Now, with video-on-demand (and live streams), the whole Tit-For-Tat system no longer works. In this situation, peers must obtain pieces in order for playback.
Not completly true. With video-on-demand it is enough that you get the occasional later piece that you can trade for more common earlier pieces. Of course, as clients are prioritizing the earlier pieces the tit-for-tat will be less efficent. Not useless though.
Yes, you're right. In my comment I simplified the situation. A peer can indeed also fetch later pieces while downloading the earlier pieces in order.
Standard BitTorrent works because of the Tit-For-Tat incentive mechanism. The whole idea is that a peer exchanges pieces with another peer, so it can achieve a higher download rate than just getting pieces from the seed.
Now, I won't go into details, but the reason you get your files in some arbitrary order is because a BitTorrent swarm is just like a marketplace. Certain pieces are rare and might have long queues, i.e. many peers are interested in it and are competing for it. Other pieces are so common, most peers are not interested in it and can thus be exchanged with fewer peers. So the trick to achieving high download speeds is to obtain the right pieces that are still valuable for further trade, while not spending too much time on obtaining such a valuable piece.
Now, with video-on-demand (and live streams), the whole Tit-For-Tat system no longer works. In this situation, peers must obtain pieces in order for playback. The problem now is that a peer that wants the next minute of the video can only get it from a peer whose playback position is further ahead. The latter peer however is not interested in pieces from the former (since it already has these) and thus no exchange will take place.
So, the solution the Tribler team came up with is the Give-to-Get incentive mechanism. A peer will only receive pieces from others if it sends its pieces to those that are interested, i.e. peers that have seen even less of the video. This requires some feedback, so a peer that receives some pieces will have to inform others that it recently has received data from a certain sender. Thus, you could say that the Give-to-Get incentive mechanism is based on reputation.
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Now, with video-on-demand (and live streams), the whole Tit-For-Tat system no longer works. In this situation, peers must obtain pieces in order for playback.
Not completly true. With video-on-demand it is enough that you get the occasional later piece that you can trade for more common earlier pieces. Of course, as clients are prioritizing the earlier pieces the tit-for-tat will be less efficent. Not useless though.
Yes, you're right. In my comment I simplified the situation. A peer can indeed also fetch later pieces while downloading the earlier pieces in order.
Standard BitTorrent works because of the Tit-For-Tat incentive mechanism. The whole idea is that a peer exchanges pieces with another peer, so it can achieve a higher download rate than just getting pieces from the seed.
Now, I won't go into details, but the reason you get your files in some arbitrary order is because a BitTorrent swarm is just like a marketplace. Certain pieces are rare and might have long queues, i.e. many peers are interested in it and are competing for it. Other pieces are so common, most peers are not interested in it and can thus be exchanged with fewer peers. So the trick to achieving high download speeds is to obtain the right pieces that are still valuable for further trade, while not spending too much time on obtaining such a valuable piece.
Now, with video-on-demand (and live streams), the whole Tit-For-Tat system no longer works. In this situation, peers must obtain pieces in order for playback. The problem now is that a peer that wants the next minute of the video can only get it from a peer whose playback position is further ahead. The latter peer however is not interested in pieces from the former (since it already has these) and thus no exchange will take place.
So, the solution the Tribler team came up with is the Give-to-Get incentive mechanism. A peer will only receive pieces from others if it sends its pieces to those that are interested, i.e. peers that have seen even less of the video. This requires some feedback, so a peer that receives some pieces will have to inform others that it recently has received data from a certain sender. Thus, you could say that the Give-to-Get incentive mechanism is based on reputation.