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RFC 735                                          DHC RHG  3 Nov 77 42083Telnet Byte Macro OptionNetwork Working Group                                   David H. CrockerRFC: #735                                                       Rand-ISDNIC: #42083                                      (Dcrocker at Rand-Unix)                                                     Richard H. Gumpertz                                              Carnegie-Mellon University                                                   (Gumpertz at CMU-10A)Obsoletes: RFC #729 (NIC #40306)                         3 November 1977Revised TELNET Byte Macro Option1. Command name and code:   BM 192. Command Meanings:   IAC WILL BM      The sender of this command REQUESTS or AGREES to use the BM      option, and will send single data characters which are to be      interpreted as if replacement data strings had been sent.   IAC WON'T BM      The sender of this option REFUSES to send single data characters      which are to be interpreted as if replacement data strings had      been sent. Any existing BM <macro byte> definitions are discarded      (i.e., reset to their original data interpretations).   IAC DO BM      The sender REQUESTS or AGREES to have the other side (sender of      WILL BM) send single data characters which are to be interpreted      as if replacement data strings had been sent.   IAC DON'T BM      The sender REFUSES to allow the other side to send single data      characters which are to be interpreted as if replacement data      strings had been sent. Any existing BM <macro byte> definitions      are to be discarded.                                   1

RFC 735                                          DHC RHG  3 Nov 77 42083Telnet Byte Macro Option   IAC SB BM <DEFINE> <macro byte> <count>                                             <replacement string> IAC SE      where:         <macro byte> is the data byte actually to be sent across the         network; it may NOT be Telnet IAC (decimal 255, but may be any         other 8-bit character.         <count> is one 8-bit byte binary number, indicating how many         <replacement string> characters follow, up to the ending IAC         SE, but not including it. Note that doubled IACs in the         definition should only be counted as one character per pair.         <replacement string> is a string of zero or more Telnet ASCII         characters and/or commands, which the <macro byte> is to         represent; any character may occur within a <replacement         string>. Note, however, that an IAC in the string must be         doubled, to be interpreted later as an IAC; to be interpreted         later as data byte 255, it must be quadrupled in the original         <replacement string> specification.      The indicated <macro byte> will be sent instead of the indicated      <replacement string>. The receiver of the <macro byte> (the sender      of the DO BM) is to behave EXACTLY as if the <replacement string>      string of bytes had instead been received from the network. This      interpretation is to occur before any other Telnet      interpretations, unless the <macro byte> occurs as part of a      Telnet command; in this case no special interpretation is to be      made. In particular, an entire Telnet subnegotiation (i.e. from      IAC SB through IAC SE) is to be considered a Telnet command in      which NO replacement should be done.      The effect of a particular <macro byte> may be negated by reseting      it to "expand" into itself.      IAC SB BM <DEFINE> X <0> IAC SE may be used to cause X to be      ignored in the data stream.      <DEFINE> is decimal 1.   IAC SB BM <ACCEPT> <macro byte> IAC SE      The receiver of the <DEFINE> for <macro byte> accepts the      requested definition and will perform the indicated replacement      whenever a <macro byte> is received and is not part of any IAC      Telnet command sequence.                                   2

RFC 735                                          DHC RHG  3 Nov 77 42083Telnet Byte Macro Option      <ACCEPT> is decimal 2.   IAC SB BM <REFUSE> <macro byte> <REASON> IAC SE      The receiver of the <DEFINE> for <macro byte> refuses to perform      the indicated translation from <macro byte> to <replacement      string> because the particular <macro byte> is not an acceptable      choice, the length of the <replacement string> exceeds available      storage, the length of the actual <replacement string> did not      match the length predicted in the <count>, or for some unspecified      reason.      <REFUSE> is decimal 3.      <REASON> may be         <BAD-CHOICE>        which is decimal 1;         <TOO-LONG>          (for receiver's storage) which is decimal                             2;         <WRONG-LENGTH>      (of actual string compared with promised                             length in <count>) which is decimal 3; or         <OTHER-REASON>      (intended for use only until this document                             can be updated to include reasons not                             anticipated by the authors) which is                             decimal zero (0).   IAC SB BM <LITERAL> <macro byte> IAC SE      The <macro byte> is to be treated as real data, rather than as      representative of the <replacement string>      Note that this subcommand cannot be used during Telnet      subcommands, since subcommands are defined to end with the next      occurrence of "IAC SE". Including this BM subcommand within any      Telnet subcommand would therefore prematurely terminate the      containing subcommand.      <LITERAL> is decimal 4.   IAC SB BM <PLEASE CANCEL> <macro byte> <REASON> IAC SE      The RECEIVER of the defined <macro byte> (i.e., the sender of IAC      DO BM) requests the sender of <macro byte> to cancel its      definition. <REASON> is the same as for the <REFUSE> subcommand.                                   3

