Data Communication and Internet Technology

Transcription

1 Data Communication and Internet Technology Lehrstuhl für Informatik 4 RWTH Aachen Prof. Dr. Otto Spaniol Page 0

2 Organization Lehrstuhl für Informatik 4 Exercises to the lecture Fortnightly Monday 11:00 12:30 h Presence exercise Exercise dates: Material (Slide copies, exercise sheets) Written exam At the end of winter term Contact information Otto Spaniol Lehrstuhl für Informatik 4, RWTH Aachen University Ahornstr. 55, Aachen Phone: 0241 / spaniol@informatik.rwth-aachen.de Page 1

3 Content Lehrstuhl für Informatik 4 1. Introduction Networks and Network Topologies Communication Protocols 2. Computer Networks Network principles Network Components (Cables, Repeaters, Hubs, Bridges, Switches, Routers) Local Area Networks (Ethernet, Token Ring, FDDI) Wide Area Networks (DQDB, ATM, SDH) Wireless Networks (WLAN) 3. Internet Protocols Internet/Intranet: the TCP/IP Reference Model Network protocols (the Internet Protocol IP, Routing in the Internet) Transport protocols (TCP and UDP) 4. Application Protocols in the Internet Higher protocols (FTP, HTTP, ,...) Page 2

4 Literature Lehrstuhl für Informatik 4 A.S. Tanenbaum: Computer Networks. 4th Edition, Prentice Hall, J.F. Kurose, K.W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet. 3rd Edition, Addison-Wesley, Cisco Systems: Internetworking Technologies Handbook. 3rd Edition, Cisco Press, J. Schiller: Mobile Communications. 2nd Edition, Addison Wesley, Page 3

5 Data Communication Data communication is the processing and the transport of digital data over connections between computers and/or other devices (generally over large distances) Data communication comprises two topical areas: Computer Networks How to connect several computers? Which media can be used for data transport? How to represent digital data on the medium? How to coordinate the access of several computers to the medium? Communication Protocols (Internet Technology) How to design uniform data units for transfer? How to achieve a reliable and efficient transfer? Page 4

6 Computing Power is cheap... Today, everybody has a computer (at work as well as privately) Possible applications: file sharing, efficient interworking (CSCW = Computer Supported Cooperative Work) And: Sharing resources lowers costs Access to foreign resources by communication networks to achieve reasonable usage Agreements for shared usage of devices which are too expensive to buy for one single organization and/or have no use for the total capacity Essential: Efficient methods to share/transfer data between the components of a system of interconnected devices Example for interworking of two parties: Client/Server principle Page 5

7 The Client/Server Principle Client Server Client Process Server Process Network Request Reply Advantages Network Cost reduction Better usage of resources Modular extensions Reliability by redundancy Page 6

8 Client/Server Systems Server Program (process) which offers a service over a network. Servers receive requests and return a result to the inquiring party. The services offered include simple operations (e.g. name server) or a complex set of operations (e.g. web server). Client Program (process) which uses a service offered by a server. Examples for Client/Server systems Client WWW Browser Program FTP Client Server WWW Server Domain Name System (DNS) FTP Server Page 7

9 Another principle: Peer-to-Peer Equal partners, no fixed client and server roles Connections between any pair of computers Establishment of a whole network of connections Best example: File Sharing, e.g. Napster, Gnutella Page 8

10 Non-technical aspects Communication networks enable a faster and cheaper exchange/distribution of information. There is however a large number of social, ethnical, cultural, juridical,... side effects. Eventually dubious or forbidden contents Responsibility Juridical aspects (legislation) Potential censorship? Control over the productivity of employees, of the whereabouts of people Annoyance through anonymous or unwanted messages (SPAM)... Page 9

11 Computer Networks Page 10

12 First Generation Computer Networks Operator Computing Center Mainframe Telephone lines Rest of the world Demultiplexer Multiplexer Terminals Terminals Peripherals Page 11

13 Introduction of Local Area Networks Building A Fixed lines Rest of the world Building B Computing Center Operator Mainframe Building C Router Terminals Peripherals Page 12

