Revised Telnet byte macro option (original) (raw)
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PROPOSED STANDARD
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 Option1. 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.
1RFC 735 DHC RHG 3 Nov 77 42083 Telnet Byte Macro Option
IAC SB BM 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 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.
2RFC 735 DHC RHG 3 Nov 77 42083 Telnet Byte Macro Option
<ACCEPT> is decimal 2.IAC SB BM 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 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 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.
3RFC 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
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 until it has received an , or DONT BM, in reply to a .
NOTE: The Telnet command character IAC (decimal 255) may be a member of a but is the ONLY character which may NOT be defined as a .
4RFC 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 , and interpreted as its corresponding , and one which is to be taken as its usual Telnet NVT value. In all other cases, however, s are to be interpreted immediately, as if their corresponding 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 ; 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 .
The in the 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 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 (obsolete) Telnet protocol, it is recommended that s normally be drawn from that pool.
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