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[tcpm] CWND increase in disordered state in Linux?

Ingemar Johansson S <ingemar.s.johansson@ericsson.com> Tue, 03 September 2024 09:47 UTC

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From: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
To: Neal Cardwell <ncardwell@google.com>
Thread-Topic: CWND increase in disordered state in Linux?
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Date: Tue, 03 Sep 2024 09:47:45 +0000
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Subject: [tcpm] CWND increase in disordered state in Linux?
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Hi
Hope you are fine

I am working on improving the TCP stack in our 5G system simulator to pass all relevant packetdrill tests.
One thing that I I try to figure out is the behavior in CA_Disordered state. One such example is the prr-ss-10pkt-lost-1.pkt test.
What I see in the current implementation is that cwnd increases from 10 to 12 when it is in disordered state.
When it gets into CA_Recovery however the ssthresh should be set to 7 but it is set to 8 because int(12*0.7) = 8. So I guess ssthresh should be set based on cwnd from when the state was last in CA_Open ?.

What confuses me is that RFC3042 says that CWND should not be modified when in disordered state, however the explanation text in tcp_may_raise_cwnd() seem to indicate that CWND can increase as long as new data is SACKed, right?.

/Ingemar
PS: CC:ed the tcpm group in case this topic is of more general interest

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