What are the two forms of software programs or processes that provide network access to the application layer?

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The OSI (Open System Interconnection) Network Model Standard

The OSI network model layers are arranged here from the lower levels starting with the physical (hardware) to the higher levels.

The 7 layers of OSI

Figure from webopedia

An application that communicates with other computers that corresponds to the application's communication service.

It defines how to transfer data on a single link

This layer defines the end-to-end packet transport, which defines the logical addresses that identify all nodes and also defines how the routes are implemented and how they are learned.

The functions of this layer include whether to choose the error recovery protocol or the error-free recovery protocol, and multiplex the input of different application data streams on the same host, and also include the function of reordering received packets that are not in the correct sequence.

It defines how to start, control and end a session, including the control and management of multiple bidirectional messages so that applications can be notified when only a portion of a continuous message has been completed, so that the data seen by the presentation layer is contiguous, In some cases, if the presentation layer has received all of the data, the data represents the presentation layer.

The main function of this layer is to define the data format and encryption.

An application that communicates with other computers that corresponds to the application's communication service.

What is application layer?

The Application Layer is the seventh layer of the seven-layer OSI model. Application layer interface directly interacts with the application and provides common web application services. The application layer also makes a request to the presentation layer. Application layer is the highest level of open systems, providing services directly for the application process.

Application layer functions

• Transport access and management

It allows a user to access, retrieve and manage files in a remote computer.

It provides the basis for email forwarding and storage facilities.

For various reasons, it can be said that the standardization of terminals has completely failed. The OSI solution to this problem is to define a virtual terminal that is really just an abstract data structure that takes the abstract state of the actual terminal. This abstract data structure can be operated by both the keyboard and the computer and reflects the current state of the data structure on the display. The computer can query this abstract data structure and change this abstract data structure so that the output appears on the screen.

In addition to the three functions above, there are some other functions: directory services, remote job entry, graphics, information communication and so on.

Application layer examples

The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or other resources connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities.

Currently, the limit on domain name length is 63 characters, including www. And .com or other extensions. Domain names are also restricted to only a subset of ASCII characters, making many other languages unable to properly represent their names and words. Punycode-based IDNA systems, which map Unicode strings to valid DNS character sets, have been validated and adopted by some registries as a workaround.

• HTTP (Hypertext Transfer Protocol)

The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed, collaborative, and hypermedia information systems.[1] HTTP is the foundation of data communication for the World Wide Web.

HTTP is a client and server-side standard for request and response (TCP). The client is the end user, the server is the website. Using a web browser, web crawler, or other tools, the client initiates an HTTP request to the specified port on the server. The responding server stores (some) resources, such as HTML files and images. (We call it) This answering server is the origin server. There may be multiple middleware between the user agent and the origin server, such as agents, gateways, or tunnels. Although the TCP / IP protocol is the most popular application on the Internet, the HTTP protocol does not require that it be used and based on the layers it supports. In fact, HTTP can be implemented on any other Internet protocol, or on other networks. HTTP assumes only that the underlying protocol (provided by the underlying protocol) is reliable and that any protocol capable of providing this guarantee can be used by it.

Typically, a request is made by the HTTP client to establish a TCP connection to the server's designated port (the default is port 80). The HTTP server listens on that port for requests sent by the client. Upon receipt of the request, the server sends a status line (to the client), such as "HTTP / 1.1 200 OK," and (response) a message body that may be a requested file, an error message, or some other information.

• FTP (File Transfer Protocol)

FTP services generally run on both ports 20 and 21. Port 20 is used to transmit data flow between the client and server, while port 21 is used to transmit control flow and is the command to import to the ftp server. When data is streamed, the control flow is idle. When the control flow, idle for a long time, the client's firewall, the session will be set to overtime, so when a large amount of data through the firewall, there will be some problems. At this point, although the file can be successfully transmitted, it can be broken by the firewall because of the control session; the transfer can result in some errors.

Application layer protocols

The application layer protocols provide rules for communication between applications. To ensure smooth communication, the application layer protocols implemented on the source host and the destination host must be the same.

Application layer protocol features:

• Define the process for both parties to the communication.
• Define the message type.
• Define the syntax of the message.
• Definition of the meaning of any informational field.
• Define the way to send the message and the expected response.
• Define interaction with the next level.

Prerequisite : OSI Model

Introduction :
The Application Layer is topmost layer in the Open System Interconnection (OSI) model. This layer provides several ways for manipulating the data (information) which actually enables any type of user to access network with ease. This layer also makes a request to its bottom layer, which is presentation layer for receiving various types of information from it. The Application Layer interface directly interacts with application and provides common web application services. This layer is basically highest level of open system, which provides services directly for application process.

Functions of Application Layer :The Application Layer, as discussed above, being topmost layer in OSI model, performs several kinds of functions which are requirement in any kind of application or communication process.

