Re: Linux TCP's Robustness to Multipath Packet Reordering

Yuchung Cheng
2011-04-27T17:40:18Z

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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