Chapter 1
Introduction
COMPUTER NETWORKS
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History
Definitions
Computer networks
Distributed systems
COMPUTER NETWORKS
History
- Key technology = Information gathering, processing, distribution
- Computer systems - highly centralized
- Merging of Computers and Communications
- computer networks
Definition
A computer network consists of computers, printers and other
equipments that are connected together so they can communicate
with each other.
COMPUTER NETWORKS
Computer networks – a collection of autonomous
computers that can exchange information
The connection:
- copper wire
- fiber optics
- microwaves
- infrared
- communication satellites
Networks come in many sizes, shapes and forms.
DISTRIBUTED SYSTEMS
In a distributed system, a collection of independent
computers appears to its users as a single coherent
system
Has a single model or paradigm that it presents to its
users.
Expl: the World Wide Web
Conclusion: A distributed system is a software system
built on the top of a network.
Uses of Computer Networks
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Business Applications
Home Applications
Mobile Users
Social Issues
Business Applications
• Initial applications
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To monitor production
To keep track of inventories
To do payroll
• Resources sharing
• Sharing information
Business Applications of Networks
A network with two clients and one server.
Business Applications of Networks (2)
The client-server model involves requests and replies.
Home Network Applications
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Access to remote information
Person-to-person communication
Interactive entertainment
Electronic commerce
Access to remote information
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Surfing the WEB for remote information (arts, business, government,
history, hobbies, recreation, science, sports, travel,
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or just for fun: comes in too many ways to mention, plus some ways
that are better to left unmentioned
Many newspapers (magazines and scientific journals) have gone on line
and can be personalized ( tell a newspaper that you want everything about
corrupt politicians)
The on-line digital library: many professional organizations (ACM, IEEE
Society) have journals and conference proceedings on-line, the book sized
notebook computer is a reality!!!!!!!!!!!
NOTE: All of these applications involve interactions between a person and
a remote data base full of information.
Person-to-person communication
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Basically the21 century answer to the 19th centuries' telephone
E-mail is used by millions of people. It contains audio, video, as well as
text and pictures.
Instant messaging: allows two people to type messages at each other in real
time
Chat room: a multiperson version
Worldwide news groups: with discussions on conceivable topic
Peer-to- peer communication
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in this form individuals who form a loose group can communicate
with others in the group.
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Every person can communicate with one or more other people
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There is no fixed division into clients and servers
Home Network Applications (2)
In peer-to-peer system there are no fixed clients and servers.
Person-to-person communication
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The next generation peer-to- peer systems eliminates the central database
by having each user maintain his own database locally as well as providing
a list of other nearby people who are members of the systems. The lookup
process can be repeated indefinitely.
Other communication-oriented application include using the Internet to
carry telephone-calls, video phone and internet radio. Telelearning
Legal applications for peer-to-peer communications:
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fans sharing public domain music, families sharing photos, e-mail is
also a peer-to peer application.
Interactive entertainment
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A huge and growing industry
Important application:
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video on demand, live television
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Game playing; if the games are played with goggles and threedimensional real-time we have a kind of worldwide shared virtual
reality
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Electronic commerce
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Home shopping is already popular and enables users to inspect the on-line
catalogs of thousand of companies. It provides a lot of abilities:
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Instant video of any product
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On-line technical support
Access to financial institutions:
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Manny people already pay their bills, manage their bank accounts and
handle their investments electronically
Electronic flea markets
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On line auctions of second-hand goods have become a massive
industry. They are more of a peer-to-peer system, sort of consumer-toconsumer.
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Some of these forms e-commerce have acquired cute little tags based
on the fact that “to” and “2” are pronounced the same.
Home Network Applications (3)
Some forms of e-commerce.
Mobile Network Users
Combinations of wireless networks and mobile computing.
Network Hardware
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Local Area Networks
Metropolitan Area Networks
Wide Area Networks
Wireless Networks
Home Networks
Internetworks
Network hardware
Types of transmission technology
• Broadcast links –single comm channel
shared by all the machines
• Point-to-point links – many
connections between individual pairs
of machines- multiple routes
Scale
Broadcast Networks (2)
Classification of interconnected processors by scale.
Broadcast Networks (2)
Classification of interconnected processors by scale
OBS
Distance is important as a classification metric because
different techniques are used at different scales.
Local Area Networks
Privately owned networks within a single building or campus
of up to a few km in size.
