Introduction to Computer Networks

1. Introduction to Computer Networks MGS 602, Fall 2012

2. What is Networking? • Networking involves connecting computers and other electronic devices for the purpose of sharing information and resources and for communication • A great deal of technology is required for one device to connect and communicate with another, and many choices for physical connections and related software are possible

3. Networking Fundamentals • An elementary network consists of two computers connected by some kind of transmission medium • Motivation: need to share data and to communicate quickly and efficiently • Sharing enables users to exchange information and route data between them as workflow demands • Can improve human communication substantially • Peripheral device sharing enables users to take advantage of peripherals and other devices attached directly to a network or to a generally available computer attached to a network

4. Network Protocols • Network protocol: common set of rules that allows two computers on a network to communicate with one another successfully • How to interpret signals, how to identify a computer on a network, how to initiate and end networked communications, and how to manage information exchange across the network medium • Examples: • TCP/IP • NetBEUI • IPX/SPX

5. OSI Model • A Seven-Layer model that describes functions for computers to talk to each other. • Proposed by ISO in 1984. • Each layer can only talk to the layer above/below it. • Reference guide for any communication system.

6. OSI Model • Layering accommodates new technologies independently. • Layer 1 gets Wired to Wireless but upper layers shouldn’t care. • Layer 7 gets email and twitter but Layer 1 shouldn’t care. • OSI and other standardization efforts allow us to mix and match hardware. • Not restricted by vendor’s proprietary hw/sw.

7. Open Vs. Proprietary Systems • Why do closed/proprietary systems exist? • Competitive Advantage • Collect fees from others • Technical openness needed for different hw/sw talking to each other. • Examples of Open-Systems in Networks: • TCP/IP, Ethernet.

9. 7 Layers of OSI model • Layer 7 – Email Client / Browser • Layer 6 – File Encryption / Format Conversion • Layer 5 – Session between Web Server and browser • Layer 4 – HTTP (port 80) / FTP (port 21) • Layer 3 – IP addresses (software) and Routing • Layer 2 – MAC addresses (hardware) • Layer 1 – Electric Signals!

11. Why care about OSI? • Guideline for network stds, devices and internetworking schemes. • Breaks operations into less-complex elements. • Helps engineers focus on modular functions. • Standardize interface for plug-and-play compatibility. • Help multivendor integration.

12. How Two Computers Communicate • TCP/IP is the most common protocol (language) used on networks • TCP/IP uses 2 addresses to identify devices on a network • Logical address (called IP address) • Physical address (called MAC address) • Just as a mail carrier needs an address to deliver mail, TCP/IP needs an address in order to deliver data to the correct device on a network • Think of the Logical address as a zip code and the Physical address as a street address

13. TCP/IP Physical Layer • Physical interface between a computer or terminal and a transmission medium • Specifies: • Characteristics of medium • Nature of signals • Data rate

14. TCP/IP Internet Layer • An Internet is an interconnection of two or more networks • Internet layer handles tasks similar to network access layer, but between networks rather than between nodes on a network • Uses IP for addressing and routing across networks • Implemented in workstations and routers

15. TCP/IP Transport Layer • Also called host-to-host layer • Reliable exchange of data between applications • Uses TCP protocols for transmission

16. TCP/IP Application Layer • Logic needed to support variety of applications • Separate module supports each type of application (e.g. file transfer)

17. TCP and UDP • Most TCP/IP applications use TCP for transport layer • TCP provides a connection (logical association) between two entities to regulate flow check errors • UDP (User Datagram Protocol) does not maintain a connection, and therefore does not guarantee delivery, preserve sequences, or protect against duplication

18. TCP/IP Applications • SMTP (Simple Mail Transfer Protocol) • Basic e-mail facility, transferring messages among hosts • FTP (File Transfer Protocol) • Sends files from one system to another on user command • Telnet • Remote login capability, allowing a user to emulate a terminal on the remote system

19. TCP Segment and Port • Source port (16 bits) • Destination port (16 bits) • Sequence number (32 bits) • Acknowledgment number (32 bits) • Data Offset (4 bits) • Flags (6 bits) : URG, ACK, PSH, RST, SYN, FIN In Ubuntu, use netstat -ln –tcpto see open ports (those that are accepting packets)

20. TCP Segment

21. IP Address • IP provides for 32-bit source and destination addresses • IPv6 (1996 standard) provides for 128-bit addresses • Migration to IPv6 will be a very slow process • IP Header: • Time to Live (8 bits) • Type of Service (8 bits) • Fragment Offset (13 bits) In Ubuntu, use ifconfig and then look for inetaddr (IP addresses associated with the local network interfaces)

22. IP Packet

23. MAC / Physical Address • MAC – Media Access Control • Unique in the world for each physical network card/interface • Network Interface Card (NIC) • Numbering assigned by IEEE organizations • First 6 characters (highest 3 bytes) => Vendor • Organizationally Unique Identifier (OUI) In Ubuntu, , use ifconfig and then look for HWaddr (MAC addresses of the local network interfaces)

24. Communication Between Two Computers • A user at Comp A types ping 10.1.1.2 at a command prompt • The network software creates a ping message • The network protocol packages the message by adding IP address of sending and destination computers and acquires the destination computer’s MAC address • The network interface software adds MAC addresses of sending and destination computers and sends the message • Comp B receives message, verifies that the addresses are correct and then sends a reply to Comp A using Steps 2 – 4 33

25. Local and Wide Area Networks • Local Area Network (LAN): small network, limited to a single collection of machines and one or more cables and other peripheral equipment • Internetwork: networked collection of LANs tied together by devices such as routers • The Internetis the best example • Wide Area Network (WAN): internetwork that spans distances measured in miles and links two or more separate LANs • Metropolitan Area Network (MAN): uses WAN technologies to interconnect LANs in a specific geographic region, such as a county or a city

26. Local and Wide Area Networks WAN LAN

27. Network Servers • Most common server roles found on networks: • Domain controller/directory servers • File and print servers • Application servers • Communication servers • E-mail/fax servers • Web servers

28. Network Servers • Domain Controller/Directory Servers • Directory services make it possible for users to locate, store, and secure information about a network and its resources. • Windows servers permit combining computers, users, groups, and resources into domains. The server handling the computers and users in a domain is called a domain controller. • File and Print Servers • Provide secure centralized file storage and sharing and access to networked printers. • Any Windows or Linux computer can act as a file and print server, however the Server version of Windows provides advanced sharing features.

29. Network Servers • Application Servers • Supply the server side of client/server applications to network clients • Differ from basic file and print servers by providing processing services as well as handling requests for file or print services • Communication Servers • Provide a mechanism for users to access a network’s resources remotely • Enable users who are traveling or working at home to dial in to the network via a modem or their existing Internet connection • E-mail/Fax Servers

30. Network Servers • Web Servers • Windows Server includes a complete Web server called Internet Information Services (IIS) as well as File Transfer Protocol (FTP) • Apache Web Server is available as a part of most Linux distributions and remains the most widely used Web server in the world • Other Network Services • Most networks require additional support services to function efficiently. The most common are Domain Name System (DNS) and Dynamic Host Configuration Protocol (DHCP) • DNS allows users to access both local and Internet servers by name rather than by address • DHCP provides automatic addressing for network clients so that network administrators do not have to assign addresses manually

31. Questions?