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Network Working Group                                         R. GellensRequest for Comments: 2066                                        UnisysCategory: Experimental                                      January 1997TELNET CHARSET OptionStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   This document specifies a mechanism for passing character set and   translation information between a TELNET client and server.  Use of   this mechanism enables an application used by a TELNET user to send   and receive data in the correct character set.   Either side can (subject to option negotiation) at any time request   that a (new) character set be used.1.   Command Names and Codes   CHARSET.......................42      REQUEST ....................01      ACCEPTED ...................02      REJECTED ...................03      TTABLE-IS ..................04      TTABLE-REJECTED ............05      TTABLE-ACK .................06      TTABLE-NAK .................07   As a convenience, standard TELNET text and codes for commands used in   this document are reproduced here (excerpted from [1]):      All TELNET commands consist of at least a two byte sequence:  the      "Interpret as Command" (IAC) escape character followed by the code      for the command.  The commands dealing with option negotiation are      three byte sequences, the third byte being the code for the option      referenced. ... [O]nly the IAC need be doubled to be sent as data,      and the other 255 codes may be passed transparently.  The      following are [some of] the defined TELNET commands.  Note that      these codes and code sequences have the indicated meaning only      when immediately preceded by an IAC.Gellens                       Experimental                      [Page 1]

RFC 2066                 TELNET CHARSET Option              January 1997      NAME          CODE  MEANING      SE            240   End of subnegotiation parameters.      SB            250   Indicates that what follows is                          subnegotiation of the indicated                          option.      WILL          251   Indicates the desire to begin                          performing, or confirmation that                          you are now performing, the                          indicated option.      WON'T         252   Indicates the refusal to perform,                          or continue performing, the                          indicated option.      DO            253   Indicates the request that the                          other party perform, or                          confirmation that you are expecting                          the other party to perform, the                          indicated option.      DON'T         254   Indicates the demand that the other                          party stop performing, or                          confirmation that you are no longer                          expecting the other party to                          perform, the indicated option.      IAC          255    Data Byte 255.2.   Command Meanings   A very simple meta-syntax is used, where most tokens represent   previously defined items (such as IAC); angle-brackets ("<>") are   used for items to be further defined; curly-braces ("{}") are used   around optional items; ellipses represent repeated sequences of   items; and quotes are used for literal strings.   IAC WILL CHARSET     The sender REQUESTS permission to, or AGREES to, use     CHARSET option subnegotiation to choose a character set.   IAC WON'T CHARSET      The sender REFUSES to use CHARSET option subnegotiation      to choose a character set.Gellens                       Experimental                      [Page 2]

RFC 2066                 TELNET CHARSET Option              January 1997    IAC DO CHARSET      The sender REQUESTS that, or AGREES to have, the other      side use CHARSET option subnegotiation to choose a      character set.   IAC DON'T CHARSET      The sender DEMANDS that the other side not use the      CHARSET option subnegotiation.   IAC SB CHARSET REQUEST { "[TTABLE ]" <Version> } <char set   list> IAC SE      Char set list:      <sep> <character set> { ... <sep> <character set> }   This message initiates a new CHARSET subnegotiation.  It can only be   sent by a side that has received a DO CHARSET message and sent a WILL   CHARSET message (in either order).   The sender requests that all text sent to and by it be encoded in one   of the specified character sets.   If the string [TTABLE] appears, the sender is willing to accept a   mapping (translation table) between any character set listed in <char   set list> and any character set desired by the receiver.   <Version>  is an octet whose binary value is the highest version   level of the TTABLE-IS message which can be sent in response.  This   field must not be zero.  See the TTABLE-IS message for the permitted   version values.   <Char set list>  is a sequence of 7-BIT ASCII printable characters.   The first octet defines the separator character (which must not   appear within any character set).  It is terminated by the IAC SE   sequence.  Case is not significant.  It consists of one or more   character sets.  The character sets should appear in order of   preference (most preferred first).   <Sep>  is a separator octet, the value of which is chosen by the   sender.  Examples include a space or a semicolon.  Any value other   than IAC is allowed.  The obvious choice is a space or any other   punctuation symbol which does not appear in any of the character set   names.Gellens                       Experimental                      [Page 3]

