Lecture 1 - Acsu Buffalo

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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?