Network Working Group                                          J. Penner
Request for Comments: 1576                                     DCA, Inc.
Category: Informational                                     January 1994

                        TN3270 Current Practices

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


   This document describes the existing implementation of transferring
   3270 display terminal data using currently available telnet
   capabilities.  The name traditionally associated with this
   implementation is TN3270.

   Information is provided to aid in the implementation of TN3270
   servers as well as client terminal emulators.

   The following areas pertaining to TN3270 implementations are covered
   in this document:

      1. the telnet options negotiated to transition from a NVT ASCII
         state to a TN3270 state ready to process incoming 3270 data
         stream commands

      2. the method for sending and receiving 3270 data

      3. the method of handling some special keys known as SYSREQ and
         ATTN using current available telnet commands

      4. the events that will transition a TN3270 session back to an NVT

Table of Contents

      1.  Motivation  . . . . . . . . . . . . . . . . . . .   2
      2.  Background  . . . . . . . . . . . . . . . . . . .   2
      3.  Telnet Options and Commands Used  . . . . . . . .   4
      4.  Connection Negotiation  . . . . . . . . . . . . .   4
      4.1 3270 Regime Option  . . . . . . . . . . . . . . .   6
      4.2 Suppress Go Ahead Option  . . . . . . . . . . . .   6
      4.3 Echo Option . . . . . . . . . . . . . . . . . . .   6
      4.4 Timing Mark Option  . . . . . . . . . . . . . . .   7

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      5.  Testing for session presence  . . . . . . . . . .   7
      6.  Handling 3270 data  . . . . . . . . . . . . . . .   7
      7.  3270 Structured Fields  . . . . . . . . . . . . .   8
      8.  The 3270 ATTN (Attention) Key . . . . . . . . . .   8
      9.  The 3270 SYSREQ Key . . . . . . . . . . . . . . .   9
      10. Items not addressed by TN3270 . . . . . . . . . .  10
      11. References  . . . . . . . . . . . . . . . . . . .  10
      12. Security Considerations . . . . . . . . . . . . .  11
      13. Author's Note . . . . . . . . . . . . . . . . . .  11
      14. Author's Address  . . . . . . . . . . . . . . . .  12

1. Motivation

   3270 display terminal data differs from traditional display terminal
   data in that it is block mode and uses EBCDIC instead of ASCII
   character representation. These two differences are the primary
   reason for the differentiation of TN3270 from standard Telnet in this

2. Background

   Existing complex IBM 3270 display terminal networks are not easily
   integrated with the increasing number of multi-platform networking
   environments, specifically TCP/IP. These complex networks include
   terminals attached to a 3270 host using SNA (Systems Network
   Architecture) and non-SNA connections. To address the issue of easily
   connecting display terminals to 3270 hosts using IP networks, several
   vendors have introduced telnet servers that provide TCP/IP users a
   connection to existing IBM mainframes by supporting display terminal
   emulation using a subset of the existing telnet protocol.  Telnet
   servers may exist on the host itself, or be connected to the host
   using SNA or non-SNA methods.

   IBM terminals are generically referred to as 3270's which includes a
   broad range of terminals and devices, not all of which actually begin
   with the numbers 327x.

   3270 terminals in the IBM SNA network environment have two sessions
   with the host computer application. One is used for communicating
   with the host application, the other is used for communicating with
   the SSCP (System Services Control Point) that links the terminal with
   the appropriate host computer.  For the purposes of TN3270, this
   distinction is not apparent or relevant since there is actually only
   a single telnet session with the host computer or server.  On an IBM
   SNA network, the 3270 terminal has a special key that toggles between
   the two sessions (SYSREQ).  A brief discussion on how some telnet
   servers deal with this is included.

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   In an SNA environment, a client session is identified by a Logical
   Unit (LU) name.  In a non-SNA environment, there is not a LU name
   associated with a client session.  The closest thing to a LU name in
   the TN3270 environment is the client's IP address.  Although some
   telnet servers are connected to the host using SNA, TN3270 clients
   using these servers have no defined way to determine the LU name
   associated with the session.

   Telnet servers that exist in non-SNA environments do not have to be
   concerned about providing TN3270 clients with support for the SNA
   functions described in this document.

