Date: Wed, 4 Nov 2009 13:17:06 -0500 From: John Baldwin <jhb@freebsd.org> To: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net> Cc: net@freebsd.org Subject: Re: Small bug with TCP zero windows Message-ID: <200911041317.06793.jhb@freebsd.org> In-Reply-To: <20091103212129.F37440@maildrop.int.zabbadoz.net> References: <200911031529.26514.jhb@FreeBSD.org> <20091103212129.F37440@maildrop.int.zabbadoz.net>
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On Tuesday 03 November 2009 4:26:48 pm Bjoern A. Zeeb wrote: > On Tue, 3 Nov 2009, John Baldwin wrote: > > > Several years ago Dillon added a feature to TCP that casued soreceive() to > > send an ACK right away if data was drained from a TCP socket that had > > previously advertised a zero-sized window. The current code requires the > > receive window to be exactly zero for this to kick in. If window scaling is > > enabled and the window is smaller than the scale, then the effective window > > that is advertised is zero. However, in that case the zero-sized window > > handling is not enabled because the window is not exactly zero. The patch > > below changes the code to check the raw window value against zero. Arguably > > it could check 'th_win' directly instead if folks would prefer that. > > hmm, looking a few lines up, there is a htons() there as well; > obviously doesn't matter for 0. th_win is set to something different > for SYNs. I guess what you are doing is ok, and even though it is not > needed, I feel that it would be easier to read it with an extra pair > of (). How about using 'if (th->th_win == htons(0))' for the condition? > > Index: tcp_output.c > > =================================================================== > > --- tcp_output.c (revision 198794) > > +++ tcp_output.c (working copy) > > @@ -992,7 +992,7 @@ > > * to read more data than can be buffered prior to transmitting on > > * the connection. > > */ > > - if (recwin == 0) > > + if (recwin >> tp->rcv_scale == 0) > > tp->t_flags |= TF_RXWIN0SENT; > > else > > tp->t_flags &= ~TF_RXWIN0SENT; > > > > > > -- > Bjoern A. Zeeb It will not break if you know what you are doing. > -- John Baldwin
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