1. Application Layer DNS Mail protocol (SMTP, POP, IMAP)
Web services (WWW, HTTP, HTTPS, FTP)
telnet
Client server and P2P application
Relation between Application layer and Transport layer.
Prepared By: Er. Bednidhi Rijal

2. Domain Name System (DNS)
There are two ways to identify a host
Host name
IP address
People prefer the hostname identifier whereas routers prefer fixed-length, hierarchically structured IP address
In order to reconcile these preferences, DNS translates hostnames to IP addresses.
The DNS is
A distributed database implemented in a hierarchy of DNS servers, and
An application-layer protocol that allows hosts to query the distributed database.
The DNS protocol runs over UDP and uses port 53
Different application layer protocols including HTTP, SMTP, and FTP use DNS to translate user supplied hostnames to IP addresses.
Prepared By: Er. Bednidhi Rijal

3. How DNS Works? (How DNS translate hostname to IP address?)
Application program calls a library procedure called the resolver, passing it the name as a parameter
The resolver sends a UDP packet to a local DNS server (at port 53)
DNS server looks up the name and returns the IP address to the resolver
Resolver returns it to the application
Armed with the IP address, the program can then establish a TCP connection with the destination or send it UDP packets
Prepared By: Er. Bednidhi Rijal

4. The same user machine runs the client side of the DNS application.
What happens when a browser running on some user’s host requests ?
The same user machine runs the client side of the DNS application.
The browser extracts the hostname, from the URL and passes the hostname to the client side of the DNS application.
The DNS client sends a query containing the hostname to a DNS server.
The DNS client eventually receives a reply, which includes the IP address for the hostname.
Once the browser receives the IP address from DNS, it can initiate a TCP connection to the HTTP server process located at port 80 at that IP address.
Prepared By: Er. Bednidhi Rijal

5. DNS Services Translating hostname to IP (Main services of DNS)
Host Aliasing
A host with a complicated hostname can have one or more alias names
Alias host-names are more mnemonic than canonical hostnames
Eg. relay1.west-coast.enterprise.com (Canonical host names)enterprise.com (alias name)
DNS can be invoked to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host
Mail server aliasing
DNS can be invoked by a mail application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host
Prepared By: Er. Bednidhi Rijal

6. Contd.. Load distribution
DNS is also used to perform load distribution among replicated servers, such as replicated Web servers.
For replicated Web servers, a set of IP addresses is thus associated with one canonical hostname.
The DNS database contains this set of IP addresses.
When clients make a DNS query for a name mapped to a set of addresses, the server responds with the entire set of IP addresses, but rotates the ordering of the addresses within each reply.
Prepared By: Er. Bednidhi Rijal

7. Name Space Maps each address to a unique name Can be categorized into:
Flat name space
Name (sequence of character without structure) is assigned to an address
E.g. Morgan, Kist college, tribhuvan university etc.
Problem: cannot be used in a large system such as the Internet because it must be centrally controlled to avoid ambiguity and duplication
Hierarchical name space
Each name is of several parts. The first part can define nature of organization, second part can the name of an organization, third part can define departments in the organization and so on.
E.g. etc.
In this case, the authority to assign and control the name spaces can be decentralized
Prepared By: Er. Bednidhi Rijal

8. Domain Name
Each 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)
Domain Names are case insensitive.
Can be:
Fully qualified domain name (FQDN)
If a label is terminated by a null string, it is called FQDN
E.g. , etc.
Partially qualified domain name (PQDN)
If a label is not terminated by a null string, it is called a PQDN
E.g. , etc.
Prepared By: Er. Bednidhi Rijal

9. Domain names and Labels
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10. The Web and HTTP
The World Wide Web (WWW) 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 project was initiated by CERN (European Laboratory for Particle Physics) to create a system to handle distributed resources necessary for scientific research.
Besides retrieving of linked documents, WWW can be used to provide electronic shopping, gamming, radio programs, or television program etc.
HTTP is the protocol used to retrieve information from the Web.
Prepared By: Er. Bednidhi Rijal

11. Overview of HTTP (HyperText Transfer Protocol)
HTTP is the Web’s application layer protocol and is used to retrieve the content from the web
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; the client uses a temporary port number
HTTP uses the services of TCP
HTTP is stateless protocol because it does not maintain any information about clients.
Prepared By: Er. Bednidhi Rijal

12. HTTP request-response behavior
When user requests a Web page (for example, clicks on a hyperlink), the browser sends HTTP request messages for the objects in the page to the server. The server receives the requests and responds with HTTP response messages that contain the objects.
Prepared By: Er. Bednidhi Rijal

