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Data Networks

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  1. Data Networks Introduction

  2. Data Networks • Organization: • Core lecture, 9 ECTS points • Lectures: Tue 10-12 (Günter-Hotzlecture hall), Thu 12-14 (HS002 except for 25th) • Tutorial: Fri 12-14 (HS002, except for May 31st , June 21st) • Written exam (25.07), re-exam (beginning of October) • Weekly quizzes on Wednesdays: 50% of points to participate in exam • Weekly exercises (optional): solutions are discussed in tutorial (sheets are uploaded one week earlier)

  3. Data Networks • Organization: • Questions about the organization: {turrini, spieler, mikeev}@cs.uni-… • Course webpage: mosi.cs.uni-saarland.de -> teaching • you may post live comments during the lecture: • everything you want to discuss immediately without interrupting the presentation • you may also post interesting links related to the material presented • feedback concerning the presentation • forum for “dead (not live)” comments/questions

  4. Data Networks • Literature: • Computer Networking by J.F Kurose and K.W. Ross, 6th edition (Chapter 1-6) • (=> Bock & Seip store on campus!) • Data Networks by Dimitri P. Bertsekas and Robert G. Gallager, 2nd edition (Chapter 3)

  5. Syllabus • 1: Introduction: Broad overview of computer networking and the Internet • 2: Application Layer: HTTP, FTP, SMTP, DNS, P2P, … • 3: Transport Layer: Multiplexing, TCP, Congestion control, … • 4: Network Layer: IP and routing, … • 5: Link Layer: Links, Access Networks, LANs, … • 6: Wireless Networks • 7: Delay Models: Little’s Theorem, Queueing Systems, … • Further reading: • Computer Networking by J.F Kurose and K.W. Ross, 6th edition (Chapter 7-9) Multimedia Networking, Security in Computer Networks, Network Management • Computer Networks by A. Tanenbaum, 4th ed • Computer Networks by L. Peterson, B. Davie, 5th Edition

  6. Syllabus What this lecture is (not) about: Fundamental principles of computer networks in general! Internet is just an instance/ an example where we can see how things are implemented in practice. Not: details of most recent developments of the Internet (currently “in”, may be “out” in two years ...)

  7. Chapter 1: introduction our goal: get broad overview and terminology more depth, detail later in course approach: use Internet as example overview: what’s the Internet? what’s a protocol? network edge; hosts (servers, Laptops, smartphones, ...) network core: packet/circuit switching, Internet structure performance: loss, delay, throughput security (no details later) protocol layers history Introduction

  8. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge 1.3 network core 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Introduction

  9. What’s the Internet: “nuts and bolts” view hundreds of millions of connected computing devices: hosts = end systems running network apps PC server wireless laptop smartphone wireless links wired links router mobile network global ISP • communication links • coaxial cable, copper, fiber, radio, … • transmission rate: bandwidth home network (WiFi) regional ISP • Packet switches: receive and forward packets (chunks of data) • routers and switches institutional network Introduction

  10. What’s the Internet: “nuts and bolts” view Internet Service Provider (ISP): End systems access the Internet through ISPs Interconnected ISPs (=> “network of networks”) classification according to size (Tier 3 (local), Tier 2 (national), Tier 1(global/international) protocolscontrol sending, receiving of msgs e.g., TCP, IP, HTTP, SMTP, … Internet standards different components of the internet have to agree on standards to interoperate IETF: Internet Engineering Task Force develops and promotes such standards IETF publishes RFCs (Requests for comments) describing/defining protocols mobile network global ISP home network regional ISP institutional network Introduction

  11. What’s the Internet: a service view Infrastructure that provides services to applications: Web, VoIP, email, distributed games, P2P, social nets,... applications are distributed (involve multiple hosts exchanging data) End systems provides applic. programming interface to apps rules for exchanging data between programms provides service options, analogous to postal service The Internet from a different perspective: mobile network global ISP home network regional ISP institutional network Introduction

  12. a human analogy: ask Got the time? “Yes?" 2:00 time answer What’s a protocol? teacher “broadcast” Hi "Questions?" Hi raise hand Introduction

  13. a human protocol and a computer network protocol: Get http://mosi.cs.uni-saarland.de Got the time? 2:00 time What’s a protocol? Hi TCP connection request Hi TCP connection response <file> Introduction

  14. What’s a protocol? network protocols: communication between machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of messages sent and received among network entities, and actions taken on message transmission or receipt Introduction

  15. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge 1.3 network core 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history just a quick overview; no technical details! Introduction

  16. A closer look at network structure: network edge: hosts: clients and servers home-based hosts: desktop computers, laptops, smartphones, tablets servers often reside in large data centers (Web, email) mobile network global ISP home network regional ISP institutional network Introduction

