered by the service-provider

      THEN the service-user is completely naive with respect to the
      protocol which the service-peers use

   For the purposes of this memo, we will use the layer-independence to
   define a Transport Service Access Point (TSAP) which appears to be
   identical to the services and interfaces offered by the ISO/CCITT
   TSAP (as defined in [ISO-8072]), but we will base the internals of
   this TSAP on TCP/IP (as defined in [RFC-793,RFC 791]), not on the
   ISO/CCITT transport and network protocols.  Hence, ISO/CCITT higher
   level layers (all session, presentation, and application entities)
   can operate fully without knowledge of the fact that they are running
   on a TCP/IP internetwork.

   The authors hope that the preceding paragraph will not come as a
   shock to most readers.  However, an ALARMING number of people seem to
   think that layering is just a way of cutting up a large problem into
   smaller ones, *simply* for the sake of cutting it up.  Although
   layering tends to introduce modularity into an architecture, and
   modularity tends to introduce sanity into implementations (both
   conceptual and physical implementations), modularity, per se, is not
   the end goal.  Flexibility IS.






Cass & Rose