[Bug 243234] TCP/ECN - RTO retransmissions sent without CWR flag in newreno and cubic

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=243234

            Bug ID: 243234
           Summary: TCP/ECN - RTO retransmissions sent without CWR flag in
                    newreno and cubic
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: rscheff at gmx.at

While investigating performance implications of RTO on ECN-enabled TCP flows,
it was noted that the recovery from a lost retransmission - using the RTO
timeout - is sent without the CWR flag in the TCP header (but the IP ECT
codepoint is cleared).

Linux has a similar misbehavior, but there it appears that someone tied the
signaling of TCP ECN header flags to the setting of the IP header ECT
codepoints - which is not correct though.

dctcp has explicit code in its _cong_signal handler, to mark the RTO
retransmission explicitly with the CWR flag.

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