The following table describes the protocol classes, their functions,
    and which Network service can be used.

    Protocol  Functions                               Network Service
    Class
    ---------------------------------------------------------------------

    TP 0      Provides a basic Transport Service.     CONS and RFC1006

    TP 2      Provides Provides all functions of      CONS and RFC1006 Ext
              TP 0. Provides multiplexing of more
              than one transport connection over a
              Network Connection or TCP connection.
              Provides flow control over CONS.

    TP 4      Provides all functions of TP 2.         CONS and CLNS
              Provides error detection and recovery.

    Some other differences are that:

    o  TP 0 relies on the upper layers to do its error correction.
       This class is disconnected if the underlying Network layer is
       disconnected.

    o  TP 2 and 4 use disconnect requests.

    o  TP 4 reassigns the OSI Transport connection to another Network
       layer connection if the existing one fails.

    When a Transport user sets up a Transport connection, a preferred
    protocol class for the connection is specified in the connection
    request. The responding Transport user must either agree to this
    protocol class, or suggest an alternative protocol class that is
    acceptable to the initiating user. If no such agreement is possible,
    the Transport connection cannot be set up.