RFC2355 - TN3270 Enhancements

Network Working Group B. Kelly
Request for Comments: 2355 Auburn University
Obsoletes: 1647 June 1998
Category: Standards Track
TN3270 Enhancements
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document describes a protocol that more fully supports 3270
devices than do traditional tn3270 practices. Specifically, it
defines a method of emulating both the terminal and printer members
of the 3270 family of devices via Telnet; it provides for the ability
of a Telnet client to request that it be assigned a specific device-
name (also referred to as "LU name" or "network name"); finally, it
adds support for a variety of functions sUCh as the ATTN key, the
SYSREQ key, and SNA response handling.
This protocol is negotiated under the TN3270E Telnet Option, and is
unrelated to the Telnet 3270 Regime Option as defined in RFC1041
[1].
TABLE OF CONTENTS
1. Introduction ............................................... 2
1.1 Changes to RFC1647 .................................... 4
2. TN3270E OVERVIEW ........................................... 5
3. COMMAND NAMES AND CODES .................................... 6
4. COMMAND MEANINGS ........................................... 7
5. DEFAULT SPECIFICATION ...................................... 9
6. MOTIVATION ................................................. 9
7. TN3270E SUB-NEGOTIATION RULES .............................. 9
7.1 DEVICE-TYPE Negotiation ................................ 9
7.1.1 Device Pools ...................................... 10
7.1.2 CONNECT Command ................................... 12
7.1.3 ASSOCIATE Command ................................. 12
7.1.4 Accepting a Request ............................... 13
7.1.5 REJECT Command .................................... 13
7.2 FUNCTIONS Negotiation .................................. 14
7.2.1 Commands .......................................... 14
7.2.2 List of TN3270E Functions ......................... 16
8. TN3270E DATA MESSAGES ...................................... 16
8.1 The TN3270E Message Header ............................. 18
8.1.1 DATA-TYPE Field ................................... 18
8.1.2 REQUEST-FLAG Field ................................ 19
8.1.3 RESPONSE-FLAG Field ............................... 19
8.1.4 SEQ-NUMBER Field .................................. 20
9. BASIC TN3270E .............................................. 20
9.1 3270 Mode and NVT Mode ................................. 21
10. DETAILS OF PROCESSING TN3270E FUNCTIONS .................... 22
10.1 The SCS-CTL-CODES Function ............................. 22
10.2 The DATA-STREAM-CTL Function ........................... 23
10.3 The BIND-IMAGE Function ................................ 24
10.4 The RESPONSES Function ................................. 25
10.4.1 Response Messages ................................. 26
10.5 The SYSREQ Function .................................... 28
10.5.1 Background ........................................ 28
10.5.2 TN3270E Implementation of SYSREQ .................. 29
11. THE 3270 ATTN KEY .......................................... 30
12. 3270 STRUCTURED FIELDS ..................................... 31
13. IMPLEMENTATION GUIDELINES .................................. 31
13.1 3270 Data Stream Notes ................................. 31
13.2 Negotiation of the TN3270E Telnet Option ............... 32
13.3 A "Keep-alive" Mechanism ............................... 32
13.4 Examples ............................................... 32
14. SECURITY CONSIDERATIONS .................................... 36
15. REFERENCES ................................................. 36
16. AUTHOR"S NOTE .............................................. 37
17. AUTHOR"S ADDRESS ........................................... 37
18. Full Copyright Statement ................................... 38
1. Introduction
Traditionally, support for 3270 terminal emulation over Telnet has
been accomplished by the de facto standard of negotiating three
separate Telnet Options - Terminal-Type [2], Binary Transmission [3],
and End of Record [4]. Note that there is no RFCthat specifies this
negotiation as a standard. RFC1041 attempted to standardize the
method of negotiating 3270 terminal support by defining the 3270
Regime Telnet Option. Very few developers and vendors ever
implemented RFC1041.
This document will refer to the existing practice of negotiating
these three Telnet Options before exchanging the 3270 data stream as
"traditional tn3270". Traditional tn3270 is documented in [10].
NOTE: Except where otherwise stated, this document does not
distinguish between Telnet servers that represent SNA devices and
those that represent non-SNA 3270 devices.
All references in this document to the 3270 data stream, 3270 data
stream commands, orders, structured fields and the like rely on [5].
References to SNA Request and Response Units rely on [6]. References
to SNA versus non-SNA operation rely on [7].
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119.
There were several shortcomings in traditional tn3270; among them
were the following:
- It provided no capability for Telnet clients to emulate the 328x
class of printers.
- There was no mechanism by which a Telnet client could request that
a connection be associated with a given 3270 device-name. This
can be of importance when a terminal session is being established,
since many host applications behave differently depending on the
network name of the terminal. In the case of printer emulation,
this capability is an absolute necessity because a large number of
host applications have some method of pre-defining printer
destinations.
- The 3270 ATTN and SYSREQ keys were not universally supported.
- There was no support for the SNA positive/negative response
process. This is particularly important if printer emulation is
to function properly, but is also useful for some terminal
applications. A positive response is used to indicate that the
previously received data has been successfully processed. A
negative response indicates some sort of error has occurred while
processing the previously received data; this could be caused by
the host application building a 3270 data stream that contains an
invalid command, or by a mechanical error at the client side,
among other things.
- There was no mechanism by which the client could Access the SNA
Bind information. The Bind image contains a detailed description
of the session between the Telnet server and the host application.
- There was no mechanism by which the server could determine whether
a client supported 3270 structured fields, or a client could
request that it receive them.
1.1 Changes to RFC1647
This document replaces RFC1647 the following is a summary of the
changes that have been incorporated:
- Reworded the Introduction to refer to traditional tn3270 in the
past tense.
Affected sections: 1.
- Added this section documenting changes to RFC1647
Affected sections: 1.1
- Clarified the specification of numeric literals contained in the
document.
Affected sections: 3. (first paragraph)
- Extensively revised several sections to
1) clarify the motivation behind and use of the ASSOCIATE
command
2) remove restrictive wording regarding the organization
and use of server maintained device pools
3) distinguish between device-names and resource-names in the
TN3270E device-type negotiation, and define a maximum length for
device-names and resource-names
Affected sections: 4. (DEVICE-TYPE REQUEST command) and 7.1.1
through 7.1.6
- Corrected the erroneous specification of the format of the
function-list sent during TN3270E functions negotiation.
