Date: Mon, 28 Mar 2011 20:38:10 +0200 From: Stefan `Sec` Zehl <sec@42.org> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-net@freebsd.org, Doug Barton <dougb@freebsd.org> Subject: Re: The tale of a TCP bug Message-ID: <20110328183810.GF23803@ice.42.org> In-Reply-To: <201103281423.52202.jhb@freebsd.org> References: <4D8B99B4.4070404@FreeBSD.org> <20110326140212.GB45402@ice.42.org> <201103281021.00673.jhb@freebsd.org> <201103281423.52202.jhb@freebsd.org>
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Hi,
On Mon, Mar 28, 2011 at 14:23 -0400, John Baldwin wrote:
>
> No, this is not really right. Your patch from your blog is the best
> fix actually. The reason we want to let 'win' be larger than
> TCP_MAXWIN is that if the remote end sends more data than we've
> advertised but we have room in the socket buffer, we want to go ahead
> and accept the data as valid and ACK it rather than dropping the data
> that is beyond rcv_adv. My change above to rcv_wnd would break this.
> Also, for the TCPS_SYN_SENT case we don't know what 'rcv_scale' is
> until just before we update 'rcv_adv'. This should be the same
> as your patch:
>
> Index: tcp_input.c
> ===================================================================
> --- tcp_input.c (revision 220098)
> +++ tcp_input.c (working copy)
> @@ -1756,7 +1756,8 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th,
> (TF_RCVD_SCALE|TF_REQ_SCALE)) {
> tp->rcv_scale = tp->request_r_scale;
> }
> - tp->rcv_adv += tp->rcv_wnd;
> + tp->rcv_adv += imin(tp->rcv_wnd,
> + TCP_MAXWIN << tp->rcv_scale);
> tp->snd_una++; /* SYN is acked */
> /*
> * If there's data, delay ACK; if there's also a FIN
>
I've applied this to my test-VM, and as expected it now passes my two
testcases. As far as I'm concerned this fixes it for me.
I'm interested to see if my adv_neg counting hack together with this
patch still registers any hits. -- If nobody beats me to it, I'll try it
out on my webserver tomorrow.
CU,
Sec
--
We may very soon have computers weighing no more than 1.5 tons.
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