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from c2e057a Prop 324: RTT should only be reset after slow start. new dabadcc Update Prop#324 clock heuristic spec for bug 40626. new 773fc92 Merge branch 'tor-gitlab/mr/75' new aab9efe prop#324: mention rearranged form for N_EWMA smoothing new 0bacc73 Merge branch 'tor-gitlab/mr/83'
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Summary of changes: proposals/324-rtt-congestion-control.txt | 32 +++++++++++++++++++------------- 1 file changed, 19 insertions(+), 13 deletions(-)
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commit dabadcc9d6a264c81ff84c3672c76e3261d4fe34 Author: Mike Perry mikeperry-git@torproject.org AuthorDate: Tue Jun 21 20:26:37 2022 +0000
Update Prop#324 clock heuristic spec for bug 40626. --- proposals/324-rtt-congestion-control.txt | 30 ++++++++++++++++-------------- 1 file changed, 16 insertions(+), 14 deletions(-)
diff --git a/proposals/324-rtt-congestion-control.txt b/proposals/324-rtt-congestion-control.txt index 78b6789..67514dc 100644 --- a/proposals/324-rtt-congestion-control.txt +++ b/proposals/324-rtt-congestion-control.txt @@ -144,20 +144,22 @@ measured by the RTT estimator, and if these heurtics detect a stall or a jump, we do not use that value to update RTT or BDP, nor do we update any congestion control algorithm information that round.
-If the time delta is 0, that is always treated as a clock stall. - -If we have measured at least 'cc_bwe_min' RTT values or we have successfully -exited slow start, then every sendme ACK, the new candidate RTT is compared to -the stored EWMA RTT. If the new RTT is either 5000 times larger than the EWMA -RTT, or 5000 times smaller than the stored EWMA RTT, then we do not record that -estimate, and do not update BDP or the congestion control algorithms for that -SENDME ack. - -Moreover, if a clock stall is detected by *any* circuit, this fact is -cached, and this cached value is used on circuits for which we do not -have enough data to compute the above heueristics. This cached value is -also exported for use by the edge connection rate calculations done by -[XON_ADVISORY]. +If the time delta is 0, that is always treated as a clock stall, the RTT is +not used, congestion control is not updated, and this fact is cached globally. + +If the circuit does not yet have an EWMA RTT or it is still in Slow Start, then +no further checks are performed, and the RTT is used. + +If the circuit has stored an EWMA RTT and has exited Slow Start, then every +sendme ACK, the new candidate RTT is compared to the stored EWMA RTT. If the +new RTT is 5000 times larger than the EWMA RTT, then the circuit does not +record that estimate, and does not update BDP or the congestion control +algorithms for that SENDME ack. If the new RTT is 5000 times smaller than the +EWMA RTT, then the circuit uses the globally cached value from above (ie: it +assumes the clock is stalled *only* if there was previously *also* a 0-delta RTT). + +If both ratio checks pass, the globally cached clock stall state is set to +false (no stall), and the RTT value is used.
2.1.2. N_EWMA Smoothing [N_EWMA_SMOOTHING]
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commit aab9efe13b3162db29e72d9364e30bc2955614ab Author: eta tor@eta.st AuthorDate: Mon Aug 8 13:14:45 2022 +0100
prop#324: mention rearranged form for N_EWMA smoothing
The formula used for N_EWMA is actually rearranged in the C tor implementation, in a way that caused the arti reimplementation to be off by a small rounding error until it was corrected.
In order to ensure other implementations don't have this issue, mention the rearranged form and add a requirement that it be used.
(background: https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/525#note_280724...) --- proposals/324-rtt-congestion-control.txt | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/proposals/324-rtt-congestion-control.txt b/proposals/324-rtt-congestion-control.txt index 78b6789..c617983 100644 --- a/proposals/324-rtt-congestion-control.txt +++ b/proposals/324-rtt-congestion-control.txt @@ -167,7 +167,11 @@ reduce the effects of packet jitter. This smoothing is performed using N_EWMA[27], which is an Exponential Moving Average with alpha = 2/(N+1):
- N_EWMA = BDP*2/(N+1) + N_EWMA_prev*(N-1)/(N+1). + N_EWMA = BDP*2/(N+1) + N_EWMA_prev*(N-1)/(N+1) + = (BDP*2 + N_EWMA_prev*(N-1))/(N+1). + +Note that the second rearranged form MUST be used in order to ensure that +rounding errors are handled in the same manner as other implementations.
Flow control rate limiting uses this function
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commit 773fc9200046124e40f40f9367e0c94736353cac Merge: c2e057a dabadcc Author: David Goulet dgoulet@torproject.org AuthorDate: Tue Jan 10 16:17:06 2023 -0500
Merge branch 'tor-gitlab/mr/75'
proposals/324-rtt-congestion-control.txt | 30 ++++++++++++++++-------------- 1 file changed, 16 insertions(+), 14 deletions(-)
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commit 0bacc73d6baee7c2dfd9a7a864163e26f6c121ef Merge: 773fc92 aab9efe Author: David Goulet dgoulet@torproject.org AuthorDate: Tue Jan 10 16:17:25 2023 -0500
Merge branch 'tor-gitlab/mr/83'
proposals/324-rtt-congestion-control.txt | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --cc proposals/324-rtt-congestion-control.txt index 7701230,c617983..952d140 --- a/proposals/324-rtt-congestion-control.txt +++ b/proposals/324-rtt-congestion-control.txt @@@ -169,14 -167,16 +169,18 @@@ reduce the effects of packet jitter This smoothing is performed using N_EWMA[27], which is an Exponential Moving Average with alpha = 2/(N+1):
- N_EWMA = BDP*2/(N+1) + N_EWMA_prev*(N-1)/(N+1). + N_EWMA = BDP*2/(N+1) + N_EWMA_prev*(N-1)/(N+1) + = (BDP*2 + N_EWMA_prev*(N-1))/(N+1). + + Note that the second rearranged form MUST be used in order to ensure that + rounding errors are handled in the same manner as other implementations.
-Flow control rate limiting uses this function +Flow control rate limiting uses this function. + +During Slow Start, N is set to `cc_ewma_ss`, for RTT estimation.
-For both RTT and SENDME BDP estimation, N is the number of SENDME acks -between congestion window updates, divided by the value of consensus +After Slow Start, for both RTT and SENDME BDP estimation, N is the number +of SENDME acks between congestion window updates, divided by the value of consensus parameter 'cc_ewma_cwnd_pct', and then capped at a max of 'cc_ewma_max', but always at least 2:
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