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Department of Computer Science Southern Illinois University Carbondale CS591 – Wireless & Network Security Lecture 2: Overview of Computer Networking Dr. Kemal Akkaya E-mail: [email protected] Kemal Akkaya Wireless & Network Security 1 What’s the Internet: “nuts and bolts” view Millions of connected computing devices: hosts = end systems Running network apps Communication links fiber, copper, radio, satellite transmission rate = bandwidth router server workstation mobile local ISP Routers: forward packets (chunks of data) Protocols control sending, receiving of msgs regional ISP e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” Loosely hierarchical Public Internet vs Private intranet Internet standards RFC: Request for Comments IETF: Internet Engineering Task Force Kemal Akkaya company network Wireless & Network Security 2 The network edge End systems (hosts) Run application programs e.g., Web, email At “edge of network” Client/Server model Client host requests, receives service from always-on server e.g. Web browser/server; email client/server Peer-Peer model Minimal (or no) use of dedicated servers e.g., Napster, Gnutella, KaZaA Kemal Akkaya Wireless & Network Security 3 What’s the Internet: a service view Communication infrastructure enables distributed applications Network edge: applications and hosts Network core: routers network of networks Access networks, physical media: communication links Applications: Web, email, games, ecommerce, file sharing Communication services provided to apps: Connectionless unreliable Connection-oriented reliable Various protocols are used for communication services Kemal Akkaya Wireless & Network Security 4 What’s a protocol? Human protocols: “What’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events Kemal Akkaya Network protocols: Machines rather than humans All communication activity in Internet governed by protocols Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt Wireless & Network Security 5 What’s a protocol? A human protocol and a computer network protocol: Hi TCP connection req Hi TCP connection response Got the time? Get http://www.awl.com/kurose-ross 2:00 Q: Other human protocols? <file> time Key Elements of a Protocol: Timing Syntax Data formats Signal levels Kemal Akkaya Speed matching Sequencing Wireless & Network Security Semantics Control information Error handling 6 Protocol “Layers” Networks are complex! Many “pieces”: Consider the following analogy: hosts routers links of various media applications protocols hardware, software ticket (purchase) ticket (complain) baggage (check) baggage (claim) Question: gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing Is there any hope of organizing structure of network? Or at least our discussion of networks? Kemal Akkaya Organization of air travel airplane routing A series of steps Wireless & Network Security 7 Layering of airline functionality ticket (purchase) ticket (complain) baggage (check) baggage (claim baggage gates (load) gates (unload) gate runway (takeoff) runway (land) takeoff/landing airplane routing airplane routing airplane routing airplane routing departure airport airplane routing intermediate air-traffic control centers ticket arrival airport Layers: Each layer implements a service via its own internal-layer actions relying on services provided by layer below Why Layering? Dealing with complex systems Explicit structure allows identification, relationship of complex system’s pieces Layered reference model for discussion Modularization eases maintenance, updating of system Change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system Kemal Akkaya Wireless & Network Security 8 Standardized Protocol Architectures Required for devices to communicate Vendors have more marketable products Customers can insist on standards based equipment Two standards: OSI Reference model Open Systems Interconnection Developed by the International Organization for Standardization (ISO) Seven layers Never lived up to early promises TCP/IP protocol suite Most widely used De facto standard Also: IBM Systems Network Architecture (SNA) Kemal Akkaya Wireless & Network Security 9 OSI - The Model A layer model Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers Why OSI did not take over the world Bad timing Bad technology Bad implementations Bad politics Kemal Akkaya Wireless & Network Security 10 The OSI Environment Kemal Akkaya Wireless & Network Security 11 OSI as Framework for Standardization Kemal Akkaya Wireless & Network Security 12 OSI Layers Transport Physical Physical interface between devices Mechanical Electrical Functional Procedural Data Link Means of activating, maintaining and deactivating a reliable link Error detection and control Higher layers may assume error free transmission Network Transport of information Higher layers do not need to know about underlying technology Not needed on direct links Kemal Akkaya Exchange of data between end systems Error free In sequence No losses No duplicates Quality of service Session Control of dialogues between applications Dialogue discipline Grouping Recovery Presentation Data formats and coding Data compression Encryption Application Means for applications to access OSI environment Wireless & Network Security 13 Use of a Relay Kemal Akkaya Wireless & Network Security 14 TCP/IP Protocol Architecture Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) Used by the global Internet No official model but a working one. Application layer Host to host or transport layer Internet layer Data link layer Physical layer Problems: Service, interface, and protocol not distinguished Not a general model No exact separate mention of physical and data link layers Sometimes called host-to-network layer Minor protocols deeply entrenched, hard to replace Kemal Akkaya Wireless & Network Security 15 Layer Descriptions Physical Layer Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates Data Link Layer Exchange of data between neighboring network nodes Invoking services like priority Internet (IP) Layer Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers Transport Layer (TCP) Reliable delivery of data Ordering of delivery Application Layer Support for user applications: HTTP, SMTP Kemal Akkaya Wireless & Network Security 16 OSI vs TCP/IP Kemal Akkaya Wireless & Network Security 17 Protocols and networks in the TCP/IP model Kemal Akkaya Wireless & Network Security 18 PDUs in TCP/IP Kemal Akkaya Wireless & Network Security 19 source message segment Ht datagram Hn Ht frame Hl Hn Ht M M M M Encapsulation application transport network link physical Hl Hn Ht M link physical Hl Hn Ht M switch destination M Ht M Hn Ht Hl Hn Ht M M application transport network link physical Kemal Akkaya Hn Ht Hl Hn Ht M M network link physical Hn Ht Hl Hn Ht M M router Wireless & Network Security 20 Some Protocols in TCP/IP Suite Kemal Akkaya Wireless & Network Security 21