added to BBR FAQ: where does the value of the BBR Drain pacing_gain come from?

[Neal Cardwell]

Hi,

We have just posted to the BBR FAQ an analytic derivation explaining the pacing gain used in BBR's "Drain" state, explaining why it's the reciprocal of the Startup cwnd gain. Quoting from the new entry in the FAQ:

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Where does the value of the BBR DRAIN pacing_gain come from?

In a nutshell, the BBR DRAIN pacing gain is derived to be the pacing gain that is selected to try to drain the queue created by STARTUP in one packet-timed round trip.

Here is a detailed derivation:
 https://github.com/google/bbr/blob/master/Documentation/startup/gain/analysis/bbr_drain_gain.pdf

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best,

neal