UDP for multiplayer game

Question

I have no experience with sockets nor multiplayer programming.
I need to code a multiplayer mode for a game I made in c++. It's a puzzle game but the game mode will not be turn-based, it's more like cooperative.
I decided to use UDP, so I've read some tutorials, and all the samples I find decribes how to create a client that sends data and a server that receives it.
My game will be played by two players, and both will send and receive data to/from the other.

Do I need to code a client and a server?
Should I use the same socket to send and receive?
Should I send and receive data in the same port?

Thanks, I'm kind of lost.

Answer 1

Read how the masters did it: http://www.bluesnews.com/abrash/chap70.shtml

Read the code:

git clone git://quake.git.sourceforge.net/gitroot/quake/quake

Open one UDP socket and use sendto and recvfrom. The following file contains the functions for the network client.

quake/libs/net/nc/net_udp.c
UDP_OpenSocket calls socket (PF_INET, SOCK_DGRAM, IPPROTO_UDP)
NET_SendPacket calls sendto 
NET_GetPacket  calls recvfrom

Answer 2

Do I need to code a client and a server?

It depends. For a two player game, with both computers on the same LAN, or both on the open Internet, you could simply have the two computers send packets to each other directly.

On the other hand, if you want your game to work across the Internet, when one or both players are behind a NAT and/or firewall, then you have the problem that the NAT and/or firewall will probably filter out the other player's incoming UDP packets, unless the local player goes to the trouble of setting up port-forwarding in their firewall... something that many users are not willing (or able) to do. In that case, you might be better off running a public server that both clients can connect to, which forwards data from one client to another. (You might also consider using TCP instead of UDP in that case, at least as a fallback, since TCP streams are in general likely to have fewer issues with firewalls than UDP packets)

Should I use the same socket to send and receive? Should I send and receive data in the same port?

You don't have to, but you might as well -- there's no downside to using just a single socket and a single port, and it will simplify your code a bit.

Answer 3

Note that this answer is all about using UDP sockets. If you change your mind to use TCP sockets, it will almost all be irrelevant.

• Do I need to code a client and a server?

Since you've chosen to to use UDP (a fair choice if your data isn't really important and benefits more from lower latency than reliable communication), you don't have much of a choice here: a "server" is a piece of code for receiving packets from the network, and your "client" is for sending packets into the network. UDP doesn't provide any mechanism for the server to communicate to the client (unlike TCP which establishes a 2 way socket). In this case, if you want to have two way communication between your two hosts, they'll each need server and client code.

Now, you could choose to use UDP broadcasts, where both clients listen and send on the broadcast address (usually 192.168.1.255 for home networks, but it can be anything and is configurable). This is slightly more complex to code for, but it would eliminate the need for client/server configuration and may be seen as more plug 'n play for your users. However, note that this will not work over the Internet.

Alternatively, you can create a hybrid method where hosts are discovered by broadcasting and listening for broadcasts, but then once the hosts are chosen you use host to host unicast sockets. You could provide fallback to manually specify network settings (remote host/port for each) so that it can work over the Internet.

Finally, you could provide a true "server" role that all clients connect to. The server would then know which clients connected to it and would in turn try to connect back to them. This is a server at a higher level, not at the socket level. Both hosts still need to have packet sending (client) and receiving (server) code.

• Should I use the same socket to send and receive?

Well, since you're using UDP, you don't really have a choice. UDP doesn't establish any kind of persistent connection that they can communicate back and forth over. See the above point for more details.

• Should I send and receive data in the same port?

In light of the above question, your question may be better phrased "should each host listen on the same port?". I think that would certainly make your coding easier, but it doesn't have to. If you don't and you opt for the 3rd option of the first point, you'll need a "connect back to me on this port" datafield in the "client's" first message to the server.