Isolating Download and Upload Operations

Upload and download operations are not always isolated transactions. Upload and download operations can be affected by other operations on messaging objects.

To counteract the lack of transaction isolation between ICS download operations and the rest of operations that occur on messaging objects at the same time, servers MUST confirm that the final ICS state does not reflect the state of the server replica at the end of the operation, but instead reflects the actual differences downloaded to a client, combined with the initial ICS state.