Re: [quicwg/base-drafts] No RTT samples, no persistent congestion (#3889)

[Kazuho Oku <[e-mail]>]

@kazuho commented on this pull request.



> @@ -1581,6 +1588,10 @@ Invoked when DetectAndRemoveLostPackets deems packets lost.
 
 ~~~
    InPersistentCongestion(lost_packets):

@marten-seemann IIUC, one trivial way of implementing `InPersistentCongestion` is as follows:
* Retain the entries in sentmap (the list of packets being sent) for at least 3 PTO.
* When receiving an acknowledgement, mark a hole in the sentmap for the packet being acked.
* When a loss is detected in a packet number space, check if that loss spans across more than 3 PTO, by traversing the sentmap. Then, if did, check the sentmap of other packet number spaces (by traversing through those sentmaps) to see if any ACK-eliciting packet sent in that period has been acked.

This approach might sound too trivial, as it is O(N) where N is the number of packets being lost (or in case there are multiple PN spaces in action, the number of inflight packets in other PN spaces). But in practice, I think this approach is sufficient in terms of performance, because N would be like 3 for the current packet number space, and because we would not be sending that many packets on Initial and Handshake packet number spaces.

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