[tor-bugs] #29084 [Core Tor/Tor]: Ensure circuit padding RTT estimate handes var cells/wide creates (was: WTF-PAD: Test and improve circuit RTT measurement)

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#29084: Ensure circuit padding RTT estimate handes var cells/wide creates
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 Reporter:  asn                                  |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  wtf-pad, tor-relay, tor-cell,        |  Actual Points:
  padding                                        |
Parent ID:                                       |         Points:  2
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor2
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Description changed by mikeperry:

Old description:

> From TODO file:
> {{{
>   - Circuit RTT measurement will break on var_cell/EXTEND2 cells
>     - Are there any heuristics we can use here?
>       - If RELAY_EARLY is only for the first cell of an EXTEND2 series,
>         we can use that. But the proposal currently says MAY, but not
> MUST
>         for this behavior.
> }}}

New description:

 The use_rtt_estimate field in the circuit padding machines lets machines
 offset the inter-packet delays by a middle-node estimated RTT value of
 packets that go from the middle to the exit/website.

 We abort this measurement if we get two or more cells back-to-back in
 either direction, as this indicates that the half-duplex request/response
 circuit setup and RELAY_BEGIN sequence has finished.

 However, if we switch to a multi-cell circuit handshake, then we will need
 to take that into account when measuring RTT.

 If RELAY_EARLY is used only for the first cell of a multi-cell EXTEND2
 payload,
 then we can just count time between RELAY_EARLIES. But the proposal
 currently says MAY, but not MUST for this behavior.

--

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29084#comment:7>
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