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Topic3-ApplicationLayer.pptxITECH1102 Networking and Security
Topic 3 – The Application Layer (Layer 5)
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Last week
Last week we looked at:
The difference between Circuit Switched networks and Packet Switched networks.
The origins of packet switched networks
The Arpanet
We saw how packet switching can lead to lost packets, duplicate packets and out of order network packets.
The role of network interface cards
How Hubs, Switches and Routers work
We introduced 3 reference networks. Home network, Campus network & the Internet.
Introduced the 5 layer Internet (TCP/IP) model of networking.
Introduced the 7 layer OSI model of networking
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This week – overview of content
Introduction to the Application Layer of the Internet Model.
Application architectures including:
Client/Server
Peer to peer
Host based
Cloud architectures
How simple networked applications work (Web Browser & FTP client)
Internet Services can be a source of considerable monetary value
The role of Network protocols
Common Application layer protocols
How data travels through a network & protocol headers
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The Application Layer (User Applications)
User applications give us the ability to interact with thousands of services and devices on the Internet and on our local network. These applications are enabled though their ability to use networks.
Internet services include:
Google search engine
Google Maps
FTP file download sites
Domino’s pizza ordering online app
There are thousands of others
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The Application Layer (Local user applications)
At the local level networks allow:
Access to printers
Access to network faxes
Access to Network attached storage
Shared resources from other local machines
Prior to the prevalence of computer networks users would transport data locally by carrying the data on removable media (Floppy disks) to the destination.
This approach is commonly referred to as “sneaker net”, so called because everyone at the time wore a type of footwear called sneakers.
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Categories of interaction
Client/Server Architecture
In this type of network the user’s application is described as a client.
Common clients include:
email clients
web clients
FTP clients etc.
Clients request services of Severs.
For example a Web client will request web pages from web servers.
Files are requested from File servers.
Information from Database severs.
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Client/Server Architectures (continued)
The role of Servers
In client/server networks a server waits for client requests and responds to those requests.
Some servers require authentication (Students J drive). Others do not (Eg. Most web servers).
In client/server networks the server runs as a service (on Windows) or as a server Daemon (on Linux or Unix).
In either case server processes listen for requests from client applications, and upon request respond to those requests.
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Strict distinction in roles
In Client/Server networking there is a strict distinction between the roles of the client and the server.
The client is a Consumer of services.
The Server is a Provider of web, file, email or other services.
The server program code is totally different from the client code. Example:
Common web client software is Firefox, Chrome, Safari etc.
Common web server software is IIS on Windows or Apache on Linux/Unix.
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Peer to Peer Architectures
In peer-to-peer networking each machine in the network can act as both a server and a client (simultaneously if required).
All machines (peers) can share resources (file and printers) and can also access any shared resource.
Peer to peer networks do not require a dedicated server because the server role can be shared amongst many peers.
Home networks that share files or printers to other computers on the network are a good example of a peer to peer network.
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Pro’s and cons of Client/server vs peer to peer.
Peer to peer can work well in small networks like home network and small businesses.
Administrative duties are distributed between the administrators of each peer, because any peer can act as a server.
In larger networks (like Fed Uni campus network) network administration cannot be distributed as with peer to peer networks.
Client/Server networking requires strong distinction between the roles of Network Administrators and network users. Network administrators control the network and its applications. Network users are only given the minimum access required to complete their day to day roles.
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Host based architectures
Host based architectures were popular is the early days of computing and still have a strong presence today.
Early system used a Mainframe with many attached dumb user terminals. The mainframe performed all the processing.
Today Mainframes are used for high volume transaction processing functions like point or sale processing (Woolworths, Coles checkouts), Bank transactions, Airline transactions, Insurance company transactions etc.
Some of the processing role has shifted to the point of sale or other terminals but the general processing architecture distinctions still exist.
Microsoft Windows also supports a host based architecture to thin clients with its Citrix Metaframe product. (Most processing is done on the server). Users only require thin clients (low powered systems) because they are only required to render the graphical screen updates from the central Citrix server.
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Cloud based Architectures
Cloud based Architectures are becoming increasingly important in modern computing.
Cloud computing providers can supply a range of services to organisations including the computing platform, operating systems and software.
This can be attractive to organisations because they no longer require in house hardware/software support for such systems or the associated costs of maintaining servers and associated infrastructure.
Cloud services do come at considerable cost.
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Infrastructure as a Service (IAAS)
The cloud provider manages all hardware, including servers, storage and networking components.
The organisation is responsible for all software, including the Operating System/s, applications and associated data.
The organisation is also responsible for virtualization software as required.
Such architectures have the advantage that the organisation no longer has the responsibility of providing hardware, associated air conditioning of server rooms, power supply issues and associated uninterruptable power backup systems.
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Platform as a service (PaaS)
The cloud provider offers the hardware, the operating system and base database or other software.
