path: root/doc/rfc/rfc2355.txt

Network Working Group                                           B. Kelly
Request for Comments: 2355                             Auburn University
Obsoletes: 1647                                                June 1998
Category: Standards Track


                          TN3270 Enhancements

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This document describes a protocol that more fully supports 3270
   devices than do traditional tn3270 practices.  Specifically, it
   defines a method of emulating both the terminal and printer members
   of the 3270 family of devices via Telnet; it provides for the ability
   of a Telnet client to request that it be assigned a specific device-
   name (also referred to as "LU name" or "network name"); finally, it
   adds support for a variety of functions such as the ATTN key, the
   SYSREQ key, and SNA response handling.

   This protocol is negotiated under the TN3270E Telnet Option, and is
   unrelated to the Telnet 3270 Regime Option as defined in RFC 1041
   [1].

TABLE OF CONTENTS

   1.  Introduction ...............................................  2
      1.1  Changes to RFC 1647 ....................................  4
   2.  TN3270E OVERVIEW ...........................................  5
   3.  COMMAND NAMES AND CODES ....................................  6
   4.  COMMAND MEANINGS ...........................................  7
   5.  DEFAULT SPECIFICATION ......................................  9
   6.  MOTIVATION .................................................  9
   7.  TN3270E SUB-NEGOTIATION RULES ..............................  9
      7.1  DEVICE-TYPE Negotiation ................................  9
          7.1.1 Device Pools ...................................... 10
          7.1.2 CONNECT Command ................................... 12



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          7.1.3 ASSOCIATE Command ................................. 12
          7.1.4 Accepting a Request ............................... 13
          7.1.5 REJECT Command .................................... 13
      7.2  FUNCTIONS Negotiation .................................. 14
          7.2.1 Commands .......................................... 14
          7.2.2 List of TN3270E Functions ......................... 16
   8.  TN3270E DATA MESSAGES ...................................... 16
      8.1  The TN3270E Message Header ............................. 18
          8.1.1 DATA-TYPE Field ................................... 18
          8.1.2 REQUEST-FLAG Field ................................ 19
          8.1.3 RESPONSE-FLAG Field ............................... 19
          8.1.4 SEQ-NUMBER Field .................................. 20
   9.  BASIC TN3270E .............................................. 20
      9.1  3270 Mode and NVT Mode ................................. 21
   10. DETAILS OF PROCESSING TN3270E FUNCTIONS .................... 22
      10.1 The SCS-CTL-CODES Function ............................. 22
      10.2 The DATA-STREAM-CTL Function ........................... 23
      10.3 The BIND-IMAGE Function ................................ 24
      10.4 The RESPONSES Function ................................. 25
         10.4.1 Response Messages ................................. 26
      10.5 The SYSREQ Function .................................... 28
         10.5.1 Background ........................................ 28
         10.5.2 TN3270E Implementation of SYSREQ .................. 29
   11. THE 3270 ATTN KEY .......................................... 30
   12. 3270 STRUCTURED FIELDS ..................................... 31
   13. IMPLEMENTATION GUIDELINES .................................. 31
      13.1 3270 Data Stream Notes ................................. 31
      13.2 Negotiation of the TN3270E Telnet Option ............... 32
      13.3 A "Keep-alive" Mechanism ............................... 32
      13.4 Examples ............................................... 32
   14. SECURITY CONSIDERATIONS .................................... 36
   15. REFERENCES ................................................. 36
   16. AUTHOR'S NOTE .............................................. 37
   17. AUTHOR'S ADDRESS ........................................... 37
   18. Full Copyright Statement ................................... 38

1.  Introduction

   Traditionally, support for 3270 terminal emulation over Telnet has
   been accomplished by the de facto standard of negotiating three
   separate Telnet Options - Terminal-Type [2], Binary Transmission [3],
   and End of Record [4].  Note that there is no RFC that specifies this
   negotiation as a standard.  RFC 1041 attempted to standardize the
   method of negotiating 3270 terminal support by defining the 3270
   Regime Telnet Option.  Very few developers and vendors ever
   implemented RFC 1041.





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   This document will refer to the existing practice of negotiating
   these three Telnet Options before exchanging the 3270 data stream as
   "traditional tn3270".  Traditional tn3270 is documented in [10].

   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.

   All references in this document to the 3270 data stream, 3270 data
   stream commands, orders, structured fields and the like rely on [5].

   References to SNA Request and Response Units rely on [6].  References
   to SNA versus non-SNA operation rely on [7].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

   There were several shortcomings in traditional tn3270; among them
   were the following:

    - It provided no capability for Telnet clients to emulate the 328x
      class of printers.

    - There was no mechanism by which a Telnet client could 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 were not universally supported.

    - There was no support for the SNA positive/negative response
      process.  This is particularly important if printer emulation is
      to function properly, but is also useful for some terminal
      applications.  A positive response is used to indicate that the
      previously received data has been successfully processed.  A
      negative response indicates some sort of error has occurred while
      processing the previously received data; this could be caused by
      the host application building a 3270 data stream that contains an
      invalid command, or by a mechanical error at the client side,
      among other things.






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    - There was no mechanism by which the client could access the SNA
      Bind information.  The Bind image contains a detailed description
      of the session between the Telnet server and the host application.

    - There was no mechanism by which the server could determine whether
      a client supported 3270 structured fields, or a client could
      request that it receive them.

1.1 Changes to RFC 1647

   This document replaces RFC 1647; the following is a summary of the
   changes that have been incorporated:

   - Reworded the Introduction to refer to traditional tn3270 in the
     past tense.

     Affected sections: 1.

   - Added this section documenting changes to RFC 1647

     Affected sections: 1.1

   - Clarified the specification of numeric literals contained in the
     document.

     Affected sections: 3. (first paragraph)

   - Extensively revised several sections to

     1) clarify the motivation behind and use of the ASSOCIATE
        command
     2) remove restrictive wording regarding the organization
        and use of server maintained device pools
     3) distinguish between device-names and resource-names in the
        TN3270E device-type negotiation, and define a maximum length for
        device-names and resource-names

     Affected sections: 4. (DEVICE-TYPE REQUEST command) and 7.1.1
                            through 7.1.6

   - Corrected the erroneous specification of the format of the
     function-list sent during TN3270E functions negotiation.

     Affected sections: 7.2.1 (first paragraph)







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   - Added a statement addressing what a client or server should do
     if an impasse is reached during TN3270E functions negotiation.

     Affected sections: 7.2.1 (last paragraph)

   - Added a DATA-TYPE of PRINT-EOJ with a value of 0x08 to support
     the end-of-job indication for printers.

     Affected sections: 8.1.1, 10.1, 10.2

   - Clarified the description of the SEQ-NUMBER Field to state that

     1) the field should be sent in network byte order (big endian)
     2) either byte that contains a 0xff must be doubled to 0xffff
        before sending, and stripped back to 0xff after receipt.

     Affected sections: 8.1.4

   - Defined the format and maximum length of the Bind image.

     Affected sections: 10.3 (fourth paragraph)

   - Clarified the misleading wording regarding allowable DATA-TYPEs
     when BIND-IMAGE has been negotiated and a BIND has been sent.

     Affected sections: 10.3 (last paragraph)

   - Clarified the use of the SEQ-NUMBER field in regards to when it
     should be reset to zero.

     Affected sections: 10.4 (last paragraph)

   - Clarified the format of the data when the DATA-TYPE field is
     SSCP-LU-DATA.

     Affected sections: 10.5.2 (fourth paragraph)

   - Reworded the Security section to refer to Kerberos.

     Affected sections: 14.

2.  TN3270E Overview

   In order to address these issues, this document proposes a new Telnet
   Option - TN3270E.  Telnet clients and servers would be free to
   negotiate support of the TN3270E option or not. If either side does
   not support TN3270E, traditional tn3270 can be used; otherwise, a
   sub-negotiation will occur to determine what subset of TN3270E will



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   be used on the session.  It is anticipated that a client or server
   capable of both types of 3270 emulation would attempt to negotiate
   TN3270E first, and only negotiate traditional tn3270 if the other
   side refuses TN3270E.

