why not enable tcp_pmtud_blackhole_detect in default

[Cui, Cheng]

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