RFC 2066                 TELNET CHARSET Option              January 1997   <Character set>  is a sequence of 7-BIT ASCII printable characters.   Case is not significant.   If a requested character set name does not start with "X-" or "x-",   it MUST be registered with the Internet Assigned Number Authority   (IANA) [2].   The receiver responds in one of four ways:   If the receiver is already sending text to and expecting text from   the sender to be encoded in one of the specified character sets, it   sends a positive acknowledgment (CHARSET ACCEPTED); it MUST NOT   ignore the message.  (Although ignoring the message is perhaps   suggested by some interpretations of the relevant RFCs ([1], [3]), in   the interests of determinacy it is not permitted.  This ensures that   the issuer does not need to time out and infer a response, while   avoiding (because there is no response to a positive acknowledgment)   the non-terminating subnegotiation which is the rationale in the RFCs   for the non-response behavior.)   If the receiver is capable of handling at least one of the specified   character sets, it can respond with a positive acknowledgment for one   of the requested character sets.  Normally, it should pick the first   set it is capable of handling but may choose one based on its own   preferences.  After doing so, each side MUST encode subsequent text   in the specified character set.   If the string [TTABLE] is present, and the receiver prefers to use a   character set not included in <char set list>, and is capable of   doing so, it can send a translate table (TTABLE-IS) response.   If the receiver is not capable of handling any of the specified   character sets, it sends a negative acknowledgment (CHARSET   REJECTED).   Because it is not valid to reply to a CHARSET REQUEST message with   another CHARSET REQUEST message, if a CHARSET REQUEST message is   received after sending one, it means that both sides have sent them   simultaneously.  In this case, the server side MUST issue a negative   acknowledgment.  The client side MUST respond to the one from the   server.   IAC SB CHARSET ACCEPTED <Charset> IAC SE      This is a positive acknowledgment response to a CHARSET REQUEST      message; the receiver of the CHARSET REQUEST message acknowledges      its receipt and accepts the indicated character set.Gellens                       Experimental                      [Page 4]

RFC 2066                 TELNET CHARSET Option              January 1997      <Charset> is a character sequence identical to one of the      character sets in the CHARSET REQUEST message.  It is terminated      by the IAC SE sequence.      Text messages which follow this response must now be coded in the      indicated character set.  This message terminates the current      CHARSET subnegotiation.   IAC SB CHARSET REJECTED IAC SE      This is a negative acknowledgment response to a CHARSET REQUEST      message; the receiver of the CHARSET REQUEST message acknowledges      its receipt but refuses to use any of the requested character      sets.  Messages can not be sent in any of the indicated character      sets.  This message can also be sent by the sender of a TTABLE-IS      message, if multiple TTABLE-NAK messages were sent in response.      This message terminates the current CHARSET subnegotiation.   IAC SB CHARSET TTABLE-IS <version> <syntax for version> IAC SE      In response to a CHARSET REQUEST message in which [TTABLE] was      specified, the receiver of the CHARSET REQUEST message      acknowledges its receipt and is transmitting a pair of tables      which define the mapping between specified character sets.      <Version> is an octet whose binary value is the version level of      this TTABLE-IS message.  Different versions have different syntax.      The lowest version level is one (zero is not valid).  The current      highest version level is also one.  This field is provided so that      future versions of the TTABLE-SEND message can be specified, for      example, to handle character sets for which there is no simple      one-to-one character-for-character translation.  This might      include some forms of multi-octet character sets for which      translation algorithms or subsets need to be sent.   Syntax for Version 1:      <sep> <char set name 1> <sep> < char size 1> < char count 1> <char      set name 2> <sep> <char size 2> <char count 2> <map 1> <map 2>      <Sep>  is a separator octet, the value of which is chosen by the      sender.  Examples include a space or a semicolon.  Any value other      than IAC is allowed.  The obvious choice is a space or any other      punctuation symbol which does not appear in either of the      character set names.      <Char set name 1> and <Char set name 2>  are sequences of 7-BIT      ASCII printable characters which identify the two character sets      for which a mapping is being specified.  Each is terminated by      <sep>.  Case is not significant.  If a character set name does notGellens                       Experimental                      [Page 5]

