Data Communications & Computer Networks


Domain Name System

         To be unambiguous, the names assigned to machines must be carefully selected from a name space with complete control over the binding between the names and IP addresses.
         To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127.

    Domain names and labels:


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

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

    Generic domain labels:


         Mapping a name to an address or an address to a name is called name-address resolution.
         DNS has two types of messages: query and response. Both types have the same format. The query message consists of a header and question records; the response message consists of a header, question records, answer records, authoritative records, and additional records.

         Two types of records are used in DNS. The question records are used in the question section of the query and response messages. The resource records are used in the answer, authoritative, and additional information sections of the response message.
         How are new domains added to DNS? This is done through a registrar, a commercial entity accredited by ICANN. A registrar first verifies that the requested domain name is unique and then enters it into the DNS database. A fee is charged.
         The DNS master file must be updated dynamically. The Dynamic Domain Name System (DDNS) therefore was devised to respond to this need. In DDNS, when a binding between a name and an address is determined, the information is sent, usually by DHCP to a primary DNS server. The primary server updates the zone. The secondary servers are notified either actively or passively.
         DNS can use either UDP or TCP. In both cases the well-known port used by the server is port 53. UDP is used when the size of the response message is less than 512 bytes because most UDP packages have a 512-byte packet size limit. If the size of the response message is more than 512 bytes, a TCP connection is used. DNS can use the services of UDP or TCP using the well-known port 53.
         It would be impossible to write a specific client/server program for each demand. The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer. TELNET is a general-purpose client/server application program.
    Local and remote log-in:

    Concept of NVT(Network Virtual Terminal):

    Some NVT control characters:


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

    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).

    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 pairsof MTAs (client and server).

    Fourth scenario in electronic mail:
         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.

    Services of user agent:
         Some examples of command-driven user agents are mail, pine, and elm.Some examples of GUI-based user agents are Eudora, Outlook, and Netscape.

         Transferring files from one computer to another is one of the most common tasks expected from a networking or internetworking environment. As a matter of fact, the greatest volume of data exchange in the Internet today is due to file transfer. 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.

    Using the data connection:

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