Revised Telnet byte macro option :: RFC0735

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  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  definitions
      are to be discarded.










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RFC 735                                          DHC RHG  3 Nov 77 42083
Telnet Byte Macro Option



   IAC SB BM   
                                              IAC SE

      where:

          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.

          is one 8-bit byte binary number, indicating how many
          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.

          is a string of zero or more Telnet ASCII
         characters and/or commands, which the  is to
         represent; any character may occur within a . 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
          specification.

      The indicated  will be sent instead of the indicated
      . The receiver of the  (the sender
      of the DO BM) is to behave EXACTLY as if the 
      string of bytes had instead been received from the network. This
      interpretation is to occur before any other Telnet
      interpretations, unless the  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  may be negated by reseting
      it to "expand" into itself.

      IAC SB BM  X <0> IAC SE may be used to cause X to be
      ignored in the data stream.

       is decimal 1.

   IAC SB BM   IAC SE

      The receiver of the  for  accepts the
      requested definition and will perform the indicated replacement
      whenever a  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



       is decimal 2.

   IAC SB BM    IAC SE

      The receiver of the  for  refuses to perform
      the indicated translation from  to  because the particular  is not an acceptable
      choice, the length of the  exceeds available
      storage, the length of the actual  did not
      match the length predicted in the , or for some unspecified
      reason.

       is decimal 3.

       may be

                 which is decimal 1;

                   (for receiver's storage) which is decimal
                             2;

               (of actual string compared with promised
                             length in ) which is decimal 3; or

               (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   IAC SE

      The  is to be treated as real data, rather than as
      representative of the 

      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.

       is decimal 4.

   IAC SB BM    IAC SE

      The RECEIVER of the defined  (i.e., the sender of IAC
      DO BM) requests the sender of  to cancel its
      definition.  is the same as for the  subcommand.





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RFC 735                                          DHC RHG  3 Nov 77 42083
Telnet Byte Macro Option



      The  sender should (but is not required to) respond by
      resetting  (i.e., sending an IAC SB BM   <1>  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 
   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
    until it has received an  , or DONT BM,
   in reply to a .

   NOTE: The Telnet command character IAC (decimal 255) may be a member
   of a  but is the ONLY character which may NOT be
   defined as a .





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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 , and interpreted as its corresponding , and
   one which is to be taken as its usual Telnet NVT value. In all other
   cases, however, s are to be interpreted immediately, as
   if their corresponding 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 ; 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 .

   The  in the  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
    definition and use of s as real data. It is
   expected that relatively few 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 s normally be drawn from
   that pool.



















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RFC, FYI, BCP