13. Non-Persistent versus Persistent Connections
If multiple objects of the web page need to be retrieved are located at different server, than it requires to create multiple new connections
However if some of the objects are located on the same server, than we have two choices
To retrieve each object using a new TCP connection. This method is called non-persistent connections
To make a (one) TCP connection and retrieve them all. This method is called persistent connections
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14. Web Caching (Proxy server)
A Web cache---also called a proxy server---is a network entity that satisfies HTTP requests on the behalf of an origin Web server.
The Web cache has its own disk storage and keeps copies of recently requested objects in this storage.
A user’s browser can be configured so that all of the user’s HTTP requests are first directed to the Web cache.
Once a browser is configured, each browser request for an object is first directed to the Web cache
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15. Contd.. Advantage: Problem:
Highly reduce the response time for a client request
Reduces the web traffic in the Internet as a whole, thereby improving performance for all applications
Problem:
The copy of an object residing in the cache may be stale
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16. Contd..
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17. HTTPS (HyperText Transfer Protocol, secured)
HTTP does not provide security but HTTPS does.
When HTTP run over the Secure Socket Layer (SSL) then it is called HTTPS
It provides confidentiality, client and server authentication, and data integrity
Prepared By: Er. Bednidhi Rijal

18. File Transfer: FTP (file transfer protocol)
FTP is an application layers protocol used to transfer files to or from a remote host
FTP is better choice for transferring large files or to transfer files using different format.
FTP uses TCP services
The user interacts with FTP through an FTP user agent.
The user first provides the hostname of the remote host, causing the FTP client process in the local host to establish a TCP connection with the FTP server process in the remote host.
The user then provides the user identification and password, which are sent over the TCP connection as part of FTP commands.
Prepared By: Er. Bednidhi Rijal

19. Contd..
Once the server has authorized the user, the user copies one or more files stored in the local file system into the remote file system (or vice versa)
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20. FTP connection
FTP uses two parallel TCP connections to transfer a file:
control connection, and
used for sending control information between the two hosts—information such as user identification, password, commands to change remote directory, and commands to “put” and “get” files
Use port number 21
data connection
The data connection is used to actually send a file.
Use port number 20.
Because FTP uses a separate control connection, FTP is said to send its control information out-of-band.
Prepared By: Er. Bednidhi Rijal

21. Contd..
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22. How Data transfer takes place using FTP?
When a user starts an FTP session with a remote host, the client side of FTP (user) first initiates a control TCP connection with the server side (remote host) on server port number 21.
The client side of FTP sends the user identification, password, commands to change the remote directory over this control connection.
When the server side receives a command for a file transfer over the control connection (either to, or from, the remote host), the server side initiates a TCP data connection to the client side.
Prepared By: Er. Bednidhi Rijal

23. Contd..
FTP sends exactly one file over the data connection and then closes the data connection.
If, during the same session, the user wants to transfer another file, FTP opens another data connection.
Thus, with FTP, the control connection remains open throughout the duration of the user session, but a new data connection is created for each file transferred within a session (i.e., the data connections are non-persistent)
Prepared By: Er. Bednidhi Rijal

24. Electronic Mail (e-mail)
It allows user to exchange messages
Architecture
A simple consists of three components
A user agent (UA)
A message transfer agent (MTA)
A message access agent (MAA)
Prepared By: Er. Bednidhi Rijal

25. User agent (UA) UA is the first component of E-mail system
It provides service to the user to make the process of sending and receiving a message easier
It is a software package that composes, reads, replies to, and forwards messages.
It also handles local mailboxes on the user computers
Two types:
Command driven UA (e.g. mail, pine and elm)
GUI-based UA (e.g. Eudora and Outlook)
The user through the UA creates mail for sending
Prepared By: Er. Bednidhi Rijal

26. Contd.. UA notifies the user of new receiving mail Mail address:
UA usually displayed a list of mail in which each line contains a summary of the information about a particular message in the mailbox
Summary of information includes: sender mail address, the subject, and the time the mail was received or sent
Mail address:
Used to deliver to its destination
Consists of two parts: local parts (defines the name of special file called the user mailbox where all mail received for the user is stored) and domain name (defines mail servers or exchangers) and is separated sign
Eg. (morgan—local parts and gmail.com—domain name)
Prepared By: Er. Bednidhi Rijal

27. Message Transfer Agent (MTA)
MTA is involved during transferring or sending mail.
Formal protocol that defines the MTA in the internet is called SMTP
Mail transfer phase: The process of transferring a message occurs in three phase
Connection Establishment:
After a client has made TCP connection 0n port 25, the SMTP server starts the connection phase
Message Transfer:
After connection has been established between the SMTP client and server, a single message between a sender and one or more recipients can be exchanged
Connection Termination:
After the message is transferred successfully, the client terminates the connection
Prepared By: Er. Bednidhi Rijal

28. Message Access Agent (MAA)
MAA is used for pulling or retrieving mail from mail box
Two protocols are available: Post Office Protocol (POP3) version 3, and Internet Mail Access Protocol version 4 (IMAP4)
Prepared By: Er. Bednidhi Rijal

