I recently had a chance to work on a iOS Client written in Swift that would talk to a UDP server. The app basically needed to send a request to the server and receive the response.
All over UDP. It turned out to be quite a simple task if done using the CocoaAsyncSocket. I set up a simple Mac OS X test server and a corresponding iOS test client. The sources are here:
My worries about evil routers blocking responses due to NATs turned out to be false. Most routers now support NAT traversal. The only exception I’ve found so far is Microsoft ISA working as a router. It cannot do the traversal thus blocking the responses.
It’s also worth mentioning the UDP hole punching technique. I haven’t used it by now, but would probably turn to that when more real-world usage data comes in.