18-441/741: Computer Networks Lecture 1: Course Overview
Swarun Kumar
Course Overview
• Administrivia • Objective
• People, course communications • Grading, course policies
• Why are networks important? • A whirlwind tour of the course
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Instructors
• Instructor
• Swarun Kumar
• swarun@cmu.edu
• Office hours: 5-6 pm Mondays (Over zoom)
• Teaching Assistants
• Atul Bansal, Joel Miller and Junbo Zhang
• [atulb, jgmiller, junboz2]@andrew.cmu.edu
• Office hours: Atul: 5-6 pm Fridays, Joel: 7-8 pm Tuesdays, Junbo: 4:30-5:30 pm Thursdays (Over zoom)
• Check canvas for zoom links
Hybrid Mode
• All lectures, office hours will be remote over zoom
• All recitations will be hybrid (schedule on canvas)
• Recitations will be offered both over zoom & in-person at WEH7500
• Room capacity of WEH7500 is 38
• Last names starting between A-L can attend odd numbered
recitations, others even numbered recitations
• You are permitted to attend any/all recitations remotely
• Zoom links available on canvas
Course Goals
• Becomefamiliarwiththeprinciplesandpractice of data networking
• Routing, transport protocols, naming, …
• Learnhowtowriteapplicationsthatusenetwork
• Use web and peer-to-peer style applications
• Gethands-onunderstandingofnetworkinternals
• Implementing different types of protocol, error recovery, conformance with standards, etc.
Course Materials
• Textbook: Computer Networks – A Systems Approach, L. Peterson and B. Davie, Morgan Kaufmann
• References
• Computer Networking – A Top-Down Approach, by J.
Kurose and K. Ross, Addison Wesley
• Computer Networks, Wetherall and Tanenbaum
• Communication Networks, by A. Leon-Garcia and I. Widjaja, , Second edition, McGraw-Hill.
• Data and Computer Communications, W. Stalling, MacMillan Publishing Company, New York.
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Course Format
• ~24lectures,1.5hrseach
• Coverthe“principlesandpractice”
• 3programmingprojects
• Howtouseandbuildnetworks/networkedapplications • Open-ended individual projects. Start early!
• 5onlinequizzes(canvas)
• Nottimed+openbook&Internet(nocollaboration)
• Midtermandfinal
• Two110-minquizzesoncanvas
• Willbetimedandopenbook&Internet(nocollaboration)
Getting Questions Answered
• Administrative: start with canvas
• If the answer is not there, please send us e-
mail
• Course material: class, office hours, piazza
• Typically requires a discussion – e-mail often does not work well
• Projects: piazza, office hours
• Piazza: others might have the same question • Office hours for more complicated issues
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Projects and Recitation Sections
• Key objective: system programming
• Differentfromwhatyou’vedonebefore!
• Project 1 – MATLAB (recommended)
• Project 2/3 – Can use C/C++ or Java
• May run indefinitely – must handle all errors!
• Interfaces specified by documented protocols
• Concurrency involved (inter and intra-machine) • Must have good test methods
• Recitationstoprovideprojectbackground, discuss programming tools and skills
• Firstrecitation:Feb12
Administrative Stuff
• Watchthecoursecanvaswebsite
• All handouts, readings, project information, ..
• If something is missing, please let us know ASAP
• Postquestionsaboutlectures/projecton piazza.
• Emailinstructor/TAwithquestionsabout grades, etc.
Grading
• Grading:
• 20% for quizzes
• 30% for projects (bonus problems – 18-741)
• 50% for two exams
• Cutoff:
• >90%or>mean+std -A
• 70-90% or>mean–std-B
• 50-70%or>mean–2*std-C
• 40-50% – D
• Else fail
• You MUST demonstrate competence in both projects and tests to pass the course
• Fail either and you fail the class!
Policy on Collaboration
• Workingtogetherisimportant
• Discuss course material in general terms • Final submission must be your own work
• Whatwedon’twanttohavetosay:werunall projects through cheat-checkers
• Allcasesofcheatingwillbereported
Policy on Late Work, Re-grading
• Submitassignmentsontime
• Only exception is documented illness and
family emergencies
• Re-gradingrequestsmustbesubmittedin writing with secretary within 1 week
• Entire exam or quiz will be re-graded
• ExamandQuizcoverage:
• All materials right before the exam/quiz • Details will be on canvas
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The Slides
• Theslidesarearesourcethatissharedbymany instructors
• Also some sharing with 15-441
• TheyincludecontributionsfromPeterSteenkiste, Hyong Kim, Srini Seshan, Dave Andersen, Hui Zhang, and others
Why take this course?
