1. Application layer
Lecture 7

2. Introduction
Having finished all the previous layers , we now come to the layer where all the applications are found.
The layers below the application layer are there to provide transport services, but they do not do real work for users.
However, even in the application layer there is a need for support protocols, to allow the applications to function.
Accordingly, we will look at an important one of these protocols. The item in question is DNS, which handles naming within the Internet.
Note that there is three real applications: electronic mail, the World Wide Web, and multimedia.

3. HTTP, FTP Application layer message TCP, UDP Transport layer segment
Protocol Layer PDU
HTTP, FTP Application layer message
TCP, UDP Transport layer segment
ICMP, IP Network layer datagram
PPP, Ethernet, Data link layer frame
IEEE
Behrouz A. Forouzan” Data communications and Networking

4. 1. Domain Name System https://www.youtube.com/watch?v=sABA3S1NtlE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. Introduction
There are several applications in the application layer of the Internet model that follow the client/server paradigm.
The client/server programs can be divided into two categories:
Application that directly used by the user, such as
Application that support other application programs.
The Domain Name System (DNS) is a supporting program that is used by other programs such as .

6. Introduction
To identify an entity, TCP\IP protocols use the IP address, which uniquely identifies the connection of a host to the Internet.
However, people prefer to use names instead of numeric addresses.
Therefore, we need a system that can map a name to an address or an address to a name, which is the DNS server

7. Figure Example of using the DNS service
DNS server
Figure shows an example of how a DNS client/server program can support an program to find the IP address of an recipient
A user of an program may know the address of the recipient
The DNS client program sends a request to a DNS server to map the address to the corresponding IP address
After that, DNS will response to the client with the IP address

8. Hierarchical Name Space
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. Name space can be
Flat name space
A name is assigned to an address.
A name in this space is a sequence of characters without structure.
Hierarchical name space
In a hierarchical name space, each name is made of several parts.
The first part can define the nature of the organization, the second part can define the name of an organization, the third part can define departments in the organization, and so on.
Important example :
For example, assume two colleges and a company
call one of their computers challenger. The first college is given a name by the central
authority such as jhda.edu, the second college is given the name berkeley.edu, and the
company is given the name smart. com. When these organizations add the name challenger
to the name they have already been given, the end result is three distinguishable
names: challenger.jhda.edu, challenger.berkeley.edu, and challenger.smart.com. The names
are unique without the need for assignment by a central authority. The central authority
controls only part of the name, not the whole.

9. DOMAIN NAME SPACE
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.

10. DOMAIN NAME SPACE Label : Domain Name:
Each node in the tree has a label, which is a string with a maximum of 63 characters.
The root label is a null string (empty string).
Domain Name:
Each node in the tree has a domain name. A full domain name is a sequence of labels separated by dots
Fully Qualified Domain Name
Partially Qualified Domain Name

11. Figure 25.3 Domain names and labels

12. DOMAIN NAME SPACE Domain :
A domain is a sub tree of the domain name space.
The name of the domain is the domain
name of the node at the top of the sub tree.

13. Hierarchy of Name Servers:
With huge information, there is a need to distribute the information among many computers called DNS servers.
One way to do this is to divide the whole space into many domains based on the first level.
we let the root stand alone and create as many domains (subtrees) as there are first-level nodes

14. DNS IN THE INTERNET
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.
Topics
Generic Domains Country Domains Inverse Domain

15. DNS IN THE INTERNET 1. Generic Domains
The generic domains define registered hosts according to their generic behavior. Each node in the tree defines a domain
Example : .com , .gov

16. 2. Country Domains
The country domains section uses two- character country abbreviations (e.g., us for United States).
Second labels can be organizational, or they can be more specific, national designations.
The United States, for example, uses state abbreviations as a subdivision of us (e.g., ca.us.).

17. 3. Inverse domain
The inverse domain is used to map an address to a name. This may happen, for example, when a server has received a request from a client to do a task.
Although the server has a file that contains a list of authorized clients, only the IP address of the client (extracted from the received IP packet) is listed.
Mapping a name to an address or an address to a name is called name-address resolution.

18. 2. Remote logging & FTP https://www.youtube.com/watch?v=sABA3S1NtlE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

19. Introduction
The main task of the Internet is to provide services for users.
The most popular applications are:
remote logging
electronic mail
file transfer

20. 1. REMOTE LOGGING
In the Internet, users may want to run application programs at a remote site and create results that can be transferred to their local site.
One way to satisfy that demand is to create a client/server application program for each desired service.
The better solution is a general-purpose client/server program that lets a user access any application program on a remote computer;

21. TELNET TELNET is a general-purpose client/server application program.
TELNET is an abbreviation for TErminaL NETwork.
It’s enable the establishment of a connection to a remote .

