Internet DRAFT - draft-ietf-tn3270e-implementation
draft-ietf-tn3270e-implementation
TN3270 Enhancements Working Group Jon Penner
Internet Draft August 1993
TN3270 Current Practices
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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
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
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reason for the differentiation of TN3270 from standard Telnet in
this document.
Background
Existing proprietary IBM networks are not easily integrated with
the increasing number of multi-platform networking environments,
specifically TCP/IP. These proprietary IBM networks are referred to
as SNA (Systems Network Architecture) in this document. To address
this issue, several vendors have introduced telnet servers that
provide a TCP/IP users a connection to existing IBM mainframes by
supporting display terminal emulation using a subset of the existing
telnet protocol. IBM now also offers host based tn3270 support over
TCP/IP.
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 2 parallel
sessions with the host computer. One is used for communicating with
the host application, the other is used for communicating with the
network control program 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.
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
telnet servers are connected to the host using SNA, tn3270 clients
using these servers have no defined way to determine the LU name
associated with the session.
Currently, support for 3270 terminal emulation over Telnet is
accomplished by the de facto standard of negotiating three separate
Telnet Options - Terminal-Type [2], Binary Transmission [3], and
End of Record [4]. This negotiation and the resulting data flow will
be described below.
RFC 1041 [1] attempted to standardize the method of negotiating
3270 terminal support by defining the 3270 Regime Telnet Option.
Historically, very few developers and vendors ever implemented
RFC 1041.
All references in this document to the 3270 datastream, SNA versus
non-SNA operation, 3270 datastream commands, orders, structured
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fields and the like rely on [6].
References to SNA Request and Response Units rely on [7].
TN3270 does not support typical SNA responses and is classified as a
non-SNA protocol. A TN3270 emulator is not aware or concerned about
how the telnet server is connected to a 3270 host application.
Current telnet server implementations are either 3270 host based or
connected to the host using the SNA protocol.
NOTE: Except where otherwise stated, this document does not
distinguish between telnet servers that represent SNA devices and
those that represent non-SNA 3270 devices.
Some typical "SNA" functions such as the SYSREQ
and ATTN keys have been mapped to existing telnet function codes
and are supported by some telnet server implementations.
There are several shortcomings in current tn3270 implementations;
among them are the following:
- It provides no capability for Telnet clients to emulate the 328x
class of printers.
- There is no mechanism by which a Telnet client can request that
a connection be associated with a given 3270 device-name. This
can be of importance when a terminal session is being
established, since many host applications behave differently
depending on the network name of the terminal. In the case of
printer emulation, this capability is an absolute necessity
because a large number of host applications have some method of
pre-defining printer destinations.
- The 3270 ATTN and SYSREQ keys are not universally supported.
- There is no support for the SNA positive/negative response
process. All data that is sent is assumed to either be handled
or ignored.
A negative response indicates some sort of error at the client
while processing the previously received data; this could be
caused by the host application building a 3270 datastream that
contains an invalid command, or by a mechanical error at the
client side, among other things.
Positive responses indicate processing of the previously received
data has completed.
- There is no mechanism by which the client can access the SNA
BIND information. The BIND image in a SNA environment
contains a detailed description of the session between the
telnet server and the host application.
- The connection negotiation does not make it clear whether clients
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should support 3270 structured fields.
These and other issues are addressed by the Internet-Draft [?]
written by Bill Kelly
Command Names and Codes
TN3270 makes use of existing Telnet commands and codes and does
not define any new commands.
BINARY 0
TERMINAL-TYPE 24
EOR 25
Additional commands may be used during a TN3270 session and are
interpreted as per their respective RFCs. Examples of this are
3270-REGIME, SUPPRESS-GO-AHEAD, and TM.
Command Meanings
See respective RFCs.
Connection Negotiation
The following example shows a TN3270-capable server and a
tn3270 client establishing a connection:
The TCP/IP port generally used to connect with is 23 (Telnet).
