Re: [quicwg/base-drafts] Rewrite section on ack range limiting (#3315)

Martin Thomson <notifications@github.com> Thu, 02 January 2020 22:49 UTC

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Date: Thu, 02 Jan 2020 14:49:13 -0800
From: Martin Thomson <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Rewrite section on ack range limiting (#3315)
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martinthomson commented on this pull request.



> -contribute to the ACK frame size.  When the receiver is only sending
-non-ack-eliciting packets, it can bundle a PING or other small ack-eliciting
-frame with a fraction of them, such as once per round trip, to enable
-dropping unnecessary ACK ranges and any state for previously sent packets.
-The receiver MUST NOT bundle an ack-eliciting frame, such as a PING, with all
-packets that would otherwise be non-ack-eliciting, in order to avoid an
-infinite feedback loop of acknowledgements.
-
-To limit receiver state or the size of ACK frames, a receiver MAY limit the
-number of ACK Ranges it sends.  A receiver can do this even without receiving
-acknowledgment of its ACK frames, with the knowledge this could cause the sender
-to unnecessarily retransmit some data.  Standard QUIC algorithms
-({{QUIC-RECOVERY}}) declare packets lost after sufficiently newer packets are
-acknowledged.  Therefore, the receiver SHOULD repeatedly acknowledge newly
-received packets in preference to packets received in the past.
+A receiver MAY limit the number of ACK Ranges it sends to limit receiver state

Yeah, this might be better as an axiomatic statement.  How about:

> A receiver limits the number of ACK Ranges ({{ack-ranges}}) it remembers and sends in ACK frames, both to limit the size of ACK frames and to avoid resource exhaustion.  A receiver SHOULD stop tracking ACK Ranges after receiving acknowledgments for ACK frames containing ACK Ranges.
>
> It is possible that retaining many ACK Ranges could cause an ACK frame to become too large.  A receiver can discard unacknowledged ACK Ranges to limit ACK frame size.  This is necessary if an ACK frame would be too large to fit in a packet, however receivers MAY also limit ACK frame size further to preserve space for other frames.
>
> When discarding unacknowledged ACK Ranges, a receiver MUST retain the largest acknowledged packet number.  A receiver SHOULD retain ACK Ranges containing newly received packets or higher-numbered packets.

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