Introduction to Computer Networks

MGS 602, Fall 2012

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

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

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

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.

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.

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.

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!

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.

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

TCP/IP Physical Layer

• Physical interface between a computer or terminal and a transmission medium

• Specifies:– Characteristics of medium– Nature of signals– Data rate

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

TCP/IP Transport Layer

• Also called host-to-host layer• Reliable exchange of data between

applications• Uses TCP protocols for transmission

TCP/IP Application Layer

• Logic needed to support variety of applications

• Separate module supports each type of application (e.g. file transfer)

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

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

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 –tcp to see open ports (those that are accepting packets)

TCP Segment

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 inet addr (IP addresses associated with the local network interfaces)

IP Packet

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)

Communication Between Two Computers

1. A user at Comp A types ping 10.1.1.2 at a command prompt

2. The network software creates a ping message

3. The network protocol packages the message by adding IP address of sending and destination computers and acquires the destination computer’s MAC address

4. The network interface software adds MAC addresses of sending and destination computers and sends the message

5. Comp B receives message, verifies that the addresses are correct and then sends a reply to Comp A using Steps 2 – 4

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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 Internet is 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

Local and Wide Area Networks

LAN

WAN

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

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.

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

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

Questions?