The World Wide Web is a network application that uses the services of TCP and IP to communicate over the Internet. When a web browser requests a page from a web server, the TCP/IP services provide a virtual connection -- a virtual circuit-between the two communicating systems. Remember that packet-switched networks don't operate like telephone networks that create an actual circuit dedicated to a particular call.
Once a connection is established and acknowledged, the two systems can communicate by sending messages. These messages can be large, such as the binary representation of an image, and TCP may fragment the data into a series of IP datagrams. An IP datagram is equivalent to the couriers' envelope in that it holds the fragment of the message along with the destination address and several other fields that manage its transmission through the network.
Each node in the network runs IP software, and IP moves the datagrams through the network, one node at a time. When an IP node receives a datagram, it inspects the address and other header fields, looks up a table of routing information, and sends it on to the next node. Often these nodes are dedicated routers-systems that form interconnections between networks-but the nodes can also include the computer systems on which the applications are running. IP datagrams are totally independent of each other as far as IP is concerned: the IP software just moves them from node to node through a network.
TCP software performs the function of gluing the fragments together at the destination using the fragment identifier field in the IP datagram header. Because IP datagrams are transmitted through the network independently, there is no guarantee they will arrive at the destination in order, and TCP stores the fragments in a buffer until all preceding fragments are received.
IP doesn't guarantee that datagrams are delivered. If an IP node receives a corrupt datagram, it throws it away. Datagrams may be missing from the stream the TCP software receives because a datagram was corrupt and not passed on from the IP software or was delayed in the network. TCP buffers the fragments to allow the out-of-order datagrams to arrive. If a missing datagram fails to arrive, TCP eventually requests that it be resent. This can cause datagrams to be received twice; however TCP recognizes and discards the duplicate datagram when it arrives.