Re: Linux TCP's Robustness to Multipath Packet Reordering

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 27 avril 2011 à 18:22 +0200, Dominik Kaspar a écrit :
> 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
> 

SACK is a win in a normal environnement, with few reorders, but some
percents of losses ;)

Given the limit of 3 blocks in SACK option, and your pretty asymetric
paths (10ms and 100ms), SACK is useless and consume 12 bytes per
frame...

You really should add traces to every tp->reordering changes done in our
TCP stack, its a 20 minutes patch, and would help you to understand
where/when its increased/decreased.