EE 122: Layering and the Internet ArchitectureOrganizing Network FunctionalityLayeringNo Layering3 LayersISO OSI Reference ModelISO OSI Reference ModelEncapsulationOSI Model ConceptsPhysical Layer (1)Datalink Layer (2)Network Layer (3)Transport Layer (4)Session Layer (5)Presentation Layer (6)Application Layer (7)OSI vs. TCP/IPLayer Violations and NotEnd-to-End ArgumentExample: Reliable File TransferDiscussionIntuitive JustificationInternet Embodies End-to-End ArgumentPropertiesSummaryEE 122: Layering and the Internet ArchitectureKevin LaiSeptember 4, [email protected] 2Organizing Network Functionality Many kinds of networking functionality- e.g., encoding, framing, routing, addressing, reliability, etc. How should they be organized? How should they interact? Layering is one answer to these [email protected] 3LayeringA technique to organize a network system into a succession of logically distinct entities, such that the service provided by one entity is solely based on the service provided by the previous (lower level) entity- no layering violationsAdvantages- Abstraction – lower layers can be changed without affecting the upper layers- Modularity – protocols easier to manage and maintain- Reuse – upper layers can reuse the functionality provided by lower layers Disadvantages- Information hiding – inefficient [email protected] 4No LayeringSMTP SSH NFSPacketradioCoaxial cableFiberopticHTTPApplicationTransmissionMedianew application has to interface to all existing media- adding new application requires O(m) work, m = number of medianew media requires all existing applications be modified- adding new media requires O(a) work, a = number of applicationstotal work in system O(ma) → eventually too much work to add apps/[email protected] 53 Layers Solution: introduce an intermediate layer that provides a single abstraction for various network technologies- O(1) work to add app/media- variation on “add another level of indirection”SMTP SSH NFSPacketradioCoaxial cableFiberopticHTTPApplicationIntermediate [email protected] 6ISO OSI Reference Model ISO – International Standard Organization OSI – Open System Interconnection Started to 1978; first standard 1979- ARPANET started in 1969; TCP/IP protocols ready by 1974 Goal: a general open standard - allow vendors to enter the market by using their own implementation and [email protected] 7ISO OSI Reference Model Seven layers- Lower three layers are peer-to-peer- Next four layers are end-to-endApplicationPresentationSessionTransportNetworkDatalinkPhysicalApplicationPresentationSessionTransportNetworkDatalinkPhysicalNetworkDatalinkPhysicalPhysical [email protected] 8Encapsulation A layer can use only the service provided by the layer immediate below it Each layer may change and add a header to data packet- higher layer’s header is treated as payloaddatadatadatadatadatadatadatadatadatadatadatadatadatadatalaik@cs.berkeley.edu 9OSI Model Concepts Service – says what a layer does Interface – says how to access the service Protocol – says how is the service implemented- a set of rules and formats that govern the communication between two [email protected] 10Physical Layer (1) Service: move the information between two systems connected by a physical link Interface: specifies how to send a bit Protocols: coding scheme used to represent a bit, voltage levels, duration of a bit Examples: coaxial cable, optical fiber links; transmitters, [email protected] 11Datalink Layer (2) Service: - framing, i.e., attach frame separators - send data frames between peers- others:• arbitrate the access to common physical media• ensure reliable transmission• provide flow control Interface: send a data unit (packet) to a machine connected to the same physical media Protocols: layer addresses, implement Medium Access Control (MAC) (e.g., CSMA/CD)…[email protected] 12Network Layer (3) Service: - deliver a packet to specified destination- perform segmentation/reassemble- others:• packet scheduling• buffer management Interface: send a packet to a specified destination Protocols: define global unique addresses; construct routing [email protected] 13Transport Layer (4) Services: - provide an error-free and flow-controlled end-to-end connection- multiplex multiple transport connections to one network connection- split one transport connection in multiple network connections Interface: send a packet to specify destination Protocols: implement reliability and flow control Examples: TCP and [email protected] 14Session Layer (5) Service:- full-duplex- access management, e.g., token control- synchronization, e.g., provide check points for long transfers Interface: depends on service Protocols: token management; insert checkpoints, implement roll-back [email protected] 15Presentation Layer (6) Service: convert data between various representations Interface: depends on service Protocol: define data formats, and rules to convert from one format to [email protected] 16Application Layer (7) Service: any service provided to the end user Interface: depends on the application Protocol: depends on the application Examples: Kazaa, SMTP, ssh, NFS, WWW [email protected] 17OSI vs. TCP/IPOSI: conceptually define services, interfaces, protocols- more layers → less efficientInternet: provide a successful implementation - don’t need so many layers in practiceInternetHost-to-networkTransportApplicationIPLANPacketradioTCP UDPSMTP SSH DNSApplicationPresentationSessionTransportNetworkDatalinkPhysicalOSI TCP/[email protected] 18Layer Violations and Not Types of violations- Higher layer interacts with layer below previous layer• e.g., application has special behavior when running on wireless link• sometimes provides useful performance enhancement- Lower layer interacts with higher layer• e.g., router (network layer device) routes based on transport or application data in packet• usually a very bad idea (see end-to-end argument) Not violations- Reducing copying of [email protected] 19End-to-End Argument How do you divide functionality across layers?- One answer: the end-to-end argument- Push as much functionality to higher layers
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