Time between PROBE_RTT

[Adi Gordon]

Hi,

Recently, I have been trying to learn and understand the BBR congestion control algorithm.

I have a question regarding the time between PROBE_RTT states.

In the https://github.com/google/bbr/blob/v2alpha/net/ipv4/tcp_bbr2.c (line 40), it mentions a 10-second duration for entering PROBE_RTT if certain conditions are met:

"*If* a flow has been continuously sending for the entire min_rtt window, and hasn't seen an RTT sample that matches or decreases its min_rtt estimate for 10 seconds, then it briefly enters PROBE_RTT to cut inflight to a minimum value to re-probe the path's two-way propagation delay (min_rtt)."

However, in the same file, in the `bbr_update_min_rtt` function (line 1145), it seems that the flow moves to the PROBE_RTT state if the `probe_rtt_min_us` was not updated for 5 seconds (probe_rtt_win_ms = 5000 line 250).

In the IETF documentation, it mentions an interval of 5 seconds for entering PROBE_RTT:

"Up to once every ProbeRTTInterval = 5 seconds, the flow enters ProbeRTT."

So my question is, why is there a difference in the time interval mentioned between the sources? Specifically, why does the first reference use 10 seconds?

Best regards,

Adi

[Neal Cardwell]

Thanks for pointing out this discrepancy between the comments and the code! The comments mention 10 seconds because that was the time interval used in BBRv1 for both the PROBE_RTT interval and the min_rtt filter window. When we changed the code to use a PROBE_RTT interval of 5 sec in BBRv2 we missed those old comments, and forgot to update them to match the new behavior.  :-) We will update those comments to make them match the code.

Thanks again!

neal

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