The client-server paradigm (do you remember the WEB ?) The web server program Web resources request answer THE INTERNET request answer The web client program
1- The server opens a channel and starts listening to requests. A SERVER Web resources THE INTERNET A CLIENT 1 ?
2- A client who knows it, sends a request and waits for the answer A SERVER Web resources THE INTERNET A CLIENT 2 2
3- The server, analyses the request and answers properly according to the protocol A SERVER Web resources THE INTERNET A CLIENT 3 3 This may involve the reading of a file
The channel which server and client use to communicate is called SOCKET A SERVER 1 When a server wants to start listening it must create a socket bound to a port. The port is specified with a number. A SERVER 2 A SERVER 3 www.thisserver.jp 4444 3333 5555 If a client wants to communicate with server 1 should try to communicate with computer www.thisserver.jp through port 4444
Internet : two different ways to deliver a message to another application The UDP: User Defined Package: like writing a letter TCP or UDP Applications’ programmers decide on this according to their needs
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....) A SERVER A CLIENT 4444 www.waseda1.jp message 4444 Once a server is listening, the client should create a datagram with the server’s address, port number and, the message www.waseda2.jp ?
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 4444 www.waseda1.jp 3333 www.waseda2.jp ?
Before the datagram leaves the client, it receives the address of the originating computer and the socket number A SERVER A CLIENT 4444 www.waseda1.jp 3333 www.waseda2.jp ! 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 4444 www.waseda1.jp 3333 www.waseda2.jp ?
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 4444 www.waseda1.jp 3333 www.waseda2.jp answer ?
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 4444 www.waseda1.jp 3333 www.waseda2.jp ?
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 4444 www.waseda1.jp 3333 www.waseda2.jp ?
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 !!!! A SERVER A CLIENT 4444 www.waseda1.jp 3333 www.waseda2.jp bla
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
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 bla bla bla Sending 1st bla Ack 1st bla Sending 2nd bla Sending 2nd bla again Ack 2nd bla No confirmation !!! LOST !!!
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 - Reliable - Complete - Valid in a certain period of time - No need of speed UDP TCP - not complete - fast - valid in a very short period of time
Mark with a + the applications that need TCP and with a = the applications that can use UDP E-Mail Video conference Temperature every second Web server and client Stock values every 5 seconds
How do we implement this video- conference ? (why the problems) Rat, vic Router in chile A Tunnel makes computers in Accessnova and Waseda look like being in the same local network A firewall in the Chilean router does not let multicast packages go in for security reasons
Attending more than a client: The sequential server A SERVER A CLIENT 4444
During the conversation the server is not listening at the port 444 A SERVER A CLIENT 4444
Only after the server is ready with the first client it can listen to the port 444 again A SERVER A CLIENT 4444
The service may be to transfer a file. The user at the client should first send the filename A SERVER A CLIENT 4444
What if the server has to wait too much for a client to type in a file name ? A SERVER A CLIENT 4444 Timeout
Concurrent Servers: there are separate processes to attend the port and to transfer the file A SERVER A CLIENT 4444
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 SERVER A CLIENT 4444
While the new process is serving the first client, the second client can contact the server at the port 4444 A SERVER A CLIENT 4444
And the server creates another process A SERVER A CLIENT 4444
Now the third client contacts the server A SERVER A CLIENT 4444
And a third slave process or thread is created A SERVER A CLIENT 4444
Every layer has the illusion to be talking to the correspondent one in the other application A SERVER A CLIENT 4444 The UDP: User Defined Package: like writing a letterRead write sequence UDP or TCP communication Internet frames and addresses electric pulses
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
For example, the RMI mechanism in JAVA (similar to CORBA) Creates and publishes a Remote Object Other applications use and share this object (data)