Transcript INE1020 Lecture Notes

Lecture 1: Internet Architecture
 Architecture of the Internet
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Network
Internetwork
Router
Protocol
 Accessing the Internet
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Residential access
Dedicated access
Institutional access
 Communicating over the Internet
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Protocol layering (covers only BASIC idea)
Internet Addressing
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1. What’s the Internet
 The Internet is a collection of networks
connected by interconnecting devices.
 The connecting devices
 are specialized computing devices, e.g. routers
 forward data from one network to another.
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1.1 Computer network
 An
interconnected collection of computers
Client-Server Model
File
Servers
Computer
Network
Workstations
(Local Processing)
Application
Servers
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1.1 Classifications of Networks
 Based on size:
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System/Storage area networks (SAN)
• within the same room
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Local area networks (LAN)
• in a close proximity
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Metropolitan area networks (MAN)
• span a city
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Wide area networks (WAN)
• connecting computers situated anywhere
• Connects many networks together
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1.2 internetwork
 A collection of networks
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1.2 Internetwork: Another example
Figure 15.3(text book) An example of internet with six networks
and three computers attached.
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1.2 The Internet Topology
See http://www.cybergeography.org/atlas/topology.html for more Internet topologies.
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1.2 The Illusion Of A Giant Network
 Any computer can send data to any other
computer providing they have IP software
installed.
 The Internet operates like a virtual network.
Fig. 15.1 (text book)
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1.2 The Reality Of Internal Structure
 The Internet contains a complex physical
structure users never see
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Interconnecting networks with routers
Fig. 15.2 (text book)
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1.2 Internet: a service view
 communication
infrastructure: enables
distributed applications:
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WWW, email, games, ecommerce, database.,
 cyberspace
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1.2 Internet: “nuts & bolts” view
 millions of connected
computing devices: hosts,
end-systems
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workstation
server
mobile
local ISP
communication links
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pc’s workstations, servers
PDA’s
router
regional ISP
fiber, copper, radio,
satellite
routers: forward packets
(chunks of data) across
networks
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company
network
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1.2 Internet: “nuts & bolts” view
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protocols: control sending,
receiving of msgs
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e.g., TCP, IP, HTTP, FTP
Internet: “network of
networks”
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router
workstation
server
mobile
local ISP
public Internet versus
private intranet
collection(s) of SAN, LAN,
MAN, WAN
regional ISP
company
network
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1.2 Connecting a Computer to a Remote
Network
 Cables connecting a
computer to a LAN are
usually short, but may be
extended.
 Modems used to send data
across wire, telephone
system & optical fibres
 Fiber optic cable can provide
connections across long
distances.
Fig 13.1 (text book)
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Requiring optical modems
Using light instead of
electricity to carry data
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1.3 Incompatible Network Technologies
 Not all networks are compatible.
 Networks:
 Vary
in speed
 Vary in electrical voltages
 Differ in addressing scheme
 Each technology designed to meet speed,
dist, cost constraints.
 It is impractical to make computers in an
enterprise use the same network technology.
 Allow groups to select a network technology
based on need
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1.3 Routers
 Special-purpose computers are
used to interconnect networks
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 Fig. 13.2 (text book)
Using standard hardware (CPU,
memory, and network
interfaces)
Running special-purpose
software
 Forwarding packets from one
network to another
 Determining where to send
packets
 Transforms packets as
necessary to meet standards
for each network
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1.3 Routers
 Interconnecting LAN to LAN
 Interconnecting LAN to WAN
Fig 13.3 (text book)
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1.4 Network Protocol
 A common language computers use to
exchange messages.
Specifying exact format and meaning of each
message
 specify actions taken during sending and receiving
of data
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1.4 Network Protocol
a human protocol and a computer network protocol:
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
Get http://gaia.cs.umass.edu/index.htm
2:00
<file>
time
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1.4 Network Protocol
human protocols:
 “what’s the time?”
 “I have a question”
 introductions
… specific messages
sent
… specific actions
taken when messages
received, or other
events
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network protocols:
 machines rather than
humans
 all communication
activity in Internet
governed by network
protocols
protocols define format, order of
messages sent and received
among network entities, and
actions taken on message
transmission, receipt
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1.4 Internet Protocol (IP)
 IP defines computer communication details.
 Specifying
how packets are formed
 Specifying how routers forward each packet
 Computers connecting to the Internet must
follow the IP rules.
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1.4 IP Software On Every Machine
 Computer hardware does not understand IP.
 Connecting
a computer to the Internet does not
mean it can use the Internet
 Computers need IP software before using the
Internet.
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2. Accessing the Internet
 User’s computers or hosts connect to the
Internet through an access network.
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Residential access:
• dial-up access vs. dedicated access.
• dial-up access advantages: least expensive.
• dedicated access advantages: higher speed of delivery
and remains connected at all times.
access
– instantaneous access
network
– continuous availability
The Internet
 Institutional access
• dedicated access.
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network
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2.1. Access Network: Residential access
 Residential access is normally through an ISP
(Internet Service Provider)
 Most ISPs offer dial-up access through a telephonebased system.
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Requires a modem and software that uses the modem.
Speed: up to 56Kbps.
Fig. 14.1
(text book)
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2.2. Access Network: dedicated access
 Three newer technologies used by ISPs to
offer inexpensive dedicated access:
 Cable modems
• use cable television wiring (most homes already have cable TV
wiring).
• Cable modems send data over coax cable but transmission does
not interfere with TV signals
– cable system is designed to carry many more signals than are
currently available, cable modem use the unused bandwidth
• advantages:
– deliver data faster than a dial-up connection.
