If the checksum is not correct, it asks for retransmission. Two protocols are most widely used. Connectionless Part of Internet protocol suite. An IP packet can be sent without a connection being established. Each packet is routed to its destination independently.
Transport layer ensures reliable service.
In case IP, lost packets arriving out of order must be reordered. Internet transport protocol. Very few applications use it. Enhanced version of transport layer. Dialog control, synchronization facilities.