networking - Connecting P2P over NAT?

Question

I started to explore the option of connecting with other using a p2p connection, so I coded a simple socket program in JAVA for android devices in which the users can share simple messages p2p (I didn't have any idea about NAT then). I got to know about NAT, so I now need to establish a TCP connection with another user which uses a server for discovery but payload is transferred p2p. I have also looked at XMPP(a very good and detailed explanation of how protocol works is here) and UPnP but I dont know how to implement them.

Another interesting question that arises is of BitTorrent because they can work on any device and even behind a NAT. I am not able to get any explanation of how BitTorrent works.

I have researched a lot but I am stuck.

My questions are:

1. A detailed explanation of BitTorrent(like here, not how torrents work) and how is it able to work around NAT ?
2. Is there a way to make a NAT entry programmatically ?
3. Is socket programming sufficient for p2p ?
4. How difficult is it to create your own protocol and how can I build one ?
5. If two devices D1 and D2 want to communicate p2p and they know each other's IP. D1 sends a request to D2 and that can't get through the D2's NAT, but there should be an entry created in D1's NAT. So when D2 tries to send something D1's NAT should discover an entry with D2's IP. Then why is the packet not allowed by it ?

Answer 1

Another interesting question that arises is of BitTorrent because they can work on any device and even behind a NAT. I am not able to get any explanation of how BitTorrent works.

This statement looks like you assume that bittorrent needs full connectivity to operate.

That is incorrect.

Behind a NAT device you will still be able to establish outgoing TCP connections. Which generally is sufficient for bittorrent as long as there are other, non-NATed (or NATed but properly port-forwarded) clients in the network that can accept incoming connnections.

NAT has no impact on the flow direction of the data because connections are bi-directional once they are established. It only is problematic for the initial connection setup.

This works perfectly fine for bittorrent because bittorent does not care from which specific node you get your data. Although better connectivity generally does improve performance.

If the identity of the node matters or one-on-one transfers are an important use-case then other p2p protocols usually attempt NAT traversal first and if that fails rely on 3rd party nodes relaying traffic between those nodes who cannot connect to each other directly.

Additionally, IPv6 support will become essential in the future to maintain end-to-end connectivity because more and more ISPs are starting to roll out carrier-grade NAT for IPv4 while IPv6 will remain non-NATed