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tcp: Don't prematurely drop receiving-only connections

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If the connection was persistent and receiving-only, several (12)
sporadic device insufficient buffers would cause the connection be
dropped prematurely:
Upon ENOBUFS in tcp_output() for an ACK, retransmission timer is
started.  No one will stop this retransmission timer for receiving-
only connection, so the retransmission timer promises to expire and
t_rxtshift is promised to be increased.  And t_rxtshift will not be
reset to 0, since no RTT measurement will be done for receiving-only
connection.  If this receiving-only connection lived long enough
(e.g. >350sec, given the RTO starts from 200ms), and it suffered 12
sporadic device insufficient buffers, i.e. t_rxtshift >= 12, this
receiving-only connection would be dropped prematurely by the
retransmission timer.

We now assert that for data segments, SYNs or FINs either rexmit or
persist timer was wired upon ENOBUFS.  And don't set rexmit timer
for other cases, i.e. ENOBUFS upon ACKs.

Discussed with:	lstewart, hiren, jtl, Mike Karels
MFC after:	3 weeks
Sponsored by:	Microsoft OSTC
Differential Revision:	https://reviews.freebsd.org/D5872
This commit is contained in:
Sepherosa Ziehau 2016-05-30 03:31:37 +00:00
parent f9e7d7e1a3
commit 425b763928
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=300981
1 changed files with 11 additions and 3 deletions
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14
sys/netinet/tcp_output.c
View File

@ -130,6 +130,16 @@ SYSCTL_INT(_net_inet_tcp, OID_AUTO, sendbuf_max, CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(tcp_autosndbuf_max), 0,
"Max size of automatic send buffer");
/*
* Make sure that either retransmit or persist timer is set for SYN, FIN and
* non-ACK.
*/
#define TCP_XMIT_TIMER_ASSERT(tp, len, th_flags) \
KASSERT(((len) == 0 && ((th_flags) & (TH_SYN | TH_FIN)) == 0) ||\
tcp_timer_active((tp), TT_REXMT) || \
tcp_timer_active((tp), TT_PERSIST), \
("neither rexmt nor persist timer is set"))
static void inline hhook_run_tcp_est_out(struct tcpcb *tp,
struct tcphdr *th, struct tcpopt *to,
long len, int tso);
@ -1545,9 +1555,7 @@ tcp_output(struct tcpcb *tp)
tp->t_softerror = error;
return (error);
case ENOBUFS:
if (!tcp_timer_active(tp, TT_REXMT) &&
!tcp_timer_active(tp, TT_PERSIST))
tcp_timer_activate(tp, TT_REXMT, tp->t_rxtcur);
TCP_XMIT_TIMER_ASSERT(tp, len, flags);
tp->snd_cwnd = tp->t_maxseg;
return (0);
case EMSGSIZE: