1. An Introduction to Internetworking
Design and Prog. of Distributed Systems

2. Why distributed systems
- Share resources
(devices & CPU)
- Communicate people
(by transmitting data)

3. We know already how computers communicate but...

4. ... how do programs communicate?
They need to establish a protocol !
- Who send the data first
- What kind of data
- How to react to the data

5. Remember this ?

6. 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

7. 1- The server opens a channel and starts listening to requests.
A SERVER ? 1
THE INTERNET
Web
resources
A CLIENT

8. 2- A client who knows it, sends a request and waits for the answer
A SERVER 2
THE INTERNET
Web
resources 2
A CLIENT

9. 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

10. 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.
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 through port 4444

11. 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

12. 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 server’s address, port number and, the message
A SERVER
A CLIENT ?
4444
4444
message

13. 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
A SERVER
A CLIENT ?
3333
4444

14. Sending datagrams with UDP protocol
Before the datagram leaves the client, it receives the address of the originating computer and the socket number
A SERVER
A CLIENT !
3333
4444

15. 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
A SERVER ?
A CLIENT
3333
4444

16. Sending datagrams with UDP protocol
The server can extract the client’s address and port number to create another datagram with the answer
A SERVER ?
A CLIENT
3333
4444
answer

17. 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 ...
A SERVER ?
A CLIENT
3333
4444

18. 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.
A SERVER
A CLIENT ?
3333
4444

19. 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 !!!!
bla
bla
A SERVER
bla
A CLIENT
bla
3333
4444

20. 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

21. 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

22. 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

23. Mark with a + the applications that need TCP and with a = the applications that can use UDP
Video conference
Web server and client
Stock values every 5 seconds
Temperature every second

24. The Multicast paradigm
PROG2
PROG1
PROG2
PROG2

25. 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

26. Attending more than a client: The sequential server
A SERVER
A CLIENT
4444
A CLIENT

27. During the conversation the server is not listening at the port 444
A CLIENT
A SERVER
A CLIENT
4444
A CLIENT

28. 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

29. The service may be to transfer a file
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

30. What if the server has to wait too much for a client to type in a file name ?
A SERVER
Timeout
A CLIENT
4444
A CLIENT

31. Concurrent Servers: there are separate processes to attend the port and to transfer the file
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT

32. 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

33. 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

34. And the server creates another process
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT

35. Now the third client contacts the server
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT

36. And a third slave process or thread is created
A CLIENT
A SERVER
4444
A CLIENT
A CLIENT

37. Every layer has the illusion to be talking to the correspondent one in the other application
A CLIENT
A SERVER
The UDP: User Defined Package: like writing a letter
Read write sequence
4444
UDP or TCP communication
A CLIENT
Internet frames and addresses
A CLIENT
electric
pulses

38. 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

39. For example, the RMI mechanism in JAVA (similar to CORBA)
Other applications use and
share this object (data)
Creates and publishes
a Remote Object