RFC1646 - TN3270 Extensions for LUname and Printer Selection

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　　Network Working Group C. Graves
Request for Comments: 1646 T. Butts
Category: Informational M. Angel
Open Connect Systems
July 1994

TN3270 Extensions for LUname and Printer Selection

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 protocol extensions to TN3270. There are two
extensions outlined in this document. The first defines a way by
which a TN3270 client can request a specific device (LUname) from a
TN3270 server. The second extension specifies how a TN3270 printer
device can be requested by a TN3270 client and the manner in which
the 3270 printer status information can be sent to the TN3270 server.
Discussions and suggestions for improvements to these enhancements
should be sent to the TN3270E Working Group mailing list
TN3270E@list.nih.gov . These extensions will be called TN3287 in this
document. This information is being provided to members of the
Internet community that want to support the 3287 data stream within
the TELNET protocol.

1. INTRODUCTION

The need to communicate with IBM mainframe systems has a number of
unique requirements associated with it. This document addresses
those needs in a TCP/IP communications network.

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.

The 3270 family of terminals and the IBM mainframe applications
systems are VERY closely coupled and it is the nature of the way the
3270s and the applications interact which require that this document
be available to provide a consistent way for the TCP/IP environment
to interact effectively with the 3270 applications of the IBM
mainframe world.

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).

One such out-of-band flow is the LUSTAT message which tells the
application that the status of the terminal has changed, and is how a
printer or screen tells the application that it is ready, or is not
ready to receive data.

There are also flows which must be able to flow in the LU-LU session
to help control the use of the terminal by applications. The block
of information sent in a session is called an RU (Request Unit) and
it tells what type of data this block contains, how long it is and if
more data (RUs) is coming along. This is a gross over simplification
of what RUs are and do, but it should help understand their use in
the TN3270 documents. Some of the VTAM/SNA terms used to describe
what an RU is requesting are: Chains/chaining which tell a session
partner that another RU is being sent or not being sent in this
transmission. Brackets which are used to indicate that a unit of
work is complete, such as when a printout of a file is complete.

The determination of what part of the VTAM/SNA protocols such as
brackets and chaining are to be used are managed by VTAM tables
called LOGMODE tables. These tables are selected when an LU-LU
session is started and set up such things as bracket, and/or chaining
protocols; and the type of terminal data contained in the RUs, such
as printer data without screen formatting data (LU type 1), 3270
screen formatted data (LU type 2) and 3270 screen formatted data for
a printer (LU type 3). The LOGMODE tables also contain the size of
the RU to be sent and received. These tables also communicate the
screen size of 3270 terminals such as 24X80 (Model 2), 27X132 (Model
5), etc. Each LU has a LOGMODE table entry hard assigned to it as
part of the VTAM configuration (often called a GEN). The selection
of these table entries can't be controlled by the terminal LU or PU.
They can only be selected by the user at connection/logon time or by
the application when the connection is established. The actual
LOGMODE entries to be used during a session are sent at session logon
time, in a special type of RU called a BIND. Once the bind has been
sent then the rules for the use of the session have been set, can't
be changed, and must be followed.

The purpose of the TN3287 protocol is to provide a general IBM 3270
host printer communications facility. Its primary goal is to allow a
method of connecting printer devices and printer-oriented processes
to each other. This protocol will allow a TN3270 Client to process
3287 print data streams.

This memo supplements and extends the STD 8, RFC854, TELNET Protocol
Specification. This memo also presents an example of the correct
implementation.

2. GENERAL CONSIDERATIONS

A TELNET connection is a Transmission Control Protocol (TCP)
connection used to transmit data with interspersed TELNET control
information.

The companion document, STD 8, RFC854 -- "TELNET Protocol
Specification" should be consulted for further information about the
TELNET command, codes and code sequences referenced in this
specification.

3. CLIENT-SERVER NEGOTIATION

The TN3270 Client and Server require a specific negotiation protocol.
After the negotiation is complete, all transmission between the
Client and Server is in TELNET Binary format with a TELNET "End-Of-
Record(EOR)" sequence at the end of each data stream.

Support for the TN3287 data stream requires that both sides:

A. Are able to exchange binary data.

B. Can establish the agreement between client and server on the
terminal type that will be used.

C. Agree to use the TELNET IAC EOR as a delimiter for inbound
and outbound TN3287 data streams

This implementation requires the options: TERMINAL-TYPE and BINARY be
successfully negotiated between the Client and Server before
processing of any print data streams.

