Re: TCP Success Story (was Re: TCP_RACK, TCP_BBR, and firewalls)

From: Junho Choi <junho.choi_at_gmail.com>
Date: Thu, 18 Jul 2024 13:00:38 UTC
Alan - this is a great result to see. Thanks for experimenting.

Just curious why bbr and rack don't co-exist? Those are two separate things.
Is it a current bug or by design?

BR,

On Thu, Jul 18, 2024 at 5:27 AM <tuexen@freebsd.org> wrote:

> > On 17. Jul 2024, at 22:00, Alan Somers <asomers@freebsd.org> wrote:
> >
> > On Sat, Jul 13, 2024 at 1:50 AM <tuexen@freebsd.org> wrote:
> >>
> >>> On 13. Jul 2024, at 01:43, Alan Somers <asomers@FreeBSD.org> wrote:
> >>>
> >>> I've been experimenting with RACK and BBR.  In my environment, they
> >>> can dramatically improve single-stream TCP performance, which is
> >>> awesome.  But pf interferes.  I have to disable pf in order for them
> >>> to work at all.
> >>>
> >>> Is this a known limitation?  If not, I will experiment some more to
> >>> determine exactly what aspect of my pf configuration is responsible.
> >>> If so, can anybody suggest what changes would have to happen to make
> >>> the two compatible?
> >> A problem with same symptoms was already reported and fixed in
> >> https://reviews.freebsd.org/D43769
> >>
> >> Which version are you using?
> >>
> >> Best regards
> >> Michael
> >>>
> >>> -Alan
> >
> > TLDR; tcp_rack is good, cc_chd is better, and tcp_bbr is best
> >
> > I want to follow up with the list to post my conclusions.  Firstly
> > tuexen@ helped me solve my problem: in FreeBSD 14.0 there is a 3-way
> > incompatibility between (tcp_bbr || tcp_rack) && lro && pf.  I can
> > confirm that tcp_bbr works for me if I either disable LRO, disable PF,
> > or switch to a 14.1 server.
> >
> > Here's the real problem: on multiple production servers, downloading
> > large files (or ZFS send/recv streams) was slow.  After ruling out
> > many possible causes, wireshark revealed that the connection was
> > suffering about 0.05% packet loss.  I don't know the source of that
> > packet loss, but I don't believe it to be congestion-related.  Along
> > with a 54ms RTT, that's a fatal combination for the throughput of
> > loss-based congestion control algorithms.  According to the Mathis
> > Formula [1], I could only expect 1.1 MBps over such a connection.
> > That's actually worse than what I saw.  With default settings
> > (cc_cubic), I averaged 5.6 MBps.  Probably Mathis's assumptions are
> > outdated, but that's still pretty close for such a simple formula
> > that's 27 years old.
> >
> > So I benchmarked all available congestion control algorithms for
> > single download streams.  The results are summarized in the table
> > below.
> >
> > Algo    Packet Loss Rate    Average Throughput
> > vegas   0.05%               2.0 MBps
> > newreno 0.05%               3.2 MBps
> > cubic   0.05%               5.6 MBps
> > hd      0.05%               8.6 MBps
> > cdg     0.05%               13.5 MBps
> > rack    0.04%               14 MBps
> > htcp    0.05%               15 MBps
> > dctcp   0.05%               15 MBps
> > chd     0.05%               17.3 MBps
> > bbr     0.05%               29.2 MBps
> > cubic   10%                 159 kBps
> > chd     10%                 208 kBps
> > bbr     10%                 5.7 MBps
> >
> > RACK seemed to achieve about the same maximum bandwidth as BBR, though
> > it took a lot longer to get there.  Also, with RACK, wireshark
> > reported about 10x as many retransmissions as dropped packets, which
> > is suspicious.
> >
> > At one point, something went haywire and packet loss briefly spiked to
> > the neighborhood of 10%.  I took advantage of the chaos to repeat my
> > measurements.  As the table shows, all algorithms sucked under those
> > conditions, but BBR sucked impressively less than the others.
> >
> > Disclaimer: there was significant run-to-run variation; the presented
> > results are averages.  And I did not attempt to measure packet loss
> > exactly for most runs; 0.05% is merely an average of a few selected
> > runs.  These measurements were taken on a production server running a
> > real workload, which introduces noise.  Soon I hope to have the
> > opportunity to repeat the experiment on an idle server in the same
> > environment.
> >
> > In conclusion, while we'd like to use BBR, we really can't until we
> > upgrade to 14.1, which hopefully will be soon.  So in the meantime
> > we've switched all relevant servers from cubic to chd, and we'll
> > reevaluate BBR after the upgrade.
> Hi Alan,
>
> just to be clear: the version of BBR currently implemented is
> BBR version 1, which is known to be unfair in certain scenarios.
> Google is still working on BBR to address this problem and improve
> it in other aspects. But there is no RFC yet and the updates haven't
> been implemented yet in FreeBSD.
>
> Best regards
> Michael
> >
> > [1]: https://www.slac.stanford.edu/comp/net/wan-mon/thru-vs-loss.html
> >
> > -Alan
>
>
>

-- 
Junho Choi <junho dot choi at gmail.com> | https://saturnsoft.net