• If you need to build foundations on computer networks (for industry / gradschool / capstone project)
• If your interviewer asks: “How does TCP work?” or explain “What an IP address is?”
• Cool (individual) project: Build your own Netflix (-ish) from scratch.. Doable remotely + Get something you can show off to potential employers (academia/industry) laterJ
• 12 units means 12 units
• It’s a popular course (long waitlist even with ~ 2x capacity)
Course Overview
• Administrivia
• Why are networks important? • What is a network?
• What is the Internet • Internet design
• A whirlwind tour of the course 16
What is a Network?
• An infrastructure that allows (distributed) “users” to communicate with each other
• People, devices, …
• By means of voice, video, text, …
• It is assumed that the infrastructure is shared by many users
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Basic Building Block: Links
Node Link Node
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Scaling the Network
(N2) Wires for everybody!
Or how about …
Or one wire
But First a bit of History
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Network Architecture
Network architecture: the plan that specifies how the network is built and operated
Network Architecture Trends
1.0E+14 1.0E+12 1.0E+10 1.0E+08 1.0E+06 1.0E+04 1.0E+02
1.0E+00
1850 1875
?
Telegraph Telephone networks networks
Internet, Optical & Wireless
networks
Next Generation Internet
1900 1925
1950 1975
2000
Information transfer per second
Telegraphs & Long-Distance Message Communications
• Drumbeats…Courier…Telegraphs
Message Switching Architecture
• Networknodeswerecreated where several telegraph lines met (Paris and other sites)
West line
South line
Network Node
North line
East line
• Store-and-ForwardOperation:
• Messagesweredecoded
• Next-hopinroutedeterminedby destination address of a message
• Eachmessagewascarriedby hand to next line
Bell’s Telephone
• AlexanderGrahamBell(1875)workingonharmonic telegraph to multiplex telegraph signals
• Discoveredvoicesignalscanbetransmitteddirectly • Microphone converts voice pressure variation (sound)
into analogous electrical signal
• Loudspeaker converts electrical signal back into sound
• BellTelephoneCompanyfoundedin1877 Signal for “ae” as in cat
sound
sound
Microphone
Electrical signal
Loudspeaker
Circuit Switching (analog telephones)
• Source first establishes a connection (circuit) to destination
• Each switch along the way stores info about connection (and possibly allocates resources)
• Source sends the data over the circuit
• No need to include the destination address with the data since the
switches know the path
• The connection is explicitly torn down
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Three Phases of a Connection
Connection set up
1.
2.
3.
4.
Telephone network
Telephone network
Telephone network
Telephone network
Telephone network
Telephone network
Pick up phone Dial tone.
Dial number
Network selects route; Sets up connection; Called party alerted
Exchange voice signals
Hang up.
Information
transfer 5.
Connection release
6.
Links and Switches in Early Telephone Networks
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Circuit Switching Discussion
• Circuits have some very attractive properties.
• Fastandsimpledatatransfer,oncethecircuitisestablished • Predictableperformance;E.g.guaranteedbandwidth
• But it also has some shortcomings.
• Howaboutburstytraffic?
• Do you need a permanent circuit to Facebook? • Circuit will be idle for significant periods of time
• Howaboutuserswithdifferentbandwidthneeds?
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Contrast this with the Internet, i.e. (Packet) Switching (our emphasis)
• Source sends information as self-contained messages that have an address.
• Source may have to break up single message in multiple packets
• Each packet travels independently to the destination host.
• Switches use the address in the packet to determine how to forward the packets
• Store and forward
• Analogy: a letter in surface mail.
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Sample Quiz Question!
• Question: “Now that VOIP (e.g. Skype audio, Whatsapp calls) is here, circuit-switched landline phones are obsolete and can be phased out”. Do you agree or disagree with this statement? Justify.
React on Zoom:
• Solution: Not necessarily. Circuit switched networks do have reliable bandwidth – something that Skype, Whatsapp do not. It is debatable whether these networks will reach the kind of reliability that (say) 911 requires. As of today, they do not, although there are exceptions.
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Today’s Lecture
• Administrivia
• Why are networks important? • What is a network?
• What is the Internet • Internet design
• A whirlwind tour of the course
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What about the Internet
• inter-net: network of networks.
• Networksareconnectedusing routers and other devices, e.g., for security, accounting, …
• Usediversetechnologies
• Managedbydifferentcos.
