The client knows that its question arrived if it gets an answer from the server

The client knows that its question arrived if it gets

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client-server system. The client knows that its question arrived if it gets an answer from the server, so asking the network protocols to guarantee it would be a waste of time. A single `message' of udp encapsulated datagrams is officially called a packet and is given a small header as shown in the figure below. Figure 7.2: udp packet header Notice that this header contains no ordering information - so the order in which the packets arrive at their destination is not guaranteed by the protocol itself. Only the integrity of the data are checked, using a checksum. 7.2.2 tcp A single message of the transmission control protocol is called a segment . The tcp protocol is called reliable or connection-oriented because sufficient handshaking is provided to guarantee the arrival and the ordering of the segments at their destination. The ordering of each message
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implies a concept of two machines being continual contact with one another. This is like a telephone conversation: both parties are in contact all the time. TCP connections are useful for sending data to servers, where no particular reply is required. For example, it would be used to send print jobs to a printer queue across a network. The sender receives no reply from the print spooler, and wants every single line of data to arrive in the correct order without having to worry. Each tcp segment has a header as shown in the figure below. Figure 7.3: TCP segment header 7.3 The physical layer As an example of a physical layer, we can take a brief look at the ethernet. Ethernet is one form of cabling which is in common use. Other kinds of cable include fibre optics (FDDI) , 10BaseT or ISDN. 7.3.1 Network connectivity To send messages from one computer to another, we have to connect computers together. One way of doing this would be connect every machine to every other machine in some bizarre `cat's cradle' of wiring. This would require network connections per machine if there were machines. It's pretty clear that this is not a good solution. Another solution is to chain machines together (see figure below) or put them in a ring. This requires only two connections per machine. Figure 7.4: Chains and rings. The disadvantage with this scheme is that each machine has to send signals forward to the next one, until they arrive at the correct machine, which costs time and resources. FDDI fibre optic transmission works like this. It is called a token ring . Modern ethernet uses neither method. Instead it uses a combination of two solutions. A basic ethernet network consists of a single cable or bus. Every machine listens into the same cable with one interface connector (see figure). Figure 7.5: Ethernet Since all machines share the same cable, only one machine can be talking at once. Each machine waits its turn to transmit data. Each host flashes its signals to all the hosts on the cable like sending Morse code with a torch. Every host sees every message but only the host with the destination address bothers to accept the message.
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