TN3270 Current Practices
RFC 1576

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Network Working Group                                          J. Penner
Request for Comments: 1576                                     DCA, Inc.
Category: Informational                                     January 1994

                        TN3270 Current Practices

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This document describes the existing implementation of transferring
   3270 display terminal data using currently available telnet
   capabilities.  The name traditionally associated with this
   implementation is TN3270.

   Information is provided to aid in the implementation of TN3270
   servers as well as client terminal emulators.

   The following areas pertaining to TN3270 implementations are covered
   in this document:

      1. the telnet options negotiated to transition from a NVT ASCII
         state to a TN3270 state ready to process incoming 3270 data
         stream commands

      2. the method for sending and receiving 3270 data

      3. the method of handling some special keys known as SYSREQ and
         ATTN using current available telnet commands

      4. the events that will transition a TN3270 session back to an NVT
         session

Table of Contents

      1.  Motivation  . . . . . . . . . . . . . . . . . . .   2
      2.  Background  . . . . . . . . . . . . . . . . . . .   2
      3.  Telnet Options and Commands Used  . . . . . . . .   4
      4.  Connection Negotiation  . . . . . . . . . . . . .   4
      4.1 3270 Regime Option  . . . . . . . . . . . . . . .   6
      4.2 Suppress Go Ahead Option  . . . . . . . . . . . .   6
      4.3 Echo Option . . . . . . . . . . . . . . . . . . .   6
      4.4 Timing Mark Option  . . . . . . . . . . . . . . .   7

TN3270 Enhancements Working Group                               [Page 1]
RFC 1576                TN3270 Current Practices            January 1994

      5.  Testing for session presence  . . . . . . . . . .   7
      6.  Handling 3270 data  . . . . . . . . . . . . . . .   7
      7.  3270 Structured Fields  . . . . . . . . . . . . .   8
      8.  The 3270 ATTN (Attention) Key . . . . . . . . . .   8
      9.  The 3270 SYSREQ Key . . . . . . . . . . . . . . .   9
      10. Items not addressed by TN3270 . . . . . . . . . .  10
      11. References  . . . . . . . . . . . . . . . . . . .  10
      12. Security Considerations . . . . . . . . . . . . .  11
      13. Author's Note . . . . . . . . . . . . . . . . . .  11
      14. Author's Address  . . . . . . . . . . . . . . . .  12

1. Motivation

   3270 display terminal data differs from traditional display terminal
   data in that it is block mode and uses EBCDIC instead of ASCII
   character representation. These two differences are the primary
   reason for the differentiation of TN3270 from standard Telnet in this
   document.

2. Background

   Existing complex IBM 3270 display terminal networks are not easily
   integrated with the increasing number of multi-platform networking
   environments, specifically TCP/IP. These complex networks include
   terminals attached to a 3270 host using SNA (Systems Network
   Architecture) and non-SNA connections. To address the issue of easily
   connecting display terminals to 3270 hosts using IP networks, several
   vendors have introduced telnet servers that provide TCP/IP users a
   connection to existing IBM mainframes by supporting display terminal
   emulation using a subset of the existing telnet protocol.  Telnet
   servers may exist on the host itself, or be connected to the host
   using SNA or non-SNA methods.

   IBM terminals are generically referred to as 3270's which includes a
   broad range of terminals and devices, not all of which actually begin
   with the numbers 327x.

   3270 terminals in the IBM SNA network environment have two sessions
   with the host computer application. One is used for communicating
   with the host application, the other is used for communicating with
   the SSCP (System Services Control Point) that links the terminal with
   the appropriate host computer.  For the purposes of TN3270, this
   distinction is not apparent or relevant since there is actually only
   a single telnet session with the host computer or server.  On an IBM
   SNA network, the 3270 terminal has a special key that toggles between
   the two sessions (SYSREQ).  A brief discussion on how some telnet
   servers deal with this is included.

TN3270 Enhancements Working Group                               [Page 2]
RFC 1576                TN3270 Current Practices            January 1994

   In an SNA environment, a client session is identified by a Logical
   Unit (LU) name.  In a non-SNA environment, there is not a LU name
   associated with a client session.  The closest thing to a LU name in
   the TN3270 environment is the client's IP address.  Although some