This module implements the OSI Connection-Oriented Transport Protocol
   specification International Standard ISO 8073; and for DEC OSF/1, the
   Connectionless-Mode Transport Service Protocol International Standard ISO
   8602. These protocols implement the OSI Reference Model Transport Layer 4.

   These protocols, as well as the NSP protocol, implement the transport
   protocols in the Digital Network Architecture.

  The entities that constitute the OSI Transport module are listed below. The
  indention indicates the hierarchical relationships  between the entities.

                               OSI Transport
                                  Application
                                  Port
                                  Template
                                  Local NSAP
                                     Remote NSAP

   The Connection-Oriented Transport Service COTS provides transparent and
   reliable transfer of data between Transport Service users.  It operates over
   the Network layer, and employs end-to-end protocols that enhance the inherent
   characteristics of the underlying network service.

   The protocol is structured to produce protocol classes that minimize the
   possible incompatibilities and implementation costs.  A protocol class is
   selected for a particular transport connection, based on the characteristics
   of the network service to be used, the requirements of the Transport Service
   user, and the availability of classes at the remote Transport Service
   provider.

   This module implements class 0, class 2, and class 4 of the COTS protocol.
   Class 0 is the simplest class.  It sets up a network connection for each
   transport connection requested, and assumes that the network connection does
   not make errors.  The transport protocol does neither sequencing nor flow
   control, as it relies on the underlying network layer to perform everything
   correctly.

   Class 2, like class 0, is designed to be used over reliable networks.  It
   differs from class 0 in that two or more transport connections may be
   multiplexed over the same network connection.

   Class 4, the most complex of the protocols, is designed to work over
   unreliable Network Services.  It can handle the error detection and recovery
   of lost transport packets.  These packets are called TPDUs Transport
   Protocol Data Units.  Class 4 runs over either CLNS or CONS.  Classes 0 and
   2 run only over CONS.

   The Routing module provides a connectionless network service CLNS; for more
   information, refer to the Routing chapter.

   The X.25 Access module, if configured into the system, provides a reliable
   connection-oriented network service CONS.

   The OSI Transport entity is the top-level entity in the hierarchy of
   entities belonging to the OSI Transport module.