Re: Linux TCP's Robustness to Multipath Packet Reordering

Dominik Kaspar
2011-04-27T16:22:56Z

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!).
- 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
>