1. Introduction 1-1 Chapter 1 Computer Networks and the Internet Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. Courtesy of J.F Kurose and K.W. Ross (All material copyright 1996-2006)

2. Introduction 1-2 Chapter 1: Introduction Our goal:  get “feel” and terminology  more depth, detail later in course  approach: m use Internet as example Overview:  what’s the Internet  what’s a protocol?  network edge  network core  access network  Internet/ISP structure  performance: loss, delay  protocol layers, service models  network modeling

3. Introduction 1-3 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

4. Introduction 1-4 What’s the Internet: “nuts and bolts” view  millions of connected computing devices: hosts = end-systems m PCs workstations, servers, smart phones, toasters, …  running network apps  communication links m fiber, copper, radio, satellite m transmission rate = bandwidth  Packet switches: forward packets (chunks of data) m routers and switches local ISP company network regional ISP router workstation server mobile

5. Introduction 1-5 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 Interconnected ISPs 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

6. Introduction 1-6 What’s the Internet: a service view  communication infrastructure provides services to applications: m Web, VoIP, email, games, e- commerce, social nets, …  provides programming interface to apps m hooks that allow sending & receiving apps to “connect” to Internet m provides service options, analogous to postal service

7. Introduction 1-7 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, receipt

8. Introduction 1-8 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

9. Introduction 1-9 A closer look at network structure:  network edge: applications and hosts  network core: m Interconnected routers m network of networks  access networks, physical media: communication links

10. Introduction 1-10 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

11. Introduction 1-11 The network edge:  end systems (hosts): m run application programs 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. Skype, KaZaA

12. Introduction 1-12 Network edge: connection-oriented service Goal: data transfer between end systems  handshaking: setup (prepare for) data transfer ahead of time m Hello, hello back human protocol m set up “state” in two communicating hosts  TCP - Transmission Control Protocol m Internet’s connection- oriented service TCP service [RFC 793]  reliable, in-order byte- stream data transfer m loss: acknowledgements and retransmissions  flow control: m sender won’t overwhelm receiver  congestion control: m senders “slow down sending rate” when network congested Not provided by connection- oriented service but by TCP!

13. Introduction 1-13 Network edge: connectionless service Goal: data transfer between end systems m same as before! m No handshaking  UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service m unreliable data transfer m no flow control m no congestion control App’s using TCP:  HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP:  streaming media, teleconferencing, DNS, Internet telephony

14. Introduction 1-14 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

15. Introduction 1-15 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”

16. Introduction 1-16 Network Core: Circuit Switching End-end resources reserved for “call”  link bandwidth, switch capacity  dedicated resources m no sharing  circuit-like (guaranteed) performance  call setup required

17. Introduction 1-17 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to calls  resource piece idle if not used by owning call (no sharing)  dividing link bandwidth into “pieces” m frequency division m time division

18. Introduction 1-18 Circuit Switching: FDMA and TDMA FDMA frequency time TDMA frequency time 4 users Example:

19. Introduction 1-19 Example: Circuit Switching  How long will it take to send a file of 640,000 bits from host A to host B over a circuit-switched network.  Suppose all links in the network are TDM with: m 24 slots and m have a bit rate of 1.536Mbps  It takes 500 msec to establish an end-to-end circuit before host A begins transmitting to B  How long will it take to send file?  Transmission rate for each circuit = 1.536 Mbps / 24 = 64 Kbps  Time to send 640 Kbits file = (640000 bits)/(64 Kbits/sec) = 10 seconds  Including circuit setup overhead, time to send file is 10.5 seconds  This calculation is independent of the # of end-to-end links and does not include propagation delays

20. Introduction 1-20 Network Core: Packet Switching each end-end data stream divided into packets  user A, B packets share network resources  each packet uses full link bandwidth  resources used as needed resource contention:  aggregate resource demand can exceed amount available  congestion: packets queue, wait for link use  store and forward: packets move one hop at a time m Node receives complete packet before forwarding Bandwidth division into “pieces” Dedicated allocation Resource reservation

