EE 122 Layering and the Internet Architecture Kevin Lai September 4 2002 Organizing 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 questions laik cs berkeley edu 2 Layering A 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 violations Advantages 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 implementations laik cs berkeley edu 3 No Layering Application Transmission Media SMTP SSH Coaxial cable NFS Fiber optic HTTP Packet radio new application has to interface to all existing media adding new application requires O m work m number of media new media requires all existing applications be modified adding new media requires O a work a number of applications total work in system O ma eventually too much work to add apps media laik cs berkeley edu 4 3 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 Application SMTP SSH NFS HTTP Intermediate layer Transmission Media Coaxial cable Fiber optic laik cs berkeley edu Packet radio 5 ISO 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 protocols laik cs berkeley edu 6 ISO OSI Reference Model Seven layers Lower three layers are peer to peer Next four layers are end to end Application Presentation Application Presentation Session Transport Network Session Transport Network Datalink Physical Network Datalink Physical Physical medium laik cs berkeley edu Datalink Physical 7 Encapsulation 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 payload data data data data data data data data data data data data data data laik cs berkeley edu 8 OSI 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 peers laik cs berkeley edu 9 Physical 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 receivers laik cs berkeley edu 10 Datalink 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 laik cs berkeley edu 11 Network 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 tables laik cs berkeley edu 12 Transport 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 UDP laik cs berkeley edu 13 Session 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 functions laik cs berkeley edu 14 Presentation Layer 6 Service convert data between various representations Interface depends on service Protocol define data formats and rules to convert from one format to another laik cs berkeley edu 15 Application 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 browser laik cs berkeley edu 16 OSI vs TCP IP OSI conceptually define services interfaces protocols more layers less efficient Internet provide a successful implementation don t need so many layers in practice Application Presentation Session Transport Network Datalink Physical OSI Application Transport Internet Host tonetwork SMTP SSH DNS TCP UDP IP LAN Packet radio TCP IP laik cs berkeley edu 17 Layer 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 packets laik cs berkeley edu 18 End to End Argument How do you divide functionality across layers One answer the end to end argument Push as much functionality to higher layers as possible while still maintaining correctness and performance J H Saltzer D P Reed D D Clark End to End Arguments in System Design ACM Transactions on Computer Systems Vol 2 No 4 1984 pp 277 288 laik cs berkeley edu 19 Example Reliable File Transfer Host A Host B Appl OS Appl OK OS Problem transfer a file composed of blocks Solution 1 transfer each block and retry if necessary and then concatenate them Solution 2 transfer the blocks verify the entire file and retry the entire file if necessary laik cs berkeley edu 20 Discussion Solution 1 not complete e g disk fills up after block is verified but before block is written to disk The receiver has to check the entire file anyway Thus full functionality can be entirely implemented at application layer no need for reliability from lower
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