Telnet byte macro option

　　Network Working Group Dave Crocker, Rand-ISD

　　Request for Comments: 729 (Dcrocker at Rand-Unix)

　　NlC: 40306 13 May l977

　　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 sing1e data characters which are to be　interpreted as if longer 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 longer data　strings had been sent.

　　IAC DO BM

　　The sender REQUESTS or AGREES to have the other side (send of　WILL BM) issue send single data characters which are to be　interpreted as if longer 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 longer data　strings had been sent.

　　IAC SB BM

　　IAC SE

　　where:

　　is the data byte actually to be sent across　the network; it may NOT be Telnet IAC (decimal 255).

　　is a one-byte binary number, indicating how many　characters follow, up to the ending IAC　SE, but not including it.

　　is a string of one or more Telnet ASCII　characters and/or commands, which the is to　represent; any character may occur within an 　string>.

　　The indicated will be sent instead of the　indicated . The receiver of the 　(the sender of the DO BM) is to behave EXACTLY as if the　of bytes had instead been received from the　network. This interpretation is to occur before any other　Telnet interpretations, unless the occurs as part　of a BM subcommand; in this case no special interpretation is　to be made.

　　Note that the effect of a particular may be　negated by reseting it to "expand" into itself.

　　is decimal 01.

　　IAC SB BM IAC SE

　　The receiver of the for accepts the　requested definition and will perform the indicating expansion　whenever a is received and is not part of a BM　subcommand.

　　is decimal 02.

　　IAC SB BM IAC SE

　　The receive of the for refuses to　perform the indicated translation from to　either because the particular 　is not an acceptable choice or because the length of the　exceeds available storage.

　　is decimal 03.

　　may be

　　which is decimal 01; or

　　which is decimal 02.

　　IAC SB BM IAC SE

　　The is to be treated as real data, rather than as　representative of the 　is decimal 03.

　　3. Default:

　　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 　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 until it has received an or , in　reply to a .

　　NOTE: The Telnet command character IAC (decimal 255) may be a　member of an but is the ONLY character which may　NOT be defined as a . 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.

　　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 　byte>, and interpreted as its corresponding , and　that same character to be taken as its usual Telnet NVT value. In　all other cases, however, s are to be interpreted　immediately, as if the had actually been sent　across the network. Expanded strings are not subject to　reinterpretation, so that recursive definitions cannot be made.

　　The in the 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 , the　string "IAC IAC" must be sent.

　　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 Telnet　protocol, it is recommended that s normally be drawn　from that pool.