The Internet today moves packets around without due regard to any special priorities. The speed a packet goes at once it starts to be transmitted is the speed of the wire (LAN, point-to-point link, dial up or what have you), on the next hop. We illustrate the range of communication technology speeds in 1.2. But if there are a lot of users, packets get held up inside routers (like letters in sorting offices at christmas). Because the Internet is designed to be interactive, rather than the slow turnaround of mail (even electronic mail) , routers generally do not hang on to packets for very long. Instead, they just "drop them on the floor" when things get too busy!
This then means that hosts have to deal with a network that loses packets. Hosts generally have conversations that last a little longer than a single packet - at the least, a packet in each direction, but usually, several in each direction.
In fact, it is worse than that. The network can automatically decide to change the routes it is using because of a problem somewhere. Then it is possible for a new route to appear that is better. Suddenly, all packets will follow the new route. But if there were already some packets half way along the old route, they may get there after some of the later packets (a bit like people driving to a party, and a some smart late driver taking a short cut and overtaking the earlier leavers).
So a host has to be prepared to put up with out of order packets, as well as lost packets.
Figure 1.2: Range of Network Performance