Application Layer

The application layer is the topmost layer of the OSI Model. As shown in the figure below, the upper three layers of the OSI model (application, presentation, and session) define functions of the single TCP/IP application layer. The application layer enables the human or software to get access to the network. It serves as the source and destination of communications across data networks. The application layer applications, services, and protocols enable humans to interact with the data network in a way that is useful. The applications are computer software programs with which the user interacts and start the data transfer process at the request. The services are programs which run in the background and give the link between the application layer and the lower layers.

The Protocols give a structure of rules that make sure services running on a particular device can send and receive data from a range of different network devices. Data Packet delivery over the network should be requested by the client from a server. If there is a P2P network, the client/server affiliation is established according to which is the source device and which the destination device is at that time of establishes. The conversions are exchanged between the application layer services at both end devices in accordance with the terms of protocol to establish and utilize these relations.

  

TCP/IP Application Layer Protocols

The Application Layer contains a protocol that is usually required by end users. The HTTP (hypertext transfer protocol) is one of the widely used application protocol, which supports the delivery of web pages to end devices. The HTTP is the base for the World Wide Web. When a browser requests a web page, the protocol sends the name of the required page to the server. The server then sends the requested page to a client.

SMTP (simple mail transfer protocol), IMAP(Internet messaging access protocol), and POP (post office protocol) maintain sending and receiving email. SMB(server message block),  FTP (file transfer protocol) and TFTP(trivial file transfer protocol)  allow clients to share files. P2P applications make it easier to share media in a distributed fashion. DNS (domain name system) resolves the IP address and name address for better human understanding. Clouds are remote locations that host application and store data so that end users do not need as many local resources, and the users can effortlessly access content from a different location. The TCP/IP application protocols identify the format and control information required for many general Internet communication functions.  Both source and destination devices are used by the application layer protocols during a communication session. The application layer enables hosts to work and play over the Internet.

Presentation and Session Layer

The Presentation Layer

There is three main function of the presentation layer.

1. Translation: Before being transmitted, formatting, or presenting, data at the source device into a compatible form for reception by the destination device. All data should be changed to bit streams. This layer is responsible for interoperability between encoding methods as different computers use different encoding methods. It translates data between the formats the network requires and the format of the computer.


2. Encryption: It carries Encrypting data for transmission and decrypting data at the receiver end.


3. Compression: This layer carries out data compression in a way that can be decompressed by the destination device. The role of compression is to decrease the number of bits to be transmitted. It is important in transmitting a big file.

The presentation layer formats data for the application layer, and it sets principles for file formats. Some well-known standards for video and graphics format are QuickTime, Motion Picture Experts Group (MPEG), Portable Network Graphics (PNG) format, Graphics Interchange Format (GIF) and Joint Photographic Experts Group (JPEG).

The Session Layer

The functions of the session layer are to create and maintain session between the source and the destination applications. This layer handles the following.

1. Dialog Control: The session layer allows two hosts to start communicating each other in half-duplex or full-duplex mode.


2. Token Management: The session layer prevents two hosts from attempting the same critical operation at the same time.


3. Synchronization: The session layer allows a process to add checkpoints which are measured as synchronization points into the stream of data.