Here are explanations of the requested concepts: 1. SMTP (Simple Mail Transfer Protocol): - Used for sending and receiving email messages between servers - Works on TCP port 25 - Client (email sender) connects to SMTP server and sends the email - SMTP server then handles delivering the email to recipient's SMTP server 2. POP (Post Office Protocol): - Used for retrieving email from a remote server to a local machine - Works on TCP port 110 (POP3) - User must first authenticate with their username and password - POP downloads all emails from the server to the local machine - Emails are then deleted from the server by default 3. IMAP (Internet Message Access Protocol):

1. 26.1
ELECTRONIC MAILELECTRONIC MAIL
One of the most popular Internet services is electronicOne of the most popular Internet services is electronic
mail (e-mail). The designers of the Internet probablymail (e-mail). The designers of the Internet probably
never imagined the popularity of this applicationnever imagined the popularity of this application
program. Its architecture consists of severalprogram. Its architecture consists of several
components that we discuss in this chapter.components that we discuss in this chapter.
Architecture
User Agent
Message Transfer Agent: SMTP
Message Access Agent: POP and IMAP
Web-Based Mail
Topics discussed in this section:Topics discussed in this section:

2. 26.2
First scenario in electronic mail

3. 26.3
When the sender and the receiver of an
e-mail are on the same system,
we need only two user agents.
Note

4. 26.4
Second scenario in electronic mail

5. 26.5
When the sender and the receiver of an
e-mail are on different systems, we
need two UAs and a pair of
MTAs (client and server).
Note

6. 26.6
Third scenario in electronic mail

7. 26.7
When the sender is connected to the
mail server via a LAN or a WAN,
we need two UAs and two pairs
of MTAs (client and server).
Note

8. 26.8
Fourth scenario in electronic mail

9. 26.9
Push versus pull in electronic email

10. 26.10
When both sender and receiver are
connected to the mail server via
a LAN or a WAN, we need two
UAs, two pairs of MTAs
and a pair of MAAs.
This is the most common situation
today.
Note

11. 26.11
Services of user agent

12. 26.12
Some examples of command-driven
user agents are mail, pine,
and elm.
Note

13. 26.13
Some examples of GUI-based user
agents are Eudora, Outlook, and
Netscape.
Note

14. 26.14
Format of an e-mail

15. 26.15
E-mail address

16. 26.16
MIME (Multipurpose Internet mail Extension)

17. 26.17
MIME header

18. 26.18
Data types and subtypes in MIME

19. 26.19
Content-transfer-encoding

20. 26.20
SMTP range

21. 26.21
Commands and responses

22. 26.22
Command format

23. 26.23
POP3 and IMAP4 (Internet mail access protocol)

24. 26.24
The exchange of commands and responses in POP3

25. 26.25
FILE TRANSFERFILE TRANSFER
Transferring files from one computer to another is oneTransferring files from one computer to another is one
of the most common tasks expected from a networkingof the most common tasks expected from a networking
or internetworking environment. As a matter of fact,or internetworking environment. As a matter of fact,
the greatest volume of data exchange in the Internetthe greatest volume of data exchange in the Internet
today is due to file transfer.today is due to file transfer.
File Transfer Protocol (FTP)
Anonymous FTP
Topics discussed in this section:Topics discussed in this section:

26. 26.26
FTP uses the services of TCP. It needs
two TCP connections.
The well-known port 21 is used for the
control connection and the well-known
port 20 for the data connection.
Note

27. 26.27
Figure FTP

28. 26.28
Figure Using the control connection

29. 26.29
Figure 26.23 Using the data connection

30. 27.30
27-3 HTTP27-3 HTTP
The Hypertext Transfer Protocol (HTTP) is a protocolThe Hypertext Transfer Protocol (HTTP) is a protocol
used mainly to access data on the World Wide Web.used mainly to access data on the World Wide Web.
HTTP functions as a combination of FTP and SMTP.HTTP functions as a combination of FTP and SMTP.
HTTP Transaction
Persistent Versus Nonpersistent Connection
Topics discussed in this section:Topics discussed in this section:

31. 27.31
HTTP uses the services of TCP on well-
known port 80.
Note

32. 27.32
Figure 27.12 HTTP transaction

33. 27.33
Figure 27.13 Request and response messages

34. 27.34
Figure 27.14 Request and status lines

35. 27.35
Table 27.1 Methods

36. 27.36
Table 27.2 Status codes

37. 27.37
Table 27.2 Status codes (continued)

38. 27.38
Figure 27.15 Header format

39. 27.39
A header line belong to one of four categories:
1.General Header
2.Request Header
3.Response Header
4.Entity Header
Note

40. 27.40
Table 27.3 General headers

41. 27.41
Table 27.4 Request headers

42. 27.42
Table 27.5 Response headers

43. 27.43
Table 27.6 Entity headers

44. 27.44
This example retrieves a document. We use the GET
method to retrieve an image with the path
/usr/bin/image1. The request line shows the method
(GET), the URL, and the HTTP version (1.1). The header
has two lines that show that the client can accept images
in the GIF or JPEG format. The request does not have a
body. The response message contains the status line and
four lines of header. The header lines define the date,
server, MIME version, and length of the document. The
body of the document follows the header (see Figure
27.16).
Example 27.1

45. 27.45
Figure 27.16 Example 27.1

46. 27.46
In this example, the client wants to send data to the
server. We use the POST method. The request line shows
the method (POST), URL, and HTTP version (1.1). There
are four lines of headers. The request body contains the
input information. The response message contains the
status line and four lines of headers. The created
document, which is a CGI document, is included as the
body (see Figure 27.17).
Example 27.2

47. 27.47
Figure 27.17 Example 27.2

48. 27.48
HTTP uses ASCII characters. A client can directly
connect to a server using TELNET, which logs into port
80 (see next slide). The next three lines show that the
connection is successful. We then type three lines. The
first shows the request line (GET method), the second is
the header (defining the host), the third is a blank,
terminating the request. The server response is seven
lines starting with the status line. The blank line at the
end terminates the server response. The file of 14,230
lines is received after the blank line (not shown here).
The last line is the output by the client.
Example 27.3

49. 27.49
Example 27.3 (continued)

50. 27.50
HTTP version 1.1 specifies a persistent
connection by default.
Note

51. 26.51
Computer Science & Engineering
Assignment
Semester- VI (I & II) Branch: CSE
Subject with Code: Computer Networks (IT-305-F)
Q:1 Explain the Following.
1. SMTP,
2. POP,
3. IMAP;
4. FTP,
5. HTTP