CS162 Operating Systems and Systems Programming Lecture 14 Protocols Layering and e2e Argument March 14 2011 Ion Stoica http inst eecs berkeley edu cs162 Goals for Today Finish Page Replacement Working Set Thrashing Introduction to networking 3 14 Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and Gagne Many slides generated from my lecture notes by Vern Paxson and Scott Shenker Ion Stoica CS162 UCB Spring 2011 Lec 14 2 What Is A Protocol A protocol is an agreement on how to communicate Includes Syntax how a communication is specified structured Format order messages are sent and received Semantics what a communication means Actions taken when transmitting receiving or when a timer expires 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 3 Examples of Protocols in Human Interactions Telephone 1 2 3 4 5 6 7 8 9 10 11 3 14 Pick up open up the phone Listen for a dial tone see that you have service Dial Should hear ringing Callee Hello Caller Hi it s Alice Or Hi it s me what s that about Caller Hey do you think blah blah blah pause Callee Yeah blah blah blah pause Caller Bye Callee Bye Hang up Ion Stoica CS162 UCB Spring 2011 Lec 14 4 Examples of Protocols in Human Interactions Asking a question 1 Raise your hand 2 Wait to be called on 3 Or wait for speaker to pause and vocalize 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 5 The Internet Protocol IP Best Effort Packet Delivery Datagram packet switching Send data in packets Header with source destination address Service it provides source Packets may be lost Packets may be corrupted Packets may be delivered out of order destination IP network 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 6 Example Transmission Control Protocol TCP Communication service Ordered reliable byte stream Simultaneous transmission in both directions Key mechanisms at end hosts Retransmit lost and corrupted packets Discard duplicate packets and put packets in order Flow control to avoid overloading the receiver buffer Congestion control to adapt sending rate to network load TCP connection source 3 14 network destination Ion Stoica CS162 UCB Spring 2011 Lec 14 7 Protocol Standardization Ensure communicating hosts speak the same protocol Standardization to enable multiple implementations Or the same folks have to write all the software Standardization Internet Engineering Task Force Based on working groups that focus on specific issues Produces Request For Comments RFCs Promoted to standards via rough consensus and running code IETF Web site is http www ietf org RFCs archived at http www rfc editor org De facto standards same folks writing the code P2P file sharing Skype your protocol here 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 8 Layering The Problem Many different applications email web P2P etc Many different network styles and technologies Circuit switched vs packet switched etc Wireless vs wired vs optical etc How do we organize this mess 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 9 The Problem cont d Application Transmission Media Skype SSH Coaxial cable NFS Fiber optic HTTP Radio Re implement every application for every technology No But how does the Internet design avoid this 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 10 Solution Intermediate Layers Introduce intermediate layers that provide set of abstractions for various network functionality technologies A new app media implemented only once Variation on add another level of indirection Application Skype SSH NFS HTTP Intermediate layers Transmission Media 3 14 Coaxial cable Fiber optic Packet radio Ion Stoica CS162 UCB Spring 2011 Lec 14 11 Network Architecture Architecture is not the implementation itself Architecture is how to organize structure the elements of the system their implementation What interfaces are supported Using what sort of abstractions Where functionality is implemented The modular design of the network 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 12 Software System Modularity Partition system into modules abstractions Well defined interfaces give flexibility Hides implementation thus it can be freely changed Extend functionality of system by adding new modules E g libraries encapsulating set of functionality E g programming language compiler abstracts away not only how the particular CPU works but also the basic computational model Well defined interfaces hide information Isolate assumptions Present high level abstractions But can impair performance 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 13 Network System Modularity Like software modularity but Implementation distributed across many machines routers and hosts Must decide How to break system into modules Layering What functionality does each module implement End to End Principle Where state is stored Fate sharing We will address these choices in turn 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 14 Layering A Modular Approach Partition the system Each layer solely relies on services from layer below Each layer solely exports services to layer above Interface between layers defines interaction Hides implementation details Layers can change without disturbing other layers 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 15 Properties of Layers OSI Model Service what a layer does Service interface how to access the service Interface for layer above Protocol peer interface how peers communicate to achieve the service Set of rules and formats that specify the communication between network elements Does not specify the implementation on a single machine but how the layer is implemented between machines 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 16 Physical Layer 1 Service move information between two systems connected by a physical link Application Present Session Transport Network Datalink Physical Interface specifies how to send and receive bits Protocol coding scheme used to represent a bit voltage levels duration of a bit Examples coaxial cable optical fiber links transmitters receivers 3 14 Ion Stoica CS162 UCB Spring 2011 Lec 14 17 Data Link Layer 2 Service Application Present Session Transport Network Datalink Physical Enable end hosts to exchange atomic messages with one another Using abstract addresses i e not just direct physical connections Perhaps over multiple physical links But using the same framing headers trailers Possible other services arbitrate access to common physical media reliable transmission flow control Interface send messages frames to other end hosts
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