   Once a client and server have agreed to use TN3270E, negotiation of
   the TN3270E suboptions can begin.  The two major elements of TN3270E
   sub-negotiation are:

    - a device-type negotiation that is similar to, but somewhat
      more complicated than, the existing Telnet Terminal-Type Option.

    - the negotiation of a set of supported 3270 functions, such as
      printer data stream type (3270 data stream or SNA Character
      Stream), positive/negative response exchanges, device status
      information, and the passing of BIND information from server to
      client.

   Successful negotiation of these two suboptions signals the beginning
   of 3270 data stream transmission. In order to support several of the
   new functions in TN3270E, each data message must be prefixed by a
   header.  This header will contain flags and indicators that convey
   such things as positive and negative responses and what type of data
   follows the header (for example, 3270 data stream, SNA Character
   Stream, or device status information).

3.  Command Names and Codes

   Please note that all numeric literals in this document specify
   decimal values, unless they are preceded by "0x", in which case a
   hexadecimal value is represented.

       TN3270E            40
         ASSOCIATE          00
         CONNECT            01
         DEVICE-TYPE        02
         FUNCTIONS          03
         IS                 04
         REASON             05
         REJECT             06
         REQUEST            07
         SEND               08

       Reason-codes
         CONN-PARTNER       00
         DEVICE-IN-USE      01
         INV-ASSOCIATE      02
         INV-NAME           03



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         INV-DEVICE-TYPE    04
         TYPE-NAME-ERROR    05
         UNKNOWN-ERROR      06
         UNSUPPORTED-REQ    07

       Function Names
         BIND-IMAGE         00
         DATA-STREAM-CTL    01
         RESPONSES          02
         SCS-CTL-CODES      03
         SYSREQ             04

4.  Command Meanings

   Refer to the Telnet Protocol Specification [8] for the meaning of
   IAC, DO, WILL, etc.

   IAC WILL TN3270E

      The sender of this command is willing to send TN3270E information
      in subsequent sub-negotiations.

   IAC WON'T TN3270E

      The sender of this command refuses to send TN3270E information.

   IAC DO TN3270E

      The sender of this command is willing to receive TN3270E
      information in subsequent sub-negotiations.

   IAC DON'T TN3270E

      The sender of this command refuses to receive TN3270E information.

   Note that while they are not explicitly negotiated, the equivalent of
   the Telnet Binary Transmission Option [3] and the Telnet End of
   Record Option [4] is implied in the negotiation of the TN3270E
   Option.  That is, a party to the negotiation that agrees to support
   TN3270E is automatically required to support bi-directional binary
   and EOR transmissions.

   IAC SB TN3270E SEND DEVICE-TYPE IAC SE

      Only the server may send this command.  This command is used to
      request that the client transmit a device-type and, optionally,
      device-name information.




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   IAC SB TN3270E DEVICE-TYPE REQUEST <device-type>
       [ [CONNECT <resource-name>] | [ASSOCIATE <device-name>] ] IAC SE

      Only the client may send this command.  It is used in response to
      the server's SEND DEVICE-TYPE command, as well as to suggest
      another device-type after the server has sent a DEVICE-TYPE REJECT
      command (see below).  This command requests emulation of a
      specific 3270 device type and model.  The REQUEST command may
      optionally include either the CONNECT or the ASSOCIATE command
      (but not both).  If present, CONNECT must be followed by
      <resource-name> and ASSOCIATE must be followed by <device-name>.
      (See the section entitled "DEVICE-TYPE Negotiation" for more
      detailed information.)

   IAC SB TN3270E DEVICE-TYPE IS <device-type> CONNECT
       <device-name> IAC SE

      Only the server may send this command.  This command is used to
      accept a client's DEVICE-TYPE REQUEST command and to return the
      server-defined device-name.

   IAC SB TN3270E DEVICE-TYPE REJECT REASON <reason-code> IAC SE

      Only the server may send this command.  This command is used to
      reject a client's DEVICE-TYPE REQUEST command.

   IAC SB TN3270E FUNCTIONS REQUEST <function-list> IAC SE

      Either side may send this command.  This command is used to
      suggest a set of 3270 functions that will be supported on this
      session.  It is also sent as an implicit rejection of a previous
      FUNCTIONS REQUEST command sent by the other side (see the section
      entitled "FUNCTIONS Negotiation" for more information).  Note that
      when used to reject a FUNCTIONS REQUEST command, the function-list
      must not be identical to that received in the previous REQUEST
      command.

   IAC SB TN3270E FUNCTIONS IS <function-list> IAC SE

      Either side may send this command.  This command is sent as a
      response to a FUNCTIONS REQUEST command and implies acceptance of
      the set of functions sent to it in the REQUEST command.  Note that
      the list of functions in the FUNCTIONS IS command must match the
      list that was received in the previous FUNCTIONS REQUEST command.







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5.  Default Specification

   WON'T TN3270E

   DON'T TN3270E

   i.e., TN3270E will not be used.

6.  Motivation

   See the section entitled "Introduction".

7.  TN3270E Sub-negotiation Rules

   Once it has been agreed that TN3270E will be supported, the first
   sub-negotiation must concern the DEVICE-TYPE (and possibly device-
   name) information.  Only after that has been successfully negotiated
   can the client and server exchange FUNCTIONS information.  Only after
   both DEVICE-TYPE and FUNCTIONS have been successfully negotiated can
   3270 data stream transmission occur.

7.1 DEVICE-TYPE Negotiation

   Device-type names are NVT ASCII strings, all upper case.

   Device-type (and device-name) negotiation begins when the server
   transmits the DEVICE-TYPE SEND command to the client.  The client
   responds with the DEVICE-TYPE REQUEST command, which must include a
   device-type and may include a resource-name or device-name request.

   Valid device-types are:

     erminals: IBM-3278-2  IBM-3278-2-E  (24 row x 80 col display)
               IBM-3278-3  IBM-3278-3-E  (32 row x 80 col display)
               IBM-3278-4  IBM-3278-4-E  (43 row x 80 col display)
               IBM-3278-5  IBM-3278-5-E  (27 row x 132 col display)
               IBM-DYNAMIC            (no pre-defined display size)

     printers: IBM-3287-1

   Note that the use of '3278' and '3287' is NOT intended to exclude any
   particular device capabilities; they are used here only because they
   are commonly known designations for a terminal and a printer member
   of the 3270 family of devices.  The intention is to simplify the
   device-type negotiation (in comparison to traditional tn3270) by
   minimizing the number of possible device-types, and by breaking the
   association of a specific piece of IBM hardware with a related set of
   data stream capabilities.  For example, negotiation of device-type



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   IBM-3278-2-E does NOT in and of itself preclude the use of any of the
   functions associated with a physical 3279 model S2B.  A client's
   ability to support the more advanced functions of the 3270 data
   stream will be indicated not by negotiation of an IBM device type and
   model number, but rather by the combination of Read Partition Query
   and Query Reply.

   All of the terminal device-types support a "primary" display size of
   24 rows by 80 columns.  The "-3", "-4" and "-5" types each support an
   "alternate" display size as noted in the above list.  The IBM-DYNAMIC
   device-type implies no pre-defined alternate display size; this value
   will be passed from the client to host applications as part of the
   Query Reply structured field, and it can represent any display size
   the client and the host application can support.

   Terminal device-types with the "-E" suffix should only be negotiated
   by clients that are willing to support some subset of the 3270
   "extended data stream".  This usually includes at a minimum support
   for extended colors and highlighting, but may also include a number
   of other functions, such as graphics capability, alternate character
   sets, and partitions.

