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rfc:rfc1647

Network Working Group B. Kelly Request for Comments: 1647 Auburn University Category: Standards Track July 1994

                        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.

Abstract

 This document describes a protocol that more fully supports 3270
 devices than do the existing 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 would be negotiated and implemented under a new Telnet
 Option and would be unrelated to the Telnet 3270 Regime Option as
 defined in RFC 1041 [1].

TABLE OF CONTENTS

 1.  Introduction ...............................................  2
 2.  TN3270E OVERVIEW ...........................................  3
 3.  COMMAND NAMES AND CODES ....................................  4
 4.  COMMAND MEANINGS ...........................................  5
 5.  DEFAULT SPECIFICATION ......................................  6
 6.  MOTIVATION .................................................  7
 7.  TN3270E SUB-NEGOTIATION RULES ..............................  7
    7.1  DEVICE-TYPE Negotiation ................................  7
        7.1.1 Device Pools ......................................  8
        7.1.2 CONNECT Command ...................................  9
        7.1.3 ASSOCIATE Command ................................. 10
        7.1.4 Device Selection Rules ............................ 10
        7.1.5 Accepting a Request ............................... 11
        7.1.6 REJECT Command .................................... 12
    7.2  FUNCTIONS Negotiation .................................. 13
        7.2.1 Commands .......................................... 13

Kelly [Page 1] RFC 1647 TN3270 Enhancements July 1994

        7.2.2 List of TN3270E Functions ......................... 14
 8.  TN3270E DATA MESSAGES ...................................... 15
    8.1  The TN3270E Message Header ............................. 16
        8.1.1 DATA-TYPE Field ................................... 16
        8.1.2 REQUEST-FLAG Field ................................ 17
        8.1.3 RESPONSE-FLAG Field ............................... 17
        8.1.4 SEQ-NUMBER Field .................................. 18
 9.  BASIC TN3270E .............................................. 18
    9.1  3270 Mode and NVT Mode ................................. 19
 10. DETAILS OF PROCESSING TN3270E FUNCTIONS .................... 20
    10.1 The SCS-CTL-CODES Function ............................. 20
    10.2 The DATA-STREAM-CTL Function ........................... 20
    10.3 The BIND-IMAGE Function ................................ 21
    10.4 The RESPONSES Function ................................. 22
       10.4.1 Response Messages ................................. 23
    10.5 The SYSREQ Function .................................... 26
       10.5.1 Background ........................................ 26
       10.5.2 TN3270E Implementation of SYSREQ .................. 27
 11. THE 3270 ATTN KEY .......................................... 28
 12. 3270 STRUCTURED FIELDS ..................................... 29
 13. IMPLEMENTATION GUIDELINES .................................. 29
    13.1 3270 Data Stream Notes ................................. 29
    13.2 Negotiation of the TN3270E Telnet Option ............... 30
    13.3 A "Keep-alive" Mechanism ............................... 30
    13.4 Examples ............................................... 31
 14. SECURITY CONSIDERATIONS .................................... 33
 15. REFERENCES ................................................. 33
 16. AUTHOR'S NOTE .............................................. 34
 17. AUTHOR'S ADDRESS ........................................... 34

1. Introduction

 Currently, support for 3270 terminal emulation over Telnet is
 accomplished by the de facto standard of negotiating three separate
 Telnet Options - Terminal-Type [2], Binary Transmission [3], and End
 of Record [4].  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.
 This document will refer to the existing practice of negotiating
 these three Telnet Options before exchanging the 3270 data stream as
 "traditional tn3270".
 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.

Kelly [Page 2] RFC 1647 TN3270 Enhancements July 1994

 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].
 There are several shortcomings in traditional tn3270; among them are
 the following:
  1. It provides no capability for Telnet clients to emulate the 328x

class of printers.

  1. There is no mechanism by which a Telnet client can request that

a connection be associated with a given 3270 device-name. This

    can be of importance when a terminal session is being
    established, since many host applications behave differently
    depending on the network name of the terminal.  In the case of
    printer emulation, this capability is an absolute necessity
    because a large number of host applications have some method of
    pre-defining printer destinations.
  1. The 3270 ATTN and SYSREQ keys are not universally supported.
  1. There is 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.
  1. There is no mechanism by which the client can access the SNA

Bind information. The Bind image contains a detailed

    description of the session between the Telnet server and the
    host application.
  1. There is no mechanism by which the server can determine whether

a client supports 3270 structured fields, or a client can

    request that it receive them.

