When game hosts join in multiplay, the position of their playable game characters must be shown to each other. For this, the position of game characters must be synchronized.
When a host sends a message to another host in internet, latency occurs due to the time it takes to transmit the message. Because of this latency and the limit of network traffic, there is also a limit in the rapidity and number of sending the position information of game character. A method that can synchronize the position of characters is Dead Reckoning.
When sending information of each game characters under control of users to other hosts in a casual online game that only a small number of gamers enjoying the game in one game world, the entire users can form a P2P group and communicate with each other. Please refer to Gameplay synchronization for small number of hosts for more details.
However, such P2P method increases significant amount of network traffic if it has used in a game with a massive number of users playing together in one game world. Thus the range of synchronization has to be limited to game characters only in the sight of each user. Please refer to Gameplay synchronization in MMO for more details.
You can also look up sample programs from Character synchronization in 3D world.