L2 The Application Layer

Question 0

What are the functions of the application layer?

Answer 0

1. Data storage (data encoding, compression)
2. Data access logic (data retrieval decisions)
3. Application/business logic (core processing logic of the application)
4. Presentation logic (information presentation and UI interaction)

Question 1

What is the difference between architecture and function in the application layer?

Answer 1

Architecture: the way in which the functions of the application layer are spread among the clients and servers in the network.

Functions: the tasks performed by the application in providing services to the user.

Question 2

What are the types of application architecture?

Answer 2

• host based (server based): the server does most of the work
• client based: the client does most of the work
• client-server: the work is shared between client and sever
• peer-to-peer: the work is shared between all communicating computers where they assume both roles of client and server

Question 3

List four different types of servers

Answer 3

Mainframe
Personal computer
Cluster (physically separate but linked. Cluster of PC’s)
Virtual servers

Question 4

List four examples of clients

Answer 4

Personal computers
Terminal (everything sent to server to be processed)
Transaction terminal (ATM)
Handheld

Question 5

Describe the types of clients

Answer 5

Fat client: powerful processing (presentation, application and data access logic)
Thin client: low processing power (only runs presentation logic and little application logic)
Ultra-thin client: very low processing power (used where theft might be a problem)

Question 6

Describe host (or server) based architecture

Answer 6

The server does practically all the work.

User input is sent to server and display information is generated by server.

Question 7

What are the advantages and disadvantages of host (server) based architectures?

Answer 7

Advantages:
Central processing: application software easy to manage
Central data storage: secure
Expensive parts of the system are protected
Disadvantages:
Central processing: single point of failure
Highly critical resource: redundancy, backup, and protection are important.
Incredibly expensive to maintain

Question 8

Describe client-based architecture

Answer 8

Client responsible for presentation, application, and do that access logic. The server is only used to store data

Question 9

What are the advantages and disadvantages of a client-based architecture?

Answer 9

Advantages:
Cost distribution
Cost reduction: powerful clients are relatively inexpensive
Disadvantages:
Server takes no part data access: file locking can be problematic
Highly critical resource: redundancy, backup, and protection become important
Inefficient network utilization: Entire file must be retrieved over the network, edited, then sent back over the network

Question 10

Describe the client/server architecture

Answer 10

The client is responsible for presentation and application logic. Server is used to store data and provide data access logic (and sometimes application logic).

Question 11

What are the advantages and disadvantages of client/server architecture?

Answer 11

Advantages:
Cost distribution
Cost-reduction: powerful clients are really expensive
Increased efficiency: processing distribution means that you can do more with less powerful processors.
Enables multivendor interoperation: allows the vendor’s client to send and retrieve data to/from any other vendor’s server.
Disadvantages:
Multivendor interworking: all vendors must follow the exact same data interchange specification.
Middleware: middleware is used when there is no direct data interchange standard between certain type of client and server.

Question 12

Describe middleware

Answer 12

Translates between the client application and the server application. Used to manage message transfer.

Question 13

In what layer is middleware implemented?

Answer 13

Mostly the application layer, but some parts of the presentation layer

Question 14

Describe two-tier client-server architecture and state what the client does as well as what the server does.

Answer 14

It has two hosts involved in end to end transactions. Client: Handles presentation and application logic
Server: Stores data and provides data access logic.

Question 15

Describe the three parts of three-tier client-server architecture and what they do.

Answer 15

It has three hosts involved in end to end transactions.
Client: Handles presentation logic
Application Server: Handles application logic
Server: stores data and provides data access logic

Question 16

Describe the parts of N-Tier client-server architecture and what they do.

Answer 16

It has four or more hosts involved in end to end transactions.
Client: Handles presentation logic
Application Server: Handles application logic
Server: stores data and provides data access logic

Question 17

Explain the advantages and disadvantages of Multi-Tier architecture.

Answer 17

Advantages
- application logic is distributed producing better load balancing
- scales easily because smaller single nodes are easy to upgrade
Disadvantages
- distributed communicating nodes put more load on the network infrastructure
- more complex & more difficult to develop and test

Question 18

What is peer-to-peer architecture?

Answer 18

Each host in the set acts as both client and server to any and all other hosts in the set. ex. Limewire and BitTorrent

Question 19

Explain the advantages and disadvantages of peer-to-peer architecture.

Answer 19

Advantages:
- Capacity of system grows as number of participating nodes increases
- Network is very resilient to failure
- Data can be stored at multiple points in the network – the same data can be access simultaneously from different nodes
Disadvantages
- Lack of centralized management means that it is harder to control the availability of data
- Malicious software can more easily infiltrate the stored data = big security risk

Question 20

What is Cloud Computing?

Answer 20

Model for enabling ubiquitous, on-demand access to shared pool of computing resources.
Cloud infrastructure is a collection of hardware and software that enables cloud computing.

Question 21

What are the essential characteristics of cloud computing?

Answer 21

• On-demand self-service
• Broad network access
• Resource pooling
• Rapid elasticity (capabilities can grow or shrink according to demand)
• Measured service (resource usage is monitored, controlled, reported and charged to)

Question 22

Name and explain three cloud service models.

Answer 22

SaaS

• Service provider provides access and use of applications
• Applications accessed through client software (e.g. Web browser)
• User does not manage any part of infrastructure or applications

PaaS

• Consumer creates or acquires their own software which they deploy on the provider’s cloud infrastructure
• User does not manage any part of infrastructure but manages the software and its deployment

IaaS

• Consumer is able to provision processing, storage, and network computing resources, and deploy arbitrary software including operating system and applications
• User does not manage underlying (physical) cloud infrastructure

Question 23

Name and explain four cloud deployment models.