   TN3270 does not support typical SNA responses and is classified as a
   non-SNA protocol.  A TN3270 emulator is not aware or concerned about
   how the telnet server is connected to a 3270 host application.

   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.

   Some typical "SNA" functions such as the SYSREQ and ATTN keys have
   been mapped to existing telnet commands and are supported by some
   telnet server implementations.

   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].  This negotiation and the resulting data flow will be
   described below.

   RFC 1041 [1] attempted to standardize the method of negotiating 3270
   terminal support by defining the 3270 Regime Telnet Option.
   Historically, very few developers and vendors ever implemented RFC

   All references in this document to the 3270 datastream, SNA versus
   non-SNA operation, 3270 datastream commands, orders, structured
   fields and the like rely on [6].

   References to SNA Request and Response Units rely on [7].  References
   to SNA and SSCP rely on [12].

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3. Telnet Options and Commands Used

   TN3270 makes use of existing Telnet options and does not define any
   additional options or commands.

       Telnet option    Value (decimal)
       -------------    ---------------
       BINARY            0
       TERMINAL-TYPE    24
       EOR              25

   Additional options may be used during a TN3270 session and are
   interpreted as per their respective RFCs. These are [1] 3270-REGIME,
   [8] SUPPRESS-GO-AHEAD, [9] ECHO and [10] TIMING-MARK. Other options
   should be rejected unless they are specifically handled by the client
   for NVT mode.

   Commands that may be encountered during a TN3270 session and are
   described in RFC 854 [11] include NOP, BREAK and Interrupt Process.

4. Connection Negotiation

   The following example shows a TN3270-capable server and a TN3270
   client establishing a connection:

   The TCP/IP port used to connect with is 23 (Telnet).

   At any place before and during the TN3270 connection negotiation
   process, other telnet commands and data may be transferred and will
   be interpreted under the existing telnet state. Some existing TN3270
   servers start a client connection using an NVT telnet dialog to
   establish parameters needed to complete the TN3270 connection to the
   desired host.

   The order of negotiating terminal type, EOR and BINARY is not
   significant, this example shows a typical TN3270 connection.




      Client:  IAC SB TERMINAL-TYPE IS <terminal type>IAC SE

      where <terminal type> is a string consisting of terminal model,
      type and support of enhanced attribute bytes; an example is IBM-
      3278-2.  The acceptable values are listed in RFC 1340, Assigned

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      Numbers [5].  Other values are in use that do not exist in [5].

      The -2 following 3278 designates the alternate screen size.  3270
      terminals have the ability to switch between the standard (24x80)
      screen size and an alternate screen size.  Model -2 is 24x80 which
      is the same as the standard size.  Model -3 is 32x80, model -4 is
      43x80 and model -5 is 27x132.

      Appending the two character string "-E" to the end of the terminal
      type signifies that the terminal is capable of handling 3270
      extended data stream. This is interpreted to mean that the
      terminal is able to handle structured fields, which are described
      below.  Some telnet server implementations also interpret this to
      mean that the terminal is capable of handling extended attributes
      (highlighting, field validation, character set, outlining, etc.)

      The 3279 series of terminals is capable of extended attributes
      while the 3278 series is not.

      Server:  IAC DO EOR IAC WILL EOR
      Client:  IAC WILL EOR IAC DO EOR
      Server:  <3270 data stream> IAC EOR
      Client:  <3270 data stream> IAC EOR
           .            .
           .            .

   To terminate the connection the socket is closed by one of the
   session partners. Typically, when the user logs off of the host, the
   telnet server closes the connection.

   If the telnet server wishes to go back to NVT mode, it may issue the
   following telnet options:

       Server:  IAC WONT BINARY
       Client:  IAC DONT BINARY


       Server:  IAC WONT EOR
       Client:  IAC DONT EOR

   Either one of the above two cases causes the connection to not
   satisfy the requirements for a valid TN3270 session. The telnet
   client would then process data from the server as though it were NVT
   ASCII data.

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   The following examples show how a TN3270 client handles the 3270-

4.1 3270 Regime Option

   Very few servers support the 3270 Regime Telnet Option.  If the
   client does not support this option and responds negatively as shown
   in the following example, the server will proceed on to the more
   typical example shown above.

