Computer Networks Test 6

• Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending emails across the Internet.
• IMAP and POP3 are the Internet mail protocols used for retrieving emails.
• MIME allows the users to exchange different kinds of data files in an email: audio, video, images, etc.
• SMPT, POP3 and SMTP, IMAP are the possible correct answer in which only SMTP, IMAP is present in option D.

PROTOCOL	PORT	DESCRIPTION
FTP	20,21	File Transfer
SMTP	25	Email
HTTP	80	World Wide Web

Hypertext transfer protocol (HTTP):

• HTTP is used to access data on the world wide web. HTTP functions as a combination of FTP and SMTP. HTTP uses the services of TCP port number 80. HTTP is a stateless protocol.
• HTTP prior to version 1.1 specified a non-persistent connection, while a persistent connection is a default.
• In a non-persistent connection, one TCP connection is made for each request/ response. The client opens a TCP connection and sends a request and server sends the response then closes the connection.

File Transfer Protocol (FTP):

• FTP stands for file transfer protocol. It is provided by TCP/IP for copying a file from one host to another. FTP can transfer a file through one of the following three modes: stream mode, block mode and compressed mode. Stream mode is the default mode.
• FTP uses two TCP connections one for data and another for control.

Simple Transfer Protocol (SMTP):

• SMTP is a push protocol, it can push the messages from the client to the server only.
• SMTP uses one TCP connection at a time

TELNET:

• TELNET is a client/ server application that allows a user to log on to a remote machine, giving the user access to the remote system.
• TELNET uses one TCP connection at a time.

Important Point:

Option 1 is not correct since HTTP might have more than one TCP connections for a single web page.

• SMTP is a simple ASCII protocol. The use of SMTP with extensions is called ESMTP (Extended SMTP).
• Extended HELO (EHLO) is an Extended Simple Mail Transfer Protocol (ESMTP) command sent by an email server to identify itself when connecting to another email server to start the process of sending an email.

Options in ESMTP:

Keyword	Description
AUTH	Client authentication
BINARYMIME	Server accepts binary messages
CHUNKING	Server accepts large messages in chunks
SIZE	Check message size before trying to send
STARTTLS	Switch to secure transport
UTF8SMTP	Internationalized addresses

When both sender and receiver are connected the mail server via a LAN or a WAN we need two user agents, two pairs of message transfer agents and one pair of message access agents. As SMTP is a push protocol, it can push the messages from the client to the server only, it cannot pull the messages from the server to the client.

Concept:

Stateless Protocols are the type of network protocols in which the client sends the request to the server and server response back according to the current state.

It does not require the server to retain session information or status about each communicating partner for multiple requests.

Explanation:

Application Layer protocol: HTTP and DNS, POP3

Transport Layer protocol: UDP

Stateless protocol: HTTP, UDP, DNS

Stateful protocol: POP3

Therefore, DNS and HTTP is a stateless application layer protocol

Important Point:

The Domain Name System (DNS) is a hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet or a private network.

It translates more readily memorized domain names to the numerical IP addresses needed for locating and identifying computer services and devices with the underlying network protocols.

The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, where hypertext documents include hyperlinks to other resources that the user can easily access.

Concept:

HTTP requests are of two types: persistent and non- persistent.

In persistent mode, a single TCP connection is required for the entire HTTP session and all the objects fetched in that session.

However, in the non- persistent mode, one TCP connection is required for each object to be fetched from the webserver.

Explanation:

The web page contains text and 4 small images, that is, total 5 objects. Therefore, in non- persistent mode, 5 TCP connections would be required. 

Important point:

If in the question, term ‘persistent’ was used instead of ‘non-persistent’, only 1 TCP connection would have been needed.

The correct order in which the following actions take place in an interaction between a web browser and a web server

Step 1:

The web browser first needs to figure out IP address of site from URL using DNS. This is typically called domain name resolution.

Step 2:

TCP connection is established mostly at port 80.

Step 3:

Once the TCP connection is established, the browser sends an HTTP request using GET method.

Step 4:

Finally, web server responds with HTTP response that is, the page we requested.

Hence, the correct sequence of actions is 4, 2, 1, 3

The basic SMTP works well, but it is limited in several respects:

→ It does not include authentication. This means that the FROM command give any sender address that it wants. This is quite useful for sending spam.

→ Another limitation is that SMTP transfers ASCII messages, not binary data. Therefore, the base64 MIME content transfer encoding was needed. However, with that encoding the mail transmission uses bandwidth inefficiently, which is an issue for large messages.

→ A third limitation is that SMTP sends messages in the clear. It has no encryption to provide a measure of privacy against prying eyes.

• FTP is out-of-band protocol since control connection and data connection operates on different port number.
• FTP is stateful protocol because it knows about the user activity and maintain a record of it
• FTP uses TCP at transport layer for reliability.