application layer protocol ( dns smtp pop ftp http)

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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 types 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.

 SMTP (Simple Mail Transfer Protocol):

• One of the most popular network service 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.
• Users 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 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.

TELNET (Terminal Network):

• TELNET is client-server application that allows a user to log onto remote machine and lets the user to 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.

FTP (File Transfer Protocol):

• FTP is the standard mechanism provided by TCP/IP for copying a file from one host to another.
• FTP differs form 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 transfer 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 headers 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.

POP (Post Office Protocol):

• POP is also called as 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 mail server to the user's hard disk.

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 respository 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.
• Domain name space is divided into three categories.
• Generic Domain: The generic domain defines registered hosts according, to their generic behaviour. 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 country abbreviations (e.g., US for United States) in place of 3 characters.
• Inverse Domain: The inverse domain is used to map an address to a name.

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