Network Working Group J. Postel
Request for Comments: 860 J. Reynolds
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
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
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.
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 | |-|-|-|-|-| | | |
+-----------+ +---------------+ +-------+
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
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.