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1: Introduction 1 Chapter I: Introduction Course on Computer Communication and Networks, EDA343/DIT 420, CTH/GU The slides are based on adaptations of.

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Presentation on theme: "1: Introduction 1 Chapter I: Introduction Course on Computer Communication and Networks, EDA343/DIT 420, CTH/GU The slides are based on adaptations of."— Presentation transcript:

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2 1: Introduction 1 Chapter I: Introduction Course on Computer Communication and Networks, EDA343/DIT 420, CTH/GU The slides are based on adaptations of the slides available by the authors of the course’s main textbook, further edited by the instructor(s): Computer Networking: A Top Down Approach, Jim Kurose, Keith Ross, Addison-Wesley. Slides with darker background contain topics discussed in less detail in class

3 1: Introduction 2 Chapter I: Introduction Overview: r what’s the Internet r types of service r ways of information transfer, routing, performance, delays, loss r protocol layers, service models r access net, physical media r backbones, NAPs, ISPs r (history) r quick look into ATM networks

4 Introduction 1-3 the Internet: “nuts and bolts” view r millions of connected computing devices: hosts = end systems m running network apps Home network Institutional network Mobile network Global ISP Regional ISP router PC server wireless laptop cellular handheld wired links access points  communication links  fiber, copper, radio, satellite  transmission rate = bandwidth  routers: forward packets (chunks of data)

5 Introduction 1-4 the Internet: “nuts and bolts” view r protocols control sending, receiving of msgs m e.g., TCP, IP, HTTP, Skype, Ethernet r Internet: “network of networks” m loosely hierarchical m public Internet versus private intranet r Internet standards m RFC: Request for comments m IETF: Internet Engineering Task Force Home network Institutional network Mobile network Global ISP Regional ISP

6 Introduction 1-5 the Internet: a service view r communication infrastructure enables distributed applications: m Web, VoIP, , games, e-commerce, file sharing r communication services provided to apps: m reliable data delivery from source to destination m “best effort” (unreliable) data delivery

7 Internet standards r RFC: Request for comments r IETF: Internet Engineering Task Force 1: Introduction 6

8 Introduction 1-7 A closer look at (any big) network’s structure: r network edge: applications and hosts  access networks, physical media: wired, wireless communication links  network core:  interconnected routers  network of networks

9 Introduction 1-8 The network edge: end systems (hosts):  run application programs e.g. in Internet Web, , …  … based on network services available at the edge client/server peer-peer types of service offered by the network to applications: connection-oriented: deliver data in the order they are sent connectionless: delivery of data in arbitrary order

10 Introduction 1-9 The Network Core r mesh of interconnected routers r fundamental question: how is data transferred through net? r packet-switching: data sent thru net in discrete “chunks” We will contrast with circuit switching: dedicated circuit per call: “classic”phone net

11 Network Core 1: Introduction 10

12 1: Introduction 11 Network Core: Packet Switching A B C 10 Mbs Ethernet 1.5 Mbs 45 Mbs D E statistical multiplexing queue of packets waiting for output link

13 1: Introduction 12 Network Core: Packet Switching each end-end data stream divided into packets r packets share network resources r resources used as needed store and forward: r packets move one hop at a time m transmit over link m wait turn at next link r =O7CuFlM4V54 =O7CuFlM4V54 m Watch the video at home; nice animation; disregard the terms used in narration; they do not follow exact protocol specifications resource contention: r aggregated resource demand can exceed amount available (bandwidth), hence … r … congestion: packets queue, wait for link use

14 1: Introduction 13 Delay in packet-switched networks packets experience delay on end-to-end path r 1. nodal processing: m check bit errors m determine output link r 2. queuing m time waiting at output link for transmission m depends on congestion level of router A B propagation transmission nodal processing queueing =O7CuFlM4V54 Nice animation; disregard the terms used in narration; they do not follow exact protocol specifications

15 1: Introduction 14 Delay in packet-switched networks 3. Transmission delay: r R=link bandwidth (bps) r L=packet length (bits) r time to send bits into link = L/R 4. Propagation delay: r d = length of physical link r s = propagation speed in medium (~2x10 8 m/sec) r propagation delay = d/s A B propagation transmission nodal processing queuing Note: s and R are very different quantities!

