Why study computer networks? They are engineering marvels! - PDF document

Why study computer networks? ❒ They are engineering marvels! ❍ Scalability, layered protocols, lots of 1: subtleties- worthy of study Introduction ❒ They are all around us! ❍ Understanding the nuts and bolts behind a technology you use every day is exciting Last Modified: ❒ They are changing the world! 1/27/2003 1:25:26 PM ❍ Profound societal changes taking place in our lifetime -1 -2 How much do you know already? Goals of this class ❒ Understand both basic computer networking concepts and their instantiation ❒ How much do you know about what happens in the current Internet when you use networking applications? ❒ Question why the current Internet is the ❒ What do you hope to learn in this class? way it is ❍ Appreciate good ❍ Understand limitations and consider solutions ❒ Gain practical skills (network analysis, network programming) -3 -4 How do we begin to make sense Network Trace Analysis? of the Internet? ❒ Network Trace Analysis - what happens on our local network when use a network ❒ Often like the blind men and the elephant? application? It was six men of Indostan To learning much inclined, The First approached the Elephant, Who went to see the Elephant And happening to fall (Though all of them were blind), Against his broad and sturdy side, That each by observation At once began to bawl: “God bless me! but Might satisfy his mind the Elephant Is very like a wall!” -5 -6

Ethereal Internet: Example Click -> get page ❒ page from local ❒ or remote computer link: ❒ http://www.cnn.com specifies - protocol (http) - location (www.cnn.com) What network traffic results? ❒ ❍ Request page ❍ Send page ❍ What else? Lots…. -7 -8 Internet: Internet: Port Numbers Locating Resource ❒ www.cnn.com is the ❒ When a packet arrives at its destination, name of a computer the operating system uses the destination (and, implicitly, of a port number to identify which application file in that computer) com should receive it. ❒ To find the address, cnn.com? IP = a.b.c.d ❒ This is called demultiplexing. the application uses a cnn.com? hierarchical directory host local service called the IP = a.b.c.d Domain Name System to translate human readable names to IP addresses -9 -10 Internet: Connection Internet: End-to-end Host A Host B ❒ The protocol (http) sets up a connection (another ❒ The end hosts worry about errors and pacing: User protocol, tcp) between the host and cnn.com to Seq=42, ACK=79, data = ‘C’ types ❍ Destination sends ACKs transfer the page ‘C’ host ACKs ❍ Source checks losses ❒ The connection transfers the page as a byte receipt of ❒ Stream of packets regulated Seq=79, ACK=43, data = ‘C’ ‘C’, echoes stream, without errors: error control and controlled by both ends: back ‘C’ ❍ retransmission of erroneous or missing bytes cnn.com host ACKs Host ❍ Pacing, sender not receipt S e q = 4 3 overwhelming the receiver of echoed , A C K = connect 8 0 ‘C’ ( flow control) O K ❍ Pacing, Sender not overwhelming the network get page ( congestion control) o s e p a g e ; c l -11 -12

Internet: Bits Routing? ❒ Will network trace analysis let us understand the ❒ Data sent over a physical medium Internet? ❒ That equipment is not aware of the meaning of ❒ It gave us very little hint that between our the bits machine and the remote machine there are many hops The Second, feeling of the tusk, 01011...011...110 01011...011...110 Cried, “Ho! what have we here So very round and smooth and sharp? Transmitter Physical Medium Receiver To me ’tis mighty clear Optical This wonder of an Elephant Copper Wireless Is very like a spear!” -13 -14 Routing Traceroute/tracert ❒ Packets flows from end to end across many links A | B | # , CRC | bytes CNN.COM: A ❒ Routers receive packet and try to forward one step closer to HOST: B destination ( routing ) ❒ The packets contain all information necessary to C indicate destination B: to (addressing) and are handled C individually ❍ Example: Caravan of cars on a road trip ❒ Packets may take different paths -15 -16 Graphical Traceroute (plus Internet Topology? DNS information ☺ ) ❒ 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!” -17 -18