   Clients that negotiate a terminal device-type with the "-E" suffix or
   the DYNAMIC type, as well as those that negotiate a printer device-
   type, must be able to accept and respond to a Read Partition Query
   command (see the section entitled "3270 Structured Fields").  This
   allows the client to indicate to host applications which subsets of
   the 3270 extended data stream the client is willing to support.

   In a VTAM/SNA environment, negotiation of IBM-DYNAMIC as the device-
   type should result in a Bind in which the Presentation Services Usage
   screen field (the eleventh byte in the logmode's PSERVIC field) is
   set to 0x03, indicating that the alternate screen size will be
   determined by the Query Reply (Usable Area).

7.1.1 Device Pools

   An explanation of the CONNECT and ASSOCIATE commands first requires a
   discussion of the organization of terminal and printer device pools
   that the server maintains and from which it selects device-names to
   assign to session requests.  Definition of a few terms is also in
   order.

   The terms "device-name", "LU name" and "network name" can be
   considered interchangeable in this document.  They refer to a
   specific terminal or printer device.





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   The term "resource-name" is less specific; it may refer to a device-
   name, but it may also be the name of a pool of printer or terminal
   devices.  Such a named pool could serve to group devices with similar
   operational or administrative characteristics.  In fact, this
   document places no restrictions on how a server makes use of
   resource-names, so long as the server can take a resource-name
   specified by the client and use it to come up with a device-name to
   assign to the session.  Note, however, that servers must avoid
   allowing ambiguity; for example, they must not allow the definition
   of a device-name with the same name as that of a pool of devices.

   Device-names and resource-names are specified as NVT ASCII strings in
   which upper and lower case are considered equivalent.  The length of
   device-names and resource-names should not exceed 8 bytes.

   A "generic session request" is one which includes neither the CONNECT
   nor the ASSOCIATE command, while a "specific session request" is one
   that includes either the CONNECT or the ASSOCIATE command.

   If a TN3270E server wishes to support traditional tn3270 clients, it
   must maintain a set of terminal device-names that can be used to
   satisfy requests from such clients for terminal sessions.  This same
   pool could be used to satisfy generic requests for terminal sessions
   from TN3270E clients.

   The server may also maintain any number of other pools of device-
   names.  For example, there could be a pool of terminal device-names
   reserved for a specific department within the organization, or a pool
   of terminal device-names that have access to certain applications on
   the host.

   For any of these terminal device pools, the TN3270E server may also
   have defined a "partner" or "paired" printer device for each terminal
   in the pool.  There should be a unique, one-to-one mapping between a
   terminal and its associated printer.  The reasoning behind such a
   configuration is to allow for those host applications that produce
   printed output bound for a printer whose device-name is determined by
   the device-name of the terminal that initiated the print request.
   These printer devices can only be assigned to specific printer
   session requests that use the ASSOCIATE command (see below).

   In addition, the TN3270E server may also maintain one or more pools
   of printer device-names that are not associated with any terminal.
   These printer devices can only be assigned to specific printer
   session requests that use the CONNECT command (see below).  This
   allows for those host applications that generate printed output bound
   for a printer whose device-name is determined by something other than
   the device-name of the terminal that initiated the print request (for



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   example, when the userid of the person signed on to a terminal
   determines the print destination).  It is also possible that a pool
   of printer device-names could be maintained to satisfy generic
   requests for printers (i.e., those that specify neither CONNECT nor
   ASSOCIATE).

7.1.2 CONNECT Command

   CONNECT can be used by the client in two ways: if the resource-name
   it specifies is a device-name, then the client is requesting a
   specific device-name.  If the specified resource-name is not a
   device-name, then the client is requesting any one of the device-
   names associated with the resource-name.

   In either case, the resource indicated by the specified resource-name
   must not conflict with the device-type; e.g., if the client requests
   DEVICE-TYPE IBM-3287-1 (a printer) and specifies CONNECT T1000001,
   but T1000001 is a device-name defined at the host as a terminal, then
   the server must deny the request.  Further, if the requested
   resource-name is a device-name already associated with some other
   Telnet session, or if it is not defined to the server, the server
   must deny the request.

7.1.3 ASSOCIATE Command

   ASSOCIATE can be used by the client only when requesting a DEVICE-
   TYPE that represents a printer, and the specified device-name must be
   that of a terminal that was returned by the server in a previous
   DEVICE-TYPE IS <device-type> CONNECT <device-name> command.

   The ASSOCIATE command requests that this session be assigned the
   device-name of the printer that is paired with the terminal named in
   the request.  If the device-type does not represent a printer, or if
   the device-name is not that of a terminal, then the server must deny
   the request.  Also, if the server does not have defined a partner
   printer for the specified terminal, it must deny the request.

   The use of the ASSOCIATE command is to be as follows:  A client first
   connects and requests a terminal from one of the terminal pools; it
   then uses the terminal device-name returned by the server (see
   "Accepting a Request", section 7.1.4 below) in a second session
   request, this time asking for the printer that is paired with the
   terminal session it just established.  This allows clients to
   associate a printer session with a terminal rather than having to
   have prior knowledge of a printer device-name.






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7.1.4 Accepting a Request

   The server must accept the client's request or deny it as a whole -
   it cannot, for example, accept the DEVICE-TYPE request but deny the
   CONNECT portion.

   If the server wishes to accept the request, it sends back the
   DEVICE-TYPE IS command confirming the requested device-type and the
   CONNECT command specifying the device-name of the terminal or printer
   assigned to this session.

   Normally, the client should accept any DEVICE-TYPE IS <device-type>
   CONNECT <device-name> sent by the server.  An exception to this would
   be if the client must (e.g., to satisfy local-site policy) be
   connected to a specific LU name and is presented with a device-name
   which does not match the one requested by the client (this could
   happen, for example, if the client requested what it thought was a
   device-name, but what was defined at the server as the name of a pool
   of devices).  In this case, the client should reject the DEVICE-TYPE
   IS command by terminating TN3270E negotiations.

7.1.5 REJECT Command

   If the server wishes to deny the request, it sends back the DEVICE-
   TYPE REJECT command with one of the following reason-codes:

   Reason code name         Explanation
   ----------------         -----------------------------------
   INV-DEVICE-TYPE          The server does not support the
                            requested device-type.

   INV-NAME                 The resource-name or device-name
                            specified in the CONNECT or ASSOCIATE
                            command is not known to the server.

   DEVICE-IN-USE            The requested device-name is
                            already associated with another
                            session.

   TYPE-NAME-ERROR          The requested device-name or
                            resource-name is incompatible
                            with the requested device-type
                            (such as terminal/printer mismatch).

   UNSUPPORTED-REQ          The server is unable to satisfy
                            the type of request sent by the
                            client; e.g., a specific terminal
                            or printer was requested but the



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                            server does not have any such pools of
                            device-names defined to it, or the
                            ASSOCIATE command was used but no
                            partner printers are defined to the
                            server.

   INV-ASSOCIATE            The client used the ASSOCIATE
                            command and either the device-type
                            is not a printer or the device-name
                            is not a terminal.

   CONN-PARTNER             The client used the CONNECT command
                            to request a specific printer but
                            the device-name requested is the
                            partner to some terminal.

   UNKNOWN-ERROR            Any other error in device type or
                            name processing has occurred.

   The process of negotiating a device-type and device-name that are
   acceptable to both client and server may entail several iterations of
   DEVICE-TYPE REQUEST and DEVICE-TYPE REJECT commands.  The client must
   make use of the reason-code specified by the server in any DEVICE-
   TYPE REJECT command(s) to minimize the amount of negotiation
   necessary.  For example, if the client initially requests that it be
   assigned a specific terminal device-name via the CONNECT command, and
   the server rejects the request with a reason-code of UNSUPPORTED-REQ,
   the client must make no further specific terminal requests in the
   negotiations.  If at any point in the process either side wishes to
   "bail out," it can simply send a WON'T (or DON'T) TN3270E command to
   the other side.  At this point both sides are free to negotiate other
   Telnet options (including traditional tn3270).

