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=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 I'm all for taking that code out. Best, George
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