Classification criterion
(1) their size
(2) the transmission technology
(3) their topology
(1) The worst case transmission time is bounded
(2) A cable to witch all machines are attached
(3) Bus
Ring
Local Area Networks
Two broadcast networks
(a) Bus
(b) Ring
Local Area Networks
(a) Bus
Expl: IEEE 802.3, called Ethernet is a bus based broadcast
network with decentralized control operating at
10Mbps to 10Gbps
(b) Ring
Expl: IEEE 802.5 (the IBM taken-ring) is a ring based
LAN operating at 4 and 16 Mbps.
Broadcast Networks
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Static allocation:
- time division into discrete intervals (slots)
- use a round robin algorithm
- wastes channel capacity
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Dynamic allocation:
– Centralized:
- a single entity (bus administration unit)
– Decentralized:
- each machine decides for itself to transmit
- many algorithms exists to bring order
Metropolitan Area Networks
A metropolitan area network based on cable TV.
Wide Area Networks
Relation between hosts on LANs and the subnet.
Wide Area Networks
Relation between hosts on LANs and the subnet
Separation of the pure communication aspects of the network
from the applications aspects ( the hosts) greatly simplifies the
network design
The subnet elements:
- transmission lines ( copper wire, optical fiber,
radiotrans. lines)
- switching elements – specialized computers
Wide Area Networks
Packet – switched network
- all packets from a given message must follow the same route
- each packet is routed separately – routing decisions are made
locally
A subnet organized according to this principle is called a store-and
– forward or packet switched network
A satellite system
- each router has an antenna
- satellite networks are inherently broadcast
Wide Area Networks (2)
The principle of PS WANs
A stream of packets from sender to receiver.
Wireless Networks
Categories of wireless networks:
• System interconnection networks
• Wireless LANs
• Wireless WANs
Wireless Networks (2)
(a) System interconnection-Bluetooth configuration
(b) Wireless LAN
Wireless Networks (3)
1) System interconnection networks
- Ex: Bluetooth, a short range wireless network
- Uses the master-slave model
- the master impose: what address to use
- when they can broadcast
- how long they transmit
- what frequencies they can use
Wireless Networks (4)
2) Wireless LANs
- Every computer has a radio modem and antenna
- Uses:
- in small offices
- in older buildings
- conference rooms
Standard for wireless LANs: IEEE 802.11
Wireless Networks (5)
3)Wireless WANs:
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Low-bandwidth wireless system
Ex: The radio network used for cellular tel.
- Operate at rate up to 50Mbps
- Over distances of tens of meters
• Cellular systems operate below 1Mbps
• The distance between the BS and the computer or telephone
is measured in km
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High bandwidth wide area wireless networks:
- high-speed Internet
- local multipoint distribution service
- Standard: IEEE 802.16
Wireless Networks (6)
(a) Individual mobile computers
(b) A flying LAN
Home Network Categories
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Computers (desktop PC, PDA, shared peripherals
Entertainment (TV, DVD, VCR, camera, stereo, MP3)
Telecomm (telephone, cell phone, intercom, fax)
Appliances (microwave, fridge, clock, furnace, airco)
Telemetry (utility meter, burglar alarm, babycam).
Internetworks
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Def: A collection of interconnected networks is called an
internetwork or internet
– Expl: a collection of LANs connected by a WAN
Subnet
WAN
Internetworks (2)
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Subnet – networks – internetworks : differences
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Subnet make sense in the context of wide area network =
collection of routers and communication lines
Subnet + hosts = network
Internetwork = Connecting a LAN and a WAN
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= Connecting 2 LANs
Network Software
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Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and Connectionless Services
Service Primitives
The Relationship of Services to Protocols
Protocol Hierarchies
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Stack of layers or levels
The name, the no., the content, the function – differ
from network to network
Each layer – services – higher layers
Each layer = virtual machine
Layer n (M1) ↔ Layer n (M2) = the rules and
conventions = PROTOCOL
• A protocol is an agreement between the
communicating parties on how communications
is to proceed
Protocol Hierarchies (2)
A five layers network
Layers, protocols, and interfaces.