21. Introduction 1-21 Packet Switching: Statistical Multiplexing Sequence of A & B packets does not have fixed pattern  statistical multiplexing. In TDM each host gets same slot in revolving TDM frame. A B C 100 Mbs Ethernet 1.5 Mbs D E statistical multiplexing queue of packets waiting for output link

22. Introduction 1-22 Packet switching versus circuit switching  1 Mbit link  each user: m 100 kbps when “active” m active 10% of time  circuit-switching: m 10 users  packet switching: m with 35 users, probability > 10 active less than.0004 Packet switching allows more users to use network! N users 1 Mbps link

23. Introduction 1-23 Packet switching versus circuit switching  Great for bursty data m resource sharing m simpler, no call setup  Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control  Q: How to provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps m still an unsolved problem (chapter 7) Is packet switching a “slam dunk winner?”

24. Introduction 1-24 Packet-switching: store-and-forward  Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps  Entire packet must arrive at router before it can be transmitted on next link: store and forward  End-to-end (e2e) delay = 3L/R (assuming zero propagation delay) Example:  L = 7.5 Mbits  R = 1.5 Mbps  one-hop delay = 5 sec  e2e delay = 15 sec R R R L

25. Introduction 1-25 Packet Switching: Message Segmenting Now break up the message into 5000 packets  Each packet 1,500 bits  1 msec to transmit packet on one link  pipelining: each link works in parallel  Delay reduced from 15 sec to 5.002 sec

26. Introduction 1-26 Network Core: Packet Switching (Cont’d) Advantages and disadvantages of segmentation  Advantages: m More efficient  less delays m Handles bit errors more efficiently Entire message does not require retransmission Less overhead in terms of bandwidth  Disadvantage: m Each packet has a header vs. one header per message m In most cases, this overhead is small

27. Introduction 1-27 Network Core: Packet Switching (Cont’d) Packet switching vs. circuit switching:  Advantages of packet switching: m Offers better bandwidth sharing than circuit switching m Simpler, more efficient, and robust  Limitations of packet switching: m Not suitable for real-time services IP telephony, video conference, etc. m Variable and unpredictable delays

28. Introduction 1-28 Packet-switched networks: forwarding  Goal: move packets through routers from source to destination m we’ll study several path selection (i.e. routing) algorithms (chapter 4)  datagram network: m destination address in packet determines next hop m routes may change during session m analogy: driving, asking directions  virtual circuit network: m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed thru call m routers maintain per-call state

29. Introduction 1-29 Network Taxonomy Telecommunication networks Circuit-switched networks FDM TDM Packet-switched networks Networks with VCs Datagram Networks Datagram network is not exclusively connection- oriented nor is it exclusively connectionless. Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

30. Introduction 1-30 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

31. Introduction 1-31 Access networks 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?

32. Introduction 1-32 Residential access: point to point access  Dialup via modem m up to 56Kbps direct access to router (often less) m Can’t surf and phone at same time: can’t be “always on”  ADSL: asymmetric digital subscriber line m up to 1 Mbps upstream (today typically < 256 kbps) m up to 8 Mbps downstream (today typically < 1 Mbps) m FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone

33. Introduction 1-33 Residential access: cable modems  HFC: hybrid fiber coax m asymmetric: up to 30Mbps downstream, 2 Mbps upstream  network of cable and fiber attaches homes to ISP router m homes share access to router  deployment: available via cable TV companies

34. Introduction 1-34 Company access: local area networks  company/univ local area network (LAN) connects end system to edge router  Ethernet: m shared or dedicated link connects end system and router m 10 Mbs, 100Mbps, Gigabit Ethernet  deployment: institutions, home LANs  LANs: chapter 5