22. Logging
users are part of the system with some right to access resources.
Each authorized user has an identification and password.
The user identification defines the user as part of the system.
To access the system, the user logs into the system with a user id or log-in name and password.
to prevent an unauthorized user from accessing the resources.

23. Network Virtual Terminal
We are dealing with heterogeneous systems.
If we want to access any remote computer in the world, we must first know what type of computer we will be connected to.
we must also install the specific terminal emulator used by that computer.
TELNET solves this problem by defining a universal interface called the network virtual terminal (NVT) character set.

24. Network Virtual Terminal
The client TELNET translates characters (data or commands) into NVT form and delivers them to the network.
The server TELNET, on the other hand, translates data and commands from NVT form into the form acceptable by the remote computer.

25. 2. FILE TRANSFER
File Transfer Protocol (FTP) is the standard mechanism provided by TCP/IP for copying a file from one host to another which is not as it seems simple and straightforward.
Before transferring, some problems must be dealt with first, such as:
Two systems may use different file name conventions.
Two systems may have different ways to represent text and data.
Two systems may have different directory structures.
All of these problems have been solved by FTP in a very simple and elegant approach.

26. FILE TRANSFER
FTP differs from other client-server applications in that it establishes two connections between the hosts.
One connection is used for data transfer, the other for control information (commands and responses).
We need to transfer only a line of command or a line of response at a time.
Separation of commands and data transfer makes FTP more efficient.
FTP uses the control connection to establish a communication between the client control process and the server control process.

27. FTP
In details

28. File Type – (data transfer connection)
FTP can transfer one of the following file types across the data connection:
ASCII file.
EBCDIC file
Image file.
The ASCII file is the default format for transferring text files.
Each character is encoded using 7-bit ASCII.
The sender transforms the file from its own representation into ASCII characters, and the receiver transforms the ASCII characters to its own representation.
The EBCDIC file used If one or both ends of the connection use EBCDIC encoding (the file format used by IBM).
The image file is the default format for transferring binary files.
The file is sent as continuous streams of bits without any interpretation or encoding.
This is mostly used to transfer binary files such as compiled programs.

29. Commands – (control connection)
Commands are divided into six groups:
access commands
file management commands
data formatting commands
port defining commands
file transferring commands
miscellaneous commands

30. File Transfer File transfer in FTP means one of three things:
Retrieving:
A file is to be copied from the server to the client.
It is done under the supervision of the RETR command.
Storing:
A file is to be copied from the client to the server.
It is done under the supervision of the STOR command.
List:
A list of directory or file names is to be sent from the server to the client.
This is done under the supervision of the LIST command.
FTP treats a list of directory or file names as a file.

31. File Transfer

32. Anonymous FTP
To use FTP, a user needs an account (user name) and a password on the remote server.
Some sites have a set of files available for public access, to enable anonymous FTP.
To access these files, a user does not need to have an account or password.
The user can use anonymous as the user name and guest as the password.
User access to the system is very limited. Some sites allow anonymous users only a subset of commands.
For example, most sites allow the user to copy some files, but do not allow navigation through the directories.

33. Another common protocol used by the Internet HTTP

34. HTTP: Hyper Transfer Protocol
What is HTTP?
Hypertext Transfer Protocol – the Web’s application layer protocol
HTTP is an request-response client-server protocol . An HTTP client sends a request message to an HTTP server.  The server, in turn, returns a response message
Prosperities:
Pipelined – requests sent as soon as it is encountered
Persistent – multiple objects can be sent over a single TCP connection between the server and the client

35. Whenever you issue a URL from your browser to get a web resource using HTTP, the browser turns the URL into a request message and sends it to the HTTP server. The HTTP server interprets the request message, and returns you an appropriate response message, which is either the resource you requested or an error message
There are 4 parts in a URL (Uniform Resource Locator )
Protocol: The application-level protocol used by the client and server, e.g., HTTP, FTP, and telnet.
Hostname: The DNS domain name or IP address of the server.
Port: The TCP port number that the server is listening for incoming requests from the clients.
Path-and-file-name: The name and location of the requested resource, under the server document base directory.

36. The end 
The end of this course