why not enable tcp_pmtud_blackhole_detect in default

Cui, Cheng Cheng.Cui at netapp.com
Thu Mar 8 16:07:15 UTC 2018


Thanks for the reply. I think you have answered most of my doubts. But I would like to be clear that, is there a very strong reason why you run this in production even it often causes unnecessary lower MSS with lower performance?

Cheng Cui

On 3/7/18, 3:35 PM, "Kevin Bowling" <kevin.bowling at kev009.com> wrote:

    We run this in production at Limelight Networks (i.e toward a broad
    spectrum of Internet hosts) and must to deal with some uncommon
    network topology. There are currently some limitations as you point
    Like you say the signaling is not perfect and we do often clamp MSS
    unnecessarily.  There is also no probing to see if we can expand the
    MSS later.
    I think those issues should be fixed up before it's enabled by default
    and I don't know anyone working on it at the moment.
    On Wed, Mar 7, 2018 at 8:35 AM, Cui, Cheng <Cheng.Cui at netapp.com> wrote:
    > Dear all,
    > Reading through the tcp blackhole detection code (support RFC 4821) in FreeBSD including the recent bug fixes, I am wondering why is it still not enabled in default? Given the fact that this implementation was a merge from xnu, and the xnu has enabled it in default, do we have a plan to enable it in default? Or is there any concern about the side-effect from it as performance regression against some false positive blackhole event like a temporary link flap, which is long enough to trigger a lower MSS but shorter than 6 RTO?
    > https://opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/netinet/tcp_timer.c.auto.html  << enabled in macOS 10.6
    > https://reviews.freebsd.org/rS322967  << bug fixes
    > https://reviews.freebsd.org/rS272720  << merge from xnu
    > Thanks,
    > --Cheng Cui
    > NetApp Scale Out Networking
    > https://netapp-meeting.webex.com/meet/chengc
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