Chapter 7: Application Layer Flashcards
(24 cards)
Job of the application layer
Ensures that data transmitted over the network is presented in standard way, and is therefore sometimes called the syntax layer.
Dynamic host configuration protocol (DHCP)
This protocol dynamically assigns IP addresses to hosts connected to the network.
It uses UDP to request an available IP address from a local server, called the DHCP server, which responds with a leased IP address that is available for a certain amount of time.
HyperText Transfer Protocol (HTTP)
This protocol allows for the transmission of specially structured information over a TCP connection that can be interpreted by applications such as web browsers. It also allows for online navigation through hyperlinks.
Cascading style sheets (CSS)
Applies a consistent appearance to web pages on the same site
File Transfer protocol (FTP)
Used to transfer files over a network. It differs from HTTP as the connection is maintained while in HTTP the connection is dropped.
Email protocols: Simple Mail Protocol (SMTP)
This sends emails. It was used to receive emails as well, but it requires the receiver to be online.
Post office Protocol version 3 (POP3)
This protocol temporarily stores emails on a server. Once the client downloads the email, the email is deleted on the server side.
Internet message access protocol (IMAP)
Redesigned version of POP3 that includes a wide variety of features to email servers that POP3 does not provide.
It does not auto-delete emails once they are downloaded, and requires an explicit delete command to delete it in server side. It also allows for folder storage, but all this means it requires high quality servers.
Webmail
Broad name that covers a variety of protocols and online services that can be used to access emails stored on a server without the use of a specialised client software.
Advantage of webmail
It usually provides a cross-platform solution that only requires a normal web browser
Disadvantage of webmail
because a webmail service has to be hosted on a web server it requires additional server resources. Webmail only acts as a email client, and the emails are still sent and received using SMTP, POP3 and IMAP.
Three types of links on the WWW
Uniform Resource Identifier (URI)
Uniform Resource Location (URL)
Uniform Resource Name (URN)
Uniform resource Identifier (URI)
Any link that references a source. It can be used for location, file or name.
Uniform Resource Location (URL)
Specific type of URI that indicates a resource’s location. Most common type of URL is the web address typed into a web browser, but a full file path can also be seen as a URL.
Uniform Resource Name (URN)
Specific type of URI that is used to uniquely identify a resource. One example is an ISBN number for publication.
How a person views a website from a network enabled device: Step 1
A URL is submitted by a user to the browser
The browser needs data from the server that contains the data for the website.
To get this data, the browser needs an IP address in order to connect to the web server.
URL needs to be converted to a IP address. This is where Domain Name System (DNS) comes in.
How a person views a website from a network enabled device: Step 2
The browser sends a lookup request to a DNS server using a UDP connection.
How a person views a website from a network enabled device: Step 3
The DNS server responds with the IP address of the web server.
How a person views a website from a network enabled device: Step 4
The browser now requests a TCP connection to the web server, typically through port 80, using the previously mentioned IP address, from the transport entity.
Once connection is established, the browser requests the applicable web page from the web server via the TCP connection.
How a person views a website from a network enabled device: Step 5
The web server, upon receiving the request, uses HTTP to send the web page file back to the browser. This can be any website file, such as HTTP or PHP. If it needs CSS, that is sent as well. It uses the same TCP connection that was used to send the request.
How a person views a website from a network enabled device: Step 6
The browser, upon receiving the file, processes the data contained in the file and displays the results to the user.
Client-side website processing
With client-side website processing, the processing for the site is done on the local machine. This is typically the graphical processing of the site, but could also include more complicated processing using scripts (JavaScript), which could increase user-friendliness of the site.
Client side should only be used on non-sensitive data as processing can be manipulated by the local machine.
Server-side Processing
When the data of a website is processed on the server. It is crucial for data security that all secure data processing is done on the server using a serve managed process.
