iPhone - Peer to Peer connection over the internet

Question

I am building an app that uses a async socket connection with a web server. For sending large amounts of data I would like to connect two iPhones into a Peer to Peer connection using an async socket. I am more familiar with GCDAsyncSocket than to the iOS SDK socket API.

I have done something similar, but only within a local network were I would user NSNetService to publish a presence and get the addresses needed from the p2p connection.

Can you give me a hint on how to connect two iPhone over the internet using a p2p socket connection?

Answer 1

Please be specific, u may use GCDAsyncSocket to connect 2 phones, u may use sample code from Github.

Answer 2

I do not know much about NSNetService but it seems that it only works in a local network. If you want to establish a p2p connection from an iphone to another you will always have the problem of several NATs (Network address translators) in your route. How to find the address and route to the peer? And how to get trough all the firewalls and NATs?

I am working on an app that needs to deal with the same problem and I have answered a similar question before which I will cite here:

Actually, it IS possible. You may want to google for something called "UDP hole punching" or "TCP hole punching".

The main approach in short: Assuming you got something like a relay server, that is some server in the internet that is publicly addressable from every private LAN that is connected to the www. No you have your two clients A and B in (different) private LANs, with some Network address translation (NAT) going on, that want to establish a peer to peer connection.

First of all both will tell the server their IP address and the port they have in their own LAN. In the UDP or TCP packet, the server will find the public address and port of the device (or the NAT (router)). So the server knows the private and the public IP address as well as the ports.

If now A wants to communicate with B, it asks the server for help. The server will send a message to B that A wants to communicate with her telling her A's public and private IP and port. A gets back B's public and private information and port.

Now here is where the magic happens. Both clients now send packets out to establish a connection simultaneously to the private and public addresses of the other party and thus punching a whole in their NATs such that incoming connections will not be blocked. Even if one party's connection establishing packets will arrive before this whole is created, the other's packets will get through to such that a connection can be created.

Beware of some NATs that scan the data for IP addresses and translate them as well, but if you encrypt your data or change the appearance of the address (complement, ...) you will be fine.

Now the master question, how can the server communicate with one of the clients without an active connection. Well in this case you can use "connection reversal" and apple's "push notifications". Use the "push notifications" (pn) to tell a client behind a NAT that there is something of interest going on and that it should contact the server. Once it has done that the connection is active and can be used in the previous described fashion.

I hope this helps some people that get to this problem although the post is quite old!

Now if you find this solution helpful and you try to implement it, please tell me if you are successful in getting the public ip and port of the server-client connection on the server side. You can also answer my question about this on stackoverflow.