  17. Non-traditional end systems Web-enabled toaster + weather forecaster IP picture frame http://www.ceiva.com/ Tweet-a-watt: monitor energy use Slingbox: watch, control cable TV remotely Internet refrigerator Internet phones Introduction

  18. A closer look at network structure: network edge: hosts: clients and servers mobile network global ISP • access networks:physical connection of end system to first router (“edge router”); wired or wireless communication links home network regional ISP • network core: • interconnected routers • network of networks institutional network Introduction

  19. Access networks and physical media How to connect end systems to edge router? different types residential access nets institutional access networks (school, company) mobile access networks edgerouter Introduction

  20. Access net: digital subscriber line (DSL) • DSL Internet accessobtainedfromlocaltelephonecompany => ISP • Market share in Germany 86% (in 2012) • use existing telephone line (copper wire, analog transmission) to DSL access multiplexer (DSLAM) of phone company • DSLAM usually located in the central office (CO)„Vermittlungsstelle“ (or it is an outdoor DSLAM) • data over DSL line goes to Internet • voice over DSL line goes to telephone net

  21. Access net: digital subscriber line (DSL) ISP voice, data transmitted at different frequencies over dedicated line to central office DSL access multiplexer central office telephone network DSL modem splitter DSLAM • use different frequencies for telephone and Internet data => splitter separates rec. signal & forwards data to DSL modem • DSLAM separates the incoming data/phone signals and translates it into digital format • rates are either limited by ISP or because of the distance to CO Introduction

  22. cable modem termination system ISP Access net: cable network cable headend … cable modem splitter CMTS • make use of the cable television infrastructure • residence: needs cable modem to exchange data with cable modem termination system (CMTS) • splitter often integrated in cable modem • both optical fibre and coaxial cable are employed (HFC: hybrid fiber coax) Introduction

  23. cable modem termination system data, TV transmitted at different frequencies over shared cable distribution network ISP Access net: cable network cable headend … cable modem splitter CMTS • network of cable/fiber attaches homes to ISP router • homes share access networkto cable headend • => transm. rate decreases in case of high traffic • => requested data packets are sent to all connected participants of a group • unlike DSL, which has dedicated access to central office Introduction

  24. often combined in single box cable or DSL modem router wireless access point (54 Mbps) wired Ethernet (100 Mbps) Access net: home network wireless devices to/from headend or central office Introduction

  25. Enterprise access networks (Ethernet) institutional link to ISP (Internet) institutional router Ethernet switch institutional mail, web servers • typically used in companies, universities, etc • transmission rates: • 10 Mbps to 100Mbps (users), • 1Gbps, 10Gbps (servers) • today, end systems typically connect into Ethernet switch Introduction

  26. Wireless access networks wireless access network connects end system to router via “access point”; uses radio signals to transmit data access point shared by several end systems wide-area wireless access • use infrastructure of cellular telephony • connect to base stations • range: few 10’s of km distinguish between wireless LANs: • IEEE 802.11 technology (WiFi) • up to 54 Mbps transmission rate • range: typically a few 10 meters to Internet to Internet maximal bandwidths Introduction

  27. Physical media bit:sent by propagating electromagnetic waves (analog signal) or optical pulses (digital signal) betweentransmitter/receiver pairs physical link: what lies between transmitter & receiver distinguish: guided media: signals propagate in solid media: copper, fiber-optic, coaxial unguided media:signals propagate freely, e.g., radio waves twisted pair (TP) copper wire two insulated copper wires arranged in regular spiral pattern least expensive, most commonly used 10 Mbps to 10 Gpbs Ethernet, DSL Introduction

  28. Physical media: coax, fiber coaxial cable: two concentric copper conductors used for HFC (hybrid fibre coaxial) access via (cable internet) fiber optic cable: • glass fiber carrying light pulses (digital signal) • high-speed point-to-point transmission (10’s-100’s Gpbs) • low error rate (immune to electromagnetic noise) • transmitters, receiver, switches are costly • long distance link (oversea) • prevalent in Internet backbone Introduction

  29. Physical media: radio radio link types: • terrestrial radio channels • short distance (e.g. Bluetooth) • LAN (e.g., WiFi) • cellular access (wide–area wireless access) • satellite links • used if DSL or cable is not available • transmission rate:10’s of Mbps • > 270 msec signal propagation delay (DSL has ony 20 ms) • typically using a geostationary satellite • location needs to be in the footprint of the satellite • influenced by meteorological factors Introduction

  30. 1.1 what is the Internet? 1.2 network edge end systems, access networks, links 1.3 network core packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Chapter 1: roadmap Introduction

