Let us discuss about some important application layer protocols in computer networks;
Domain Name System:
Domain Name System (DNS) is a hierarchical naming system that is built on a distributed database for computers or servers containing resources which is connected to the Internet or a private network.
Simple Mail Transfer Protocol:
The Simple Mail Transfer Protocol (SMTP) is used to transfer electronic mail from one user to another.
This is performs by using the email client software that is User Agents, which is uses by the user.
The User Agents help the user to format and type the e-mail and store it until internet is accessible.
When an email is ready to be sending, then the sending process is handled by Message Transfer Agent which comes inbuilt in email client software.
Message Transfer Agent uses SMTP to forward the email to another Message Transfer Agent that is present at the Server side.
While SMTP is used by end user to only send the emails, the Server generally uses SMTP for both purposes that are sending as well as receiving emails.
SMTP uses TCP port number 25 and 587.
Client software uses IMAP that is Internet Message Access Protocol to receive emails.
File Transfer Protocol:
The File Transfer Protocol (FTP) is the most widely used protocol for transferring files over the network.
FTP uses TCP/IP for communication and it works on TCP port 21.
FTP works on both Client side and Server side model where a client requests for file from Server and server responds to the request and then send the file back to the client.
FTP uses TCP port 20 for exchanging controlling information and the actual data is sent over TCP port 21.
First the client requests the server for a file.
Server receives the request then; it opens a TCP connection for the client and transfers the file.
After the transferring of files is completed; the server closes the connection.
For the second file, client requests again and the server reopens a new TCP connection; in this way the process of File Transfer Protocol works.
Post Office Protocol (POP):
The Post Office Protocol version that is known as POP is a simple mail retrieval protocol used by User Agents that is client email software to retrieve mails from mail server.
When a client needs to retrieve mails from the server, then it opens a connection with the server on TCP port 110 and after that user can access their mails and download those to the local computers.
POP works in two modes:-
The most common mode is the delete mode, which deletes the e-mails from the remote server after the e-mails are downloaded in local systems.
The second mode is known as the keep mode, which does not delete e-mail from the mail server and it gives the user an option to access mails later on mail server.
Hyper Text Transfer Protocol (HTTP):
The Hyper Text Transfer Protocol generally known as 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; so when a user wants to access any HTTP page on the internet, then the client machine at user end initiates a TCP connection to server on port 80.
Now Server accepts the client request and responds to it, so the client becomes authorized to access web pages.
Client uses web browsers to access the web pages, which are responsible for initiating, maintaining and finally closing the TCP connections.
HTTP is a stateless protocol that is why the Server need not maintains any information about earlier requests by clients.
Let us now discuss about some important network layer protocols in TCP/IP model architecture.
IP (Internet Protocol):
The Internet Protocol (IP) is the network layer communications protocol that works in the Internet layer of TCP/IP model.
It is uses in order to address the datagram with the label of source and destination information.
ARP (Address Resolution Protocol):
The Address Resolution Protocol that is known as ARP in short and it is a network layer protocol of TCP/IP model.
It is uses for discovering the MAC address that is associated with a given internet layer address, which is typically known as the IPv4 address.
Therefore this mapping of IP Address with the MAC Address is a critical function in the Internet protocol suite.
ICMP (Internet Control Message Protocol):
The Internet Control Message Protocol, which is known as ICMP; it is a supporting protocol in the Internet protocol suite, which is used by networking devices, like the routers, it sends error messages and important operational information.
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