tirgul 5 networking

8/3/2019 Tirgul 5 Networking

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Networking 1

Web Programming courseDan Goldwasser

[email protected]


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Outline

Network concepts

OSI reference model

TCP\IP

Protocols

Client server introduction


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What is a Network?

... communication system

for connecting end-systems

End-systems a.k.a. hostsPCs, workstationsdedicated computers

network components


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History and Motivation

Early computers were highly centralized.

Single point of failure

User has to go to the computer.

Proliferation of low cost computers made itpossible to get past these 2 primary

disadvantages (with a network).


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Motivation

Sharing of resources is more efficient

Price/Performance

Use each piece of equipment for what it isbest at

Centralize administration

Computers as communication tools


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Computer Networks are now

everywhere

PCs Mainframes

Automated Tellers

Embedded Systems

Communications Systems

The Internet


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Networked Computers -

Traditional Uses

Communication (email)

File exchange, disk sharing Sharing peripherals (printers, tape drives)

Remote execution


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Wide variety of types of

networks circuit switched

telephone system

packet switched:

The Internet (TCP/IP)


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Outline

Network concepts

OSI reference model

TCP\IP

Protocols



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Network Models

Using a formal model allows us to dealwith various aspects of Networks

abstractly. We will look at a popular model (OSI

reference model).

The OSI reference model is a layeredmodel.


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Layering

Divide a task into pieces and then solveeach piece independently (or nearly so).

Establishing a well defined interfacebetween layers makes porting easier.

Major Advantages:

Code Reuse

Extensibility


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OSI Reference Model

The International Standards Organization(ISO) proposal for the standardization of the

various protocols used in computer networks(specifically those networks used to connectopen systems) is called the Open Systems

Interconnection Reference Model (1984), orsimply the OSI model.


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OSI Model

Although the OSI model is a just a model(not a specification), it is generally regarded

as the most complete model (as well itshould be - nearly all of the popular networkprotocol suites in use today were developed

before the OSI model was defined).


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OSI 7 Layer Model:

7 Application

6 Presentation

5 Session

4 Transport

3 Network2 Data-Link

1 Physical

High level protocols

Low level protocols


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Simplified Network Model

Process

Transport

Network

DataLink

Process

Transport

Network

Data Link

Interface Protocols

Peer-to-peer

Protocols


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The Physical Layer

Responsibility:

transmission of raw bits over acommunication channel.

Issues:

mechanical and electrical interfaces

time per bit

distances


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The Data Link Layer -

Data Link Control Responsibility:provide an error-free communication link

Issues: framing(dividing data into chunks)

header & trailer bits

addressing

10110110101 01100010011 10110000001


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The Network Layer Responsibilities:

path selection between end-systems (routing).

subnet flow control.

fragmentation & reassembly

translation between different network types.

Issues:

packetheaders

virtual circuits


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The Transport Layer Responsibilities:provides virtual end-to-end links between

peer processes.

end-to-end flow control Issues:

headers

error detectionreliable communication


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The Application Layer

Responsibilities:

anything not provided by any of the other

layers

Issues:

application level protocols

appropriate selection of type of service


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Layering & Headers Each layer needs to add some control

information to the data in order to do its

job.

This information is typically prependedto the data before being given to thelower layer.

Once the lower layers deliver the thedata and control information - the peerlayer uses the control information.


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Headers

Process

Transport

Network

Data

Link

Process

Transport

Network

Data Link

DATA

DATA

DATA

DATA

H

H

H

H

HH


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Outline

Network concepts

OSI reference model

TCP\IP

Protocols



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TCP/IP & OSI

In OSI reference model terminology -the TCP/IP protocol suite covers thenetworkand transportlayers.

TCP/IP can be used on many data-link layers (can support manynetwork hardware implementations).


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Internet Protocol

The IP in TCP/IP

IP is the network layer

packet delivery service (host-to-host). translation between different data-link

protocols.


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IP Datagrams

IP provides connectionless,unreliable delivery of IP datagrams.

Connectionless: each datagram isindependent of all others.

Unreliable:there is no guarantee thatdatagrams are delivered correctly or

even delivered at all.


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Transport Layer & TCP/IP

TCP is only part of the TCP/IPtransport layer - the other part isUDP (User Datagram Protocol).


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TCP UDP

IP

802.3

Process Layer

Transport Layer

Network Layer

Data-Link Layer

Process Process

ICMP, ARP&

RARP


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Ports

TCP/IP uses an abstractdestination point called a protocol

port. Ports are identified by a positive

integer.

Operating systems provide somemechanism that processes useto specify a port.


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UDP User Datagram Protocol

UDP is a transport protocol

communication between processes

UDP uses IP to deliver datagramsto the right host.

UDP uses portsto providecommunication services toindividual processes.


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UDP

Datagram Delivery

Connectionless

Unreliable

Minimal

Source Port Destination Port

Length Checksum

Data

UDP Datagram Format


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TCP

Transmission Control Protocol TCP is an alternative transport layer

protocol supported by TCP/IP.

