How the game will search for other online users and will display the list of all users ?
You'll need a server that returns a list of online players to your iPhone client is some kind of data format, maybe XML or JSON.
How one can request someone to join the game and then visible to other users ?
The easiest approach would be the person who wants to to join the game sends that command to the server, the server tells that other person they want to join. Wait for a response. If player two says "yes", returns to the server which forwards to the first player.
It's basically a series of commands sent to and from the server. This is how all multiplayer games work - for example the Quake engine sends very compact commands, as little as 4 bytes for things like "get me all players on a server". Given the flakeyness of the iPhone connection this would be a good model to copy, as the Quake networking code was designed for 56k modems.
How the connection and session will work between players of a table ? (Sockets ?)
The connection could either be a continuous stream (UDP would be best), or polling from the client. You'll need to consider scaling for both, as it's possible 100 people playing could bring your server to its knees.
The session will be dropped if one player loses their connection. Alternatively as it's turn based, each player can simply send a command when they're online and the other client picks this up when he's online - there's no need for session in this model, and the commands are stored on the server in a datastore.
What sort of network programming is necessary as part of the server and client ?
I'll broaden this out:
- An in depth knowledge of byte and bit manipulation including bit shifting
- A knowledge of creating and reading UDP packets
- Network programming in C (Objective-C may simplify a lot of this)
- The server: writing daemons on Linux, or services on Windows to listen for commands
- A knowledge of SOAP, XML or JSON if you're not concerned with bandwidth and connection drops