At any place before and during the tn3270 connection negotiation
process, other telnet commands and data may be transferred and
will be interpreted under the existing telnet state. Some existing
tn3270 servers start a client connection using an NVT telnet
dialog to establish parameters needed to complete the tn3270
connection to the desired host.
The order of negotiating terminal type, EOR and BINARY is not
significant, this example shows a typical tn3270 connection.
Server: IAC DO TERMINAL-TYPE
Client: IAC WILL TERMINAL-TYPE
Server: IAC SB TERMINAL-TYPE SEND IAC SE
Client: IAC SB TERMINAL-TYPE IS <terminal type>IAC SE
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where <terminal type> is a string consisting of terminal model,
type and support of enhanced attribute bytes; and example is
IBM-3278-2. The acceptable values are listed in RFC 1340,
Assigned Numbers [5]. Other values are in use that do not exist
in [5].
The -2 following 3278 designates the alternate screen size.
3270 terminals have the ablity to switch between the
standard (24x80) screen size and an alternate screen size.
Model -2 is 24x80 which is the same as the standard size.
Model -3 is 32x80, model -4 is 43x80 and model -5 is 27x132.
Appending the two character string "-E" to the end of the
terminal type signifies that the terminal is capable of handling
3270 extended data stream. This is interpreted to mean that the
terminal is able to handle structured fields, which are
described below. Some telnet server implementations also
interpret this to mean that the terminal is capable of handling
extended attributes (highlighting, field validation, character
set, outlining, etc.) [6].
The 3279 series of terminals is capable of extended attributes
while the 3278 series is not.
Server: IAC DO EOR IAC WILL EOR
Client: IAC WILL EOR IAC DO EOR
Server: IAC DO BINARY IAC WILL BINARY
Client: IAC WILL BINARY IAC DO BINARY
Server: <3270 data stream>
Client: <3270 data stream>
. .
. .
To terminate the connection the socket is closed by one of
the session partners. Typically, when the user logs off of the
host, the telnet server closes the connection.
If the telnet server wishes to go back to NVT mode, it may issue
the following commands:
Server: IAC WONT BINARY
Client: IAC DONT BINARY
or
Server: IAC WONT EOR
Client: IAC DONT EOR
Either one of the above two cases causes the connection to not
satisfy the requirements for a valid TN3270 session. The telnet
client should then process data from the server as though it
were NVT ASCII data.
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Handling 3270 data
The 3270 data stream consists of a command and its associated data.
Commands include but are not limited to erase screen, erase and write
to screen and read current screen; see [6] for a complete description
of 3270 commands and parameters.
The reason for negotiating the EOR telnet option [4] is to provide a
method for separating these commands since no length information is
specified. 3270 commands are interpreted by the telnet client in
their entirety. Each 3270 command and possible data is terminated
with the IAC EOR sequence.
The Binary option [3] is also required since 3270 data may contain
the FF (hexadecimal) or IAC character. When this character is
encountered during a tn3270 connection it is handled as per the
Binary RFC [3].
3270 Structured Fields
3270 structured fields provide a much wider range of features than
"old-style" 3270 data, such as support for graphics, partitions and
IPDS printer datastreams. A structured field is a 3270 data type
that allows non 3270 data to be embedded within 3270 data. Briefly,
a structured field consists of the structured field command followed
by one or more data blocks. Each data block has a length and a
structured field identifier, followed optionally by additional data.
Not every TN3270 client can be expected to support all structured
field functions. There must be a mechanism by which those clients
that are capable of supporting some or all structured field
functions can indicate their wishes. This is typically done by
adding "-E" to the end of the terminal type string. That is, when the
terminal identifies itself as being able to handle extended
attributes, it also is capable of being able to send and receive
structured fields.
The design of 3270 structured fields provides a convenient means to
convey the level of support (including no support) for the various
structured field functions. This mechanism is the Read Partition
Query command, which is sent from the host application to the
client. The client responds with a Query Reply, listing which,
if any, structured field functions it supports.
A TN3270 client that supports structured fields will
respond to a Read Partition Query command with the appropriate reply.
The sequence of events of when a client receives a Read Partition
Query and it does not support structured fields is left up to the
client implementation. Typically clients can identify at least this
structured field and reply with a null set.
