Is the snd_cwnd_clamp in bbr_set_cwnd() configurable?
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Tao Tse
Jul 30, 2020, 11:13:55 PM7/30/20
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Hi,
According to the source code of bbr_set_cwnd() in tcp_bbr.c, the final cwnd value is being limited by the snd_cwnd_clamp parameter.
Does anyone know that is the snd_cwnd_clamp configurable in userspace? If yes, how? If no, how and where does the value be decided?
Thanks!
--
xtao
Neal Cardwell
Jul 31, 2020, 11:05:35 AM7/31/20
to Tao Tse, BBR Development
That cwnd clamp can be set from user space using the "ip" tool, using
the "route" command, which sets the maximum cwnd for a particular
route.
The documentation for this is in the man page for ip-route, e.g.:
https://www.systutorials.com/docs/linux/man/8-ip-route/
cwnd NUMBER (Linux 2.3.15+ only)
the clamp for congestion window. It is ignored if the lock flag
is not used.
From your source code cross-reference:
https://elixir.bootlin.com/linux/latest/C/ident/snd_cwnd_clamp
...the references to focus on are the ones in tcp_metrics.c, where
snd_cwnd_clamp comes from the TCP_METRIC_CWND:
tp->snd_cwnd_clamp = tcp_metric_get(tm, TCP_METRIC_CWND);
...and TCP_METRIC_CWND comes from the routing attribute RTAX_CWND
tm->tcpm_vals[TCP_METRIC_CWND] = dst_metric_raw(dst, RTAX_CWND);
...and that routing attribute is set from the "ip" tool, using the
"route" command using the "cwnd" attribute:
https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/ip/iproute.c?h=v5.7.0#n47
[RTAX_CWND] = "cwnd",
best,
neal
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the "route" command, which sets the maximum cwnd for a particular
route.
The documentation for this is in the man page for ip-route, e.g.:
https://www.systutorials.com/docs/linux/man/8-ip-route/
cwnd NUMBER (Linux 2.3.15+ only)
the clamp for congestion window. It is ignored if the lock flag
is not used.
From your source code cross-reference:
https://elixir.bootlin.com/linux/latest/C/ident/snd_cwnd_clamp
...the references to focus on are the ones in tcp_metrics.c, where
snd_cwnd_clamp comes from the TCP_METRIC_CWND:
tp->snd_cwnd_clamp = tcp_metric_get(tm, TCP_METRIC_CWND);
...and TCP_METRIC_CWND comes from the routing attribute RTAX_CWND
tm->tcpm_vals[TCP_METRIC_CWND] = dst_metric_raw(dst, RTAX_CWND);
...and that routing attribute is set from the "ip" tool, using the
"route" command using the "cwnd" attribute:
https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/ip/iproute.c?h=v5.7.0#n47
[RTAX_CWND] = "cwnd",
best,
neal
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Tao Tse
Aug 3, 2020, 4:23:35 AM8/3/20
to BBR Development
Thank you, neal!
Further more, what's the default value of
[RTAX_CWND]? I can't find the code about it, where is it defined?
--
xtao
Neal Cardwell
Aug 3, 2020, 9:04:06 AM8/3/20
to Tao Tse, BBR Development
My quick reading is that the default for RTAX_CWND is to not have an RTAX_CWND set. There does not seem to be a specific default value of RTAX_CWND.
best,
neal
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