14 Global Networking Building A Local Server Router Switch Clients Computing Center Rest of the world (Internet) Fixed lines, ISDN, Provider... Router Server Network and system administrator Router Building B Backbone Local Server Clients Switch Peripherals Switch Mainframe Router Page 13

15 Classification of Networks Point-to-Point Network A pair of computers is directly connected by one cable Broadcast Network One-to-all (e.g.: radio, television) All connected stations are sharing one transmission channel For ensuring that the data are sent the correct receiver, they have to be marked with the destination address of the receiving computer Data are being packed into packets with the Unicast Address of the receiver ( = destination address) Every computer connected controls each received packet for its destination address. Only the addressed computer processes the data, all others are simply deleting them. To address all connected stations at once, so-called Broadcast Addresses are used Page 14

16 Classification of Networks Classification by Distance 1 m 10 m Room 100 m Building 1 km Campus 10 km Town 100 km Country 1000 km Continent km Planet Personal Area Network (PAN) Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) Internet Page 15

17 Local Area Networks Communication infrastructure for a restricted geographical area (10 m up to some km) Usually maintained by one local organization Linked are PCs/Workstations/..., for exchanging information and sharing peripherals and resources Transmission capacity up to 1,000 Mbit/s Transmission delay of a message in the range of milliseconds (~10 ms) Simple connection structures ( Simple is beautiful ) Topologies Bus Star Ring Tree Meshed network LAN Page 16

18 LANs: Bus Lehrstuhl für Informatik 4 Terminating resistor Ω B Ω Example: Ethernet A Bus Broadcast Network: if station A intends to send data to station B, the message reaches all connected stations. Only station B processes the data, all other stations are ignoring it. - (+) Passive coupling of stations - Restriction of the extension and number of stations to connected + Simple, cheap, easy to connect new stations + No choose of path to target (= routing) necessary + The breakdown of a station does not influence the rest of the network Page 17

19 LANs: Star Lehrstuhl für Informatik 4 Star A B Example: Fast Ethernet Designated computer as central station: a message of station A is forwarded to station B via the central station Broadcast network (Hub) or point-topoint connections (Switch) Expensive central station Vulnerability through central station (Redundancy possible) + Definite path, no routing + N connections for N stations + Easy connection of new stations Page 18

20 LANs: Tree Lehrstuhl für Informatik 4 Branch 1 Branch 2 A B C D Repeater Router Backbone Tree Topology: Connection of several busses or stars Branching elements can be active (Router) or passive (Repeater) + Bridging of large distances + Adaptation to given geographical structure + Minimization of the cable length necessary Page 19

21 LANs: Ring Lehrstuhl für Informatik 4 Example: Token Ring, FDDI B A Ring Broadcast Network Chain of point-to-point connections Active stations: messages are regenerated by the stations (Repeater) Breakdown of the whole network in case of failure of one single station or connection + Large extent possible + Easy connection of new stations +Only N connections for N stations Variant: bidirectional ring stations are connected by two opposed rings Page 20

22 LANs: Meshed Networks Fully Meshed Network Point-to-Point connections between all stations For N stations, 2 connections are needed Connecting a new station is a costly process + No routing + Redundant paths N( N 1) + Maximal connection availability through routing integration Partly meshed network: cheaper, but routing, flow control and congestion control become necessary (Wide Area Networks) Page 21

23 LANs: Examples Ethernet (IEEE 802.3, 10 MBit/s) - originally the standard network - available in an immense number of variants Token Ring (IEEE 802.5, 4/16/100 MBit/s) - for a long time the Ethernet competitor - extended to FDDI (Fiber Distributed Data Interface) Fast Ethernet (IEEE 802.3u, 100 MBit/s) - at the moment the most widely spread network - extension of Ethernet for small distances Gigabit Ethernet (IEEE 802.3z, 1,000 MBit/s) - very popular at the moment; also, 10 GBit/s are already possible for Metropolitan Area Networks Page 22

24 Metropolitan Area Network (MAN) Designed for larger distances than a LAN, usage e.g. in a whole town Similar technologies as in a LAN In general, only 1 or 2 cables without additional components Difference to LANs: Time slots MAN Example: Distributed Queue Dual Bus (DQDB, IEEE 802.6) Page 23