Affected sections: 7.2.1 (first paragraph)
- Added a statement addressing what a client or server should do
if an impasse is reached during TN3270E functions negotiation.
Affected sections: 7.2.1 (last paragraph)
- Added a DATA-TYPE of PRINT-EOJ with a value of 0x08 to support
the end-of-job indication for printers.
Affected sections: 8.1.1, 10.1, 10.2
- Clarified the description of the SEQ-NUMBER Field to state that
1) the field should be sent in network byte order (big endian)
2) either byte that contains a 0xff must be doubled to 0xffff
before sending, and stripped back to 0xff after receipt.
Affected sections: 8.1.4
- Defined the format and maximum length of the Bind image.
Affected sections: 10.3 (fourth paragraph)
- Clarified the misleading wording regarding allowable DATA-TYPEs
when BIND-IMAGE has been negotiated and a BIND has been sent.
Affected sections: 10.3 (last paragraph)
- Clarified the use of the SEQ-NUMBER field in regards to when it
should be reset to zero.
Affected sections: 10.4 (last paragraph)
- Clarified the format of the data when the DATA-TYPE field is
SSCP-LU-DATA.
Affected sections: 10.5.2 (fourth paragraph)
- Reworded the Security section to refer to Kerberos.
Affected sections: 14.
2. TN3270E Overview
In order to address these issues, this document proposes a new Telnet
Option - TN3270E. Telnet clients and servers would be free to
negotiate support of the TN3270E option or not. If either side does
not support TN3270E, traditional tn3270 can be used; otherwise, a
sub-negotiation will occur to determine what subset of TN3270E will
be used on the session. It is anticipated that a client or server
capable of both types of 3270 emulation would attempt to negotiate
TN3270E first, and only negotiate traditional tn3270 if the other
side refuses TN3270E.
Once a client and server have agreed to use TN3270E, negotiation of
the TN3270E suboptions can begin. The two major elements of TN3270E
sub-negotiation are:
- a device-type negotiation that is similar to, but somewhat
more complicated than, the existing Telnet Terminal-Type Option.
- the negotiation of a set of supported 3270 functions, such as
printer data stream type (3270 data stream or SNA Character
Stream), positive/negative response exchanges, device status
information, and the passing of BIND information from server to
client.
Successful negotiation of these two suboptions signals the beginning
of 3270 data stream transmission. In order to support several of the
new functions in TN3270E, each data message must be prefixed by a
header. This header will contain flags and indicators that convey
such things as positive and negative responses and what type of data
follows the header (for example, 3270 data stream, SNA Character
Stream, or device status information).
3. Command Names and Codes
Please note that all numeric literals in this document specify
decimal values, unless they are preceded by "0x", in which case a
hexadecimal value is represented.
TN3270E 40
ASSOCIATE 00
CONNECT 01
DEVICE-TYPE 02
FUNCTIONS 03
IS 04
REASON 05
REJECT 06
REQUEST 07
SEND 08
Reason-codes
CONN-PARTNER 00
DEVICE-IN-USE 01
INV-ASSOCIATE 02
INV-NAME 03
INV-DEVICE-TYPE 04
TYPE-NAME-ERROR 05
UNKNOWN-ERROR 06
UNSUPPORTED-REQ 07
Function Names
BIND-IMAGE 00
DATA-STREAM-CTL 01
RESPONSES 02
SCS-CTL-CODES 03
SYSREQ 04
4. Command Meanings
Refer to the Telnet Protocol Specification [8] for the meaning of
IAC, DO, WILL, etc.
IAC WILL TN3270E
The sender of this command is willing to send TN3270E information
in subsequent sub-negotiations.
IAC WON"T TN3270E
The sender of this command refuses to send TN3270E information.
IAC DO TN3270E
The sender of this command is willing to receive TN3270E
information in subsequent sub-negotiations.
IAC DON"T TN3270E
The sender of this command refuses to receive TN3270E information.
Note that while they are not eXPlicitly negotiated, the equivalent of
the Telnet Binary Transmission Option [3] and the Telnet End of
Record Option [4] is implied in the negotiation of the TN3270E
Option. That is, a party to the negotiation that agrees to support
TN3270E is automatically required to support bi-directional binary
and EOR transmissions.
IAC SB TN3270E SEND DEVICE-TYPE IAC SE
Only the server may send this command. This command is used to
request that the client transmit a device-type and, optionally,
device-name information.
IAC SB TN3270E DEVICE-TYPE REQUEST <device-type>
[ [CONNECT <resource-name>] [ASSOCIATE <device-name>] ] IAC SE
Only the client may send this command. It is used in response to
the server"s SEND DEVICE-TYPE command, as well as to suggest
another device-type after the server has sent a DEVICE-TYPE REJECT
command (see below). This command requests emulation of a
specific 3270 device type and model. The REQUEST command may
optionally include either the CONNECT or the ASSOCIATE command
(but not both). If present, CONNECT must be followed by
<resource-name> and ASSOCIATE must be followed by <device-name>.
(See the section entitled "DEVICE-TYPE Negotiation" for more
detailed information.)
IAC SB TN3270E DEVICE-TYPE IS <device-type> CONNECT
<device-name> IAC SE
Only the server may send this command. This command is used to
accept a client"s DEVICE-TYPE REQUEST command and to return the
server-defined device-name.
IAC SB TN3270E DEVICE-TYPE REJECT REASON <reason-code> IAC SE
Only the server may send this command. This command is used to
reject a client"s DEVICE-TYPE REQUEST command.
IAC SB TN3270E FUNCTIONS REQUEST <function-list> IAC SE
Either side may send this command. This command is used to
suggest a set of 3270 functions that will be supported on this
session. It is also sent as an implicit rejection of a previous
FUNCTIONS REQUEST command sent by the other side (see the section
entitled "FUNCTIONS Negotiation" for more information). Note that
when used to reject a FUNCTIONS REQUEST command, the function-list
must not be identical to that received in the previous REQUEST
command.
