[Bug 214171] [tcp] Out-of-order transmission severely degrades performance

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=214171

            Bug ID: 214171
           Summary: [tcp] Out-of-order transmission severely degrades
                    performance
           Product: Base System
           Version: 10.3-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: noah.bergbauer at tum.de

Downloading data from a FreeBSD server over a fast (~700Mbps with 0.1% loss)
internet connection that suffers from a lot of packet reordering, I'm seeing
very bad transmission rates (~4Mbps whereas Linux gets ~40Mbps on the same
connection).

Traffic analysis shows that when the client (Windows in my tests) receives
out-of-order packets, it first sends out SACKs until the missing packet arrives
(usually just ~1ms late). Then it ACKs everything normally.

Linux reacts to this by just carrying on - after all, no packets were dropped.
FreeBSD however appears to react to each SACK immediately and indepentently,
assuming that the late packets were actually lost. Because of that, it
retransmits them and lowers the transmission rate (I tested with several
different congestion control modules and it affects all of them).

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