Date: Fri, 12 Oct 2012 14:15:04 -0400 From: George Neville-Neil <gnn@neville-neil.com> To: John Baldwin <jhb@freebsd.org> Cc: net@freebsd.org Subject: Re: Dropping TCP options from retransmitted SYNs considered harmful Message-ID: <3DBAA027-B5D9-44AA-A00D-0496985D4FEA@neville-neil.com> In-Reply-To: <201210121213.11152.jhb@freebsd.org> References: <201210121213.11152.jhb@freebsd.org>
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On Oct 12, 2012, at 12:13 , John Baldwin <jhb@freebsd.org> wrote:
> Back in 2001 FreeBSD added a hack to strip TCP options from retransmitted SYNs
> starting with the 3rd SYN in this block in tcp_timer.c:
>
> /*
> * Disable rfc1323 if we haven't got any response to
> * our third SYN to work-around some broken terminal servers
> * (most of which have hopefully been retired) that have bad VJ
> * header compression code which trashes TCP segments containing
> * unknown-to-them TCP options.
> */
> if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
> tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
>
> There is even a PR for the original bug report: kern/1689
>
> However, there is an unintended consequence of this change that can be
> disastrous. Specifically, suppose you have a FreeBSD client connecting to a
> server, and that the SYNs are arriving at the server successfully, but the
> first few return SYN/ACKs are dropped. Eventually a SYN/ACK makes it through
> and the connection is established.
>
> The server (based on the first SYN it saw) believes it has negotiated window
> scaling with the client. The client, however, has broken what it promised in
> that first SYN and believes it is not using any window scaling at all. This
> causes two forms of breakage:
>
> 1) When the server advertises a scaled window (e.g. '8' for a 64k window
> scaled at 13), the client thinks it is an unscaled window ('8') and
> sends data to the server very slowly.
>
> 2) When the client advertises an unscaled window (e.g. '65535' for a 64k
> window), the server thinks it has a huge window (65535 << 13 == 511MB)
> to send into.
>
> I'm not sure that 2) is a problem per se, but I have definitely seen instances
> of 1) (and examined the 'struct tcpcb' in kgdb on both the server and client
> end of the connections to verify they disagreed on the scaling).
>
> The original motivation of this change is to work around broken terminal
> servers that were old when this change was added in 2001. Over 10 years later
> I think we should at least have an option to turn this work-around off, and
> possibly disable it by default.
>
> Thoughts?
>
I'm all for taking that code out.
Best,
George
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