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CMU CS 15441 - Lecture: Design Philosophy & Applications

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Lecture 4 Design Philosophy & ApplicationsLecture OverviewInternet ArchitecturePrioritiesSurvivabilityFate SharingTypes of ServiceVarieties of NetworksThe “Other” goalsAccountabilityProject 1Project 1 goalsFTP: The File Transfer ProtocolFtp: Separate Control, Data ConnectionsFtp Commands, ResponsesHTTP BasicsHow to Mark End of Message?HTTP RequestSlide 19Slide 20HTTP Request ExampleHTTP ResponseSlide 23HTTP Response ExampleCookies: Keeping “state”Cookies: Keeping “State” (Cont.)Typical Workload (Web Pages)HTTP 1.1 - new featuresPacket DelaySlide 30A Word about UnitsApplication-level DelaySome ExamplesSustained ThroughputOne more detail: TCPHTTP 0.9/1.0Single Transfer ExamplePerformance IssuesNetscape SolutionPersistent Connection SolutionSlide 41Persistent HTTPPersistent Connection PerformanceRemaining ProblemsBack to performanceBandwidth SharingFair Sharing of BandwidthBut It is Not that SimpleNetwork Service ModelsOther RequirementsReadings1Lecture 4Design Philosophy &ApplicationsDavid AndersenSchool of Computer ScienceCarnegie Mellon University15-441 Networking, Spring 2005http://www.cs.cmu.edu/~srini/15-441/S05/2Lecture OverviewLast time:»Protocol stacks and layering»OSI and TCP/IP models»Application requirements from transport protocolsInternet ArchitectureProject informationApplication examples.»ftp»httpApplication requirements.»“ilities”»Sharing3Internet ArchitectureBackground»“The Design Philosophy of the DARPA Internet Protocols” (David Clark, 1988).Fundamental goal: Effective network interconnectionGoals, in order of priority:1. Continue despite loss of networks or gateways2. Support multiple types of communication service3. Accommodate a variety of networks4. Permit distributed management of Internet resources5. Cost effective6. Host attachment should be easy7. Resource accountability4PrioritiesThe effects of the order of items in that list are still felt today»E.g., resource accounting is a hard, current research topicLet’s look at them in detail5SurvivabilityIf network disrupted and reconfigured»Communicating entities should not care!»No higher-level state reconfiguration»Ergo, transport interface only knows “working” and “not working.” Not working == complete partition.How to achieve such reliability?»Where can communication state be stored?Network HostFailure handing Replication “Fate sharing”Net Engineering Tough SimpleSwitches Maintain state StatelessHost trust Less More6Fate SharingLose state information for an entity if (and only if?) the entity itself is lost.Examples:»OK to lose TCP state if one endpoint crashes–NOT okay to lose if an intermediate router reboots»Is this still true in today’s network?–NATs and firewallsSurvivability compromise: Heterogenous network -> less information available to end hosts and Internet level recovery mechanismsConnection State StateNo State7Types of ServiceRecall from last time TCP vs. UDP»Elastic apps that need reliability: remote login or email»Inelastic, loss-tolerant apps: real-time voice or video»Others in between, or with stronger requirements»Biggest cause of delay variation: reliable delivery–Today’s net: ~100ms RTT–Reliable delivery can add seconds.Original Internet model: “TCP/IP” one layer»First app was remote login…»But then came debugging, voice, etc.»These differences caused the layer split, added UDPNo QoS support assumed from below»In fact, some underlying nets only supported reliable delivery–Made Internet datagram service less useful!»Hard to implement without network support»QoS is an ongoing debate…8Varieties of NetworksDiscussed a lot of this last time - »Interconnect the ARPANET, X.25 networks, LANs, satellite networks, packet networks, serial links…Mininum set of assumptions for underlying net»Minimum packet size»Reasonable delivery odds, but not 100%»Some form of addressing unless point to pointImportant non-assumptions:»Perfect reliability»Broadcast, multicast»Priority handling of traffic»Internal knowledge of delays, speeds, failures, etc.Much engineering then only has to be done once9The “Other” goalsManagement»Today’s Internet is decentralized - BGP»Very coarse tools. Still in the “assembly language” stageCost effectiveness»Economies of scale won out»Internet cheaper than most dedicated networks»Packet overhead less important by the yearAttaching a host»Not awful; DHCP and related autoconfiguration technologies helping. A ways to go, but the path is thereBut…10AccountabilityHuge problem.Accounting»Billing? (mostly flat-rate. But phones are moving that way too - people like it!)»Inter-provider payments–Hornet’s nest. Complicated. Political. Hard.Accountability and security»Huge problem.»Worms, viruses, etc.–Partly a host problem. But hosts very trusted.»Authentication–Purely optional. Many philosophical issues of privacy vs. security.… Questions before we move on to the project?11Project 1Out today, due 2/24»Intermediate validation deadline for basic functionsGet started early. Get started early. Get …Project partners»Choose very soon»Mail to David Craft, [email protected]Project is an IRC server (Internet Relay Chat)»Text-based chat protocol. Features, in order:1. Basic server (connect, channels, talk, etc.)•can do now2. Link-state routing to send messages to users across servers1. OSPF lecture (2/10). Book: Chapter 4 (4.2)3. Multicast routing to let channels span servers1. MOSPF lecture (2/15). Paper: Deering “Multicast Routing”12Project 1 goalsSkill with real network applications»Select, dealing with multiple streams of data, remote clients and servers»Protocol “grunge” - headers, layers, packets, etc.»Be able to implement a [whatever] server.Meet a real protocol»Create it from the specFamiliarity with routing protocols and techniquesDon’t be dismayed by the size of the handout. It breaks down into reasonable chunks.13FTP: The File Transfer ProtocolTransfer file to/from remote hostClient/server model»Client: side that initiates transfer (either to/from remote)»Server: remote hostftp: RFC 959ftp server: port 21file transferFTPserverFTPuserinterfaceFTPclientlocal filesystemremote filesystemuser at host14Ftp: Separate Control, Data ConnectionsFtp client contacts ftp server at port 21, specifying TCP


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CMU CS 15441 - Lecture: Design Philosophy & Applications

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