25 Wide Area Network (WAN) Bridging of any distance Connects LANs and MANs over large distances Irregular topology, based on current needs Consists out of stations which are connected through point-to-point with each other Mostly quite complex interconnection of subnetworks which are owned by independent organizations WAN Router LAN Host Page 24

26 Wireless Networks System Interconnections (PANs) Direct connection between the components of a computer (Example: Bluetooth) Wireless LANs Communication of computers connected by a base station (Access Point) in a local area, or direct connection between computers (Example: IEEE Wireless LAN, WLAN) Range of meters Transmission capacity of up to 100 MBit/s Wireless MANs/WANs E.g. common telecommunication networks like GSM. Range of several kilometers ( worldwide") Transmission capacity below 1 MBit/s IEEE WirelessMAN (IEEE ) as MAN for data transmission Page 25

27 Standards Organizations - IEEE Institute of Electrical and Electronic Engineers - IEEE Standardization e.g. of the IEEE 802.X- Standards for Local Area Networks Overview and Architecture of LANs Logical Link Control (LLC) CSMA/CD ( Ethernet ) Token Bus Token Ring DQDB (Distributed Queue Dual Bus) Broadband Technical Advisory Group (BBTAG) Fiber Optic Technical Advisory Group (FOTAG) Integrated Services LAN (ISLAN) Interface Standard for Interoperable LAN Security (SILS) Wireless LAN (WLAN) Demand Priority (HP s AnyLAN) Cable modems Personal Area Networks (Bluetooth) WirelessMAN Page 26

28 Communication Protocols Page 27

29 Why Protocols? To enable understanding in communication, all communication partners have to speak the same language. Data formats and their semantics Control over media access Priorities Handling of transmission errors Sequence control Flow control mechanisms Segmentation and composition of long messages Multiplexing Routing A protocol is defined as the whole set of agreements between application processes with the purpose of a common communication Page 28

30 Implementation of Protocols Solution 1: Write one large Communication Program which fulfills all requirements needed to establish a communication process. Advantage: efficient data exchange for a given application. Disadvantage: No flexibility! Adoptions require large efforts. Solution 2: Write a set of small programs specialized to special tasks of the communication process. For each application, the needed programs can be combined. Advantage: Very flexible, since single components can be exchanged. Disadvantage: Fixed structures of program interworking; adds more complexity and overhead. Accepted today: solution 2. The implementation takes place in layer models. Page 29

31 Example: Exchange of ideas between philosophers Philosopher A Language: Chinese Thoughts about world politics Philosopher B Language: Spanish Interpreter A Language: Chinese additionally: English Uninterpreted sentences, i.e. no knowledge about politics Interpreter B Language: Spanish additionally: English Technical Expert A Recognizes single characters and sends them in Morse Uninterpreted characters in correct order Electrical signals Network Technical Expert B Recognizes single characters and sends them in Morse Page 30

32 Standardization Indispensable for the area-wide practical use of communication systems: Standardization On the national as well as the international level! Successful standardization is quite difficult due to: Complex technical problems have to be solved The involved parties, e.g. companies are often working against each other Confidentially restrictions hinder the information flow Consequence: Standardization processes are very slow (due to many, often non-technical reasons). Page 31

33 Standards Organizations - ISO International Standards Organization - ISO Organisation, which is working on a volunteer basis (since 1946). Members: standards organizations in approx. 90 countries Deals with a very broad range of standards 200 Technical Committees (TC) for specific tasks (e.g. TC97 for computer and information processing) TCs consist of subcommittees comprising in turn several working groups Interworking with ITU-T regarding telecommunication standards, (ISO is a member of ITU-T). Pioneering work of ISO regarding data communication: the ISO/OSI reference model Notice: only the concept is pioneering not the products developed from those concepts! (OSI: Open Systems Interconnection) Page 32

34 The ISO/OSI Reference Model Reduce the complexity of a communication process (all details to be considered) through layers. 7 layers: 7 Application Common services for the end user Criticism of the model: Presentation Session Transport Network Data Link Physical Network-independent end-to-end data transfer Addressing and routing of packets Securing of frames ; Flow Control Signal representation, character transmission Layer 5 and 6 are rarely being implemented Generally to much overhead some details are unnecessary, some are overloaded Transmission medium ( Layer 0 ) Page 33

