Re: [quicwg/base-drafts] Fix to ECN section regarding validation (#2113)

[Benjamin Saunders <[e-mail]>]

Ralith commented on this pull request.

It would also be helpful to briefly mention that the local reference count is maintained (by copying verified ECN blocks from ACKs?)

>  
 * The total increase in ECT(0), ECT(1), and CE counters reported in the ACK
   frame MUST be at least the total number of QUIC packets newly acknowledged in
-  this ACK frame.
+  this ACK frame. Detects if the network remark ECT(0), ECT(1) or CE to

```suggestion
  this ACK frame. Detects if the network remarks ECT(0), ECT(1) or CE to
```

>  
 * The total increase in ECT(0), ECT(1), and CE counters reported in the ACK
   frame MUST be at least the total number of QUIC packets newly acknowledged in
-  this ACK frame.
+  this ACK frame. Detects if the network remark ECT(0), ECT(1) or CE to
+  Not-ECT.
+
+If a sender receives an ACK that contains no new acknowledgments, for example
+due to reordering of the ACKs, then ECN counter comparison SHOULD NOT be
+performed. Also if sender do not have state to determine if a particular PSN

```suggestion
performed. If the sender does not have state to determine if a particular PSN
```

>  
 * The total increase in ECT(0), ECT(1), and CE counters reported in the ACK
   frame MUST be at least the total number of QUIC packets newly acknowledged in
-  this ACK frame.
+  this ACK frame. Detects if the network remark ECT(0), ECT(1) or CE to
+  Not-ECT.
+
+If a sender receives an ACK that contains no new acknowledgments, for example
+due to reordering of the ACKs, then ECN counter comparison SHOULD NOT be
+performed. Also if sender do not have state to determine if a particular PSN
+is newly acknowledge or not, then the comparison SHOULD NOT be performed.

The implication here seems to be that an implementation should retain extra state to distinguish between a packet that is deemed lost but not yet acknowledged and a packet that was previously acknowledged. Is that intended?

If these two cases are instead conflated, e.g. by forgetting about sent packets once either acknowledged or deemed lost, then at worst the number of newly acknowledged packets in an ACK would sometimes be under-counted, and a verification with a higher rate of false positives seems preferable to going to extra effort to decide to skip verification outright.

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