SFTP Transfer DL reduce Upload Bandwith

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zillakongretry
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SFTP Transfer DL reduce Upload Bandwith

#1 Post by zillakongretry » 2021-05-24 05:48

Newbee here please be gentle :)

I've been using Filezilla for transfering files from a remote server and its been great so far I love Filezilla
Thank you Filezilla such an awesome software :)

I tested this some day ago, but whenever I start transfering files from remote server to local PC, the following happened (using Speedtest)
1. DL Speed/bandwith decrease. Works as expected :)
2. UL Speed/bandwith also decrease. For example, if I don't download anything I got 25Mbit per second upload, but if I Download the UPLOAD speed decrease to 7-12 Megabit per second.
I need the upload too for streaming via media servers hehe
This is despite there is no, none, yadda other upload activity happening in my notebook. I registered almost solely to ask if this is the fault of my ISP or if this is actually a normal case, or maybe there's something wrong with Filezilla?

Hopefully someone can clarify :)

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botg
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Re: SFTP Transfer DL reduce Upload Bandwith

#2 Post by botg » 2021-05-24 06:20

It's normal for TCP transfers in one direction to affect transfers in the other direction, especially if saturating the available bandwidth. Main reason is TCP ACKs the receiver of data packets sends back to the sender. If they are delayed due to lack of bandwidth, speeds will plummet.

If using Wifi, another reason is that it only operates half-duplex, at any point in time on a channel, only one device may send data. On busy channels this increases latency. In a simple two-device situation (Access point + single laptop) if data flows both directions, each direction only gets about half (assuming the collision avoidance is fair) the total available bandwidth. Often it's even less due to the use of backoff time.

Try setting a speed limit in FileZilla that's low enough so that you can still upload at the desired speed.

zillakongretry
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Re: SFTP Transfer DL reduce Upload Bandwith

#3 Post by zillakongretry » 2021-05-24 13:28

I see... (I don't actually understand the networking stuff xd)
So basically its normal to experience drops in upload if DL bandwith is set at around half or more DL Bandwith
Thank you so much for the explanation... (I think I owe my isp an apology, seeing its not really their fault. :D )

So my best bet is to lower the DL transfer speed or pause the queue in order to maintain upload speed right? Unless there is other way to bypass the limitations? (I am using Usb to Ethernet if that matters)

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boco
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Re: SFTP Transfer DL reduce Upload Bandwith

#4 Post by boco » 2021-05-24 17:57

It's nodody's fault, it is in the design of the TCP protocol. The data stream happens in packets (of MaxMTU size or lower). However, if you send anything anywhere, you want an acknowledgement that it arrived safely, right? That's what the ACK packets do. Once the ACK packet arrived, the sender knows that packet arrived good and can carry on.
If you saturate the bandwidth with downloads, the number of ACK packets your machine sends back to the sender can get very large. As ACK packets move in the opposite direction of the data stream, they hurt your upload bandwidth. Unfortunately, on modern high-bandwidth connections, the max. upload speed is much lower than the download speed, thus it'll be reaching the upload limit sooner.

Commonly recommended is setting download speed to ca. 80% of the maximum. On modern machines and network cards, you could also leverage Priority or QOS features to control the flow. Firewalls and security software usually hurt performance to a degree.
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botg
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Re: SFTP Transfer DL reduce Upload Bandwith

#5 Post by botg » 2021-05-25 07:32

As the old adage goes: No amount of tuning and bandwidth optimization will be as effective as simply getting bigger pipes.

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