Network Working Group                            Dave Crocker, Rand-ISD
Request for Comments: 729                        (Dcrocker at Rand-Unix)
NlC: 40306                                       13 May l977




                       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 sing1e  data characters which are to  be
       interpreted as if longer 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  longer  data
       strings had been sent.

  IAC DO BM

       The sender REQUESTS or AGREES to have the other side (send  of
       WILL BM) issue  send single  data characters which  are to  be
       interpreted as if longer 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  longer  data
       strings had been sent.

  IAC SB BM <DEFINE> <macro byte> <count>
                  <expansion 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).


RFC #729 Telnet Byte Macro Option                                Page 2



          <count> is a  one-byte binary number,  indicating how  many
          <expansion string> characters follow, up to the ending  IAC
          SE, but not including it.

          <expansion string> is a string of one or more Telnet  ASCII
          characters and/or commands,  which the <macro  byte> is  to
          represent; any  character may  occur within  an  <expansion
          string>.

       The indicated  <macro  byte>  will  be  sent  instead  of  the
       indicated <expansion string>. The receiver of the <macro byte>
       (the sender  of the  DO BM)  is to  behave EXACTLY  as if  the
       <expansion 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 BM subcommand; in this case no special interpretation  is
       to be made.

       Note that the effect of a particular <macro byte> may be
       negated by reseting it to "expand" into itself.

       <DEFINE> is decimal 01.


  IAC SB BM <ACCEPT> <macro byte> IAC SE

       The receiver  of the  <DEFINE> for  <macro byte>  accepts  the
       requested definition and will perform the indicating expansion
       whenever a <macro byte>  is received and is  not part of a  BM
       subcommand.

       <ACCEPT> is decimal 02.


  IAC SB BM <REFUSE> <macro byte> <REASON> IAC SE

       The receive  of  the  <DEFINE> for  <macro  byte>  refuses  to
       perform  the  indicated  translation  from  <macro  byte>   to
       <expansion string> either because the particular <macro  byte>
       is not  an acceptable  choice  or because  the length  of  the
       <expansion string> exceeds available storage.

  <REFUSE> is decimal 03.

  <REASON> may be

          <BAD CHOICE> which is decimal 01; or

          <TOO LONG> which is decimal 02.


RFC #729 Telnet Byte Macro Option                                Page 3




  IAC SB BM <LITERAL> <macro byte> IAC SE

       The <macro byte> is to be treated as real data, rather than as
       representative of the <expansion string>

       <LITERAL> is decimal 03.




3. Default:

  WON'T BM -- DON'T BM

       No reinterpretation of data is allowed.

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 sub-commands  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 multiple NOT characters. Except for the ability to offer
  a blanket refusal,  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> or <REFUSE>,  in
  reply to a <DEFINE>.

  NOTE: The  Telnet command  character  IAC (decimal  255) may  be  a
  member of an <expansion string> but is the ONLY character which may
  NOT be  defined  as  a  <macro  byte>.  ALL  OTHER  Telnet  command
  characters use IAC  as a  preface and therefore  occupy a  SEPARATE
  portion of the data space than do regular data bytes.

RFC #729 Telnet Byte Macro Option Page 4



  With  the  exception  of  IAC,  data  transfered  as  part  of   BM
  subcommands are NOT to be interpreted. They are to be taken 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 <expansion string>, and
  that same character to be taken  as its usual Telnet NVT value.  In
  all other  cases,  however, <macro  byte>s  are to  be  interpreted
  immediately, as if  the <expansion string>  had actually been  sent
  across  the  network.    Expanded  strings  are   not  subject   to
  reinterpretation, so that recursive definitions cannot be made.

  The <count>  in  the <DEFINE>  subcommand  allows 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. To include  IAC as part  of a <DEFINE>,  the
  string "IAC IAC" must be sent.

  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  Telnet
  protocol, it is  recommended that <macro  byte>s normally be  drawn
  from that pool.

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