Following are list of functions which are performed by Application Layer of OSI Model –

• Application Layer provides a facility by which users can forward several emails and it also provides a storage facility.
• This layer allows users to access, retrieve and manage files in a remote computer.
• It allows users to log on as a remote host.
• This layer provides access to global information about various services.
• This layer provides services which include: e-mail, transferring files, distributing results to the user, directory services, network resources and so on.
• It provides protocols that allow software to send and receive information and present meaningful data to users.
• It handles issues such as network transparency, resource allocation and so on.
• This layer serves as a window for users and application processes to access network services.
• Application Layer is basically not a function, but it performs application layer functions.
• The application layer is actually an abstraction layer that specifies the shared protocols and interface methods used by hosts in a communication network.
• Application Layer helps us to identify communication partners, and synchronizing communication.
• This layer allows users to interact with other software applications.
• In this layer, data is in visual form, which makes users truly understand data rather than remembering or visualize the data in the binary format (0’s or 1’s).
• This application layer basically interacts with Operating System (OS) and thus further preserves the data in a suitable manner.
• This layer also receives and preserves data from it’s previous layer, which is Presentation Layer (which carries in itself the syntax and semantics of the information transmitted).
• The protocols which are used in this application layer depend upon what information users wish to send or receive.
• This application layer, in general, performs host initialization followed by remote login to hosts.

Working of Application Layer in the OSI model :In the OSI model, this application layer is narrower in scope. 

The application layer in the OSI model generally acts only like the interface which is responsible for communicating with host-based and user-facing applications. This is in contrast with TCP/IP protocol, wherein the layers below the application layer, which is Session Layer and Presentation layer, are clubbed together and form a simple single layer which is responsible for performing the functions, which includes controlling the dialogues between computers, establishing as well as maintaining as well as ending a particular session, providing data compression and data encryption and so on.

At first, client sends a command to server and when server receives that command, it allocates port number to client. Thereafter, the client sends an initiation connection request to server and when server receives request, it gives acknowledgement (ACK) to client through client has successfully established a connection with the server and, therefore, now client has access to server through which it may either ask server to send any types of files or other documents or it may upload some files or documents on server itself.

Features provided by Application Layer Protocols :To ensure smooth communication, application layer protocols are implemented the same on source host and destination host.

The following are some of the features which are provided by Application layer protocols-

• The Application Layer protocol defines process for both parties which are involved in communication.
• These protocols define the type of message being sent or received from any side (either source host or destination host).
• These protocols also define basic syntax of the message being forwarded or retrieved.
• These protocols define the way to send a message and the expected response.
• These protocols also define interaction with the next level.

Application Layer Protocols: The application layer provides several protocols which allow any software to easily send and receive information and present meaningful data to its users.
The following are some of the protocols which are provided by the application layer.

• TELNET: Telnet stands for Telecommunications Network. This protocol is used for managing files over the Internet. It allows the Telnet clients to access the resources of Telnet server. Telnet uses port number 23.
• DNS: DNS stands for Domain Name System. The DNS service translates the domain name (selected by user) into the corresponding IP address. For example- If you choose the domain name as www.abcd.com, then DNS must translate it as 192.36.20.8 (random IP address written just for understanding purposes). DNS protocol uses the port number 53.
• DHCP: DHCP stands for Dynamic Host Configuration Protocol. It provides IP addresses to hosts. Whenever a host tries to register for an IP address with the DHCP server, DHCP server provides lots of information to the corresponding host. DHCP uses port numbers 67 and 68.
• FTP: FTP stands for File Transfer Protocol. This protocol helps to transfer different files from one device to another. FTP promotes sharing of files via remote computer devices with reliable, efficient data transfer. FTP uses port number 20 for data access and port number 21 for data control.
• SMTP: SMTP stands for Simple Mail Transfer Protocol. It is used to transfer electronic mail from one user to another user. SMTP is used by end users to send emails with ease. SMTP uses port numbers 25 and 587.
• HTTP: HTTP stands for Hyper Text Transfer Protocol. It is the foundation of the World Wide Web (WWW). HTTP works on the client server model. This protocol is used for transmitting hypermedia documents like HTML. This protocol was designed particularly for the communications between the web browsers and web servers, but this protocol can also be used for several other purposes. HTTP is a stateless protocol (network protocol in which a client sends requests to server and server responses back as per the given state), which means the server is not responsible for maintaining the previous client’s requests. HTTP uses port number 80.
• NFS: NFS stands for Network File System. This protocol allows remote hosts to mount files over a network and interact with those file systems as though they are mounted locally. NFS uses the port number 2049.
• SNMP: SNMP stands for Simple Network Management Protocol. This protocol gathers data by polling the devices from the network to the management station at fixed or random intervals, requiring them to disclose certain information. SNMP uses port numbers 161 (TCP) and 162 (UDP).

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