Telnet byte macro option :: RFC0729

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   
                    IAC SE
	where:

	    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



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

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

	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
	 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 BM subcommand; in this case no special interpretation  is
	to be made.

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

	 is decimal 01.


   IAC SB BM   IAC SE

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

	 is decimal 02.


   IAC SB BM    IAC SE

	The receive  of  the   for    refuses  to
	perform  the  indicated  translation  from     to
	 either because the particular 
	is not  an acceptable  choice  or because  the length  of  the
	 exceeds available storage.

    is decimal 03.

    may be

	    which is decimal 01; or

	    which is decimal 02.


RFC #729 Telnet Byte Macro Option				 Page 3




   IAC SB BM   IAC SE

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

	 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 
   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  until  it has received an   or ,  in
   reply to a .

   NOTE: The  Telnet command  character  IAC (decimal  255) may  be  a
   member of an  but is the ONLY character which may
   NOT be  defined  as  a  .  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.

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   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 , and interpreted as its corresponding , and
   that same character to be taken  as its usual Telnet NVT value.  In
   all other  cases,  however, s  are to  be  interpreted
   immediately, as if  the   had actually been  sent
   across  the  network.    Expanded  strings  are   not  subject   to
   reinterpretation, so that recursive definitions cannot be made.

   The   in  the   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 ,  the
   string "IAC IAC" must be sent.

   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  Telnet
   protocol, it is  recommended that s normally be  drawn
   from that pool.

RFC, FYI, BCP