CAIDA: Layout showing Major Internet Map ISPs ❒ 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 -19 -20 Location (longitude) CAIDA’s skitter plot Internet growth and change Top 15 ASes are in North America (14 in US, 1 in Canada) Highly connected Many links US to Asia and Europe; few direct Asia/Europe Links ❒ Maps not even a complete “snapshot” Asia ❒ Even if they were, what about change over time? Few connections ❍ Growth trends (years)? Europe ❍ Cycles through months/weeks/days? Skitter data ❍ Failures? Stability? 16 monitors probing ❒ Summarizing the dynamic Internet approximately 400,000 The Fourth reached out an eager hand, destinations And felt about the knee. 626,773 IP “What most this wondrous beast is like addresses 1,007.723 IP links Is mighty plain,” quoth he; “ ‘Tis clear enough the Elephant 48,302 (52%) of globally routable Is very like a tree!” network prefixes North America -21 -22 The Internet in 1997 The Internet around 1990 (Post NSFNET) -23 -24

Internet Domain Survey Periodic Cycles Number of Internet Hosts ❒ Cycles of a day "Source: Internet Software Consortium (http://www.isc.org/)". ❍ People come to work in one part of the globe, go 180000000 home in another 160000000 140000000 ❒ Week 120000000 ❍ Weekends vs weekdays 100000000 80000000 ❒ Holidays 60000000 ❒ … 40000000 20000000 0 Aug-81 Aug-83 Aug-85 Aug-87 Aug-89 Aug-91 Aug-93 Aug-95 Aug-97 Aug-99 Aug-01 -25 -26 Stability? Internet principles? ❒ Despite any growth trends or periodic ❒ Besides the details of protocols, topology cycle, we also never have the Internet as a and growth trends, are there fundamental whole “timeless” principles of the Internet? ❒ Machines disconnecting/connecting The Fifth, who chanced to touch the ear, ❒ Stability of routes? Said: “E’en the blindest man Can tell what this resembles most; ❒ Viruses? Attacks? Deny the fact who can This marvel of an Elephant Is very like a fan! -27 -28 Packet Switching Protocol ❒ We’ve already seen this ❒ If two entities are going to communicate, they must agree on the expected order and ❒ Packets indicate their destination meaning of messages they exchange. ❒ No predetermined path for a packet to ❍ Hi …Hi…Got the time?….two oclock take SUCCESSFUL PROTOCOL EXCHANGE ❒ Each intermediate note routes the packet ❍ Hi…Don’t bother meXX ABORTED PROTOCOL closer to its destination ❍ Allo…Hello..Quelle heuere a’til …..XX<blank stare> PROTOCOL MISMATCH -29 -30

Protocol Layered Architectures ❒ Human beings are able to handle lots of A protocol defines the format and the complexity in their protocol processing. order of messages exchanged between ❍ Ambiguously defined protocols communicating entities as well as the ❍ Many protocols all at once actions taken on the receipt or the ❒ How do computers manage complex transmission of a message. protocol processing? ❍ Specify well defined protocols to enact. ❍ Decompose complicated jobs into layers that each have a well defined task -31 -32 Layered Architectures Protocol stack user X English user Y ❒ Break-up design problem intro smaller, more manageable problems. SMTP e-mail client e-mail server ❒ Modular design: easy to extend/modify. ❒ Difficult to implement (careful with TCP TCP server TCP server interaction of layers for efficiency). IP IP server IP server IEEE 802.3 standard ethernet ethernet driver/card driver/card electric signals -33 -34 Protocol encapsulation Physical Mail user X user Y CEO X CEO Y “Hello” “Lunch?” e-mail client e-mail server Secretary X Secretary Y “Hello” “Lunch?” “Hello” “Lunch?” TCP server TCP server Mail Room X Mail Room Y “Hello” “Lunch?” IP server IP server Postman X Postman Y ethernet ethernet Post office X Post office Y “Hello” “Lunch?” driver/card driver/card -35 -36