Protocol Hierarchies (3)
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Peers: » Processes
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» Hardware devices
» Human beings
Communicate using the protocol
No data are transferred from layer n (M1) to layer n (M2)
Data and information control are passed to the layer below
Interfaces – between adjacent layers
• defines which primitive operations and services the lower
layer makes available to the upper one
Protocol Hierarchies (4)
• Defining clean interfaces between layer
• Each layer perform a specific collection of well
understood functions
• Well designed interfaces make simple to replace the
implementation of one layer
Def: A set of layers and protocols is called a network
architecture
Specifications of an architecture:
- to write the program
- to build the hardware for each layer
A list of protocols used by a certain system, one protocol
per layer is called a protocol stack
Protocol Hierarchies (5)
Example information flow supporting virtual communication in layer 5.
Design Issues for the Layers
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Addressing
Data transfer
Error Control
Flow Control
Multiplexing
Routing
Connection-Oriented and Connectionless
Services
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Connection oriented (co)
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Connectionless service (cs)
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Modeled after the telephone system
Modeled after postal system
Quality of service: (co), (cs)
Reliable services
Connection-Oriented and Connectionless
Services
Six different types of service.
Service Primitives (2)
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A service is specified by a set of primitives
(operations) available to a user to accesses the service
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Modeled after the telephone system
Protocol stack in the operating system – systems calls
Primitives depends on the service being provided
Service Primitives
Five service primitives for implementing a simple connectionoriented service.
Service Primitives (3)
Packets sent in a simple client-server interaction on a
connection-oriented network.
Services to Protocols Relationship
The relationship between a service and a protocol.
Reference Models
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The OSI Reference Model
The TCP/IP Reference Model
A Comparison of OSI and TCP/IP
A Critique of the OSI Model and Protocols
A Critique of the TCP/IP Reference Model
Reference Models (2)
The OSI
reference
model.
Reference Models (3)
Network layer – Controls the operation of the subnet
- Packets routes:
- based on static tables
- determined at the start of each conversation
- highly dynamic – determined for each packet reflect the network load
- Controls congestion
- Provides Qos (delay, transit time, jitter,…)
- Allows heterogeneous networks to be interconnected
Note In broadcast networks routing problem is simple → network
layer is thin or nonexistent
Reference Models (4)
Transport layer – Functions
- Accepts data from above (session layer)
- Split it into smaller units
- Pass these to the network layer and
- Ensure that the pieces arrive correctly at the other end
- Determines what type of service to provide to the session
layer and to the users
- error-free channel that delivers messages or bytes
in order in which they are sent
- transports isolated messages
- broadcasts messages to multiple destinations
Note End-to-end layer
Reference Models (5)
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Arpanet – DoD (USA), Internet
TCP/IP Reference Model
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1974 – Cerf and Kahn
1985 Leiner & al.
1988 Clark
Connections to remain intact as long as source and destination
machines are functioning
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A flexible architecture
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Transferring files
Real-time speech transmission
Reference Models (6)
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Solution: Packet-switching network based on a connectionless
internetwork layer called INTERNET LAYER
Internet (generic sense) layer (TCP/IP) – function:
– Permits hosts to inject packets into any network and have
them travel independently to the destination (potentially)
on a different network
Analogy with the mail system
Defines an official packet format and protocol, called IP
(Internet Protocol) – Packet routing is the major issue here
Similar in functionality to the network layer in OSI model
Reference Models (7)
The TCP/IP reference model.
Reference Models (8)
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Transport layer (TCP/IP) – function: allow peers entities on
the source and destination hosts to carry on a conversation
End-to end protocols:
– TCP (Transmission Control Protocol)
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Reliable connection oriented
Handles flow control
UDP ( User Datagram Protocol)
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Unreliable connectionless protocol
For applications without TCP/IP sequencing and TCP/IP flow
control
Applications in which prompt delivery is much important than
accurate delivery
Reference Models (9)
Protocols and networks in the TCP/IP model initially.
