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+
+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:
+
+ - It provides no capability for Telnet clients to emulate the 328x
+ class of printers.
+
+ - There is no mechanism by which a Telnet client can request that
+ a connection be associated with a given 3270 device-name. This
+ can be of importance when a terminal session is being
+ established, since many host applications behave differently
+ depending on the network name of the terminal. In the case of
+ printer emulation, this capability is an absolute necessity
+ because a large number of host applications have some method of
+ pre-defining printer destinations.
+
+ - The 3270 ATTN and SYSREQ keys are not universally supported.
+
+ - There is no support for the SNA positive/negative response
+ process. 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.
+
+ - 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.
+
+ - 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:
+
+ - 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
+
+ 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
+
+
+ - a generic terminal request can only be satisfied from the
+ generic terminal pool (device-names TG000001 - TG000003).
+
+ - 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).
+
+ - a generic printer request can only be satisfied from the
+ generic printer pool (device-names PG000001 - PG000003).
+
+ - 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:
+
+ - 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
+
+
+
+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:
+
+ -----------------------------------------------------------
+ | 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.
+
+
+
+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:
+
+ - 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.
+
+
+
+
+
+
+
+
+
+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:
+
+ - 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
+ 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:
+
+ - 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.
+
+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]
+