The organisation’s programmers can code database or other applications in any language they like. They also have control over their own data.
PaaS is not restricted to database infrastructure.
This allows the organisation to build their own custom applications without the need to worry about the operating system, database system or hardware.
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Software as a Service (SaaS)
Here the cloud provider provides everything including the software.
User’s access the software from the Internet and are able to customise their user interface.
Commonly email is outsourced by organisations to cloud providers.
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Common Internet Application
Web Browser
Allow us to view web pages
Complete order forms over the Internet
Pizza order
Census online
Download programs and other files from the web
Search for information
Run web applications from the browser (Eg. draw.io)
Microsoft Office online
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Other network enabled applications
Email client
Compose and send email
Read email
File transfer services
Many Internet sites allow downloading of all sorts of files
Phone apps
Weather forecasting applications
Banking apps
Considerable business value can be gained by developing popular web services (Examples: Facebook, Google, Twitter, Whatsapp etc.)
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A close look a some web applications
Application 1: Web browser query to Google
Open Browser to www.google.com (This is a request to display the Google search page dialog)
Submit a query (outbound request)
Web server replies with a list of suggested web sites (inbound)
The browser renders the suggested links
You select one of the options (outbound request)
That web page is displayed on your browser screen
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File transfer from an FTP site
Application 2 – Download a file using an FTP application
Run the local FTP client program (Installed by default in Windows)
The FTP program asks for the Internet address of the FTP server
User enters the address (Eg ftp.novell.com)
FTP client requests a connection to the FTP server
Server requests a username
User responds with valid credentials
FTP server provides access to available downloadable files
User requests file download
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Network protocols
All networked applications, including those described above must adhere to a strict set of rules (called protocols) for network communication to succeed.
For Web applications the application layer transfer protocol is HTTP (Hypertext Transfer Protocol).
For FTP file transfers the FTP client and server applications must comply with the application layer File Transfer Protocol.
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Other Application Layer protocols
There are many other Application Layer protocols in the TCP/IP protocol suite. Some familiar ones include:
BitTorrent
NTP (Network Time Protocol)
POP (Post Office Protocol)
SMB (Server Message Block)
SSH (Secure Shell)
Each of these protocols define the rules that applications and associated server programs must comply with for successful communications between one another.
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The role of application layer protocols
Communications protocols do the following:
Establish consistent rules between sender and receiver.
Specify how data inside messages is structured and the types of messages that are sent between source and destination.
Handle message dialogue, for instance which entity communicates first and how the response should be handled.
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Basic aim of networking
As a rule one party in a network transaction makes a request of the other party according to the rules of the appropriate application protocol.
We require the network to ensure that the data sent in either direction arrives intact.
For instance in the FTP example above we require:
The username and password entered at the client to arrive unchanged at the FTP server.
We require client requested files sent from the server to be exactly the same at the client as they originated on the server.
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How do the client and server applications interact
Lets start with an FTP client program on your PC and a remote FTP server
User Program (FTP client Application)
Application Layer
Other layers
FTP Server (FTP program)
Application Layer
Other layers
Client’s
Networking
software
Servers
Networking
software
Login and other FTP protocol details are handled at the client and server application layers.
The two application layers must exchange usernames, passwords etc. to implement the FTP protocol.
Data Transfers occur between the FTP client application and the FTP server application.
All transfers must use the physical network.
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Network data exchanges use all layers
Client program data must pass through all underlying network layers to reach the Physical Network.
So FTP data must traverse the following layers in turn:
Transport layer
Network Layer
Data Link Layer
Data being sent traverses down through the software layers of the protocol stack to the physical network.
Received data rises up though the layers from the physical network.
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Protocol Headers
As data passes through the protocol stack, Protocol headers are added at each layer.
The transport layer normally adds a TCP header.
The network layer adds an IP header.
The Data Link layer add an Ethernet header.
The headers supply information to the associated layer of the destination machine.
The information in these headers informs the destination where the data came from and other essential details.
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Visualization of client HTTP request from Firefox.
Application
Transport
Network
Data Link
User Program (Firefox)
Physical
Step 1:
User opens browser and enters URL to a web site.
HTTP request is created at the Application layer.
Step 2:
HTTP request is passed to the Transport layer.
Transport layer information (TCP) is added.
Step 3:
The process continues.
IP information is added at the Network layer.
Step 4:
Ethernet information is added at the Data Link Layer.
Then the resulting frame that is sent to the network media.
The reverse process occurs at the server end.
Headers are removed at each layer as the data moves up through the network layers.
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What happens at layers 2,3 & 4.
As we progress through the course we will discuss the details concerning each of the other layers.
Topic 4 - Data Link layer
Topic 5 - Network layer
Topic 6 - Transport layer
Topic 7 - Physical layer
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Next Week
The Data Link Layer
Local delivery of network traffic
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