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Don't drop options from the third retransmitted SYN by default. If the

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SYNs (or SYN/ACK replies) are dropped due to network congestion, then the
remote end of the connection may act as if options such as window scaling
are enabled but the local end will think they are not.  This can result in
very slow data transfers in the case of window scaling disagreements.

The old behavior can be obtained by setting the
net.inet.tcp.rexmit_drop_options sysctl to a non-zero value.

Reviewed by:	net@
MFC after:	2 weeks
This commit is contained in:
jhb 2013-01-09 20:27:06 +00:00
parent 8dc1694ed8
commit 20f8f82daf
1 changed files with 7 additions and 1 deletions
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8
sys/netinet/tcp_timer.c
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@ -119,6 +119,11 @@ SYSCTL_INT(_net_inet_tcp, OID_AUTO, keepcnt, CTLFLAG_RW, &tcp_keepcnt, 0,
/* 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");
@ -595,7 +600,8 @@ 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|TF_SACK_PERMIT);
/*
* If we backed off this far, our srtt estimate is probably bogus.