Re: [quicwg/base-drafts] Make in_flight the criterion for declaring loss (#2104)

[ianswett <[e-mail]>]

I think strictly speaking, neither option may be correct, unfortunately.

Packets that are in_flight count towards bytes in flight, so need to be removed from flight eventually.  And they may never get acknowledged.  The transport and recovery drafts are either a bit unclear or in conflict about whether one should ACK a PADDING only packet that arrives out of order.

The transport draft currently says "To avoid creating an indefinite feedback loop, an endpoint MUST NOT send an ACK frame in response to a packet containing only ACK or PADDING frames, even if there are packet gaps which precede the received packet."

In recovery it says "Out-of-order packets SHOULD be acknowledged more quickly, in order to accelerate loss recovery. The receiver SHOULD send an immediate ACK when it receives a new packet which is not one greater than the largest received packet number."

I think this is an excellent reason to move the text about "Generating Acknowlegements" into transport, because having it in two places increases the chances of inconsistencies.

I tend to think that the best thing to do is take Martin's change, but also say that if an in-flight packet arrives out of order, an immediate acknowledgement should be sent, because it counts towards bytes in flight.

Again, having a packet that counts towards bytes in flight, but doesn't generate an ACK, generates a bunch of edge cases I dislike, but I think we're stuck with it.

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