Date: Thu, 25 Aug 2016 19:52:41 +0000 From: "Cui, Cheng" <Cheng.Cui@netapp.com> To: "freebsd-transport@freebsd.org" <freebsd-transport@freebsd.org> Subject: question about if a recent Linux patch on window scaling is required in FreeBSD Message-ID: <D3E4C4C9.147A0%Cheng.Cui@netapp.com>
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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= =3D6 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=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. 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=20 other side, if the other side is FreeBSD, I think FreeBSD will also fail 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. 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. 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? Thanks and apologize in advance if I did not do enough research, --Cheng Cui
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