Networks Overview. Dr. Yingwu Zhu

Transcription

1 Networks Overview Dr. Yingwu Zhu 1

2 Networking is everywhere! Internet, ad-hoc wireless networks, sensor networks Networking devices: Computers, PDAs, i-pods, sensor nodes, others Networking services Web, s, P2P file sharing, VoIP, VOD, multimedia streaming Social networks Changing our lives in many ways! 2

3 Introduction Goal: get context, overview, feel of networking more depth, detail later in other courses approach: descriptive use Internet as example Overview: what s the Internet what s a protocol? network edge network core access net, physical media performance: loss, delay protocol layers, service models history 3

4 What s the Internet: nuts and bolts view millions of connected computing devices: hosts, end-systems pc s workstations, servers PDA s phones, toasters running network apps communication links fiber, copper, radio, satellite routers: forward packets (chunks) of data thru network router local ISP company network server workstation mobile regional ISP 4

5 What s the Internet: nuts and bolts view protocols: control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP Internet: network of networks loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force router local ISP company network server workstation mobile regional ISP 5

6 What s the Internet: a service view communication infrastructure enables distributed applications: WWW, , games, e- commerce, database., voting, more? communication services provided: connectionless connection-oriented 6

7 Perspective Network users: Does the network support the users applications Reliability Error free service Speed of data transfer Network designers: Cost efficient network design Good utilization of network resources Cost of building the network Types of services to be supported 7

8 Perspective Network providers: Network administration and customer service Maximize Revenue Minimize Operations Expenses Survivability and Resiliency (Why) 8

9 A closer look at network structure: network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links 9

10 Connectivity We may want a set of hosts (or devices) to be directly connected! WHY? We may not always strive for global connectivity! WHY? Building Blocks to connect at the physical level: links: coax cable, optical fiber... nodes: general-purpose workstations (though sometimes very specialized) 10

11 Basic Connectivity Two types of direct connectivity: point-to-point multiple access 11

12 Discussions If all computers had to be directly connected, networks would be either very limited or expensive and unmanageable! Question: If n computers were to be completely and directly connected by point-to-point links of cost C, what would be the total cost of the net? Question: Would it be less expensive to use a multiple-access network? What are the drawbacks and limitations? 12

13 Building Blocks Switches, Routers, Gateways Special network components responsible for moving packets across the network from source to destination. Network hosts, workstations, etc. they generally represent the source and sink (destination) of data traffic (packets) We can recursively build large networks by interconnecting networks via gateways and routers. 13

14 An Interconnection Network 14

15 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 15

16 What s a protocol? a human protocol and a computer network protocol: Hi Hi Got the time? 2:00 time TCP connection req. TCP connection reply. Get <file> Q: Other human protocol? 16

17 Protocols Building blocks of a network architecture Each protocol object has two different interfaces service interface: defines operations on this protocol peer-to-peer interface: defines messages exchanged with peer Term protocol is overloaded specification of peer-to-peer interface module that implements this interface 17

18 The network edge: end systems (hosts): run application programs e.g., WWW, at edge of network client/server model client host requests, receives service from server e.g., WWW client (browser)/ server; client/server peer-peer model: host interaction symmetric e.g.: teleconferencing, Gnutella, Kazza 18

19 Network edge: connection-oriented service Goal: data transfer between end sys. handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human protocol set up state in two communicating hosts TCP - Transmission Control Protocol Internet s connectionoriented service TCP service [RFC 793] reliable, in-order bytestream data transfer loss: acknowledgements and retransmissions flow control: sender won t overwhelm receiver congestion control: senders slow down sending rate when network congested 19

20 Network edge: connectionless service Goal: data transfer between end systems same as before! UDP - User Datagram Protocol [RFC 768]: Internet s connectionless service unreliable data transfer no flow control no congestion control App s using TCP: HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP ( ) App s using UDP: streaming media, teleconferencing, Internet telephony 20

21 The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete chunks 21

22 Different Types of Switching Different Types of Switching: Circuit Switching (telephone network) dedicated circuit, sending and receiving bit streams Message Switching Packet Switching store and forward, sending and receiving packets Virtual Circuit Switching Cell Switching (ATM) What are Packets? Data to be transmitted is divided into discrete blocks 22

23 Network Core: Circuit Switching End-end resources reserved for call link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required 23

24 Cost-Effective Resource Sharing Must share (multiplex) network resources among multiple users. Common Multiplexing Strategies Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM): Frequency band bandwidth Multiplexing multiple logical flows over a single physical link. 24

25 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 frequency division time division 25

26 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, Bandwidth division into pieces Dedicated allocation Resource reservation 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 transmit over link wait turn at next link 26

27 Network Core: Packet Switching A 10 Mbs Ethernet On-demand sharing statistical multiplexing C B queue of packets waiting for output link 1.5 Mbs 45 Mbs D E 27

28 Network Core: Packet Switching Packet-switching: store and forward behavior 28

29 Packet switching versus circuit switching Packet switching allows more users to use network! 1 Mbit link each user: 100Kbps when active active 10% of time circuit-switching: 10 users packet switching: with 35 users, probability > 10 active less than.004 N users 1 Mbps link 29

30 Packet switching versus circuit switching Is packet switching a slam dunk winner? Great for bursty data resource sharing no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem! 30

31 Packet-switched networks: routing Goal: move packets among routers from source to destination we ll study several path selection algorithms datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions 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 ATM 31

32 Access networks and physical media 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

33 Residential access: point to point access Dialup via modem up to 56Kbps direct access to router (conceptually) ISDN: intergrated services digital network: 128Kbps alldigital connect to router ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home 33

