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Persistent congestion pseudocode to match text #4010

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Persistent congestion pseudocode to match text #4010

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c036ffa
Persistent congestion pseudocode to match text
martinthomson Aug 18, 2020
033500d
0 means not set
martinthomson Aug 18, 2020
f144481
Fix the AllPacketsLost thing too
martinthomson Aug 19, 2020
2c029b3
Merge branch 'master' into pc-starts-now
martinthomson Aug 19, 2020
e1fc8a9
`.append()` is clear than `+=`
martinthomson Aug 19, 2020
d02b6a0
Rename and use a direct result
martinthomson Aug 25, 2020
58b1411
Add an implementation tip
martinthomson Aug 25, 2020
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6 changes: 3 additions & 3 deletions draft-ietf-quic-recovery.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1517,7 +1517,7 @@ variables as follows:
bytes_in_flight = 0
congestion_recovery_start_time = 0
ssthresh = infinite
first_rtt_sample = never
first_rtt_sample = 0
for pn_space in [ Initial, Handshake, ApplicationData ]:
ecn_ce_counters[pn_space] = 0
~~~
Expand Down Expand Up @@ -1551,7 +1551,7 @@ newly acked_packets from sent_packets.
// Remove from bytes_in_flight.
bytes_in_flight -= acked_packet.sent_bytes

if (first_rtt_sample is never):
if (first_rtt_sample == 0):
first_rtt_sample = now()

// Do not increase congestion_window if application
Expand Down Expand Up @@ -1621,7 +1621,7 @@ Invoked when DetectAndRemoveLostPackets deems packets lost.

// Only look for persistent congestion if the period
// starts after getting the first RTT sample.
assert(first_rtt_sample is not never)
assert(first_rtt_sample != 0)
if (largest_lost.time_sent - pc_period > first_rtt_sample):
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Suggested change
if (largest_lost.time_sent - pc_period > first_rtt_sample):
if (smallest_lost.time_sent - pc_period > first_rtt_sample):

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largest_lost is the name of the variable we have. This says that the span of length pc_period that ends at the time that the largest lost was sent must start after we first got an RTT sample.

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FWIW, this is AllPacketsLost is not a fair representation of the algorithm in the latest PR. Only the largest ack-eliciting packet is relevant, and the function needs to ensure that it only considers the first ack-eliciting packet. But I guess we're abstracting over much of that.

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Yes, we're glossing over a few implementation details with AreAllPacketsLost() below, but I think this is detailed enough.

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See my comment below. You can change the param here to lost_packets. I would suggest the following:

We could make AreAllPacketsLostInPeriod() return the starting point of the persistent congestion period, and use that up here. Meaning call that function first and then check to see if that period starts after first_rtt_sample.

return false

Expand Down