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Exxonen
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Packets loss in L2TP/IPSec tunnel

Tue Jan 14, 2020 9:34 pm

Hi!

I have site-to-site L2TP/IPSec connection between CCR1009 over high-latency link (200ms)
both running 6.46.1
one side has 30M/30M connection, another one 300M/15M.

SMB/FTP Speed is pretty bad across this tunnel, so I decided to debug it:
I've set up packet capture on both CCRs and ran bandwidth test limited to 1mbit (to make sure that channel saturation is not a factor) from machine in one subnet to machine in another one.
basically I have sniffers on both ends of tunnel.

In capture results I can see packets that enter tunnel on one side, but never exit another, thus triggering "fast retransmission". Nothing is fast about it though, due to high latency.
tx/rx errors/drops on both sides of tunnel don't increase.
Number of packets lost is pretty small: approx 10 out of 10000, or 10 per 60 secs.

Also I observe out of order packets time to time, which is not as bad, but still suspicious.
I am not sure either of this may cause speed problems, but fast retransmits definitely affect wnd size.

questions:
1) is it normal to have packets reordered while in tunnel?
2) is it normal for tunnel to drop packets quietly?
3) any typical suspects in settings to look at?

thanks.
Andrey

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