Code change:
9. <del> There are tradeoffs inherent in a sender sending an ACK_FREQUENCY frame to the receiver.  As such it is recommended that implementers experiment with different strategies and find those which best suit their applications and congestion controllers.  There are, however, noteworthy considerations when devising strategies for sending ACK_FREQUENCY frames. </del> <ins> This section provides some guidance on a sender's choice of acknowledgment frequency and discusses some additional considerations.  Implementers can select an appropriate strategy to meet the needs of their applications and congestion controllers. </ins> 9.1. <del> A sender relies on receipt of acknowledgements to determine the amount of data in flight and to detect losses, e.g. when packets experience reordering, see QUIC-RECOVERY.  Consequently, how often a receiver sends acknowledgments determines how long it takes for losses to be detected at the sender. </del> <ins> A sender needs to be responsive to notifications of congestion, such as a packet loss or an ECN CE marking.  Also, window-based congestion controllers that strictly adhere to packet conservation, such as the one defined in QUIC-RECOVERY, rely on receipt of acknowledgments to send additional data into the network, and will suffer degraded performance if acknowledgments are delayed excessively. To enable a sender to respond to potential network congestion, a sender SHOULD cause a receiver to send an acknowledgement at least once per RTT if there are unacknowledged ack-eliciting packets in flight.  A sender can accomplish this by sending an IMMEDIATE_ACK frame once per round-trip time (RTT), or it can set the Ack-Eliciting Threshold and Request Max Ack Delay values to be no more than a congestion window and an estimated RTT, respectively. </ins> 9.2. <del> Many congestion control algorithms have a startup mechanism during the beginning phases of a connection.  It is typical that in this period the congestion controller will quickly increase the amount of data in the network until it is signalled to stop.  While the mechanism used to achieve this increase varies, acknowledgments by the peer are generally critical during this phase to drive the congestion controller's machinery.  A sender can send ACK_FREQUENCY frames while its congestion controller is in this state, ensuring that the receiver will send acknowledgments at a rate which is optimal for the the sender's congestion controller. </del> <ins> Receiving an acknowledgement can allow a sender to release new packets into the network.  If a sender is designed to rely on the timing of peer acknowledgments ("ACK clock"), delaying acknowledgments can cause undesirable bursts of data into the network.  A sender MUST limit such bursts.  In keeping with Section 7.7 of QUIC-RECOVERY, a sender can either employ pacing or cause a receiver to send an acknowledgement for at least each initial congestion window of received data. </ins> 9.3. <del> Congestion controllers that are purely window-based and strictly adherent to packet conservation, such as the one defined in QUIC- RECOVERY, rely on receipt of acknowledgments to move the congestion window forward and send additional data into the network.  Such controllers will suffer degraded performance if acknowledgments are delayed excessively.  Similarly, if these controllers rely on the timing of peer acknowledgments (an "ACK clock"), delaying acknowledgments will cause undesirable bursts of data into the network. </del> <ins> Acknowledgements are fundamental to reliability in QUIC. Consequently, delaying or reducing the frequency of acknowledgments can cause loss detection at the sender to be delayed. A QUIC sender detects loss using packet thresholds on receiving an acknowledgement (Section 6.1.1 of QUIC-RECOVERY); delaying the acknowledgement therefore delays this method of detecting losses. Reducing acknowledgement frequency reduces the number of RTT samples that a sender receives (Section 5 of QUIC-RECOVERY), making a sender's RTT estimate less responsive to changes in the path's RTT. As a result, any mechanisms that rely on an accurate RTT estimate, such as time-threshold loss detection (Section 6.1.2 of QUIC- RECOVERY) or Probe Timeout (Section 6.2 of QUIC-RECOVERY), will be less responsive to changes in the path's RTT, resulting in either delayed or unnecessary packet transmissions. To limit these consequences of reduced acknowledgement frequency, a sender SHOULD cause a receiver to send an acknowledgement at least once per RTT if there are unacknowledged ack-eliciting packets in flight.  A sender can accomplish this by sending an IMMEDIATE_ACK frame once per round-trip time (RTT), or it can set the Ack-Eliciting Threshold and Request Max Ack Delay values to be no more than a congestion window and an estimated RTT, respectively. A sender might use timers to detect loss of PMTUD probe packets.  A sender SHOULD bundle an IMMEDIATE_ACK frame with any PTMUD probes to avoid triggering such timers. </ins> 9.4.

Related discussion:
text: I'd suggest 5/4, but otherwise agree with you NAME
Should the Max ACK Delay be restricted to 1/8 or 1/4 RTT as there is inherent 1RTT time spent while sending data and receiving ACKs? That would make a total of 1 + 1/8 or 1 +1/4 RTT total/
I think this is best as a sender-side SHOULD, since it knows what time threshold it's using. For example, we use 1/4 RTT, but have considered making it adaptive(ie: 1/16 RTT to 1 RTT) based on observed reordering. Also, if your RTT is 100us, and your timers have coarser granularity, 1/8 RTT or 1/4 RTT just doesn't make sense. I'm not sure what does, because I'm not an expert in datacenter networking, but that's why I think this is a SHOULD, not a MUST. Of course, it's also unenforceable, which is a good reason for a SHOULD.
+1 to SHOULD. The only argument I might disagree with NAME is that it is unenforceable (IMO a receiver can enforce by capping Ack Delay to a fraction of the RTT that the receiver has observed). But I do not think there's any reason to require the receiver to enforce, considering the fact that a sender can send as many packets as it likes, regardless of ACKs the receiver sends (or lack of).
I am fine with SHOULD and with a mention of how a sender would compute Max ACK Delay when he sets ignore-order = true. An example is fine too.
I believe this is the same issue as
There are two things that are impacted by setting a high value for ack delay: congestion control and time-threshold loss detection. On congestion control, we need some protection for the network, so we should recommend something. On loss detection, a sender that chooses and sets an ack delay higher than the time-threshold loss detection time period risks triggering unnecessary retransmissions.