16 1: Introduction 15 Visualize deleys: Circuit, message, packet switching r store and forward behavior + other delays’ visualization (fig. from “Computer Networks” by A. Tanenbaum,)

17 Introduction Network Core: Circuit Switching End-end resources reserved/dedicated for “call” r link bandwidth, switch capacity r dedicated resources: no sharing r circuit-like (guaranteed) performance r call setup required

18 1: Introduction 17 Packet switching versus “classical” circuit switching r 1 Mbit link r each user: m 100Kbps when “active” m active 10% of time (bursty behaviour) r circuit-switching: m 10 users r packet switching: m with 35 users, probability > 10 active less than (  almost all of the time same queuing behaviour as circuit switching) Packet switching allows more users to use the network! N users 1 Mbps link

19 Queueing delay (revisited) … 1: Introduction 18 r R=link bandwidth (bps) r L=packet length (bits) r a=average packet arrival rate traffic intensity = La/R r La/R ~ 0: average queueing delay small r La/R -> 1: delays become large r La/R > 1: more “work” arriving than can be serviced, average delay infinite! Queues may grow unlimited, packets can be lost

20 1: Introduction 19 … “Real” Internet delays and routes (1)… r What do “real” Internet delay & loss look like? r Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: m sends three packets that will reach router i on path towards destination m router i will return packets to sender m sender times interval between transmission and reply. 3 probes

21 1: Introduction 20 …“Real” Internet delays and routes (2)… 1 cs-gw ( ) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms 4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms 5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms ( ) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms ( ) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms traceroute: gaia.cs.umass.edu to Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu * means no reponse (probe lost, router not replying) trans-oceanic link

22 1: Introduction 21 Packet switching properties r PS: Good: Great for bursty data m resource sharing m no call setup r PS: Not so good? Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control r ure=related ure=related r Q: How to provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps m Some routing policies can help (cf next slide)

23 1: Introduction 22 Packet-switched networks: routing r Goal: move packets among routers from source to destination m Challenge 1: path selection algorithms m Challenge2: Important design issue: datagram network: –destination address determines next hop –routes may change during session virtual circuit network: –each packet carries tag (virtual circuit ID), tag determines next hop –fixed path determined at call setup time, remains fixed thru call –routers maintain per-call state

24 23 Virtual circuits: “source-to-dest path behaves almost like a circuit” r call setup, teardown for each call before data can flow m signaling protocols to setup, maintain, teardown VC r every router maintains “state” for each passing connection r resources (bandwidth, buffers) may be allocated to VC application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

25 1: Introduction 24 Network Taxonomy Telecommunication networks Circuit-switched networks Packet-switched networks Networks with VCs Datagram Networks Datagram network (eg Internet) cannot be characterized either connection-oriented or connectionless. Internet provides both connection-oriented (TCP) and connectionless services (UDP), at the network edge, to apps.

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

27 Introduction Throughput r 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

28 Introduction Throughput (more) r 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

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

30 1: Introduction 29 Access networks and physical media

31 1: Introduction 30 Access networks and physical media Q: How to connect end systems to edge router? r residential access nets r institutional access networks (school, company) r mobile access networks Keep in mind: r bandwidth (bits per second) of access network? r shared or dedicated?

32 telephone network Internet home dial-up modem ISP modem (e.g., AOL) home PC central office  Uses existing telephony infrastructure  Home is connected to central office  up to 56Kbps direct access to router (often less)  Can’t surf and phone at same time: not “always on” Dial-up Modem

33 telephone network DSL modem home PC home phone Internet DSLAM Existing phone line: splitter Central Office: multiplexer Digital Subscriber Line (DSL)  Also uses existing telephone infrastruture  Commonly up to 2.5 Mbps upstream (more typically < 1 Mbps)  Commonly up to 24 Mbps downstream (more typically < 10 Mbps)  dedicated physical line to telephone central office

