Differential D32717

tcp: Rack at times can miscalculate the RTT from what it thinks is a persists probe respone.
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Authored by rrs on Oct 28 2021, 7:54 PM.

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rGaeda85278255: tcp: Rack at times can miscalculate the RTT from what it thinks is a persists…

Summary

Turns out that if a peer sends in a window update right after rack fires off
a persists probe, we can mis-interpret the window update and calculate
a bogus RTT (very short). We still process the window update and send
the data but we incorrectly generate an RTT. We should be only doing
the RTT stuff if the rwnd is still small and has not changed.

Test Plan

We can use packet drill to generate the situation but will have to
examine the BB logs to validate that the invalid RTT measurement
is not made.

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Event Timeline

rrs created this revision.Oct 28 2021, 7:54 PM

Herald added 1 blocking reviewer(s): transport. · View Herald TranscriptOct 28 2021, 7:54 PM

Herald added a subscriber: melifaro. · View Herald Transcript

rrs requested review of this revision.Oct 28 2021, 7:54 PM

tuexen accepted this revision.Oct 28 2021, 8:11 PM

This revision is now accepted and ready to land.Oct 28 2021, 8:11 PM

Opps there is another issue here, a double bump of rtt_shift that should not be when persists happen. Only
bump it in the timer code.

This revision now requires review to proceed.Oct 28 2021, 8:19 PM

tuexen accepted this revision.Oct 28 2021, 8:28 PM

This revision is now accepted and ready to land.Oct 28 2021, 8:28 PM

Closed by commit rGaeda85278255: tcp: Rack at times can miscalculate the RTT from what it thinks is a persists… (authored by rrs). · Explain WhyOct 29 2021, 7:20 AM

This revision was automatically updated to reflect the committed changes.

rrs added a commit: rGaeda85278255: tcp: Rack at times can miscalculate the RTT from what it thinks is a persists….

Herald added a subscriber: imp. · View Herald TranscriptOct 29 2021, 7:20 AM

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sys/

netinet/

tcp_stacks/

19 lines

Diff 97634

sys/netinet/tcp_stacks/rack.c

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