Re: Linux TCP's Robustness to Multipath Packet Reordering

Dominik Kaspar
2011-04-27T19:56:53Z

Hi Yuchung,

Yes, FACK was enabled (as it is by default), but as Alexander already
pointed out, it should be disabled automatically when TCP detects
reordering.

However, I am not so sure how well this automatic turning off FACK is
done by Linux... I see a tendency that in situations with persistent
packet reordering, TCP with FACK enabled gets a lower performance than
if FACK is disabled right from the beginning of a connection.

Greetings,
Dominik

On Wed, Apr 27, 2011 at 7:39 PM, Yuchung Cheng <ycheng@google.com> wrote:
> Hi Dominik,
>
> On Wed, Apr 27, 2011 at 9:22 AM, Dominik Kaspar <dokaspar.ietf@gmail.com> wrote:
>>
>> Hi Carsten,
>>
>> Thanks for your feedback. I made some new tests with the same setup of
>> packet-based forwarding over two emulated paths (600 KB/s, 10 ms) +
>> (400 KB/s, 100 ms). In the first experiments, which showed a step-wise
>> adaptation to reordering, SACK, DSACK, and Timestamps were all
>> enabled. In the experiments, I individually disabled these three
>> mechanisms and saw the following:
>>
>> - Disabling timestamps causes TCP to never adjust to reordering at all.
>> - Disabling SACK allows TCP to adapt very rapidly ("perfect" aggregation!).
>
> Did you enable tcp_fack when sack is enabled? this may make a (big)
> difference. FACK assumes little network reordering and mark packet
> losses more aggressively.
>
>> - Disabling DSACK has no obvious impact (still a step-wise throughput).
>>
>> Is there an explanation for why turning off SACK can be beneficial in
>> the presence of packet reordering? That sounds pretty
>> counter-intuitive to me... I thought SACK=1 always performs better
>> than SACK=0. The results are also illustrated in the following plot.
>> For each setting, there are three runs, which all exhibit a similar
>> behavior:
>>
>> http://home.simula.no/~kaspar/static/mptcp-emu-wlan-hspa-02-sack.png
>>
>> Greetings,
>> Dominik
>>
>> On Wed, Apr 27, 2011 at 11:57 AM, Carsten Wolff <carsten@wolffcarsten.de> wrote:
>> > Hi all,
>> >
>> > On Tuesday 26 April 2011, John Heffner wrote:
>> >> First, TCP is definitely not designed to work under such conditions.
>> >> For example, assumptions behind RTO calculation and fast retransmit
>> >> heuristics are violated.  However, in this particular case my first
>> >> guess is that you are being limited by "cwnd moderation," which was
>> >> the topic of recent discussion here.  Under persistent reordering,
>> >> cwnd moderation can inhibit the ability of cwnd to grow.
>> >
>> > it's not just cwnd moderation (of which I'm still in favor, even though I lost
>> > the argument by inactivity ;-)).
>> >
>> > Anyway, there are a lot of things in reordering handling that can be improved.
>> > Our group (Alexander, Lennart, Arnd, myself and others) has worked on the
>> > problem for a long time now. This work resulted in an algorithm that is in
>> > large parts TCP-NCR (RFC4653), but also utilizes information gathered by
>> > reordering detection for determination of a good DupThresh, fixes a few
>> > problems in RFC4653 and improves on the reordering detection in Linux when the
>> > connection has no timestamps option. We implemented "pure" TCP-NCR and our own
>> > variant in Linux using a modular framework similar to the congestion control
>> > modules. A lot of measurements and evaluation have gone into the comparison of
>> > the three algorithms. We are now very close(TM) to a final patch, that is more
>> > suited for publication on this list and integrates our algorithm into tcp*.
>> > [hc] without introducing the overhead of that modular framework.
>> >
>> > Greetings,
>> > Carsten
>> >
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