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prop#324: mention rearranged form for N_EWMA smoothing

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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_2807244)
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eta 2022-08-08 13:14:45 +01:00
parent b5e2002983
commit aab9efe13b
1 changed files with 5 additions and 1 deletions
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proposals/324-rtt-congestion-control.txt
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@ -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