7.2 FUNCTIONS Negotiation

   Once the DEVICE-TYPE negotiation has successfully completed (i.e,
   when the client receives a DEVICE-TYPE IS command that is
   acceptable), the client must initiate the FUNCTIONS negotiation by
   sending the FUNCTIONS REQUEST command to the server.  After this
   initial REQUEST command, both sides are free to transmit FUNCTIONS
   REQUEST and FUNCTIONS IS commands as needed.

7.2.1 Commands

   The FUNCTIONS REQUEST command contains a list of the TN3270E
   functions that the sender would like to see supported on this
   session.  All functions not in the list are to be considered
   unsupported.  The list is terminated by the IAC code that precedes



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   the SE command.  Functions may appear in any order in the list.

   Upon receipt of a FUNCTIONS REQUEST command, the recipient has two
   choices:

   - it may respond in the positive (meaning it agrees to support
     all functions in the list, and not to transmit any data related to
     functions not in the list).  To do this, it sends the FUNCTIONS IS
     command with the function-list exactly as it was received.  At this
     point, FUNCTIONS negotiation has successfully completed.

   - it may respond in the negative by sending a FUNCTIONS
     REQUEST command in which the function-list differs from the one it
     received (and not simply in the order of appearance of functions in
     the list; at least one function must have been added to, or removed
     from, the list).

     To avoid endlessly looping, both parties must not add to the
     function-list it receives any function that it has previously added
     and that the other side has removed.

     The process of sending FUNCTIONS REQUEST commands back and forth
     continues until one side receives a function-list it is willing to
     live with.  It uses the FUNCTIONS IS command to accept the list,
     and, once this command is received by the other side, all necessary
     negotiation has been completed.  At this point, 3270 data stream
     transmission can begin.

     Note that it is possible that the function-list agreed to is null;
     this is referred to as "basic TN3270E".  See the section entitled
     "Basic TN3270E" for more information.

     If an impasse is reached during FUNCTIONS negotiation (for example,
     if a client requested and was granted a DEVICE-TYPE representing a
     printer, but refuses to accept either the SCS-CTL-CODES or DATA-
     STREAM-CTL function), then the "offended" party should terminate
     the negotiation by sending an IAC DON'T (or WON'T) TN3270E.














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7.2.2 List of TN3270E Functions

   The following list briefly describes the 3270 functions that may be
   negotiated in the function-list:

   Function Name       Description
   -------------       -----------
   SCS-CTL-CODES       (Printer sessions only).  Allows the use
                       of the SNA Character Stream (SCS) and SCS
                       control codes on the session.  SCS is
                       used with LU type 1 SNA sessions.

   DATA-STREAM-CTL     (Printer sessions only).  Allows the use
                       of the standard 3270 data stream.  This
                       corresponds to LU type 3 SNA sessions.

   RESPONSES           Provides support for positive and
                       negative response handling.  Allows the
                       server to reflect to the client any and
                       all definite, exception, and no response
                       requests sent by the host application.

   BIND-IMAGE          Allows the server to send the SNA Bind
                       image and Unbind notification to the
                       client.

   SYSREQ              Allows the client and server to emulate
                       some (or all, depending on the server) of
                       the functions of the SYSREQ key in an SNA
                       environment.

   See the section entitled "Details of Processing TN3270E Functions"
   for a more detailed explanation of the meaning and use of these
   functions.

   If in the process of functions negotiation an unrecognized function
   code is recieved, the recipient should simply remove that function
   code from the list and continue normal functions negotiation.

8.  TN3270E Data Messages

   3270 device communications are generally understood to be block
   oriented in nature.  That is, each partner buffers data until an
   entire "message" has been built, at which point the data is sent to
   the other side.  The "outbound message" (from host to device)
   consists of a 3270 command and a series of buffer orders, buffer
   addresses, and data, while the "inbound message" contains only buffer
   orders, addresses and data.  The end of a message is understood to be



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   the last byte transmitted (note that this discussion disregards SNA
   chaining).  The Telnet EOR command is used to delimit these natural
   blocks of 3270 data within the Telnet data stream.

   In TN3270E, each 3270 message must be prefixed with a TN3270E header,
   which consists of five bytes and whose format is defined below (see
   the section entitled "The TN3270E Message Header").  A "data message"
   in TN3270E therefore has the following construction:

          <TN3270E Header><data><IAC EOR>

   It should be noted that it is possible that, for certain message
   types, there is no data portion present.  In this case, the TN3270E
   data message consists of:

          <TN3270E Header><IAC EOR>

   If either side wishes to transmit the decimal value 255 and have it
   interpreted as data, it must "double" this byte.  In other words, a
   single occurrence of decimal 255 will be interpreted by the other
   side as an IAC, while two successive bytes containing decimal 255
   will be treated as one data byte with a value of decimal 255.

   It is strongly recommended that Telnet commands (other than IAC IAC)
   should be sent between TN3270E data messages, with no header and no
   trailing IAC EOR.  If a TN3270E data message containing either IAC IP
   (to be interpreted as 3270 Attention) or IAC AO (to be interpreted as
   SYSREQ) is received, the receiver should defer processing the command
   until the 3270 data has been processed (see the appropriate sections
   for discussion of 3270 Attention and SYSREQ).  If a TN3270E data
   message containing any other IAC-command sequence (other than IAC
   IAC) is received, it is implementation dependent when the IAC-command
   sequence will be processed, but it must be processed.  The receiver
   may process it immediately, which in effect causes it to be processed
   as if it had been received before the current TN3270E data message,
   or the processing may be deferred until after the current TN3270E
   data message has been processed.  It is because of this ambiguity
   that the presence of Telnet commands within a TN3270E data message
   (i.e., between the header and the trailing IAC EOR) is not
   recommended; neither clients nor servers should send such data.











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8.1 The TN3270E Message Header

   As stated earlier, each data message in TN3270E must be prefixed by a
   header, which consists of five bytes and is formatted as follows:

      -----------------------------------------------------------
      | DATA-TYPE | REQUEST-FLAG | RESPONSE-FLAG |  SEQ-NUMBER  |
      -----------------------------------------------------------
         1 byte        1 byte         1 byte         2 bytes

8.1.1 DATA-TYPE Field

   The DATA-TYPE field indicates how the data portion of the message is
   to be interpreted by the receiver.  Possible values for the DATA-TYPE
   field are:

   Data-type Name   Code                Meaning
   --------------   ----   ---------------------------------
   3270-DATA        0x00   The data portion of the message
                           contains only the 3270 data stream.

   SCS-DATA         0x01   The data portion of the message
                           contains SNA Character Stream data.

   RESPONSE         0x02   The data portion of the message
                           constitutes device-status information
                           and the RESPONSE-FLAG field indicates
                           whether this is a positive or negative
                           response (see below).

   BIND-IMAGE       0x03   The data portion of the message is
                           the SNA bind image from the session
                           established between the server and the
                           host application.

   UNBIND           0x04   The data portion of the message is
                           an Unbind reason code.

   NVT-DATA         0x05   The data portion of the message is to
                           be interpreted as NVT data.

   REQUEST          0x06   There is no data portion present in
                           the message.  Only the REQUEST-FLAG
                           field has any meaning.

   SSCP-LU-DATA     0x07   The data portion of the message is
                           data from the SSCP-LU session.




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   PRINT-EOJ        0x08   There is no data portion present in
                           the message.  This value can be sent
                           only by the server, and only on a
                           printer session.

8.1.2 REQUEST-FLAG Field

   The REQUEST-FLAG field only has meaning when the DATA-TYPE field has
   a value of REQUEST; otherwise, the REQUEST-FLAG field must be ignored
   by the receiver and should be set to 0x00 by the sender.  Possible
   values for the REQUEST-FLAG field are:

   Request-Flag Name   Code                Meaning
   -----------------   ----   ---------------------------------
   ERR-COND-CLEARED    0x00   The client sends this to the server
                              when some previously encountered
                              printer error condition has been
                              cleared.  (See the section entitled
                              "The RESPONSES Function" below.)

