What are the three application layer protocols?

There are several protocols which work for users in Application Layer. Application layer protocols can be broadly divided into two categories:

• Protocols which are used by users.For email for example, eMail.

• Protocols which help and support protocols used by users.For example DNS.

Few of Application layer protocols are described below:

Domain Name System

The Domain Name System (DNS) works on Client Server model. It uses UDP protocol for transport layer communication. DNS uses hierarchical domain based naming scheme. The DNS server is configured with Fully Qualified Domain Names (FQDN) and email addresses mapped with their respective Internet Protocol addresses.

A DNS server is requested with FQDN and it responds back with the IP address mapped with it. DNS uses UDP port 53.

Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) is used to transfer electronic mail from one user to another. This task is done by means of email client software (User Agents) the user is using. User Agents help the user to type and format the email and store it until internet is available. When an email is submitted to send, the sending process is handled by Message Transfer Agent which is normally comes inbuilt in email client software.

Message Transfer Agent uses SMTP to forward the email to another Message Transfer Agent (Server side). While SMTP is used by end user to only send the emails, the Servers normally use SMTP to send as well as receive emails. SMTP uses TCP port number 25 and 587.

Client software uses Internet Message Access Protocol (IMAP) or POP protocols to receive emails.

File Transfer Protocol

The File Transfer Protocol (FTP) is the most widely used protocol for file transfer over the network. FTP uses TCP/IP for communication and it works on TCP port 21. FTP works on Client/Server Model where a client requests file from Server and server sends requested resource back to the client.

FTP uses out-of-band controlling i.e. FTP uses TCP port 20 for exchanging controlling information and the actual data is sent over TCP port 21.

The client requests the server for a file. When the server receives a request for a file, it opens a TCP connection for the client and transfers the file. After the transfer is complete, the server closes the connection. For a second file, client requests again and the server reopens a new TCP connection.

Post Office Protocol (POP)

The Post Office Protocol version 3 (POP 3) is a simple mail retrieval protocol used by User Agents (client email software) to retrieve mails from mail server.

When a client needs to retrieve mails from server, it opens a connection with the server on TCP port 110. User can then access his mails and download them to the local computer. POP3 works in two modes. The most common mode the delete mode, is to delete the emails from remote server after they are downloaded to local machines. The second mode, the keep mode, does not delete the email from mail server and gives the user an option to access mails later on mail server.

Hyper Text Transfer Protocol (HTTP)

The Hyper Text Transfer Protocol (HTTP) is the foundation of World Wide Web. Hypertext is well organized documentation system which uses hyperlinks to link the pages in the text documents. HTTP works on client server model. When a user wants to access any HTTP page on the internet, the client machine at user end initiates a TCP connection to server on port 80. When the server accepts the client request, the client is authorized to access web pages.

To access the web pages, a client normally uses web browsers, who are responsible for initiating, maintaining, and closing TCP connections. HTTP is a stateless protocol, which means the Server maintains no information about earlier requests by clients.

HTTP versions

• HTTP 1.0 uses non persistent HTTP. At most one object can be sent over a single TCP connection.

• HTTP 1.1 uses persistent HTTP. In this version, multiple objects can be sent over a single TCP connection.

What is the Application Layer?

The application layer is the topmost layer of the OSI and TCP/IP models. The application layer in the TCP/IP model is created by combining the top three layers: the application layer, the presentation layer, and the session layer.

Application Layer Protocols PDF

An application layer is an abstraction layer that specifies the shared communications protocols and interface methods that host in network use. It is the layer closest to the end user, implying that both the application layer and the end user can interact with the software application directly.

Application Layer Protocols

The application layer protocols used to make communication between sender and receiver faster, more efficient, reliable, and secure. These protocols will be discussed further below.

• SMTP
• TELNET
• FTP
• MIME
• POP
• HTTP
• DNS

Simple Mail Transfer Protocol(SMTP)

One of the most popular application layer protocols for network services is electronic mail (e-mail). The TCP/IP protocol that supports electronic mail on the Internet is called Simple Mail Transfer Protocol (SMTP).

SMTP transfers messages from senders' mail servers to the recipients' mail servers using TCP connections. In SMTP, users are based on e-mail addresses. 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 a server-side, which executes on the 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.

Terminal Network(TELNET)

TELNET is an application layer protocol in which a client-server application allows a user to log onto a remote machine and lets the user access any application program on a remote computer. 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.

File Transfer Protocol(FTP)

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, and control connection uses PORT 21. 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). The FTP is data reliably and efficiently.

Multipurpose Internet Mail Extensions (MIME)

The Multipurpose Internet Mail Extensions (MIME) is an extension of SMTP that allows the transfer of multimedia messages. If binary data is included in a message, MIME headers are used to inform the receiving mail agent that is as follows:

• Content-Transfer-Encoding: The header alerts the receiving user agent that the message body has been ASCII encoded and the type of encoding used.
• Content-Type: The header informs the receiving mail agent about the type of data in the message.

Post Office Protocol(POP)

POP(Post Office Protocol) is also called the POP3 protocol. This is a protocol used by a mail server in conjunction with SMTP to receive and holds mail for hosts.POP3 mail server receives e-mails and filters them into the appropriate user folders.

When a user connects to the mail server to retrieve his mail, the messages are downloaded from the mail server to the user's hard disk.

Hypertext Transfer Protocol(HTTP)

Hypertext Transfer Protocol(HTTP) is used mainly to access World Wide Web (www) data. The Hypertext Transfer Protocol (HTTP) is 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 HTTP protocol transfers data in plain text, hypertext, audio, video, etc. HTTP utilizes TCP connections to send client requests and server replies. It is a synchronous protocol that works by making both persistent and non-persistent connections.

Domain Name System(DNS)

In Domain Name System(DNS), TCP/IP protocol uses the IP address that uniquely identifies a host's connection to the Internet to identify an entity. 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.

DNS in the Internet: DNS is a protocol that can be used on different platforms.

Domain name space is divided into three categories.

• Generic Domain: The generic domain defines registered hosts according to their generic behavior. Each node in the tree defines a domain which is an index to the domain name space database.

• Country Domain: The country domain section follows the same format as the generic domain but uses 2 characters of country abbreviations (e.g., the US for the United States) instead of 3 characters.
• Inverse Domain: The inverse domain maps an address to a name.

Overview of Services

Application Layer Protocol Examples

Examples of application layer protocols include the X.400 Message Handling Service Protocol allowing email transfer between compatible systems.

• The Simple Network Management Protocol (SNTP) provides remote host management.
• Use the Hypertext Transfer Protocol (HTTP) for message or text communications.
• To accept and store mail for hosts, a mail server employs the POP (Post Office Protocol) protocol in conjunction with SMTP.

What are application layer protocols?

What three application layer protocols are the part of TCP or IP protocol suite?

Which three protocols operate at the application layer of the TCP IP model choose three 3 a Arp B TCP C UDP D FTP E POP3 F DHCP?