Topic 4- Application Layer

Question 1

When programmer creates network applications,

the programmer specifies some details

Answer 1

• Syntax and semantics of messages to be

exchanged • Client or server initiates interaction • Error Control • When to terminate communication

Question 2

Two broad types of application-layer protocols that depend on intended use:

Answer 2

Private communication

Standardised service

Question 3

Private communication application

Answer 3

Private communication: communicate over the
Internet with intention for private use
• Straightforward interaction: code can be written
without formal protocol specification

Question 4

Standardised Service

Answer 4

• Standardised service: expectation that many
programmers will create server software to offer the
service or client software to access the service
• Must be documented independent of
implementation
• Specification must be precise and unambiguous

Question 5

Application-layer protocols specify two aspects of interaction:

Answer 5

Representation
Transfer

Details on pictures doc

Question 6

Www standards

Answer 6

HTML
URL
HTTP

More details on the pictures document

Question 7

HTML Characteristics

Answer 7

Uses textual representation
Describes pages that contain multimedia Follows a declarative rather than procedural paradigm
Provides markup specifications instead of formatting Permits embedded hyperlink in an arbitrary object
Allows a document to include metadata

Further explanation on the pictures document

Question 8

HTML- Declarative Markup Language

Answer 8

• Declarative: allows one to specify what is to be done,
not how to do it
• Markup language: gives general guidelines for display
and does not include detailed formatting instructions
• Allows a page to specify level of importance of a
heading • Does not require author to specify the exact font,
typeface, point size, or spacing for the heading

Question 9

HTML Extension

Answer 9

• HTML Extensions: created to allow the specification of an exact font, typeface, point size, and formatting.

Question 10

Importance of Markup Language- Two advantages:

Answer 10

1- Allows a browser to adapt the page to the underlying
display hardware

Computer screen or smartphone

2-Web page can be formatted for a high-resolution or
low-resolution display

Question 11

HTTP Characteristic

Answer 11

1. Uses textual control messages
2. Transfers binary data files
3. Can download or upload data
4. Incorporates caching

Question 12

get put head post

Answer 12

in the picture document

Question 13

cashing

Answer 13

Caching is an important optimization for web access as users tend to visit the same web sites repeatedly. Much of the content on the web consist of large images in formats such as Graphics Image Format (GIF) and Joint Picture Encoding Group (JPEG). Such images often contain backgrounds or banners that do not change frequently. Web browsers can reduce download times significantly by saving a copy of the image in a cache on the disk of the user. Each time the web browser visits the same site again, it uses the cached copy to reduce loading time. However, the web browser must be able to tell whether a cached copy is stale. That is there is an updated version of the image on the server and the cached copy needs to be updated.

When the web browser contacts server, the response header shows the time document was modified. In this case, the web browser saves the Last-Modified date along with the cached copy. Each time a request to the same site is made, the web browser makes HEAD request to the server. The Last-Modified on the server is then compared with the Last-Modified on the cache. If the cached version is stale, the web browser downloads the new version. The algorithm to update the cache is given in Algorithm 4i.

However, HTTP allows web site to include No-cache header that specifies a given item should not be cached. In most cases, web browsers do not cache small items. The reason is that the time to download the item with a GET request is approximately same as time to make a HEAD request, which is relatively fast. Additionally, keeping many small items in cache increases cache lookup times, which defeats the purpose of quick access.

Question 14

File Transfer Protocol (FTP)

Answer 14

allows sending a copy of a file from one computer to another. It provides a powerful mechanism for the exchange of data. Most FTP applications are one-to-one.

Question 15

file

Answer 15

A file is the fundamental storage abstraction that holds an arbitrary object like a document, spreadsheet, computer program, graphic image, or data.

Question 16

There are seven characteristics of FTP as follows:

Answer 16

1. Arbitrary file contents meaning that it can transfer any type of data.
2. Bidirectional transfer as it supports download or upload of files.
3. Supports authentication & ownership where each file have ownership and access restrictions.
4. Ability to browse folders,
5. Textual control messages as commands are sent as ASCII text.
6. Accommodates heterogeneity as it hides the details of individual computer OS.
7. FTP is mostly invisible as it is invoked automatically by browser when user requests for file download.

Question 17

ftp client server model

Answer 17

To be efficient, FTP uses a client server model. Firstly, the client establishes a command connection to the FTP server and sends requests to which the server responds. This command connection is also known as control connection. Secondly, when the FTP server needs to download or upload a file, it opens a data connection, which is used to transfer files. However, for data connection, it inverts the client server relationship. The server opens the data connection instead of the client. Figure 5a illustrate the communication sequence in FTP.

Question 18

There are four common commands in FTP.

Answer 18

1-The open and close are used for establishing command connection
2-the ls and cd are used to list and change directory, respectively.
3-The get and put are used to open download and upload data
4- the quit is used to close the FTP.