8.1.3 RESPONSE-FLAG Field

   The RESPONSE-FLAG field only has meaning for certain values of the
   DATA-TYPE field.  For DATA-TYPE field values of 3270-DATA and SCS-
   DATA, the RESPONSE-FLAG is an indication of whether or not the sender
   of the data expects to receive a response.  In this case the possible
   values of RESPONSE-FLAG are:

   Response-Flag Name  Code                Meaning
   ------------------  ----   ---------------------------------
   NO-RESPONSE         0x00   The sender does not expect the
                              receiver to respond either
                              positively or negatively to this
                              message.  The receiver must
                              therefore not send any response
                              to this data-message.

   ERROR-RESPONSE      0x01   The sender only expects the
                              receiver to respond to this message
                              if some type of error occurred, in
                              which case a negative response must
                              be sent by the receiver.

   ALWAYS-RESPONSE     0x02   The sender expects the receiver to
                              respond negatively if an error
                              occurs, or positively if no errors
                              occur.  One or the other must
                              always be sent by the receiver.



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   For a DATA-TYPE field value of RESPONSE, the RESPONSE-FLAG is an
   actual response to a previous data message (which must by definition
   have had a DATA-TYPE of either 3270-DATA or SCS-DATA and a RESPONSE-
   FLAG value of either ERROR-RESPONSE or ALWAYS-RESPONSE).  In this
   case the possible values of RESPONSE-FLAG are:

   Response-Flag Name  Code                Meaning
   ------------------  ----   ---------------------------------
   POSITIVE-RESPONSE   0x00   The previous message was received
                              and executed successfully with
                              no errors.

   NEGATIVE-RESPONSE   0x01   The previous message was received
                              but an error(s) occurred while
                              processing it.

   Accompanying status information will be found in the data portion of
   the message.

   For any other values of the DATA-TYPE field, the RESPONSE-FLAG field
   must be ignored by the receiver and should be set to 0x00 by the
   sender.

8.1.4 SEQ-NUMBER Field

   The SEQ-NUMBER field is only used when the RESPONSES function has
   been agreed to.  It contains a 2 byte binary number, and is used to
   correlate positive and negative responses to the data messages for
   which they were intended.  This field must be sent in network byte
   order ("big endian").  If either byte contains a 0xff, it should be
   doubled to 0xffff before sending and stripped back to 0xff upon
   receipt; this is standard IAC escaping.  See the section entitled
   "The RESPONSES Function" for further information on the use of the
   SEQ-NUMBER field.  When the RESPONSES function is not agreed to, this
   field should always be set to 0x0000 by the sender and ignored by the
   receiver.

9.  Basic TN3270E

   As has been stated earlier, whether or not the use of each of the
   TN3270E functions is allowed on a session is negotiated when the
   connection is established.  It is possible that none of the functions
   are agreed to (in this case, the function-list in the FUNCTIONS
   REQUEST and FUNCTIONS IS commands is null).  This mode of operation
   is referred to as "basic TN3270E".  Note that, since neither the
   SCS-CTL-CODES function nor the DATA-STREAM-CTL function is agreed to,
   basic TN3270E refers to terminal sessions only.




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   Basic TN3270E requires the support of only the following TN3270E
   header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          3270-DATA
              DATA-TYPE          NVT-DATA

   The REQUEST-FLAG, RESPONSE-FLAG and SEQ-NUMBER fields are not used in
   basic TN3270E.

9.1 3270 Mode and NVT Mode

   At any given time, a TN3270E connection can be considered to be
   operating in either "3270 mode" or "NVT mode".  In 3270 mode, each
   party may send data messages with the DATA-TYPE flag set to 3270-
   DATA; sending a DATA-TYPE flag set to NVT-DATA constitutes a request
   to switch modes.  In NVT mode, each party may send data messages with
   the DATA-TYPE flag set to NVT-DATA; sending 3270-DATA is a request to
   switch modes.  The connection is initially in 3270 mode when TN3270E
   operation is successfully negotiated.  When a party receives a
   message with a DATA-TYPE different from the mode it is operating in,
   the mode of operation for the connection is switched.  Switching
   modes results in the client performing the equivalent of a 3270
   Erase/Reset operation, as described in [5], using the default
   partition (screen) size.  The server cannot assume the client
   preserves any attributes of the previous environment across a mode
   switch.

   Note that even when sending NVT-DATA, each side should buffer data
   until an entire message is built (for the client, this would normally
   mean until the user presses Enter).  At that point, a complete
   TN3270E data message should be built to transmit the NVT data.

   Typically, NVT data is used by a server to interact with the user of
   a client.  It allows the server to do this using a simple NVT data
   stream, instead of requiring a 3270 data stream.  An example would be
   a server which displays a list of 3270 applications to which it can
   connect the client.  The server would use NVT data to display the
   list and read the user's choice.  Then the server would connect to
   the application, and begin the exchange of 3270 data between the
   application and the client.









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10.  Details of Processing TN3270E Functions

   Agreement by both parties to a specific function in the FUNCTIONS
   REQUEST function-list implies agreement by each party to support a
   related set of values in the TN3270E header.  It also implies a
   willingness to adhere to the rules governing the processing of data
   messages with regard to the agreed upon function.  Either party that
   fails to accept header values associated either with agreed upon
   functions or with basic TN3270E, or attempts to use header values
   associated with a function that is not a part of basic TN3270E and
   was not agreed upon, will be considered non-conforming and in
   violation of the protocol.  The following sections detail for each
   TN3270E function the associated header values and processing rules.

10.1 The SCS-CTL-CODES Function

   This function can only be supported on a 3270 printer session.

   Agreement to support this function requires that the party support
   the following TN3270E header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          SCS-DATA
              DATA-TYPE          PRINT-EOJ

   A client representing a printer device uses this function to indicate
   its willingness to accept a data stream that includes SCS control
   codes.  For the purposes of NVT mode versus 3270 mode, SCS-DATA must
   be treated exactly like 3270-DATA (i.e., it can cause a switch from
   NVT mode to 3270 mode).

   When a printer device-type has been negotiated, either the SCS-CTL-
   CODES function or the DATA-STREAM-CTL function, or both, must be
   negotiated.  This enables the server to know when it should and
   should not accept a session with a host application on behalf of the
   client.  If only the SCS-CTL-CODES function is agreed to, then the
   server will not establish sessions with host applications that would
   send 3270 data stream control.  If both SCS-CTL-CODES and DATA-
   STREAM-CTL are agreed to, then the server will establish sessions
   both with host applications that would send SCS control codes and
   with those that would send 3270 orders.

   The server should send a TN3270E message with DATA-TYPE set to
   PRINT-EOJ at the end of each print job to indicate to the client that
   it may now take whatever action is appropriate for its environment
   (e.g., close a disk or spool file, etc.).  The server may have
   multiple criteria for determining when it should send a PRINT-EOJ,



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   such as receipt of SNA End Bracket from the host application, or
   expiration of a pre-defined timeout value.

10.2 The DATA-STREAM-CTL Function

   This function can only be supported on a 3270 printer session.

   Agreement to support this function requires that the party support
   the following TN3270E header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          3270-DATA
              DATA-TYPE          PRINT-EOJ

   A client representing a printer device uses this function to indicate
   its willingness to accept a data stream that includes 3270 orders and
   attributes.

   When a printer device-type has been negotiated, either the SCS-CTL-
   CODES function or the DATA-STREAM-CTL function, or both, must be
   negotiated.  This enables the server to know when it should and
   should not accept a session with a host application on behalf of the
   client.  If only the DATA-STREAM-CTL function is agreed to, then the
   server will not establish sessions with host applications that would
   send SCS control codes in a data stream.  If both SCS-CTL-CODES and
   DATA-STREAM-CTL are agreed to, then the server will establish
   sessions both with host applications that would send SCS control
   codes and with those that would send 3270 orders.