RFC 735                                          DHC RHG  3 Nov 77 42083Telnet Byte Macro Option      The <macro byte> sender should (but is not required to) respond by      resetting <macro byte> (i.e., sending an IAC SB BM <DEFINE> <macro      byte> <1> <macro byte> IAC SE).      If the receiver absolutely insists on cancelling a given macro,      the best it can do is to turn off the entire option, with IAC DONT      BM, wait for an acknowledging IAC WONT BM and then restart the      option, with IAC DO BM. This will reset all other macroes as well      but it will allow the receiver to REFUSE with code BAD CHOICE      if/when the foreign site attempts to redefine the macro in      question.3. Default:   WON'T BM -- DON'T BM      No reinterpretation of data bytes is done.4. Motivation for the option:   Subcommands for Telnet options currently require a minimum of five   characters to be sent over the network (i.e., IAC SB <Option name>   IAC SE). For subcommands which are employed infrequently, in absolute   numbers and in relation to normal data, this overhead is tolerable.   In other cases, however, it is not. For example, data which is sent   in a block- oriented fashion may need a "block separator" mark. If   blocks are commonly as small as five or ten bytes, then most of the   cross-net data will be control information. The BM option is intended   as a simple data compression technique, to remove this overhead from   the communication channel.5. Description of the option   The option is enabled through the standard Telnet Option negotiation   process. Afterwards, the SENDER of data (the side which sends the IAC   WILL BM) is free to define and use mappings between single and   replacement NVT characters. Except for the ability to refuse   particular definitions, the receiver of data has no control over the   definition and use of mappings.   The sender (of the WILL BM) is prohibited from using or redefining a   <macro byte> until it has received an <ACCEPT> <REFUSE>, or DONT BM,   in reply to a <DEFINE>.   NOTE: The Telnet command character IAC (decimal 255) may be a member   of a <replacement string> but is the ONLY character which may NOT be   defined as a <macro byte>.                                   4

RFC 735                                          DHC RHG  3 Nov 77 42083Telnet Byte Macro Option   Within any Telnet command (i.e., any sequence beginning with IAC)   macro replacement may NOT take place. Data are to be interpreted only   as their normal character values. This avoids the problem of   distinguishing between a character which is to be taken as a <macro   byte>, and interpreted as its corresponding <replacement string>, and   one which is to be taken as its usual Telnet NVT value. In all other   cases, however, <macro byte>s are to be interpreted immediately, as   if their corresponding <replacement string>s had actually been sent   across the network. Expanded strings are not subject to   reinterpretation, so that recursive definitions cannot be made.   Telnet commands may be included in <replacement strings>; however,   they must be totally contained within the macro or must begin within   the macro and terminate outside of it. In particular, they may NOT   begin outside a macro and continue or terminate inside one, since no   macro replacement takes place while processing any Telnet command.   Note that when skipping data due to Telnet SYNCH (INS/DM) processing,   BM macro replacement should still take place, since (for example)   "IAC DM" would be a valid <replacement string>.   The <count> in the <DEFINE> subcommand is intended to allow the   receiver to allocate storage. IAC interpretation is not over-ridden   during BM subcommands so that IAC SE will continue to safely   terminate malformed subcommands.   The BM option is notably inefficient with regard to problems during   <macro byte> definition and use of <macro byte>s as real data. It is   expected that relatively few <macro byte>s will be defined and that   they will represent relatively short strings. Since the Telnet data   space between decimal 128 and decimal 254 is not normally used,   except by implementations employing the original (obsolete) Telnet   protocol, it is recommended that <macro byte>s normally be drawn from   that pool.                                   5