2. Introduction1-1 Part of slides provided by J.F Kurose and K.W. Ross, All Rights Reserved Chapter 1 Computer Networks and the Internet Computer networking - A top-down approach featuring the internet 4th Edition, 2008 Addison Wesley James F. Kurose, Keith W. Ross ISBN 0-321-49770-8 Communication Networks P. Demeester

3. Introduction1-2 Chapter 1: Introduction Our goal:  Introduction to computer networks  Limit to >80% of the technologies used today  Technologies : TCP+UDP, IP, Ethernet  Describe applications (file transfer, e-mail, Web access,…)

4. Introduction1-3 Chapter 1 outline 1.1 What is the Internet? [1.2 Network edge] [1.3 Network core] [1.4 Network access and physical media] 1.5 Internet structure and ISPs [1.6 Delay & loss in packet-switched networks] 1.7 Protocol layers, service models [1.8 History]

5. Introduction1-4 Move towards all-IP  VoIP  Skype  Video over IP  Belgacom TV  Internet of Things  Smart environments  Operators are transforming their network towards all IP

6. Introduction1-5 BT's 21st Century Network : current network IP ATM PSTN DSL KStream PSTN DPCN PDH Fibre Copper DWSS ASDH End User ~5k nodes ~2k nodes ~400 nodes ~100 nodes ~15 nodes MSH -SDH ~1k nodes Mesh -SDH Inter-node transmission provided by SDH/PDH platforms CWSS Today: multiple services on multiple platforms Other Networks

7. Introduction1-6 BT's 21st Century Network : Future IP-MPLS-WDM DSL Fibre & Copper Agg Box End User ~5k nodes ~100 nodes Class 5 Call Server GFP on SDH Content WWW ISP PSTN services migrate to IP Other Networks accessaggregationcore

8. Introduction1-7 ISP What’s the Internet: “nuts and bolts” view router workstation server mobile ISP : Internet Service Provider How to address the terminals ? How to find them ? How to route the information ? …

9. Introduction1-8 What’s the Internet: “nuts and bolts” view  millions of connected computing devices: hosts, end-systems m PCs workstations, servers m PDAs, phones, toasters running network apps  communication links m fiber, copper, radio, satellite m transmission rate = bandwidth  routers: forward packets (chunks of data) local ISP regional ISP company network router workstation server mobile ISP : Internet Service Provider

10. Introduction1-9 What’s the Internet: “nuts and bolts” view  protocols control sending, receiving of msgs m e.g., TCP, IP, HTTP, FTP, PPP  Internet: “network of networks” m loosely hierarchical m public Internet versus private intranet  Internet standards m RFC: Request for comments m IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile

11. Introduction1-10 A closer look at network structure  network edge: applications and hosts  network core: m routers m network of networks  access networks, physical media: communication links

12. Introduction1-11 The network edge  end systems (hosts): m run application programs m e.g. Web, email m at “edge of network”  client/server model m client host requests / receives service from always-on server m e.g. Web browser/server; email client/server  peer-peer model: m minimal (or no) use of dedicated servers m e.g. Gnutella, KaZaA

13. Introduction1-12 The network core  mesh of interconnected routers  the fundamental question: how is data transferred through net? m circuit switching: dedicated circuit per call: telephone net m packet-switching: data sent thru net in discrete “chunks”

14. Introduction1-13 Q: How to connect end systems to edge router?  residential access nets  institutional access networks (school, company)  mobile access networks Keep in mind:  bandwidth (bits per second) of access network?  shared or dedicated? Access networks and physical media

15. Introduction1-14 Network taxonomy Telecommunication networks FDMTDM Networks with VCs Datagram Networks Circuit-switched networks Packet-switched networks Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

16. Introduction1-15 Chapter 1 outline 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History

17. Introduction1-16  roughly hierarchical  at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T), national/international coverage m treat each other as equals Tier 1 ISP Tier-1 providers interconnect (peer) privately NAP Tier-1 providers also interconnect at public network access points (NAPs) Internet structure: network of networks

