Movatterモバイル変換

Network Working Group                                         C. HedrickRequest for Comments: 1372                            Rutgers UniversityObsoletes: RFC1080                                            D. Borman                                                     Cray Research, Inc.                                                            October 1992Telnet Remote Flow Control OptionStatus of This Memo   This RFC specifies an IAB standards track protocol for the Internet   community, and requests discussion and suggestions for improvements.   Please refer to the current edition of the "IAB Official Protocol   Standards" for the standardization state and status of this protocol.   Distribution of this memo is unlimited.Introduction   This document specifies an extended version of the Telnet Remote Flow   Control Option,RFC 1080, with the addition of the RESTART-ANY and   RESTART-XON suboptions.1.  Command Names and Codes   TOGGLE-FLOW-CONTROL   33       OFF                    0       ON                     1       RESTART-ANY            2       RESTART-XON            32.  Command Meanings   IAC WILL TOGGLE-FLOW-CONTROL      Sender is willing to enable and disable flow control upon command.   IAC WONT TOGGLE-FLOW-CONTROL      Sender refuses to enable and disable flow control.  Nothing is      implied about whether sender does or does not use flow control.      It is simply unwilling to enable and disable it using this      protocol.   IAC DO TOGGLE-FLOW-CONTROL      Sender is willing to send commands to enable and disable flow      control.Hedrick & Borman                                                [Page 1]

RFC 1372           Telnet Remote Flow Control Option        October 1992   IAC DONT TOGGLE-FLOW-CONTROL      Sender refuses to send command to enable and disable flow control.   IAC SB TOGGLE-FLOW-CONTROL OFF IAC SE      Sender requests receiver to disable flow control.   IAC SB TOGGLE-FLOW-CONTROL ON IAC SE      Sender requests receiver to enable flow control.   IAC SB TOGGLE-FLOW-CONTROL RESTART-ANY IAC SE      Sender requests that when flow control is enabled, the receiver      allow any character (except another XOFF) to restart output.   IAC SB TOGGLE-FLOW-CONTROL RESTART-XON IAC SE      Sender requests that when flow control is enabled, the receiver      allows only the XON character to restart output.3.  Default Specification   The default specification for this option is      WONT TOGGLE-FLOW-CONTROL DONT TOGGLE-FLOW-CONTROL   meaning flow control information will not be exchanged in either   direction.4.  Motivation   This memo describes a method of remotely toggling flow control   between a user telnet process and the attached terminal.  Only flow   control of data being transmitted from the telnet process to the   terminal is considered.  Many systems will also allow flow control of   data from the terminal to the telnet process, however there is seldom   need to change this behavior repeatedly during the session.   There are two common ways of doing flow control: hardware and   software.  Hardware flow control uses signals on wires dedicated for   this purpose.  Software flow control uses one or two specific   characters sent along the same path as normal input data.  Most   commonly, XOFF (control-S) and XON (control-Q) are used to stop and   start output, respectively.  The option described herein is useful   primarily where software flow control is being used.  (Since hardware   flow control does not preempt any characters, there is normally noHedrick & Borman                                                [Page 2]

RFC 1372           Telnet Remote Flow Control Option        October 1992   need to disable it.)  It should also be noted that flow control can   either be generated automatically by the terminal when its buffers   are close to overflowing, or manually by the user, when he/she cannot   read the information as fast as it is being displayed, and unread   information will start scrolling off the screen.   The primary difficulty with software flow control is that it preempts   one or two characters.  Host software often requires the user to be   able to input every possible ASCII character.  (Certain editors are   notorious for having XOFF and XON as commonly-used commands.)  For   this reason, operating systems often allow programs to disable flow   control.  While it is disabled, the characters that normally signal   flow control may be read as normal input.  In a telnet environment,   flow control is normally done by the user telnet process, not by the   host computer.  In addition, many operating systems, when flow   control is enabled, the user may specify whether the XOFF character   is the only character that is allowed to re-enable the output of   data, or whether any typed character should re-enable the flow of   data.  Thus this RFC defines a way to propagate flow control status   from the host computer to the user telnet process.5.  Description of the Option   Use of the option requires two phases.  In the first phase, the   telnet processes agree that one of them will TOGGLE-FLOW-CONTROL.   WILL and DO are used only in this first phase.  In general there will   be only one exchange of WILL and DO for a session.  Subnegotiations   must not be issued until DO and WILL have been exchanged.  It is   permissible for either side to turn off the option by sending a WONT   or DONT.  Should this happen, no more subnegotiations may be sent,   unless the option is re-enabled by another exchange of DO and WILL.   Once the hosts have exchanged a WILL and a DO, the sender of the DO   TOGGLE-FLOW-CONTROL is free to send subnegotiations to enable and   disable flow control in the other process, and to send   subnegotiations for recommendations on when to restart output.   Normally, the sender of the DO will be a host, and the other end will   be a user telnet process, which is connected to a terminal.  Thus the   protocol is normally asymmetric, however it may be used in both   directions without confusion should need for this arise.   As soon as the DO and WILL have been exchanged, the sender of the   WILL must enable flow control.  This allows flow control to begin in   a known state.  The decision of whether to restart output only when   the XON character is received, or when any character received, starts   in a system dependent state.  (This is to make it consistent with   older implementations of the TOGGLE-FLOW-CONTROL option that do not   understand the RESTART-ANY and RESTART-XON suboptions.)  The senderHedrick & Borman                                                [Page 3]