34 Introduction data, TV transmitted at different frequencies over shared cable distribution network cable modem splitter … cable headend CMTS ISP cable modem termination system  HFC: hybrid fiber coax  asymmetric: up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate  network of cable, fiber attaches homes to ISP router  homes share access network to cable headend  unlike DSL, which has dedicated access to central office Access net: cable network 1- 33

35 1: Introduction 34 Institutional access: local area networks r company/univ local area network (LAN) connects end system to edge router r E.g. Ethernet: m shared or dedicated cable connects end system and router m 10 Mbs, 100Mbps, Gigabit Ethernet r deployment: institutions, home LANs

36 Introduction Wireless access networks r shared wireless access network connects end system to router m via base station aka “access point” r wireless LANs: m b/g (WiFi): 11 or 54 Mbps r wider-area wireless access m provided by telco operator m ~1Mbps over cellular system m next up (?): WiMAX (10’s Mbps) over wide area base station mobile hosts router

37 1: Introduction 36 Home networks Typical home network components: r DSL or cable modem r router/firewall/NAT r Ethernet r wireless access point wireless access Point (54 Mbps) wireless devices router/ Firewall NAT Cable or DSL modem to/from cable headend Ethernet

38 1: Introduction 37 Physical Media r physical link: transmitted data bit propagates across link m guided media: signals propagate in solid media: copper, fiber m unguided media: signals propagate freely e.g., radio

39 1: Introduction 38 Physical media: wireless r signal carried in electromagnetic spectrum r Omnidirectional: signal spreads, can be received by many antennas r Directional: antennas communicate with focused el- magnetic beams and must be aligned (requires higher frequency ranges) r propagation environment effects: m reflection m obstruction by objects m interference

40 Signal can fade with distance, can get obstructed, can take many different paths between sender and receiver due to reflection, scattering, diffraction Properties: Attenuation, Multipath propagation signal at sender signal at receiver

41 1: Introduction 40 Physical Media: Twisted pair Twisted Pair (TP) r two insulated copper wires m Category 3: traditional phone wires, 10 Mbps Ethernet m Category 5 TP: more twists, higher insulation: 100Mbps Ethernet

42 1: Introduction 41 Physical Media: coax, fiber Coaxial cable: r wire (signal carrier) within a wire (shield) m baseband: single channel on cable (common use in 10Mbs Ethernet) m broadband: multiple channels on cable (FDM; commonly used for cable TV) Fiber optic cable: r glass fiber carrying light pulses r low attenuation r high-speed operation: m 100Mbps Ethernet m high-speed point-to-point transmission (e.g., 5 Gps) r low error rate

43 1: Introduction 42 Back to Layers-discussion

44 1: Introduction 43 Protocol “Layers” Networks are complex! r 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

45 1: Introduction 44 Why layering? Dealing with complex systems: r explicit structure allows identification, relationship of complex system’s pieces m layered reference model for discussion r modularization eases maintenance/es 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

46 1: Introduction 45 Terminology: Protocols, Interfaces r Each layer offers services to the upper layers (shielding from the implementation details) m service interface: across layers in same host r Layer n on a host carries a conversation with layer n on another host m PROTOCOL, host-to-host interface: defines messages exchanged with peer entity r Network architecture (set of layers, interfaces) vs protocol stack (protocol implementation)

47 1: Introduction 46 What’s a protocol? a human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection req. TCP connection reply. Get time host-to-host interface: defines messages exchanged with peer entity: format, order of msgs sent and received among network entities and actions taken on msg transmission, receipt

48 1: Introduction 47 The OSI Reference Model r ISO (International Standards Organization) defines the OSI (Open Systems Inerconnect) model to help vendors create interoperable network implementation r Reduce the problem into smaller and more manageable problems: 7 layers m a layer should be created where a different level of abstraction is needed; each layer should perform a well defined function) m The function of each layer should be chosen with an eye toward defining internationally standardized protocols r ``X dot" series (X.25, X. 400, X.500) OSI model implementation (protocol stack)

49 1: Introduction 48 Internet protocol stack r application: ftp, smtp, http, etc r transport: tcp, udp, … r network: routing of datagrams from source to destination m ip, routing protocols r link: data transfer between neighboring network elements m ppp, ethernet r physical: bits “on the wire” application transport network link physical

