The Application layer is the top most layer of an OSI model. It is responsible to provide service to the users. The application layer enables the user, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.
The Application layer includes the following functions:
- Identifying communication partners: The application layer identifies the availability of communication partners for an application with data to transmit.
- Determining resource availability: The application layer determines whether sufficient network resources are available for the requested communication.
- Synchronizing communication: All the communications occur between the applications requires cooperation which is managed by an application layer.
APPLICATION LAYER PROTOCOLS:
An application layer protocol defines how application processes (clients and servers), running on different end systems, pass messages to each other. In particular, an application layer protocol defines:
· The type of messages, e.g., request messages and response messages.
· The syntax of the various message types, i.e., the fields in the message and how the fields are delineated.
· The semantics of the fields, i.e., the meaning of the information that the field is supposed to contain;
· Rules for determining when and how a process sends messages and responds to messages.
Various Application layer protocols are:
SMTP (Simple Mail Transfer Protocol):
- The TCP/IP protocol that supports electronic mail (e-mail) on the Internet is called Simple Mail Transfer Protocol (SMTP).
- SMTP transfers messages from sender’s mail servers to the recipient’s mail servers using TCP connections.
- SMTP provides services for mail exchange between users on the same or different computers.
- Following the client/server model:
- SMTP has two sides: a client side which executes on a sender’s mail server, and server side which executes on recipient’s mail server.
- Both the client and server sides of SMTP run on every mail server.
- When a mail server sends mail (to other mail servers), it acts as an SMTP client.
- When a mail server receives mail (from other mail servers) it acts as an SMTP server
POP (Post Office Protocol):
- POP is also called as POP3 protocol.
- It is an application layer internet standard protocol.
- Since POP supports offline access to the messages, thus requires less internet usage time.
- POP does not allow search facility.
- In order to access the messages, it is necessary to download them.
- It allows only one mailbox to be created on server.
- It is not suitable for accessing non mail data.
TELNET (Terminal Network):
· TELNET is an application protocol that allows a user to communicate with a remote device. A user on a client machine can use a software (known as a Telnet client) to access a command-line interface of another, remote machine that is running a Telnet server program.
· TELNET is often used by network administrators to access and manage remote devices. A network administrator can access the device by telnetting to the IP address or hostname of a remote device. The network administrator will then be presented with a virtual terminal that can interact with the remote host.
- TELNET uses the NVT (Network Virtual Terminal) system to encode characters on the local system.
- On the server (remote) machine, NVT decodes the characters to a form acceptable to the remote machine.
- TELNET is a protocol that provides a general, bi-directional, eight-bit byte-oriented communications facility.
- Many application protocols are built upon the TELNET protocol
- Telnet services are used on PORT 23.
FTP (File Transfer Protocol):
- FTP is the standard mechanism provided by TCP/IP for copying a file from one host to another.
- FTP differs from other client-server applications because it establishes 2 connections between hosts.
- Two connections are: Data Connection and Control Connection.
- Data Connection uses PORT 20 for the purpose and control connection uses PORT 21 for the purpose.
- FTP is built on a client-server architecture and uses separate control and data connections between the client and the server.
- One connection is used for data transfer, the other for control information (commands and responses).
- It transfers data reliably and efficiently.
Multipurpose Internet Mail Extensions (MIME):
- It is an extension of SMTP that allows the transfer of multimedia messages.
- If binary data is included in a message MIME header are used to inform the receiving mail agent:
- Content-Transfer-Encoding: Header alerts the receiving user agent that the message body has been ASCII encoded and the type of encoding used.
- Content-Type: Header informs the receiving mail agent about the type of data included in the message.
HTTP (Hypertext Transfer Protocol):
- This is a protocol used mainly to access data on the World Wide Web (www).
- The Hypertext Transfer Protocol (HTTP) the Web’s main application-layer protocol although current browsers can access other types of servers
- A repository of information spread all over the world and linked together.
- The HTIP protocol transfer data in the form of plain text, hyper text, audio, video and so on.
- HTTP utilizes TCP connections to send client requests and server replies.
- It is a synchronous protocol which works by making both persistent and non-persistent connections.
Domain Name System (DNS):
- To identify an entity, TCP/IP protocol uses the IP address which uniquely identifies the connection of a host to the Internet.
- DNS is a hierarchical system, based on a distributed database, that uses a hierarchy of Name Servers to resolve Internet host names into the corresponding IP addresses required for packet routing by issuing a DNS query to a name server.
- However, people refer to use names instead of address. Therefore, we need a system that can map a name to an address and conversely an address to name.
- In TCP/IP, this is the domain name system.
- DNS in the Internet: DNS is protocol that can be used in different platforms.
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