How to let player find an opponent online?

I am going to make a game like Mortal Kombat for iOS. How can I make a function to let player to find an opponent and fight him online? For instance. Player "Bob" wants to play online. He presses the button "Find an opponent" to find a free player and waits. Meanwhile the other player named "Jack" does the same thing. Eventually they are redirected to the world and fight... Do you know how to do that? Do I need external network engines such as Photon or uLink? Please send me a piece of code or send me a link if it has been discussed before.