Lecture 1 - Acsu Buffalo

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.

–
N
ot 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

33

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

W
AN

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?