Telnet Timing Mark Option(2)

　　(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

　　RFC860May 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 , , .

　　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.

　　RFC860May 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.