Re: [quicwg/base-drafts] Complicated Retransmission Corner Cases (#765)

[janaiyengar <[e-mail]>]

So, GQUIC tracks delivery state of packets, in part because loss recovery and congestion control work at these scales. As I wrote to Martin earlier, GQUIC has a data structure that holds per-packet metadata about what frames were contained in a sent packet, and a deque of these per-packet structs. Once acked you can remove the entry. When marked as lost, we hang on to it with the packet number of the retransmission (which is a new packet in the same data structure), to allow spurious rtx detection, and to prevent future retransmissions. We also remember one past transmission in a chain of retransmitted packets, to avoid having an indefinitely long chain of past transmissions. (You'd want to be careful around removing old state around TLP, just to be safe.)

@MikeBishop In addition to reliability, an ack also contributes towards RTT measurement, and a late ack is a very useful signal, which moves the RTT estimate to be higher than it otherwise would have been. I would strongly recommend holding on to packets for a while after it is marked as lost.

@martinduke I think you can make the implementation a ton easier if you don't try to re-bundle retransmissions. Meaning, simply send retransmittable frames from a lost packet in a retransmission, without trying to pack it maximally with mixing and matching. Yes, it's a pain if the PTMU reduces mid-connection, but that situation is rare enough (I've never really seen it in practice) that I'm comfortable in punting on that in practice. Does that make sense?

I think it's a good idea to keep the data in the streams and have frame-level metadata per-packet that has offsets into the stream about data that has been sent. This allows you to keep one copy of stream data without having to maintain a separate copy for sent data.

@martinduke I'd love to look at a PR if you could send one along.

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