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CMU 15441 Computer Networking - Lecture

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1Lecture 2Protocol Stacks and LayeringDavid AndersenSchool of Computer ScienceCarnegie Mellon University15-441, Computer Networks2Last Time!The Big Picture» Goals: – Efficiency– “ilities” (scalability, manageability, availability),– Ease of creating applications» Challenges:– Scale– Geography– Heterogeneity (** today’s focus!)!A few specific details:» Circuits vs. packets» Little bit about routing» Service model and how to construct services (** today!)3Today’s Lecture!Last time: “Big picture”!Today:» General architectural principles for networks» Introduces a few concrete models & examples!Where we are going:» Tuesday: Socket programming review++ (for project)» Thursday: Application examples (still high level)» After that: Burrowing into the details, ground up!Today’s specifics:» What is a protocol.» Protocol stacks.» Some history.» Standards organizations.» Application layer.4Why protocols and layering?!Interoperability!Reuse!Hiding underlying details5What is a Protocol!An agreement between parties on how communication should take place.!Protocols may have to define many aspects of the communication.!Syntax:» Data encoding, language, etc.!Semantics:» Error handling, termination, ordering of requests, etc.!Protocols at hardware, software, all levels!!Example: Buying airline ticket by typing.!Syntax: English, ascii, lines delimited by “\n”Friendly greetingMuttered replyDestination?PittsburghThank you6Interfaces!Each protocol offers an interface to its users, and expects one from the layers on which it builds» Syntax and semantics strike again– Data formats– Interface characteristics, e.g. IP service model!Protocols build upon each other» Add value– E.g., a reliable protocol running on top of IP» Reuse– E.g., OS provides TCP, so apps don’t have to rewrite7Protocol andService LevelsApplicationEnd-to-endCoreNetwork8A Layered Network ModelThe Open Systems Interconnection (OSI) Model.ApplicationPresentationSessionTransportNetworkData linkPhysical1234567NetworkData linkPhysicalApplicationPresentationSessionTransportNetworkData linkPhysical9OSI Motivation!Standard way of breaking up a system in a set of components, but the components are organized as a set of layers.» Only horizontal and vertical communication» Components/layers can be implemented and modified in isolation!Each layer offers a service to the higher layer, using the services of the lower layer.!“Peer” layers on different systems communicate via a protocol.» higher level protocols (e.g. TCP/IP, Appletalk) can run on multiple lower layers» multiple higher level protocols can share a single physical network!“It’s only a model!” - TCP/IP has been crazy successful, and it’s not based on a rigid OSI model. But the OSI model has been very successful at shaping thought.10OSI Functions!(1) Physical: transmission of a bit stream.!(2) Data link: flow control, framing, error detection.!(3) Network: switching and routing.!(4) Transport: reliable end to end delivery.!(5) Session: managing logical connections.!(6) Presentation: data transformations.!(7) Application: specific uses, e.g. mail, file transfer, telnet, network management.Multiplexing takes place in multiple layers11Looking at protocols!Hop by hop / link protocols» Ethernet!End-to-end protocols» TCP, apps, etc.!Management / “control plane” protocols» Routing, etc.– Can be either link or e2e themselves– Definition somewhat vague.!Standards» File formats, etc.!E.g., JPEG, MPEG, MP3, …Categories not solid / religious, just a way to view things.12Heterogenous Sources of Components!Application: web server/browser, mail, distributed game,..!Presentation/session.» Often part of application» Sometimes a library!Transport/network.» Typically part of the operating system!Datalink.» Often written by vendor of the network interface hardware!Physical.» Hardware: card and linkApplicationPresentationSessionTransportNetworkData linkPhysical13Motivation: Many many Network ComponentsApplicationOperating SystemProtocol SoftwareComputerLinksRouter HardwareRouter Software(many protocols)Bridge HW/SWApplicationOperating SystemComputerNetwork Interface14Protocols for Interoperability!Many implementations of many technologies:!Hosts running FreeBSD, Linux, Windows, MacOS, …!People using Mozilla, Explorer, Opera, …!Routers made by cisco, juniper, …!Hardware made by IBM, Dell, Apple, …!And it changes all the time.!Phew!!But they can all talk together because they use the same protocol(s)» Application level protocols: HTTP, SMTP, POP, IMAP, etc.» Hardware protocols (ethernet, etc)15Protocols for Abstraction & Reuse!Multiple choices of protocol at many layers» Physical: copper, fiber, air, carrier pigeon» Link: ethernet, token ring, SONET, FDDI» Transport: TCP, UDP, SCTP!But we don’t want to have to write “a web (HTTP) browser for TCP networks running IP over Ethernet on Copper” and another for the fiber version…» Reuse! Abstraction!» Protocols provide a standard interface to write to» Layers hide the details of the protocols below16Multiplexing and Demultiplexing!There may be multiple implementations of each layer.» How does the receiver know what version of a layer to use?!Each header includes a demultiplexing field that is used to identify the next layer.» Filled in by the sender» Used by the receiver!Multiplexing ooccurs at multiple layers. E.g., IP, TCP, …IPTCPIPTCPV/HL TOS LengthID Flags/OffsetTTL Prot. H. ChecksumSource IP addressDestination IP addressOptions..17Example: Sending a Web PageHttp hdr Web pageTCP header. . .ApplicationpayloadApplicationPresentationSessionTransportNetworkData linkPhysical18Limitations of theLayered Model!Some layers are not always cleanly separated.» Inter-layer dependencies in implementations for performance reasons» Some dependencies in the standards (header checksums)!Higher layers not always well defined.» Session, presentation, application layers!Lower layers have “sublayers”.» Usually very well defined (e.g., SONET protocol)!Interfaces are not always well standardized.» It would be hard to mix and match layers from independent implementations, e.g., windows network apps on unix (w/out compatability library)» Many cross-layer assumptions, e.g. buffer management19The TCP/IP ModelApplication(pluslibraries)TCP/UDPIP/ICMPData linkPhysicalApplicationPresentationSessionTransportNetworkData linkPhysical20Local Area


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