This is part of the book "Introduction to Networking" at www.net-intro.com

The next layer up from the Internetworking layer is the Transport layer. A key element of the Internetworking layer is that it does not attempt to guarantee delivery of any particular packet.   The Internetworking layer is nearly perfect, but sometimes packets can be lost or misrouted. But users of the network want to reliably send entire files or messages across the Internet. A network is not much good to us if all it can do is send packets that are received most of the time. For the network to be useful, all of the packets need to be reassembled into the right order to reconstruct the message on the receiving system. The network must also deal with packets that arrive out of order or never arrive at all. The Transport layer is where we handle reliability and message reconstruction on the destination computer.  Just like the IP layer, the Transport layer adds a small amount of data to each packet to help solve the problems of packet reassembly and re-transmission.