1-11: IntroductionLast Modified: 1/27/2003 1:25:26 PM-2Why study computer networks?❒ They are engineering marvels!❍ Scalability, layered protocols, lots of subtleties- worthy of study❒They are all around us!❍ Understanding the nuts and bolts behind a technology you use every day is exciting❒They are changing the world!❍ Profound societal changes taking place in our lifetime -3How much do you know already?❒ How much do you know about what happens when you use networking applications?❒ What do you hope to learn in this class?-4Goals of this class❒ Understand both basic computer networking concepts and their instantiation in the current Internet❒ Question why the current Internet is the way it is ❍ Appreciate good ❍ Understand limitations and consider solutions❒Gain practical skills (network analysis, network programming)-5How do we begin to make sense of the Internet?❒ Often like the blind men and the elephant?It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind-6Network Trace Analysis?❒ Network Trace Analysis - what happens on our local network when use a network application? The First approached the Elephant, And happening to fall Against his broad and sturdy side, At once began to bawl: “God bless me! but the Elephant Is very like a wall!”2-7Ethereal-8Internet: Example❒ Click -> get page❒ page from localor remote computer❒ link:http://www.cnn.comspecifies- protocol (http)- location (www.cnn.com)❒ What network traffic results?❍ Request page❍ Send page❍ What else? Lots….-9Internet:Locating Resource❒ www.cnn.com is the name of a computer (and, implicitly, of a file in that computer)❒ To find the address, the application uses a hierarchical directory service called theDomain Name System to translate human readable names to IP addresseslocalcomhostcnn.com?cnn.com?IP = a.b.c.dIP = a.b.c.d-10Internet: Port Numbers❒ When a packet arrives at its destination, the operating system uses the destination port number to identify which application should receive it.❒ This is called demultiplexing. -11Internet: Connection❒ The protocol (http) sets up a connection (another protocol, tcp) between the host and cnn.com to transfer the page❒ The connection transfers the page as a byte stream, without errors: error controlHostcnn.comconnectOKget pagepage; close-12Internet: End-to-end❒ The end hosts worry about errors and pacing:❍ Destination sends ACKs❍ Source checks losses ❒ Stream of packets regulated and controlled by both ends: ❍ retransmission of erroneous or missing bytes❍ Pacing, sender not overwhelming the receiver (flow control)❍ Pacing, Sender not overwhelming the network (congestion control)Host AHost BSeq=42, ACK=79, data = ‘C’Seq=79, ACK=43, data = ‘C’Seq=43, ACK=80Usertypes‘C’host ACKsreceipt of echoed‘C’host ACKsreceipt of‘C’, echoesback ‘C’3-13Internet: Bits❒ Data sent over a physical medium ❒ That equipment is not aware of the meaning of the bits01011...011...110Transmitter Physical Medium Receiver01011...011...110OpticalCopperWireless-14Routing?❒ Will network trace analysis let us understand the Internet?❒ It gave us very little hint that between our machine and the remote machine there are many hopsThe Second, feeling of the tusk, Cried, “Ho! what have we here So very round and smooth and sharp? To me ’tis mighty clear This wonder of an Elephant Is very like a spear!” -15Routing❒ Packets flows from end to end across many links❒ Routers receive packet and try to forward one step closer to destination (routing) ❒ The packets contain all information necessary to indicate destination (addressing) and are handled individually❍ Example: Caravan of cars on a road trip❒ Packets may take different pathsCHOST: BCNN.COM: AA | B | # , CRC | bytesB: toC-16Traceroute/tracert-17Graphical Traceroute (plus DNS information ☺)-18Internet Topology?❒ Traceroute gave us one slice through the Internet ❒ What does that tell us about the routes in the entire Internet?The Third approached the animal, And happening to take The squirming trunk within his hands, Thus boldly up and spake: “I see,” quoth he, “the Elephant Is very like a snake!”4-19Internet Map❒ What does the Internet really look like?❍ That is a actually a hard question to answer❍ Internet Atlas Project• http://www.caida.org/projects/internetatlas/• Techniques, software, and protocols for mapping the Internet, focusing on Internet topology, performance, workload, and routing data-20CAIDA: Layout showing Major ISPs-21CAIDA’s skitter plotHighly connectedFew connectionsLocation (longitude)Skitter data16 monitors probing approximately 400,000 destinations626,773 IP addresses1,007.723 IP links48,302 (52%) of globally routable network prefixesEuropeNorth America AsiaTop 15 ASes are in North America (14 in US, 1 in Canada)Many links US to Asia and Europe; few direct Asia/Europe Links -22Internet growth and change❒ Maps not even a complete “snapshot”❒ Even if they were, what about change over time?❍ Growth trends (years)? ❍ Cycles through months/weeks/days?❍ Failures? Stability?❒Summarizing the dynamic InternetThe Fourth reached out an eager hand, And felt about the knee. “What most this wondrous beast is like Is mighty plain,” quoth he; “ ‘Tis clear enough the Elephant Is very like a tree!” -23The Internet around 1990-24The Internet in 1997 (Post NSFNET)5-25Internet Domain SurveyNumber of Internet Hosts020000000400000006000000080000000100000000120000000140000000160000000180000000Aug-81Aug-83Aug-85Aug-87Aug-89Aug-91Aug-93Aug-95Aug-97Aug-99Aug-01"Source: Internet Software Consortium (http://www.isc.org/)". -26Periodic Cycles❒ Cycles of a day❍ People come to work in one part of the globe, go home in another❒Week❍ Weekends vs weekdays❒Holidays❒ …-27Stability?❒ Despite any growth trends or periodic cycle, we also never have the Internet as a whole❒ Machines disconnecting/connecting❒ Stability of routes?❒ Viruses? Attacks?-28Internet principles?❒ Besides the details of protocols, topology and growth trends, are there fundamental “timeless” principles of the Internet?The Fifth, who chanced to touch the ear, Said: “E’en the blindest man Can tell what this resembles most; Deny the fact who can This marvel of an Elephant Is very like a