Re: [quicwg/base-drafts] Improve ACK_ECN frame encoding (e.g., use bit-vector) (#1439)

[Kazuho Oku <[e-mail]>]

> If the concern is too many bits in the frame for high throughput scenarios, the bitmask could be capped at 64. In those high throughput cases, you should have enough ACKs to reassemble the markings, it's just that you wouldn't have a whole lot of redundancy for the marking signal. That bounds the size of the frame.

@martinthomson Or, considering the fact that we are requiring recipients of the ECN-CE flag to send back the information as soon as possible, and that we also want to restrict the frequency of feeding in the CE event to the congestion controller to maybe once per RTT, defining an ACK_WITH_CE frame that carries just one PN that had the CE bit set might be sufficient.

```fe
+ 0                   1                   2                   3
		
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
+|                     Largest Acknowledged (i)                ...
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
+|                          ACK Delay (i)                      ...
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
+|                       ACK Block Count (i)                   ...
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
+|                          ACK Blocks (*)                     ...
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
+|                         CE ACK Index (i)                    ...
		
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
		
```

In case of this example, "CE ACK Index" will designate the index of the PN among the PNs that were carried using the ACK_WITH_CE frame. Ordinary ACK frame can be used when none of the packets being acknowledged had the ECN-CE bit set.

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