Re: [quicwg/base-drafts] Unify TLP and RTO into Probe Timeout (#2114)

[Praveen Balasubramanian <[e-mail]>]

pravb commented on this pull request.



> -
-QUIC includes MaxAckDelay in all probe timeouts, because it assumes the ack
-delay may come into play, regardless of the number of ack-eliciting
-packets in flight. TCP's TLP assumes if at least 2 ack-eliciting packets are
-in flight, acks will not be delayed.
-
-A PTO value of at least 1.5*SRTT ensures that the ACK is overdue.  The 1.5 is
-based on {{?TLP}}, but implementations MAY experiment with other constants.
-
-To reduce latency, it is RECOMMENDED that the sender set and allow the TLP timer
-to fire twice before setting an RTO timer. In other words, when the TLP timer
-expires the first time, a TLP packet is sent, and it is RECOMMENDED that the TLP
-timer be scheduled for a second time. When the TLP timer expires the second
-time, a second TLP packet is sent, and an RTO timer SHOULD be scheduled {{rto}}.
+~~~
+PTO = max(smoothed_rtt + 4*rttvar + max_ack_delay, kGranularity)

Sorry just noticed another difference. If peer advertises max_ack_delay as 0 (which seems to be legal), then first PTO becomes Srtt + 4 * RttVar, whereas in TCP the first PTO is 2* Srtt. If variance is low this may be too aggressive? Should we also do PTO = max (PTO, 2 *Srtt) ?

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