18. Introduction1-17 Tier-1 ISP: e.g., Sprint Sprint US backbone network

19. Introduction1-18  “Tier-2” ISPs: smaller (often regional) ISPs m Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier 1 ISP NAP Tier-2 ISP - Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet - Tier-2 ISP is customer of tier-1 provider Tier-2 ISPs also peer privately with each other, interconnect at NAP Internet structure: network of networks

20. Introduction1-19  “Tier-3” ISPs and local ISPs m last hop (“access”) network (closest to end systems) Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Internet structure: network of networks

21. Introduction1-20  a packet passes through many networks! Tier 1 ISP NAP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP Internet structure: network of networks

22. Introduction1-21 Routers in the Internet : Global In 2005 the number of (pubic) routers is in the 100.000 - 200.000 range. The average number of hops a packet has to cross is in the order of 13.

23. Introduction1-22 Routers in the Internet : Zoom-In

24. Introduction1-23 Routers in the Internet : Global www.cybergeography.org/atlas/geographic.html NSFNET 1992

25. Introduction1-24 Routers in the Internet : Global 2001

26. Introduction1-25 Routers in the Internet : Global 2000

27. Introduction1-26 Chapter 1 outline 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models 1.8 History

28. Introduction1-27 Protocol “Layers” Networks are complex!  many “pieces”: m hosts m routers m links of various media m applications m protocols m hardware, software Question: Is there any hope of organizing the structure of a network? Or at least in our discussion of networks?

29. Introduction1-28 An example : postal services writer friend “application” service : friend reads book they agree and make a “connection” they use the unreliable postal services “network”

30. Introduction1-29 client server An example : web access Internet Protocol 10 Mbit/s local area network RFC 1011 : “Internet Protocols” router TCP-Connection 100 Gbit/s WDM transatlantic optical cable 2 Mbit/s ISDN link local telephone switch 56.6 kbit/s modem connection 140 Mbit/s PDH link URL: www.ietf.org IETF web page IP-datagram find info about Internet on the Web !

31. Introduction1-30 What’s a protocol ? human protocols:  “what’s the time?”  “I have a question”  introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols:  machines rather than humans  all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, reception

32. Introduction1-31 What’s a protocol ? a human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection req TCP connection response Get http://www.awl.com/kurose-ross time

33. Introduction1-32 Why layering ? Dealing with complex systems:  explicit structure allows identification of functions, relationship between complex system’s possible m layered reference model  modularization eases maintenance and updating of system m change of implementation of layer’s service is transparent to rest of system m e.g., change in postal service implementation doesn’t affect rest of system (as long as interfaces are the same)

34. Introduction1-33 Internet protocol stack  application: supporting network applications m FTP, SMTP, HTTP  transport: host-host data transfer m TCP, UDP  network: routing of datagrams from source to destination m IP, routing protocols  data link: data transfer between neighboring network elements m PPP, Ethernet  physical: bits “on the wire” 5. Application Layer4. Transport Layer3. Network Layer2. Data Link Layer1. Physical Layer

35. Introduction1-34 Each layer:  distributed  “entities” implement layer functions in the node  entities perform actions, exchange messages with peers application transport network data link physical application transport network data link physical application transport network data link physical application transport network data link physical network data link physical Layering: logical communication

36. Introduction1-35 E.g.: transport  take data from application layer  add addressing, reliability check, info to form “segment”  send segment to peer  wait for peer to ack receip application transport network data link physical application transport network data link physical application transport network data link physical application transport network data link physical network data link physical data Layering: logical communication

37. Introduction1-36 application transport network data link physical application transport network data link physical application transport network data link physical application transport network data link physical network data link physical data Layering: “physical” communication

38. Introduction1-37 Application Layer : File Transfer Protocol OPEN connection TCP control connection Login, passwordOK Request directory TCP transfer connection directory TCP transfer connection file transfer CLOSE connection FTP server Request file transfer

39. Introduction1-38 Login, password OK read directory directory Request file transfer file transfer application layer : FTP transport layer : TCP client layer FTP-server “peer to peer” communication server layer Remark : client - server SAP SAP : Service Access point Application/Transport Layer

