What computers talk about and how. (Networking & the Internet.)Brief historyModern InternetToday: A Peek Underneath the ‘NetCaveat: Internet ≠ W W WTheme 1:The (shaky) foundation of the Internet: TCP/IP ProtocolHopping along“Best effort transmission”DiscussionSome mechanismsTheme 2:Political and Military Setup in Medieval Europe (?)PowerPoint PresentationFirst example of decentralization: Physical networkThe Second Decentralization: Domain Name SystemWhat happens when you type a URL?Theme 3.CongestionHow does a good netizen respond to congestion?What’s in the future?What computers talk about and how.(Networking & the Internet.)COS 116: 4/3/2008Sanjeev AroraBrief historyLocal area networks & university networksMilitary communication networksARPANET [’68] (a.k.a. DARPANET), etc.Early 1980s: US government decides on new way to connect various networks: the “Internet” 1989: World Wide Web; html, browsers1998: Responsibility for Internet naming system handed over to private non-profit corporation ICANN.Rest of InternetRest of InternetModern InternetCollection of computers (including devices, servers, etc.) connected by wires, optical cables, wireless, etc.To join, need:Device capable of “speaking the right protocol” (TCP/IP)IP “address” given by an Internet providerConnection to provider’s servers (via modem, DSL, wireless, etc.)Your PCIP Address: 128.156.16.201Today: A Peek Underneath the ‘NetDominant technological artifact of second half of 20th centuryInteresting example of design of a large, heterogeneous system (decentralized, yet fairly robust).Why?Caveat: Internet ≠ W W WInternet: network connecting computers, devices, etc.WWW: hyperlinked content (webpages) stored on servers; requested and served using http protocolBuilt on top of the internetInternetInternetTheme 1:Building reliability on top of unreliable protocolsThe (shaky) foundation of the Internet: TCP/IP ProtocolAll transmissions broken up into packetsDestination address Book-keeping info Data32 bitsOften about 1500 bytes(but can vary)A Packet:Hopping alongInternet is actually a bunch of connected computers called routersPackets hop from router to router until they reach destinationInternetInternetSee, for example: http://network-tools.com“Best effort transmission”Packet not guaranteed to arrive in a timely fashion (or ever!)If many packets sent, may arrive out of orderInternetInternetSenderReceiverDiscussionIs there some unreliable communications device you use everyday?How do you cope with the cellphone’s unreliability?Some mechanismsRetransmission (“Could you say that again?”)Timeout (“Let me hang up and try redialing?”)Acknowledgements (“Finally understood you. Go on.”)(In TCP/IP: if sequence of packets, number them and sort at receiver end.)Theme 2:Decentralized controlWhat is a suitable postal system for this “army”?Political and Military Setup in Medieval Europe (?)KingDukeDukeKnightKnightPeasantsCountCountHow should a peasant in one town send mail to a peasant in another town?Discussion Time• What happens if a knight leaves the army?First example of decentralization: Physical network12 major providersMany local providersPrinceton SchoolsMcCarterUSLECPrinceton homes &businessesThe Second Decentralization: Domain Name System.com.edu.net.uk.in.princeton.edu.cs.princeton.edu .econ.princeton.eduWhat happens when you type a URL?Address translated by asking appropriate DNS server up/down the DNS hierarchywww.nytimes.com query to .com server 199.239.136.200Physical routing of packets up/down the physical network hierarchy based upon addressOther stuffTheme 3.Dependence upon the kindness of strangersCongestionQueue full packets are droppedRouter 1Router 2QueueHow does a good netizen respond to congestion?Packets getting dropped? Halve the transmission rateAll packets getting through? Increase transmission rate a little.Done in all TCP/IP software But, no enforcement mechanism! (Allows “cheating”, as well as VoIP Telephony, Streaming media, etc.)What’s in the future?128-bit instead of 32-bit addresses. Can send email to your toaster. (Especially if it lives in Asia)Mechanisms for pricing, security, quality of service, etc.NSF’s GENI
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