Re: [quicwg/base-drafts] What to ack in an ACK frame (#764)

[martinduke <[e-mail]>]

Yes, it is clear that we should not include Ack blocks that have already been acked, and that we SHOULD include timestamps exactly once per packet. There is no stated recommendation on in-flight acks. Possibilities:
    (i) Do not repeat them in subsequent acks
    (ii) Repeat them only if they do not require additional ack blocks
    (iii) Repeat them only if they require neither additional ack blocks nor longer block lengths.
    (iv) Always repeat them until acked

These have significant pros and cons. The benefits of not adding more ACK overhead are obvious. The benefits of repeating acks are obvious. However,

(A) Repeating in-flight acks may increase largest_acked so that we cannot include certain unsent timestamps.
(B) _not_ repeating in-flight acks may create packet number gaps too large to send subsequent ack blocks.

These are subtle issues, and the algorithms to optimize the outcomes are not simple at all. Some sort of implementation guidance, or even narrowing of what is allowed in the frame, would be helpful.

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