Computer Networks An Introduction

1. Computer NetworksAn Introduction Prepared by S.Perry (February 2010)

2. To present a comprehensive view of the principles and fundamental concepts in Computer Networks • To learn about the basics in design and implementation of network protocols • To provide an understanding of the components of a network and how they are connected. • To acquire some hands-on experience Unit Objectives

3. Introduction • Fundamental concepts • Basic definitions • Network architecture • Communication Basics • Media and signals • Asynchronous and synchronous communication • Relationship among bandwidth, throughput, and noise • Frequency-division and time-division multiplexing Presentation Outline

4. Networking and network technologies • Packing switching • Framing, parity, and error detection • Local and wide area technologies • Network addressing • Connection, wiring and extension (repeaters, bridges, hubs, switches) • Forwarding and measuring of delay and throughput Presentation Outline (continued)

5. Internets and Internetworking • Motivation and concept • Internet Protocol (IP) datagram format and addressing • Internet routers and routing • Transmission Control Protocol (TCP) Presentation Outline (continued)

6. Network Applications • Domain Name System (DNS) • File Transfer Protocol (FTP) • Remote Login Protocol (TELNET) • Email Transfer (SMTP) • Web technologies and protocol (HTTP) • Putting all pieces together Presentation Outline (continued)

7. A collection of computers (PCs, workstations) and other devices (e.g. printers, credit card readers) are all interconnected (each is called a “node”) • Components: • Hosts (computers) • Links (coaxial cable, twisted pair, optical fiber, radio, satellite) • Switches/routers (intermediate systems) • Goal: provide ubiquitous access to resources (e.g., database servers, Web), allow remote users to communicate (e.g., email) • User runs applications What is a Computer Network?

8. Application Application Frame Networks connect applications on different stations What is a Computer Network?

9. Frame Server Station Client Station Stations are computers and other devices such as cellphones and PDAs Server Station Mobile Client Station What is a Computer Network?

10. Frame Server Station Client Station Stations communicate by sending messages called Frames Server Station Mobile Client Station What is a Computer Network?

11. Switch Frame Switch Switch Switch Frames may pass through multiple switches; Each switch reads the frame And passes it on What is a Computer Network?

12. Trunk Link Trunk Link Trunk Link Trunk links connect switches Higher capacity than access links Often optical fiber Trunk Link Trunk Link What is a Computer Network?

13. In summary, a network is a system of hardware, software and transmission components that collectively allow two application programs on two different stations connected to the network to communicate well What is a Computer Network?

14. Point-to-Point Communication What is a Computer Network? • Multiple Access Communication

15. Switched Networks • Circuit - switched network: public telephone network (dedicated circuit per call) • Packet switched network: Internet (collection of networks where data is sent in chunks) What is a Computer Network?

16. Set up a connection path (circuit) between the source and the destination (permanent for the lifetime of the connection) • All bytes follow the same dedicated path • Used in telephony • Advantages: dedicated resources • Disadvantages: not very efficient (lower utilization, e.g., a person talks < 35% of the time during a call) • While A talks to C, B cannot talk to D on the same line. Circuit-Switching

17. Packets • Data are chopped up into small blocks called packets (e.g., ~ 4500 bytes) • Each packet carries extra information to allow it to reach its destination Packets

18. Packets from different sources are interleaved • Efficient use of resources (since they are used on a demand): statistical multiplexing. Nobody reserves a lane on a freeway • Can accommodate bursty traffic (as opposed to circuit-switching where transmission is at constant rate). Packet-Switching

19. Store and forward: intermediate nodes (e.g., routers) store (buffer) incoming packets, process them and forward them to the appropriate outgoing link. • Allows for flexibility and robustness. Packets can travel through alternate paths (adaptive routing). • Undesired situations such congestion, long delays may occur. Features of a Packet-Switching

20. Packets can travel on different networks/links that may have different line speeds Packet Switched Networks: Example

21. Packet-Switched Networks: Topologies

22. In the 60’s and 70’s the Internet (ARPANET) was a small network connecting universities, research labs and government agencies. Main application: email, FTP. Motivation: share & research • Today it is a global, non-regulated communications network with millions of hosts and users. Main applications: Web, multimedia (audio/video), email. Motivation: commercialization • A large number of different network technologies and standards exist: LANs, WANs, B-ISDN, Optical Nets, Wireless, Satellite. What is the Internet?

