Question

How does one determine which peer you are connected to has the fastest connection(upload rate)?

Does the actual connection of the peer dominate who is fastest or will the peer who needs the most chunks cause him to upload the fastest as less people are downloading from him?

I want to write an algorithm which takes all the peers in the peer list returned from the tracker and determine wither which peers are closer using a ping and timing the response or some other way.

Thanks

Answer 1

A ping (ICMP echo request/reply) will give you the latency of a peer, but not the available bandwidth the peer has. You want the bandwidth since TCP is good at doing bandwidth*delay products and figuring out how to make a connection fast, even if it roundtrips a satellite.

What you do is to connect to all of them. Having 40 peers connected is not uncommon. And then you decide upon which to unchoke based on their current rates towards you (until you become a seeder). It also has to be fairly dynamic, since available bandwidth change over time. The best advice I can give is to read

http://www.bittorrent.org/bittorrentecon.pdf

which gives the general idea of how to implement the economics. But many clients do different things than the paper, so reading code is another option.

So: You want to measure bandwidth, not latency. Hence, ping is the wrong tool for the job. Measuring bandwidth is most easily done by tracking the rate at which you send packets to a peer.

I think that the choking/unchoking algorithm and selecting peers to unchoke is one of the hardest parts to get right in a client. It is best solved with pen, paper and brain, not by sitting in front of the computer writing code.