What happens when I download the same torrent file using BitTorrent in two different pcs and using the same internet connection?

Question

If I have two laptops and connect them to the same internet connection then download the same torrent file using a BitTorrent software, what do the trackers see? and how will the torrent software behave??

Let's say in Laptop A I start downloading the torrent file then after 30 minutes, I download the same torrent file on Laptop B.

Will the two laptop become peers and send each other data? Will Laptop B get data from Laptop A?

Will the trackers in the torrent file think that there is only one laptop/one peer trying to download the file?

Or will the tracker only see one public IP address since they are using the same internet connection?

Farquhar

Answer 1

Will the two laptop become peers and send each other data? Will Laptop B get data from Laptop A?

Assuming that two BitTorrent clients are behind the same NAT:

If the torrent is public (the private flag is not set) and both clients support Local Peer Discovery, they can find each other that way.

If the NAT support Hairpinning and preserve the same external and internal port for at least one of the clients, there is a good chance that they can connect to each other that way too.

Will the trackers in the torrent file think that there is only one laptop/one peer trying to download the file?
Or will the tracker only see one public IP address since they are using the same internet connection?

They will share the same IP. If the tracker can differ between them with the help of the port they connect from and the different peer_ids they use, depends on the specific implementation the tracker use.

Glide

Thank you for the answer! If Local Peer Discovery is supported, even if the torrent file has thousands of seeders, they would s till find each other? And if not, it's less likely but there's still a chance? So local peer discovery is the deciding factor if laptop B can get data from laptop A right? Also, laptop B should be able to download the file faster since it'll be getting them from laptop A? – Farquhar

Seems resonable that a client prioritize peers from LPD, but that depends on the implementation. The speed between them should only be limited by the local LAN speed, if the client don't have any additional limits set. For example: uTorrent has a advanced setting: net.limit_excludeslocal that must be set to true for full speed if global speed limits are set. – Glide