CCNAHelp.Com

7 layer OSI Model

The Transport Layer

:

In technical terms the server is said to be listening on a port. A port is nothing but a file pointer. It is only logical because all the incoming requests need buffering (say 1000 clients accessing the mail server at the same instance of time). All the requests are buffered in a file and the server continuously checks the file for requests.

Unless we are talking about a multithreaded server, the requests would be catered on FIFO (First In First Out) basis. Mail servers are mostly multithreaded in that they can cater to several requests at a time. Well there is a difference between having only one service personnel catering to the customers at the ticket counter and having 100 service personnel, that's the difference between a single threaded server and multithreaded server. Now like I said the server application is designed to be listening continuously on a port and yet again I would like to remind you that a port is nothing but a file pointer. In the case of a mail server the applications running are SMTP and POP3. SMTP is the application which takes care of allowing the users to access their mail either by directly checking it on the server or by downloading the mails to the individual clients.

SMTP listens on port 23 in accordance with the standards. POP3 is the application which takes care of receiving mails, say when you send a mail to your friend your mail would initially get into the POP3 buffer of your mail server and from there it would be sent to the POP3 buffer of you friend's mail server. When your friend accesses his mail-server to read his mails he is first authenticated by the server (Remember the username and password screen) and then the SMTP application interfaces with the POP3 buffer interface and obtains the mails for the your friends and either dumps it his local pc or allows him to read it on the server itself. POP3 listens on port 110 according to standards.

Continue...