User Datagram Protocol: The unreliable and connectionless transport protocol is known as User Datagram Protocol. This is a lower version of TCP that is why it consists far less latency rate than TCP and UDP does not give Flow control, sequencing and acknowledgement of sending data however it provides error checking facility by employing Checksum.
So we can state that as the UDP does not conatin Sequence number and Acknowledgement number it does not provide reliable service and no surety of sending data as this Sequence number and Acknowledgement number is used to initialize session while sending data from source to destination.