IAC SB TN3270E FUNCTIONS IS <function-list> IAC SE
Either side may send this command. This command is sent as a
response to a FUNCTIONS REQUEST command and implies acceptance of
the set of functions sent to it in the REQUEST command. Note that
the list of functions in the FUNCTIONS IS command must match the
list that was received in the previous FUNCTIONS REQUEST command.
5. Default Specification
WON"T TN3270E
DON"T TN3270E
i.e., TN3270E will not be used.
6. Motivation
See the section entitled "Introduction".
7. TN3270E Sub-negotiation Rules
Once it has been agreed that TN3270E will be supported, the first
sub-negotiation must concern the DEVICE-TYPE (and possibly device-
name) information. Only after that has been successfully negotiated
can the client and server exchange FUNCTIONS information. Only after
both DEVICE-TYPE and FUNCTIONS have been successfully negotiated can
3270 data stream transmission occur.
7.1 DEVICE-TYPE Negotiation
Device-type names are NVT ASCII strings, all upper case.
Device-type (and device-name) negotiation begins when the server
transmits the DEVICE-TYPE SEND command to the client. The client
responds with the DEVICE-TYPE REQUEST command, which must include a
device-type and may include a resource-name or device-name request.
Valid device-types are:
erminals: IBM-3278-2 IBM-3278-2-E (24 row x 80 col display)
IBM-3278-3 IBM-3278-3-E (32 row x 80 col display)
IBM-3278-4 IBM-3278-4-E (43 row x 80 col display)
IBM-3278-5 IBM-3278-5-E (27 row x 132 col display)
IBM-DYNAMIC (no pre-defined display size)
printers: IBM-3287-1
Note that the use of "3278" and "3287" is NOT intended to exclude any
particular device capabilities; they are used here only because they
are commonly known designations for a terminal and a printer member
of the 3270 family of devices. The intention is to simplify the
device-type negotiation (in comparison to traditional tn3270) by
minimizing the number of possible device-types, and by breaking the
association of a specific piece of IBM hardware with a related set of
data stream capabilities. For example, negotiation of device-type
IBM-3278-2-E does NOT in and of itself preclude the use of any of the
functions associated with a physical 3279 model S2B. A client"s
ability to support the more advanced functions of the 3270 data
stream will be indicated not by negotiation of an IBM device type and
model number, but rather by the combination of Read Partition Query
and Query Reply.
All of the terminal device-types support a "primary" display size of
24 rows by 80 columns. The "-3", "-4" and "-5" types each support an
"alternate" display size as noted in the above list. The IBM-DYNAMIC
device-type implies no pre-defined alternate display size; this value
will be passed from the client to host applications as part of the
Query Reply structured field, and it can represent any display size
the client and the host application can support.
Terminal device-types with the "-E" suffix should only be negotiated
by clients that are willing to support some subset of the 3270
"extended data stream". This usually includes at a minimum support
for extended colors and highlighting, but may also include a number
of other functions, such as graphics capability, alternate character
sets, and partitions.
Clients that negotiate a terminal device-type with the "-E" suffix or
the DYNAMIC type, as well as those that negotiate a printer device-
type, must be able to accept and respond to a Read Partition Query
command (see the section entitled "3270 Structured Fields"). This
allows the client to indicate to host applications which subsets of
the 3270 extended data stream the client is willing to support.
In a VTAM/SNA environment, negotiation of IBM-DYNAMIC as the device-
type should result in a Bind in which the Presentation Services Usage
screen field (the eleventh byte in the logmode"s PSERVIC field) is
set to 0x03, indicating that the alternate screen size will be
determined by the Query Reply (Usable Area).
7.1.1 Device Pools
An explanation of the CONNECT and ASSOCIATE commands first requires a
discussion of the organization of terminal and printer device pools
that the server maintains and from which it selects device-names to
assign to session requests. Definition of a few terms is also in
order.
The terms "device-name", "LU name" and "network name" can be
considered interchangeable in this document. They refer to a
specific terminal or printer device.
The term "resource-name" is less specific; it may refer to a device-
name, but it may also be the name of a pool of printer or terminal
devices. Such a named pool could serve to group devices with similar
operational or administrative characteristics. In fact, this
document places no restrictions on how a server makes use of
resource-names, so long as the server can take a resource-name
specified by the client and use it to come up with a device-name to
assign to the session. Note, however, that servers must avoid
allowing ambiguity; for example, they must not allow the definition
of a device-name with the same name as that of a pool of devices.
Device-names and resource-names are specified as NVT ASCII strings in
which upper and lower case are considered equivalent. The length of
device-names and resource-names should not exceed 8 bytes.
A "generic session request" is one which includes neither the CONNECT
nor the ASSOCIATE command, while a "specific session request" is one
that includes either the CONNECT or the ASSOCIATE command.
If a TN3270E server wishes to support traditional tn3270 clients, it
must maintain a set of terminal device-names that can be used to
satisfy requests from such clients for terminal sessions. This same
pool could be used to satisfy generic requests for terminal sessions
from TN3270E clients.
The server may also maintain any number of other pools of device-
names. For example, there could be a pool of terminal device-names
reserved for a specific department within the organization, or a pool
of terminal device-names that have access to certain applications on
the host.
For any of these terminal device pools, the TN3270E server may also
have defined a "partner" or "paired" printer device for each terminal
in the pool. There should be a unique, one-to-one mapping between a
terminal and its associated printer. The reasoning behind such a
configuration is to allow for those host applications that produce
printed output bound for a printer whose device-name is determined by
the device-name of the terminal that initiated the print request.
These printer devices can only be assigned to specific printer
session requests that use the ASSOCIATE command (see below).
In addition, the TN3270E server may also maintain one or more pools
of printer device-names that are not associated with any terminal.
These printer devices can only be assigned to specific printer
session requests that use the CONNECT command (see below). This
allows for those host applications that generate printed output bound
for a printer whose device-name is determined by something other than
the device-name of the terminal that initiated the print request (for
example, when the userid of the person signed on to a terminal
determines the print destination). It is also possible that a pool
of printer device-names could be maintained to satisfy generic
requests for printers (i.e., those that specify neither CONNECT nor
ASSOCIATE).
