Re: Linux TCP's Robustness to Multipath Packet Reordering

Yuchung Cheng
2011-04-27T21:41:43Z

AFAIK, FACK is disabled throughout the life of the connection after
sender detects reordering degree > 3.

But Alex said the reordering has some bugs. I suspect these bugs may
affect FACK/sack auto-tuning. Maybe Alex could describe the reordering
bugs?

On Wed, Apr 27, 2011 at 12:56 PM, Dominik Kaspar
<dokaspar.ietf@gmail.com> wrote:
> 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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>>
>