U de chile santiago sem 2011 2 nelson baloian teaching assistant jonathan frez
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U de Chile, Santiago, Sem. 2011-2 Nelson Baloian, Teaching assistant: Jonathan Frez. Programming distributed application using TCP/IP. Content. 0. Introduction (concepts of distributed systems) 1. TCP/IP client & server programming Client programming: a simple client (date, echo)

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U de chile santiago sem 2011 2 nelson baloian teaching assistant jonathan frez

U de Chile, Santiago, Sem. 2011-2

Nelson Baloian,

Teaching assistant: Jonathan Frez

Programming distributed application using TCP/IP


Content

Content

0. Introduction (concepts of distributed systems)

1. TCP/IP client & server programming

  • Client programming:

    • a simple client (date, echo)

    • a pop3 client

    • A SMTP client

      2. Server programming (and their clients)

    • Simple client-server example with serializing example

    • File Servers: simple whole file iterative server (not secure)

    • Simple whole file robust server

    • Simple whole file concurrent server

    • Stateless random access file server

    • TCP/IP Chat with awareness

    • A simple extensible web server

    • Parallel downloading techniques

    • Awareness in a TC/IP peer to peer environment and the latecomers problem


Content1

Content

3. UDP programming

  • simple UDP client-server example

  • a "ping" program –

  • multicasting

  • multicasting chat

  • awareness in a multicasting environment

  • broadcasting vs. multicasting

    4. RMI Client-server programming

  • a simple example will be used to show: remiregistry, concurrency automatic stub distribution

  • a sequential file server with state

  • Automatic teller machine example

  • RMI-based chat with awareness

    5- Introduction to servlets and jsp

  • principles - parameters (from request and parameter file)

  • using forms

  • implementing state with cookies/sessions

    6- JDBC


Evaluation

Evaluation

  • Attending to classes (at least 80%)

  • 2 Controles (1 al terminar el capitulo UDP, 2 en semana antes de que se acabenlasclases

  • Participation in classes

  • Our webpage at http://133.9.108.158/~nbaloian/

  • Jonathan will also assist you in every doubt you may have regarding assignments


Schedule

Schedule

  • Mondays and Wednesdays from 2:00pm to 3:15


How to follow the lectures

How to follow the lectures

  • During classes:

    • Download and run the examples by yourself

    • Keep the programs in your computer and write comments (many do not have any)

    • Follow the class: I will ask you to tell me what do the programs do

  • After classes:

    • Read the lecture again,


Why distributed systems

Why distributed systems

- Share resources (25 years ago)

- Communicate people (now)

  • Performance, scalability (always)

  • Fault tolerant systems (always)


Which distributed programs do i use daily

Which distributed programs do I use daily ?

1- Chat

2- email

3- p2p file sharing

4- web browser-server

5- database software

6- file server


Can we deduce how were they developed

Can we deduce how were they developed ?

1- Programming language and resources used

2- Connection style

3- Communications architecture

4- Software architecture

5- Server design (if any)


Internet two different ways to deliver a message to another application

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


Every layer has the illusion of talking to the same one located at the other host

A CLIENT

A SERVER

A CLIENT

A CLIENT

Every layer has the illusion of talking to the same one located at the other host

The UDP: User Defined Package: like writing a letter

Read write sequence

4444

UDP or TCP communication

Internet frames and addresses

electric

pulses


Implementation of communications in a tcp ip network

Implementation of Communications in a TCP/IP Network

  • At a low level (¿future “assembler of the communications”?)

  • Based on the “sockets” & “ports” abstractions

  • Originally developed for BSD UNIX but now present in almost all systems (UNIX, LINUX, Macintosh OS, Windows)

  • The destination of a message is determined by the computer’s IP number and the port number

  • Every machine has 2**16 ports

  • The origin of the message is also a socket but most of the times the port number is not important

  • Ports are associated to services (programs)


The 3 basic communication forms

The 3 basic communication forms

  • UDP communication reflects almost what really happens over the internet. An application sends a packet trough a socket addressed to a certain IP number and port. There should be another application on that host listening to packets coming to that port (which is agreed beforehand)

  • TCP simulates a data flow. A client must establish a communication with the server before starting sending/receiving data. The server must be waiting for such request.

