[tcpm] A question about PRR/loss recovery

Yi Huang <huanyi@microsoft.com> Fri, 16 April 2021 19:08 UTC

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From: Yi Huang <huanyi@microsoft.com>
To: "tcpm@ietf.org" <tcpm@ietf.org>
Thread-Topic: A question about PRR/loss recovery
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Date: Fri, 16 Apr 2021 19:08:28 +0000
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Subject: [tcpm] A question about PRR/loss recovery
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Hi tcpm and PRR authors,

We are revisiting PRR implementation in Windows and I noticed there might be a problem in our implementation (and it is actually not specific to PRR).

PRR algorithm computes a sndcnt on each ACK during loss recovery and thus use it to accurately regulate bytes in-flight.

In the RFC (6937 or bis), I see this statement and note "local" and "response to each ACK".
   We introduce a local variable "sndcnt", which indicates exactly how
   many bytes should be sent in response to each ACK.

In Windows' TCP implementation, new data can be also sent out inline during loss recovery from app send context (app calling send()) and they are not regulated by sndcnt/PRR. Instead, we just check if CWND allows us to send in this case. So, effectively, we use two variables controlling the number of bytes in flight during loss recovery, i.e. sndcnt and CWND. It apparently seems a bad behavior because it ignores PRR's sendcnt but I just want to double check with the PRR authors and the community.

Thanks,
Yi