This implementation supports host applications that can mix LU 1 and
LU 3 type data in the data stream.

3.1 TN3287 SERVER

The maximum Request Unit (RU) size is server specific, but should not
exceed 4 kilobytes.

The LU type is determined by the bind from the mainframe application.
The server, when bound, must remember LU 1 or LU 3 type.

The server will automatically unbind the session upon receipt of a
TELNET CLOSE command. The printer will be reported to VTAM as
powered down until a new TELNET connection is established.

3.2 TN3287 CLIENT

The TN3287 Client is a TN3270 client created specifically to print
mainframe 3270 print data. The client emulates the IBM device type
that it identifies itself to the TN3270 server as, in this case, an
IBM 3287 model 1 type printer. The design of this printer protocol
is aligned with the way printing occurs in the IBM host and how 3270
printers function. These printer extensions DO NOT support a 3270
printer client that cannot accept both types LU 1 and LU 3 printer
streams. No IBM printer operates in this fashion, and as a result,
no TN3270 server could function properly with mainframe applications
if it didn't allow for a mixing of LU 1 and LU 3 data streams. The
common way in which this can occur is printer sharing between
multiple IBM host applications, such as CICS and JES. Since there is
no restriction, the JES can be configured to output LU 1 data
streams, and the CICS can be configured for LU 3 data streams.
Therefore, the server will identify what LU type the current
application connected to the server is using. If that type is LU 1,
ALL message records sent to the Client will be preceded by one byte
of binary zeros (0x00). If the first byte is not zeros, then that
byte will be a valid LU type 3 Write-Command-Code(WCC), which can
NEVER be zeros. Thus, the client can tell the LU type of data as
each record is received.

This protocol does allow for the client to shutdown if the client
does not wish to support both LU types. This is accomplished by
detecting an invalid data type from the received record, and
notifying the user that the mainframe application has sent LU type x
print data and should be configured for LU type y printing.

4. COMMAND STRUCTURE

1. All TELNET commands consist of at least a two-byte sequence:
the "Interpret-as-Command(IAC)" escape character followed by
the code for the command.

NOTE: Since the TELNET IAC character (255 decimal) is used as a
delimiter (together with EOR) in the inbound and outbound data
streams, a data byte within the data stream itself that has the same
value as the IAC command is sent as two bytes (255, 255) and one byte
is discarded.

4.1 TELNET COMMANDS

Command meaning - WILL and DO commands are used to obtain and grant
permission for the subsequent subnegotiation. Both sides must
exchange WILL TERM-TYPE and DO TERM-TYPE before subnegotiation.

The actual exchange of information is done within the option
subcommand.

<IAC DO TERMINAL-TYPE> Sender requests that the other party begin
terminal-type sub-negotiation.

<IAC WILL TERMINAL-TYPE> Sender is willing to send terminal-type
information in a subsequent sub-negotiation.

<IAC SB TERMINAL-TYPE SEND IAC SE> Sender requests the receiver to
transmit his terminal-type.

<IAC SB TERM TYPE IS IBM-3287-1 IAC SE> Sender is stating the name
of his terminal-type. The code for <IS> is 0. Optionally, a
specific Logical Unit (LU) can be requested by using the TERMINAL-
TYPE string below. If no LUname is specified, the first available
3287 LU is selected.

IAC SB TERM-TYPE IS IBM-3287-1 @ LUNAME IAC SE

<IAC DO BINARY> Sender requests that sender of the data starts
transmitting or confirms that the sender of data is expected to
transmit characters that are to be interpreted as 8 bits of binary
data by the receiver.

<IAC WILL BINARY> Sender requests permission to begin transmitting,
or confirms it will now begin transmitting binary data.

An <EOR> is sent at the end of each SNA Request Unit (RU) end of
chain, in either direction. The first byte following the <EOR> is a
Write-Command-Code(WCC) for LU 3 data streams.

An <AO> is sent at the end of the SNA RU and end of bracket. This
signifies the end of the print output or file by the IBM host
application and possibly a change of LU type.

