Question

For my course, we have to build a bittorrent client in Java. I’m having a tough time conceptualizing how connections are dealt with. I’m going to go step-by-step and point out the part that’s bothering me. If I make an incorrect statement please correct me but for the most part I understand the protocol, it’s more of an issue with connections from incoming peers and dealing with ports and sockets.

So when you connect with the tracker, you tell it the port you will be listening to, for peers wanting to connect with you.

The peer dictionary you get from the tracker includes all those peers ports they are listening on. To connect with those peers, you just create a socket on a random port and then transmit the handshake.

Now my question is, is if you’re the peer on the other side. That is, if you’re listening on the port you gave the tracker and a peer sends you a handshake on that port how do you communicate to the peer that you want it to transmit to a different port.

I’ve gotten very confused with ports and sockets trying to figure out how to deal with downloading from more than one peer and uploading as well but here are some assumptions I’ve made:

1. For every peer you connect with you use a new port/socket which you upload(if they’re an active peer)/download with.
2. The port given to the tracker is used only for listening to incoming peers.

I think maybe my understanding of ports and sockets maybe flawed which is what’s keeping me from grasping the problem. I hope I worded this correctly too. Thanks.

Answer 1

While your server listens on a specific port for incoming connections, the underlying protocol will switch to a new port as part of the handshake. You don’t have to tell the peer to use a different port.