使用 GKSession 进行对等时如何获得精确的时钟同步？

Question

I made a simple game which connects to other peers using GKSession from GameKit. It was easy to set up but I discovered some problems:

• Latency varies a lot. Sometimes message arrives instantly on other devices. Sometimes latency is > 1 second for sending data to other peers. Data is only a string with 10 chars.

• My game depends on precise clock synchronisation. The game has music playing and it sounds odd when it doesn't start playing the same time on all connected devices.

I couldn't find anything in the documentation about how to snychroize timing. The problem is that the "master" pier which starts the game immediately begins playing the music and then all other piers receive the message a little bit later and thus start playing after delay.

Then I tried to delay playing the game music after sending the start message to all piers, but sometimes latency is lower or higher and I just can't get a good reliable sync.

Is there a open source framework which makes peer to peer and clock sync more reliable and easier to use on iOS?

Answer 1

Unfortunately, you're in for a world of hurt here. It is very difficult to do precise clock synchronization over a network interface. You would need your devices to sync to within 20ms here.

I would recommend doing the following: send a bunch of ping packets at 50ms intervals, make the client respond immediately. Take the average of this to give you average round trip time (RTT) and then halve that to estimate the one-way latency. Then send your 'start clock' message and spin in a loop until you've waited for your one-way latency and begin playing the music on your end. The client should play its music immediately upon receiving 'start clock'.

There is a more robust long-term sync solution outlined here: http://en.wikipedia.org/wiki/Precision_Time_Protocol