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.
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.