Date: Tue, 30 Aug 2016 14:19:12 +0000 From: "Cui, Cheng" <Cheng.Cui@netapp.com> To: "freebsd-transport@freebsd.org" <freebsd-transport@freebsd.org> Subject: Re: question about if a recent Linux patch on window scaling is required in FreeBSD Message-ID: <27FDDABD-4726-41B1-8A49-FF3274F9AAD3@netapp.com> In-Reply-To: <D3E4C4C9.147A0%Cheng.Cui@netapp.com> References: <D3E4C4C9.147A0%Cheng.Cui@netapp.com>
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Refresh this question. Can anyone make a comment?
Thanks,
--Cheng Cui
NetApp Scale Out Networking
On 8/25/16, 3:52 PM, "Cui, Cheng" <Cheng.Cui@netapp.com> wrote:
Hello everyone,
=20
I hope this email could reach you well, because I found related
discussions about this topic on window scaling and the case of window
shrinking (or retraction or loss of precision). And I try to make this
question simple.
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There is a recent Linux patch at receiver side to round-up advertised
window due to precision loss of window scaling. It reaches my attention
because the same problem could also happen between a pair of Linux and
FreeBSD nodes, and I am not aware of any similar patch in FreeBSD yet.
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The Linux patch is this:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?=
id=3D6
07bfbf2d55dd1cfe5368b41c2a81a8c9ccf4723
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And I quote some description of the Linux patch below:
> If the sender uses up the entire window before it is shrunk, this can
> have chaotic effects on the connection. When sending ACKs,
> tcp_acceptable_seq() will notice that the window has been shrunk sinc=
e
> tcp_wnd_end() is before tp->snd_nxt, which makes it choose tcp_wnd_en=
d()
>as=20
> sequence number. This will fail the receivers checks in tcp_sequence(=
)
>however=20
> since it is before it's tp->rcv_wup, making it respond with a dupack.
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I think the Linux's behavior is right ("ACK-only packets should be sent
with the largest in-window sequence number that has ever been sent." re=
f:
https://www.ietf.org/mail-archive/web/tcpm/current/msg10512.html), it
actually chooses "tp->snd_una+tp->snd_wnd" (tcp_wnd_end()) instead of
tp->snd_nxt, as it thought tp->snd_nxt is out of window, in case of
precision loss which made the receiver's advertise-window smaller. But =
at
the=20
other side, if the other side is FreeBSD, I think FreeBSD will also fai=
l
the=20
check since "tp->snd_una+tp->snd_wnd" is before it's tp->rcv_nxt, and
ignore=20
the sequence number in the packet.
=20
I also sent an email to tcpm@ietf.org asking if this Linux patch is RFC
7323=20
(window scaling part) compliant, but I have not get any reply yet.
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So my question here is: Is there any recent change in FreeBSD to
accommodate the=20
Linux behavior ("tp->snd_una+tp->snd_wnd" as sequence number)? If not, =
do
we=20
consider to apply the same way as in the Linux patch?
=20
Thanks and apologize in advance if I did not do enough research,
--Cheng Cui
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