• The Internet: the interconnected set of networks of the Internet Service Providers (ISPs)
• About~23,000“transit”ISPs make up the Internet
• Manymore“edge”networks
Internet
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What is the Objective of the Internet?
• Enable communication between diverse applications on diverse devices (“computers”)
• Web, peer-to-peer, video streaming, audio conferencing, …
• Over very diverse infrastructures: WiFi, cellular, data center
networks, corporate networks, …
• In contrast: previous networks were special purpose and fairly homogeneous in terms of technology
• Must understand application needs/demands
• Traffic data rate and loss sensitivity
• Traffic pattern (bursty or constant bit rate)
• Traffic target (multipoint or single destination, mobile or fixed)
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Packet Switching – Statistical Multiplexing
• Switchesarbitratebetweeninputs
• Cansendfromanyinputthat’sready
• Links are never idle when there is traffic to send • (Efficiency!)
Packets
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Multiplexing
• Need to share network resources
• How? Switched network
• Party “A” gets resources sometimes • Party “B” gets them sometimes
• Interior nodes act as “Switches”
• Many challenges: fairness, efficiency, …
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Internet Design
• In order to inter-operate, all participating networks must follow a common set of rules
• Example: requirements for packets:
• Address format, header information, packet size limit, ..
• Also: what is the “service model”, i.e., the
commitment made to applications
• Internet: best-effort – packets can get lost, etc.
• But some applications need reliable data delivery, a minimal bandwidth guarantee, low latency, …
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Success Factors for New Services
• Technology not only factor in success of a new service • Three factors considered in new telecom services
New
Technology
Can there be demand for the service?
Market
Can it be implemented cost-effectively?
Service
Regulation
Is the service allowed?
Standards
• New technologies very costly and risky • Standards allow players to share risk
and benefits of a new market • Reduced cost of entry
• Interoperability and network effect
• Compete on innovation
• Completing the value chain
• Chips, systems, equipment vendors, service
providers • Example:
• 802.11 LAN, IP, HTTP/SMTP/…
Today’s Lecture
• Administrivia
• Why are networks important? • What is a network?
• What is the Internet • Internet design
• A whirlwind tour of the course
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Whirlwind Tour of the Course
• Infrastructure: hardware (or close to it)
• Core networking protocols: IP, dealing with
errors and congestion, routing, …
• Tools: caching, CDNs, SDNs, middleboxes, …
• Making it work well: security, management, …
• IP everywhere: the Internet, last mile, wireless, mobility, data center, video, IP-TV, skype, …
• Focus is on today’s Internet but also trends
• What will the Internet look like in 10, 20, 30 years?
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Infrastructure
• Ethernet is very old, so why is it so fast?
• Can’t they find something better?
• Wireless: 2G, 3G, 4G and (now) 5G.. How’s the speedup achieved?
• What are the limits of some of the technologies?
• Both physical and protocol limits
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Core Networking Protocols
Think: traffic on the roads
•How do I found a path to my destination •How do I specify addresses
•What if my car breaks down?
•How do I deal with traffic jams
•…
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Optimizing Performance
• Intuitively: lots of bandwidth!
• But there is more to it:
• Latency is often more critical!
• How voice and video – can I offer guarantees?
• Can I beat the speed of light? • Hint: this can make you rich
• Why did we use peer to peer networks? • And why did they (mostly) go away?
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Making the Network Work Well
• Goodtechnologyisonlyasmallpartofthepuzzle – deployment and management issues are equally (or more) critical
• Involves many people, high cost
• HowdoIsecuremynetwork?
• Lots of bad guys: DOS, compromised hosts, privacy leaks, botnets, …
• HowImanageresources,reduceoperatorerrors, deal with failures, …
• And how does it differ in LAN, WAN, wireless, …
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IP Everywhere
• Using IP technology has become attractive
• Cheap commodity hardware, lots of tools, people trained in the
technology, end-to-end support, … • The (public) Internet: our focus
• How do you optimize “the web”: CDNs, caching, … • Data centers: very special requirements
• Map-reduce, 3-tier business apps, load balancing, … • IP TV, voice/video conferencing:
• Very high QoE expectations
• Wireless and mobile apps
• For many users, primary way of accessing Internet
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Course Schedule (Bird’s eye view)
• Feb-Mar:“Thehardware”,“Theprotocols” • Physical Layer
• Data Link
• Network
• Transport
• April:“Makingitwork”,“Theusecases” • Software Defined Networking
• Security
• Future Internet
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Next Lecture
Protocol Stack: an overview
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