[quicwg/base-drafts] Reduce restrictions on valid RTT samples (#2568)

Jana Iyengar <notifications@github.com> Sat, 30 March 2019 20:41 UTC

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Subject: [quicwg/base-drafts] Reduce restrictions on valid RTT samples (#2568)
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The RTT estimator currently does not use RTT samples if the largest_acked is not ack-eliciting. This can lead to the endpoint missing RTT samples unnecessarily, in two ways.

First, checking only for the largest_acked to be ack-eliciting means that we ignore other newly acked packets that were ack-eliciting. The peer is expected to send an ack within max_ack_delay if it has received any ack-eliciting packets, not just the largest acked one. This is probematic if the connection has a pattern that results in more non-ack-eliciting packets being the largest_acked. I propose that we change the condition for a valid RTT sample to be if _any_ newly acked is ack-eliciting (in addition to the ACK frame carrying a new largest_acked).

Second, non-ack-eliciting packets are a perfectly fine source of RTT samples, with the caveat that ack_delay might be larger than max_ack_delay; a receiver that has only received non-ack-eliciting packets might delay acknowledging these packets much longer than otherwise. I propose that we don't drop all RTT samples associated with non-ack-eliciting packets, but only those with unusable ack_delay. Basically, I propose that we use the RTT sample if ack_delay < max_ack_delay.

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