Revised Telnet byte macro option(2)

　　is decimal 4.

　　IAC SB BM IAC SE

　　The RECEIVER of the defined (i.e., the sender of IAC

　　DO BM) requests the sender of to cancel its　definition. is the same as for the subcommand.

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　　RFC735DHC RHG 3 Nov 77 42083　Telnet Byte Macro Option

　　The sender should (but is not required to) respond by　resetting (i.e., sending an IAC SB BM 　byte> <1> 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 　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　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 .

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　　RFC735DHC 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 　byte>, 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.