BitTorrent Optimistic Unchoke/Bandwith probing

Question

While thinking about how BitTorrent works, a few questions come to my mind. Would appreciate if somebody can share a few possible responses.

1. Suppose a BitTorrent gets 50 peers from the tracker and then it establishes connection with 20 of them to form the peer-set. Is this peer-set randomly selected or based on their bandwith? (i understand that the peers which will be unchoked are selected based on their offered bandwidth) Subsequently, how is this bandwidth determined for each connection (a ping can give us the latency but not the bandwidth i assume)

2. The optimistic unchoke leads to the problem of free-riders in the system. Considering an unchoke might not always result in better peers, why is not possible to discard this policy at all? (I assume this policy helps peers with slow bandwith to fulfill requests, why cannot BitTorrent adopt a policy to probe the bandwith of the optimistic peer without sending data packets; and have another (maybe the 5th connection) for low-bandwidth peers so that they don't starve. This 5th channel will transmit at only a fraction of the bandwith compared to the other 4 channels) This may at least discourage free-riding?

Answer 1

1. traditionally the peers are selected at random. Some clients may have had weak biases based on previous interactions with the peers or CIDR distance. However, there is a recent proposal (which uTorrent and libtorrent implemens) suggests a consistent but uniformly distributed peer selection/priority algorithm. For more information, see this blog post. The unchoke algorithm is triggered every 15 seconds. The peers are then sorted by the number of bytes they sent during the last 15 seconds. The ones sending the most are then unchoked, and the rest are choked. So, the download rate is the 15 second average.

2. If you don't optimistically unchoke peers, there's no way for you to prove to them that you are better than the other peers in their unchoke set, and they will never unchoke you back. Without optimistic unchokes (also assuming you don't have the allow-fast extension), there is no way to start a download. When you first join, you won't have any pieces, you can't trade the first piece, you have to rely on being optimistically unchoked. Estimating someone's bandwidth without sending bulk data is hard and probably unreliable. Even if you got a good estimate of someones capacity, that wouldn't necessarily mean that capacity was available to you. The current mechanism is very robust in that it doesn't need to make assumptions about the network equipment between the peers (like the packet-train bandwidth estimation needs to do) and it looks at actual data.