2140709

Computer Networks

Unit-2
Application Layer

Prof. Maulik D. Trivedi

9998265805
maulik.trivedi@darshan.ac.in
 Network Applications
▪ A Network application is an application running on one host and provides a
communication to another application running on a different host.
▪ A network application development is writing programs that run on different
end systems and communicate with each other over the network.
▪ In the Web application there are two different programs that communicate
with each other:
✔ Browser program running in the user's host.
✔ Web server program running in the Web server host.

Host Host

Unit: 2 - Application Layer 2 Darshan Institute of Engineering & Technology

 Network Applications - Examples
▪ Email
▪ Web
▪ Remote Login
▪ P2P File Sharing
▪ Multi-user Network Games
▪ Streaming Stored Video
(YouTube)
▪ Voice Over IP (Skype)
▪ Real-time Video Conference
▪ Social Networking

Unit: 2 - Application Layer 3 Darshan Institute of Engineering & Technology

 Network Application Architecture
1. Client-Server architecture
2. P2P (Peer to Peer) architecture

Unit: 2 - Application Layer 4 Darshan Institute of Engineering & Technology

 1. Client-Server Architecture
Server:
Client
✔ Its always-on host.
✔ It has a fixed IP address.
✔ Large cluster of host – Data Centers.
✔ E.g. Web Server

Client:
✔ It communicate with server.
✔ Its not like continuously connected.
✔ May have dynamic IP addresses.
✔ Do not communicate directly with
each other.
✔ E.g. PCs, Mobiles
Server

Unit: 2 - Application Layer 5 Darshan Institute of Engineering & Technology

 2. P2P Architecture
▪ Peers (end systems) directly
Peer communicate.
▪ Get peers request service from
other peers, provide service to
other peers.
✔ Self Scalability – New peers
bring new service capacity, as
well as new service demands.
▪ Peers are alternatingly
connected and change IP
Peer addresses.
✔ Complex management
Peer

Unit: 2 - Application Layer 6 Darshan Institute of Engineering & Technology

 Process Communicating
▪ What is Process?
▪ A process is an instance of a program running in a computer.
▪ We can say that process is program under execution.
▪ Within same host, two processes communicate using inter-process
communication (IPC).
▪ Process in different hosts communicate by exchanging messages.
▪ Client process: A process that initiates communication.
▪ Server process: A process that waits to be contacted.
Process
Process P2
P1

Unit: 2 - Application Layer 7 Darshan Institute of Engineering & Technology

 Socket
▪ A process sends messages into, and receives messages from; the
network through a software interface called a socket.
▪ A process is similar to a house and its socket is similar to its door.
✔ Sending process passes message out door.
✔ Sending process relies on transport infrastructure on other side of door to
deliver message to socket at receiving process.
Process
application application
proce socke proce controlled by
ss t ss app developer

transport transport
network network controlled
link
by OS
link Internet
Socket physical physical

Unit: 2 - Application Layer 8 Darshan Institute of Engineering & Technology

 Transport Services to Applications
▪ Recall that a socket is the interface
between the application process
and the transport layer protocol. Reliable Data
Throughput
Transfer
▪ For develop an application, choose
available transport layer protocol.
▪ Pick the protocol with the services
that best match the needs of your
application. Timing Security
▪ Example: Choose either Train or
Airplane transport for travel
between two cities.
▪ Classify services with four
parameters.

Unit: 2 - Application Layer 9 Darshan Institute of Engineering & Technology

 Transport Services to Applications
▪ Reliable Data Transfer
✔ Many applications (e.g., email, file Blur image
transfer, financial applications) require
100% reliable data transfer
✔ Required guarantee that data sent by
one end of application is delivered
correctly and completely to the other
end of application.
✔ This guaranteed data delivery service is
called Reliable Data Transfer.
✔ When it will fail to deliver reliable data
transfer, it is acceptable for loss-
tolerant applications.
✔ Loss-tolerant Applications (e.g.,
audio/video) can tolerate some loss.

