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Unity Networking Sample Using One NetworkView
by Darrel Cusey on 02/21/13 07:10:00 pm   Expert Blogs   Featured Blogs

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The following blog post, unless otherwise noted, was written by a member of Gamasutra’s community.
The thoughts and opinions expressed are those of the writer and not Gamasutra or its parent company.

 

fps_sample_screenshot

This Unity Package can be downloaded here (updated):  fps_sample_2013_02_21.unitypackage

Unity is a great tool for Indie developers, but there are parts of it that can really make you spin around in circles.  Networking is definitely one of those parts.  I set out to make a simple peer-to-peer FPS sample project with two goals in mind:

  1. Make it as simple as possible
  2. Avoid (as much as possible) the "exotic" Unity networking components

The components I decided to avoid were NetworkViews, Network.Instantiate, and RemoveRPCs.  I picked these three because NetworkViews (even when set to "Unreliable") can create a lot of unnecessary network traffic if they are not managed very carefully.  Network.Instantiate also generates so much network traffic that the game can pause and skip every time a new player joins.  And finally, RemoveRPCs (RPC = "Remote Procedure Call") only works when you are using Network.Instantiate, so it goes out the window as well.  I really wanted to do this as a true peer-to-peer, but Unity's built-in networking is geared strictly for client-server networking.

Update:  I recently found a good way to do Server Discovery for the LAN, please post a comment if you'd like me to post that code as well :)

So, what does that leave us with?  It leaves us with...

Unity Networking Sample Using One NetworkView

If you start digging through LOTS of other Unity Networking tutorials, they'll follow the "standard route" of adding a NetworkView to your player prefab and then relying on Unity to decide when to send updates, and how much data will be sent.  Using this setup, EACH player will have its own chatty NetworkView sending out information to everyone else more or less continuously.

We're not going to take the standard route.

However, we're still going to need a single NetworkView because that's the only way Unity can communicate over the network (since we don't want to re-create the wheel and build our own Socket manager in .NET).  However, a single NetworkView component attached to an empty GameObject can be set to State Synchronization=OFF, and Observed=NONE.  This will allow us to send RPCs back and forth between client and server without all the overhead and without all the unnecessary complexity.

Which would leave us with something like this:

fps_sample_overview

At the same time, we still want a way to guarantee a unique ID is assigned to each player.  To do this, we could use any of .NET's functions specifically designed for this task.  But if we did that, then we'd have to import at least another package -- which isn't really worth it just for one function.  A better option is to just use AllocateViewID() to get a unique object on the server even though our actual player object won't have their own unique network views.  Yes, that's totally cheating :)

Having just a single NetworkView over which RPCs can be sent means there are a few extra responsibilities we have to take on:

  • Maintaining a dictionary between GameObjects and NetworkPlayer objects so that we can easily lookup one from the other.
  • Managing the creating and destruction of player objects on all clients (from the server) as players join or disconnect.
  • Devising a way for clients to be able to distinguish between messages that are meant for them and messages that are meant for other players

To help explain how this is accomplished, let's take a look at a process flow diagram.  Did I mention that I <3 process flow diagrams.  I should probably mention that before we get too far into things and PFDs start flying all over the place.

simple_fps

If you haven't done so already, grab the Unity package and open up the networkController script so you can follow along.

OK, let's look at the important parts, starting with our first RPC in OnPlayerConnected()

networkView.RPC("JoinPlayer", RPCMode.All, newViewID, Vector3.zero, p);

In OnPlayerConnected(), we execute this RPC call to all connected clients using the server's world networkView.  Notice that we are using RPCMode.All - which will send the JoinPlayer() RPC to all players and the server.  The player object must be created on all clients and the server, and this is the simplest way of accomplishing that.

Also, notice that the server AND the client is maintaining the HashTable of all players.  This is done in the JoinPlayer() RPC with this simple command:

players.Add(p,newPlayer);

...which is removed when a player disconnects with something very similar in the DisconnectPlayer() RPC:

players.Remove(player);

Now, in the JoinPlayer() RPC, we have this critical comparison:

if(p.ipAddress==LocalAddress)

In Start(), we determine the computer's Local IP Address using Dns.GetHostEntry(Dns.GetHostName()) from .NET's System.Net.Socket package and then use this to compare it against the NetworkPlayer structure's "ipAddress" attribute that is being sent from the server.

Now, at this point, you're probably asking, "well what about everyone else?"  After all, it's all well and good that the player and server know about each  other now, but what if there are already players in the game?  How does the server tell the new player about existing players.  Before that can be done, the client must wait until it is fully connected to the server, which is why we wait until the client determines that it is connected before requesting the complete player list from the server.

We use the built-in function OnConnectedToServer() to get our timing just right, and then request the player list:

void OnConnectedToServer()
{ 
   networkView.RPC("SendAllPlayers", RPCMode.Server);
}

Because we use RPCMode.All in the original JoinPlayer() RPC, the server has instantiated a GameObject for every client that is joined.  To get a list of all players, the server only needs to run this code once every time a new player joins in the SendAllPlayers() function:

GameObject[] goPlayers = GameObject.FindGameObjectsWithTag("Player");

For each member of goPlayers, the server extracts the NetworkPlayer and the allocated NetworkViewID, and sends those on using more RPC calls to the JoinPlayer() function.  However, we make sure that we are NOT sending the requestor's data back to them by doing this comparison:

if(gonp.ToString() != info.sender.ToString())

Because the allocated NetworkViewID is guaranteed unique within the server session, this is far more reliable than using RPCMode.Others (which, despite Unity's claims to the contrary, actually DOES send a message back to the original sender).

The client ALSO checks the incoming NetworkPlayer data structure's ipAddress against its own LocalAddress -- but this is really just a cross-check to insure that no client can get a doppelganger :)

...because no one like doppelgangers.

So, that's pretty much it, a complete FPS Networking Sample that only uses a single world NetworkView that generates minimal network traffic.

So is this the end of our peer-to-peer networking dream?  Hmmm, you'll notice that the Hashtable of all players is being maintained on all clients.  So, in reality, each client already knows everything the server knows.

If you are guessing that I've already thought of a way that any given client can:

  • detect when the server disconnects,
  • using ipAddresses, determine if it should take the place of the server,
  • turn itself into the server,
  • and finally tell all the other clients that it is now the server...

...well, if you've guessed all that, then you should really consider getting checked out to see if you are psychic or something :D

To see what I've got so far, grab the Unity Package here:  fps_sample_2013_02_21.unitypackage

As always, thanks for reading!

Follow me on Twitter @Tulrath 


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