Connectionless service

The second case here is a connectionless service. This is used when multiple messages bear no relation to one another, and thus these protocols do not need any connection negotiation step before sending any data. An example of this is UDP, which provides no guarantees of the sequence or reliability of transmitted messages (it does, however, have a checksum field to guarantee the correctness of the datagram). One should note that the protocol running above UDP is always free to implement reliability if that is desired. Interestingly, IP routing is also a connectionless service. The UDP header is shown as follows:

It's easy to see that the header here is far smaller than a TCP header. It also lacks a number of fields that TCP uses to manage the connection and tune it according to network congestion and so on. Since UDP does not have those fields, it cannot provide those guarantees.