KryoNet UDP connection error

[leudast1215]

I'm in the process of adding multiplayer to my indie game by creating a server/client relationship using KryoNet. I started with a simple proof of concept which worked well. I've integrated it into the game up to the point where I have data transfering back and forth (using sendTCP()). This is when I noticed a fatal connection issue.

I first start the server, then start the client. The client connects to the server and data is exchanged as expected. I then close the client connection using client.stop() and reconnect with client.start() and client.connect(timeout, "127.0.0.1", 1337, 1338) once again. Everything works fine. However, if I close and then restart the client program entirely I am unable to connect to the server (which is still running).

This is the error on the client:

java.net.SocketTimeoutException: Connected, but timed out during UDP registration: /127.0.0.1:1338

at com.esotericsoftware.kryonet.Client.connect(Client.java:186)

at com.esotericsoftware.kryonet.Client.connect(Client.java:110)

at com.zyin.game.Game.connectToServer(Game.java:439)

at com.zyin.game.gui.screen.ScreenMultiplayer.lambda$0(ScreenMultiplayer.java:42)

at com.zyin.game.gui.screen.Button.onMouseClicked(Button.java:318)

at com.zyin.game.gui.screen.Screen.onMouseClicked(Screen.java:144)

at com.zyin.game.MouseInputHandler.handleMouseInput(MouseInputHandler.java:59)

at com.zyin.game.Game.loop(Game.java:321)

at com.zyin.game.Game.run(Game.java:203)

at com.zyin.game.Game.main(Game.java:191)

This is the error on the server:

00:12  WARN: [kryonet] Error reading UDP from unregistered address: /127.0.0.1:55408

com.esotericsoftware.kryonet.KryoNetException: Error during deserialization.

at com.esotericsoftware.kryonet.UdpConnection.readObject(UdpConnection.java:89)

at com.esotericsoftware.kryonet.Server.update(Server.java:274)

at com.esotericsoftware.kryonet.Server.run(Server.java:356)

at java.lang.Thread.run(Thread.java:745)

Caused by: com.esotericsoftware.kryo.KryoException: Buffer underflow.

at com.esotericsoftware.kryo.io.Input.require(Input.java:162)

at com.esotericsoftware.kryo.io.Input.readInt(Input.java:308)

at com.esotericsoftware.kryo.serializers.FieldSerializer$IntField.read(FieldSerializer.java:341)

at com.esotericsoftware.kryo.serializers.FieldSerializer.read(FieldSerializer.java:221)

at com.esotericsoftware.kryo.Kryo.readClassAndObject(Kryo.java:732)

at com.esotericsoftware.kryonet.KryoSerialization.read(KryoSerialization.java:58)

at com.esotericsoftware.kryonet.UdpConnection.readObject(UdpConnection.java:83)

... 3 more

The error is outputted once every 100ms until the timeout is reached.

If I change the connection to a TCP only connection isntead of also using UDP it works fine. I tried this by changing my connect statement to client.connect(timeout, "127.0.0.1", 1337).

I've attempted to debug through the KryoNet code and didn't notice anything blaringly wrong, but it's also hard to do it due to the multithreaded nature of the code, timeouts constantly being hit, and update() being called so frequently.

But looking through the code and the error thrown leads me to believe that the newly created TCP connection isn't creating an entry in Server.pendingConnections for the to-be-made UDP connection. Or the RegisterUDP packet is having an incorrect connectionID associated with it. Both of which I could not confirm by stepping through the code.

I am new to this networking stuff so it is possible I'm missing something simple. Right now my best guess is it either has something to do with KryoNet's threaded implementation, or the way I close the connection on the client.

I can post code samples by request, but won't post the whole project online.

[Joachim Durchholz]

Am 16.07.2017 um 02:01 schrieb leudast1215:
> I've attempted to debug through the KryoNet code and didn't notice
> anything blaringly wrong, but it's also hard to do it due to the
> multithreaded nature of the code, timeouts constantly being hit, and
> update() being called so frequently.

In code of this nature, it's better to crank up logging.
Add logging statements if you find the existing ones do not give you the
information you need to diagnose this. I'm pretty sure the project will
be interested to know where the logging was too scarce so it can be
improved, but that's the second step.

> But looking through the code and the error thrown leads me to believe
> that the newly created TCP connection isn't creating an entry in
> Server.pendingConnections for the to-be-made UDP connection. Or the
> RegisterUDP packet is having an incorrect connectionID associated with
> it. Both of which I could not confirm by stepping through the code.
>
> I am new to this networking stuff so it is possible I'm missing
> something simple. Right now my best guess is it either has something to
> do with KryoNet's threaded implementation, or the way I close the
> connection on the client.

Usually it's a bug in the way connections are setup or closed, but from
your description it sounds that you might have found a use case that
Kryonet isn't covering well... or maybe we need to determine whether
your use case is a good approach worth supporting in the first place;
however, even then the diagnostics might leave room for improvement.

HTH
Jo

[leudast1215 .]

Thanks everyone! Much appreciated. I'll get some feed back to you guys as soon as I can with the results. 

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