   The server should send a TN3270E message with DATA-TYPE set to
   PRINT-EOJ at the end of each print job to indicate to the client that
   it may now take whatever action is appropriate for its environment
   (e.g., close a disk or spool file, etc.).  The server may have
   multiple criteria for determining when it should send a PRINT-EOJ,
   such as receipt of SNA End Bracket from the host application, or
   expiration of a pre-defined timeout value.














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10.3 The BIND-IMAGE Function

   This function can only be supported when the TN3270E server
   represents SNA terminals and printers.

   Agreement to support this function requires that the party support
   the following TN3270E header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          BIND-IMAGE
              DATA-TYPE          UNBIND
              DATA-TYPE          SSCP-LU-DATA

   When BIND-IMAGE is in effect, the server must inform the client when
   an SNA session has been established with a host application, and when
   such a session has been terminated.  It uses DATA-TYPE values of
   BIND-IMAGE and UNBIND to convey this information.

   When establishing an SNA session on behalf of a client, the server
   will receive a Bind RU from the host application.  It will also
   receive a Start Data Traffic RU.  Once both of these have been
   responded to positively by the server, it must then inform the client
   of the presence of this session by sending it a data message with the
   DATA-TYPE flag set to BIND-IMAGE.  The data portion of this message
   must contain the bind image exactly as it was received in the Bind RU
   that the server accepted on behalf of the client.  The format and
   maximum length of this bind image are defined in [6].

   When an SNA session between the server and a host application is
   terminated, the server must send a data message to the client with
   the DATA-TYPE flag set to UNBIND.  If the server was notified of the
   session termination via an SNA Unbind RU, it should include the
   Unbind reason code in the data portion of the message it sends to the
   client.  If the server itself requested the SNA session termination
   (for example, as part of SYSREQ key processing), it should set the
   data portion of the UNBIND message to 0x01, indicating "normal end of
   session".

   Another aspect of the BIND-IMAGE function alters the allowable DATA-
   TYPE flag values slightly from the behavior described in the section
   entitled "Basic TN3270E".  When BIND-IMAGE is in effect, data
   messages with DATA-TYPE set to 3270-DATA or SCS-DATA are not allowed
   before the first BIND-IMAGE is received by the client; only SSCP-LU-
   DATA or NVT-DATA can be used to transmit user- oriented data.  The
   same applies to data messages exchanged after an UNBIND is sent and
   before another BIND-IMAGE is received by the client.  Once the client
   receives a BIND-IMAGE data message, the allowable DATA-TYPE values,



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   in addition to SSCP-LU-DATA, now include 3270-DATA and/or SCS-DATA,
   depending on whether a terminal or printer device-type was
   negotiated, and whether a printer client agreed to DATA-STREAM-CTL or
   SCS-CTL-CODES, or both.  (See the section entitled "The SYSREQ
   Function" for further discussion of the SSCP-LU session in an SNA
   environment.)

10.4 The RESPONSES Function

   This function can be supported for both terminal and printer sessions
   connected to both SNA and non-SNA servers.

   Agreement to support this function requires that the party support
   the following TN3270E header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          RESPONSE
              DATA-TYPE          REQUEST
              RESPONSE-FLAG      -all values-
              REQUEST-FLAG       ERR-COND-CLEARED
              SEQ-NUMBER         binary values from 0-32767

   Whenever a data message is sent with a DATA-TYPE of either SCS-DATA
   or 3270-DATA, the sender must set the RESPONSE-FLAG field to either
   NO-RESPONSE, ERROR-RESPONSE, or ALWAYS-RESPONSE.  It is anticipated
   that the client side will normally set RESPONSE-FLAG to NO-RESPONSE.
   The server, if it represents an SNA device, should set RESPONSE-FLAG
   to reflect the response value set in the RH of the RU that generated
   this data message - Definite Response resulting in a RESPONSE-FLAG
   value of ALWAYS-RESPONSE, Exception Response resulting in ERROR-
   RESPONSE being set, and No Response causing a setting of NO-RESPONSE.
   A non-SNA server should set RESPONSE-FLAG to ERROR-RESPONSE.

   In addition, the sender must keep a count of the messages with a
   DATA-TYPE of 3270-DATA or SCS-DATA that it sends on a given TN3270E
   session.  This counter should start at zero for the first such
   message, and be incremented by one for each subsequent message.  Note
   that this counter is independent of any SNA sequence numbers, and
   should not be reset to zero as a result of Bind or Unbind.  If the
   counter reaches the maximum of 32767, it should be restarted at zero.
   The sender must place this value in the SEQ-NUMBER field of the
   TN3270E header before it sends the message.  Note that the SEQ-NUMBER
   field must be set regardless of the value of the RESPONSE-FLAG field.







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10.4.1 Response Messages

   Whenever a data message with a DATA-TYPE of either SCS-DATA or 3270-
   DATA is received, the receiver must attempt to process the data in
   the data portion of the message, then determine whether or not it
   should send a data message with a DATA-TYPE of RESPONSE.  If the data
   message it has just processed had a RESPONSE-FLAG value of NO-
   RESPONSE, or if it had a value of ERROR-RESPONSE and there were no
   errors encountered while processing the data, then no RESPONSE type
   message should be sent.  Otherwise, a data message should be sent in
   which the header DATA-TYPE field is set to RESPONSE, and in which the
   SEQ-NUMBER field is a copy of the SEQ-NUMBER field from the message
   to which this response corresponds.  The RESPONSE-FLAG field in this
   header must have a value of either POSITIVE-RESPONSE or NEGATIVE-
   RESPONSE.  A POSITIVE-RESPONSE should be sent if the previously
   processed message's header specified ALWAYS-RESPONSE and no errors
   were encountered in processing the data.  A NEGATIVE-RESPONSE should
   be sent when

    1) the previously processed message specified ERROR-RESPONSE
       or ALWAYS-RESPONSE and

    2) some kind of error occurred while processing the data.

   Normally only the client will be constructing and sending these
   RESPONSE messages.  A negative response sent by the client to the
   server is the equivalent of a Unit Check Status [7].  All references
   to device status and sense codes in this section rely on [7].

   The data portion of a RESPONSE message must consist of one byte of
   binary data.  The value of this byte gives a more detailed account of
   the results of having processed the previously received data message.
   The possible values for this byte are:

           For a RESPONSE-FLAG value of POSITIVE-RESPONSE -

             Value            Meaning
             -----            -------
             0x00      Successful completion (when sent by the client,
                       this is equivalent to "Device End").

           For a RESPONSE-FLAG value of NEGATIVE-RESPONSE -

             Value            Meaning
             -----            -------
             0x00      An invalid 3270 command was received
                       (equivalent to "Command Reject").




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             0x01      Printer is not ready (equivalent to
                       "Intervention Required").

             0x02      An illegal 3270 buffer address or order
                       sequence was received (equivalent to
                       "Operation Check").

             0x03      Printer is powered off or not connected
                       (equivalent to "Component Disconnected").

   When the server receives any of the above responses, it should pass
   along the appropriate information to the host application.  The
   appropriate information is determined by whether the server
   represents an SNA or a non-SNA device.

   An SNA server should pass along a POSITIVE-RESPONSE from the client
   as an SNA positive Response Unit to the host application.  It should
   translate a NEGATIVE-RESPONSE from the client into an SNA negative
   Response Unit in which the Sense Data Indicator bit is on and which
   contains one of the following sense codes:

          RESPONSE-FLAG        Equivalent        SNA Sense Code
          -------------        ----------        --------------
              0x00           Command Reject        0x10030000

              0x01        Intervention Required    0x08020000

              0x02           Operation Check       0x10050000

              0x03        Component Disconnected   0x08310000

   A non-SNA server should pass along a POSITIVE-RESPONSE from the
   client by setting the Device End Status bit on.  It should reflect a
   NEGATIVE-RESPONSE from the client by setting the Unit Check Status
   Bit on, and setting either the Command Reject, Intervention Required,
   or Operation Check Sense bit on when responding to the Sense command.