7.1.2 CONNECT Command
CONNECT can be used by the client in two ways: if the resource-name
it specifies is a device-name, then the client is requesting a
specific device-name. If the specified resource-name is not a
device-name, then the client is requesting any one of the device-
names associated with the resource-name.
In either case, the resource indicated by the specified resource-name
must not conflict with the device-type; e.g., if the client requests
DEVICE-TYPE IBM-3287-1 (a printer) and specifies CONNECT T1000001,
but T1000001 is a device-name defined at the host as a terminal, then
the server must deny the request. Further, if the requested
resource-name is a device-name already associated with some other
Telnet session, or if it is not defined to the server, the server
must deny the request.
7.1.3 ASSOCIATE Command
ASSOCIATE can be used by the client only when requesting a DEVICE-
TYPE that represents a printer, and the specified device-name must be
that of a terminal that was returned by the server in a previous
DEVICE-TYPE IS <device-type> CONNECT <device-name> command.
The ASSOCIATE command requests that this session be assigned the
device-name of the printer that is paired with the terminal named in
the request. If the device-type does not represent a printer, or if
the device-name is not that of a terminal, then the server must deny
the request. Also, if the server does not have defined a partner
printer for the specified terminal, it must deny the request.
The use of the ASSOCIATE command is to be as follows: A client first
connects and requests a terminal from one of the terminal pools; it
then uses the terminal device-name returned by the server (see
"Accepting a Request", section 7.1.4 below) in a second session
request, this time aSKINg for the printer that is paired with the
terminal session it just established. This allows clients to
associate a printer session with a terminal rather than having to
have prior knowledge of a printer device-name.
7.1.4 Accepting a Request
The server must accept the client"s request or deny it as a whole -
it cannot, for example, accept the DEVICE-TYPE request but deny the
CONNECT portion.
If the server wishes to accept the request, it sends back the
DEVICE-TYPE IS command confirming the requested device-type and the
CONNECT command specifying the device-name of the terminal or printer
assigned to this session.
Normally, the client should accept any DEVICE-TYPE IS <device-type>
CONNECT <device-name> sent by the server. An exception to this would
be if the client must (e.g., to satisfy local-site policy) be
connected to a specific LU name and is presented with a device-name
which does not match the one requested by the client (this could
happen, for example, if the client requested what it thought was a
device-name, but what was defined at the server as the name of a pool
of devices). In this case, the client should reject the DEVICE-TYPE
IS command by terminating TN3270E negotiations.
7.1.5 REJECT Command
If the server wishes to deny the request, it sends back the DEVICE-
TYPE REJECT command with one of the following reason-codes:
Reason code name Explanation
---------------- -----------------------------------
INV-DEVICE-TYPE The server does not support the
requested device-type.
INV-NAME The resource-name or device-name
specified in the CONNECT or ASSOCIATE
command is not known to the server.
DEVICE-IN-USE The requested device-name is
already associated with another
session.
TYPE-NAME-ERROR The requested device-name or
resource-name is incompatible
with the requested device-type
(such as terminal/printer mismatch).
UNSUPPORTED-REQ The server is unable to satisfy
the type of request sent by the
client; e.g., a specific terminal
or printer was requested but the
server does not have any such pools of
device-names defined to it, or the
ASSOCIATE command was used but no
partner printers are defined to the
server.
INV-ASSOCIATE The client used the ASSOCIATE
command and either the device-type
is not a printer or the device-name
is not a terminal.
CONN-PARTNER The client used the CONNECT command
to request a specific printer but
the device-name requested is the
partner to some terminal.
UNKNOWN-ERROR Any other error in device type or
name processing has occurred.
The process of negotiating a device-type and device-name that are
acceptable to both client and server may entail several iterations of
DEVICE-TYPE REQUEST and DEVICE-TYPE REJECT commands. The client must
make use of the reason-code specified by the server in any DEVICE-
TYPE REJECT command(s) to minimize the amount of negotiation
necessary. For example, if the client initially requests that it be
assigned a specific terminal device-name via the CONNECT command, and
the server rejects the request with a reason-code of UNSUPPORTED-REQ,
the client must make no further specific terminal requests in the
negotiations. If at any point in the process either side wishes to
"bail out," it can simply send a WON"T (or DON"T) TN3270E command to
the other side. At this point both sides are free to negotiate other
Telnet options (including traditional tn3270).
7.2 FUNCTIONS Negotiation
Once the DEVICE-TYPE negotiation has successfully completed (i.e,
when the client receives a DEVICE-TYPE IS command that is
acceptable), the client must initiate the FUNCTIONS negotiation by
sending the FUNCTIONS REQUEST command to the server. After this
initial REQUEST command, both sides are free to transmit FUNCTIONS
REQUEST and FUNCTIONS IS commands as needed.
7.2.1 Commands
The FUNCTIONS REQUEST command contains a list of the TN3270E
functions that the sender would like to see supported on this
session. All functions not in the list are to be considered
unsupported. The list is terminated by the IAC code that precedes
the SE command. Functions may appear in any order in the list.
Upon receipt of a FUNCTIONS REQUEST command, the recipient has two
choices:
- it may respond in the positive (meaning it agrees to support
all functions in the list, and not to transmit any data related to
functions not in the list). To do this, it sends the FUNCTIONS IS
command with the function-list exactly as it was received. At this
point, FUNCTIONS negotiation has successfully completed.
- it may respond in the negative by sending a FUNCTIONS
REQUEST command in which the function-list differs from the one it
received (and not simply in the order of appearance of functions in
the list; at least one function must have been added to, or removed
from, the list).
To avoid endlessly looping, both parties must not add to the
function-list it receives any function that it has previously added
and that the other side has removed.
The process of sending FUNCTIONS REQUEST commands back and forth
continues until one side receives a function-list it is willing to
live with. It uses the FUNCTIONS IS command to accept the list,
and, once this command is received by the other side, all necessary
negotiation has been completed. At this point, 3270 data stream
transmission can begin.
Note that it is possible that the function-list agreed to is null;
this is referred to as "basic TN3270E". See the section entitled
"Basic TN3270E" for more information.
If an impasse is reached during FUNCTIONS negotiation (for example,
if a client requested and was granted a DEVICE-TYPE representing a
printer, but refuses to accept either the SCS-CTL-CODES or DATA-
STREAM-CTL function), then the "offended" party should terminate
the negotiation by sending an IAC DON"T (or WON"T) TN3270E.
