    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.