35. Introduction 1-35 Wireless access networks  shared wireless access network connects end system to router m via base station aka “access point”  wireless LANs: m 802.11b/g (WiFi): 11-54 Mbps  wider-area wireless access m provided by telco operator m 3G ~ 384 kbps Will it happen?? m GPRS in Europe/US base station mobile hosts router

36. Introduction 1-36 Home networks Typical home network components:  ADSL or cable modem  router/firewall/NAT  Ethernet  wireless access point wireless access point wireless laptops router/ firewall cable modem to/from cable headend Ethernet (switched)

37. Introduction 1-37 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

38. Internet structure: network of networks  End systems connect to Internet via access ISPs (Internet Service Providers)  Residential, company and university ISPs  Access ISPs in turn must be interconnected.  So that any two hosts can send packets to each other  Resulting network of networks is very complex  Evolution was driven by economics and national policies  Let’s take a stepwise approach to describe current Internet structure

39. Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … …

40. Internet structure: network of networks Option: connect each access ISP to every other access ISP? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … … … … … connecting each access ISP to each other directly doesn’t scale: O(N 2 ) connections.

41. Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement. global ISP

42. Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. ISP B ISP A ISP C

43. Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. which must be interconnected ISP B ISP A ISP C IXP peering link Internet exchange point

44. Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and regional networks may arise to connect access nets to ISPs ISP B ISP A ISP C IXP regional net

45. Internet structure: network of networks access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users ISP B ISP A ISP B IXP regional net Content provider network

46. Introduction Internet structure: network of networks  at center: small # of well-connected large networks m “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage m content provider network (e.g, Google): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs 1-46 access ISP access ISP access ISP access ISP access ISP access ISP access ISP IXP Regional ISP Tier 1 ISP Google access ISP

47. Introduction Tier-1 ISP: e.g., Sprint … to/from customers peering to/from backbone … … … … POP: point-of-presence 1-47

48. Introduction 1-48 Internet structure: network of networks  “Tier-2” ISPs: smaller (often regional) ISPs m Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier 1 ISP IXP 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 IXP

49. Introduction 1-49 Internet structure: network of networks  “Tier-3” ISPs and local ISPs m last hop (“access”) network (closest to end systems) Tier 1 ISP IXP 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

50. Introduction 1-50 Internet structure: network of networks  a packet passes through many networks! Tier 1 ISP IXP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP

51. Introduction 1-51 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

52. Introduction 1-52 How do loss and delay occur? packets queue in router buffers  packet arrival rate to link exceeds output link capacity  packets queue, wait for turn A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers

53. Introduction 1-53 Four sources of packet delay  1. nodal processing: m check bit errors m determine output link A B propagation transmission nodal processing queueing  2. queueing m time waiting at output link for transmission m depends on congestion level of router

54. Introduction 1-54 3. Transmission delay:  R=link bandwidth (bps)  L=packet length (bits)  time to send bits into link = d trans = L/R 4. Propagation delay:  d = length of physical link  s = propagation speed in medium (~2x10 8 m/sec)  propagation delay = d prop = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities!  d trans  d prop Four sources of packet delay

55. Introduction 1-55 Caravan analogy  cars “propagate” at 100 km/hr  toll booth takes 12 sec to service car (transmission time)  car~bit; caravan ~ packet  Q: How long until caravan is lined up before 2nd toll booth?  Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec  Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr  A: 62 minutes toll booth toll booth ten-car caravan 100 km

56. Introduction 1-56 Caravan analogy (more)  Cars now “propagate” at 1000 km/hr  Toll booth now takes 1 min to service a car  Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?  Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.  1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! toll booth toll booth ten-car caravan 100 km

57. Introduction 1-57 Nodal delay  d proc = processing delay m typically a few microsecs or less  d queue = queuing delay m depends on congestion  d trans = transmission delay m = L/R, significant for low-speed links  d prop = propagation delay m a few microsecs to hundreds of msecs

