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RFC 1646 - 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.


   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.


   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, RFC 854, TELNET Protocol
   Specification.  This memo also presents an example of the correct


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

   The companion document, STD 8, RFC 854 -- "TELNET Protocol
   Specification" should be consulted for further information about the
   TELNET command, codes and code sequences referenced in this


   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.


      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.


   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

   <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 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.


                     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.


                               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

               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"

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

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

      Note: To request a specific LU the TERMINAL-TYPE string would be:
      (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

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

      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

      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)


   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


   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


   [1] Postel, J., and J. Reynolds, "TELNET Protocol Specification", STD
       8, RFC 854, USC/Information Services Institute, May 1983.

   [2] VanBokkeln, J., "TELNET Terminal-Type Option" RFC 1091, FTP
       Software Inc., February 1989.

   [3] Postel, J., and J. Reynolds, "TELNET Binary Transmission", STD
       27, RFC 856, 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
       EMail: angel@oc.com


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