RFC 1372           Telnet Remote Flow Control Option        October 1992   of the DO should send either a RESTART-ANY or RESTART-XON suboption   to put the restart characteristics to a know state.  Some clients   might not be able to support both of the RESTART-ANY and RESTART-XON   modes due to system limitations, and would then choose to ignore   these commands.  There is no way for the server to be notified of   this condition, but a client should make every attempt to honor the   state requested by the RESTART-ANY and RESTART-XON modes.  Should the   option be disabled by exchange of DONT and WONT, flow control may   revert to an implementation-defined default state.  It is not safe to   assume that flow control will remain in the state requested by the   most recent subnegotiation.   In most implementations of software flow control, when enabled, the   XOFF and XON characters are never propagated to the server; they are   typically eaten by the terminal driver between the telnet client and   the attached terminal.  In most implementations that support the   RESTART-ANY functionality, the typed character that re-enables the   output is not eaten by the terminal driver, unless it is the XON   character.   Currently, only four command codes are defined for the   subnegotiations: flow control off (code 0), flow control on (code 1),   restart output on any character (code 2) and restart output only on   XON (code 3).  None of these codes requires any additional data,   however it is possible that additional commands may be added.  Thus   subnegotiations having command codes other than those defined in this   document should be silently ignored.   This option does not deal with the issue of allowing the DO side of   the connection to inform the WILL side which characters should be   used for XON and XOFF.  That functionality is already supplied by the   LINEMODE [1] option.Hedrick & Borman                                                [Page 4]

RFC 1372           Telnet Remote Flow Control Option        October 19926.  Example   Here is an example of the use of this option:       Client                           Server                                        IAC DO TOGGLE-FLOW-CONTROL       IAC WILL TOGGLE-FLOW-CONTROL       [ The server is now free to send commands to change flow control.         Note that the client must now have enabled flow control, but         that whether it is restart on XON only or on any character is         client specific.  ]                                        IAC SB TOGGLE-FLOW-CONTROL                                        RESTART-ANY IAC SE       [ The client should now switch to allowing output to restart when         the user types any character, if the client is able to support         that functionality.  ]                                        IAC SB TOGGLE-FLOW-CONTROL OFF                                        IAC SE                                        IAC SB TOGGLE-FLOW-CONTROL ON                                        IAC SEReferences   [1] Internet Engineering Task Force, Telnet Working Group,       D. Borman, Editor, "Telnet Linemode Option",RFC 1184,       Cray Research, Inc., October 1990.Acknowledgments   The original specification for this option was written by Charles   Hedrick, and published asRFC 1080.  The RESTART-ANY and RESTART-XON   suboptions were defined and added to this version of the document by   David Borman.Security Considerations   Security issues are not discussed in this memo.Hedrick & Borman                                                [Page 5]

RFC 1372           Telnet Remote Flow Control Option        October 1992Authors' Addresses   David Borman   Cray Research, Inc.   655F Lone Oak Drive   Eagan, MN 55121   Phone: (612) 452-6650   Email: dab@CRAY.COM   Charles Hedrick   Director, LCSR Computing Facility   Rutgers University   227 CORE Building   P.O. Box 879   Piscataway, NJ  08855-0879   Phone: (908) 932-3088   Email: hedrick@cs.rutgers.edu   Mailing List: telnet-ietf@CRAY.COMHedrick & Borman                                                [Page 6]