Date: Fri, 12 Oct 2012 22:29:48 +0200 From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> To: George Neville-Neil <gnn@neville-neil.com> Cc: John Baldwin <jhb@freebsd.org>, net@freebsd.org Subject: Re: Dropping TCP options from retransmitted SYNs considered harmful Message-ID: <E11D6C3C-0A77-48B3-8D4B-7E606E71ED4C@lurchi.franken.de> In-Reply-To: <3DBAA027-B5D9-44AA-A00D-0496985D4FEA@neville-neil.com> References: <201210121213.11152.jhb@freebsd.org> <3DBAA027-B5D9-44AA-A00D-0496985D4FEA@neville-neil.com>
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On Oct 12, 2012, at 8:15 PM, George Neville-Neil wrote:
>=20
> On Oct 12, 2012, at 12:13 , John Baldwin <jhb@freebsd.org> wrote:
>=20
>> Back in 2001 FreeBSD added a hack to strip TCP options from =
retransmitted SYNs=20
>> starting with the 3rd SYN in this block in tcp_timer.c:
>>=20
>> /*
>> * 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 =3D=3D TCPS_SYN_SENT) && (tp->t_rxtshift =3D=3D =
3))
>> tp->t_flags &=3D ~(TF_REQ_SCALE|TF_REQ_TSTMP);
>>=20
>> There is even a PR for the original bug report: kern/1689
>>=20
>> However, there is an unintended consequence of this change that can =
be=20
>> disastrous. Specifically, suppose you have a FreeBSD client =
connecting to a=20
>> server, and that the SYNs are arriving at the server successfully, =
but the=20
>> first few return SYN/ACKs are dropped. Eventually a SYN/ACK makes it =
through=20
>> and the connection is established.
>>=20
>> The server (based on the first SYN it saw) believes it has negotiated =
window=20
>> scaling with the client. The client, however, has broken what it =
promised in=20
>> that first SYN and believes it is not using any window scaling at =
all. This=20
>> causes two forms of breakage:
>>=20
>> 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.
>>=20
>> 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 =3D=3D =
511MB)
>> to send into.
>>=20
>> I'm not sure that 2) is a problem per se, but I have definitely seen =
instances=20
>> of 1) (and examined the 'struct tcpcb' in kgdb on both the server and =
client=20
>> end of the connections to verify they disagreed on the scaling).
>>=20
>> The original motivation of this change is to work around broken =
terminal=20
>> servers that were old when this change was added in 2001. Over 10 =
years later=20
>> I think we should at least have an option to turn this work-around =
off, and=20
>> possibly disable it by default.
>>=20
>> Thoughts?
>>=20
>=20
> I'm all for taking that code out.
ACK.
>=20
> Best,
> George
>=20
>=20
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>=20
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