Application Layer (HTTP,FTP,SMTP,DNS)

Application Layer


·         HTTP is a stateless  protocol

·         When all request are sent over the same TCP connection then it is called a persistent TCP connection and when request are sent over different TCP connections then is called non-persistent TCP connection

·         In persistent connection the server leaves the connection open and a whole web page can be received in the same connection. No need to wait for a response in this connection.

·         The get method is used when the browser request an object and the requested object is identified in the URL field.

·         The post method is used then the entity body will not be empty unlike get method where it is empty.

·         When the Head method is used the server responds with a HTTP message but leaves out the requested object. This is used mainly for debugging purposes.

·         The PUT method is used by applications to upload objects to the web server and the Delete method is used to delete objects from the web server.

·         In the HTTP message the Client uses the CONNECTION: close to tell the server it does not want to bother with the persistent connections. When the server uses the connection: Close it says that it is closing the connection.

·         The conditional GET statement is a request message that uses a GET message and has a IF MODIFIED SINCE: header line included.

·         The user agent field notifies the server which browser is being used.

·         HTTP sends its control information INBAND



·         Runs on top of TCP

·         FTP opens two connections control connection(sending control Information) and data connection(sending data)

·         FTP sends it control information OUTOFBAND

·         FTP maintains state.

·         Only one FTP file is sent in one data connection. To send multiple files multiple data connections are established.

·         Only one control connection is established throughout the session.

·         FTP commands are readable by people


·         Asynchronous communication

·         A line consisting of a single period indicates the end of message to the server.

·         SMTP uses persistent connections.

·         SMTP is basically a push protocol while HTTP is a PULL protocol. SMTP places restriction on the data while SMTP does not place restrictions on the data. HTTP encapsulates only one object while SMPT encapsulates the whole message.

·         The header line and body are separated by a blanlk line.

·         We cannot use SMTP to retrieve mails from a server as it is a push protocol and not a pull protocol.

·         POP3 there are three phases authorization,  transaction and update. IN POP3 u are given an option of either downloading and delte the mail or downloading and keep.

·         IMAP more features and more complicated than POP#. Allows the user to maintain remote folders.


·         DNS is a distributed database and an application layer protocol that allows host to query the distributed database.

·         DNS servers are UNIX machines that run on BIND server. DNS uses UDP and port no. 53

·         Different  Services are  host aliasing, Mail server aliasing, load distribution.

·         DNS distributed database stores resource records and there are different types of 4 types.


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