difference between p2p and client-server node design

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I have some time of experience in server-client application development with c++ native call (epoll,etc), now i am trying to do something with p2p with netty. But I couldn't make clear different between p2p and server-client design. IMHO, p2p is just like a server application but also will make connection to other peer(server) like a client, am I right? which protocol tcp/udp is more common to use in a p2p design? The file sharing p2p applications I have seen are all tcp base. The p2p example from netty guide are using udt, i guess it would be a good use case for peer-to-peer gaming, am I right?

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You are correct in assuming P2P peers are applications with both server and client functionality. Server being the fact that it receives connections from other peers and client being the fact that it connects outward to other peers.

TCP is possible but due to most home/corporate/ISP networks using NAT, peers would be required to setup port forwarding on their routers. This is not practical/possible for the average computer user. A technique using UDP called UDP hole punching solves this (this may still not work for some NATs).

In UDP hole punching, peers would contact a third party "tracker"/"coordination"/"relay" server that would establish the connection between two peers. Lets say you have two peers, A and B. Peer A would contact the server and "punch" a hole through the NAT. The server would receive the details of the punched hole (IP Address, UDP Port). Peer B would do the same. The server would then send the IP Address and UDP port of peer A to peer B and vice-versa. Now the two peers can communicate directly.

P2P can be useful for just about everything server-client can do, if implemented right. Areas of interest in P2P include determining trust between peers, preventing someone from creating many fake peers to overthrow the network, establishing identity, etc. These are problems that have well known solutions in a client-server model.

P2P has been used for:

Filesharing - Bittorrent, Gnutella

Money exchange - Bitcoin

Marketplaces - OpenBazaar

Messaging - Bitmessage

References:

http://www.brynosaurus.com/pub/net/p2pnat/