   In the case of Intervention Required or Component Disconnected being
   passed by the server to the host application, the host would normally
   refrain from sending any further data to the printer.  If and when
   the error condition at the client has been resolved, the client must
   send to the server a data message whose header DATA-TYPE field is set
   to REQUEST, and whose REQUEST-FLAG is set to ERR-COND-CLEARED.  Note
   that this message has no data portion.  Upon receipt of this message,
   the server should pass along the appropriate information to the host
   application so that it may resume sending printer output.  Again, the
   form of this information depends on whether the server represents an
   SNA or a non-SNA device.



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   An SNA server should reflect an ERR-COND-CLEARED to the host
   application by sending an SNA LUSTAT RU with one of the following
   sense codes:

    - if the previous error condition was an Intervention
      Required, the server should send sense code 0x00010000

    - if the previous error condition was Component
      Disconnected, the server should send sense code 0x082B0000

   A non-SNA server should set the corresponding bits in the Ending
   Status and Sense Condition bytes.

10.5 The SYSREQ Function

   This function can only be supported when the TN3270E server
   represents SNA devices.

   Agreement to support this function requires that the party support
   the following TN3270E header values:

             Header field         Value
             ------------         -----
              DATA-TYPE          SSCP-LU-DATA

   The 3270 SYSREQ key can be useful in an SNA environment when the ATTN
   key is not sufficient to terminate a process.  (See the section
   entitled "The 3270 ATTN Key" for more information.)

10.5.1 Background

   In SNA, there is a session between the host application (the PLU, or
   Primary Logical Unit) and the TN3270E server representing the client
   (the SLU, or Secondary Logical Unit).  This is referred to as the
   PLU-SLU session, and it is the one on which normal communications
   flow.  There is also a session between the host telecommunications
   access method (the SSCP, or System Services Control Point) and the
   SLU, and it is referred to as the SSCP-LU session.  This session is
   used to carry various control information and is normally transparent
   to the user; normal 3270 data stream orders are not allowed in this
   data.  For more information, refer to [7].

   The terminal display and keyboard are usually "owned" by the PLU-SLU
   session, meaning any data the user types is sent to the host
   application.  The SYSREQ key is used to toggle ownership of the
   keyboard and display between the PLU-SLU session and the SSCP-LU
   session.  In other words, the user is able to press SYSREQ and then
   communicate directly with the host SSCP.  The user may then enter any



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   valid Unformatted Systems Services commands, which are defined in the
   USS table associated with the SLU.  The most common USS command users
   employ is "LOGOFF," which requests that the SSCP immediately
   terminate the PLU-SLU session.  The usual reason for requesting such
   an action is that the host application (the PLU) has stopped
   responding altogether.

   Whenever the keyboard and display are owned by the SSCP-LU session,
   no data is allowed to flow in either direction on the PLU-SLU
   session.  Once "in" the SSCP-LU session, the user may decide to
   switch back to the PLU-SLU session by again pressing the SYSREQ key.

10.5.2 TN3270E Implementation of SYSREQ

   The design of some TN3270E servers allows them to fully support the
   SYSREQ key because they are allowed to send USS commands on the
   SSCP-LU session.  Other TN3270E servers operate in an environment
   which does not allow them to send USS commands to the SSCP; this
   makes full support of the SYSREQ key impossible.  For such servers,
   TN3270E provides for emulation of a minimal subset of functions,
   namely, for the sequence of pressing SYSREQ and typing LOGOFF that
   many users employ to immediately terminate the PLU-SLU session.

   The Telnet Abort Output (AO) command is the mechanism used to
   implement SYSREQ key support in TN3270E because, in a real SNA
   session, once the user presses the SYSREQ key, the host application
   is prevented from sending any more output to the terminal (unless the
   user presses SYSREQ a second time), but the user's process continues
   to execute.

   In order to implement SYSREQ key support, TN3270E clients that have
   agreed to the SYSREQ function 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 transmit a Telnet AO
   command to the server.

   Upon receipt of the AO command, a TN3270E server that has agreed to
   the SYSREQ function should enter what will be loosely termed
   "suspended mode" for the connection.  If a server that has not agreed
   to the SYSREQ function receives an AO command, it should simply
   ignore it.  Any attempt by the host application to send data to the
   client while the connection is "suspended" should be responded to by
   the server with a negative response, sense code 0x082D, indicating an
   "LU Busy" condition.  The server should not transmit anything to the
   client on behalf of the host application.  While the connection is
   "suspended," any data messages exchanged between the client and
   server should have the DATA-TYPE flag set to SSCP-LU-DATA; the data
   stream will be as defined in [7], specifically the section entitled



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   "Operation in SSCP-SLU Session."

   At this point, the behavior of the server depends upon whether or not
   it is allowed to send USS commands on the SSCP-LU session.  Servers
   that have this ability should simply act as a vehicle for passing USS
   commands and responses between the client and the SSCP.

   Servers that are not allowed to send USS commands on the SSCP-LU
   session should behave as follows:

   - if the user transmits the string LOGOFF (upper or lower case),
     the server should send an Unbind SNA RU to the host application.
     This will result in termination of the PLU-SLU session.  If the
     BIND-IMAGE function was agreed upon, then the server should also
     send a data message to the client with the DATA-TYPE flag set to
     UNBIND and the data portion set to 0x01.

   - if the user transmits anything other than LOGOFF, the server
     should respond with the string "COMMAND UNRECOGNIZED" to the
     client.  The server should not send anything to the host
     application on behalf of the client.

   Regardless of which kind of server is present (i.e., whether or not
   it may send USS commands on the SSCP-LU session), while the
   connection is suspended, the user may press the "SYSREQ" key again.
   This will result in the transmission of another AO to the server.
   The server should then send to the host application an LUSTAT RU with
   a value of 0x082B indicating "presentation space integrity lost". The
   server will then "un-suspend" the Telnet connection to the client,
   meaning it will allow the host application to once again send data to
   the client.

11.  The 3270 ATTN 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.  The Telnet Interrupt Process (IP)
   command was defined expressly for such a purpose, so it is used to
   implement support for the 3270 ATTN key.  This requires two things:

       - TN3270E 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 IP command to the server.

       - TN3270E servers should translate the IP command received from
         a TN3270E client into the appropriate form and pass it along to
         the host application as an ATTN key.  In other words, the



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         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
   TN3270E server representing non-SNA 3270 devices should ignore any
   Telnet IP commands it receives from a client.

12.  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 data streams. It would be unreasonable to expect all
   TN3270E clients to support all possible structured field functions,
   yet there must be a mechanism by which those clients that are capable
   of supporting some or all structured field functions can indicate
   their wishes.

   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 device.
   The device responds with a Query Reply structured field(s) listing
   which, if any, structured field functions it supports.

   The Query Reply is also used to indicate some device capabilities
   which do not require the use of structured fields, such as extended
   color support and extended highlighting capability.  Most host
   applications will use Read Partition Query to precisely determine a
   device's capabilities when there has been some indication that the
   device supports the "extended data stream".

   Therefore, all TN3270E clients that negotiate a terminal device-type
   that contains a "-E" suffix, the DYNAMIC terminal type, or a printer
   device-type, must be able to respond to a Read Partition Query
   command.  Note that these clients must support both the Read
   Partition Query (Type 02), and all forms of the Read Partition Query
   List (Type 03).

13.  Implementation Guidelines

13.1 3270 Data Stream Notes

   Implementors of TN3270E clients should note that the command codes
   for the various 3270 Read and Write commands have different values
   depending on how the server is connected to the host (local versus
   remote, SNA versus non-SNA).  Clients should be coded to check for
   the various possible values if they wish to be compatible with the
   widest range of servers.  See [7] for further details.



