RFC 1646 (rfc1646) - Page 2 of 13

TN3270 Extensions for LUname and Printer Selection

RFC 1646                   TN3270 Extensions                   July 1994


   IBM mainframe applications systems have existed for almost two
   decades now and are used to serve tens of thousands of users daily.
   For this reason it is usually the need of a mainframe environment to
   add TCP/IP network support WITHOUT writing new applications to run
   with the TCP network.  The TN3270 series of documents addresses how
   this can be done and maintain compatibility with those mainframe
   application systems.

   One of the unique characteristics of the 3270 terminals is their
   ability to communicate status information in an out-of-band data
   flow.  These status's are in turn used by the applications systems to
   support error recovery, and conflict resolutions, examples of these
   are printer out of paper, and terminal powered up.  The terminals are
   also half duplex and block mode in their operations, which results in
   the need to communicate when blocks are being sent, when they end,
   and when they cannot be sent.  This document describes these
   characteristics in IBM VTAM/SNA terms.  Some VM mainframe application
   systems do not use VTAM, so for those systems these terms don't
   apply.  For any systems which use VTAM these terms apply and are
   dealt with in some way by the TCP/IP to VTAM interface.

   VTAM/SNA is a hierarchical network and some of that hierarchy needs
   to be addressed by the TCP network attaching to it if the
   applications systems are to continue to provide the same applications
   support that they have provided to the 3270 terminals.

   The 3270 terminal environment consists of a terminal controller with
   terminals attached to that controller.  In VTAM/SNA this controller
   is called a PU (Physical Unit) and the terminals called LUs (Logical
   Units).  The PU is used to communicate management information to the
   VTAM/SNA system, and the LU is used by the application to communicate
   with the terminal.  VTAM/SNA identifies each LU and PU in a network
   by a unique name.  These names are referred to as LUnames and
   PUnames, and is how the network is managed and the applications
   identify what terminals are being communicated with in the network.
   The actual connection between a terminal and the applications is
   referred to as a session, and it is this session which has both in-
   band and out-of-band information flows sent between the applications
   and the terminals.

   VTAM/SNA 3270 terminals actually have two sessions when communicating
   with the applications.  One session is directly connected with the
   application and the other session is connected directly to VTAM.  It
   is the session with VTAM, also called the SSCP, that is used to
   communicate the out-of-band information flows.  This session is
   called the SSCP-LU session, and the session with the application is
   called the LU-LU session (in VTAM an applications is just another
   Logical Unit).



Graves, Butts & Angel