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

Kelly [Page 3] RFC 1647 TN3270 Enhancements July 1994

 sub-negotiation will occur to determine what subset of TN3270E will
 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:
  1. a device-type negotiation that is similar to, but somewhat

more complicated than, the existing Telnet Terminal-Type Option.

  1. 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

     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-DEVICE-NAME    03
       INV-DEVICE-TYPE    04
       TYPE-NAME-ERROR    05
       UNKNOWN-ERROR      06

Kelly [Page 4] RFC 1647 TN3270 Enhancements July 1994

       UNSUPPORTED-REQ    07
     Function Names
       BIND-IMAGE         00
       DATA-STREAM-CTL    01
       RESPONSES          02
       SCS-CTL-CODES      03
       SYSREQ             04

4. Command Meanings

 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.
 IAC SB TN3270E DEVICE-TYPE REQUEST <device-type>
        [CONNECT | 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

Kelly [Page 5] RFC 1647 TN3270 Enhancements July 1994

    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 and ASSOCIATE must both 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.

5. Default Specification

 WON'T TN3270E
 DON'T TN3270E
 i.e., TN3270E will not be used.

Kelly [Page 6] RFC 1647 TN3270 Enhancements July 1994

6. Motivation

 See the section entitled "Introduction".

7. TN3270E Sub-negotiation Rules

 All TN3270E commands and parameters are NVT ASCII strings in which
 upper and lower case are considered equivalent.
 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 (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 device-name request.
    Valid device-types are:
     terminals: 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 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.

Kelly [Page 7] RFC 1647 TN3270 Enhancements July 1994

    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.  (The
       terms "device-name", "LU name" and "network name" can be
       considered interchangeable in this document.)  Also, for the
       purposes of this discussion, the term "generic session request"
       will be used to describe a request for a session by a Telnet
       client (either traditional or TN3270E) that does not include a
       request for a specific device-name.  The term "specific session
       request" will be used to describe a request for a session by a
       TN3270E client that includes a request for a specific device-
       name (either via CONNECT or ASSOCIATE).
       As is the case with traditional tn3270, the TN3270E server must
       maintain a set of terminal device-names.  A generic request for

Kelly [Page 8] RFC 1647 TN3270 Enhancements July 1994

       a terminal session would result in the server selecting any
       available device-name from this pool.  The server, however, may
       also maintain a separate pool of terminal device-names which
       can only be used to satisfy specific terminal session requests.
       This is to ensure that a terminal device that has some
       significance to host applications (and is therefore likely to
       be the target of a specific session request) is not
       "accidentally" assigned to a generic request and winds up
       associated with a client that has no use for it.  Note that the
       reverse situation is allowed.  That is, a specific terminal
       session request could ask for a device-name that happens to be
       in the "generic terminal pool".
       For each terminal device (in both the "generic" and the
       "specific" pools), the TN3270E server could also have defined a
       "partner" or "paired" printer device.  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 a pool 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 example, when the userid of
       the person signed on to a terminal determines the print
       destination).
       Finally, it is possible that a pool of printer device-names
       could be maintained and used only to satisfy generic requests
       for printers.
    7.1.2 CONNECT Command
       CONNECT is used by the client to request that the server assign
       a specific device-name to this Telnet session; it may be used
       when requesting either a terminal or a printer session.  The
       specified device-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 defined at the

Kelly [Page 9] RFC 1647 TN3270 Enhancements July 1994

       host as a terminal, then the server should deny the request.
       Further, if the requested device-name is already associated
       with some other Telnet session, or if it is not defined to the
       server, the server should 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. 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 should
       deny the request.  It is anticipated that the device-name
       specified in this request would be one returned by the server
       when accepting a previous terminal session request (see the IS
       command below).  Since no means of authentication has been
       provided for, it is possible that the printer paired with the
       terminal specified in the ASSOCIATE command has already been
       assigned to some other Telnet session; in this case, the server
       should deny the request.
    7.1.4 Device Selection Rules
       To summarize, assume a TN3270E server has the following device
       pools defined to it (device-names that begin with a "T" are
       terminal devices; those that begin with a "P" are printers):
        Generic Terminal Pool              Specific Terminal Pool
        ---------------------              ----------------------
        TG000001 <--> PTG00001             TS000001 <--> PTS00001
        TG000002 <--> PTG00002             TS000002 <--> PTS00002
        TG000003 <--> PTG00003             TS000003 <--> PTS00003
        Generic Printer Pool               Specific Printer Pool
        --------------------               ----------------------
             PG000001                            PS000001
             PG000002                            PS000002
             PG000003                            PS000003
       Note that the only pool that absolutely must be defined to the
       server is the generic terminal pool.  The absence of other
       pools (or of partner printers for a terminal pool) simply means
       that the server is unable to satisfy as wide a variety of
       requests as would be possible if all pools were defined to it.
       Given the above configuration, the following rules apply:

Kelly [Page 10] RFC 1647 TN3270 Enhancements July 1994

  1. a generic terminal request can only be satisfied from the

generic terminal pool (device-names TG000001 - TG000003).

  1. a specific terminal request (allowable only via the CONNECT

command) can be satisfied from either the generic or the

         specific terminal pool, although it is anticipated that the
         majority of such requests would ask for terminals in the
         specific terminal pool (TS000001 - TS000003).
  1. a generic printer request can only be satisfied from the

generic printer pool (device-names PG000001 - PG000003).

  1. a specific printer request may come in one of two forms:
         via ASSOCIATE: the request can only be satisfied using the
                        partner of the specified terminal, which
                        may be in the generic or the specific
                        terminal pool; therefore, devices in the
                        ranges PTG00001 - PTG00003 and PTS00001 -
                        PTS00003 can be used to satisfy the request.
         via CONNECT:   the request can be satisfied either from
                        the generic or the specific printer pools
                        (although, as with specific terminal requests,
                        it is likely that most such requests will name
                        printers in the specific printer pool); this
                        request cannot be satisfied with the partner
                        printer of a terminal in either the specific or
                        the generic terminal pools.
    7.1.5 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 Telnet session.  This device-name
       may be the one directly requested (via CONNECT) by the client,
       the one indirectly requested (via ASSOCIATE) by the client, or
       one chosen by the server if the client specified neither
       CONNECT nor ASSOCIATE.

Kelly [Page 11] RFC 1647 TN3270 Enhancements July 1994

    7.1.6 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-DEVICE-NAME          The 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
                                Telnet session.
       TYPE-NAME-ERROR          The requested device-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
                                server does not have such a pool 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.

Kelly [Page 12] RFC 1647 TN3270 Enhancements July 1994

       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 should 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 should 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 the DEVICE-TYPE IS command), the client
    should 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 3270
       functions that the sender would like to see supported on this
       session.  All functions not in the list are to be considered
       unsupported.  The function-list consists of a string of 2-byte
       entries separated from one another by a single space character.
       The list is terminated by the IAC code that precedes the SE
       command.  Functions may appear in any order in the list.
       Upon receipt of a FUNCTIONS REQUEST command, the recipient has
       two choices:
  1. 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.
  1. 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

Kelly [Page 13] RFC 1647 TN3270 Enhancements July 1994

       of functions in the list; at least one function must have
       been added to, or removed from, the list).
       To avoid endlessly looping, neither party should 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.
    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.

Kelly [Page 14] RFC 1647 TN3270 Enhancements July 1994

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

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
 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

Kelly [Page 15] RFC 1647 TN3270 Enhancements July 1994

 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.
 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:
  1. ———————————————————-

| DATA-TYPE | REQUEST-FLAG | RESPONSE-FLAG | SEQ-NUMBER |

  1. ———————————————————-

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.

Kelly [Page 16] RFC 1647 TN3270 Enhancements July 1994

       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.
    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

Kelly [Page 17] RFC 1647 TN3270 Enhancements July 1994

                                  occur.  One or the other must
                                  always be sent by the receiver.
       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.  See the section
       entitled "The RESPONSES Function" for further information.
       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.

Kelly [Page 18] RFC 1647 TN3270 Enhancements July 1994

 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.

Kelly [Page 19] RFC 1647 TN3270 Enhancements July 1994

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
    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 should 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.
 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:

Kelly [Page 20] RFC 1647 TN3270 Enhancements July 1994

        Header field         Value
        ------------         -----
         DATA-TYPE          3270-DATA
    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.
 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.

