TN3270 Current Practices
RFC 1576
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RFC - Informational
(January 1994; No errata)
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Author |
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Jon Penner
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2013-03-02
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RFC 1576 (Informational)
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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
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