      Server:  IAC DO 3270-REGIME
      Client:  IAC WONT 3270-REGIME
       Normal negotiation:
         ...  (see above)

4.2 Suppress Go Ahead Option

   The Suppress Go Ahead option [8] is requested by some servers. The
   Suppress Go Ahead option RFC lists the default as being go aheads are
   transmitted to signal the receiver to begin transmitting.  Since
   TN3270 negotiates binary and end-of-record and is a block mode
   protocol, the telnet go ahead character is not sent.  Most servers do
   not negotiate this option even though they do not use the telnet go
   ahead character.


4.3 Echo Option

   The Echo option [9] is negotiated by those servers that make use of
   the telnet NVT mode to allow the user to enter information prior to
   negotiating the options necessary for TN3270.  This information
   includes but is not limited to user identification, password and
   destination 3270 host.  Some servers accept the default for this
   option which is for the client to not do a local echo of characters
   the user enters at the keyboard. This allows the server to decide if
   it should echo characters back to the client (or not in the case of
   password). Echoing characters back to the client causes slow response
   time since every character is typically echoed individually. Because
   of this, some servers negotiate for the client to do it's own local
   echoing (except for passwords). The following example illustrates
   this case.

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      Server:  IAC DO ECHO
      Client:  IAC WILL ECHO
        (Client does local display of all characters)
      Server:  IAC WONT ECHO
      Client:  IAC DONT ECHO
        (Client enters password - not locally displayed or remotely
      Server:  IAC DO ECHO
      Client:  IAC WILL ECHO
     (Client resumes local display of all characters)

4.4 Timing Mark Option

   The Timing Mark option [10] is used by some servers to test for the
   continued presence of a TN3270 client. The following example will
   assure the server the client is still alive.

      Server:  IAC DO TIMING-MARK

5. Testing for session presence

   The NOP command (hexadecimal F1) [11] is used by some servers to test
   for the continued presence of a TN3270 client. If a client has
   terminated abnormally, TCP/IP send errors will occur. The Timing Mark
   option, described above, is also used to test for presence.

      Server:  IAC NOP
      Client:  <ignore / no response>

6. Handling 3270 data

   The 3270 data stream consists of a command and its associated data.
   Commands include but are not limited to erase screen, erase and write
   to screen and read current screen; see [6] for a complete description
   of 3270 commands and parameters.

   The reason for negotiating the EOR telnet option [4] is to provide a
   method for separating these commands since no length information is
   specified. 3270 commands are interpreted by the telnet client in
   their entirety.  Each 3270 command and possible data is terminated
   with the IAC EOR sequence.

   The Binary option [3] is also required since 3270 data may contain
   the FF (hexadecimal) or IAC character. When this character is
   encountered during a TN3270 connection it is handled as per the
   Binary RFC [3].

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7. 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 datastreams. A structured field is a 3270 data type that
   allows non 3270 data to be embedded within 3270 data. Briefly, a
   structured field consists of the structured field command followed by
   one or more data blocks. Each data block has a length and a
   structured field identifier, followed optionally by additional data.

   Not every TN3270 client can be expected to support all structured
   field functions.   There must be a  mechanism by which those clients
   that are capable of supporting some or all structured field functions
   can indicate their wishes. This is typically done by adding "-E" to
   the end of the terminal type string. That is, when the terminal
   identifies itself as being able to handle extended attributes, it
   also is capable of being able to send and receive structured fields.

   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 client.
   The client responds with a Query Reply, listing which, if any,
   structured field functions it supports.

   A TN3270 client that supports structured fields will respond to a
   Read Partition Query command with the appropriate reply.  The
   sequence of events when a client receives a Read Partition Query and
   does not support structured fields is left up to the client
   implementation.  Typically clients can identify at least this
   structured field and reply with a null set.

8. The 3270 ATTN (Attention) 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.  A majority of the telnet servers
   currently accept the telnet IAC BREAK (code 243) [11] sequence to
   signal this event.

   Use of this key requires two things:

    - The TN3270 clients 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 transmits a Telnet BREAK command to the server.

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    - The TN3270 servers translate the BREAK command received from
      a TN3270 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 would send
      a SIGNAL RU to the host application.

