email and dns.. in computer networks ppt

ELECTRONIC MAIL ELECTRONIC MAIL One of the most popular Internet services is electronic mail One of the most popular Internet services is electronic mail (e-mail). The designers of the Internet probably never (e-mail). The designers of the Internet probably never imagined the popularity of this application program. Its imagined the popularity of this application program. Its architecture consists of several components that we discuss architecture consists of several components that we discuss in this chapter. in this chapter.

Figure 26.6 First scenario in electronic mail

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

Figure 26.7 Second scenario in electronic mail

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

Figure 26.8 Third scenario in electronic mail

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

Figure 26.9 Fourth scenario in electronic mail

Figure 26.10 Push versus pull in electronic email

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

Figure 26.11 Services of user agent

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

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

Figure 26.12 Format of an e-mail

Figure 26.17 Commands and responses

Protocols used in Email Message Transfer Agent: SMTP

NAME SPACE NAME SPACE To be unambiguous, the names assigned to machines To be unambiguous, the names assigned to machines must be carefully selected from a name space with must be carefully selected from a name space with complete control over the binding between the names complete control over the binding between the names and IP addresses. and IP addresses.

DOMAIN NAME SPACE DOMAIN NAME SPACE To have a hierarchical name space, a domain name To have a hierarchical name space, a domain name space was designed. In this design the names are space was designed. In this design the names are defined in an inverted-tree structure with the root at defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 the top. The tree can have only 128 levels: level 0 (root) to level 127. (root) to level 127.

Figure 25.3 Domain names and labels

DISTRIBUTION OF NAME SPACE DISTRIBUTION OF NAME SPACE The information contained in the domain name space The information contained in the domain name space must be stored. However, it is very inefficient and also must be stored. However, it is very inefficient and also unreliable to have just one computer store such a huge unreliable to have just one computer store such a huge amount of information. In this section, we discuss the amount of information. In this section, we discuss the distribution of the domain name space. distribution of the domain name space.

A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads information from the primary, it is called zone transfer. Note

DNS IN THE INTERNET DNS IN THE INTERNET DNS is a protocol that can be used in different DNS is a protocol that can be used in different platforms. In the Internet, the domain name space platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic (tree) is divided into three different sections: generic domains, country domains, and the inverse domain. domains, country domains, and the inverse domain.

Table 25.1 Generic domain labels

RESOLUTION RESOLUTION Mapping a name to an address or an address to a Mapping a name to an address or an address to a name is called name-address resolution. name is called name-address resolution.

email and dns.. in computer networks ppt

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email and dns.. in computer networks ppt