Content Introduction TCP Clients Iterative TCP Servers Concurent

Content • • • Introduction TCP Clients Iterative TCP Servers Concurent TCP Servers UDP Multicasting RMI-Corba Web-based systems http: //www. dcc. uchile. cl/~nbaloian/cc 50 h/2004

Evaluation • 4 Homeworks • 2 Tests • Final Exam

Introduction October 2003

Why distributed systems - Share resources - Communicate people - Performance, scalability - Fault tolerant systems

We know already how computers communicate but. . .

. . . how do programs communicate? PROG 1 PROG 2 They need to establish a protocol ! - Who send the data first - What kind of data - How to react to the data

Every layer has the illusion of talking to the same one located at the other host A CLIENT The UDP: User. Read Defined write Package: sequence like writing a letter A SERVER 4444 UDP or TCP communication A CLIENT Internet frames and addresses A CLIENT electric pulses

Decisions when Developing a Distributed System • Which service from the transport layer are we going to use (TCP, UDP, or a middleware) • Software architecture: replicated, centralized • Communications architecture: centralized, networked • Server design: concurrent, iterative, stateless, with state • Etc…

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

Nowadays there is a lot of middleware which make distributed programming much easier Libraries for distributed programming (middleware) RPC, CORBA, RMI

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) • What are the protocol ports of a server machine ? – It is a virtual address inside the machine at a server listening to client requirements asking for a certain service. In most Unix machines there are “well known ports” which are associated to a server program providing a service trough a protocol. Port number and protocol should be well known.

The client-server paradigm (do you remember the WEB ? ) The web server program Web resources answer request THE INTERNET answer The web client program request

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

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

3 - The server, analyses the request and answers properly according to the protocol A SERVER Web resources 3 THE INTERNET 3 This may involve the reading of a file A CLIENT

The Client-Server Model invocación Cliente resultado Servidor 2 Servidor 1 Cliente Servidor 3

Services Provided by Multiple Servers Server 1 Client Server 2 Client Server 3

Proxy servers & caches Server 1 Client Proxy/cache Client Server 2

Peer-top-peer Applications Application + Coordination

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): P 2 P applications • There is a commincation server and all applications open one communication channel with it (star configuration): multiple chat servers.

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

Star communication architecture • The applications open a channel with the server and send their communication requests to the server. This Las aplicaciones envían sus requerimientos de comunicación a un servidor y éste se encarga de mandarlas a su punto de destino final. • 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.

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 Architecture Data view Data Appl

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 Architecture 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)

Full centralized Architecture view / commands

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 asociated to services (programms)

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 comming 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 sucha request. • Multicast fits well for group communication when the group is not well defined beforehand (sponaneous 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

Protocolos for communication • Every service is normally identifyed by a port – – Web: HTTP (port 80) Mail: SMTP File transfer protocol: FTP (21) telnet: 22/23 • Servers with/without Connection – connectionless style: UDP – connection-oriented style TCP

The channel which server and client use to communicate (either int TCP or UDP) 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 A SERVER 1 4444 A SERVER 2 3333 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

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. waseda 1. jp A SERVER www. waseda 2. jp A CLIENT ? 4444 www. waseda 1. jp 4444 message

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. waseda 1. jp A SERVER www. waseda 2. jp A CLIENT ? 4444 3333

Sending datagrams with UDP protocol Before the datagram leaves the client, it receives the address of the originating computer and the socket number www. waseda 1. jp A SERVER www. waseda 2. jp A CLIENT ! 4444 3333

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. waseda 1. jp www. waseda 2. jp ? A SERVER 4444 3333 A CLIENT

Sending datagrams with UDP protocol The server can extract the client’s address and port number to create another datagram with the answer www. waseda 1. jp www. waseda 2. jp ? A SERVER 4444 answer 3333 A CLIENT

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. waseda 1. jp www. waseda 2. jp ? A SERVER 4444 3333 A CLIENT

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. waseda 1. jp A SERVER www. waseda 2. jp A CLIENT ? 4444 3333

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. waseda 2. jp www. waseda 1. jp bla A SERVER 4444 bla bla A CLIENT 3333

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 Sending 1 st bla Ack 1 st bla Sending 2 nd bla Ack 2 nd bla Sending 3 rd bla Ack 3 rd 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 Sending 1 st bla Ack 1 st bla Sending 2 nd bla LOST !!! No confirmation !!! Sending 2 nd bla again Ack 2 nd bla

The Window for improving efficiency The transmitter will handle a set of not acknowledged packets Sending 1 st bla Sending 2 nd bla Sending 3 rd bla Ack 1 st bla Ack 2 nd bla Ack 3 rd bla

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

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

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 - not complete info - fast - valid in a very short period of time - history not important TCP - Reliable - Complete - Valid in a certain period of time - No need of speed

Mark with a + the applications to use TCP and with a = those to use UDP E-Mail Video conference Web server and client Stock values every 5 seconds Temperature every second