For both class 0 and class 1 connectionless services, best effort delivery service is maintained across switchovers. No state information is maintained between primary and backup SCCP layers other than the status of remote signaling points or subsystems.
It is possible for messages to be lost on a switchover for both classes of connectionless service. The application-level protocol running above SCCP is responsible for detecting and recovering lost messages.
For both classes of service, segmented messages in the process of being transmitted or received are lost or discarded on a switchover. If the remaining segments of a partially reassembled incoming message that was lost or discarded due to a switchover are received by the (new) primary, they are detected and discarded. If any of these segments has the return option set, the segment is returned to the sender in an XUDTS message with a return cause of Segmentation Failed for ITU or Error in Message Transport for ANSI.