Can peers in P2P locate each other without a tracker?
What happens when their IP changes?
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Can peers in p2p locate each other without a tracker?
Yes and No.
The standard BitTorrent technology do uses a tracker for each peer to 'report' back to so they can be found by other peers. Once they found each other, they can then go directly peer to peer and ignores the tracker.
If you read the specification on BitTorrent protocol available here, there is an official extension to the BitTorrent protocol called "Distributed Hash Table" quoted below:
Distributed Hash Table
This extension is to allow for the tracking of peers downloading torrents without the use of a standard tracker. A peer implementing this protocol becomes a "tracker" and stores lists of other nodes/peers which can be used to locate new peers.
So yes you can go without using 3rd party tracker, but in return YOU (or the other peer) became the tracker for the other peer to go to, and find you.
What happens when their IP changes ?
As above, due to the tracker assisting peers to connect to each other, any IP changes will be reported to the tracker, and then the peer can simply connect to the updated information on the tracker.
Hope this helps.