4.2 COMMAND VALUES

TELNET COMMAND CODE
IAC Interpret as Command 255
DO 253
WILL 251
SB SuBnegotiation option 250
SE Subnegotiation End 240
TERMINAL-TYPE 24
SEND 1
IS 0

EOR End-Of-Record 25
BINARY 0
AO Abort Output 245
IP Interrupt Process 244
AYT Are You There 246
BREAK 243

NOTE: The above codes and code sequences have the indicated meaning
only when immediately preceded by an "Interpret as Command (IAC)".

5. TN3270 Printer Status Message

The status message can be sent at any time. It must be sent every
time the TN3270 Server sends an End-of-Record(EOR) indicator to the
TN3270 Client, or when a printing error occurs at the Client. The
Printer Status Message is only sent by the TN3270 Client. Once the
End-Of-Record IAC is processed, the TN3270 Client sends the status
message to the server when it is ready to receive more print data.

MESSAGE DESCRIPTION: SOH % R S1 S2 IAC EOR

SOH = 0X01
% = 0X6C
R = 0XD9
S1 = Status/Sense Byte 0
S2 = Status/Sense Byte 1
IAC = Telnet IAC Character
EOR = Telnet EOR Character

5.1 Status/Sense Byte description

5.1.1. S/S Byte 0:

High Order Low Order

_____________________________________________________________
| |
| 0 1 2 3 4 5 6 7 |
|___________________________________________________________|

Bit Number: Bit Definition:

0 Always Zero

1 Always Zero

2 Always Zero

3 Always Zero

4 Always Zero

5 Unit Specify - is set due to an error
condition. The reason for the error
condition will be indicated in S/S Byte 1.
See Note 1*.

6 Device End - when this bit sent in response
to a data message it indicates the client
has successfully processed the data message
from the server and notifies the server to
send a new data message to the client when
available. See Note 2*.

7 Always Zero

Note 1*: A negative response to the Server's data message would be
S/S Byte 0 Bit 5 "Unit Specify condition". The possible Unit Specify
conditions are listed below. (See Section 3.2 for bit settings for
the Unit Specify conditions listed below.)

Unit Specify Condition: SNA Sense Code sent to host:

Command Rejected 0X10030000
Intervention Required 0X08020000
Data Check 0X10010000
Operation Check 0X10050000
Component Disconnected (LU) 0X08020000

Note 2*: Device End - A positive response to the Server's data
message would be the "Device End" bit (S/S Byte 0 Bit 6) to indicate
a ready to receive data from the host condition. This will also be
sent after clearing a previous Unit Specify condition of
"Intervention Required".

5.1.2. S/S Byte 1:

High Order Low Order

______________________________________________________________
| |
| 0 1 2 3 4 5 6 7 |
|____________________________________________________________|

Bit Number: Bit Definition:

0 Always Zero

1 Always Zero

2 Command Rejected (CR) -- This bit
indicates an invalid 3270 command
generated.

3 Intervention Required - Printer Not Ready.
See Note 3*.

4 Component Disconnected - Printer is powered
off or printer cable not connected. See
Note 4*.

5 Data Check - Invalid print data

6 Always Zero

7 Operation Check - An illegal buffer address
or incomplete order sequence

Note 3*: The Intervention Required is cleared by sending an S/S
message with the "Device End" bit (Bit 6 of S/S byte 0). The LUSTAT
sent to the host is 0X00010000. The IBM host interprets this as a
"printer now ready" condition.

Note 4*: The Component disconnected is cleared by sending an S/S
message with the "Device End" bit (Bit 6 of S/S byte 0). The LUSTAT
sent to the host is 0X082B0000. The IBM host interprets this as a
"printer now ready -- presentation space integrity may be lost"
condition.

6. The following is an example of the Client-Server negotiation
process.

Server: IAC DO TERMINAL-TYPE
Client: IAC WILL TERMINAL-TYPE
Server: IAC SB TERMINAL-TYPE SEND IAC SE
Client: IAC SB TERMINAL-TYPE IS IBM-3287-1 IAC SE

Note: To request a specific LU the TERMINAL-TYPE string would be:
IAC SB TERMINAL-TYPE IS IBM-3287-1 @ LUNAME IAC SE
(The client has specified its terminal type is an IBM-3287-1)

Server: IAC DO END-OF-RECORD
Client: IAC WILL END-OF-RECORD
Server: IAC WILL END-OF-RECORD
Client: IAC DO END-OF-RECORD

(The Server and Client have both agreed to transmit End-Of-Record
(EOR)).

