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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

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RFC 860 May 1983

 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

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RFC 860 May 1983

 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.

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RFC 860 May 1983

       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.

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