Application Layer - OSI Model

At the very top of the OSI Reference Model stack of layers, the application layer. The application layer is the one that is used by network applications. These programs are what actually implement the functions performed by users to accomplish various tasks over the network.

It's important to understand that what the OSI model calls an “application” is not exactly the same as what we normally think of as an “application”. In the OSI model, the application layer provides services for user applications to employ. For example, when you use your Web browser, that actual software is an application running on your PC. It doesn't really “reside” at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and HTTP is subtle, but important.

The reason for pointing this out is because not all user applications use the application layer of the network in the same way. Sure, your Web browser does, and so does your e-mail client and your Usenet news reader. But if you use a text editor to open a file on another machine on your network, that editor is not using the application layer. In fact, it has no clue that the file you are using is on the network: it just sees a file addressed with a name that has been mapped to a network somewhere else. The operating system takes care of redirecting what the editor does, over the network.

Similarly, not all uses of the application layer are by applications. The operating system itself can (and does) use services directly at the application layer.

That caveat aside, under normal circumstances, whenever you interact with a program on your computer that is designed specifically for use on a network, you are dealing directly with the application layer. For example, sending an e-mail, firing up a Web browser, or using an IRC chat program—all of these involve protocols that reside at the application layer.

There are dozens of different application layer protocols that enable various functions at this layer. Some of the most popular ones include HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP and IRC. Lots of alphabet soup, sorry. J I describe all of these and more in the chapter on higher-layer protocols and applications.

As the “top of the stack” layer, the application layer is the only one that does not provide any services to the layer above it in the stack—there isn't one! Instead, it provides services to programs that want to use the network, and to you, the user. So the responsibilities at this layer are simply to implement the functions that are needed by users of the network. And, of course, to issue the appropriate commands to make use of the services provided by the lower layers.

DOD model

This model is sometimes called the DOD model since it was designed for the department of defense It is also called the TCP/IP four layer protocol, or the internet protocol. It has the following layers:

• Link - Device driver and interface card which maps to the data link and physical layer of the OSI model.
• Network - Corresponds to the network layer of the OSI model and includes the IP, ICMP, and IGMP protocols.
• Transport - Corresponds to the transport layer and includes the TCP and UDP protocols.
• Application - Corresponds to the OSI Session, Presentation and Application layers and includes FTP, Telnet, ping, Rlogin, rsh, TFTP, SMTP, SNMP, DNS, your program, etc.
• The four layer TCP/IP protocol. Each layer has a set of data that it generates.
• The Link layer corresponds to the hardware, including the device driver and interface card. The link layer has data packets associated with it depending on the type of network being used such as ARCnet, Token ring or ethernet. In our case, we will be talking about ethernet.
• The network layer manages the movement of packets around the network and includes IP, ICMP, and IGMP. It is responsible for making sure that packages reach their destinations, and if they don't, reporting errors.

The transport layer is the mechanism used for two computers to exchange data with regards to software. The two types of protocols that are the transport mechanisms are TCP and UDP.

The application layer refers to networking protocols that are used to support various services such as FTP, Telnet, BOOTP, etc. Note here to avoid confusion, that the application layer is generally referring to protocols such as FTP, telnet, ping, and other programs designed for specific purposes which are governed by a specific set of protocols defined with RFC's (request for comments). However a program that you may write can define its own data structure to send between your client and server program so long as the program you run on both the client and server machine understand your protocol. For example when your program opens a socket to another machine, it is using TCP protocol, but the data you send depends on how you structure it.

Firewall

A Hardware Firewall is a network device that is connected upstream from a server. The Firewall blocks unwanted traffic from a server before the traffic ever reaches the server. The main advantage to having a Hardware Firewall is that a server only has to handle 'good' traffic and no resources are wasted dealing with the 'bad' traffic.Configuring a Firewall is as simple as creating a set of rules to allow access to certain ip addresses and ports from specific internet addresses.

Adding a Firewall to a Server:

To add a Firewall to a server, click on the link under the Security->hardware firewall tab in the customer portal. This page will display a list of servers on the account and which servers are eligible to be protected by a Firewall, which ones are already protected by a Firewall, and which ones cannot be protected by a Firewall due to network configuration.

To add a Firewall to your server, assuming the server is eligible for a Firewall, click the 'add' link and instructions will be displayed on how to have Firewall protection added to the server. Once a Firewall has been added to a server, an 'edit' link will be available to configure the Firewall.

Common Ports:

 
FTP - 21
SSH - 22
Telnet - 23
SMTP - 25
DNS - 53
HTTP - 80
POP3 - 110
IMAP - 143
HTTPS - 443
MSSQL - 1433
MySQL - 3306
Remote Desktop - 3389
PostgreSQL - 5432
VNC Web - 5800
VNC Client - 5900
Urchin - 9999 or 10000

Network Connections and Layers

A computer network is a telecommunication network that allows any set of computers or devices connected to each other with the ability to exchange data. The connections between the devices are established using either cable or wireless media.  Internet is a part of computer network where each and every computing device connected with each other. In a nutshell, Internet is a network of networks. Internet is a system architecture that has revolutionized communications by allowing various computer networks around the world to interconnect. Some of the internet standards are IETF (Internet Engineering Task Force) and RFC (Request for comments).

Internet Layers

• Application Layer: It is mainly responsible for supporting network applications. It Includes different protocols such as HTTP, FTP, SMTP
• Transport Layer: It is mainly responsible for transporting application – layer messages between the client and server sides of an application. It includes two protocols such as  TCP, UDP
• Network Layer: It is responsible for routing diagrams from one host to another. This layer has two principle components such as IP protocol and Internet Transport protocol
• Link layer: The network layer routes a packet through a series of packet switches between the source and destination. To move a packet from one node to other node, network layer must rely on the link layer
• PhysicaNetwork l Layer: It moves the individual bits within the frame from one to the next.


HTTP - HTTP abbreviated as Hypertext transfer protocol, web’s application-layer protocol and it is a heart of web. When the client and server interact with each other, HTTP defines the structure of the message, when the user request particular address with the help of URL (Uniform Resource Locator), it reaches the destination and fetches the records.

FTP – It involve in application layer and it is abbreviated as File Transfer protocol is a protocol for transferring a file from one host to another host. Both HTTP and FTP are file transfer protocols and have many common characteristics for illustration both run on top of TCP, Internet’s connection-oriented etc and at the same time both have differences.

SMTP - Simple mail Transfer protocol is a protocol for internet electronic mail. It uses the reliable data transfer service of TCP

Connectionless and connection oriented service
In order to establish connection and access application includes email, file transfer etc between internet and communicating devices can be done in two ways. They are 

Connection-Oriented Service

When an application uses the connection-oriented service, the client and the server (residing in different end systems) send control packets to each other before sending packets with real data (such as e-mail messages). This is called handshaking between the two points,  connection- oriented service is provided by TCP (Transmission control Protocol). Flow control, congestion control can be done in Connection-oriented service

Connectionless Service

There is no handshaking with the connectionless service. When one side of an application wants to send packets to another side of an application, the sending application simply sends the packets. Moreover, the service makes no provision for flow control or congestion control. The connectionless service is provided by UDP (User Datagram Protocol)

Four Principles of Quality of Service (QoS) for Network Applications

•    Packet Classification
•    Isolation: Scheduling and policing
•    High resonance utilization
•    Call admission