Server: IAC DO TRANSMIT-BINARY
Client: IAC WILL TRANSMIT-BINARY
Server: IAC WILL TRANSMIT-BINARY
Client: IAC DO TRANSMIT-BINARY
(The Server and Client have both agreed to use binary
transmission)

Server: 0x00 (3270 PRINT DATA)
Client: (S/S with DEV END) IAC EOR
Server: 0x00 (3270 PRINT DATA) IAC EOR

NOTE: LU 1 type data is prefaced with a 0x00 character. LU 3
type data is not prefaced with a special character. This
character will precede print data in each chain, and should be
discarded before the print data is processed. An <IAC EOR> must
be received before changing to LU 1 or LU 3 type data.

Client: (S/S with IR) IAC EOR (This indicates a paper jam
at printer.)
Client: (S/S with DE) IAC EOR (This indicates the clearing
of above condition.)
Server: 0x00 (3270 PRINT DATA) (This indicates start of LU 1
data)

Server: (3270 PRINT DATA)

Server: (3270 PRINT DATA)
Server: (3270 PRINT DATA) IAC EOR
Client: (S/S with DE) IAC EOR
Server: 0x00 (LAST 3270 PRINT DATA) IAC EOR

Client: (S/S with DE) IAC EOR
Server: IAC AO
(The Abort Output <AO> signifies the end-of-bracket -- end of
print job)

7. SECURITY CONSIDERATIONS

This document does not specify a security methodology to insure that
the client requesting a printer LU name is authorized to access that
LU. Currently, this is left up to individual server implementations.
The design of the protocols described in this document allow for the
future incorporation of the RFCs regarding encryptions and
authentication protocols and services. However, before this may
occur, certain extensions may be required to the protocols defined in
this document or to the encryptions and authentication services and
protocols.

8. ERROR CONDITIONS

After a client and server have successfully completed negotiation, a
number of potential error conditions may be detected by the server
which require notifying the client and aborting the connection.

When an error condition is detected by the server, the client must be
negotiated back into NVT mode by the server sending a "WONT/DONT
BINARY" TELNET sequence and the client responding with the
appropriate "DONT/WONT BINARY" TELNET sequence.

The server should immediately send the appropriate error message to
the client as an ASCII string and then close the connection. The
error message should be prefixed by a numeric identifier to precisely
notify the client of the specific error condition. The error message
sent to the client should be routed to the proper console or log for
corrective action.

Below is a list of error conditions identified by numeric value,
error text, meaning of the error and recovery procedure.

Message: "01 No LU's of the type configured"

Meaning: The configuration definition on the server
does not include the LU type requested.

Recovery: Notify your Systems Administrator as this
is a permanent error condition.

Message: "02 Requested LU unavailable"

Meaning: The requested LU is not available at
this time.

Recovery: This may be a temporary error and may
be retried periodically. If the condition
persists contact your Systems Administrator.

Message: "03 Requested LU type is inconsistent with configuration"

Meaning: The LU requested does not match the terminal
type in the server configuration.

Recovery: Notify your Systems Administrator as this
is a permanent error condition.

Message: "04 Requested LU is not configured"

Meaning: The LU is not defined in server configuration.

Recovery: Notify your Systems Administrator as this
is a permanent error condition.

When a client receives a message not defined in the above list, the
message should be displayed to a console or log and the connection to
the server should be closed. No other recovery should be attempted
as this is most likely a fatal error condition. (Notify your Systems
Administrator.)

9. REFERENCES

[1] Postel, J., and J. Reynolds, "TELNET Protocol Specification", STD
8, RFC854, USC/Information Services Institute, May 1983.

[2] VanBokkeln, J., "TELNET Terminal-Type Option" RFC1091, FTP
Software Inc., February 1989.

[3] Postel, J., and J. Reynolds, "TELNET Binary Transmission", STD
27, RFC856, USC/Information Services Institute, May 1983.

Authors' Addresses

Cleve Graves
2711 LBJ Freeway
Dallas, Texas 75234

Phone: (214) 484-5200
EMail: cvg@oc.com

Thomas Butts
2711 LBJ Freeway
Dallas, Texas 75234

Phone: (214) 484-5200
EMail: tom@oc.com

Michelle Angel
2711 LBJ Freeway
Dallas, Texas 75234

Phone: (214) 484-5200

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