From: Jamie Gloudon <jamie.gloudon@gmail•com>
To: Eric Dumazet <erdnetdev@gmail•com>
Cc: rick.jones2@hp•com, netdev@vger•kernel.org
Subject: Re: Is keepalive behaving as expected in 3.7.0+/net-next?
Date: Sat, 29 Dec 2012 15:47:55 -0400 [thread overview]
Message-ID: <50df48f4.0e5b650a.36fa.1452@mx.google.com> (raw)
In-Reply-To: <1356645265.30414.1542.camel@edumazet-glaptop>
On Thu, Dec 27, 2012 at 01:54:25PM -0800, Eric Dumazet wrote:
> On Fri, 2012-12-21 at 14:05 -0800, Rick Jones wrote:
> > I was looking to do a bit more documentation clean-up and thought I
> > would work on the descriptions of the "keepalive" sysctls, but first I
> > wanted to see if they behaved as the existing descriptions suggested:
> >
> > > tcp_keepalive_time - INTEGER
> > > How often TCP sends out keepalive messages when keepalive is enabled.
> > > Default: 2hours.
> > >
> > > tcp_keepalive_probes - INTEGER
> > > How many keepalive probes TCP sends out, until it decides that the
> > > connection is broken. Default value: 9.
> > >
> > > tcp_keepalive_intvl - INTEGER
> > > How frequently the probes are send out. Multiplied by
> > > tcp_keepalive_probes it is time to kill not responding connection,
> > > after probes started. Default value: 75sec i.e. connection
> > > will be aborted after ~11 minutes of retries.
> >
> > I interpreted all that that as: When a connection is idle, TCP will
> > send a keepalive probe every tcp_keepalive_time seconds. If a response
> > to a keepalive probe is not received, TCP will resend (retransmit) it
> > every tcp_keepalive_intvl seconds.
> >
> > However, what I see is that on a connection where the remote is indeed
> > still there, only the first keepalive probe is sent after
> > tcp_keepalive_time, and thereafter it is sent every tcp_keepalive_intvl
> > seconds.
> >
> > Now, some of this may relate to my being impatient - rather than wait
> > two hours for the first probe, I set tcp_keepalive_time to 3 seconds,
> > and tcp_keepalive_intvl to 7 seconds. I then kicked-off a ./configure
> > --intervals-enable netperf TCP_RR test with a burst of one and a wait
> > time of 90 seconds and got the following (trimmed) trace:
> >
> > 13:43:46.879133 IP netnextraj.43054 > netnextraj2.srvr: Flags [S], seq
> > 807869796, win 14600, options [mss 1460,sackOK,TS val 133470 ecr
> > 0,nop,wscale 7], length 0
> > 13:43:46.880091 IP netnextraj2.srvr > netnextraj.43054: Flags [S.], seq
> > 1522345902, ack 807869797, win 14480, options [mss 1460,sackOK,TS val
> > 136186 ecr 133470,nop,wscale 4], length 0
> > 13:43:46.880114 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 1, win 115, options [nop,nop,TS val 133470 ecr 136186], length 0
> > 13:43:46.880306 IP netnextraj.43054 > netnextraj2.srvr: Flags [P.], seq
> > 1:11, ack 1, win 115, options [nop,nop,TS val 133470 ecr 136186], length 10
> > 13:43:46.880948 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 11, win 905, options [nop,nop,TS val 136187 ecr 133470], length 0
> > 13:43:46.880964 IP netnextraj2.srvr > netnextraj.43054: Flags [P.], seq
> > 1:11, ack 11, win 905, options [nop,nop,TS val 136187 ecr 133470], length 10
> > 13:43:46.881161 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 11, win 115, options [nop,nop,TS val 133470 ecr 136187], length 0
> >
> > The first probe above comes after 3 seconds - tcp_keepalive_time - at
> > 13:43:49
> >
> > 13:43:49.886752 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 11, win 115, options [nop,nop,TS val 134222 ecr 136187], length 0
> >
> > And it does seem to elicit a response:
> >
> > 13:43:49.887530 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 11, win 905, options [nop,nop,TS val 136938 ecr 133470], length 0
> >
>
>
> > Now it starts sending probes every 7 seconds (tcp_keepalive_intvl):
> >
> > 13:43:56.903576 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 11, win 115, options [nop,nop,TS val 135976 ecr 136938], length 0
> > 13:43:56.904480 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 11, win 905, options [nop,nop,TS val 138693 ecr 133470], length 0
> > 13:44:03.910744 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 11, win 115, options [nop,nop,TS val 137728 ecr 138693], length 0
> > 13:44:03.911623 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 11, win 905, options [nop,nop,TS val 140444 ecr 133470], length 0
> >
> > I;ve deleted the next 9 or so probes... It continues, and doesn't
> > terminate the connection, so I assume it was happy with the responses to
> > the probes.
