· 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.