Add freebsd-net to increase the size of audience.
Thanks,
--Cheng Cui
NetApp Scale Out Networking
On 8/30/16, 10:19 AM, "Cui, Cheng" <cheng....@netapp.com> wrote:
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....@netapp.com> wrote:
Hello everyone,
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.
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.
The Linux patch is this:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6
07bfbf2d55dd1cfe5368b41c2a81a8c9ccf4723
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 since
> tcp_wnd_end() is before tp->snd_nxt, which makes it choose
tcp_wnd_end()
>as
> sequence number. This will fail the receivers checks in tcp_sequence()
>however
> since it is before it's tp->rcv_wup, making it respond with a dupack.
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."
ref:
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
other side, if the other side is FreeBSD, I think FreeBSD will also fail
the
check since "tp->snd_una+tp->snd_wnd" is before it's tp->rcv_nxt, and
ignore
the sequence number in the packet.
I also sent an email to t...@ietf.org asking if this Linux patch is RFC
7323
(window scaling part) compliant, but I have not get any reply yet.
So my question here is: Is there any recent change in FreeBSD to
accommodate the
Linux behavior ("tp->snd_una+tp->snd_wnd" as sequence number)? If not,
do
we
consider to apply the same way as in the Linux patch?
Thanks and apologize in advance if I did not do enough research,
--Cheng Cui
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