The application layer allows users to interface with networks through application layer protocols like HTTP, SMTP, POP3, FTP, Telnet, and DHCP. It provides the interface between applications on different ends of a network. Common application layer protocols include DNS for mapping domain names to IP addresses, HTTP for transferring web page data, and SMTP/POP3 for sending and receiving email messages. The client/server and peer-to-peer models describe how requests are made and fulfilled over the application layer.

1. Application Layer
Functionality and
Protocols
Application Layer Protocols

2. Applications Layer – allows user to interface with the
network!

3. Application Layer – Provides the interface between
the applications on either end of the network.

4. Protocols and networks

5. Protocols
 DNS – Matches domain names with IP addresses
 HTTP – Used to transfer data between clients/servers using a web browser
 SMTP & POP3 – used to send email messages from clients to servers over the
internet
 FTP – allows the download/upload of files between a client/server
 Telnet – allows users to login to a host from a remote location and take control
as if they were sitting at the machine (virtual connection)
 DHCP – assigns IP addresses, subnet masks, default gateways, DNS servers,
etc. To users as they login the network

6. Application layer software
 2 types
Applications – Provide the human (user) interface. Relies on lower
layers to complete the communication process.
Services – establish an interface to the network where protocols provide
the rules and formats that govern how data is treated..

7. How data requests occur & are filled
 Client/server model
Advantages:
Centralized administration
Security is easier to enforce
 Peer-to-peer networking and applications

8. Client/Server Model
 Client –
device requesting information (initiates the data exchange)
Can also UPLOAD data to the servers
 Server – device responding to the request
How does it handle multiple request from multiple users and keep everything
in order?
Relies on support from the lower layer functions to distinguish between
services and conversations.
Server relies on a service called a server daemon – runs in the background and
‘listens’ for requests for that service. It can then exchange messages as
appropriate & send requested data.
 Examples:
E-mail Client on an employee computer issues a request to the e-mail server for
any unread e-mail. The server responds by sending the e-mail to the client.
Conversations can originate with either party.

9. Peer-to-Peer (P2P) Network Model
 Two or more computers are connected and are able to share resources
without having a dedicated server
 Every end device can function as a client or server on a ‘per request’ basis
 Resources are decentralized (information can be located anywhere)
 Difficult to enforce security and policies
 User accounts and access rights have to be set individually on each peer
device

10. Common Port Numbers
 TCP
FTP – 20-21
Telnet – 23
SMTP – 25
DNS – 53 (Both TCP & UDP)
HTTP – 80
 UDP
DHCP – 67 & 68
POP – 110

11. Domain Name System (DNS)
 Each terminal in network has an unique IP address and a name (name space).
 A name space that maps each address to a unique name can be organized in
two ways: flat or hierarchical
 In a flat name space, a name is assigned to an address. A name in this space is
a sequence of characters without structure.
 In a hierarchical name space, each name is made of several parts.

12.  An example given below in which TCP/IP uses a DNS client and a DNS server to
map a name to an address. A user wants to use a file transfer client to access the
corresponding file transfer server running on a remote host. The user knows only
the file transfer server name, such as afilesource.com. However, the TCP/IP suite
needs the IP address of the file transfer server to make the connection. The
following six steps map the host name to an IP address:
 The user passes the host name to the file transfer client.
 The file transfer client passes the host name to the DNS client.
 Each computer, after being booted, knows the address of one DNS server. The DNS client
sends a message to a DNS server with a query that gives the file transfer server name using
the known IP address of the DNS server.
 The DNS server responds with the IP address of the desired file transfer server.
 The DNS server passes the IP address to the file transfer client.
 The file transfer client now uses the received IP address to access the file transfer Server.

13. Domain Name System (DNS)
 DNS is used for designing hierarchical name space
Example of hierarchical name space: www.staffs.ac.uk, www.fcet.staffs.ac.uk,
gawains.staffs.ac.uk, blackboard.staffs.ac.uk
 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

14. Domain Name and Label

15. 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 ends in a null label,
which means the last character is a dot because the null string is nothing.
 If a label is terminated by a null string, it is called a fully qualified
domain name (FQDN).

16. DNS Server
 Stores domain name space information within its domain/sub-
domain.

17. DSN Services and Protocol
DNS Servers resolve names to IP addresses. It would be
difficult to remember the IP address of every website we like to
visit, but we can remember names.

