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

TELNET Byte Macro Option

1. Command name and code:

   BM 19

2. Command Meanings:

        he sender of this  command REQUESTS or AGREES  to use the  BM
        ption, and will send sing1e  data characters which are to  be
        nterpreted as if longer data strings had been sent.
        he  sender  of  this  option  REFUSES  to  send  single  data
        haracters which  are  to be  interpreted  as if  longer  data
        trings had been sent.
        he sender REQUESTS or AGREES to have the other side (send  of
        ILL BM) issue  send single  data characters which  are to  be
        nterpreted as if longer data strings had been sent.
        he sender REFUSES to allow the other side to send single data
        haracters which  are  to be  interpreted  as if  longer  data
        trings had been sent.

IAC SB BM <DEFINE> <macro byte> <count>

                   <expansion string> IAC SE
          <macro byte> is the  data byte actually  to be sent  across
          the network; it may NOT be Telnet IAC (decimal 255).
          <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
        he indicated  <macro  byte>  will  be  sent  instead  of  the
        ndicated <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
        etwork. This  interpretation is  to  occur before  any  other
        elnet interpretations, unless the <macro byte> occurs as part
        f a BM subcommand; in this case no special interpretation  is
        o be made.

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

DEFINE> is decimal 01.

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

        he receiver  of the  <DEFINE> for  <macro byte>  accepts  the
        equested definition and will perform the indicating expansion
        henever a <macro byte>  is received and is  not part of a  BM

ACCEPT> is decimal 02.


        he receive  of  the  <DEFINE> for  <macro  byte>  refuses  to
        erform  the  indicated  translation  from  <macro  byte>   to
        expansion string> either because the particular <macro  byte>
        s 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.

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

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

LITERAL> is decimal 03.

3. Default:

  • 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>.
   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.