Content

1
Content

• Introduction
• TCP Clients
• Iterative TCP Servers
• Concurent TCP Servers
• UDP
• Multicasting
• http//www.dcc.uchile.cl/nbaloian/tampere

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Evaluation

• 4 Homeworks
• Final Exam

3
Introduction

• October 2003

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Why distributed systems

• - Share resources
• - Communicate people
• Performance, scalability
• Fault tolerant systems

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

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

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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 Client-Server Model
Servidor2
invocación
Cliente
Servidor1
resultado
Cliente
Servidor3
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Services Provided by Multiple Servers
Server 1
Client
Server 2
Client
Server 3
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Proxy servers caches
Server 1
Client
Proxy/cache
Client
Server 2
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Peer-top-peer Applications
Application Coordination
Application Coordination
Application Coordination
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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.

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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

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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.

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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

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Replicated Architecture
Data
Data
Data
view
Data
Appl
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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)

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Semi-replicated Architecture
Data
Data
Data
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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)

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Full centralized Architecture
view / commands
view / commands
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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
  computers IP number and the port number
• Every machine has 216 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)

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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

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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

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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
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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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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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
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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

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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

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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 info
• - fast
• - valid in a very short period of time
• history not important

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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