Answer 23

1. Private cloud - exclusive use
2. Community cloud - for exclusive use of a closed community of users that have shared requirements and concerns
3. Public cloud - use by the general public
4. Hybrid cloud - composed of two or more distinct cloud infrastructure types

Question 24

What is virtualization?

Answer 24

A method of decoupling an infrastructure service (e.g. application and resources required to run it) from the physical hardware on which it runs. can be applied to Servers, Storage, Desktops, Applications, Networks

Question 25

Name four criteria that need to be considered before choosing an architecture.

Answer 25

1. Scalability 2. Acquisition Cost 3. Development & Maintenance Cost 4. Security

Question 26

Look at slide 37 for architecture comparing chart (not sure how to make a question for this but seems important)

Answer 26

slide 37

Question 27

Name some examples of things that use the application layer.

Answer 27

- The World-Wide Web - Email - File Transfer - Videoconferencing - Internet Telephony - Instant Messaging

Question 28

What is the world wide web? note: the internet and the web are not the same!

Answer 28

It is a collection of hyperlinked documents accessible via the Internet. It is comprised of Hypertext – A text document which contains links to other documents URL - Uniform Resource Locator - A string indicating where and how to access the linked document

Question 29

Explain the difference between the world wide web and the internet.

Answer 29

The Internet is the interconnection of every networked device on the planet that has a route into the global backbone network and is reachable from any authorized device, making the Web possible The Web is the set of hyperlinked documents and Application Layer processes that use the Internet for interconnectivity and communication among the processes - it runs over the internet.

Question 30

What type of architecture is the web built on?

Answer 30

two-tier client-server architecture. It is comprised on HTTP requests and HTTP responses.

Question 31

What is File Transfer Protocol?

Answer 31

It is used to transfer files between hosts. It requires a reliable transport mechanism and uses TCP (Transmission Control Protocol) in order to ensure error free, in-order delivery delivery of data to the application layer process. FTP uses 2 peer connections to transfer data. Each connection has a TCP port number.

Question 32

What file transfer modes exist for FTP (File Transfer Protocol)?

Answer 32

1. ASCII (TYPE A) - used if the file consists of text characters that can be represented by the ASCII character set 2. Binary (TYPE I) - used if the file consists of non-text data (e.g. graphics).

Question 33

What user connection modes exist for FTP (File Transfer Protocol)?

Answer 33

1. Closed site - user logs in with a username and password and can upload files, create directories and delete files. 2. Open site - the user logs in with a username "anonymous" and uses their email as the password. The user usually only has read-only access (download) privileges.

Question 34

Explain what Email is and who the main actors are?

Answer 34

Email is essentially a client-server application. The User Agent - is the email client application ex. outlook The Mail Transfer Agent - is the email server application. It maintains and manages individual mailboxes, sends email to clients and transfers emails between servers

Question 35

What standards exist for Email?

Answer 35

1. SMTP (Simple mail transfer protocol) - Used when sending mail from client to server or from server to server 2. Post Office Protocol – POP - Used when retrieving mail from server to client, Downloads mail from server to local drive before reading 3. Internet Message Access Protocol – IMAP - Used when retrieving mail from server to client

Question 36

Explain how email works in a two-tier architecture exchange.

Answer 36

The email gets sent from Alice's computer to the email server. That email then gets sent to the second email server. Bob can then use IMAP/POP to retrieve the email from the second server.

Question 37

What packet elements exist in an SMTP transmission?

Answer 37

Header Element Fields: - FROM, TO, DATE, SUBJECT, Message-ID, etc. Body Element: - DATA field

Question 38

Explain how email works in a three-tier architecture exchange.

Answer 38

Used between thin client and a server. An HTTP request gets sent from Alices web client to the webserver + email client and an HTTP response gets sent back to Alices Web Client. The email then gets sent to the first email server and then gets sent to the second email server. Bob then sends an HTTP request from his web client to the other webserver + email client. The webserver + email client uses IMAP to retrieve the email from the email server. An HTTP response then gets sent to Bob's web client.

Question 39

What is a MIME?

Answer 39

SMTP was designed to transfer text data only (7-bit ASCII) and the solution was to use Multipurpose Internet Mail Extension (MIME) to specifies the content type and a set of transfer encodings to send images

Question 40

How does MIME work?

Answer 40

Non-text (e.g. graphics, audio, etc.) data is transformed by the encoding scheme into text characters. The encoded data is attached to the email, along with header information stating its Content-Type and Content-Encoding Transferring Encoded Data - Email client contains software that encodes data before sending and decodes data on receipt - Encoded data may be added to email message as an attachment - Encoded data may be inline (e.g. to handle characters in language other than English)

Question 41

What is Video Conferencing?

Answer 41

- Allows geographically dispersed remote meetings - Cheaper than moving people to same location - Client app combines audio and video in single data stream - Desktop videoconferencing uses PC hardware and software to communicate with server

Question 42

What is Instant messaging (IM)?

Answer 42

- IM client contacts IM server whenever user is connected to Internet - When friend connects to Internet, server notifies both - Message from client is sent to server, then to friend

Question 43

With regards to networks, what are the implications for management?

Answer 43

The network is an enabler - Sharing data increases value and knowledge, which allows better business decisions to be made - Applications running on the network is the means by which data is shared - The network becomes critical to the organization Dramatic increase in network applications - More applications running on the network implies more traffic on the network - Increased traffic means that congestion may become a problem, requiring network redesign and prioritization - Increased operating cost