Re: [quicwg/base-drafts] ACK generation recommendation (#3304)

[Christian Huitema <[e-mail]>]

I have been experimenting with various ACK reduction schemes in picoquic, larely as part of experimentation with satellite link. There are two distinct motivations: CPU reduction, and asymmetric links in which the return path is much narrower than the data path.

In the asymmetric scenario, acking every two packets leads to a lot queuing and losses on the return path. The end to end delay becomes very large, and the default congestion controller becomes very confused. In these paths, ack reduction is essential. The right thing to do is probably to write control loop that increases the ACK interval when detecting ACK congestion. I have not written that yet, and did something simpler: just go to interval=10 when the delay is long and the bandwidth is high enough.

The actual algorithm in Picoquic is:
```
if more than 128 packets received:
    if rtt_min > 100ms and receive_rate > 10MB/s:
        ack_gap = 10
    else if no_holes_detected
        ack_gap = 4
else if FIN received on last active stream:
    ack_gap=1
else
    ack_gap=2
```
The max_ack_delay is set to:
```
    max( 1ms, min(25ms, RTT/4))
```


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