Simple queue: Download speed is affected by Upload limit

RouterOS Beginner Basics
1

I have set a simple queue for a client.
He uses mostly torrents, so its always uploading and downloading and he is complaining about unstable/lower than his line speed.
The Upload limit of his queue is set to 512k and download to 4MB

I have noticed that when the upload speed hits the limit it brings down the download speed too.
Is this normal?

For eg, if he is downloading at around 3MB, and as soon as his uploading reaches 512k then his download drops to around 1.7mb and fluctuates around 2mb

How do I set a stable speed for him?
(I have unshaped bandwidth from my provider)

2

BitTorrent uses multiple TCP sessions. If the upload becomes saturated due to the BitTorrent client attempting uploads then sessions being used for download may also show reduction in throughput due to their ACKs being delayed/lost.

3

I see, thanks for the info. So is there a solution?

4

You could mark packets in Mangle where packet-size=150-9900, queue those, and leave small packets like ACK’s unlimited. I believe 150 is the smallest packet µTorrent can use for payload.

I have observed a strange effect where libtorrent/rtorrent clients would download no faster than their upload (our download), even though their download was not being marked and affected. This probably doesn’t matter in this case, where our download is higher.