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+
+RFC 735                                          DHC RHG  3 Nov 77 42083
+Telnet Byte Macro Option
+
+
+
+Network Working Group                                   David H. Crocker
+RFC: #735                                                       Rand-ISD
+NIC: #42083                                      (Dcrocker at Rand-Unix)
+                                                     Richard H. Gumpertz
+                                              Carnegie-Mellon University
+                                                   (Gumpertz at CMU-10A)
+
+Obsoletes: RFC #729 (NIC #40306)                         3 November 1977
+
+                    Revised TELNET Byte Macro Option
+
+1. Command name and code:
+
+   BM 19
+
+2. 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 42083
+Telnet 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 42083
+Telnet 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 42083
+Telnet 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 42083
+Telnet 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
+\ No newline at end of file