why not enable tcp_pmtud_blackhole_detect in default

[Kevin Bowling]

Cheng,

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
out.

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.

Regards,

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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