  • Multicast fits well for group communication when the group is not well defined beforehand (spontaneous networking). It is also based in the sending of UDP packages but all “interested” applications may receive it. It does not require a central server


Programming distributed application using tcp ip

Protocolos for communication

  • Every service is normally identifyed by a port

    • Web: HTTP (port 80)

    • Mail: SMTP (port 25)

    • File transfer protocol: FTP (21)

    • telnet: 22/23More common services ports at:http://www.chebucto.ns.ca/~rakerman/port-table.html

  • Servers with/without Connection

    • connectionless style: UDP

    • connection-oriented style TCP


The socket

A SERVER 1

A SERVER 2

A SERVER 3

The 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.informatik.de

4444

3333

5555

If a client wants to communicate with server 1 should try to

communicate with computer www.informatik.de through port 4444


Udp communication with datagrams

A CLIENT

A SERVER

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

www.informatik.de

www.waseda2.jp

?

4444

www.waseda1.jp

4444

message


Sending datagrams with udp protocol

A CLIENT

A SERVER

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.informatik.de

?

3333

4444


Programming distributed application using tcp ip

Sending datagrams with UDP protocol

A CLIENT

A SERVER

Before the datagram leaves the client, it receives the address of the originating computer and the socket number

www.waseda2.jp

www.informatik.de

!

3333

4444


Sending datagrams with udp protocol1

A CLIENT

A SERVER

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.informatik.de

?

3333

4444


Sending datagrams with udp protocol2

A CLIENT

A SERVER

Sending datagrams with UDP protocol

The server can extract the client’s address and port number to create another datagram with the answer

www.waseda2.jp

www.informatik.de

?

3333

4444

answer


Sending datagrams with udp protocol3

A CLIENT

A SERVER

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.informatik.de

?

3333

4444


Tcp communication with data flow

A CLIENT

A SERVER

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.informatik.de

?

3333

4444


Tcp communication with data flow1

A CLIENT

A SERVER

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.informatik.de

bla

bla

bla

bla

3333

4444


Tcp how is reliability achieved

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 blabla

Sending 1st bla

Ack 1st bla

Sending 2nd bla

Ack 2nd bla

Sending 3rd bla

Ack 3rd bla


What if a message get lost

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 blabla

Ack 1st bla

Sending 2nd bla

LOST !!!

Sending 2nd bla again

No confirmation !!!

Ack 2nd bla


The window for improving efficiency

The Window for improving efficiency

The transmitter will handle a set of not acknowledged packets

Sending 1st bla

Sending 2nd bla

Sending 3rd bla

Ack 1st bla

Ack 2nd bla

Ack 3rd bla


Programming distributed application using tcp ip

TCP or UDP Protocol: decision at the transport level

  • What does it means for the programmer/designer:

    • By choosing one or the other protocol for establishing a connection between machines the programmer/designer decides about the reliability and speed of the communication.

      • TCP provides high reliability: data are only sent if the communication was established. An underlying protocol is responsible for retranslating, ordering, eliminating duplicate packages

      • UDP reflects just what the internet does with the packages: best effort delivery, no checking.

    • Also the programming style is quite different :

      • With TCP the data is sent a flow (of bytes, in principle) which can be written, read as if they were stored in a file.

      • With UDP the programmer must assemble the package and send it to the internet without knowing if it will arrive its pretended destination


Programming distributed application using tcp ip

When to use one or another

  • Considerations

    • TCP imposes a much higher load to the network than UDP (almost 6 times)

    • We can expect high package loss when the information travels trough many routers.

    • Inside a LAN UDP communications may be reliable is there is not much traffic. Although with some congestion we can expect some packages to be lost inside the LAN

  • In general, it is recommended especially for beginners (but also to skilled programmers) to use only TCP to develop distributed applications. Not only it is more reliable but the programming style is also simpler. UDP is normally used if the application needs to implement hardware supported broadcasting or multicasting, or if the application cannot tolerate the overload of TCP


Mark with a the applications to use tcp and with a those to use udp

Mark with a + the applications to use TCP and with a = those to use UDP

Video conference

E-Mail

Web server and client

Stock values every 5 seconds

Temperature every second


The client server paradigm do you remember the web

Theweb

server program

Web

resources

Theweb

client program

The client-server paradigm(do you remember the WEB ?)

answer

request

THE INTERNET

answer

request


1 the server opens a channel and starts listening to requests

A SERVER

Web

resources

A CLIENT

1- The server opens a channel and starts listening to requests.

?

1

THE INTERNET


2 a client who knows it sends a request and waits for the answer

A SERVER

Web

resources

A CLIENT

2- A client who knows it, sends a request and waits for the answer

2

THE INTERNET

2


3 the server analyses the request and answers properly according to the protocol

A SERVER

Web

resources

A CLIENT

3- The server, analyses the request and answers properly according to the protocol

3

THE INTERNET

3

This may involve the

reading of a file


Programming distributed application using tcp ip

Why Client/Server ?

