[Bug 235256] [net] IW10 without ABC leads to initial window of 11 Segments

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235256

            Bug ID: 235256
           Summary: [net] IW10 without ABC leads to initial window of 11
                    Segments
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: srichard at netapp.com
 Attachment #201465 text/plain
         mime type:

Created attachment 201465
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=201465&action=edit
packetdrill script to validate IW10 behavior without ABC

Checking the Initial Window while disabling Appropriate Byte Counting:

net.inet.tcp.rfc3465=0
net.inet.tcp.initcwnd_segments=10

Transmits 11 packets (also exceeding the hardcoded limit of 14600 octets). 

After a casual inspection, this may be present since BSD10 when modular
congestion control moved relevant code around.

Possibly this is due to the final ACK of the 3WHS getting processed normally,
and without ABC checking that this is an empty ACK, immeditately increasing
cwnd

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