Game Center GKMatch GKSendDataReliable packet lost

Question

I have been using GKMatch for quite a while successfully in an app. I have been chasing down and issue with the game occasionally stopping and have tracked it down to packets being sent but not received. This happens only occasionally but I can't seem to track down why it happens.

All messages are sent using GKSendDataReliable.

Logging has shown that the packet is being sent from one device successfully, but it is never received at the target device.

//Code sample of sending method....
//self.model.match is a GKMatch instance    
-(BOOL) sendDataToAllPlayers:(NSData *)data error:(NSError **)error {
        [self.model.debugger addToLog:@"GKManager - sending data"];
        return [self.model.match sendDataToAllPlayers:data withDataMode:GKSendDataReliable error:error];
    }

...

//Code sample of receiving method....
// The match received data sent from the player.
-(void)match:(GKMatch *)match didReceiveData:(NSData *)data fromPlayer:(NSString *)playerID {
    [self.model.debugger addToLog:@"GKManager - received data"];
    [super didReceiveData:data fromPlayer:playerID];
}

What I see happen is that periodically (maybe 1 in 100 messages) is sent without error from the 'sendDataToAllPlayers' method, but the receiving device never hits the 'didReceiveData' method. My understanding is that using GKSendDataReliable should send messages and then won't send another until it receives an acknowledgement. Messages aren't received but new messages are sent from the same device.

The sending method returns 'YES' and error is nil, but the didReceiveData is never hit...!

Has anyone ever seen this? Does anyone have any ideas what this could be? I don't know what else I could do to debug this.

Answer 1

I confirm the bug. I made an example project consistently reproducing the issue: https://github.com/rabovik/GKMatchPacketLostExample. I tested it on a weak internet connection (iPad 3 with Wi-Fi and iPhone 4S with EDGE; both on iOS 6.1.3), and some packets regularly get lost without any error from Game Center API. Moreover sometimes device stops receiving any data, while another still sends and receives messages successfully.

So if we are sure the bug exists, the only possible workaround is to add an extra transport layer for truly reliable delivery.

I wrote a simple lib for this purpose: https://github.com/rabovik/RoUTP. It saves all sent messages until acknowledgement for each received, resends lost and buffers received messages in case of broken sequence. In my tests combination "RoUTP + GKMatchSendDataUnreliable" works even beter than "RoUTP + GKMatchSendDataReliable" (and of course better than pure GKMatchSendDataReliable which is not really reliable).