The OSI Model and the TCP/IP Protocol Suite

Transcription

1 The OSI Model and the TCP/IP Protocol Suite 1

2 Example 1 Assume Maria and Ann are neighbors with a lot of common ideas. However, Maria speaks only Spanish, and Ann speaks only English. Since both have learned the sign language in their childhood, they enjoy meeting in a cafe a couple of days per week and exchange their ideas using signs. Occasionally, they also use a bilingual dictionary. Communication is face to face and Happens in one layer as shown in the following Figure. 2

3 Example 2 Now assume that Ann has to move to another town because of her job. Before she moves, the two meet for the last time in the same cafe. Although both are sad, Maria surprises Ann when she opens a packet that contains two small machines. The first machine can scan and transform a letter in English to a secret code or vice versa. The other machine can scan and translate a letter in Spanish to the same secret code or vice versa. Ann takes the first machine; Maria keeps the second one. The two friends can still communicate using the secret code, as shown in Figure. 3

4 4

5 Topics Discussed in the Section Hierarchy Layer 0: Tranport the letter Layer 1: drop in the mailbox Layer 2: Translate to secret code Layer 3: Write letter Services Sender site: uses the services of the layer below it Receiver site: provide service to the layer above it 5

6 THE OSI MODEL Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. Almost three-fourths of countries in the world are represented in the ISO. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. 6

7 The OSI model 7

8 OSI layers 8

9 Layered Architecture Organization of the layers Layer 1,2,3 --- network support layers Layer 5,6,7 --- user support layers Layer link the two subgroups and ensure the format is compatible. 9

10 An exchange using the OSI model 10

11 Layered Architecture Encapsulation: The data part of a package at level N is carrying the whole packet (data + overhead) from level N+1 11

12 Physical Layer Physical characteristics of interfaces and media Representation of bits Data rate Synchronization of bits Line configuration Physical topology Mesh topology; star topology; ring topology; bus topology Transmission mode Simplex mode; half-duplex mode; full-duplex mode 12

13 Note The physical layer is responsible for moving individual bits from one (node) to the next. 13

14 Data Link Layer Framing Physical addressing Flow control Error control Access control 14

15 Network Layer Logical addressing Routing 15

16 Transport Layer Service-point addressing Segmentation and reassembly Connection control Flow control Error control 16

17 Session Layer Dialog control Synchronization 17

18 Presentation Layer Translation Encryption Compression 18

19 Application Layer Network virtual terminal File transfer, access, and management (FTAM) services Directory services 19

20 Summary of OSI Layers 20

21 TCP/IP PROTOCOL SUITE The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similarly to the ones in the OSI model. 21

22 TCP/IP and OSI model 22

23 A private internet 23

24 Communication at the physical layer Legend Source Destination Physical layer A R1 R3 R4 B Physical layer Link 1 Link 3 Link 5 Link

25 Note The unit of communication at the physical layer is a bit. 25

26 Communication at the data link layer Legend Source Destination D Data H Header A R1 R3 R4 B Data link Data link Physical Link 1 Link 3 Link 5 Link 6 Physical D2 H2 Frame D2 H2 Frame D2 H2 Frame D2 H2 Frame 26

27 Note The unit of communication at the data link layer is a frame. 27

28 Communication at the network layer Legend Source Destination D Data H Header A R1 R3 R4 B Network Network Data link Data link Physical Physical D3 H3 Datagram D3 H3 Datagram 28

29 Note The unit of communication at the network layer is a datagram. 29

30 Communication at transport layer Transport A Legend Source Destination D Data R1 R3 R4 H Header B Transport Network Network Data link Data link Physical Physical D4 H4 Segment D4 H4 Segment 30

31 Note The unit of communication at the transport layer is a segment, user datagram, or a packet, depending on the specific protocol used in this layer. 31

32 Communication at application layer A B Application Legend Source Destination D Data H Header Application Transport Network Data link Physical R1 R3 R4 Transport Network Data link Physical D5 D5 Message D5 D5 Message 32

33 Note The unit of communication at the application layer is a message. 33

34 ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical address, logical address, port address, and applicationspecific address. Each address is related to a one layer in the TCP/IP architecture. 34

35 Addresses in the TCP/IP protocol suite 35

36 physical addresses Data 1 packet accepted Data 4 36

37 logical addresses A P Data A P Data A P Data Physical addresses changed A P Data A P Data A P Data Physical addresses changed 37

38 Note The physical addresses will change from hop to hop, but the logical addresses remain the same. 38

39 port numbers A Sender Receiver P Data Data a j Data a j Data A P a j Data A P a j Data H2 A P a j Data H2 A P a j Data Internet 39

40 Note The physical addresses change from hop to hop, but the logical and port addresses usually remain the same. 40

41 CONNECTING DEVICES LANs or WANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs and WANs together we use connecting devices. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: repeaters (or hubs), bridges (or two-layer switches), and routers (or three-layer switches). 41

42 Topics Discussed in the Section Repeaters (hubs) Bridges (two-layer switches) Routers (three-layer switches) 42

43 Connecting devices 43

44 Repeater or hub Sent Maintained 44

45 Note A repeater forwards every bit; it has no filtering capability. 45

46 Bridge Bridge table Address Port 71:2B:13:45:61: :2B:13:45:61: :2B:13:45:61: :2B:13:45:61:

47 Learning bridge Address a. Original Port Address Port 71:2B:13:45:61: :2B:13:45:61:13 4 c. After D sends a frame to B Address Port 71:2B:13:45:61: :2B:13:45:61: :2B:13:45:61:42 2 d. After B sends a frame to A Address Port 71:2B:13:45:61: :2B:13:45:61:13 71:2B:13:45:61: :2B:13:45:61:12 3 e. After C sends a frame to D M M M M 47

48 Note A bridge has a table used in filtering decisions. A bridge does not change the physical (MAC) addresses in a frame. 48

49 Routers (three-layer switches) Can connect LANs and WANs together Has a physical and logical (IP) address for each of its interfaces Act only on the destination Change the physical address of the package (both source and destination) when it forwards the packet 49

50 Routing example 50

51 Note A router is a three-layer (physical, data link, and network) device. A router changes the physical addresses in a packet. 51

52 Note A repeater or a bridge connects segments of a LAN. A router connects independent LANs or WANs to create an internetwork (internet). 52