35 Layer Tasks Lehrstuhl für Informatik 4 1. Physical layer This layer is responsible for transmitting single bits over the medium. Signal representation is defined here to ensure that a sent 1 is understood by the receiver as 1. For this, e.g. on a copper cable it is defined, which voltage is used to represent a 1 resp. a 0 and how long this voltage has to be for one bit. Moreover details are being defined like the type of cables, meaning of pins of network connectors, transmission direction on the cable (uni-/bidirectional), 2. Data Link Layer Ensures an error-free data transmission between two neighbored hosts (e.g. in a sub-network). Therefore the incoming data are segmented into so-called frames which are being transmitted separately. The receiver, which identifies the start and the end of a frame e.g. with a bit pattern, checks if the transmission has been correct (e.g. with the help of a checksum). Additionally, flow control is used to control the re-transmission of corrupt frames and protect the receiver from overload. An additional task in broadcast networks is the control of medium access, i.e. the stations are coordinated in some way to prevent from access conflicts. Page 34

36 Layer Tasks Lehrstuhl für Informatik 4 3. Network Layer This layer is responsible for the data transmission over larger distances and between heterogeneous sub-networks. The main task is (worldwide) uniform addressing of hosts and choosing a path through the whole network (routing). A necessary prerequisite for doing so is among other things a common address range and an agreement about a maximum size of the transferred data units. Intermediate stations (the routers) manage tables with routing information and use the uniform addresses to make a decision about the best path to the receiver. 4. Transport Layer (ISO/OSI) Layer 4 manages end-to-end communication between two processes. It is responsible for ensuring that the received data are complete and in correct order. For this, again flow control is used (sequence numbers, acknowledgements) to detect missing or wrong ordered data units. Beneath this, the current network state is considered to not only adapt to the receiver, but to the network capacities as well. Addressing is a topic here as well. On the transport layer, a single communication process on receiver side is addressed. Page 35

37 Layer Tasks Lehrstuhl für Informatik 4 5. Session Layer This layer (like the transport layer) manages reliable data transport between the computers. However also additional services are being offered, like e.g. the possibility for dialogue control. I.e. it can be defined in which direction the transmission can take place. Closely related with this topic is the token management which also belongs to level 5. During the transmission so called tokens can be exchanged. With certain operations only the communication partner which owns the token is allowed to conduct the operation. Token management is also used here for other purposes, i.e. a set of tokens exist to coordinate several operations. One important operation is to set synchronization points in the communication process, to restart the transmission at the point it has ended in case of a connection loss. Page 36

38 Layer Tasks Lehrstuhl für Informatik 4 6. Presentation Layer The task of this layer is to display the data to transmitted that way, that they can be handled from a lot of different systems. So computers code a string with ASCII characters, others use Unicode, some for integers the 1-, other the 2-complement. Instead of defining a new transmission syntax and semantics for every application, it is tried to provide a universally valid solution. Specific data are encoded in an abstract (and commonly recognized) data format before the transmission and are being translated back by the receiver into its own personal data format. 7. Application Layer (ISO/OSI) In this layer (standard-) protocols are being provided which can be used from a whole set of applications/systems. One example is file transfer. On the application layer a universally valid protocol including an interface of file transfer is being provided. For systems from different manufacturers only the link-up into the local file system has to be realized. Other examples are file transfer, , remote operations etc. Page 37

39 Interplay between the Layers Layer (n-1) offers its functionality to the above lying layer n as a communication service. Layer n enhances the data to be sent with control information (Header) and sends the data together with the header as Protocol Data Units (PDU). Two communication partners on layer n exchange PDUs by using the communication service of the nearest lower lying layer (n-1). For layer (n-1), these PDUs are the data to be transmitted. Layer n n-pdu Layer n Layer (n-1) H Data Layer (n-1) (n-1)-pdu H: Header, e.g. control information of the layer Page 38

