Re: [quicwg/base-drafts] Move Generating Acknowledgements to Transport (#2916)

Nick Banks <> Wed, 11 September 2019 19:58 UTC

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Date: Wed, 11 Sep 2019 12:58:18 -0700
From: Nick Banks <>
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Subject: Re: [quicwg/base-drafts] Move Generating Acknowledgements to Transport (#2916)
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nibanks commented on this pull request.

-Packets containing only ACK frames are not congestion controlled, so there are
-limits on how frequently they can be sent.  An endpoint MUST NOT send more than
-one ACK-frame-only packet in response to receiving an ACK-eliciting packet
-(one containing frames other than ACK and/or PADDING).  An endpoint MUST NOT
-send a packet containing only an ACK frame in response to a non-ACK-eliciting
-packet (one containing only ACK and/or PADDING frames), even if there are
-packet gaps which precede the received packet. Limiting ACK frames avoids an
-infinite feedback loop of acknowledgements, which could prevent the connection
-from ever becoming idle. However, the endpoint acknowledges non-ACK-eliciting
-packets when it sends an ACK frame.
+A receiver's delayed acknowledgment timer SHOULD NOT exceed the current RTT
+estimate or the value it indicates in the `max_ack_delay` transport parameter.
+This ensures an acknowledgment is sent at least once per RTT when packets
+needing acknowledgement are received.  The sender can use the receiver's
+`max_ack_delay` value in determining timeouts for timer-based retransmission.

Does this text `SHOULD NOT exceed the current RTT estimate` mean the peer could/should use a min of the max_ack_delay and the current RTT when computing probe timeouts?
PTO = smoothed_rtt + max(4*rttvar, kGranularity) + min(max_ack_delay, smoothed_rtt)

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