Subject: The timestamps in the ACK list

From: Christian Huitema <[email-masked]>
Date: Wed, 6 Sep 2017 12:20:54 -0700

There is already an open issue regarding the role of the time stamp list
in the ACK frame, https://github.com/quicwg/base-drafts/issues/698. But
there seems to be a more fundamental issue, which is: why do we need
these time stamps at all?

The ACK already convey the "largest received PN" and the delay between
when that was received and when the ACK was sent. That gives a pretty
good estimate of the RTT of the connection, several times per RTT.
That's certainly enough for computing retransmission timers, and
probably enough even for delay based congestion control algorithms.

The only use I can think of is measurement of out of order deliveries.
Is that the goal? Because if it is, we certainly need to say so, so the
receivers put the time stamps of the right packets in the list. And we
may need to revisit the ACK structure, because having something like
"max out of order sequence number delta" and "max out of order delivery
delay" would probably be much easier to understand.

-- 
Christian Huitema