7.2.2 List of TN3270E Functions
The following list briefly describes the 3270 functions that may be
negotiated in the function-list:
Function Name Description
------------- -----------
SCS-CTL-CODES (Printer sessions only). Allows the use
of the SNA Character Stream (SCS) and SCS
control codes on the session. SCS is
used with LU type 1 SNA sessions.
DATA-STREAM-CTL (Printer sessions only). Allows the use
of the standard 3270 data stream. This
corresponds to LU type 3 SNA sessions.
RESPONSES Provides support for positive and
negative response handling. Allows the
server to reflect to the client any and
all definite, exception, and no response
requests sent by the host application.
BIND-IMAGE Allows the server to send the SNA Bind
image and Unbind notification to the
client.
SYSREQ Allows the client and server to emulate
some (or all, depending on the server) of
the functions of the SYSREQ key in an SNA
environment.
See the section entitled "Details of Processing TN3270E Functions"
for a more detailed explanation of the meaning and use of these
functions.
If in the process of functions negotiation an unrecognized function
code is recieved, the recipient should simply remove that function
code from the list and continue normal functions negotiation.
8. TN3270E Data Messages
3270 device communications are generally understood to be block
oriented in nature. That is, each partner buffers data until an
entire "message" has been built, at which point the data is sent to
the other side. The "outbound message" (from host to device)
consists of a 3270 command and a series of buffer orders, buffer
addresses, and data, while the "inbound message" contains only buffer
orders, addresses and data. The end of a message is understood to be
the last byte transmitted (note that this discussion disregards SNA
chaining). The Telnet EOR command is used to delimit these natural
blocks of 3270 data within the Telnet data stream.
In TN3270E, each 3270 message must be prefixed with a TN3270E header,
which consists of five bytes and whose format is defined below (see
the section entitled "The TN3270E Message Header"). A "data message"
in TN3270E therefore has the following construction:
<TN3270E Header><data><IAC EOR>
It should be noted that it is possible that, for certain message
types, there is no data portion present. In this case, the TN3270E
data message consists of:
<TN3270E Header><IAC EOR>
If either side wishes to transmit the decimal value 255 and have it
interpreted as data, it must "double" this byte. In other words, a
single occurrence of decimal 255 will be interpreted by the other
side as an IAC, while two successive bytes containing decimal 255
will be treated as one data byte with a value of decimal 255.
It is strongly recommended that Telnet commands (other than IAC IAC)
should be sent between TN3270E data messages, with no header and no
trailing IAC EOR. If a TN3270E data message containing either IAC IP
(to be interpreted as 3270 Attention) or IAC AO (to be interpreted as
SYSREQ) is received, the receiver should defer processing the command
until the 3270 data has been processed (see the appropriate sections
for discussion of 3270 Attention and SYSREQ). If a TN3270E data
message containing any other IAC-command sequence (other than IAC
IAC) is received, it is implementation dependent when the IAC-command
sequence will be processed, but it must be processed. The receiver
may process it immediately, which in effect causes it to be processed
as if it had been received before the current TN3270E data message,
or the processing may be deferred until after the current TN3270E
data message has been processed. It is because of this ambiguity
that the presence of Telnet commands within a TN3270E data message
(i.e., between the header and the trailing IAC EOR) is not
recommended; neither clients nor servers should send such data.
8.1 The TN3270E Message Header
As stated earlier, each data message in TN3270E must be prefixed by a
header, which consists of five bytes and is formatted as follows:
-----------------------------------------------------------
DATA-TYPE REQUEST-FLAG RESPONSE-FLAG SEQ-NUMBER
-----------------------------------------------------------
1 byte 1 byte 1 byte 2 bytes
8.1.1 DATA-TYPE Field
The DATA-TYPE field indicates how the data portion of the message is
to be interpreted by the receiver. Possible values for the DATA-TYPE
field are:
Data-type Name Code Meaning
-------------- ---- ---------------------------------
3270-DATA 0x00 The data portion of the message
contains only the 3270 data stream.
SCS-DATA 0x01 The data portion of the message
contains SNA Character Stream data.
RESPONSE 0x02 The data portion of the message
constitutes device-status information
and the RESPONSE-FLAG field indicates
whether this is a positive or negative
response (see below).
BIND-IMAGE 0x03 The data portion of the message is
the SNA bind image from the session
established between the server and the
host application.
UNBIND 0x04 The data portion of the message is
an Unbind reason code.
NVT-DATA 0x05 The data portion of the message is to
be interpreted as NVT data.
REQUEST 0x06 There is no data portion present in
the message. Only the REQUEST-FLAG
field has any meaning.
SSCP-LU-DATA 0x07 The data portion of the message is
data from the SSCP-LU session.
PRINT-EOJ 0x08 There is no data portion present in
the message. This value can be sent
only by the server, and only on a
printer session.
8.1.2 REQUEST-FLAG Field
The REQUEST-FLAG field only has meaning when the DATA-TYPE field has
a value of REQUEST; otherwise, the REQUEST-FLAG field must be ignored
by the receiver and should be set to 0x00 by the sender. Possible
values for the REQUEST-FLAG field are:
Request-Flag Name Code Meaning
----------------- ---- ---------------------------------
ERR-COND-CLEARED 0x00 The client sends this to the server
when some previously encountered
printer error condition has been
cleared. (See the section entitled
"The RESPONSES Function" below.)
8.1.3 RESPONSE-FLAG Field
The RESPONSE-FLAG field only has meaning for certain values of the
DATA-TYPE field. For DATA-TYPE field values of 3270-DATA and SCS-
DATA, the RESPONSE-FLAG is an indication of whether or not the sender
of the data expects to receive a response. In this case the possible
values of RESPONSE-FLAG are:
Response-Flag Name Code Meaning
------------------ ---- ---------------------------------
NO-RESPONSE 0x00 The sender does not expect the
receiver to respond either
positively or negatively to this
message. The receiver must
therefore not send any response
to this data-message.
ERROR-RESPONSE 0x01 The sender only expects the
receiver to respond to this message
if some type of error occurred, in
which case a negative response must
be sent by the receiver.