Unit: 2 - Application Layer 10 Darshan Institute of Engineering & Technology

 Transport Services to Applications
▪ Throughput
✔ some apps (e.g., multimedia) require at least amount of throughput to be
“effective”
✔ Bandwidth sensitive application, specific throughput required.
✔ Elastic application can use of as much, or as little, throughput as happens to
be available.
▪ Timing
✔ some apps (e.g., Internet telephony, interactive games) require low delay to
be “effective”
▪ Security
✔ In the sending host, encrypt all data transmitted by the sending process.
✔ In the receiving host, decrypt the data before delivering the data to the
receiving process.

Unit: 2 - Application Layer 11 Darshan Institute of Engineering & Technology

 Internet Transport Protocols Services
▪ TCP Service: ▪ UDP Services:
✔ Connection-Oriented: A setup ✔ Connectionless: No connection
required between client and before two processes start to
server processes communicate.
✔ Reliable data transfer between ✔ Unreliable data transfer between
sending and receiving process
without error and proper order sending and receiving process
✔ Congestion control: To control ✔ It does not provide congestion
sender when network control.
overloaded ✔ It Does not provide. Reliability,
✔ It does not provide, Timing, at flow control, throughput
least throughput guarantee (not guarantee, security.
preferred in real-time
application)

Unit: 2 - Application Layer 12 Darshan Institute of Engineering & Technology

 Internet Applications
▪ Popular internet applications with their application layer and their
underlying transport protocol.

Application-Layer Underlying Transport

Applications Protocol (Service)
Protocol
Email SMTP TCP
Remote Terminal Telnet TCP
Access
Web HTTP TCP
File Transfer FTP TCP
Streaming Media HTTP(YouTube), RTP TCP or UDP
Internet Telephony SIP, RTP(Skype) Typically UDP
No loss, Elastic
Loss-tolerant Bandwidth
Unit: 2 - Application Layer 13 Darshan Institute of Engineering & Technology
WEB & HTTP
 Web
▪ Early 1990, Internet was used only by
researchers, academics, and university
students.
▪ New application WWW arrived in 1994
by Tim Berners-Lee.
▪ World Wide Web - is an information
where documents and other web
resources are identified by URL,
interlinked by hypertext links, and can
be accessed via the Internet.
▪ On demand available, What they want,
When they want it.
▪ Unlike TV and Radio.
▪ Navigate through Websites.
15
Unit: 2 - Application Layer 15 Darshan Institute of Engineering & Technology
 Web and HTTP
▪ Web page consists of objects.
▪ Object can be HTML file, JPEG image, Java applet, audio file etc.…
▪ Web page consists of base HTML-file which includes several
referenced objects.
Web Page (e.g Total five objects)

▪ each object is addressable by a Uniform Resource Locator (URL),

like;
www.someschool.edu/someDept/
pic.gif
host name path name

Unit: 2 - Application Layer 16 Darshan Institute of Engineering & Technology

 HTTP
▪ HyperText Transfer Protocol – Application layer protocol
▪ it is implemented in two programs.
1. Client Program
2. Server Program
▪ Exchanging HTTP message each others.
▪ HTTP defines the structure of these messages and how web
HTTP
client – web server exchange Request
messages.

HTTP
Response

Unit: 2 - Application Layer 17 Darshan Institute of Engineering & Technology

 HTTP – Cont…
▪ HTTP
✔ Hyper-Text Transfer Protocol
✔ It is Application layer protocol
HT
✔ Client: A browser that requests, TP
req
PC ues
receives, (using HTTP protocol) HT t
(Web Browser) TP
and “displays” Web objects. res
pon
se
✔ E.g. PC, Mobile
t
✔ Server: Web server sends (using u es
req
HTTP protocol) objects in TP nse Server
HT po (Apache Web
es
response to requests. Pr
T Server)
HT
✔ E.g. Apache Web Server
Mobile
(Web Browser)

Unit: 2 - Application Layer 18 Darshan Institute of Engineering & Technology

 HTTP - Cont...
▪ A client initiates TCP connection (creates socket) to server using
port 80.
▪ A server accepts TCP connection from client.
▪ HTTP messages (application-layer protocol messages) exchanged
between browser (HTTP client) and Web server (HTTP server).
▪ HTTP is “stateless protocol”, server maintains no information
about past client requests.
▪ HTTP connection types are:
1. Non-persistent HTTP
2. Persistent HTTP