TCP provides:

Connection-oriented

Reliable

Full-duplex

Byte-Stream


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Connection-Oriented

Connection orientedmeans that avirtual connection is establishedbefore any user data istransferred.

If the connection cannot beestablished - the user program isnotified (finds out).

If the connection is everinterrupted - the user program(s)is finds out there is a problem.


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Reliable

Reliablemeans that everytransmission of data is

acknowledged by the receiver. If the sender does not receive

acknowledgement within a

specified amount of time, thesender retransmits the data.


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Byte Stream

Streammeans that the connectionis treated as a stream of bytes.

The user application does notneed to package data in individual

datagrams (as with UDP).


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Addressing in TCP/IP

Each TCP/IP address includes:

Internet Address

Protocol (UDP or TCP)

Port Number


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TCP vs. UDP

Q: Which protocol is better ?

A: It depends on the application.

TCP provides a connection-oriented, reliable,byte stream service (lots of overhead).

UDP offers minimal datagram delivery service(as little overhead as possible).


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TCP/IP Summary

IP: network layer protocolunreliable datagram delivery between

hosts.

UDP: transport layer protocol unreliable datagram delivery between

processes.

TCP: transport layer protocol reliable, byte-stream delivery

between processes.


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Outline

Network concepts

OSI reference model

TCP\IP

Protocols



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Whats a Protocol?

An agreed upon convention forcommunication.

both endpoints need to understandtheprotocol.

Protocols must be formally defined and

unambiguous!


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Interface and Peer-to-peer

Protocols Interface protocols

describe thecommunicationbetween layers on thesame endpoint.

Peer-to-peer protocolsdescribecommunicationbetween peersat the same layer.

Process

Transport

Network

Data Link

Process

Transport

Network

Data Link

Interface

Protocols

Peer-to-peer

Protocols


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Application layer protocols

We will focus on application layerprotocols-

HTTPSMTP

FTP


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HTTP Usage

HTTP is the protocol that supportscommunication between web browsers and webservers.

A Web Server is a HTTP server

The RFC states that the HTTP protocol

generally takes place over a TCP connection,but the protocol itself is not dependent on aspecific transport layer.


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Request - Response

HTTP has a simple structure:

client sends a request

server returns a reply.

HTTP can support multiple request-reply

exchanges over a single TCP connection.


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HTTP Structure

Request-Line

Headers...

Content...

blank line

The Request Method can be:

GET HEAD PUT

POST DELETE TRACE

OPTIONS


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Methods GET: retrieve information identified by

the URI.

HEAD: retrieve meta-information aboutthe URI.

POST: send information to a URI andretrieve result.


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Methods (cont.)

PUT: Store information in locationnamed by URI.

DELETE: remove entityidentified byURI.


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Class URL

Class URL represents a UniformResource Locator, a pointer to a

"resource" on the World Wide Web.A resource can be something assimple as a file or a directory, or itcan be a reference to a morecomplicated object, such as aquery to a database or to a search

engine

HttpURLConnection

URLConnection

URL

The abstract class URLConnection isthe superclass of all classes that represent

a communications link between theapplication and a URL. Instances of thisclass can be used both to read from and towrite to the resource referenced by the URL

HttpURLConnectionA URLConnection with supportfor HTTP-specific features

(extends URLConnection)
http://java.sun.com/j2se/1.4.2/docs/api/java/net/HttpURLConnection.htmlhttp://java.sun.com/j2se/1.4.2/docs/api/java/net/HttpURLConnection.htmlhttp://java.sun.com/j2se/1.4.2/docs/api/java/net/HttpURLConnection.htmlhttp://java.sun.com/j2se/1.4.2/docs/api/java/net/HttpURLConnection.html


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Amaze your friends with Java!public class URLReader {

public static void main(String args[ ]){try{

URL url = new URL("http://www.yahoo.com/");BufferedReader in = new BufferedReader(new InputStreamReader(url.openStream()));String inputLine;FileWriter fw=new FileWriter("url.html");BufferedWriter bw=new BufferedWriter(fw);while ((inputLine = in.readLine()) != null){

bw.write(inputLine);}

in.close();

}catch(Exception e){System.out.println("Error - aborting");}

}}


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Outline

Network concepts

OSI reference model

TCP\IP

Protocols



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Client - Server

A serveris a process - not a machine !

A server waits for a request from aclient.

A client is a process that sends arequest to an existing server and(usually) waits for a reply.


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Client - Server Examples

Server returns the time-of-day.

Server returns a document. Server prints a file for client.

Server does a disk read or write. Server records a transaction.


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Servers

Servers are generally more complex (moreinteresting).

2 Basic types of servers: Iterative- server handles one client at a time.

Concurrent- server handles many clients at a

time


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Summary

What is the OSI model? Why use layers? Whatis the difference between interface protocol andpeer-to-peer protocols in the OSI model?

What is TCP\IP ? To which of the OSI layersdoes it address? What is UDP? What is thedifference between TCP and UDP?

What is a protocol? Why are they needed?

What is the idea behind the client server model?


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Questions?

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