Dropping TCP options from retransmitted SYNs considered harmful

[John Baldwin]

On Friday, October 12, 2012 12:13:11 pm John Baldwin 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?

How about this:

Index: tcp_timer.c
===================================================================
--- tcp_timer.c	(revision 241579)
+++ tcp_timer.c	(working copy)
@@ -118,6 +118,11 @@ SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt, CTLFL
 	/* max idle probes */
 int	tcp_maxpersistidle;
 
+static int	tcp_rexmit_drop_options = 0;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, rexmit_drop_options, CTLFLAG_RW,
+    &tcp_rexmit_drop_options, 0,
+    "Drop TCP options from 3rd and later retransmitted SYN");
+
 static int	per_cpu_timers = 0;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, per_cpu_timers, CTLFLAG_RW,
     &per_cpu_timers , 0, "run tcp timers on all cpus");
@@ -578,7 +583,8 @@ tcp_timer_rexmt(void * xtp)
 	 * header compression code which trashes TCP segments containing
 	 * unknown-to-them TCP options.
 	 */
-	if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
+	if (tcp_rexmit_drop_options && (tp->t_state == TCPS_SYN_SENT) &&
+	    (tp->t_rxtshift == 3))
 		tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
 	/*
 	 * If we backed off this far, our srtt estimate is probably bogus.

Any other suggestions on the sysctl name?

-- 
John Baldwin