Brandon Schlinker dc0b888cfa Improve / fix smoothed_rtt calculation ...

Summary:
RFC9002 states the following in section 5.3

> 5.3. Estimating smoothed_rtt and rttvar
> Therefore, when adjusting an RTT sample using peer-reported acknowledgment delays, an endpoint [...] MUST NOT subtract the acknowledgment delay from the RTT sample if the resulting value is smaller than the min_rtt. This limits the underestimation of the smoothed_rtt due to a misreporting peer

However, today we subtract the ACK delay from an RTT sample used to estimate `smoothed_rtt` if it is the first RTT sample, as we will not yet have a minimum. This can lead to underestimation of `smoothed_rtt` if the peer is misreporting, and furthermore can create a situation during which `smoothed_rtt` is lower than `min_rtt`.

Resolving by not allowing `smoothed_rtt` to be adjusted by ACK delay for the first RTT sample, since doing so would lead to `smoothed_rtt` being lower than `min_rtt`.

Reviewed By: jbeshay

Differential Revision: D34792648

fbshipit-source-id: e0d6207ff73cdc13e0b40193abd27abce7142499

2022-03-28 23:26:30 -07:00

39 KiB

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