tcp keep-alive message sent without timestamp option

[Yongmin Cho]

Hi, all.

I have checked tcp keep-alive in freebsd head.
According to RFC7323, tcp timestamp option must be sent with
keep-alive packet after timestamp option has been negotiated.
So I have tested this on linux-3.13.0.
tcp keep-alive message is sent with timestamp option on linux-3.13.0.
But on freebsd head, tcp keep-alive packet is sent without timestamp option
after negotiated. So I made patch file based on freebsd head.
Please check this patch file. any feedback will be welcome.

Thank you in advance for your answers!

According in RFC7323:
    Once TSopt has been successfully negotiated, that is both <SYN>
    and <SYN,ACK> contain TSopt, the TSopt MUST be sent in every non-<RST>
    segment for the duration of the connection, and SHOULD be sent in
    an <RST> segment (see Section 5.2 for details).  The TCP SHOULD
    remember this state by setting a flag, referred to as Snd.TS.OK,
    to one. If a non-<RST> segment is received without a TSopt, a TCP SHOULD
    silently drop the segment. A TCP MUST NOT abort a TCP connection
    because any segment lacks an expected TSopt.
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