34 Residential access: cable modems HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1 Mbps downstream network of cable and fiber attaches homes to ISP router shared access to router among home issues: congestion, dimensioning deployment: available via cable companies, e.g., MediaOne, Comcast 34

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

36 Wireless access networks shared wireless access network connects end system to router wireless LANs: radio spectrum replaces wire e.g., Lucent Wavelan 10 Mbps wider-area wireless access CDPD: wireless access to ISP router via cellular network (base stations) router base station mobile hosts 36

37 Physical Media physical link: transmitted data bit propagates across link guided media: signals propagate in solid media: copper, fiber unguided media: signals propagate freely, e.g., radio Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps ethernet Category 5 TP: 100Mbps ethernet 37

38 Physical Media: coax, fiber Coaxial cable: wire (signal carrier) within a wire (shield) baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet Fiber optic cable: glass fiber carrying light pulses high-speed operation: 100Mbps Ethernet high-speed point-to-point transmission (e.g., 5 Gps) low error rate 38

39 Physical media: radio signal carried in electromagnetic spectrum no physical wire bidirectional propagation environment effects: reflection obstruction by objects interference Radio link types: microwave e.g. up to 45 Mbps channels LAN (e.g., wavelan) 2Mbps, 11Mbps wide-area (e.g., cellular) e.g. CDPD, 10 s Kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 Msec end-end delay geosynchronous versus LEOS 39

40 Different Types of Links Sometimes you install your own! Category 5 twisted pair Mbps, 100m 50-ohm coax (ThinNet) Mbps, 200m 75-ohm coax (ThickNet) Mbps, 500m Multimode fiber 100Mbps, 2km Single-mode fiber Mbps, 40km 40

41 Bigger Pipes! Sometimes leased from the phone company Service to ask for ISDN T1 T3 STS-1 STS-3 STS-12 STS-24 STS-48 Bandwidth you get 64 Kbps Mbps Mbps Mbps Mbps Mbps Gbps Gbps STS: Synchronous Transport Signal 41

42 Delay in packet-switched networks packets experience delay on end-to-end path four sources of delay at each hop A transmission nodal processing: check bit errors determine output link queueing time waiting at output link for transmission depends on congestion level of router propagation B nodal processing queueing 42

43 Delay in packet-switched networks Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s A transmission Note: s and R are very different quantitites! propagation B nodal processing queueing 43

44 Performance Bandwidth (throughput) Amount of data that can be transmitted per time unit Example: 10Mbps link versus end-to-end Notation KB = 2 10 bytes Mbps = 10 6 bits per second 44

45 Performance Bandwidth related to bit width a) 1 second b) 1 second 45

46 Latency (delay) Time it takes to send message from point A to point B Example: 24 milliseconds (ms) Sometimes interested in in round-trip time (RTT) Components of latency Latency = Propagation + Transmit + Queue + Proc. Propagation = Distance / SpeedOfLight Transmit = Size / Bandwidth 46

47 Transmission and Propagation Delays Propagation delay The propagation delay over a link is the time it takes a bit to travel from on end of the link to the other = d/s Transmission delay It is the amount of time it takes to push the packet onto the link =L/B Total latency over the link = transmission delay + propagation delay 47

48 Bandwidth v.s. Latency Consider a standard 6250 bpi magnetic tape that holds 180 Megabytes. A station wagon can easily transport 200 tapes. Suppose the source and destination are an hour s drive apart. Calculate the effective throughput: megabits in 3600 seconds or 80 Mbps What is the moral of this story? 48

49 Bandwidth v.s. Latency Never underestimate the bandwidth of a station wagon full of tapes pacing down the highway. But What happens to the latency? 49

50 The hard limit Speed of light 3.0 x x x 10 8 Notes meters/second in a vacuum meters/second in a cable meters/second in a fiber no queuing delays in direct link bandwidth not relevant if Size = 1 bit process-to-process latency includes software overhead software overhead can dominate when Distance is small 50

51 Relative importance of bandwidth and latency small message (e.g., 1 byte): 1ms vs. 100ms dominates 1Mbps vs. 100Mbps large message (e.g., 25 MB): 1Mbps vs. 100Mbps dominates 1ms vs. 100ms Consider two channels of 1Mbps and 100 Mbps respectively. For a 1 byte message, the available bandwidth is relatively insignificant given a RTT of 1 ms. The transmit delay for each channel is 8 s and 0.08 s, respectively. 51

52 Delay x Bandwidth Product Delay Bandwidth e.g., 100ms RTT and 45Mbps Bandwidth = 560KB of data We have to view the network as a buffer. This may have interesting consequences: How much data did the sender transmit before a response can be received? 52

53 Application Needs bandwidth requirements: burst versus peak rate jitter: variance in latency (inter-packet gap) Average Bandwidth Requirement is Not enough: consider a source with an avg. BW-requirement of 2Mbps. If the application generates 1 Mbit in one second interval and 3Mbit in a second, a channel that can support 2 Mbps max. will have a tough time. Other Quality of Service (QOS) Parameters: max. and min. delay max. and min. bandwidth demand rates for dynamic increase of demands Cell-Loss Rate 53

54 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! 54

55 What Goes Wrong in the Network? Different types of Error: Bit-level errors (electrical interference) 1 in in copper - 1 in in fiber Packet-level errors (congestion) Link and node failures How should we deal with these types of error? What are the consequences of errors? 55

56 Other types of problems in the Network? Messages are delayed Messages are deliver out-of-order Third parties eavesdrop The key problem is to fill in the gap between what applications expect and what the underlying technology provides. 56