> >
> > 13:45:13.990746 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 11, win 115, options [nop,nop,TS val 155248 ecr 156213], length 0
> > 13:45:13.991578 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 11, win 905, options [nop,nop,TS val 157965 ecr 133470], length 0
> >
> > Now the next netperf transaction happens:
> >
> > 13:45:16.879222 IP netnextraj.43054 > netnextraj2.srvr: Flags [P.], seq
> > 11:21, ack 11, win 115, options [nop,nop,TS val 155970 ecr 157965],
> > length 10
> > 13:45:16.880033 IP netnextraj2.srvr > netnextraj.43054: Flags [P.], seq
> > 11:21, ack 21, win 905, options [nop,nop,TS val 158687 ecr 155970],
> > length 10
> > 13:45:16.880220 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 21, win 115, options [nop,nop,TS val 155970 ecr 158687], length 0
> >
> > But the next keepalive probe is tcp_keepalive_intvl seconds after the
> > last one, rather than that many, or tcp_keepalive_time seconds after the
> > connection was last "active."
> >
> > 13:45:20.998739 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 21, win 115, options [nop,nop,TS val 157000 ecr 158687], length 0
> > 13:45:20.999754 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 21, win 905, options [nop,nop,TS val 159717 ecr 155970], length 0
> > 13:45:28.006747 IP netnextraj.43054 > netnextraj2.srvr: Flags [.], ack
> > 21, win 115, options [nop,nop,TS val 158752 ecr 159717], length 0
> > 13:45:28.007624 IP netnextraj2.srvr > netnextraj.43054: Flags [.], ack
> > 21, win 905, options [nop,nop,TS val 161469 ecr 155970], length 0
> >
> > Is this the expected behaviour? If I reverse the values - make
> > tcp_keepalive_time 7 and tcp_keepalive_intvl 3, it seems that all the
> > probes are after 7 seconds.
> >
> > rick jones
>
> Not sure if it makes sense to have
> tcp_keepalive_intvl > tcp_keepalive_time
>
> time should be an order of magnitude bigger than intvl.
>
> keepalive timer is not reset each time we receive a valid frame, it
> would be very expensive.
>
> Its a long period timer.
>
> First interval is tcp_keepalive_time, and subsequent interval are
> tcp_keepalive_intvl
>
> Each time timer is fired (once every 7200 seconds), we re-arm it with
> the observed elapsed time (keepalive_time_elapsed)
>
> Fixing this would require to add a timestamp in inet socket, to remember
> time of next/last probe, and firing the timer using
> min(keepalive_time_when(tp), keepalive_intvl_when(tp))
>
> Probably not worth it.
>
>
Make a lot of sense. However, I got the impression from Rick that having tcp_keepalive_intvl > tcp_keepalive_time behaved correctly in older versions of the kernel.
Regards,
Jamie Gloudon
next prev parent reply other threads:[~2012-12-29 19:54 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-21 22:05 Is keepalive behaving as expected in 3.7.0+/net-next? Rick Jones
2012-12-27 21:54 ` Eric Dumazet
2012-12-29 19:47 ` Jamie Gloudon [this message]
2012-12-30 20:51 ` Rick Jones
2012-12-31 1:50 ` Jamie Gloudon
2013-01-03 20:13 ` Rick Jones
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