Reference Models (10)
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Application layer (TCP/IP) – contains all higher level
protocols
– Early included protocols:
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Added later protocols:
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TELNET – virtual terminal
FTP - file transfer
SMTP – electronic mail
DNS – Domain name system
NNTP
HTTP
Host – to – Network layer
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Host has to connect to the network using some protocol
Protocol used is not defined, varies from host to host and network to
network
Comparing OSI and TCP/IP Models
In common:
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Stack of independent protocols
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Functionality of layers roughly similar
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Transport layer (transport providers) provide an end-to-end
network independent transport service
The layers above transport are application-oriented users of
the transport services
Many differences between the two models and not between
the protocols
Comparing OSI and TCP/IP Models(2)
Concepts central to the OSI model
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Services
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Interfaces
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Protocols
OSI model contribution:
Makes distinction between these three concepts explicit
Comparing OSI and TCP/IP Models (3)
TCP/IP did not originally distinguish between
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Services
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Interfaces
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Protocols – came first and the model was just a
description of the existing protocols
The model did not fit any other protocol stacks
More specific and obvious differences
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The number of layers
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OSI - 7 layers
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TCP/IP - 4 layers
Network, transport, application layers included in both models
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The area of connectionless versus conn. - oriented
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OSI
• supports both c-l and c-o communication in the network
layer
• Only c – o communication in the transports layer
TCP/IP
• Only c-l communication in the network layer
• C-l and c-o communication in the transport layer- giving
the users a chance – important for simple request-response
protocols
A Critique of the OSI Model and Protocols
Why OSI did not take over the world
• Bad timing
• Bad technology
• Bad implementations
• Bad politics
A Critique of the OSI Model and Protocols(2)
The time at which a standard is established is critical to its
success.
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The amount of activity surrounding a new subject (
burst of research activity)
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Standards elaboration
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Corporations discover the subject and make investments
Note
– The OSI standards protocols got crushed
– The TCP/IP protocols were already in widespread
use by research universities by the time the OSI
protocols appeared
A Critique of the OSI Model and Protocols(3)
D. Clark’s theory of standards: the apocalypse of the two elephants.
A Critique of the OSI Model and Protocols (4)
Bad technology
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The choice of 7 layers was more political then technical
Extraordinarily complex
Some functions reappear in each layer
Recommandation
– Error control must be done in highest layer
A Critique of the OSI Model and Protocols (5)
Bad implementations
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The initial implementations were huge, unwieldy and
slow
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People associate OSI with poor quality .
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TCP/IP was part of Berkeley UNIX and was quite good
( not to mention free). People began using it quickly.
Bad politics
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Many people, especially in academia thought TCP/IP as
part of UNIX.
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OSI was widely thought to be creature of the European
telecommunication ministries, European Community
and later U.S. Government
A Critique of the TCP/IP Reference Model
Problems:
• Service, interface, and protocol not distinguished
• Not a general model
• Host-to-network “layer” not really a layer
• No mention of physical and data link layers
• Many protocols were ad hoc produced – now they
are hard to replace
Conclusions
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The OSI model (minus the session and presentation layers)
has proven to be exceptionally useful for for discussing
computer networks
OSI protocols have not become popular
The TCP/IP model is practically nonexistent
Protocols are widely used
In this course
– We will use a modified OSI model
– Concentrate on TCP/IP and related protocols
Hybrid Model
The hybrid reference model to be used in this course.
Example Networks
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The Internet
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Connection-Oriented Networks:
X.25, Frame Relay, and ATM
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Ethernet
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Wireless LANs: 802:11
The ARPANET
(a) Structure of the telephone system.
(b) Baran’s proposed distributed switching system.
The ARPANET (2)
The original ARPANET design.
The ARPANET (2)
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Subnet
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IMPs (Interface Message Processors) – minicomputers
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Connected by 56kbps transm. lines
Reliability – at least 2 connections/IMP
Datagram subnet
Network nodes
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IMP + host in the same room
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Messages (host → IMP) at up to 8063 bits
Software
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Subnet software
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The IMP-end of the HOST-IMP connection
The IMP-IMP protocol
A source IMP to destination IMP ( for reliability)
Outside the subnet software
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The Host -end of the HOST-IMP connection
The HOST-HOST protocol
The application software
ARPANET, NSFNET
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TCP/IP was created to handle communications over
internetworks
A convenient program interface to the network – sockets
DNS (Domain Name System) was created to organize
machines into domains and map hosts names into addresses. It
became a generalized distributed database system for storing
information related to naming
NSFNET:
– Consists of a backbone network and the regional networks.
– Was connected to the ARPANET
ANS (Advance Network and Services)
ANSNET-1990 – NAP ( Network Access Point)
NSFNET
The NSFNET backbone in 1988.
Internet Usage
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The number of networks, machines and users connected to the
ARPANET grew rapidly after TCP/IP became the only
official protocol on 1983. When NSFNET and ARPANET
were interconnected the growth became exponential.
the glue that holds the Internet together is the TCP/IP
reference model and the TCP/IP protocol stack.
Traditional applications (1970 – 1990)
E-mail
News
Remote login
File transfer
New one: www
Architecture of the Internet
Overview of the Internet.