18. World Wide Web
 The Web is a repository of information in which the documents, called
web pages, are distributed all over the world and related documents are
linked together.
 The WWW is a distributed client-server service, in which a client using a
browser can access a service using a server. However, the service
provided is distributed over many locations called sites. Each site holds
one or more web pages. Each web page, however, can contain some links
to other web pages in the same or other sites

19.  Web Client (Browser):
Each browser usually consists of three parts: a controller, client protocols, and
interpreters. The controller receives input from the keyboard or the mouse and
uses the client programs to access the document. After the document has been
accessed, the controller uses one of the interpreters(HTML, Java, or JavaScript)
to display the document on the screen. The client protocol can be one of the
protocols such as HTTP or FTP.
 Web Server
The web page is stored at the server. Each time a request arrives, the
corresponding document is sent to the client.
 Web Documents
The documents in the WWW can be grouped into three broad categories:
–Static
–Dynamic
–active

20.  Static Documents
Static documents are fixed-content documents that are created and stored in a
server. The client can get a copy of the document only. In other words, the
contents of the file are determined when the file is created, not w.hen it is used.
 Dynamic Documents
A dynamic document is created by a web server whenever a browser requests the
document. When a request arrives, the web server runs an application program or
a script that creates the dynamic document. The server returns the result of the
program or script as a response to the browser that requested the document.
Because a fresh document is created for each request, the contents of a dynamic
document may vary from one request to another. A very simple example of a
dynamic document is the retrieval of the time and date from a server.
 Active Documents
For many applications, a program or a script needs to be run at the client site.
These are called active documents. For example, suppose a program that creates
animated graphics on the screen or a program that interacts with the user. When a
browser requests an active document, the server sends a copy of the document or
a script. The document is then run at the client (browser) site.

21. HyperText Transfer Protocol (HTTP)
 The HyperText Transfer Protocol (HTTP) is used to define how the client-server
programs can be written to retrieve web pages from the Web. An HTTP client
sends a request; an HTTP server returns a response. The server uses the port
number 80.
 HTTP uses the services of TCP. This means that, before any transaction between
the client and the server can take place, a connection needs to be established
between them. After the transaction, the connection should be terminated.

22. WWW Service and HTTP
 Steps:
1) URL is typed in the address bar.
2) Browser checks with DNS server to convert it to an IP address
3) Connects to the server requested
4) Using HTTP or HTTPS protocol requirements, the browser sends a GET
request to the server to ask for the desired html document (usually
index.html)
5) The server sends the HTML code for the web page to the browser.
6) The browser interprets the HTML code and formats the page to fit the
browser window.

23. WWW Service and HTTP
HTTP/HTTPS are
some of the MOST
used application
protocols!

24. E-mail services and SMTP/POP protocols
 E-mail is the most popular network service.
 E-mail client (when people compose e-mail) is called Mail User Agent
(MUA)
 MUA allows messages to be sent/retrieved to and from your mailbox
 Requires several applications and services
POP or POP3 – deliver email from server to client (incoming messages)
SMTP – handles outbound messages from clients

25. E-mail services and SMTP/POP protocols
 What do servers require?
1) Must be running SMTP!
2) Also operates
1) Mail Transfer Agent (MTA) – used to forward email
1)Receives email from the clients MUA
2)Uses SMTP to route email between SERVERS!
3)Passes email to the MDA for final delivery
2) Mail Delivery Agent (MDA) – receives messages from MUA
or from the MTA on another server
3) For two e-mail servers to talk – MUST run SMTP and MTA in
order to transfer mail between the 2 servers!

26. E-mail services and SMTP/POP protocols

27. FTP
 Commonly used application layer protocol
 Allows for the transfer of files between clients/servers.
 Requires 2 connections to the server
1) Commands – uses TCP port 21
2) Actual data – uses TCP port 20

28. DHCP
 Dynamic Host Configuration Protocol – enables devices to obtain IP
addresses, subnet masks, gateways, DNS server information, etc. from a
DHCP server.
 An IP address that is not being used is assigned from a range of available
addresses
 Not permanently assigned – only leased for a specific period of time
(usually 24 hours – 7 days)
 If the host logs off or the power is lost, the IP address they were using is
returned to the pool to be re-assigned to another host when needed.
 This is how you are able to use Wi-Fi at various places in the world!
 Don’t use DHCP for devices such as servers, printers, routers, switches,
etc. These should be statically assigned.

29. Telnet
 Developed in the early 1970’s – among the oldest of the application layer
protocols and services in the TCP/IP protocol suite.
 Allows users to follow text-based terminal devices over the network using
software.
 A connection is known as a ‘virtual terminal (vty)’ session.
 Can be run from the command prompt on a PC.
 You can use the device as if you were sitting there with all the rights and
priorities that you username will offer you.
 TELNET requires a logging name and password, it is vulnerable to hacking
because it sends all data including the password in plaintext (not encrypted). A
hacker can eavesdrop and obtain the logging name and password. Because of
this security issue, the use of TELNET has diminished in favor of another
protocol, Secure Shell (SSH).

30. Telnet