Kelly [Page 21] RFC 1647 TN3270 Enhancements July 1994

    When an SNA session between the server and a host application is
    terminated, the server should 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 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

Kelly [Page 22] RFC 1647 TN3270 Enhancements July 1994

    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
    session.  This counter should start at zero for the first such
    message, and be incremented by one for each subsequent message.
    If the counter reaches the maximum of 32767, it should be
    restarted at zero.  The sender should 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.
    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].

Kelly [Page 23] RFC 1647 TN3270 Enhancements July 1994

       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").
           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:

Kelly [Page 24] RFC 1647 TN3270 Enhancements July 1994

           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.
       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:
  1. if the previous error condition was an Intervention

Required, the server should send sense code 0x00010000

  1. 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.

Kelly [Page 25] RFC 1647 TN3270 Enhancements July 1994

 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 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

Kelly [Page 26] RFC 1647 TN3270 Enhancements July 1994

       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 (except TN3270E responses)
       exchanged between the client and server should have the DATA-
       TYPE flag set to SSCP-LU-DATA.
       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.

Kelly [Page 27] RFC 1647 TN3270 Enhancements July 1994

       Servers that are not allowed to send USS commands on the SSCP-
       LU session should behave as follows:
  1. 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.
  1. 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:
  1. 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.
  1. 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
       server representing an SLU in an SNA session should send
       a SIGNAL RU to the host application.

Kelly [Page 28] RFC 1647 TN3270 Enhancements July 1994

 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.

Kelly [Page 29] RFC 1647 TN3270 Enhancements July 1994

 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 two possible means of providing a keep-alive
    mechanism in TN3270E: the Telnet IAC NOP command [8], and the
    Telnet DO TIMING-MARK option [9].  Both methods have their
    advantages and disadvantages.  It is recommended that TN3270E
    clients and servers that support keep-alives should accept both
    NOPs and TIMING-MARKs, 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 in order to determine whether or not the
    partner is still alive.  Alternatively, network administrators may
    wish to configure only one side to send TIMING-MARKs or NOPs; 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 the NOP or
    TIMING-MARK should be sent on each session.
    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.

Kelly [Page 30] RFC 1647 TN3270 Enhancements July 1994

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

Kelly [Page 31] RFC 1647 TN3270 Enhancements July 1994

    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
         (3270 data stream is exchanged)
    The following example shows a TN3270E-capable server and a
    TN3270E-capable client establishing first a generic 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

Kelly [Page 32] RFC 1647 TN3270 Enhancements July 1994

      Client:  IAC SB TN3270E DEVICE-TYPE REQUEST IBM-3278-2 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)

14. Security Considerations

 Security issues are not addressed in this document.  It is
 anticipated that once authentication mechanisms have become well
 established, use of them can be made 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, IBM
     Corporation, January 1988.
 [2] VanBokkelen, J., "Telnet Terminal-Type Option", RFC 1091, FTP
     Software, Inc., February 1989.
 [3] Postel, J., and J. Reynolds, "Telnet Binary Transmission", STD
     27, RFC 856, USC/Information Sciences Institute, May 1983.
 [4] Postel, J., "Telnet End of Record Option", RFC 885, USC/
     Information Sciences Institute, December 1983.
 [5] "3270 Information Display System - Data Stream Programmer's
     Reference", publication number GA24-0059, IBM Corporation.

Kelly [Page 33] RFC 1647 TN3270 Enhancements July 1994

 [6] "SNA Formats", publication number GA27-3136, IBM Corporation.
 [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, USC/Information Sciences Institute, May 1983.
 [9] Postel, J., and J. Reynolds, "Telnet Timing Mark Option", STD 31,
     RFC 860, USC/Information Sciences Institute, May 1983.

16. Author's Note

 Portions of this document were drawn from the following sources:
  1. A White Paper written by Owen Reddecliffe, WRQ Corporation,

October 1991.

  1. Experimental work on the part of Cleve Graves and Michelle

Angel, OpenConnect Systems, 1992 - 1993.

  1. Discussions at the 1993 IETF meetings.
  1. Discussions on the "TN3270E" list, 1993-94.

17. Author's Address

 Bill Kelly
 Division of University Computing
 144 Parker Hall
 Auburn University, AL  36849
 Phone: (205) 844-4512
 EMail: kellywh@mail.auburn.edu

Kelly [Page 34]

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