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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 <macro byte> 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 <macro byte> definitions
    are to be discarded.

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

 IAC SB BM <DEFINE> <macro byte> <count>
                                           <replacement 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, but may be any
       other 8-bit character.

       <count> is one 8-bit byte binary number, indicating how many
       <replacement string> 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.

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

    The indicated <macro byte> will be sent instead of the indicated
    <replacement string>. The receiver of the <macro byte> (the sender
    of the DO BM) is to behave EXACTLY as if the <replacement string>
    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
    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 <macro byte> may be negated by reseting
    it to "expand" into itself.

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

    <DEFINE> is decimal 1.

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

    The receiver of the <DEFINE> for <macro byte> accepts the
    requested definition and will perform the indicated replacement
    whenever a <macro byte> is received and is not part of any IAC
    Telnet command sequence.

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

    <ACCEPT> is decimal 2.

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

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

    <REFUSE> is decimal 3.

    <REASON> may be

       <BAD-CHOICE>        which is decimal 1;

       <TOO-LONG>          (for receiver's storage) which is decimal
                           2;

       <WRONG-LENGTH>      (of actual string compared with promised
                           length in <count>) which is decimal 3; or

       <OTHER-REASON>      (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 <LITERAL> <macro byte> IAC SE

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

    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.

    <LITERAL> is decimal 4.

 IAC SB BM <PLEASE CANCEL> <macro byte> <REASON> IAC SE

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

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

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

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

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

 The <count> in the <DEFINE> 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
 <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 (obsolete) Telnet
 protocol, it is recommended that <macro byte>s normally be drawn from
 that pool.