Page 11CS 640: Introduction to Computer NetworksAditya AkellaLecture 1Introductionhttp://www.cs.wisc.edu/~akella/CS640/F072Goals of This Class• Understand principles and practice of networking• How are modern networks designed? Operated? Managed?• Performance and design trade-offs in network protocols and applications• How do network applications work? How to write applications that use the network?– Hands-on approach to understand network internals• How will different aspects of networking evolve in the future?3Goal of Networking• Enable communicationbetween network applicationson different end-points– End-points? computers, cell phones….– Application? Web, Peer to Peer, Streaming video, IM– Communication? transfer bits or information across a “network”• Network must understand application needs/demands– What data rate?– Traffic pattern? (bursty or constant bit rate)– Traffic target? (multipoint or single destination, mobile or fixed)– App sensitivity? (to delay, “jitter”, loss)– Difficulty: Network may not know these in the first place!• How does the application “use” the network?– Peer to peer: how to find nearest host– Web: how to modulate sending rate? Coexist with other users/apps?Page 24Defining a “Network”• Network = nodes + links– Will build on this soon• Intentionally vague. There are several different networks:– The Internet– Wisc CS network– Telephone network– Home wireless networks– Others – sensor nets, “On Star”, cellular networks• Our focus on Internet– Also explore important common issues and challenges5Challenges for Networking• Accommodate different geographic scopes– The Internet vs. home network• Enable scale– CS network vs. the Internet• Seamlessly integrate different application types– Email vs. video conferencing• Independent administration and Trust– Corporate network – owned by one entity– Internet owned and managed by 17,000 network providers• Independent, conflicting interests6Network Building Block: Links• “Physical”-layer questions– Wired or wireless– Voltage (Electrical) or wavelength (optical)• “Link”-layer issues: How to send data? – Medium access – can either side talk at once?– Data format?Node Link NodePage 37• … But what if we want more hosts?• How many additional wires per host?• Scalability?Basic Building Block: LinksWires for everybody?How many wires?8Key Idea: Multiplexing• Multiplex: share network resources– Resources need “provisioning”– Grow at slower rate than number of nodes• How to share? Switched network– Party “A” gets resources sometimes– Party “B” gets them sometimes• Interior nodes act as “Switches”AB9Circuit Switching• Source first establishes a circuit to destination– Switches along the way stores info about connection• Possibly allocate resources• Different srs-dst’s get different paths• Source sends the data over the circuit– No address required since path is established beforehand• The connection is explicitly set up and torn down• Switches use TDM (digital) or FDM (analog) to transmit data from various circuitsPage 410Switching in the Telephone Network11Circuit Switching Discussion• Positives–Fast and simpledata transfer, once the circuit has been established– Predictable performance since the circuit provides isolationfrom other users• E.g. guaranteed max bandwidth• Negatives– How about bursty traffic• Circuit will be idle for significant periods of time• Also, can’t send more than max rate– Circuit set-up/tear down is expensive– Also, reconfiguration is slow• Fast becoming a non-issue12Packet Switching• Source sends information as self-contained packets – Packets have an address.– Source may have to break up single message in multiple packets• Packets travel 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 mailPage 513Benefits ofStatistical MultiplexingPacketsBetter Link UtilizationTDM: Flow gets chance in fixed time-slotsSM: Flow gets chance on demand; no need to wait for slot14Packets vs. Circuits• Efficient – Can send from any input that is ready– No notion of wastage of resources that could be used otherwise• Contention (i.e. no isolation)– Congestion– Delay• Accommodates bursty traffic– But need packet buffers• Address look-up and forwarding– Need optimization• Packet switching pre-dominant– Circuit switching used on large time-scales, low granularities15Internet[work]Internetwork• A collection of interconnected networks• Networks: Different depts, labs, etc.• Router: node that connects distinct networks• Host: network endpoints (computer, PDA, light switch, …)• Together, an independently administered entity– Enterprise, ISP, etc.EEMECSPage 616Internetwork Challenges• Many differences between networks– Address formats– Performance –bandwidth/latency– Packet size– Loss rate/pattern/handling– Routing• How to translate and inter-operate?– Routers are key to many of these issuesInternet[work]802.3Frame relayATM17“The Internet”• Internet vs. internet • The Internet: the interconnected set of networks of the Internet Service Providers (ISPs) and end-networks, providing data communications services.– Network of internetworks, and more– About 17,000 different ISP networks make up the Internet– Many other “end” networks– 100,000,000s of hosts18• Extra Slides…– We will cover these topics in greater detail in future lecturesInternet Design IssuesPage 719InternetComputer 1 Computer 2Need:(1) naming, (2) addressing and (3) routing(4) …Some Key “Internet”Design Issues20Key Issues:Naming/AddressingWhat’s the address for www.wisc.edu?It is 144.92.104.243Translates human readable names to logical endpointsLocal DNS ServerComputer 121Key Issues:RoutingRRRRRHHHHRRHRRouters send packet towards destinationH: HostsR: RoutersPage 822Key Issues:Network Service Model• What is the service model?– Defines what to expect from the network–Best-effort:packets can get lost, no guaranteed delivery• What if you want more?– Performance guarantees (QoS)– Reliability• Corruption• Lost packets– In-order delivery for file chunks– Etc…23What if the Data gets Corrupted?InternetGET inrex.htmlGET
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