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Most of the time (all of the time?) when I download a large file using Safari from any website I see 1/3 as much data being uploaded. So if I'm seeing 3MB/s downloading I'll see about 1MB/s uploading. I'm not doing anything else.

What is being uploaded?

To be clear, when the App Store is downloading updates this doesn't happen. 3MB/s downloading in the App Store will show about 80KB/s uploading, which is commensurate with my vague memory of how TCP/IP verifies packets received.

Is Safari automatically doing something like Bit Torrent?

2

No, Safari is not automatically doing anything like BitTorrent. No need to worry about that.

When downloading a file over HTTP/HTTPS (normally the case with Safari), the system needs to upload as well in order to facilitate the transfer of the file. This cannot be avoided no matter which browser is used.

Normally the internal "house keeping" that is uploaded is small compared to the amount of downloaded data. However, it can vary depending on configuration settings, network circumstances, etc. I.e. for example if you choose to have a low MTU settings, if you have lots of network errors (failures and retransmits), etc. - the amount of upload overhead can be larger than normal.

However 1 MB/s compared to 3 MB/s seems quite excessive. I would investigate further if the upload really stems from Safari or not. Try:

(a) downloading from other sites to see if the effect persists,

(b) download with a different browser than Safari to see if the effect persists,

(c) ensure that you haven't got other programs running that might be uploading, for example a continouus backup program (like Crashplan, Backblaze or similar, or even Time Machine) that simultaneously uploads the downloaded file to your cloud backup or NAS,

(d) if all else fails, create a dump of your network traffic and examine with Wireshark or similar to see what exactly is transferred over the network.

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    As an alternative to WireShark, which can be quite the jumble to deal with for HTTP/S traffic, I would suggest first trying Charles Proxy[0], which focusses on HTTP/S traffic by acting as a proxy server. It's a paid application but does have a free trial period. [0]: charlesproxy.com – Hugo Jan 25 '18 at 22:15

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