Announcements Electronic copy of P D Chapter 1 available see announcements Web page Homework 1 due date postponed 1 week now Wed Sept 20 Also bug fixes for Problem 2 Internet Design Principles Make sure you re on the mailing list EE 122 Intro to Communication Networks Vern Dilip away next week SIGCOMM no office hours Fall 2006 MW 4 5 30 in Donner 155 Vern Paxson Sukun lectures Mon Sept 11 No Lecture Wed Sept 13 TAs Dilip Antony Joseph and Sukun Kim http inst eecs berkeley edu ee122 Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley Please see note in announcements page re printers to use for hardcopy of the notes 1 2 Overview End System Computer on the Net Roles played by end systems Clients servers peer to peer Internet Architecture layering The End to End Principle 3 Clients and Servers Also known as a host 4 Clients and Servers Client program Client program Running on end host Requests service E g Web browser Running on end host Requests service E g Web browser GET index html Server program Running on end host Provides service E g Web server GET index html 5 Site under construction 6 1 Clients Are Not Necessarily Human Client Server Communication Example Web crawler or spider Client sometimes on Spider client Server is always on Initiates a request to the server when interested E g Web browser on your laptop or cell phone Doesn t communicate directly with other clients Needs to know the server s address Automated client program Tries to discover download many Web pages Forms the basis of search engines like Google Start with a base list of popular Web sites Download the Web pages Parse the HTML files to extract hypertext links Download these Web pages too And repeat and repeat and repeat Services requests from many client hosts E g Web server for the www cnn com Web site Doesn t initiate contact with the clients Needs a fixed well known address Per Project 2 7 Peer to Peer Communication 8 Client and Server Processes No always on server at the center of it all Program vs process Hosts can come and go and change addresses Hosts may have a different address each time Program collection of code Process a running program on a host Example peer to peer file sharing Communication between processes Any host can request files send files query to find where a file is located respond to queries and forward queries Scalability by harnessing millions of peers Each peer acting as both a client and server Same end host inter process communication Governed by the operating system on the end host Though can use network protocols for this too Different end hosts exchanging messages Governed by the network protocols Client and server processes 9 The Problem Application Transmission Media Client process process that initiates communication Server process process that waits to be contacted 10 Solution Intermediate Layers Skype SSH Coaxial cable NFS Fiber optic Introduce intermediate layers that provide set of abstractions for various network functionality technologies HTTP A new app media implemented only once Variation on add another level of indirection Packet radio Application Skype SSH NFS HTTP Intermediate layers Re implement every application for every technology No But how does the Internet design avoid this Transmission Media 11 Coaxial cable Fiber optic Packet radio 12 2 Network Architecture Computer System Modularity Partition system into modules abstractions Architecture is not the implementation itself Well defined interfaces give flexibility Architecture is how to organize structure the elements of the system their implementation E g libraries encapsulating set of functionality Change implementation of modules Extend functionality of system by adding new modules E g programming language compiler abstracts away not only how the particular CPU works What interfaces are supported Using what sort of abstractions but also the basic computational model Where functionality is implemented The modular design of the network Well defined interfaces hide information 13 Network System Modularity Isolate assumptions Present high level abstractions But can impair performance 14 Layering A Modular Approach Like software modularity but Paritition the system Implementation distributed across many machines routers and hosts Must decide 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 How to break system into modules Layering Where modules are implemented Application End to End Principle Application to application channels We will address these choices in turn Host to host connectivity 15 Properties of Layers OSI Model Link hardware 16 Physical Layer 1 Service move signals information between two systems connected by a physical link Service what a layer does Service interface how to access the service Interface for layer above Interface specifies how to send bits Protocol peer interface how peers communicate to achieve the service Protocol coding scheme used to represent bits voltage levels duration of a bit Set of rules and formats that govern the communication between network elements Does not govern the implementation on a single machine but how the layer is implemented between machines 17 Examples coaxial cable optical fiber links transmitters receivers 18 3 Data link Layer 2 Inter Network Layer 3 Service Service Framing attach frame separators Deliver data frames from one peer to another Deliver a packet to specified inter network destination Inter network across multiple networks link technologies Perform segmentation reassembly Perhaps across multiple hops What if different link technologies have different size limits Possible others Possible others arbitrate access to common physical media per hop reliable transmission per hop flow control packet scheduling buffer management Interface send a data unit packet to a machine connected to the same physical network Interface send a packet to a specified internetwork destination Protocols addressing link layer specific Medium Access Control MAC e g CSMA CD Carrier Sense Multiple Access Collision Detection Protocols define inter network addresses globally unique construct routing tables 19 Transport Layer 4 20 Application Layer 7 not 5 Service any service provided to the end user Service Provide end to end communication between
View Full Document
Unlocking...