Role of Application Layer

• The Application Layer is the upper-most layer in the OSI & TCP/IP models.
• This layer manipulates the information so that any user can access the network very easily.
• This layer is the highest level of a system, which provides services directly for the application processes.
• This layer allows the user to use the services of the network, virtual terminals.
• Therefore this layer is also used to create network-based applications.
• This layer gives several user facilities such as user login, naming network devices, formatting messages, and e-mails, transfer of files, etc.
• This layer also handles errors and recovery of the message as a whole.

Main functionalities of Application Layer are as follows:

Mail services: An application layer enables the basic service of email forwarding and storage.

Directory services: An application can provide the distributed database sources and give global information access for several facilities, objects.

File Transfer, Access, and Management (FTAM): An application layer is responsible for users accessing the files on a remote computer, retrieving the files from a computer, and managing the files on a remote computer.

Application Layer provides several protocols that allow the software to send and receive information and present meaningful data to users.

Several Application Layer Protocol are as follows:

• TELNET

• HyperText Transfer Protocol (HTTP)

• File Transfer Protocol (FTP)

• Network File System (NFS)

• Domain Name System (DNS)

• Dynamic Host Configuration Protocol (DHCP)

• Simple Mail Transfer Protocol (SMTP)

• Post Office Protocol (POP)

• Simple Network Management Protocol (SNMP)

Let's see Simple Mail Transfer Protocol (SMTP) in brief.

Simple Mail Transfer Protocol (SMTP)

• SMTP is mainly used for sending emails efficiently and reliably over the internet.
• SMTP is a push protocol and uses TCP at the transport layer.
• SMTP uses persistent TCP connections, so it can send multiple emails at once.
• SMTP is a connection-oriented, stateless, and in-band protocol.
• SMTP uses port number 25.
• SMTP is a general set of interaction guidelines that allow the software to transmit electronic mail over the internet.

• It set up communication rules between servers and allows the exchange of emails between the users on the same or different computers.

• SMTP is a pure text-based protocol. It can only handle the messages containing 7 bit ASCII text and can not transfer other types of data like images, video, audio, etc.

• SMTP can not transfer executable files and binary objects.
• MIME extends the limited capabilities of email. It enables the users to send and receive graphics, audio files, video files, etc in the message.MIME was specially designed for SMTP.
• SMTP is a push protocol. It can not be used at the receiver’s side. At the receiver’s side, a pull protocol like POP3, IMAP is needed.
• Sender and receiver can not run SMTP between their machines at the same time. Because machines can not always be ON.
• Hence, the functionality has been divided between the client and the mail server. The mail server receives the mail on behalf of its client and manages the mailbox of the client.
• SMTP is not suitable for client authentication. SMTP does not require authentication. It allows anyone on the Internet to send emails to anyone or even to a large group of people.
• Sometimes SMTP Auth stands for SMTP Authentication has been provided for authentication purposes.

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