  31. mesh of interconnected routers What do routers do? packet-switching: hosts run network apps and break application-layer messages into packets forward packetsfrom one router to the next, across links on path from source to destination packets may be buffered and queue while passing network nodes The network core Introduction

  32. Host: sends packets of data two packets, L bits each 1 2 R: link transmission rate host How many seconds does it take to transmit one L-bit paket? L (bits) R (bits/sec) time needed to transmit L-bit packet into link packet transmission delay = = host sending function: • takes application message • breaks into smaller chunks, known as packets, of length L bits (packetization) • transmits packet into access network at transmission rate R (bits/sec) • link transmission rate = link bandwidth

  33. Host: sends packets of data two packets, L bits each 1 2 R: link transmission rate host host sending function: • takes application message • breaks into smaller chunks, known as packets, of length L bits • transmits packet into access network at transmission rate R (bits/sec) • transmission delay = time for pushing the packet‘s bits into the link • propagation delay = traveling across the wire

  34. Packet-switching: store-and-forward mechanism takes L/R seconds to transmit (push out) L-bit packet into link at R bps store and forward:entire packet must arrive at router before it can be transmitted on next link one-hop numerical example: L = 7.5 Mbits R = 1.5 Mbps one-hop transmission delay = 5 sec L bits per packet 1 3 2 source destination R bps R bps • end-end delay (two links with bandwidth R)= 2L/R (assuming zero propagation delay) more on delay shortly … Introduction

  35. Packet-switching: store-and-forward takes L/R seconds to transmit (push out) L-bit packet into link at R bps store and forward:entire packet must arrive at router before it can be transmitted on next link L bits per packet 1 3 2 source destination R bps R bps • Question: • What is the end-end delay if a packet is sent via N links (and N-1) routers? N x L/R • end-end delay = 2L/R (assuming zero propagation delay) Introduction

  36. Packet Switching: queueing delay, loss queuing and loss: • routers have one buffer per outgoing link => store packets that the router is about to send into the link • If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: • packets will queue, wait to be transmitted on link • packets can be dropped (lost) if memory (buffer) fills up C R = 100 Mb/s A D R = 1.5 Mb/s B E queue of packets waiting for output link Introduction

  37. Key functions in paket switching routing:determines source-destination route taken by packets • routing algorithms forwarding:move packets from router’s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 1 0111 2 3 dest address in arriving packet’s header Network Layer

  38. Paket vs. circuit switching two fundamental approaches for moving data through a network: packets switching and circuit switching - in circuit-switched networks resources are reserved - links are separated into circuit segments - fraction of each links transmission capacity is reserved for the duration of the connection restaurant analogy: reserve a table in advance! - in packet-switched networks resources are NOT reserved (=> no guarantees!) restaurant analogy: just go there without reservation! Introduction

  39. Alternative core: circuit switching resources (buffers, links, transmission rate) are reserved for “call” between source & destination: Here, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. dedicated resources: no sharing circuit-like (guaranteed) constant transmission rate circuit segment idle if not used by call (no sharing) no store and forward => transmission time independent of number of links Commonly used in traditional telephone networks Introduction

  40. Circuit switching: FDM versus TDM Example: 4 users Frequency-division multiplexing frequency time Time-division multiplexing frequency time Introduction

  41. Packet switching versus circuit switching example: users share 1000 kb/s link each user: wants to transmit 100 kb/s when “active” active 10% of time circuit-switching: can have at most ? users packet switching: with 35 users, probability > 10 active at same time is less than .0004 packet switching allows more users to use network! Q: how did we get value 0.0004? Q: what happens if > 35 users ? N users ….. 1000 kb/s link 10 => 3 timesmoreusersatthe same performancewith prob 99,96 % Introduction

  42. great for bursty data resource sharing simpler, no call setup excessive congestion possible: packet delay and loss protocols needed for reliable data transfer, congestion control Problem: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video streaming still an unsolved problem ... packet switching Packet switching versus circuit switching Introduction

  43. 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 Let’s now take a stepwise approach to describe the current Internet structure.

  44. Internet structure: network of networks Question: given thousands 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 … …

  45. Internet structure: network of networks Option: connect each access ISP to every other access ISP? … … … … connecting each access ISP to each other directly doesn’t scale: O(N2) connections. … … … … 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 … … …

  46. Internet structure: network of networks Option: connect each access ISP to a single global transit ISP? Access ISPs (customers) have to pay global ISP for traffic => customer and provider ISPs need economic agreement. … … … … globalISP 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 … …

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

  48. Internet structure: network of networks But if one global ISP is viable business, there will be competitors …. which must be interconnected Internet exchange point (multiple ISPs peer together) … … ISP B ISP C ISP A IXP IXP … … 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 peering link … …

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

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