29. SMTP (Simple Mail Transfer Protocol)
SMTP is used for sending Mail
SMTP accepts incoming connections and copies messages from them into the appropriate mailboxes.
If a message cannot be delivered, an error report containing the first part of the undeliverable message is returned to the sender.
SMTP is a simple ASCII protocol (i.e. cannot be used for sending image, video, Unicode etc.)
Transferring of mail occurs in three phases (i.e. connection establishment, mail transfer, and connection termination) which is described below:
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30. Contd..
After establishing the TCP connection to port 25, the sending machine waits for the receiving machine to talk first. The server starts by sending its identity and telling whether it is prepared to receive mail.
If it is not, the client releases the connection and tries again later.
If the server is willing to accept , the client announces whom the is coming from and whom it is going to.
If such a recipient exists at the destination, the server gives the client the go-ahead to send the message. Then the client sends the message and the server acknowledges it.
When all the has been exchanged in both directions, the connection is released.
Prepared By: Er. Bednidhi Rijal

31. MIME (Multipurpose Internet Extension)
This protocol is used along with SMTP and it extends the capability of SMTP hence called a supplementary protocol
It allows non-ASCII data (image, audio, video, non-Latin alphabets, text in different language like newari, japnese, chinese etc.) to be sent through
MIME transforms non ASCII data to standard ASCII data at the sender site and transforms back to the original data at the receiving site
Prepared By: Er. Bednidhi Rijal

32. POP3 (Post Office Protocol version 3)
POP3 is simple but limited in functionality
It is used to pull or retrieve messages from mail server
The client POP3 software is installed on the recipient computer, the server POP3 software is installed on the mail server.
Mail access starts with the clients when the user needs to download its form the mailbox on the mail server
The client opens a connection to the server on TCP port 110
It then sends its user name and password to access to the mailbox
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33. Contd..
The user can then list and retrieve the mail messages, one by one
POP3 has two modes:
Delete mode:
The mail is deleted from the mailbox after each retrieval
Keep mode:
The mail remains in the mailbox after retrieval
Limitation
It does not allow the user to organize mail on the server
The user cannot have different folders on the server
It does not allow the user to partially check the contents of the mail before downloading
Prepared By: Er. Bednidhi Rijal

34. Internet Mail Access Protocol, version 4 (IMAP4)
IMAP4 is more powerful and more complex than POP3
It completely fulfill the deficiency exist in POP3 and provides additional functionality which are listed below:
A user can check the prior to downloading
A user can search the contents of the for a specific string of characters prior to downloading
A user can partially download
A user can create, delete or remove mailboxes on the mail server
A user can create a hierarchy of mailboxes in a folder for storage
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35. TELNET (Terminal NETwork)
It allows a user on one computer to log into the remote computer and use the services available there
Although it requires logging name and password, it is vulnerable to hacking because it sends all data including the password in plaintext (not encrypted)
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36. Secure Shell (SSH)
It is also used for remote logging (alternative of TELNET and is used when security is concerned) and is more secured (i.e. less vulnerable to hacking)
It sends all data including the password in encrypted form
Prepared By: Er. Bednidhi Rijal

37. Network Application Architecture
Possible structure of applications:
client-server Architecture
peer-to-peer (P2P) Architecture
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38. Client-server architecture
always-on host
Services requests from many other hosts, called clients
permanent IP address
data centers for scaling
clients:
communicate with server
may be intermittently connected
may have dynamic IP addresses
do not communicate directly with each other
Client server architecture are often infrastructure intensive, since they require service providers to purchase, install, and maintain server farms.
Example of Client server architecture: Web application, FTP, Telenet, and .
Prepared By: Er. Bednidhi Rijal

39. Peer-to-Peer (P2P) Architecture
Minimal or no reliance on always-on servers
Arbitrary end systems directly communicate
peers request service from other peers, provide service in return to other peers
self-scalability – new peers bring new service capacity, as well as new service demands
P2P architectures are cost effective, since they do not require power-full server.
peers are intermittently connected and change IP addresses
complex management
P2P application includes: Bit Torrent, eMule and LimeWire, Internet Telephony (Eg. Skype), and IPTV (eg. PPLive)
Prepared By: Er. Bednidhi Rijal

40. Relationship between Application layer and Transport layer
Transport layer distinguish each process running at application layer uniquely with port number.
Transport layer encapsulate the data either in UDP datagram or TCP segment received from the Application layer and send it to the Network layer at the source and at the destination it generates the original data and send to the appropriate process at the application layer.
Transport layer provides logical connection between processes.
Prepared By: Er. Bednidhi Rijal

41. ASSIGNMENT Why HTTP is called stateless protocol?
What is web? How WWW works?
What is DNS? Explain the importance of DNS.
What are function of DNS? Explain how it works?
What is ? Explain its merit and demerit. Explain the architecture of
What is the difference between SMTP and MIME?
What is the difference between POP3 and IMAP4?
What is the difference between SSH and TELNET?
What is FTP? Explain how data transfer takes place using FTP?
Differentiate between persistence and non persistence connection.
Write the standard port number for FTP, DNS, SMTP, POP3, IMAP4, SSH, TELNET, HTTP, HTTPS.
Write one-one example of FQDNS and PQDNS.
Prepared By: Er. Bednidhi Rijal