Processing data synchronization sessions includes processing a request to initiate a synchronization session from one or more clients to update one or more data records that include one or more data items. The one or more clients are selectively identified as one of a trusted client and an untrusted client based on one or more properties of the synchronization session, wherein the trusted client is configured to support all data classes defined by the server. The one or more clients are detected as being configured to support field level differencing that sends changed data items only for the one or data records or record level differencing that sends all of the data items included in the one or more data records.