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Ch 1. Computer Networks and the Internet

Myungchul Kim mckim@icu.ac.kr. Ch 1. Computer Networks and the Internet. What is the Internet?. One sentence definition? A nuts-and-bolts description A service description A nuts-and-bolts description Hosts or end systems A network of communication links and packet switches

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Ch 1. Computer Networks and the Internet

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  1. Myungchul Kim mckim@icu.ac.kr Ch 1. Computer Networks and the Internet

  2. What is the Internet? • One sentence definition? • A nuts-and-bolts description • A service description • A nuts-and-bolts description • Hosts or end systems • A network of communication links and packet switches • Transmission rate • Packets • Packet switches: routers and link-layer switches • Route or path • Internet Service Providers (ISPs) • Protocols: TCP and IP • Internet Standards: Request for comments (RFCs) by IETF • Intranet

  3. A service description • An infrastructure for providing services to distributed applications: remote login, electronic mail, Web surfing, instant messaging, VoIP, audio and video streaming, Internet telephony, distributed games, peer-to-peer (P2P) file sharing, IPTV… • Application Programming Interface (API) • Protocols • Figure 1.2.

  4. Definition of a Protocol • Defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event • Similar to a human analogy: there are specific messages we send, and specific actions we take in response to the received reply messages or other events

  5. The Network Edge • Host = end system: clients and servers • Peer-to-peer: acts as both a client and a server • Access networks: connect an end system to its edge router • Residential access • Company access • Wireless access • Residential access • Digital subscriber line (DSL): point-to-point • Hybrid fiber-coaxial cable (HFC): cable modems, shared • Very-high speed DSL (VDSL)

  6. Company access • Ethernet : shared • Wireless access • Wireless LAN • IEEE 802.11 WiFi • 3G Wireless: HSDPA (High-Speed Downlink Packet Access) • IEEE 802.16 WiMax • WiBro • Physical media • Twisted-pair copper wire • Coaxial cable • Fiber optics • Terretrial radio channels: wireless LAN, the cellular access • Satellite Radio channels

  7. The Network Core • Circuit switching • Reserved for the communication session • A circuit: at the guaranteed constant rate • Telephone network • Packet switching • The network resources on demand • Internet • Best effort • Multiplexing in Circuit-switched networks • The dedicated circuits are idle during silent periods • Frequency-division multiplexing (FDM) or Time-division multiplexing (TDM)

  8. Fig 1.6.

  9. Packet switching • Message -> packets • Routers = packet switches • Store-and-forward transmission: the switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link -> store-and-forward delay • Output queue -> queueing delay • Packet loss • Fig 1.7

  10. Packet switching vs Circuit switching • Packet switching is not suitable for real-time services? • Sharing of network resources -> statistical multiplexing of resources

  11. Figure 1.11

  12. ISPs and Internet Backbones • Tier-1 ISPs: Internet Backbone • Tier-2 ISPs: regional or national coverage • Access ISPs • Points of Presence (POPs): the points at which the ISP connects to other ISPs

  13. Delay and loss in Packet-switched networks • Fig 1.18 • Processing delay • Examine the packet’s header and determine where to direct the packet • Check for bit-level errors • Microseconds or less • Queuing delay • A packet waits to be transmitted onto the link • Depends on the number of earlier-arriving packets that are queued and waiting for transmission across the link. • congestion • Microseconds to milliseconds.

  14. Transmission delay • Store-and-forward delay • Transmit all of the packet’s bits into the link • L/R where L bits = length of the packet, R = 10 Mbps for a 10 Mbps Ethernet link • Microseconds to milliseconds • Propagation delay • Propagation speed of the link • d/s where d = distance and s = the propagation speed of the link • Milliseconds • Comparing transmission and propagation delay • d nodal = d proc+ d queue + d trans + d prop • d prop: hundreds of milliseconds for two routers by a satellite link • d trans : hundreds of milliseconds for low-speed dial-up modem links • d proc: at the max rate of a router

  15. Queuing delay • Traffic intensity La/R where a = the average rate of packets arrival at the queue (packets/sec), L bits of a packet, R = the transmission rate (bits/sec), and the infinite queue. • If La/R > 1, the queue will tend to increase without bound and the queuing delay will approach infinity. • If La/R ≤ 1, the nature of the arriving traffic impacts the queuing delay. Periodically or in bursts or random • Fig 1.19

  16. Packet loss • A queue has finite capacity. • Performance of a node = delay + packet loss • End-to-end delay • d end-end = N (d proc+ d trans + d prop) for N-1 routers where the network is uncongested.

  17. Traceroute • Repeats experiment three times to get the round-trip delays between souce and destination • The queuing delay is varying with time. -> the round-trip delays are varying. • (Next slide) • Other delays • Media accessing delays in WiFi, Ethernet, … • Packetization delays in VoIP

  18. “Real” Internet delays and routes traceroute: gaia.cs.umass.edu to www.eurecom.fr Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136ms trans-oceanic link * means no response (probe lost, router not replying)

  19. pipe that can carry fluid at rate Rsbits/sec) pipe that can carry fluid at rate Rcbits/sec) Throughput • throughput: rate (bits/time unit) at which bits transferred between sender/receiver • instantaneous: rate at given point in time • average: rate over long(er) period of time link capacity Rcbits/sec link capacity Rsbits/sec server, with file of F bits to send to client server sends bits (fluid) into pipe

  20. Rs > RcWhat is average end-end throughput? Rsbits/sec Rcbits/sec Rcbits/sec bottleneck link link on end-end path that constrains end-end throughput Throughput (more) • Rs < RcWhat is average end-end throughput? Rsbits/sec

  21. Protocol layers and their service models • A layered architecture allows us to discuss a well-defined, specific part of a large and complex system. • Protocol stack • Service model • Layer (n-1) is said to offer services to layer (n)

  22. Layer functions • Error control • Flow control • Segmentation and reassembly • Multiplexing • Connection setup • Drawbacks of layering • Duplicated lower-layer functionality • Accessing an information in another layer

  23. Application layer: HTTP, SMTP, FTP, DNS • Transport layer: TCP, UDP • Network layer: IP, routing • Link layer: Ethernet, PPP, WiFi • Physical layer

  24. The internet protocol stack

  25. network link physical link physical M M M Ht M Hn Hn Hn Hn Ht Ht Ht Ht M M M M Hn Ht Ht Hl Hl Hl Hn Hn Hn Ht Ht Ht M M M Encapsulation source message application transport network link physical segment datagram frame switch destination application transport network link physical router

  26. Networks under attack • Network security • The bad guys can put malware into your host via the Internet • Botnet, Self-replicating, Viruses, Worms, Trojan hoars • The bad guys can attack servers and network infrastructure • Denial-of-service (DoS) attacks, Distributed DoS attacks • The bad guys can sniff packets • A packet sniffer: Ethereal • The bad guys can masquerade as someone you trust • IP spoofing: with a false source address • Authentication • The bad guys can modify or delete messages • Man-in-the-middle attacks • Integrity of the data

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