– provide continuous connectivity.
– use existing wiring system.
• chief disadvantage:
– customers share the wiring (share bandwidth) - data transmission
may slow down if many customer send data at the same time; but
even at the slowest speed it still operate faster than dial-up modem
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2.2. Access Network : dedicated access
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DSL (Digital Subscriber Line) technology
• use conventional telephone wiring
• unlike dial-up modem (which encode data with sound),
DSL does not use the telephone system. Instead DSL
transfer data by using the underlying telephone wires to
send electrical signals
• does not affect normal telephone service
• e.g. ADSL: asymmetric digital subscriber line.
– 6.144 Mbps downstream (data flowing to user), 576Kbps
upstream (data flowing from user).
• advantages
– use existing wiring
– provide continuous connectivity.
– does not share bandwidth with other subscribers (unlike cable
modem)
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2.2. Access Network : dedicated access
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Wireless access similar to cellular phones
• use radio transmissions
• a transmitter runs all the time to to provide continuous
access
• chief advantage is the ability to reach remote areas.
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2.3. Access Network: Institutional access
 Institutional access is mainly through
company/university local area network (LAN).
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E.g. using Ethernet:
• shared cable connects user computers
• speeds: 10 Mbs, 100Mbps, Gigabit Ethernet
Router
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3. Communicating over the Internet
 To communicate over the Internet, the
computers must:
use a common language or a protocol to govern the
exchange of messages.
 have a way to address one another.
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 Protocol:
 specifies exact format, order of messages sent
and received among network entities, and actions
taken on message transmission and receipt.
 Addressing:
 defines where to deliver the messages.
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3.1 Protocol Layering
 Internet has a large collections of protocols
organized in a layering model.
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Application: enables the user, whether human or software,
to access the network.
Transport: responsible for reliable source-to-destination
(end-to-end) delivery of the entire message.
Network: responsible for routing a packet (also called
datagram) from source-to-dest (possibly) across multiple
(different) networks.
Data link (also called network interface): specify how to
organize data into frames and how to deliver a frame over a
network.
Physical: coordinates the functions required to transmit a
bit stream over a physical medium.
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3.1 Protocol Layering
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3.1 Protocol Layering
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3.1 Protocol Layering
 TCP/IP Protocol Suite.
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3.1 Protocol Layering
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Two key protocols are:
• IP (Internet Protocol)
– Oversee end-to-end delivery of individual packets
– Does not recognize relationships among packets
– Treats each packet independently, as if they belong to different
message
• TCP (Transport Control Protocol)
– Ensure whole message arrive intact and in order
– (eg, check if packets are missing or arrive out of order)
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3.1 Router Overrun with Datagrams
 Router becomes overrun with
datagrams
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Eg, routers 1 & 2 each
receives 5000 datagrams
both routers send all
datagrams they received
(5000+5000) across
network d to router 3
but router 3 can only handle
5000 datagrams
Analogy: traffic jam
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Eg, cars from roads a and
b attempt to merge into
road d, causing traffic
congestion
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3.1 Transmission Control Protocol
 When a router is overrun with datagrams, some of them will be
discarded.
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As a result some datagrams will be lost
TCP checks for lost datagrams
 When hardware in a router or network system fails, other
routers start sending datagrams through new (alternative)
paths.
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As a result, some datagrams arrive in a different order
than they were sent
TCP checks for out-of-order datagrams
 Network hardware failure sometimes result in duplication of
datagrams
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TCP checks for duplication of datagrams
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3.1 Recovering Lost Datagrams: TCP
Retransmission
 TCP includes an identification of each datagram.
 Ignores duplicate copies
 Receiver uses identification (sequence numbers) to
put out-of-order datagram back in order
 Recovers lost datagrams
 Uses timers
 Sends an acknowledgement
back to the source - this guarantees that all data
arrives
 If timer expires before an acknowledgement
arrives, TCP assumes the datagram is lost and
retransmit the datagram
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3.2 Internet Addresses
 Every Computer Is Assigned A Unique
Address
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Each computer attached to the Internet must be
assigned a unique address.
 IP Addresses Are Not Random
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Computers on the same network have the same
prefix (Netid).
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3.2. Internet Addressing
 IP or Internet address is a 32-bit (4 byte)
address that uniquely defines every
computing devices on the Internet.
 Decimal notation to make it easier to read.
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3.2 Classes of IP addresses
 Designers chose a compromise - multiple address formats
that allow both large and small prefixes
 Each format is called an address class
 Class of an address is identified by first four bits
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3.2 Dotted decimal notation
 Class A, B and C all break between prefix and suffix on byte
boundary
 Dotted decimal notation is a convention for representing 32-bit
internet addresses in decimal
 Convert each byte of address into decimal; display separated by
periods (``dots'')
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3.2 IP Address classes at a glance
 Class A, B and C are primary classes
 Used for ordinary host addressing
 Class D is used for multicast, a limited form of broadcast
 Class E is reserved
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3.2. Internet Addressing: An Example
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Further Reading
 Chapters 13, 14, 15, & 16 of the textbook.
 Note: This lecture is designed with the objective
of providing an introduction to the architecture of
the Internet and communicating over the internet.
 Details of these topics are beyond the scope of
this course and will NOT be taught or discussed.
Students who wish to invest more time on studying
more details for these topics are referred to:
 Forouzan Chapter 2.
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Doug Comer, Computer Networks & Internets with Internet
Applications, chapter 17, 18, 20 & 24
Details of protocol layering will be covered in IEG3310
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