The following steps takes place when the new peer Alice joins BitTorrent without possessing any chunks:
1. From the set of participating peers, the tracker randomly picks a subset of peers (say 30).
2. After selecting the peers, the tracker provides the IP address of these (30 peers) peers to Alice.
3. Now, Alice tries to establish concurrent TCP (Transmission Control Protocol) connections with all the peers on the list. The peers that are successfully connected to Alice are called as neighboring peers of Alice.
Key mechanism for Alice to obtain her first chunk:
• Each participating peer determines top four neighbors and sends data to only those top four neighbors.
• But, each peer randomly picks one additional neighbor for every 30 seconds and sends data chunks to the randomly selected neighbor. The randomly chosen peer is said to be optimistically unchoked.
• Because of this random neighbor selection, new peers will get a chance to receive the chunks. Thus, Alice will get the first chunk even if she has no chunks and cannot become top-four up-loaders for any of the other peers (as she has nothing to upload).
• Assume, Alice joined newly in BitTorrent without possessing any chunks and one of the neighbors of Alice selected randomly Alice as optimistically unchoked peer. Then the neighbor that picks Alice at random sends chunks to Alice, even though Alice is not in the top four uploaders of the that neighbor.
Therefore, Alice will get her first chunk, when Alice is selected as optimistically unchoked peer by one of her neighbors, as a result of random neighbor selection mechanism.
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