Transcript What is TCP/IP? - Dept. of IE, CUHK Personal Web Server

Objectives
• A basic understanding of the following
– What is Internet?
– What is client-server computing?
– What is LAN? What is Ethernet?
– What is Internet protocol?
– How to write your web page?
– What is broadband network?
– What is MPEG1, MPEG2, and MP3?
– What is e-commerce?
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Internet
• Internet
– What is LAN?
– What is Internet?
– What is TCP/IP?
– What is domain name?
– Application - how does email work?
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What is Local Area Network (LAN)?
• Problem
– What is the simplest way to connect the
following computers together?
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What is Local Area Network (LAN)?
• Mesh
– too many links !
• Bus
– single cable
linking all
computers
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LAN
• Network address
– to send data to a destination, we need an
address
– each computer is connected to the LAN by a
network card
– each card has a unique network address
• the data we sent has two parts
– header: contains the source and destination
address (like an envelop)
– body:
contains the information (like a
letter)
• together, it is called a packet
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Contention problem
• only one computer can
use the cable at a time
• e.g. a -> c
c
b
d
a
• if more than one
computer uses the
cable to send data,
collision !
• e.g. a -> c, b -> d
e
c
b
d
a
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Contention problem
• Contention problem
– How to solve the problem that more than one
computer want to transmit at the same time
• classic problem in network
– many solutions
– example
• polling
• divide into time slots (time division)
• divide into frequency slots (frequency
division)
• CDMA (most complex)
• ALOHA-based (Ethernet, GSM)
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ALOHA-based protocol
• A classroom game
– imagine you are in a classroom and you want
to raise a question
• Rules
– you must close your eyes
– only one student can speak at a time
– if more than one student speak, the teacher
cannot hear the question
• Question
– how to raise questions in an orderly fashion
without any coordinator?
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ALOHA-based protocol
• solution:
– you listen before you raise question !!
– if someone is speaking, do not raise question!!
– thereby avoid collision
– the first person who raises the question speak
first
– in our computer LAN, the first computer who
sends data get the channel
• the computer then sends the destination
address of the receiving computer, followed
by the data
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ALOHA-based protocol
• But what if two students raise question at the
same time?
• Solution
– each student rolls a dice and wait for a random
period of time
– if student A gets a 1, and student B gets a 3,
then the student A will wait for 1 second, and B
will wait for 3 seconds
– after one second, A will speak out first
– after three seconds, B will try to speak out, but
B will listen first; if A is still talking, B will wait
until A has finished
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ALOHA-based protocol
• In computer network, this means each computer
listens to the cable, if the cable is idle (no signal),
sends immediately; otherwise, waits
• if more than one computer send data
simultaneously, we have a collision
• in this case, each computer will wait for a random
period of time, the one who sends first will grab
the channel
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Ethernet
• This type of networking protocol has been in use
for nearly thirty years !!
• It is called Ethernet and currently over 90% of the
LANs in used today are based on it
• however, with the abundance
of processing power, the
topology of Ethernet has
changed from a shared bus
to a star topology
S
• communication is via a switch
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GSM
• It is also the principle used in mobile network like
GSM
– each mobile phone transmits signal using a
frequency slot
– but the number of frequency slots is limited,
so it is possible that two mobile phones may
compete for the same frequency slot at the
same time
– if there is a collision, each mobile phone will
wait for a random period of time, the one who
wakes up first will get the frequency slot
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What is Internet?
• Problem of interconnecting LAN
– by late 70, many different types of LAN existed
– What is the problem in connecting the LANs?
Network 1
Network A
c
3
d
b
a
e
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2
1
5
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What is Internet?
• Addressing problem
– to send data to a destination, you need an
address
– different technology uses different naming
convention for the address
– LANs cannot communicate with each other
• needs a standard way to address computers
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IP address
• IP address
– the universal address is called IP address
– IP stands for Internet Protocol
– it is a 12-digit number (e.g. 137.189.123.108)
• we therefore have two set of addresses
– physical address
• address used by the LAN
• must use this address for delivery
– Internet address
• just a unique number
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Post office scenario
• Limitation of a LAN
– LAN can deliver address that is supported by
its own LAN technology
– the physical address
• e.g. the Ethernet address in the card
– cannot send address to another LAN that is
other kind of technology (e.g. token ring)
– need a middleman to forward the packet
– router
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Data
Data
IP address
147.189.123.108
d
a
147.189.123.108
147.189.123.108
1
d
2
d
1
1
router
147.189.123.108
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What is Internet?
• Internet communicates in exactly the same way as
our postal service
• the packet we sent (data + IP address) is called
datagram
– (as we sometimes call air mail letters as
aerogramme)
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What is Internet?
• Problem with datagram (and letters)
– like letters, if we send a sequence of datagrams,
the following may happen
• the datagram is lost, needs to re-send again
• the datagram is not lost, we have to eliminate
the duplicate packet
• the datagrams arrived out of sequence
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What is TCP (Transmission control protocol) ?
• Use of TCP
– IP only deals with the addressing problem,
cannot deal with error
• such as lost, duplicate, and out-of-sequence
datagram
– to deal with error, we need to add a sequence
number to each of the datagram we sent, and
check whether the sequence is correctly received
at the other side
– the software to do this is called TCP
– TCP is used to provide reliable communication
on Internet
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Data
012234
147.189.123.108
d
a
TCP
sequence number checked
IP
1
2
d
012234
147.189.123.108
147.189.123.108
d
Data
1
1
router
147.189.123.108
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Summary
• communication on Internet is based on TCP/IP
• it is operates in exactly the same way as our
postal service
• this is really all you need to know about TCP/IP
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What is domain name?
• Is 147.189.123.108 easy to remember?
• NO !!
– A 12 digits long telephone number is hard for
everyone to remember
• Solution?
– remember a name, not a number
– let the directory on Internet (DNS) translate
the computer name to IP number for you,
transparently
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What is domain name?
3. Datagram to 147.189.123.08
Your computer
147.189.123.108.
2. 147.189.123.109
1. www.xyz.com?
Internet Directory (DNS)
www.xyz.com 147.189.123.109
www.zzz.com 147.190.231.68
...
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Application
• Email
• Example:
mchang @ ie.cuhk.edu.hk
Name of my mail box
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Name of the computer that
collects mails to my dept
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