RFC 2066                 TELNET CHARSET Option              January 1997      start with "X-" or "x-", it MUST be registered with IANA.  <Char      set name 1> MUST be chosen from the <char set list> in the CHARSET      REQUEST message.  <Char set name 2> can be arbitrarily chosen.      Text on the wire MUST be encoded using <char set name 2>.      <Char size 1>  and <char size 2>  are single octets each.  The      binary value of the  octet is the number of bits nominally      required for each character in the corresponding table.  It SHOULD      be a multiple of eight.      <Char count 1> and <char count 2>  are each three-octet binary      fields in Network Byte Order [6].  Each specifies how many      characters (of the maximum 2**<char size>) are being transmitted      in the corresponding map.      <Map1> and <Map 2>  each consist of the corresponding <char count>      number of characters.  These characters form a mapping from all or      part of the characters in one of the specified character sets to      the correct characters in the other character set.  If the      indicated <char count> is less than  2**<char size>, the first      <char count> characters are being mapped, and the remaining      characters are assumed to not be changed (and thus map to      themselves).  That is, each map contains characters 0 through      <char count> -1.  <Map 1> maps from <char set name 1> to <char set      name 2>.  <Map 2> maps from <char set name 2> to <char set name      1>.  Translation between the character sets is thus an obvious      process of using the binary value of a character as an index into      the appropriate map.  The character at that index replaces the      original character.  If the index exceeds the <char count> for the      map, no translation is performed for the character.      [Note to implementers: since TELNET works in octets, it is      possible for octets of value 255 to appear "spontaneously" when      using multi-octet or non-8-bit characters.  All octets of value      255 (other than IAC) MUST be quoted to conform with TELNET      requirements.  This applies even to octets within a table, or text      in a multi-octet character set.]   IAC SB CHARSET TTABLE-ACK IAC SE      The sender acknowledges the successful receipt of the translate      table.  Text messages which follow this response must now be coded      in the character set specified as <char set name 2> of the      TTABLE-IS message.  This message terminates the current CHARSET      subnegotiation.Gellens                       Experimental                      [Page 6]

RFC 2066                 TELNET CHARSET Option              January 1997   IAC SB CHARSET TTABLE-NAK IAC SE      The sender reports the unsuccessful receipt of the translate table      and requests that it be resent.  If subsequent transmission      attempts also fail, a TTABLE-REJECTED or CHARSET REJECTED message      (depending on which side sends it) should be sent instead of      additional futile TTABLE-IS and TTABLE-NAK messages.   IAC SB CHARSET TTABLE-REJECTED IAC SE      In response to a TTABLE-IS message, the receiver of the TTABLE-IS      message acknowledges its receipt and indicates it is unable to      handle it.  This message terminates the current CHARSET      subnegotiation.      Any system which supports the CHARSET option MUST fully support      the CHARSET REQUEST, ACCEPTED, REJECTED, and TTABLE-REJECTED      subnegotiation messages.  It MAY optionally fully support the      TTABLE-IS, TTABLE-ACK, and TTABLE-NAK messages.  If it does fully      support the TTABLE-IS message, it MUST also fully support the      TTABLE-ACK and TTABLE-NAK messages.3.   Default   WON'T CHARSET   DON'T CHARSET4.   Motivation for the Option   Many TELNET sessions need to transmit data which is not in 7-bit   ASCII.  This is usually done by negotiating BINARY, and using local   conventions (or terminal type kluges) to determine the character set   of the data.  However, such methods tend not to interoperate well,   and have difficulties when multiple character sets need to be   supported by different sessions.   Many computer systems now utilize a variety of character sets.   Increasingly, a server computer needs to document character sets or   translate transmissions and receptions using different pairs of   character sets on a per-application or per-connection basis.  This is   becoming more common as client and server computers become more   geographically disperse.  (And as servers are consolidated into   ever-larger hubs, serving ever-wider areas.)  In order for files,   databases, etc. to contain correct data, the server must determine   the character set in which the user is sending, and the character set   in which the application expects to receive.Gellens                       Experimental                      [Page 7]

