Re: [quicwg/base-drafts] Rework section on persistent congestion (#3961)

[ianswett <[e-mail]>]

@ianswett approved this pull request.



> +
+This duration allows a sender to send as many packets, including some in
+response to PTO expiration, as TCP does with Tail Loss Probe ({{RACK}}), before
+establishing persistent congestion, as TCP does with a Retransmission Timeout
+({{?RFC5681}}).
+
+The RECOMMENDED value for kPersistentCongestionThreshold is 3, which is
+approximately equivalent to two TLPs before an RTO in TCP.
+
+This design does not use consecutive PTO events to establish persistent
+congestion, since application patterns impact PTO expirations. For example, a
+sender that sends small amounts of data with silence periods between them
+restarts the PTO timer every time it sends, potentially preventing the PTO timer
+from expiring for a long period of time, even when no acknowledgments are being
+received. The use of a duration enables a sender to establish persistent
+congestion without depending on the occurrence of PTOs.

We use expiry elsewhere, so maybe this is slightly clearer?

```suggestion
congestion without depending on the expiry of PTOs.
```

> +This design does not use consecutive PTO events to establish persistent
+congestion, since application patterns impact PTO expirations. For example, a
+sender that sends small amounts of data with silence periods between them
+restarts the PTO timer every time it sends, potentially preventing the PTO timer
+from expiring for a long period of time, even when no acknowledgments are being
+received. The use of a duration enables a sender to establish persistent
+congestion without depending on the occurrence of PTOs.
+
+### Establishing Persistent Congestion
+
+A sender establishes persistent congestion on receiving an acknowledgement if at
+least two ack-eliciting packets are declared lost, and:
+
+* a prior RTT sample existed when both packets were sent;
+
+* the duration between the send times of these two packets exceeds the

I think this is still ambiguous if more than 2 packets are lost, so I'm reusing language below.

```suggestion
* the duration between the send times of the oldest and newest lost packet exceeds the
```

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