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.










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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.





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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.





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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>.





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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.



















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