Department of Computer Science Southern Illinois University Carbondale

1. Department of Computer Science Southern Illinois University Carbondale CS591 – Wireless & Network Security Lecture 2: Overview of Computer Networking Dr. Kemal Akkaya E-mail: kemal@cs.siu.edu Wireless & Network Security 1

2. Millions of connected computing devices: hosts = end systems Running network apps Communication links fiber, copper, radio, satellite transmission rate = bandwidth Routers: forward packets (chunks of data) Protocols control sending, receiving of msgs 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 What’s the Internet: “nuts and bolts” view router workstation server mobile local ISP regional ISP company network Wireless & Network Security 2

3. 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 The network edge Wireless & Network Security 3

4. 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, e-commerce, file sharing Communication services provided to apps: Connectionless unreliable Connection-oriented reliable Various protocols are used for communication services What’s the Internet: a service view Wireless & Network Security 4

5. Human protocols: “What’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events Network protocols: Machines rather than humans All communication activity in Internet governed by protocols What’s a protocol? Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt Wireless & Network Security 5

6. A human protocol and a computer network protocol: What’s a protocol? Hi TCP connection req Hi TCP connection response Get http://www.awl.com/kurose-ross Got the time? <file> 2:00 time Q: Other human protocols? • Key Elements of a Protocol: • Syntax • Data formats • Signal levels • Timing • Speed matching • Sequencing • Semantics • Control information • Error handling Wireless & Network Security 6

7. Networks are complex! Many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? Protocol “Layers” ticket (complain) baggage (claim) gates (unload) runway landing airplane routing ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing airplane routing • Consider the following analogy: Organization of air travel • A series of steps Wireless & Network Security 7

8. Layering of airline functionality ticket ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing ticket (complain) baggage (claim gates (unload) runway (land) airplane routing baggage gate takeoff/landing airplane routing airplane routing airplane routing departure airport intermediate air-traffic control centers 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 Wireless & Network Security 8

9. 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) Wireless & Network Security 9

10. 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 Wireless & Network Security 10

11. The OSI Environment Wireless & Network Security 11

12. OSI as Framework for Standardization Wireless & Network Security 12

13. OSI Layers • Transport • 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 • 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 Wireless & Network Security 13

14. Use of a Relay Wireless & Network Security 14

15. 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 Wireless & Network Security 15

16. 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 Wireless & Network Security 16

17. OSI vs TCP/IP Wireless & Network Security 17

18. Protocols and networks in the TCP/IP model Wireless & Network Security 18

19. PDUs in TCP/IP Wireless & Network Security 19

20. Encapsulation network link physical link physical M M Ht Ht M M Hn Hn Hn Hn Ht Ht Ht Ht M M M M Hl Hl Hl Hl Hl Hl Hn Hn Hn Hn Hn Hn Ht Ht Ht Ht Ht Ht M M M M M M source message application transport network link physical segment datagram frame switch destination application transport network link physical router Wireless & Network Security 20

21. Some Protocols in TCP/IP Suite Wireless & Network Security 21