58. Introduction 1-58 Queueing delay (revisited)  R=link bandwidth (bps)  L=packet length (bits)  a=average packet arrival rate traffic intensity = La/R  La/R ~ 0: average queueing delay small  La/R -> 1: delays become large  La/R > 1: more “work” arriving than can be serviced, average delay infinite! La/R ~ 0 La/R -> 1

59. Introduction 1-59 Packet loss  queue (aka buffer) preceding link in buffer has finite capacity  when packet arrives to full queue, packet is dropped (aka lost)  lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area)

60. Introduction 1-60 Throughput  throughput: rate (bits/time unit) at which bits transferred between sender/receiver m instantaneous: rate at given point in time m average: rate over longer period of time server, with file of F bits to send to client link capacity R s bits/sec link capacity R c bits/sec pipe that can carry fluid at rate R s bits/sec) pipe that can carry fluid at rate R c bits/sec) server sends bits (fluid) into pipe

61. Introduction 1-61 Throughput (more)  R s < R c What is average end-end throughput? R s bits/sec R c bits/sec  R s > R c What is average end-end throughput? R s bits/sec R c bits/sec link on end-end path that constrains end-end throughput bottleneck link

62. Introduction 1-62 Throughput: Internet scenario 10 connections (fairly) share backbone bottleneck link R bits/sec RsRs RsRs RsRs RcRc RcRc RcRc R  per-connection end-end throughput: min(R c,R s,R/10)  in practice: R c or R s is often bottleneck

63. Introduction 1-63 Chapter 1: roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access 1.5 Internet structure and ISPs 1.6 Delay & loss in packet-switched networks 1.7 Protocol layers, service models

64. Introduction 1-64 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 structure of network? Or at least our discussion of networks?

65. Introduction 1-65 Organization of air travel  a series of steps ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing

66. Introduction 1-66 ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival airport intermediate air-traffic control centers airplane routing ticket (complain) baggage (claim gates (unload) runway (land) airplane routing ticket baggage gate takeoff/landing airplane routing Layering of airline functionality Layers: each layer implements a service m via its own internal-layer actions m relying on services provided by layer below

67. Introduction 1-67 Why layering? Dealing with complex systems:  explicit structure allows identification, relationship of complex system’s pieces m layered reference model for discussion  modularization eases maintenance, updating of system m change of implementation of layer’s service transparent to rest of system m e.g., change in gate procedure doesn’t affect rest of system  layering considered harmful?

68. Introduction 1-68 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  link: data transfer between neighboring network elements m PPP, Ethernet  physical: bits “on the wire” application transport network link physical

69. Introduction ISO/OSI reference model  presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions  session: synchronization, checkpointing, recovery of data exchange  Internet stack “missing” these layers! m these services, if needed, must be implemented in application m needed? application presentation session transport network link physical 1-69

70. Introduction 1-70 source application transport network link physical HtHt HnHn M segment HtHt datagram destination application transport network link physical HtHt HnHn HlHl M HtHt HnHn M HtHt M M network link physical link physical HtHt HnHn HlHl M HtHt HnHn M HtHt HnHn M HtHt HnHn HlHl M router switch Encapsulation message M HtHt M HnHn frame

71. Introduction 1-71 Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical Each layer:  distributed  “entities” implement layer functions at each node  entities perform actions, exchange messages with peers

72. Introduction 1-72 Layering: logical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data E.g.: transport  take data from app  add addressing, reliability check info to form “datagram”  send datagram to peer  wait for peer to ack receipt  analogy: post office data transport ack

73. Introduction 1-73 Layering: physical communication application transport network link physical application transport network link physical application transport network link physical application transport network link physical network link physical data

74. Introduction 1-74 Introduction: Summary Covered a “ton” of material!  Internet overview  what’s a protocol?  network edge, core, access network m packet-switching versus circuit-switching  Internet/ISP structure  performance: loss, delay  layering and service models You now have:  context, overview, “feel” of networking  more depth, detail to follow!