Implicitly acknowledging table updates by acknowleding header blocks

[martinthomson]

When you acknowledge a header block, you also implicitly acknowledge the largest reference used in that header block. You can't acknowledge it otherwise.

Should we require that an encoder track this information so that a decoder can reduce the number of acknowledgments that it sends? It's more complicated to do that, but it can save some bytes and the associated churn. I haven't implemented delayed acknowledgments for QPACK, but this would possibly make that more effective.

[afrind]

The encoder must already know the set of headers referenced in a block, so it can trivially compute the largest reference without keeping any additional state. I think we can say that acknowledging a header block implicitly acknowledges its largest reference and save the bytes on the wire.

[MikeBishop]

Given that the entries are usually added to the table precisely because a header block is going to use them, this would probably cover acknowledgements most of the time. I'd be okay with that; you're already having to track which entries the block referenced, so this really isn't much additional state, if any. Just an additional action with that state when you pull a block out of the queue.

[martinthomson]

It sounds like this is OK. However, I would caution that if you are avoiding blocking for new insertions, then this could deadlock. Decoders need to be aware of that.

[afrind]

@martinthomson : are you saying that a decoder can't rely exclusively on this ack mechanism or risk deadlock -- eg: if largest reference is less than base index the decoder should be sure to ack the remainder. Or something else?

[martinthomson]

My point was that unless we require encoders to understand this "implicit" or indirect acknowledgement, then we risk them getting into a situation where the decoder thinks that it acknowledged something, but the encoder still believes it to be outstanding.

[afrind]

Yes, if we allow decoders to do this then it's a MUST for encoders to support it.