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I just witnessed something that blew my mind. I was downloading something on BitTorrent, and it was downloading at a fairly high rate (~1MB/s). The upload speed was very low (~15kB/s courtesy of lots of seeders).

What surprised me though as that while BitTorrent was running (and downloading), I couldn't access any webpage, or ping any website. What was even more weird was that Windows was showing me the "I'm connected to the wireless router but there is no internet connection" symbol.

Microsoft diagnostics said it couldn't find the DNS server (which explains why I couldn't access/ping webpages). When I looked at the networking tab, of course it showed me plenty of activity courtesy of BitTorrent.

Everything went back to normal as soon as the download finished.

I suppose my question here is: is it possible for BitTorrent to completely hog my bandwidth, leading Windows to believe that my computer is not connected to the internet? How does Windows actually determine if my computer is connected to the internet?

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For how Windows determines internet connectivity, see this SuperUser question. Basically, Windows tries to access a Microsoft hosted text document and judges your connectivity depending on the response (timeout, 403 denied, 200 OK, etc).

Yes, BitTorrent (or anything else) using excessively large chunks of your bandwidth can saturate your connection. Basically, you're transferring so much at once that other packets end up timing out and getting dropped. Imagine a water pipe, if you try to put too much in it will fill up faster than it can be emptied and start overflowing, losing the water that overflows.

A typical IP (Internet Protocol) request requires you to send some packets to the server you are requesting from, which will then send some packets back - its response. This requires both some upstream and downstream bandwidth. When your upstream bandwidth becomes saturated, you can't send the request. When your downstream bandwidth becomes saturated, you can't receive the response. It is entirely possible for a BitTorrent client to use all your available bandwidth.

Generally, it is best to set your BitTorrent client to only utilise up to 80% of your upstream and downstream bandwidth, as determined by speed tests. If you require low latency, e.g. online gaming, the percentage should be even lower.

It's also possible for BitTorrent to overload home modems and routers by opening too many connections, overflowing their NAT tables. The maximum number of connections should therefore be kept at a fairly low level. A maximum of 300 connections globally should be alright. An overflowing NAT table's symptoms will vary from router to router, but often cause them to freeze.

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Yes, it is possible for BitTorrent to completely hog your bandwidth. However, there is typically code within the bittorrent client to address the usage of the Internet while the program is running. It, in theory works to accommodate your actions of pinging or browsing with bittorrent to allow them to happen at the same time. Sometimes using Transmission-GTK on GNU/Linux I can't even ping a website if the download speed is quite high.

Windows' networking can be quite finicky sometimes. I have had Windows 7 tell me that I have 1 bar of wifi service, but I'm downloading a 1 MB~ a second, or that I just lost connection while browsing the Internet, still browsing the Internet. Later it says I'm connected. Software bugs?

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