ALWAYS-RESPONSE 0x02 The sender expects the receiver to
respond negatively if an error
occurs, or positively if no errors
occur. One or the other must
always be sent by the receiver.
For a DATA-TYPE field value of RESPONSE, the RESPONSE-FLAG is an
actual response to a previous data message (which must by definition
have had a DATA-TYPE of either 3270-DATA or SCS-DATA and a RESPONSE-
FLAG value of either ERROR-RESPONSE or ALWAYS-RESPONSE). In this
case the possible values of RESPONSE-FLAG are:
Response-Flag Name Code Meaning
------------------ ---- ---------------------------------
POSITIVE-RESPONSE 0x00 The previous message was received
and executed successfully with
no errors.
NEGATIVE-RESPONSE 0x01 The previous message was received
but an error(s) occurred while
processing it.
Accompanying status information will be found in the data portion of
the message.
For any other values of the DATA-TYPE field, the RESPONSE-FLAG field
must be ignored by the receiver and should be set to 0x00 by the
sender.
8.1.4 SEQ-NUMBER Field
The SEQ-NUMBER field is only used when the RESPONSES function has
been agreed to. It contains a 2 byte binary number, and is used to
correlate positive and negative responses to the data messages for
which they were intended. This field must be sent in network byte
order ("big endian"). If either byte contains a 0xff, it should be
doubled to 0xffff before sending and stripped back to 0xff upon
receipt; this is standard IAC escaping. See the section entitled
"The RESPONSES Function" for further information on the use of the
SEQ-NUMBER field. When the RESPONSES function is not agreed to, this
field should always be set to 0x0000 by the sender and ignored by the
receiver.
9. Basic TN3270E
As has been stated earlier, whether or not the use of each of the
TN3270E functions is allowed on a session is negotiated when the
connection is established. It is possible that none of the functions
are agreed to (in this case, the function-list in the FUNCTIONS
REQUEST and FUNCTIONS IS commands is null). This mode of operation
is referred to as "basic TN3270E". Note that, since neither the
SCS-CTL-CODES function nor the DATA-STREAM-CTL function is agreed to,
basic TN3270E refers to terminal sessions only.
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.
10. Details of Processing TN3270E Functions
Agreement by both parties to a specific function in the FUNCTIONS
REQUEST function-list implies agreement by each party to support a
related set of values in the TN3270E header. It also implies a
willingness to adhere to the rules governing the processing of data
messages with regard to the agreed upon function. Either party that
fails to accept header values associated either with agreed upon
functions or with basic TN3270E, or attempts to use header values
associated with a function that is not a part of basic TN3270E and
was not agreed upon, will be considered non-conforming and in
violation of the protocol. The following sections detail for each
TN3270E function the associated header values and processing rules.
10.1 The SCS-CTL-CODES Function
This function can only be supported on a 3270 printer session.
Agreement to support this function requires that the party support
the following TN3270E header values:
Header field Value
------------ -----
DATA-TYPE SCS-DATA
DATA-TYPE PRINT-EOJ
A client representing a printer device uses this function to indicate
its willingness to accept a data stream that includes SCS control
codes. For the purposes of NVT mode versus 3270 mode, SCS-DATA must
be treated exactly like 3270-DATA (i.e., it can cause a switch from
NVT mode to 3270 mode).
When a printer device-type has been negotiated, either the SCS-CTL-
CODES function or the DATA-STREAM-CTL function, or both, must be
negotiated. This enables the server to know when it should and
should not accept a session with a host application on behalf of the
client. If only the SCS-CTL-CODES function is agreed to, then the
server will not establish sessions with host applications that would
send 3270 data stream control. If both SCS-CTL-CODES and DATA-
STREAM-CTL are agreed to, then the server will establish sessions
both with host applications that would send SCS control codes and
with those that would send 3270 orders.
The server should send a TN3270E message with DATA-TYPE set to
PRINT-EOJ at the end of each print job to indicate to the client that
it may now take whatever action is appropriate for its environment
(e.g., close a disk or spool file, etc.). The server may have
multiple criteria for determining when it should send a PRINT-EOJ,
such as receipt of SNA End Bracket from the host application, or
expiration of a pre-defined timeout value.
10.2 The DATA-STREAM-CTL Function
This function can only be supported on a 3270 printer session.
Agreement to support this function requires that the party support
the following TN3270E header values:
Header field Value
------------ -----
DATA-TYPE 3270-DATA
DATA-TYPE PRINT-EOJ
A client representing a printer device uses this function to indicate
its willingness to accept a data stream that includes 3270 orders and
attributes.
When a printer device-type has been negotiated, either the SCS-CTL-
CODES function or the DATA-STREAM-CTL function, or both, must be
negotiated. This enables the server to know when it should and
should not accept a session with a host application on behalf of the
client. If only the DATA-STREAM-CTL function is agreed to, then the
server will not establish sessions with host applications that would
send SCS control codes in a data stream. If both SCS-CTL-CODES and
DATA-STREAM-CTL are agreed to, then the server will establish
sessions both with host applications that would send SCS control
codes and with those that would send 3270 orders.
The server should send a TN3270E message with DATA-TYPE set to
PRINT-EOJ at the end of each print job to indicate to the client that
it may now take whatever action is appropriate for its environment
(e.g., close a disk or spool file, etc.). The server may have
multiple criteria for determining when it should send a PRINT-EOJ,
such as receipt of SNA End Bracket from the host application, or
expiration of a pre-defined timeout value.
10.3 The BIND-IMAGE Function
This function can only be supported when the TN3270E server
represents SNA terminals and printers.
Agreement to support this function requires that the party support
the following TN3270E header values:
Header field Value
------------ -----
DATA-TYPE BIND-IMAGE
DATA-TYPE UNBIND
DATA-TYPE SSCP-LU-DATA
When BIND-IMAGE is in effect, the server must inform the client when
an SNA session has been established with a host application, and when
such a session has been terminated. It uses DATA-TYPE values of
BIND-IMAGE and UNBIND to convey this information.