50 1: Introduction 49 Internet protocol stack r Architecture simple but not as good as OSI‘s m no clear distinction between interface-design and implementations; m hard to re-implement certain layers r Successful protocol suite (de-facto standard) m was there when needed (OSI implementations were too complicated) m freely distributed with UNIX

51 1: Introduction 50 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: r distributed r “entities” implement layer functions at each node r entities perform actions, exchange messages with peers

52 1: Introduction 51 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 r take data from app r add addressing, form “datagram” r send datagram to peer r (possibly wait for peer to ack receipt) data transport ack

53 1: Introduction 52 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

54 1: Introduction 53 Protocol layering and data Each layer takes data from above r adds header information to create new data unit r passes new data unit to layer below application transport network link physical application transport network link physical source destination M M M M H t H t H n H t H n H l M M M M H t H t H n H t H n H l message segment datagram frame

55 1: Introduction 54 Internet structure: network of networks r roughly hierarchical r national/international backbone providers (NBPs)- tier 1 providers m e.g. BBN/GTE, Sprint, AT&T, IBM, UUNet/Verizon, TeliaSonera m interconnect (peer) with each other privately, or at public Network Access Point (NAPs: routers or NWs of routers) r regional ISPs, tier 2 providers m connect into NBPs; e.g. Tele2 r local ISP, company m connect into regional ISPs NBP A NBP B NAP regional ISP local ISP local ISP

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

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

58 Introduction Internet structure: network of networks r a packet passes through many networks Tier 1 ISP Tier-2 ISP local ISP local ISP local ISP local ISP local ISP Tier 3 ISP local ISP local ISP local ISP

59 Security prelude 1: Introduction 58

60 Introduction Network Security r The field of network security is about: m how adversaries can attack computer networks m how we can defend networks against attacks m how to design architectures that are immune to attacks r Internet not originally designed with (much) security in mind m original vision: “a group of mutually trusting users attached to a transparent network” m Internet protocol designers playing “catch-up” m Security considerations in all layers!

61 Introduction Bad guys can put malware into hosts via Internet r Malware can get in host from a virus, worm, or trojan horse. r Spyware malware can record keystrokes, web sites visited, upload info to collection site. r Infected host can be enrolled in a botnet, used for spam and DDoS attacks. r Malware is often self-replicating: from an infected host, seeks entry into other hosts

62 Introduction Bad guys can put malware into hosts via Internet r Trojan horse m Hidden part of some otherwise useful software m Today often on a Web page (Active-X, plugin) r Virus m infection by receiving object (e.g., attachment), actively executing m self-replicating: propagate itself to other hosts, users  Worm:  infection by passively receiving object that gets itself executed  self- replicating: propagates to other hosts, users Sapphire Worm: aggregate scans/sec in first 5 minutes of outbreak (CAIDA, UWisc data)

63 Introduction Bad guys can attack servers and network infrastructure r Denial of service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic 1. select target 2. break into hosts around the network (see botnet) 3. send packets toward target from compromised hosts target

64 Introduction The bad guys can sniff packets Packet sniffing: m broadcast media (shared Ethernet, wireless) m promiscuous network interface reads/records all packets (e.g., including passwords!) passing by A B C src:B dest:A payload  Wireshark software used for end-of-chapter labs is a (free) packet-sniffer

65 Introduction The bad guys can use false source addresses r IP spoofing: send packet with false source address A B C src:B dest:A payload

66 Introduction The bad guys can record and playback r record-and-playback : sniff sensitive info (e.g., password), and use later m password holder is that user from system point of view A B C src:B dest:A user: B; password: foo

67 1: Introduction 66 Chapter 1: Summary Covered a “ton” of material! r what’s the Internet r what’s a protocol? r network edge (types of service) r network core (ways of transfer, routing, performance, delays, loss) r access net, physical media r protocol layers, service models r backbones, NAPs, ISPs r (history) r Security concerns r quick look into ATM networks (historical and service/resource- related perspective) You now hopefully have: r context, overview, “feel” of networking r more depth, detail later in course


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