Network Working Group David H. Crocker
RFC: #735 Rand-ISD
NIC: #42083 (Dcrocker at Rand-Unix)
Richard H. Gumpertz
(Gumpertz at CMU-10A)
Obsoletes: RFC #729 (NIC #40306) 3 November 1977 Revised TELNET Byte Macro Option
1. Command name and code:
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.
IAC SB BM <DEFINE> <macro byte> <count>
<replacement string> IAC SE
<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.
<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
<REFUSE> is decimal 3.
<REASON> may be
<BAD-CHOICE> which is decimal 1;
<TOO-LONG> (for receiver's storage) which is decimal
<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
<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.
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
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>.
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