Transcript Distributed Computing

Distributed Computing
Networking
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Master-slave:
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A large, central computer controls small
devices
I/O
Peer-to-peer
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Allows arbitrary communication between
computers;
Does not distinguish between connected
computers;
An ex: Internet
Distributed computing---any interaction among
computers that share access to a peer-to-peer
network.
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Distributed computing on the
Internet
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E-mail
FTP
WWW
Buy a book from Amazon
Print a document
Transmit a fax
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Client-Server Paradigm
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The scheme used by all types of distributed
applications;
Server:
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a computer that offers services;
Ex: Web server, e-mail server, ftp …
Client: a computer that uses these services;
A client addresses a request to a server which
replies accordingly;
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Client-Server Paradigm (cont.)
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Client and Server have to use the same protocol;
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TCP/IP for communication over the Internet;
HTTP for Web page transfer;
The server has to be running before the client
contacts it;
A server-computer usually runs several copies of the
same program
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A computer may run more than one program at the same
time
The operating system switches between programs
 a server may provide services to several clients
simultaneously
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Client programs
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Is an arbitrary application program that becomes a
client temporarily when remote access is needed, but
also performs other computation locally
Is invoked directly by a user, and executes for one
session;
Runs locally on a user personal computer
Initiates contact with a server
Can access multiple services as needed, but actively
contacts one remote server at a time.
Does not require special hardware, or sophisticated
operating system
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Server programs
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Is a special-purpose program, dedicated to
providing one service
Can handle several clients at the same time;
Is invoked automatically when the system
boots, and continues to execute;
Runs on a shared computer ( not personal)
Waits passively for contact from arbitrary
remote clients;
Requires powerful hardware and a
sophisticated operating system
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Names for computers
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each computer is assigned a unique address,
called Internet address or IP address;
Each address is a 32-bit binary number;
each address is divided into two parts: a
prefix and a suffix;
Prefix: identifies the physical network to
which the computer is attached;
Suffix: identifies each computer attached to
that network;
Ex: 128.6.25.4; 207.189.97.220
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Names for computers (cont.)
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IP addresses are difficult to remember and
use
Instead, people prefer alphabetic, mnemonic
names for computers
Internet allows:
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A user to name its machine;
A user to enter a computer name instead of an IP
address;
And provides a service that automatically
translates between a name and an address
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Names for computers (cont.)
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May denote
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the type of computer: pc1,pc2,..
functionality: www, mail, router, admin,…
name of the owner: john, jane,…
Characters from comic strips: calvin,…
Roman gods: andromeda, pegasus,…
However: each computer must have a
unique name
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Making names unique
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The full name of a computer consists of
its local name and a suffix, denoting the
company;
Ex: rbs.rutgers.edu,
www.barnesandnoble.com
IN USA, the last part of the name
identifies the type of the company;
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Structure of computer names
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Internet naming scheme is called: Domain Name System (DNS)
A DNS name is formed by a sequence of names separated by
periods; ex: andromeda.rutgers.edu
DNS specifies values for the most significant segment, called
top-level DNS;
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Getting a domain name
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Each organization must apply for a name
under one of the existing top-level domains;
Ex:
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Foobar company might apply for foobar under
top-level domain com;
If approved Foobar Corporation will be assigned
the domain foobar.com;
A computer called www, inside Foobar
Corporation, will have the name: www.foobar.com
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Domain names within an
organizations
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DNS does not specify
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how many segments in a name;
What each segment represent
 it is up to each organization to choose how many
segments and what they denote;
Large organizations, having many computers may
introduce additional hierarchical structure;
EX: consider that Foobar has two divisions: one that
makes candy bars, and one for soap.
Then a computer john, in soap division will have the
full name of john.soap.foobar.com
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Domains names outside US
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Most countries append the 2 letter
country code to each domain name;
Ex: interop.co.jp, minster.york.ac.uk
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Translating a name to an
equivalent IP address
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A name must be translated into an IP address
before computers can communicate;
Each organization has a domain name server
that maintains the list of all computers in
that organization and their IP address;
When an application needs to translate an
address:
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Contacts the DNS
Sends it the name
The DNS responds with the IP address
an example of client-server paradigm
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Domain Name Servers
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A computer needs to know the location
of one DNS;
If the local DNS does not store the IP
address of the name given, it contacts
other DNSs , until it finds the response;
The client always receives the answer
from the local DNS.
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Conclusion
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IP addresses: 4 numbers separated by
periods; ex: 128.10.25.21
Domain name: strings separated by
periods: mine.rbs.rutgers.edu
However, there is no connection
between the name and the digits
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