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Nope, it isn't necessary. It just helps with peering, and the actual download will start faster.

Are you sure? I always thought that the peers have to be seeded by the trackers.

How would you as a peer connect to any other peers without some kind of seed?

What IPs do you connect to?

Clients usually have some public, harcoded nodes to bootstrap the search: https://stackoverflow.com/questions/1181301