40 The whole Communication Process Application process Data Application process Application Layer H Data Application Layer Presentation Layer H A-PDU Presentation Layer Session Layer H P-PDU Session Layer Transport Layer H S-PDU Transport Layer Network Layer H T-PDU Network Layer Data Link Layer H N-PDU T Data Link Layer Physical Layer Physical Layer Bit stream Transmission medium Page 39

41 Standards Organizations - IETF Internet Engineering Task Force - IETF Forum for the technical coordination of the work regarding Arpanet, the precursor of the Internet (since 1986). Evolution to a large, open, and international community of administrators, vendors and researchers. Works on evolution of the Internet architecture and the smooth operation of the Internet. Several working groups on Internet protocols, applications, routing, security, Standard draft proposals can become a full standard only if an implementation of the proposal is successfully tested at two independent locations for at least four month. Result of such a standardization process: the resounding success of the Internet protocols TCP/IP Page 40

42 The TCP/IP Reference Model Application Layer Application Layer Presentation Layer Don t exist Session Layer Transport Layer Transport Layer Network Layer Internet Layer Data Link Layer Physical Layer ISO/OSI Host-to-Network Layer TCP/IP Page 41

43 The Tasks of the TCP/IP Layers Host-to-Network Layer (corresponds to ISO/OSI 1-2) Not defined exactly. The design does not matter, it is only defined that a host must be connected to the network via a protocol in a way that it is able to send and receive IP datagrams. The protocol design is left over to other standards to cover heterogeneous networks of all kinds. Internet Layer (corresponds to ISO/OSI 3) The term Internet refers here to the interworking of different networks, therefore not on the Internet itself. The protocol enables communication between hosts over the own network borders. In the Internet, the transmission is connectionless, meaning that the data are segmented into packets which are addressed and sent independently into the network. On each network border, a router takes over the forwarding of the packets. The choice of path can be dynamic, depending on the current network load. As a result, single packets can get lost by overload situations or received in wrong order. Such faults are not handled (this task is left over to the transport layer). In contrast to ISO, only one packet format is defined, together with a connectionless protocol, the Internet Protocol (IP). Page 42

44 The Layers of TCP/IP Transport Layer (corresponds to ISO/OSI 4) This layer covers the communication between the end systems. To adapt to different applications, two protocols are defined. TCP (Transmission Control Protocol) is a reliable, connection-oriented protocol to protect the transmission of a byte stream between two hosts. The byte stream is segmented to fit into IP packets. On the receiving side the packets are reassembled in the original order with the purpose of restoring the original data stream. It also includes flow control to adapt to the receiver s capabilities and to overcome the faults caused by the connectionless IP. UDP (User Datagram Protocol) is an unreliable and connectionless protocol ( best effort ). No error correction is integrated, thus the transmission is used when the speed of the data transmission is more important than the reliability (speech, video). Application Layer (corresponds to ISO/OSI 7) This layer defines common communication services. This comprises TELNET (remote work on another computer), FTP (file transfer), SMTP (electronic mail), DNS ( phonebook for the Internet), HTTP (used for World Wide Web), etc. Page 43

45 OSI vs. TCP/IP 1. Time Lehrstuhl für Informatik 4 The TCP/IP protocols were already widely used before OSI had finished the standardization activities. 2. Freedom from obligation A reference model like OSI is free from obligation. It only defines what is to be done, but not how to do it. Result: incompatibility of products. 3. Complicatedness Very high and partly unneeded expense in the OSI specification (thousands of pages of specification descriptions). By the wish to consider all special cases, lots of options were included, making the products lavish, unhandy, and far too expensive - The option is the enemy of the standard! Page 44

46 OSI vs. TCP/IP Lehrstuhl für Informatik 4 4. Political reasons OSI was dominated too much by Europe especially from the national telecommunication companies which had lucrative monopolies. The real market power was in the USA nobody was interested in OSI over there. 5. Hurriedly product implementation The first OSI products were implemented too fast (driven by the success of TCP/IP protocols), were covered with faults, and had an overall low performance. In contrast, the theoretically far more unmodern TCP/IP protocols were continuously modified and improved. They were of a high quality level and successfully tested before deployment and cheap to buy due to high production numbers. Page 45