It is a communication protocol model (listener/caller)

  • TCP/IP does not provide any mechanism which would start running a program in a computer when a message arrives. A program must be executing BEFORE the message arrives in order to establish a communication (daemons).

  • Is there really no other mean to communicate ?

    • Multicasting (but the sender does not know who is receiving and in this case there is no dialogue)

  • Most programs do not act as pure servers or client

    • It very frequent to have a server of o a certain program act as a client of another

    • Sometimes a group of programs are client and servers from each other at the same time!


  • Communication architectures for distributed applications

    • Servers as Clients

      • Programms do not behave as pure servers or as pure clients. For example, a file server can ask another compter for a timestamt to register the last change of a file.

      • When all application must behave at the same time as client and server we can organize the communication in two basic ways:

        • Every application can open a communication channel with each other application (network configuration): P2P applications

        • There is a commincation server and all applications open one communication channel with it (star configuration): multiple chat servers.

    Communication Architectures for Distributed Applications


    The client server model

    Server 2

    Client

    Server 1

    Client

    Server 3

    The Client-Server Model

    request

    answer


    Services provided by multiple servers

    Server 1

    Client

    Server 2

    Client

    Server 3

    Services Provided by Multiple Servers


    Proxy servers caches

    Server 1

    Client

    Proxy/cache

    Client

    Server 2

    Proxy servers & caches


    Peer to peer applications p2p

    Application

    +

    Coordination

    Application

    +

    Coordination

    Application

    +

    Coordination

    Peer-to-peer Applications (p2p)


    Network communication architecture

    • Every application opens an exclusive channel qith each other application present in the session

    • There may be up to n*(n-1)/2 channels open for n applications

    • Advantages:

      • It avoids bottlencks in the communications

    • Drawbacks:

      • All applications must be aware of all other taking part in the session

      • The dynamic is more complicated when managing consistency when applications enter and quit the session

    Network communication architecture


    Star communication architecture

    • The applications open a channel with the server and send their communication requests to the server. This server takes the message and forwards it to its final destination

    • There are up to n channels open for n applications

    • Advantages:

      • The managing og the communication parameters is more easy to manage

      • The problem of incomming and outgoing of applications is more easy to tackle

    • Drawbacks:

      • The server can get oveloaded

      • The channels may get overloaded.

    Star communication architecture


    Replicated architecutres

    • Every application has a copy of the application and the data

    • The modifications (data) are distributed to all participants in some way

    • Synchronization is normally achieved by distributing the events, not the state of the data

    • Problems with latecommers

    • Communication architecture may be that of a star or network type

    Replicated Architecutres


    Replicated architecture

    view

    Data

    Appl

    Replicated Architecture

    Data

    Data

    Data


    Semi replicated architectures

    • Data are kept centralized by a single application

    • Every client mantains its own actualized view of the data

    • There is a single data model, while the views and controllers are replicated

    • Permits the use of different interfaces (browser)

    • Synchronisation by events or by state

    • Communication architecture normaly centralized (the data are located at the server)

    Semi-replicated Architectures


    Semi replicated architecture

    Semi-replicated Architecture

    Data

    Data

    Data


    Centralized architecture

    • Data and view are mantained centralized

    • Every client has a graphic server for displaying the view

    • Synchonization by state (the view)

    • Communications architecture centralized

    • It provoques a big traffic of data over the network (the whole view is transmitted)

    • Are frecuently of general use (like netmeeting)

    Centralized Architecture


    Full centralized architecture

    Full centralized Architecture

    view / commands

    view / commands


    Nowadays there is a lot of middleware which make distributed programming much easier

    Libraries for distributed

    programming (middleware)

    Nowadays there is a lot of middleware which make distributed programming much easier

    RPC, CORBA, RMI


    Goals of the middleware

    Goals of the Middleware

    • Provide a framework for making development of distributed system easier

    • Hide (encapsulate) communications details

    • Make distributing programming similar to local programming

    • Standardization of communication protocols and data format

    • This help comes not for free !!!


    Web based distributed systems

    Web-Based Distributed Systems

    • Applications talk to each other using HTTP protocol

    • Trough port 80 (usually the only available)

    • Thin clients – Fat Servers

    • Use of Web and Application servers

    • The server is “extended” to implement new services


    J2ee basic architecture

    J2EE Basic Architecture

    Web Server

    Client:

    Web Browser

    Database Server

    Application

    Server

    HTML Pages

    Servlets

    JSP pages

    EJB

    Communication via

    JDBC


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