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13.2 Negotiation of the TN3270E Telnet Option

   Since TN3270E is a Telnet Option governed by [8], both client and
   server are free to attempt to initiate negotiation of TN3270E by
   sending a DO TN3270E command.  However, just as is usually the case
   with the Telnet DO TERMINAL-TYPE, it is anticipated that the server
   will normally be the one sending the DO TN3270E, and the client will
   be responding with a WILL or a WON'T TN3270E.

13.3 A "Keep-alive" Mechanism

   In many environments, it is very helpful to have in place a mechanism
   that allows timely notification of the loss of a 3270 session.
   TN3270E does not require that any form of keep-alive mechanism be
   employed by either clients or servers, but implementors wishing to
   support such a mechanism should consider the following guidelines.

   There are at least three possible means of providing a keep-alive
   mechanism in TN3270E: the TCP Keepalive, the Telnet IAC NOP command
   [8], and the Telnet DO TIMING-MARK option [9].  Each method has its
   advantages and disadvantages.  It is recommended that TN3270E clients
   and servers that support keep-alives should support all three
   methods, and that both sides should always respond to TIMING-MARKs.

   Note that both clients and servers could be configured to "actively"
   implement keep-alives.  That is, both sides could send a TIMING-MARK
   or a NOP or issue a TCP Keepalive in order to determine whether or
   not the partner is still alive.  Alternatively, network
   administrators may wish to configure only one side to send keep-
   alives; in this case, the other side would be a "passive" participant
   which simply responds to the keep-alives it receives.

   Implementors who want their code to be capable of being an "active"
   keep-alive participant should make their client or server
   configurable so that administrators can set which, if any, keep-alive
   mechanism should be employed, and how often it should be used.

   Upon failure of a session on which keep-alives are used, both parties
   should make the proper notifications.  A client should give the user
   some indication of the failure, such as an error code in the Operator
   Information Area of the screen.  A server should notify the host
   application that the session has been terminated, for example by
   sending an UNBIND with type CLEANUP in an SNA environment.

13.4 Examples

   The following example shows a TN3270E-capable server and a
   traditional tn3270 client establishing a connection:



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        Server:  IAC DO TN3270E
        Client:  IAC WON'T TN3270E
        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-3278-2 IAC SE
        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
           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing a generic pool (non-specific) terminal
   session:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-2 IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-2 CONNECT
                        anyterm IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST RESPONSES IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS RESPONSES IAC SE
           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing a terminal session where the client
   requests a specific device-name:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-5-E
                        CONNECT myterm IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-5-E CONNECT
                        myterm IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST RESPONSES
                        BIND-IMAGE IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS RESPONSES BIND-IMAGE
                        IAC SE
           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing a terminal session where the client
   requests a resource-name and is returned a device-name chosen by the
   server:




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       Server:  IAC DO TN3270E
       Client:  IAC WILL TN3270E
       Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
       Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-5-E
                       CONNECT pool1 IAC SE
       Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-5-E CONNECT
                       term0013 IAC SE
       Client:  IAC SB TN3270E FUNCTIONS REQUEST BIND-IMAGE IAC SE
       Server:  IAC SB TN3270E FUNCTIONS IS BIND-IMAGE IAC SE
          (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client attempting to establish a terminal session; multiple
   attempts are necessary because the device-name initially requested by
   the client is already in use:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-5
                        CONNECT myterm IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE REJECT REASON
                        DEVICE-IN-USE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-2
                        CONNECT herterm IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-2 CONNECT
                        herterm IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST RESPONSES IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS RESPONSES IAC SE
           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing a printer session where the client
   requests a specific device-name, and where some amount of 3270
   function negotiation is required before an agreement is reached:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3287-1 CONNECT
                        myprt IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3287-1 CONNECT
                        myprt IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST DATA-STREAM-CTL IAC
        Server:  IAC SB TN3270E FUNCTIONS REQUEST DATA-STREAM-CTL
                        RESPONSES IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST DATA-STREAM-CTL IAC
        Server:  IAC SB TN3270E FUNCTIONS IS DATA-STREAM-CTL IAC SE



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           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing first a specific terminal session, then a
   printer session where the "partner" printer for the assigned terminal
   is requested:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-2 CONNECT
                        termxyz IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-2 CONNECT
                        termxyz IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST RESPONSES IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS RESPONSES IAC SE
           (3270 data stream is exchanged)
             .            .
             .            .
           (user decides to request a printer session,
            so client again connects to Telnet port on server)
        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3287-1
                        ASSOCIATE termxyz IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3287-1 CONNECT
                        termxyz's-prt IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST SCS-CTL-CODES
                        RESPONSES IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS SCS-CTL-CODES RESPONSES
                        IAC SE
           (3270 data stream is exchanged)

   The following example shows a TN3270E-capable server and a TN3270E-
   capable client establishing first a terminal session where a
   resource-name was requested and a server chosen device-name was
   returned, then a printer session where the "partner" printer for the
   assigned terminal is requested:

        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-5 CONNECT
                        poolxyz IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3278-5 CONNECT
                        terma IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST RESPONSES IAC SE



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        Server:  IAC SB TN3270E FUNCTIONS IS RESPONSES IAC SE
           (3270 data stream is exchanged)
             .            .
             .            .
           (user decides to request a printer session,
            so client again connects to Telnet port on server)
        Server:  IAC DO TN3270E
        Client:  IAC WILL TN3270E
        Server:  IAC SB TN3270E SEND DEVICE-TYPE IAC SE
        Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3287-1
                        ASSOCIATE terma IAC SE
        Server:  IAC SB TN3270E DEVICE-TYPE IS IBM-3287-1 CONNECT
                        terma's-prt IAC SE
        Client:  IAC SB TN3270E FUNCTIONS REQUEST SCS-CTL-CODES
                        RESPONSES IAC SE
        Server:  IAC SB TN3270E FUNCTIONS IS SCS-CTL-CODES RESPONSES
                        IAC SE
           (3270 data stream is exchanged)

14.  Security Considerations

   These extensions to telnet do not provide any security features
   beyond that of ordinary telnet; so a TN3270E session is no more
   secure than an ordinary telnet session.  Once standard authentication
   and/or privacy mechanisms for telnet have been defined, these may
   also be usable by TN3270E.  One of the important uses of
   authentication would be to answer the question of whether or not a
   given user should be allowed to "use" a specific terminal or printer
   device-name.

15.  References

   [1] Rekhter, J., "Telnet 3270 Regime Option", RFC 1041, January 1988.

   [2] VanBokkelen, J., "Telnet Terminal-Type Option", RFC 1091,
   February 1989.

   [3] Postel, J., and J. Reynolds, "Telnet Binary Transmission", STD
   27, RFC 856, May 1983.

   [4] Postel, J., "Telnet End of Record Option", RFC 885, December
   1983.

   [5] "3270 Information Display System - Data Stream Programmer's
   Reference", publication number GA24-0059, IBM Corporation.

   [6] "SNA Formats", publication number GA27-3136, IBM Corporation.




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   [7] "3174 Establishment Controller Functional Description",
   publication number GA23-0218, IBM Corporation.

   [8] Postel, J., and J. Reynolds, "Telnet Protocol Specification", STD
   8, RFC 854, May 1983.

   [9] Postel, J., and J. Reynolds, "Telnet Timing Mark Option", STD 31,
   RFC 860, May 1983.

   [10] J. Penner, "TN3270 Current Practices", RFC 1576, January, 1994.

16.  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.

    - Discussions at the 1993 IETF meetings.

    - Discussions on the "TN3270E" list, 1993-94 and 1997.

17. Author's Address

    Bill Kelly
    Division of University Computing
    144 Parker Hall
    Auburn University, AL  36849

    Phone: (334) 844-4512
    EMail: kellywh@mail.auburn.edu

















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18.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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