   The ATTN key is not supported in a non-SNA environment; therefore, a
   TN3270 server representing non-SNA 3270 devices ignores any Telnet
   BREAK commands it receives from a client.

9. The 3270 SYSREQ Key

   The 3270 SYSREQ key is useful in an environment where the telnet
   server is attached to the host using SNA. The SYSREQ key is useful in
   this environment when the host application becomes locked or the user
   wishes to terminate the session without closing the Telnet

   The Telnet Interrupt Process (IP) command [11] is interpreted by some
   telnet servers as a SYSREQ key. Other servers recognize the 3270 Test
   Request key as a SYSREQ key.  In an SNA environment, pressing this
   key toggles the terminal between the host application session and the
   SSCP session.  Usually the user will enter LOGOFF once this key has
   been pressed to terminate the application session and then select a
   new host to connect to.  Sometimes, if SYSREQ is pressed again, the
   host application will become unlocked and normal activities may then

   It is entirely up to the telnet server to interpret this command and
   send the appropriate commands to the host as well as format the
   resulting host data for display on the telnet client. The data format
   during the SSCP session is in a slightly different format than normal
   3270 data. Since the telnet server has no way to pass this data
   directly to the telnet client, it must either handle it entirely and
   ignore SYSREQ events or convert it to 3270  data to present to the

   To implement SYSREQ key support, TN3270 clients 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 would either
   transmit a Telnet IP command or Test Request key to the server,
   depending on the server implementation.

   TN3270 servers representing non-SNA 3270 terminals may ignore any
   Telnet IP commands or Test Request keys they receive from a client.

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10. Items not addressed by TN3270

   There are several items that are not supported by current TN3270
   implementations; among them are the following:

    - TN3270 provides no capability for 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.

    - The 3270 ATTN and SYSREQ keys are not universally supported.

    - There is no support for the SNA positive/negative response
      process. All data that is sent is assumed to either be handled
      or ignored.  The lack of SNA response processing in TN3270 is
      part of what makes TN3270 efficient.
      A negative response indicates some sort of error at the client
      while processing the previously received data; this could be
      caused by the host application building a 3270 datastream that
      contains an invalid command, or by a mechanical error at the
      client side, among other things.
      Positive responses indicate processing of the previously received
      data has completed.

    - There is no mechanism by which the client can access the SNA
      BIND information.  The BIND image in a SNA environment
      contains a detailed description of the session between the
      telnet server and the host application.

    - The connection negotiation does not make it clear whether
      clients should support 3270 structured fields.

11. References

   [1] Rekhter, Y., "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.

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RFC 1576                TN3270 Current Practices            January 1994

   [4] Postel, J., "Telnet End of Record Option", RFC 885,
       USC/Information Sciences Institute, December 1983.

   [5] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

   [6] "3270 Information Display System - Data Stream Programmer's
       Reference", publication number GA23-0059, IBM Corporation.

   [7] "Systems Network Architecture - Formats", publication number
       GA27-3136, IBM Corporation.

   [8] Postel, J., and J. Reynolds, "Telnet Suppress Go Ahead Option",
       STD 29, RFC 858, USC/Information Sciences Institute, May 1983.

   [9] Postel, J., and J. Reynolds, "Telnet Echo Option", STD 28, RFC
       857, USC/Information Sciences Institute, May 1983.

  [10] Postel, J., and J. Reynolds, "Telnet Timing Mark Option", STD 31,
       RFC 860, USC/Information Sciences Institute, May 1983.

  [11] Postel, J., and J. Reynolds, "Telnet Protocol Specification", STD
       8, RFC 854, USC/Information Sciences Institute, May 1983.

  [12] "Systems Network Architecture - Concepts and Products",
       publication number GC30-3072, IBM Corporation.

12. Security Considerations

   Security issues are not discussed in this memo.

13. 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 March 1993 IETF meeting and TN3270 BOF at
      Interop August 1993.

    - Discussions on the "TN3270E" list, 1993.

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14. Author's Address

   Jon Penner
   DCA, Inc.
   2800 Oakmont Drive
   Austin, TX 78664

   Phone: (512) 388-7090 FAX
          or dca/g=Jon/s=Penner/

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