Unit: 2 - Application Layer 19 Darshan Institute of Engineering & Technology

NON-PERSISTENT HTTP &
PERSISTENT HTTP CONNECTION
 Non-persistent & Persistent Connection
▪ In Client-Server communication, Client
making a series of requests to server,
Server responding to each of the
requests.
▪ Series of requests may be made back
to back or periodically at regular time
interval.
▪ So, Application developer need to
make an important decision;
✔ Should each request/response pair be
sent over a separate TCP connection.
✔ OR should all of the requests and
corresponding responses be sent over
same TCP connection?
21
Unit: 2 - Application Layer 21 Darshan Institute of Engineering & Technology
 1. Non-persistent HTTP
▪ A non-persistent connection is closed after the server sends the
requested object to the client.
▪ The connection is used exactly for one request and one response.
▪ For downloading multiple objects it required multiple
connections.
▪ Non-persistent connections are the default mode for HTTP/1.0.
▪ Example:
▪ Transferring a webpage from server to client, webpage consists of
a base HTML file and 10 JPEG images.
▪ Total 11 object are reside on server.

Unit: 2 - Application Layer 22 Darshan Institute of Engineering & Technology

 1. Non-persistent HTTP – Cont.…
URL: www.someSchool.edu/someDepartment/home.index
1a. HTTP client initiates TCP connection to
HTTP server (process) at 1b. HTTP server at host
www.someSchool.edu on port 80 www.someSchool.edu waiting for TCP
connection at port 80. “accepts”
2. HTTP client sends HTTP request connection, notifying client
message (containing URL) into TCP 3. HTTP server receives request
connection socket. Message indicates message, forms response message
that client wants object containing requested object, and
someDepartment/home.index sends message into its socket
4. HTTP server closes TCP connection.
5. HTTP client receives response message
containing html file, displays html. Parsing
html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10
jpeg objects Time
Unit: 2 - Application Layer 23 Darshan Institute of Engineering & Technology
 1. Non-persistent HTTP: Response time
▪ RTT(round-trip time): A time
for a small packet to travel initiate TCP
from client to server and vice connection
RTT
versa.
request
▪ HTTP response time: file
✔ one RTT to initiate TCP time to
RTT transmit
connection. file
✔ one RTT for HTTP request and file
receive
first few bytes of HTTP response d
to return.
time time
✔ File transmission time

Non-persistent HTTP response time = 2RTT + file transmission time

Unit: 2 - Application Layer 24 Darshan Institute of Engineering & Technology

 2. Persistent HTTP
▪ Server leaves the TCP connection open after sending responses.
▪ Subsequent HTTP messages between same client and server sent
over open connection.
▪ The server closes the connection only when it is not used for a
certain configurable amount of time.
▪ It requires as little as one round-trip time (RTT) for all the
referenced objects.
▪ With persistent connections, the performance is improved by
20%.
▪ Persistent connections are the default mode for HTTP/1.1.

Unit: 2 - Application Layer 25 Darshan Institute of Engineering & Technology

 HTTP Message Format
▪ Two types:
1. Request Message
2. Response Message

Unit: 2 - Application Layer 26 Darshan Institute of Engineering & Technology

 1. HTTP Request Message
▪ It is in ASCII format which means that human-readable format.
▪ HTTP request message consist three part:
1. Request line
2. Header line
3. Carriage return carriage return character
line-feed character
request line
(GET, POST, GET /index.html HTTP/1.1\r\n
HEAD commands) Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\
header n
lines Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
carriage return Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n
(line feed at start Keep-Alive: 115\r\n
Connection: keep-alive\r\n
of line indicates \r\n
end of header lines)
Unit: 2 - Application Layer 27 Darshan Institute of Engineering & Technology
 1. HTTP Request Message - Format
▪ The request line has three fields: Method field, URL field, and
HTTP version field.
▪ The method field can take on several different values, including
GET, POST, HEAD, PUT, and DELETE.
▪ In above message, browser is requesting the object
/somedir/page.html and version is self-explanatory; browser
implements version HTTP/1.1.
▪ The header line Host: www-net.cs.umass.edu specifies the host on
which the object resides.
▪ User agent indicate browser name and version.