23. A huge and arbitrary collection of heterogeneous nets. A network of networks! • More than 240 million hosts • Growing exponentially– doubling every 18 months • Hierarchically structured • LANs (e.g., Ethernet) • CANs (e.g., FDDI) • National/global (e.g., ATM or optical backbone) • Fully distributed operation (i.e., no centralized system or computer) The Internet Today-- Complicated

24. Multiple Networks Connected by Routers Path of a Packet is its Route Single Network Routers Packet Route Single Network An Internet

25. A Network Example

26. Resource sharing (i.e., accommodate many users over the same link or through the same router) • Addressing and routing (i.e., how does an email message finds its way to the receiver) • Reliability and recovery: guarantee end-to-end delivery • Traffic management: monitoring and policing the network! Regulate traffic Issues

27. There is a number of measures that characterize and capture the performance of a network • It is not enough that networks work • They must work well • Quality of service (QoS) defines quantitative measures of service quality • Speed • Delay (Latency) • Reliability • Security (not a QoS measure but crucial) Network Performance

28. Speed • Bits per second (bps) • Multiples of 1,000 (not 1,024) • Kilobits per second (kbps)  Note the lower case “k” • Megabits per second (Mbps) • Gigabits per second (Gbps) • Terabits per second (Tbps) • Related to link bandwidth Network Performance

29. Congestion and Latency • Congestion because traffic chronically or momentarily exceeds capacity • Latency delay measured in milliseconds (ms), microseconds ( ). • Especially bad for some services such as voice communication or highly interactive applications Network Performance

30. Delay: • Transmission time: time it takes to transmit a packet (depends on the link speed) = packet size/ speed • Propagation delay: time for a bit to travel across a link (depends on the distance, physical medium) • Queuing delay: waiting time inside a buffer • Processing delay: time to process a packet • RTT (round-trip time): time for a bit to travel to the destination and come back Network Performance

31. Reliability • Availability – percentage of time the network is available to users for transmission and reception • Error rate – percentage of lost or damaged messages or bits. • Examples: • Bit errors (bits are flipped, e.g., due to electrical signal interference.) • Packet loss (packets may be dropped due to insufficient buffer space.) • Packet delays (e.g., due to large queue size) • Nodes or links can fail (go down) • Malicious users Reliability and Recovery

32. As a consequence: • Packets delivered to the wrong destination • Long delays on packets • Packets delivered out-of-order • Duplicate packets • Recovery: • Implement error-control mechanism • Hop by hop (I.e., between nodes) • End-to-end (source-to-destination). • Retransmissions • End-to-end security (e.g., encryption, authentication) Reliability and Recovery

33. Overload and Congestion • Overload: Too many packets occur in a subnetwork in the same time, which prevent each other and in such a way the throughput decreases • Congestion: the queues in the routers are too long, the buffers are full. • As a consequence some packages are dropped if the buffers of the routers are overloaded • In extreme case: grid-lock, lock-up (often used in a DNS (Denial of Service attack)

34. Users run application programs (web, email, ftp) at the hosts interconnected through a network • Hosts need to communicate in a meaningful way. User should not be concerned with the underlying network • Network supports process-to-process (uni- or bi-directional) communication among the hosts • Applications need to take into consideration limitations imposed by the networks physical characteristics User Applications

35. The Need for a Protocol Architecture • Procedures to exchange data between devices can be complex • High degree of cooperation required between communicating systems • destination addressing, path • readiness to receive • file formats, structure of data • how commands are sent/received and acknowledged

36. Set of rules that specify the format and meaning of messages exchanged between computers across a network • Format is sometimes called syntax • Meaning is sometimes called semantics • Example from everyday life: traffic laws! What is a Protocol?

37. Currently, Internet is mostly based on the TCP/IP protocol suite (designed in late 70’s) • TCP/IP became popular as it was bundled with the UNIX/C environment • ISO is still influential in designing networks • Other architectures: ATM. Frame Relay Internet Protocol Architecture

38. Set of rules or conventions to exchange blocks of formatted data • Syntax: data format • Semantics: control information (coordination, error handling) • Timing: speed matching, sequencing • Actions: what happens when an event occurs Key Features of a Protocol

40. Repeater: connects network segments logically to one network • Hub: multiport repeater • Bridge: datalink level connection of two networks • Switch: multiport bridge • Router: connects networks that are compatible in transport level • subnetworks are connected to the interfaces of the repeater • Gateway (proxy server): router between two individual network. The “Way Out” Network Tools

41. Characteristics of High-Speed LANs

42. Router Router LW1 LR1 LW2 LR2 Lw3 • The following affectperformance metrics • Overhead: CPU time to put packet on wire • Throughput: Maximum number of bytes per second • Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers • Latency: time until first bit of packet arrives at receiver • Raw transfer time + overhead at each routing hop • Contributions to Latency • Wire latency: depends on speed of light on wire • about 1–1.5 ns/foot • Router latency: depends on internals of router • Could be < 1 ms (for a good router) Performance Considerations