RFC 2066                 TELNET CHARSET Option              January 1997   In some cases, it is sufficient to determine the character set of the   end user (because every application on the server expects to use the   same character set, or because applications can handle the user's   character set), but in other cases different server applications   expect to use different character sets.  In the former case, an   initial CHARSET subnegotiation suffices.  In the latter case, the   server may need to initiate additional CHARSET subnegotiations as the   user switches between applications.   At a minimum, the option described in this memo allows both sides to   be clear as to which character set is being used.  A minimal   implementation would have the server send DO CHARSET, and the client   send WILL CHARSET and CHARSET REQUEST.  The server could then   communicate the client's character set to applications using whatever   means are appropriate.  Such a server might refuse subsequent CHARSET   REQUEST messages from the client (if it lacked the ability to   communicate changed character set information to applications, for   example).  Another system might have a method whereby various   applications could communicate to the TELNET server their character   set needs and abilities, which the server would handle by initiating   new CHARSET REQUEST negotiations as appropriate.   In some cases, servers may have a large set of clients which tend to   connect often (such as daily) and over a long period of time (such as   years).  The server administrators may strongly prefer that the   servers not do character set translation (to save CPU cycles when   serving very large numbers of users).  To avoid manually configuring   each copy of the user TELNET software, the administrators might   prefer that the software supports translate tables.  (If the client   software received a translate table from the server and stored it,   the table would only need to be sent once.)5.   Description of the Option   When the client TELNET program is able to determine the user's   character set it should offer to specify the character set by sending   IAC WILL CHARSET.   If the server system is able to make use of this information, it   replies with IAC DO CHARSET.  The client TELNET is then free to   request a character set in a subnegotiation at any time.   Likewise, when the server is able to determine the expected character   set(s) of the user's application(s), it should send  IAC DO CHARSET   to request that the client system specify the character set it is   using.  Or the server could send IAC WILL CHARSET to offer to specify   the character sets.Gellens                       Experimental                      [Page 8]

RFC 2066                 TELNET CHARSET Option              January 1997   Once a character set has been determined, the server can either   perform the translation between the user and application character   sets itself, or request by additional CHARSET subnegotiations that   the client system do so.   Once it has been established that both sides are capable of character   set negotiation (that is, each side has received either a WILL   CHARSET or a DO CHARSET message, and has also sent either a DO   CHARSET or a WILL CHARSET message), subnegotiations can be requested   at any time by whichever side has sent a WILL CHARSET message and   also received a DO CHARSET message (this may be either or both   sides).  Once a CHARSET subnegotiation has started, it must be   completed before additional CHARSET subnegotiations can be started   (there must never be more than one CHARSET subnegotiation active at   any given time).  When a subnegotiation has completed, additional   subnegotiations can be started at any time.   If either side violates this rule and attempts to start a CHARSET   subnegotiation while one is already active, the other side MUST   reject the new subnegotiation by sending a CHARSET REJECTED message.   Receipt of a CHARSET REJECTED or TTABLE-REJECTED message terminates   the subnegotiation, leaving the character set unchanged.  Receipt of   a CHARSET ACCEPTED or TTABLE-ACK message terminates the   subnegotiation, with the new character set in force.   In some cases, both the server and the client systems are able to   perform translations and to send and receive in the character set(s)   expected by the other side.  In such cases, either side can request   that the other use the character set it prefers.  When both sides   simultaneously make such a request (send CHARSET REQUEST messages),   the server MUST reject the client's request by sending a CHARSET   REJECTED message.  The client system MUST respond to the server's   request.  (See the CHARSET REQUEST description, above.)   When the client system makes the request first, and the server is   able to handle the requested character set(s), but prefers that the   client system instead use the server's (user application) character   set, it may reject the request, and issue a CHARSET REQUEST of its   own.  If the client system is unable to comply with the server's   preference and issues a CHARSET REJECTED message, the server can   issue a new CHARSET REQUEST message for one of the previous character   sets (one of those which the client system originally requested).   The client system would obviously accept this character set.   While a CHARSET subnegotiation is in progress, data SHOULD be queued.   Once the CHARSET subnegotiation has terminated, the data can be sent   (in the correct character set).Gellens                       Experimental                      [Page 9]

