Network Working Group                                          J. Postel
Request for Comments: 860                                    J. Reynolds
                                                                     ISI
Obsoletes: NIC 16238                                            May 1983

                       TELNET TIMING MARK OPTION
	

This RFC specifies a standard for the ARPA community.  Hosts on the ARPA
Internet are expected to adopt and implement this standard.

1.  Command Name and Code

   TIMING-MARK          6

2.  Command Meanings

   IAC DO TIMING-MARK

      The sender of this command REQUESTS that the receiver of this
      command return a WILL TIMING-MARK in the data stream at the
      "appropriate place" as defined in section 4 below.

   IAC WILL TIMING-MARK

      The sender of this command ASSURES the receiver of this command
      that it is inserted in the data stream at the "appropriate place"
      to insure synchronization with a DO TIMING-MARK transmitted by the
      receiver of this command.

   IAC WON'T TIMING-MARK

      The sender of this command REFUSES to insure that this command is
      inserted in the data stream at the "appropriate place" to insure
      synchronization.

   IAC DON'T TIMING-MARK

      The sender of this command notifies the receiver of this command
      that a WILL TIMING-MARK (previously transmitted by the receiver of
      this command) has been IGNORED.

3.  Default

   WON'T TIMING-MARK, DON'T TIMING-MARK

      i.e., No explicit attempt is made to synchronize the activities at
      the two ends of the TELNET connection.

4.  Motivation for the Option

   It is sometimes useful for a user or process at one end of a TELNET
   connection to be sure that previously transmitted data has been
   completely processed, printed, discarded, or otherwise disposed of.
   This option provides a mechanism for doing this.  In addition, even
   if the option request (DO TIMING-MARK) is refused (by WON'T
   TIMING-MARK) the requester is at least assured that the refuser has
   received (if not processed) all previous data.

   As an example of a particular application, imagine a TELNET
   connection between a physically full duplex terminal and a "full
   duplex" server system which permits the user to "type ahead" while
   the server is processing previous user input.  Suppose that both
   sides have agreed to Suppress Go Ahead and that the server has agreed
   to provide echoes.  The server now discovers a command which it
   cannot parse, perhaps because of a user typing error.  It would like
   to throw away all of the user's "type-ahead" (since failure of the
   parsing of one command is likely to lead to incorrect results if
   subsequent commands are executed), send the user an error message,
   and resume interpretation of commands which the user typed after
   seeing the error message.  If the user were local, the system would
   be able to discard the buffered input; but input may be buffered in
   the user's host or elsewhere.  Therefore, the server might send a DO
   TIMING-MARK and hope to receive a WILL TIMING-MARK from the user at
   the "appropriate place" in the data stream.

   The "appropriate place", therefore (in absence of other information)
   is clearly just before the first character which the user typed after
   seeing the error message.  That is, it should appear that the timing
   mark was "printed" on the user's terminal and that, in response, the
   user typed an answering timing mark.

   Next, suppose that the user in the example above realized that he had
   misspelled a command, realized that the server would send a DO
   TIMING-MARK, and wanted to start "typing ahead" again without waiting
   for this to occur.  He might then instruct his own system to send a
   WILL TIMING-MARK to the server and then begin "typing ahead" again.
   (Implementers should remember that the user's own system must
   remember that it sent the WILL TIMING-MARK so as to discard the
   DO/DON'T TIMING-MARK when it eventually arrives.)  Thus, in this case
   the "appropriate place" for the insertion of the WILL TIMING-MARK is
   the place defined by the user.

   It should be noted, in both of the examples above, that it is the
   responsibility of the system which transmits the DO TIMING-MARK to
   discard any unwanted characters; the WILL TIMING-MARK only provides
   help in deciding which characters are "unwanted".

5.  Description of the Option

   Suppose that Process A of Figure 1 wishes to synchronize with B. The
   DO TIMING-MARK is sent from A to B.  B can refuse by replying WON'T
   TIMING-MARK, or agree by permitting the timing mark to flow through
   his "outgoing" buffer, BUF2.  Then, instead of delivering it to the
   terminal, B will enter the mark into his "incoming" buffer BUF1, to
   flow through toward A.  When the mark has propagated through B's
   incoming buffer, B returns the WILL TIMING-MARK over the TELNET
   connection to A.

      PROCESS A    TELNETconnection    PROCESS B           Terminal
      +-----------+                +---------------+ Timing+-------+
      |           |WILL TIMING MARK|     BUF 1     |  Mark |       |
      |           |<---------------|--|-|-|-|-|-|--|<------|       |
      |           |                |  |-|-|-|-|-|  |   ^   |       |
      |           |                |     BUF 2     |   ^   |       |
      |           |--------------->|--|-|-|-|-|-|--|------>|       |
      |           | DO TIMING MARK |  |-|-|-|-|-|  |       |       |
      +-----------+                +---------------+       +-------+
                                     (NVT process).ME;
                         Figure 1

   When A receives the WILL TIMING-MARK, he knows that all the
   information he sent to B before sending the timing mark been
   delivered, and all the information sent from B to A before turnaround
   of the timing mark has been delivered.

   Three typical applications are:

      A. Measure round-trip delay between a process and a terminal or
         another process.

      B. Resynchronizing an interaction as described in section 4 above.
         A is a process interpreting commands forwarded from a terminal
         by B. When A sees an illegal command it:

         i.   Sends <carriage return>, <line feed>, <question mark>.

         ii.  Sends DO TIMING-MARK.

         iii. Sends an error message.

         iv.  Starts reading input and throwing it away until it
              receives a WILL TIMING-MARK.

         v.   Resumes interpretation of input.

         This achieves the effect of flushing all "type ahead" after the
         erroneous command, up to the point when the user actually saw
         the question mark.

      C.  The dual of B above.  The terminal user wants to throw away
         unwanted output from A.

         i.   B sends DO TIMING-MARK, followed by some new command.

         ii.  B starts reading output from A and throwing it away until
              it receives WILL TIMING-MARK.

         iii. B resumes forwarding A's output to the terminal.

         This achieves the effect of flushing all output from A, up to
         the point where A saw the timing mark, but not output generated
         in response to the following command.