How to advertise a TCP host via UDP?

Question

I have an application (essentially a game) that is broadcasting game state data via UDP to many connected clients on a private LAN.

UDP works fine for broadcasting game state. Not having to configure the clients is important for this app. The client just read the UDP datagram stream and build up state as it goes.

But now I need the clients to reliably download a few pieces large data payload from the server. TCP is way better then UDP for that.

But we still rather not have to configure each and every clients with the host info. It would be better to just embed an service advertisement in the broadcast UDP stream and then have each client see the advertisement and connect to the TCP host with no extra configuration on the endpoints.

Is there an standard way, or better, example code of advertising a TCP service via UDP. Preferably in C++.

I think it really depends on how your client is coded to act on messages from UDP. Can you explain a little how it is that your client acts on UDP packets? for instance what is the basic structure of a packet you use? If the client never had any code for creating a TCP stream you'll have to modify the client so that it does have a way... — u8sand
I expect to have to extend the client and server code. The UDP protocol is mostly fixed size records but I can extend it with a 'custom datagram' that I have my own data handler for. I am thinking I merely need to send the new service advertisement enum along with the local host name of the server and the open port number the host is listen too. When the client datagram handler sees the service, it just starts a tcp connection in a thread with the hostname. — meissnersd
In addition to finding a solution, you should also beware about man-in-the-middle attacks: a malicious program could inject/modify the stream to ask the client to download a malicious update from another ip/port than yours... (granted, one in a position to alter the stream is probably already in a good position to do a lot of other things... but can't hurt to try to prevent the preventable parts) — Olivier Dulac
yeah I was thinking about man-in-the-middle. I will need to make some sort of secure handshake. — meissnersd

usr usr · Accepted Answer · 2013-01-07T16:16:34

The client needs to know the IP and port of the TCP server, that is all. If you can embed that info into your protocol it will work.

Actually, the UDP clients probably know the IP already because the UDP packets have a sender IP. Maybe this fact can help you.

How to advertise a TCP host via UDP?

2 Answers