RFC 2066                 TELNET CHARSET Option              January 1997   Note that regardless of CHARSET negotiation, translation only applies   to text (not commands), and only occurs when in BINARY mode [4].  If   not in BINARY mode, all data is assumed to be in NVT ASCII [1].   Also note that the CHARSET option should be used with the END OF   RECORD option [5] for block-mode terminals in order to be clear on   what character represents the end of each record.   As an example of character set negotiation, consider a user on a   workstation using TELNET to communicate with a server.  In this   example, the workstation normally uses the Cyrillic (ASCII) character   set [2] but is capable of using EBCDIC-Cyrillic [2], and the server   normally uses EBCDIC-Cyrillic.  The server could handle the (ASCII)   Cyrillic character set, but prefers that instead the client system   uses the EBCDIC-Cyrillic character set.  (This and the following   examples do not show the full syntax of the subnegotiation messages.)                 CLIENT                        SERVER             WILL CHARSET                   WILL CHARSET             DO CHARSET                     DO CHARSET             CHARSET REQUEST Cyrillic                 EBCDIC-Cyrillic                                            CHARSET ACCEPTED EBCDIC-                                               CyrillicGellens                       Experimental                     [Page 10]

RFC 2066                 TELNET CHARSET Option              January 1997   Now consider a case where the workstation can't handle EBCDIC-   Cyrillic, but can accept a translate table:                 CLIENT                        SERVER              WILL CHARSET                   WILL CHARSET              DO CHARSET                     DO CHARSET              CHARSET REQUEST [TTABLE] 1                 Cyrillic                                             CHARSET TTABLE-IS 1 Cyrillic                                               EBCDIC-Cyrillic              CHARSET TTABLE-ACK   For another example, consider a case similar to the previous case,   but now the user switches server applications in the middle of the   session (denoted by ellipses), and the new application requires a   different character set:                CLIENT                        SERVER              WILL CHARSET                   WILL CHARSET              DO CHARSET                     DO CHARSET              CHARSET REQUEST [TTABLE] 1                 Cyrillic EBCDIC-INT                                             CHARSET TTABLE-IS 1 Cyrillic                                               EBCDIC-Cyrillic              CHARSET TTABLE-ACK              . . .                          . . .                                             CHARSET REQUEST EBCDIC-INT              CHARSET ACCEPTED EBCDIC-INTGellens                       Experimental                     [Page 11]

RFC 2066                 TELNET CHARSET Option              January 19976.   Security Considerations   Security issues are not discussed in this memo.7.   References   [1] Postel, J. and J. Reynolds, "Telnet Protocol       Specification", STD 8,RFC 854, ISI, May 1983.   [2] Reynolds, J., and J. Postel, "Assigned Numbers",       STD 2,RFC 1700, ISI, October 1994.   [3] Postel, J. and J. Reynolds, "Telnet Option       Specifications", STD 8,RFC 855, ISI, May 1983.   [4] Postel, J. and J. Reynolds, "Telnet Binary       Transmission", STD 27,RFC 856, ISI, May 1983.   [5] Postel, J., "Telnet End-Of-Record Option",RFC 885,       ISI, December 1983.   [6] Postel, J., "Internet Official Protocol Standards",       STD 1,RFC 1920, IAB, March 1996.8.   Author's Address   Randall Gellens   Unisys Corporation   25725 Jeronimo Road   Mail Stop 237   Mission Viejo, CA  92691   USA   Phone:  +1.714.380.6350   Fax:    +1.714.380.5912   EMail:  Randy@MV.Unisys.ComGellens                       Experimental                     [Page 12]