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 also 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, and can send and receive data from a range of different network devices. The client should request from the server, the delivery of data packet over the network. In the case of a P2P network, the affiliation of client/server establishes, according to 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 set up and use these relations.
TCP/IP Application Layer Protocols
The end devices usually require the application layer protocols. For example, the end devices receive web pages using HTTP (hypertext transfer protocol) application, which is one of the widely used application protocol.
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. For example, the servers SMTP (simple mail transfer protocol), IMAP(Internet messaging access protocol), and POP (post office protocol) keep up 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 place. The TCP/IP application protocols show the format and control information required for many general Internet communication functions. Both source and destination devices use the application layer protocols during a communication session. The application layer also 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.
- Translation: Before transmitting, formatting, or presenting, data at the source device into a compatible form for reception by the destination device. All data should change 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.
- Encryption: It carries Encrypting data for transmission and decrypting data at the receiver end.
- 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 transmitted bits. It is important in transmitting a big file.
The presentation layer formats data for the application layer, and it sets principles for file formats. For example, 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 keep up session between the source and the destination applications. This layer handles the following.
- Dialog Control: The session layer allows two hosts to start communicating each other in half-duplex or full-duplex mode.
- Token Management: The session layer prevents two hosts from attempting the same critical operation at the same time.
- Synchronization: The session layer allows a process to add checkpoints which are measured as synchronization points into the stream of data.