40. Introduction1-39 Transmission Control Protocol TCP connection  connection oriented (virtual connection in software of terminals : state)  3-way handshake protocol  point to point, full duplex  exchange of segments (=unit of data) (during a file transfer the file will be cut in pieces : segments)  reliable transport (=acknowledgement, retransmission, timers, …)  flow control (sender won’t overwhelm receiver)  congestion control (senders “slow down sending rate” when network congested)  segments are sent over a network of routers in the IP layer  typically used for http, ftp, smtp, pop, …

41. Introduction1-40 3-way handshake If set-up segment is lost ==> time-outs Client side Server side Setup TCP connection progressing time SYN : SYNchronization ACK : ACKnowledgment C : Client side S : Server side TCP connection setup

42. Introduction1-41 TCP for file transfer time-out 0001-1000 1001-2000 2001-3000 3001-4000 4001-5000 0001-1000 1001-2000 2001-3000 3001-4000 4001-5000 TCP connection for file transfer

43. Introduction1-42 User Datagram Protocol UDP audio server  very simple “protocol”  alternative to complex TCP  unreliable and connectionless  no flow control or congestion control  unidirectional  no extra delays due to acknowledgments  typically used for real-time and control applications

44. Introduction1-43 UDP real time audio send as quick as possible no extra delay due to acknowledgment no retransmissions 0001-1000 1001-2000 2001-3000 3001-4000 4001-5000 0001-1000 1001-2000 2001-3000 3001-4000 4001-5000 UDP for real time audio transfer

45. Introduction1-44 network layer : IP Transport/Network Layer server layer transport layer : TCP client layer TCP connection SERVICE : Transfer TCP segment from source to destination SOLUTION : Encapsulate TCP segment in IP datagram (destination indicated !). IP network layer will transfer IP datagram over links and routers to destination. router link IP datagrams

46. Introduction1-45 01101011101011010101 10101101010101110101 00110101011100010101 01101011100001010100 11111101010100000110 server Data file Encapsulation example: FT /TCP/IP IP datagram TCP segment TCP, S-D IP header IP payload TCP payload FTP TCP header 01101011101011010101 FTP IP NETWORK LAYER TCP TRANSPORT LAYER FTP APPLICATION LAYER 01101011101011010101

47. Introduction1-46 A B C D E Y:to B Z:to D Y:to C Z:to E Y:to Y Source:X Destination:Y Content:TCP Store and Forward Routing Table Buffer Router Link computer host terminal W X Y Z Network Layer : Internet Protocol

48. Introduction1-47  Unidirectional  Datagram (“packet”) based store and forward  Connectionless  Flexibility is provided by network elements (routers)  Best effort : no guarantee on delay, delivery, … (no Quality of Service : QoS)  IP layer is the server layer of the TCP layer and the client layer of the Data Link Layer Network Layer : Internet Protocol

49. Introduction1-48 Physical layer data link layer server layer network layer : IP client layer point-to-point shared twisted pair fiber server layer client layer Network/Data Link/Physical Layer

50. Introduction1-49 Data Link Layer point-to-point example :  PPP protocol (Point-to-Point Protocol)  establish a connection between two routers or a router and a terminal  can use e.g. an SDH VC-4 150 Mbit/s connection between 2 routers or a telephone ISDN connection of 64 kbit/s between a terminal at home and the access router of an ISP. shared medium example :  Ethernet  MAC : CSMA/CD Les 1-2

51. Introduction1-50 transport application network data link physical terminal network data link physical data link physical router data link physical switch transport application network data link physical terminal IP FTPHTTPDNS...SNMP TCP UDP PPP... CSMA/CD fiber...coax Summary

52. Introduction1-51 FTP 20,21 HTTP 80 DNS 53...SNMP 161,162 TCP 6 UDP 17 Identification : Application : port number FTP port 20 (data), 21 (control) TCP or UDP : protocol number TCP : 6 UDP : 17 Host : IP address (e.g. 157.193.122.1) Note : some terminology : Application : message Transport : segment Network : datagram (or packet) Data Link : frame IP 157.193. 122.1 Summary

53. Introduction1-52 Table of contents  1.1 What is the Internet?3  1.5 Internet structure and ISPs15  1.7 Protocol layers, service models26  Summary50  Table of contents52