Unit: 2 - Application Layer 28 Darshan Institute of Engineering & Technology

 2. HTTP Response Message
▪ HTTP response message consist of three part:
1. Status line
2. Header line
3. Data (Entity body)
status line
HTTP/1.1 200 OK\r\n
(protocol Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n
status code Server: Apache/2.0.52 (CentOS)\r\n
status phrase) Last-Modified: Tue, 30 Oct 2007 17:00:02
GMT\r\n
header ETag: "17dc6-a5c-bf716880"\r\n
lines Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n
Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-1\
data, e.g., r\n
requested \r\n
HTML file data data data data data ...

Unit: 2 - Application Layer 29 Darshan Institute of Engineering & Technology

 2. HTTP Response Message - Format
▪ The status line has three fields: protocol version field, status code and
corresponding status message.
▪ In below example, the status line indicates that the server is using
HTTP/1.1 and that everything is OK.
HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n
Server: Apache/2.0.52 (CentOS)\r\n
Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n
ETag: "17dc6-a5c-bf716880"\r\n
Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n
Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-1\r\n \r\n
data data data data data ...

Date: header
Content-Type:
Server:
Last-Modified:
Content-Length:
headerline indicates
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line line the
indicates
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indicates
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Unit: 2 - Application Layer 30 Darshan Institute of Engineering & Technology
 HTTP Response Status Codes
▪ A status code appears in 1st line in server-to-client response
message.
▪ Some sample codes:
✔ 200 OK
▪ Request succeeded, requested object later in this message
✔ 301 Moved Permanently
▪ Requested object moved, new location specified later in this message(Location)
✔ 400 Bad Request
▪ Request message not understood by server
✔ 404 Not Found
▪ Requested document not found on this server
✔ 505 HTTP Version Not Supported
▪ Requested http version not support

Unit: 2 - Application Layer 31 Darshan Institute of Engineering & Technology

 User-Server interactions: Cookie
▪ A small text file that is stored in the user's computer either temporarily for that
session only or permanently on the hard disk.
▪ Cookies provide a way for the Web site to recognize you and keep track of your
preferences.
▪ The cookie technology has four components:
1. A cookie header line in the HTTP response message
2. A cookie header line in the HTTP request message
3. A cookie file kept on the user’s end system and managed by the user’s browser
4. A back-end database at the Web site

• Use of Cookies
• Authorization
• Recommendations
• Shopping carts
• User session state (Web, Email)

Unit: 2 - Application Layer 32 Darshan Institute of Engineering & Technology

 Cookies - Example
client server

ebay 8734 usual http request msg server

cookie file creates ID
usual http response
set-cookie: 1678 1678 for user create backend
ebay 8734 entry database
amazon 1678
usual http request msg
cookie: 1678 cookie- access
specific
usual http response msg action
one week later:
access
ebay 8734 usual http request msg
amazon 1678 cookie: 1678 cookie-
specific
usual http response msg action

Unit: 2 - Application Layer 33 Darshan Institute of Engineering & Technology

 Web Caches (Proxy Server)
▪ It 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.

HT proxy
TP ue st
r equ serve req
clien HTTP est
r HTTP
p o nse
res es r origin
t pon TP
se HT server
t
ues
req se
T P o n
HT res
p
TP
HT

clien origin
t server

Unit: 2 - Application Layer 34 Darshan Institute of Engineering & Technology

 Web Caches (Proxy Server) – Cont...
▪ A user’s browser can be configured so, user’s HTTP requests are
first directed to the Web Cache.
▪ A browser sends all HTTP requests to cache.
▪ As an example, suppose a browser is requesting the object
http://www.someschool.edu/campus.gif
▪ Object in cache returns to client browser.
▪ Otherwise cache requests object from origin server, then returns
object to client browser.
▪ Reduce response time for client request.
▪ Reduce traffic on an institution’s access link.
▪ Internet dense with caches: Insufficiency for content providers to
effectively deliver content.

Unit: 2 - Application Layer 35 Darshan Institute of Engineering & Technology

 Web Caches (Proxy Server) – Example
▪ Example: Institutional Network
and Internet
▪ Reduce response time for client
request.
▪ Reduce traffic on an institution’s
access link.
▪ Internet dense with caches:
Insufficiency for content providers
to effectively deliver content.

