3 The server should loop around and around waiting for maultiple requests while

3 the server should loop around and around waiting

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3. The server should loop around and around, waiting for maultiple requests, while the client sends only one request and exits when it gets a reply. 7. TCP/IP Networks In the last chapter we looked at some of the high level considerations for enabling transparent communication over a network. The next thing to look at is how such a scheme of protocols is achieved in practice. 7.1 The protocol hierarchy 7.1.1 The OSI model We begin by returning to the `most important idea in computing' - namely hierarchies. As we have noted before, the most practical way of solving complicated problems is to create `layers of detail'. At any level in a hierarchy, the details of the lower levels are invisible - so we never see the irrelevant pieces of the computing puzzle we are trying to solve. The International Standards Organization (ISO) has defined a standard model for describing communications across a network, called the OSI model, for Open Systems Interconnect (reference model) . The OSI model is a seven layered monster. It does not have to be taken literally - it might not be natural to separate all of these parts in every single program - but it is useful as a way of discussing the logically distinct parts of network communication. The layers are described as follows. 7 Application layer Program which sends data 6 Presentation layer XDR or user routines 5 Session layer RPC / sockets 4 Transport layer tcp or udp 3 Network layer IP internet protocol 2 Data link layer ethernet (protocols) 1 Physical layer ethernet (electronics) At the lowest level, the sending of data between two machine takes place by manipulating voltages along wires. This means we need a device driver for the signaller, and something to receive the data at the other end - a way of converting the signals into bytes; then we need a way of structuring the data so that they make sense. Each of these elements is achieved by a different level of abstraction. 1. Physical layer. This is the problem of sending a signal along a wire, amplifying it if it gets weak, removing noise etc. If the type of cable changes (we might want to reflect signals off a satellite or use fibre optics) we need to convert one kind of
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signal into another. Each type of transmission might have its own accepted ways of sending data (i.e. protocols). 2. Data link layer. This is a layer of checking which makes sure that what as sent from one end of a cable to the other actually arrived. This is sometimes called handshaking . 3. Network layer. This is the layer of software which remembers which machines are talking to other machines. It establishes connections and handles the delivery of data by manipulating the physical layer. The network layer needs to know something about addresses - i.e. where the data are going, since data might flow along many cables and connections to arrive where they are going.
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