When establishing an SNA session on behalf of a client, the server
will receive a Bind RU from the host application. It will also
receive a Start Data Traffic RU. Once both of these have been
responded to positively by the server, it must then inform the client
of the presence of this session by sending it a data message with the
DATA-TYPE flag set to BIND-IMAGE. The data portion of this message
must contain the bind image exactly as it was received in the Bind RU
that the server accepted on behalf of the client. The format and
maximum length of this bind image are defined in [6].
When an SNA session between the server and a host application is
terminated, the server must send a data message to the client with
the DATA-TYPE flag set to UNBIND. If the server was notified of the
session termination via an SNA Unbind RU, it should include the
Unbind reason code in the data portion of the message it sends to the
client. If the server itself requested the SNA session termination
(for example, as part of SYSREQ key processing), it should set the
data portion of the UNBIND message to 0x01, indicating "normal end of
session".
Another ASPect of the BIND-IMAGE function alters the allowable DATA-
TYPE flag values slightly from the behavior described in the section
entitled "Basic TN3270E". When BIND-IMAGE is in effect, data
messages with DATA-TYPE set to 3270-DATA or SCS-DATA are not allowed
before the first BIND-IMAGE is received by the client; only SSCP-LU-
DATA or NVT-DATA can be used to transmit user- oriented data. The
same applies to data messages exchanged after an UNBIND is sent and
before another BIND-IMAGE is received by the client. Once the client
receives a BIND-IMAGE data message, the allowable DATA-TYPE values,
in addition to SSCP-LU-DATA, now include 3270-DATA and/or SCS-DATA,
depending on whether a terminal or printer device-type was
negotiated, and whether a printer client agreed to DATA-STREAM-CTL or
SCS-CTL-CODES, or both. (See the section entitled "The SYSREQ
Function" for further discussion of the SSCP-LU session in an SNA
environment.)
10.4 The RESPONSES Function
This function can be supported for both terminal and printer sessions
connected to both SNA and non-SNA servers.
Agreement to support this function requires that the party support
the following TN3270E header values:
Header field Value
------------ -----
DATA-TYPE RESPONSE
DATA-TYPE REQUEST
RESPONSE-FLAG -all values-
REQUEST-FLAG ERR-COND-CLEARED
SEQ-NUMBER binary values from 0-32767
Whenever a data message is sent with a DATA-TYPE of either SCS-DATA
or 3270-DATA, the sender must set the RESPONSE-FLAG field to either
NO-RESPONSE, ERROR-RESPONSE, or ALWAYS-RESPONSE. It is anticipated
that the client side will normally set RESPONSE-FLAG to NO-RESPONSE.
The server, if it represents an SNA device, should set RESPONSE-FLAG
to reflect the response value set in the RH of the RU that generated
this data message - Definite Response resulting in a RESPONSE-FLAG
value of ALWAYS-RESPONSE, Exception Response resulting in ERROR-
RESPONSE being set, and No Response causing a setting of NO-RESPONSE.
A non-SNA server should set RESPONSE-FLAG to ERROR-RESPONSE.
In addition, the sender must keep a count of the messages with a
DATA-TYPE of 3270-DATA or SCS-DATA that it sends on a given TN3270E
session. This counter should start at zero for the first such
message, and be incremented by one for each subsequent message. Note
that this counter is independent of any SNA sequence numbers, and
should not be reset to zero as a result of Bind or Unbind. If the
counter reaches the maximum of 32767, it should be restarted at zero.
The sender must place this value in the SEQ-NUMBER field of the
TN3270E header before it sends the message. Note that the SEQ-NUMBER
field must be set regardless of the value of the RESPONSE-FLAG field.
10.4.1 Response Messages
Whenever a data message with a DATA-TYPE of either SCS-DATA or 3270-
DATA is received, the receiver must attempt to process the data in
the data portion of the message, then determine whether or not it
should send a data message with a DATA-TYPE of RESPONSE. If the data
message it has just processed had a RESPONSE-FLAG value of NO-
RESPONSE, or if it had a value of ERROR-RESPONSE and there were no
errors encountered while processing the data, then no RESPONSE type
message should be sent. Otherwise, a data message should be sent in
which the header DATA-TYPE field is set to RESPONSE, and in which the
SEQ-NUMBER field is a copy of the SEQ-NUMBER field from the message
to which this response corresponds. The RESPONSE-FLAG field in this
header must have a value of either POSITIVE-RESPONSE or NEGATIVE-
RESPONSE. A POSITIVE-RESPONSE should be sent if the previously
processed message"s header specified ALWAYS-RESPONSE and no errors
were encountered in processing the data. A NEGATIVE-RESPONSE should
be sent when
1) the previously processed message specified ERROR-RESPONSE
or ALWAYS-RESPONSE and
2) some kind of error occurred while processing the data.
Normally only the client will be constructing and sending these
RESPONSE messages. A negative response sent by the client to the
server is the equivalent of a Unit Check Status [7]. All references
to device status and sense codes in this section rely on [7].
The data portion of a RESPONSE message must consist of one byte of
binary data. The value of this byte gives a more detailed account of
the results of having processed the previously received data message.
The possible values for this byte are:
For a RESPONSE-FLAG value of POSITIVE-RESPONSE -
Value Meaning
----- -------
0x00 Successful completion (when sent by the client,
this is equivalent to "Device End").
For a RESPONSE-FLAG value of NEGATIVE-RESPONSE -
Value Meaning
----- -------
0x00 An invalid 3270 command was received
(equivalent to "Command Reject").
0x01 Printer is not ready (equivalent to
"Intervention Required").
0x02 An illegal 3270 buffer address or order
sequence was received (equivalent to
"Operation Check").
0x03 Printer is powered off or not connected
(equivalent to "Component Disconnected").
When the server receives any of the above responses, it should pass
along the appropriate information to the host application. The
appropriate information is determined by whether the server
represents an SNA or a non-SNA device.
An SNA server should pass along a POSITIVE-RESPONSE from the client
as an SNA positive Response Unit to the host application. It should
translate a NEGATIVE-RESPONSE from the client into an SNA negative
Response Unit in which the Sense Data Indicator bit is on and which
contains one of the following sense codes:
RESPONSE-FLAG Equivalent SNA Sense Code
------------- ---------- --------------
0x00 Command Reject 0x10030000
0x01 Intervention Required 0x08020000
0x02 Operation Check 0x10050000
0x03 Component Disconnected 0x08310000
A non-SNA server should pass along a POSITIVE-RESPONSE from the
client by setting the Device End Status bit on. It should reflect a
NEGATIVE-RESPONSE from the client by setting the Unit Check Status
Bit on, and setting either the Command Reject, Intervention Required,
or Operation Check Sense bit on when responding to the Sense command.
