Sync iPhone AddressBook with a Backend Server

There are lots of applications doing AddressBook synchronization with their Backend servers to cross check which contacts in your AddressBook are using their application and which users needs to be invited to their application.

For the first time it may do a full sync, but after that it shouldn't be a full sync.

My first question is, What is the best way to sync the full AddressBook with a Backend Server?

Second question is, How to sync ONLY the contacts which has modified recently?

If there's any sample application or a tutorial please share with me.

Thanks in advance.