Spreading of a file in a P2P system

> By Matthieu Latapy and Abdelhamid Salah Brahim

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In a peer-to-peer (P2P) file exchange system, there is no central server that would store all available files and distribute them to users. Instead, the system self-organises in a way such that exchanges occur directly between users, between peers.

Here we represent the spreading of a file F among users of such a system: each node represents a peer, and each peer P is linked to the peers which provide F to P. The dynamics consists in appearance of new nodes and links.

In general, a new node P arrives with links to one or several other peers, which means that they were the first to provide F to P. Afterwards, P may be linked to other peers because they provide F to P, or because P itself provides F to them.

One may observe many facts in this video. First, it clearly shows that many peers are linked to only one other, which means that they asked for F but never provided it. This may be due to interrupted downloads, or to free-riding: it is well known that many peers refuse to provide files although they download some.

More interestingly, one may notice that we observe cascade phenomena: a peer provides F to another one, which provides it in turn to another one, and so on. This is essential to the system, as peers generally disconnect rather rapidly, and this illustrates an important feature of peer-to-peer exchanges: even though the initial provider is not in the system anymore, F is still available.

One may also notice that some peers appear disconnected from the others. This means that they are provider of F when they arrive in the system: they probably downloaded it previously or elsewhere.

(New nodes and links appear in red; afterwards, the color of nodes indicates their time of arrival: the first arrived nodes are white, and then darker and darker.)

See also this other video of a (simplier) spreading phenomena.

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