In the case of Intervention Required or Component Disconnected being
passed by the server to the host application, the host would normally
refrain from sending any further data to the printer. If and when
the error condition at the client has been resolved, the client must
send to the server a data message whose header DATA-TYPE field is set
to REQUEST, and whose REQUEST-FLAG is set to ERR-COND-CLEARED. Note
that this message has no data portion. Upon receipt of this message,
the server should pass along the appropriate information to the host
application so that it may resume sending printer output. Again, the
form of this information depends on whether the server represents an
SNA or a non-SNA device.
An SNA server should reflect an ERR-COND-CLEARED to the host
application by sending an SNA LUSTAT RU with one of the following
sense codes:
- if the previous error condition was an Intervention
Required, the server should send sense code 0x00010000
- if the previous error condition was Component
Disconnected, the server should send sense code 0x082B0000
A non-SNA server should set the corresponding bits in the Ending
Status and Sense Condition bytes.
10.5 The SYSREQ Function
This function can only be supported when the TN3270E server
represents SNA devices.
Agreement to support this function requires that the party support
the following TN3270E header values:
Header field Value
------------ -----
DATA-TYPE SSCP-LU-DATA
The 3270 SYSREQ key can be useful in an SNA environment when the ATTN
key is not sufficient to terminate a process. (See the section
entitled "The 3270 ATTN Key" for more information.)
10.5.1 Background
In SNA, there is a session between the host application (the PLU, or
Primary Logical Unit) and the TN3270E server representing the client
(the SLU, or Secondary Logical Unit). This is referred to as the
PLU-SLU session, and it is the one on which normal communications
flow. There is also a session between the host telecommunications
access method (the SSCP, or System Services Control Point) and the
SLU, and it is referred to as the SSCP-LU session. This session is
used to carry various control information and is normally transparent
to the user; normal 3270 data stream orders are not allowed in this
data. For more information, refer to [7].
The terminal display and keyboard are usually "owned" by the PLU-SLU
session, meaning any data the user types is sent to the host
application. The SYSREQ key is used to toggle ownership of the
keyboard and display between the PLU-SLU session and the SSCP-LU
session. In other words, the user is able to press SYSREQ and then
communicate directly with the host SSCP. The user may then enter any
valid Unformatted Systems Services commands, which are defined in the
USS table associated with the SLU. The most common USS command users
employ is "LOGOFF," which requests that the SSCP immediately
terminate the PLU-SLU session. The usual reason for requesting such
an action is that the host application (the PLU) has stopped
responding altogether.
Whenever the keyboard and display are owned by the SSCP-LU session,
no data is allowed to flow in either direction on the PLU-SLU
session. Once "in" the SSCP-LU session, the user may decide to
switch back to the PLU-SLU session by again pressing the SYSREQ key.
10.5.2 TN3270E Implementation of SYSREQ
The design of some TN3270E servers allows them to fully support the
SYSREQ key because they are allowed to send USS commands on the
SSCP-LU session. Other TN3270E servers operate in an environment
which does not allow them to send USS commands to the SSCP; this
makes full support of the SYSREQ key impossible. For such servers,
TN3270E provides for emulation of a minimal subset of functions,
namely, for the sequence of pressing SYSREQ and typing LOGOFF that
many users employ to immediately terminate the PLU-SLU session.
The Telnet Abort Output (AO) command is the mechanism used to
implement SYSREQ key support in TN3270E because, in a real SNA
session, once the user presses the SYSREQ key, the host application
is prevented from sending any more output to the terminal (unless the
user presses SYSREQ a second time), but the user"s process continues
to execute.
In order to implement SYSREQ key support, TN3270E clients that have
agreed to the SYSREQ function should provide a key (or combination of
keys) that is identified as mapping to the 3270 SYSREQ key. When the
user presses this key(s), the client should transmit a Telnet AO
command to the server.
Upon receipt of the AO command, a TN3270E server that has agreed to
the SYSREQ function should enter what will be loosely termed
"suspended mode" for the connection. If a server that has not agreed
to the SYSREQ function receives an AO command, it should simply
ignore it. Any attempt by the host application to send data to the
client while the connection is "suspended" should be responded to by
the server with a negative response, sense code 0x082D, indicating an
"LU Busy" condition. The server should not transmit anything to the
client on behalf of the host application. While the connection is
"suspended," any data messages exchanged between the client and
server should have the DATA-TYPE flag set to SSCP-LU-DATA; the data
stream will be as defined in [7], specifically the section entitled
"Operation in SSCP-SLU Session."
At this point, the behavior of the server depends upon whether or not
it is allowed to send USS commands on the SSCP-LU session. Servers
that have this ability should simply act as a vehicle for passing USS
commands and responses between the client and the SSCP.
Servers that are not allowed to send USS commands on the SSCP-LU
session should behave as follows:
- if the user transmits the string LOGOFF (upper or lower case),
the server should send an Unbind SNA RU to the host application.
This will result in termination of the PLU-SLU session. If the
BIND-IMAGE function was agreed upon, then the server should also
send a data message to the client with the DATA-TYPE flag set to
UNBIND and the data portion set to 0x01.
- if the user transmits anything other than LOGOFF, the server
should respond with the string "COMMAND UNRECOGNIZED" to the
client. The server should not send anything to the host
application on behalf of the client.
Regardless of which kind of server is present (i.e., whether or not
it may send USS commands on the SSCP-LU session), while the
connection is suspended, the user may press the "SYSREQ" key again.
This will result in the transmission of another AO to the server.
The server should then send to the host application an LUSTAT RU with
a value of 0x082B indicating "presentation space integrity lost". The
server will then "un-suspend" the Telnet connection to the client,
meaning it will allow the host application to once again send data to
the client.
11. The 3270 ATTN Key
The 3270 ATTN key is interpreted by many host applications in an SNA
environment as an indication that the user wishes to interrupt the
execution o