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Date: Tue, 31 Jul 2007 10:28:14 -0700
From: Sridhar Samudrala <sri@...ibm.com>
To: Neil Horman <nhorman@...driver.com>
Cc: Wei Yongjun <yjwei@...fujitsu.com>, netdev@...r.kernel.org,
lksctp-developers@...ts.sourceforge.net
Subject: Re: [Lksctp-developers] [PATCH] SCTP: drop SACK if ctsn is not
less than the next tsn of assoc
On Tue, 2007-07-31 at 07:37 -0400, Neil Horman wrote:
> On Tue, Jul 31, 2007 at 12:44:27PM +0800, Wei Yongjun wrote:
> > If SCTP data sender received a SACK which contains Cumulative TSN Ack is
> > not less than the Cumulative TSN Ack Point, and if this Cumulative TSN
> > Ack is not used by the data sender, SCTP data sender still accept this
> > SACK , and next SACK which send correctly to DATA sender be dropped,
> > because it is less than the new Cumulative TSN Ack Point.
> > After received this SACK, data will be retrans again and again even if
> > correct SACK is received.
> > So I think this SACK must be dropped to let data transmit correctly.
> >
> > Following is the tcpdump of my test. And patch in this mail can avoid
> > this problem.
> >
> > 02:19:38.233278 sctp (1) [INIT] [init tag: 1250461886] [rwnd: 54784] [OS: 10] [MIS: 65535] [init TSN: 217114040]
> > 02:19:39.782160 sctp (1) [INIT ACK] [init tag: 1] [rwnd: 54784] [OS: 100] [MIS: 65535] [init TSN: 100]
> > 02:19:39.798583 sctp (1) [COOKIE ECHO]
> > 02:19:40.082125 sctp (1) [COOKIE ACK]
> > 02:19:40.097859 sctp (1) [DATA] (B)(E) [TSN: 217114040] [SID: 0] [SSEQ 0] [PPID 0xf192090b]
> > 02:19:40.100162 sctp (1) [DATA] (B)(E) [TSN: 217114041] [SID: 0] [SSEQ 1] [PPID 0x3e467007]
> > 02:19:40.100779 sctp (1) [DATA] (B)(E) [TSN: 217114042] [SID: 0] [SSEQ 2] [PPID 0x11b12a0a]
> > 02:19:40.101200 sctp (1) [DATA] (B)(E) [TSN: 217114043] [SID: 0] [SSEQ 3] [PPID 0x30e7d979]
> > 02:19:40.561147 sctp (1) [SACK] [cum ack 217114040] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:40.568498 sctp (1) [DATA] (B)(E) [TSN: 217114044] [SID: 0] [SSEQ 4] [PPID 0x251ff86f]
> > 02:19:40.569308 sctp (1) [DATA] (B)(E) [TSN: 217114045] [SID: 0] [SSEQ 5] [PPID 0xe5d5da5d]
> > 02:19:40.700584 sctp (1) [SACK] [cum ack 290855864] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:40.701562 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:40.701567 sctp (1) [DATA] (B)(E) [TSN: 217114047] [SID: 0] [SSEQ 7] [PPID 0xca47e645]
> > 02:19:40.701569 sctp (1) [DATA] (B)(E) [TSN: 217114048] [SID: 0] [SSEQ 8] [PPID 0x6c0ea150]
> > 02:19:40.701576 sctp (1) [DATA] (B)(E) [TSN: 217114049] [SID: 0] [SSEQ 9] [PPID 0x9cc1994f]
> > 02:19:40.701585 sctp (1) [DATA] (B)(E) [TSN: 217114050] [SID: 0] [SSEQ 10] [PPID 0xb1df4129]
> > 02:19:41.098201 sctp (1) [SACK] [cum ack 217114041] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:41.283257 sctp (1) [SACK] [cum ack 217114042] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:41.457217 sctp (1) [SACK] [cum ack 217114043] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:41.691528 sctp (1) [SACK] [cum ack 217114044] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:41.849636 sctp (1) [SACK] [cum ack 217114045] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:41.975473 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:42.021229 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:42.196495 sctp (1) [SACK] [cum ack 217114047] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:42.424319 sctp (1) [SACK] [cum ack 217114048] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:42.586924 sctp (1) [SACK] [cum ack 217114049] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:42.744810 sctp (1) [SACK] [cum ack 217114050] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:42.965536 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:43.106385 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:43.218969 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:45.374101 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:45.489258 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:49.830116 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:49.984577 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> > 02:19:58.760300 sctp (1) [DATA] (B)(E) [TSN: 217114046] [SID: 0] [SSEQ 6] [PPID 0x87d8b423]
> > 02:19:58.931690 sctp (1) [SACK] [cum ack 217114046] [a_rwnd 54784] [#gap acks 0] [#dup tsns 0]
> >
> >
> > Signed-off-by: Wei Yongjun <yjwei@...fujitsu.com>
> >
> > --- net/sctp/sm_statefuns.c.orig 2007-07-29 18:11:01.000000000 -0400
> > +++ net/sctp/sm_statefuns.c 2007-07-29 18:14:49.000000000 -0400
> > @@ -2880,6 +2880,15 @@ sctp_disposition_t sctp_sf_eat_sack_6_2(
> > return SCTP_DISPOSITION_DISCARD;
> > }
> >
> > + /* If Cumulative TSN Ack is not less than the Cumulative TSN
> > + * Ack which will be send in the next data, drop the SACK.
> > + */
> > + if (!TSN_lt(ctsn, asoc->next_tsn)) {
> > + SCTP_DEBUG_PRINTK("ctsn %x\n", ctsn);
> > + SCTP_DEBUG_PRINTK("next_tsn %x\n", asoc->next_tsn);
> > + return SCTP_DISPOSITION_DISCARD;
> > + }
> > +
> > /* Return this SACK for further processing. */
> > sctp_add_cmd_sf(commands, SCTP_CMD_PROCESS_SACK, SCTP_SACKH(sackh));
> >
> >
> >
> Whats the behavior on this in the event that a sack is received in which the
> ctsn falls within a a missing space in a stream of gap acks? I.e. what if the
> sack being sent falls into a hole between the ack point and the first gap ack
> range? Does this patch impact that at all?
>
> Also, what is this:
> 02:19:40.700584 sctp (1) [SACK] [cum ack 290855864] ....
>
> That ack value seems rather out of range for the rest of the trace. Was that
> part of your test? If so, what caused it?
Yes. This SACK seems to be totally out of range and may be causing the problem.
I would expect the following check in sctp_sf_eat_sack_6_2() to drop any SACKs
with CTSN value lower than the earlier SACKs.
/* i) If Cumulative TSN Ack is less than the Cumulative TSN
* Ack Point, then drop the SACK. Since Cumulative TSN
* Ack is monotonically increasing, a SACK whose
* Cumulative TSN Ack is less than the Cumulative TSN Ack
* Point indicates an out-of-order SACK.
*/
if (TSN_lt(ctsn, asoc->ctsn_ack_point)) {
SCTP_DEBUG_PRINTK("ctsn %x\n", ctsn);
SCTP_DEBUG_PRINTK("ctsn_ack_point %x\n", asoc->ctsn_ack_point);
return SCTP_DISPOSITION_DISCARD;
}
Thanks
Sridhar
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