Application Layer-Data Communication and Computer Networks-Lecture Slides, Slides of Data Communication Systems and Computer Networks

Download Application Layer-Data Communication and Computer Networks-Lecture Slides and more Slides Data Communication Systems and Computer Networks in PDF only on Docsity!

Application Layer

Domain Name System

Application layer The application layer is responsible for providing

services to the user.

Domain Name System

 A user of an e-mail program may know the e-mail address of the recipient, however IP protocol needs the IP address.

 The DNC client program sends a request to a DNS server to map the e-mail address to the corresponding IP address.

Domain Name Space

 This tree structure can have only 128 levels from 0 to 127.

Label

Each node in the tree has a label, which is string with a maximum of 63 characters.

Domain Name

Each node in the tree has a domain name; a full domain name is a sequence of labels separated by dots (.).

 The domain names are always read from the node up to the root.

 The last label is the label of the root (null), this means that a full domain name always end in a null label, which means the last character is a dot because the null string is nothing.

Domain names and labels

Domain

Distributed of Name Space

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.

Hierarchy of Name Servers

The solution to these problems is to distribute the information among many computers called DNS servers.

Note

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 in the Internet

 DNS in the Internet is divided into three domains

1. Generic domains
2. Country domains
3. Inverse domains
4. Generic Domains define registered hosts according to their generic behaviors.

2. Country Domains

uses two-character country abbreviations (e.g. us for united states).

 Second label can be organizational , or they can be more specific national designations.

3. Inverse Domain

is used to map an address to a name.

For example

 When a server received a request from a client to do a task.

 Server has a file of authorized users, the IP address of the client is extracted from the received IP packet, to resolve this IP address to the send to the DNS server map an address to a name to determine if the client is on the authorized list.

DNS Messages

DNS has two types of messages.

1. Query Message consist of header and question records. 2. Response Message. consist of header, question records, answer records, authoritative records and additional records.

Header for both query and response the format of the header is same.

 Header is 12 bytes.

 Identification subfield is used by client to match the response with the query.  The client uses a different identification number each time it sends a query.  The server duplicates this number in the corresponding response.

Flag subfield that defines the type of messages, type of answer requested.

 Number of question records

 Number of answer records (all 0’s in the query message).

 Number of authoritative records (all 0’s in the query message).

 Number of additional records (all 0’s in the query message).

Question Section

This section consisting of one or more question records.

 It is present on both query and response messages.

Answer Section

This is a section of one or more resource records.

It is present only on response messages.

Types of Records

 Question Record

is used by the client to get information from a server. This contains the domain name.

 Resource Record

Each domain (each node on the tree) is associated with a record called the resource record.

Registrars

 The new domains added to DNS, this is done through a registrar.

 A registrar verifies that the requested domain name is unique and then enters it into the DNS database.

 A fee is charged.

 The name and addresses for registrars can be found at

http://www.intenic.net

 To register, the organization needs to give the name of its server and the IP address of the server.

Domain name: ws.wonderful.com IP address: 200.200.200.