Date:	Tue, 22 Nov 2011 21:06:04 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	Jesse Young <jlyo@...o.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Missing TCP SYN on loopback, retransmits after 1s

Offhand, I'd guess you're overflowing the TCP SYN queue.  (You can try
tuning tcp_max_syn_backlog.)

  -John


On Tue, Nov 22, 2011 at 7:13 PM, Jesse Young <jlyo@...o.org> wrote:
> Hi all,
>
> I am experiencing packet loss over TCP/IPv[46], which causes 1 second
> delays when connect()ing to a socket. This happens even on loopback, and
> on multiple kernels. On the older kernels, the connect() time is nearly
> 3 seconds, I believe this is due to a recent TCP connect retrasmit
> parameter changed in the kernel.
>
> 1. Linux dc-s1000-2114 2.6.32-35-server #78-Ubuntu SMP Tue Oct 11
>    16:26:12 UTC 2011 x86_64 GNU/Linux
> 2. Linux dc-a1000-2131.cleversafelabs.com 2.6.39.4-2-clevos+ #1 SMP
>    Tue Nov 8 09:06:49 CST 2011 x86_64 x86_64 x86_64 GNU/Linux
> 3. Linux telperion.jlyo.org 3.1.0-4-ARCH #1 SMP PREEMPT Mon Nov 7
>    22:47:18 CET 2011 x86_64 Intel(R) Core(TM) i7-2630QM CPU @ 2.00GHz
>    GenuineIntel GNU/Linux
>
> I have created some test cases which reify this problem, the first set
> of tests use select() multiplexing, and have some problems, however,
> they exhibit odd behavior as well, especially in the difference between
> tcp4 and tcp6.
>
> Please note: these tests will quickly exaust the amount of available
> ephemeral TCP ports on your system, which will cause any TCP connect()
> calls in other processes to return with EADDRNOTAVAIL. However, ports
> will become available after a short while.
>
> The first test fails super quick, while the others haven't timed out
> so far.  NOTE: The second test requires /proc/sys/net/ipv6/bindv6only
> to be set to 1.
>
> ./packetloss :: ::1
> ./packetloss :: 127.0.0.1
> ./packetloss 0.0.0.0 127.0.0.1
>
> The other tests run a client and server in different processes.
> Run the "close" daemon using one of:
> ./closed ::
> ./closed 0.0.0.0
>
> And flood connect() pings against 8009, the port closed listens on.
> ./tcping -f -p8009 ::1
> ./tcping -f -p8009 127.0.0.1
>
> Wait for a pause, then ^C, and notice the max statistic is ~1000ms.
>
> These tests have been rn between machines on a relativley noiseless
> ethernet LAN with similar results.
>
> What's also puzzling, is that I see no packet drop reporting in
> $ ifconfig lo
> lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 16436  metric 1
> inet 127.0.0.1  netmask 255.0.0.0
> inet6 ::1  prefixlen 128  scopeid 0x10<host>
> loop  txqueuelen 0  (Local Loopback)
> RX packets 276411482  bytes 15822880567 (14.7 GiB)
> RX errors 0  dropped 0  overruns 0  frame 0
> TX packets 276411482 bytes 15822880567 (14.7 GiB)
> TX errors 0 dropped 0 overruns 0 carrier 0 collisions
>
> I'm thinking this may be a bug in the TCP/IP stack, however, I'm not
> certain if I'm missing a socket option, or some other configuration
> that may elimiate this behavior.
>
> If there's anything else I can help you with, please don't hesitate
> to Cc me.
>
> Thanks,
> Jesse
>
> Attached: syndrop.pcap
>
> Get the code here
> https://github.com/jlyo/packetloss
> git clone git://github.com/jlyo/packetloss.git
>
> https://github.com/jlyo/tcping
> git clone git://github.com/jlyo/tcping.git
>
> https://github.com/jlyo/closed
> git clone git://github.com/jlyo/closed.git
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