Source:  Both Sites      Meaning: This site has not implemented the following control code.   Code X'88'      Source:  Both Sites      Meaning: [2]Return to control mode, i.e. next byte will be a               control signal, possible a new DATA TYPE.   Some special TELNET control signals are required to permit the user   on some systems to send control information to the using site TELNET   process.  These do not require a corresponding control code for   transmission.  The local TELNET control signals are:      1. Transmit all data to this point.      2. Suppress transmission of end of line, send all other data.O'Sullivan                                                      [Page 6]RFC 137                     TELNET Protocol                   April 1971   Data is to be forwarded to the NCP for transmission as convenient,   but at least at the end of line, end of line suppression, and   transmit signals.  If the normal line length of the sending site is   greater than the allocation given by the receiving site, the sending   sites TELNET process or TELNET server will be responsible for   breaking the line into convenient lengths and turning them over to   the NCP for transmission.   This document will be revised as necessary to provide conventions for   data types, in addition to the NVT ASCII type.Footnotes:   [1] A one-byte DATA TYPE signal is sent as the first byte of data   over a connection.  A default is employed if the first byte over a   connection has the high order bit set to zero, and it is assumed that   the seven-bit ASCII NVT convention will be employed.  After initial   connection, the DATA TYPE may be changed (see code X'88').  Most   implementations and applications may expect the DATA TYPES to be   symmetrical at any point in time,(i.e. both using a serving site   using the same DATA TYPE.).   [2] A site receiving a DATA TYPE signal is to respond with a double   X'88' if the new DATA TYPE is acceptable.O'Sullivan                                                      [Page 7]RFC 137                     TELNET Protocol                   April 1971|<------- 32 ------->|<-8->|<-8->|<-- 16 -->|<-8->|<---+--------------------+-----+-----+----------+-----+------------------|       leader       |  x  |size |  count   |  x  | TEXT+--------------------+-----+-----+----------+-----+------------------|<---- level 1  ---->|    message leader|<------------------  level 2  ------------------>|                  message preamble                                                         level 3                                                  |<- message text..-->                 Figure 1. Network Message on Link 2-31           Indicating Portions of Interest to Various Levels       USING HOST                                       Serving HOST -----------------------+                      +----------------------                        |                      |    \                   |                      |                  /     \ -----------------|  +-+            +-+  |-----------------/      \           NCP   |  |I|            |I|  |   NCP          /       \       ^      <--->|M|---NETWORK--|M|<--->      ^      /        \ -----|-----+  |  |P|            |P|  |  +-----|-----/         \     v     |  |  +-+            +-+  |  |     v    /USER      \          |  |                      |  | TELNET  /  USERPROCESSES,  ) TELNET |--|                      |--|Protocol(   PROCESSSub     <===>        |  |                      |  |Routing<--->SubSystems,  /    ^     |  |                      |  |    ^^    \SystemsETC      /-----|-----+  |                      |  +----||-----\ETC        / TTY  v      <---> Local     Local  <===> TTY vv      \       /  Handles       |   Terminals Terminals|   Handles    <===>      /-----------------|                      |-----------------\     /                  |                      |                  \                        |                      | -----------------------+                      +----------------------<---> Current TELNET paths<===> Candidate future TELNET paths          Figure 2. Current and Candidate Future TELNET PathsO'Sullivan                                                      [Page 8]RFC 137                     TELNET Protocol                   April 1971+---------------------------+----+----+----+----+----+----+----+----+|\ b8  ->                   | 0  | 0  | 0  | 0  | 0  | 0  | 0  | 0  || \ b7  ->                  | 0  | 0  | 0  | 0  | 1  | 1  | 1  | 1  ||  \ b6  ->                 | 0  | 0  | 1  | 1  | 0  | 0  | 1  | 1  ||B  \ b5  ->                | 0  | 1  | 0  | 1  | 0  | 1  | 0  | 1  || I  +---+---+---+---+------+----+----+----+----+----+----+----+----++  T | b | b | b | b |\COL->|    |    |    |    |    |    |    |    |\   S| 4 | 3 | 2 | 1 | \    |    |    |    |    |    |    |    |    | \   |   |   |   |   | |\   | 0  | 1  | 2  | 3  | 4  | 5  | 6  | 7  |  \  | | | | | | | | | v \  |    |    |    |    |    |    |    |    |   \ | v | v | v | v |ROW \ |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 0 | 0 |   0  |NUL |DLE | SP | 0  | @  | P  | \  | p  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 0 | 1 |   1  |SOH |DC1 | !  | 1  | A  | Q  | a  | q  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 1 | 0 |   2  |STX |DC2 | "  | 2  | B  | R  | b  | r  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 1 | 1 |   3  |ETX |DC3 | #  | 3  | C  | S  | c  | s  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 0 | 0 |   4  |EOT |DC4 | $  | 4  | D  | T  | d  | t  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 0 | 1 |   5  |ENQ |NAC | %  | 5  | E  | U  | e  | u  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 1 | 0 |   6  |ACK |SYN | &  | 6  | F  | V  | f  | v  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 1 | 1 |   7  |BEL |ETB | '  | 7  | G  | W  | g  | w  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 0 | 0 |   8  | BS |CAN | (  | 8  | H  | X  | h  | x  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 0 | 1 |   9  | HT | EM | )  | 9  | I  | Y  | i  | y  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 1 | 0 |  10  | LF |SUB | *  | :  | J  | Z  | j  | z  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 1 | 1 |  11  | VT |ESC | +  | ;  | K  | [  | k  | {  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 0 | 0 |  12  | FF | FS | ,  | <  | L  | \  | l  | |  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 0 | 1 |  13  | CR | GS | -  | =  | M  | ]  | m  | }  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 1 | 0 |  14  | S0 | RS | .  | >  | N  | ^  | n  | ~  |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 1 | 1 |  15  | S1 | US | /  | ?  | O  | _  | o  |DEL |     +---+---+---+---+------+----+----+----+----+----+----+----+----+         Code Structure      8   7   6   5   4   3   2   1                        --- --- --- --- --- --- --- --- --- ---O'Sullivan                                                      [Page 9]RFC 137                     TELNET Protocol                   April 1971+---------------------------+----+----+----+----+----+----+----+----+|\ b8  ->                   | 1  | 1  | 1  | 1  | 1  | 1  | 1  | 1  || \ b7  ->                  | 0  | 0  | 0  | 0  | 1  | 1  | 1  | 1  ||  \ b6  ->                 | 0  | 0  | 1  | 1  | 0  | 0  | 1  | 1  ||B  \ b5  ->                | 0  | 1  | 0  | 1  | 0  | 1  | 0  | 1  || I  +---+---+---+---+------+----+----+----+----+----+----+----+----++  T | b | b | b | b |\COL->|    |    |    |    |    |    |    |    |\   S| 4 | 3 | 2 | 1 | \    |    |    |    |    |    |    |    |    | \   |   |   |   |   | |\   | 8  | 9  | 10 | 11 | 12 | 13 | 14 | 15 |  \  | | | | | | | | | v \  |    |    |    |    |    |    |    |    |   \ | v | v | v | v |ROW \ |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 0 | 0 |   0  |'80'|    |'A0'|    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 0 | 1 |   1  |'81'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 1 | 0 |   2  |'82'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 0 | 1 | 1 |   3  |'83'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 0 | 0 |   4  |'84'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 0 | 1 |   5  |'85'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 1 | 0 |   6  |'86'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 0 | 1 | 1 | 1 |   7  |'87'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 0 | 0 |   8  |'88'|    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 0 | 1 |   9  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 1 | 0 |  10  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 0 | 1 | 1 |  11  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 0 | 0 |  12  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 0 | 1 |  13  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 1 | 0 |  14  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+     | 1 | 1 | 1 | 1 |  15  |    |    |    |    |    |    |    |    |     +---+---+---+---+------+----+----+----+----+----+----+----+----+ 'XX' = HEX designation for codes assigned to TELNET Control Signals.            Figure 3. Official Network Virtual Terminal CodeO'Sullivan                                                     [Page 10]RFC 137                     TELNET Protocol                   April 1971        [This RFC was put into machine readable form for entry]         [into the online RFC archives by Sergio Kleiman, 8/01]O'Sullivan                                                     [Page 11]