An Introduction to Internetworking

1
An Introduction to Internetworking
Design and Prog. of Distributed Systems
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Why distributed systems
- Share resources (devices CPU) - Communicate
people (by transmitting data)
3
We know already how computers communicate but...
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... how do programs communicate?
PROG1
PROG2
They need to establish a protocol ! - Who send
the data first - What kind of data - How to
react to the data
5
Remember this ?
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The client-server paradigm(do you remember the
WEB ?)
answer
The web server program
request
THE INTERNET
Web resources
answer
request
The web client program
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1- The server opens a channel and starts
listening to requests.
A SERVER
?
1
THE INTERNET
Web resources
A CLIENT
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2- A client who knows it, sends a request and
waits for the answer
A SERVER
2
THE INTERNET
Web resources
2
A CLIENT
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3- The server, analyses the request and answers
properly according to the protocol
A SERVER
3
THE INTERNET
Web resources
3
This may involve the reading of a file
A CLIENT
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The channel which server and client use to
communicate is called SOCKET
When a server wants to start listening it must
create a socket bound to a port. The port is
specified with a number.
www.thisserver.jp
4444
A SERVER 1
3333
A SERVER 2
A SERVER 3
5555
If a client wants to communicate with server 1
should try to communicate with computer
www.thisserver.jp through port 4444
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Internet two different ways to deliver a
message to another application
Applications programmers decide on this
according to their needs
The UDP User Defined Package like writing a
letter
TCP or UDP
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UDP communication with datagrams
DATAGRAM an independent, self-contained message
sent over the internet whose arrival, arrival
time and content are not guaranteed (like regular
mail in some countries....)
Once a server is listening, the client should
create a datagram with the servers address, port
number and, the message
www.waseda1.jp
www.waseda2.jp
A SERVER
A CLIENT
?
4444
www.waseda1.jp
4444
message
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Sending datagrams with UDP protocol
Then it should open a socket and send the
datagram to the internet. The routing algorithm
will find the way to the target computer
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
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Sending datagrams with UDP protocol
Before the datagram leaves the client, it
receives the address of the originating computer
and the socket number
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
!
3333
4444
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Sending datagrams with UDP protocol
After the datagram is sent, the client computer
may start hearing at the port created for sending
the datagram if an answer from the server is
expected
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
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Sending datagrams with UDP protocol
The server can extract the clients address and
port number to create another datagram with the
answer
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
answer
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Sending datagrams with UDP protocol
Finally is sends the datagram with the answer to
the client. When a datagram is sent there is no
guarantee that it will arrive to the destination.
If you want reliable communication you should
provide a checking mechanism, or use ...
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
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TCP communication with data flow
With TCP a communication channel between both
computers is built and a reliable communication
is established between both computers. This
allows to send a data flow rather tan datagrams.

www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
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TCP communication with data flow
After the client contacts the server, a reliable
channel is established. After this, client and
server may begin sending data through this
channel. The other should be reading this data
They need a protocol !!!!
www.waseda2.jp
www.waseda1.jp
bla
bla
A SERVER
A CLIENT
bla
bla
3333
4444
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TCP How is reliability achieved ?
The internet itself works only with the datagram
paradigm. Internet frames are may get lost
(destroyed) For every frame delivered carrying a
part of the data flow there is a confirmation!
Sending bla bla bla
Sending 1st bla
Ack 1st bla
Sending 2nd bla
Ack 2nd bla
Sending 3rd bla
Ack 3rd bla
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What if a message get lost ?
The server waits a certain amount of time. If it
does not receive any confirmation it sends the
message again.
Sending 1st bla
Sending bla bla bla
Ack 1st bla
Sending 2nd bla
LOST !!!
Sending 2nd bla again
No confirmation !!!
Ack 2nd bla
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When do programmers should use UDP or TCP ?
- TCP generates 6 times more traffic than UDP -
It is also slower to send and receive the
messages
UDP
TCP
- Reliable - Complete - Valid in a certain period
of time - No need of speed
- not complete - fast - valid in a very short
period of time
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Mark with a the applications that need TCP and
with a the applications that can use UDP
Video conference
E-Mail
Web server and client
Stock values every 5 seconds
Temperature every second
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The Multicast paradigm
PROG2
PROG1
PROG2
PROG2
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How do we implement this video-conference ? (why
the problems)
A firewall in the Chilean router does not let
multicast packages go in for security reasons
Rat, vic
Rat, vic
Rat, vic
Router in chile
Rat, vic
A Tunnel makes computers in Accessnova and Waseda
look like being in the same local network
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Attending more than a client The sequential
server
A CLIENT
A SERVER
A CLIENT
4444
A CLIENT
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During the conversation the server is not
listening at the port 444
A CLIENT
A SERVER
A CLIENT
4444
A CLIENT
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Only after the server is ready with the first
client it can listen to the port 444 again
A CLIENT
A SERVER
A CLIENT
4444
A CLIENT
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The service may be to transfer a file. The user
at the client should first send the filename
A CLIENT
A SERVER
A CLIENT
4444
A CLIENT
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What if the server has to wait too much for a
client to type in a file name ?
A CLIENT
A SERVER
Timeout
A CLIENT
4444
A CLIENT
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Concurrent Servers there are separate processes
to attend the port and to transfer the file
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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After the client contacts the server, the server
creates another process to attend the client and
keeps listening to the port 4444 for another
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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While the new process is serving the first
client, the second client can contact the server
at the port 4444
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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And the server creates another process
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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Now the third client contacts the server
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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And a third slave process or thread is created
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT
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Every layer has the illusion to be talking to the
correspondent one in the other application
A CLIENT
The UDP User Defined Package like writing a
letter
Read write sequence
A SERVER
4444
UDP or TCP communication
A CLIENT
Internet frames and addresses
A CLIENT
electric pulses
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There are now a lot of resources between the
application and transport layer which make
distributed programming much easier
Libraries for distributed programming (middleware)
RPC, CORBA, RMI
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For example, the RMI mechanism in JAVA (similar
to CORBA)
Other applications use and share this object
(data)
Creates and publishes a Remote Object