Connectionless versus Connection-Oriented Routing
Connectionless Routing means each packet is treated separately and makes its own way through the network.
Different packets may take different routes through the network depending on the type of routing used and the amount of traffic.
Because packets following different routes may travel at different speeds, they may arrive out of sequence
The network layer of the sending computer puts a sequence number on each packet in addition to information about the message stream to which the packet belongs.
The network layer reassembles the packets in the correct order before passing to the application layer.