Re: [quicwg/base-drafts] are flow control frames really idempotent? (#1612)

[Kazuho Oku <[e-mail]>]

@marten-seemann 
> I didn't say it's impossible, but it would add a lot of code for saving a tiny number of bytes, occasionally.

FWIW, my understanding is it's actually simple to implement the optimal way. What you would do is:
* transmit the maximum when `max(acked_max, inflight_max)` is below the threshold
* when receiving an ack for the max frame, update the `acked_max` and `inflight_max`
* when deeming a packet that contained the max frame as lost, reset `inflight_max` to `acked_max` if the number of max frames in flight is zero, otherwise you leave `inflight_max` as-is. Then, you check if transmission of the maximum is necessary by referring to the rule above

This strategy works well in QUIC v1, because packets are deemed lost in the order they are sent, and there is a guarantee in the logic that packets that are sent later contain a equal-to or greater maximum. The logic is not "perfect" when multipath is involved, though I would assume that it would work well enough in practice.

Regarding the issue, I am fine with allowing receivers to raise protocol error when they see the issue. I would oppose to making that a MUST though.

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