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Implementation Rules
All commands and parameters are governed by their respective RFCs
The 3270 ATTN (Attention) Key
The 3270 ATTN key is interpreted by many host applications in an
SNA environment as an indication that the user wishes to interrupt
the execution of the current process. A majority of the telnet
servers currently accept the telnet IAC BREAK (code 243) sequence to
signal this event.
Use of this key requires two things:
- TN3270 clients should provide as part of their keyboard
mapping a single key or a combination of keys that map to
the 3270 ATTN key. When the user presses this key(s), the
client should transmit a Telnet BREAK command to the server.
- TN3270 servers should translate the BREAK command received from
a TN3270 client into the appropriate form and pass it along
to the host application as an ATTN key. In other words, the
server representing an SLU in an SNA session should send
a SIGNAL RU to the host application.
The ATTN key is not supported in a non-SNA environment; therefore,
a TN3270 server representing non-SNA 3270 devices should ignore
any Telnet BREAK commands it receives from a client.
The 3270 SYSREQ Key
The 3270 SYSREQ key is useful in an environment where the
telnet server is attached to the host using SNA. The SYSREQ key
is useful in this environment when the host application becomes
locked and the user wishes to terminate the session without
closing the Telnet connection.
The Telnet Interrupt Process (IP) command is interpreted by some
telnet servers as a SYSREQ key. Other servers recognize the 3270
Test Request key as a SYSREQ key. In an SNA environment, pressing
this key toggles the terminal between the host application session
and the network control program session. Usually the user
will enter LOGOFF once this key has been pressed to terminate
the application session and then select a new host to connect to.
Sometimes, if SYSREQ is pressed again, the host application will
become unlocked and normal activities may then proceed.
It is entirely up to the telnet server to interpret this command and
send the appropriate commands to the host as well as format the
resulting host data for display on the telnet client. The data
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format during the network control program session is in a slightly
different format than normal 3270 data. Since the telnet server
has no way to pass this data directly to the telnet client, it must
either handle it entirely and ignore SYSREQ events or convert it to
3270 data to present to the client. There is no restriction that
prevents the server from negotiating out of the TN3270 state to
NVT to present this data.
In order to implement SYSREQ key support, TN3270
clients should provide a key (or combination of keys) that is
identified as mapping to the 3270 SYSREQ key. When the user presses
this key(s), the client should either transmit a Telnet IP command
or Test Request key to the server, depending on the server
implementation.
TN3270 servers representing non-SNA 3270 terminals may ignore any
Telnet IP commands or Test Request keys they receive from a client.
References
[1] Rekhter, J., "Telnet 3270 Regime Option", RFC 1041, IBM
Corporation, January 1988.
[2] VanBokkelen, J., "Telnet Terminal-Type Option", RFC 1091,
FTP Software, Inc., February 1989.
[3] Postel, J., and J. Reynolds, "Telnet Binary Transmission",
RFC 856, USC/Information Sciences Institute, May 1983.
[4] Postel, J., "Telnet End of Record Option", RFC 885, USC/
Information Sciences Institute, December 1983.
[5] Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1340,
USC/Information Sciences Institute, July 1992.
[6] "3270 Information Display System - Data Stream Programmer's
Reference", publication number GA23-0059, IBM Corporation.
[7] "Systems Network Architecture - Formats",
publication number GA27-3136, IBM Corporation.
Author's Note
Portions of this document were drawn from the following sources:
- A White Paper written by Owen Reddecliffe, WRQ Corporation,
October 1991.
- Experimental work on the part of Cleve Graves and Michelle
Angel, OpenConnect Systems, 1992 - 1993.
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- Discussions at the March 1993 IETF meeting.
- Discussions on the "TN3270E" list, Spring 1993.
- An Internet-draft written by Bill Kelly, Auburn University,
July, 1993
Author's Address
Jon Penner
DCA, Inc.
2800 Oakmont Drive
Austin, TX 78664
Phone: (512) 244-3871
Email: jjp@bscs.uucp
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