Unit: 2 - Application Layer 36 Darshan Institute of Engineering & Technology

 FTP (File Transfer Protocol)
▪ File Transfer Protocol (FTP) is the commonly used protocol for
exchanging files over the Network or Internet. Example: Filezilla
▪ FTP uses the Internet's TCP/IP protocols to enable data transfer.
▪ FTP uses client-server architecture.
▪ FTP promotes sharing of files via remote computers with reliable
and efficient data transfer.

FTP user FTP File transfer FTP

interface client server
User or
Host

Local file system Remote file system

Unit: 2 - Application Layer 37 Darshan Institute of Engineering & Technology
 FTP (File Transfer Protocol) – Cont…
▪ FTP client connect FTP server at port 21 using TCP.
▪ FTP uses two parallel TCP connections to transfer a file,
1. Control Connection: Used for sending control information between two hosts.
2. Data Connection: To send a file.
▪ Control Information like user identification, password, commands to change
remote directory, commands to “put” and “get” files
▪ Client will browses remote file directory, sends commands over control
connection.
▪ FTP server maintains “state” about user like current directory, earlier
authentication.

Unit: 2 - Application Layer 38 Darshan Institute of Engineering & Technology

 Electronic Mail (Email)
▪ Email is an asynchronous communication medium in which people send and
read messages as convenient for them.
▪ Modern Email has many powerful features like:
✔ A messages with attachments
✔ Hyperlinks
✔ HTML-formatted text
✔ Embedded photos
▪ Email is fast, easy to distribute, and inexpensive.
▪ High level view of Internet mail system and its key components.
1. User agents
2. Mail servers
3. Simple Mail Transfer Protocol (SMTP)

Unit: 2 - Application Layer 39 Darshan Institute of Engineering & Technology

 Email - Cont…
User Agent user
▪ User agents allow users to read, reply to, agent
forward, save, and compose messages.
mail user
▪ E.g. Microsoft Outlook and Apple Mail. server agent
Mail servers:
▪ A mailbox contains incoming messages
SMTP mail user
for user. server agent
▪ A message queue of outgoing (to be SMTP
sent) mail messages.
SMTP SMTP user
agent
mail
▪ It is a principal application layer protocol server
between mail servers to send email user
messages. agent
✔ client: sending mail to server
user
✔ server: receiving mail from other agent
different mail server Outgoing
message queue
user mailbox
Unit: 2 - Application Layer 40 Darshan Institute of Engineering & Technology
 SMTP
▪ Simple Mail Transfer Protocol used in sending and receiving e-
mail.
▪ It use TCP to reliably transfer email message from client to server
using port 25.
▪ It restricts the body (not just the headers) of all mail messages to
simple 7-bit ASCII.
▪ SMTP does not use intermediate mail servers for sending mail.
▪ If receiving end mail server is down, the message remains in
sending end mail server and waits for a new attempt.

Unit: 2 - Application Layer 41 Darshan Institute of Engineering & Technology

 SMTP - Example

1. Alice uses user agent to compose 4. SMTP client sends Alice’s

message to message over the TCP connection.
computer@darshan.ac.in 5. Bob’s mail server places the
2. Alice’s1)user
2)Alice
3) Alice’s
4)Client
5)
SMTPuses
Bob’s
agent user
user
side agent
client
mail
of
sends agent
sends
SMTP sends
server
messagetoplaces
compose
message
Alice’s
opens TCP message
messagetoinover
her
theconnection
message
message mail
tomailbox.
the
in
Bob’s
6) Bob invokes his user agent to read message
with
Bob’s
TCP
server;
computer@darshan.ac.in
to her mail server; message placed
messageBob’s inserver
connection
mail
placed message
mailbox
6. queue.
Bob invokes his user agent to
in message queue. read message.
3. Client side of SMTP opens TCP
connection with Bob’s mail server.

Unit: 2 - Application Layer 42 Darshan Institute of Engineering & Technology

 Mail Access Protocols (POP3 and IMAP)

▪ POP3
✔ Post Office Protocol – Version 3
▪ IMAP
✔ Internet Mail Access Protocol
▪ A mail access protocol, such as POP3, is used to transfer mail from
the recipient’s mail server to the recipient’s user agent.

Unit: 2 - Application Layer 43 Darshan Institute of Engineering & Technology

 POP3 – Post Office Version 3
▪ POP3 is an extremely simple mail access protocol.
▪ With the TCP connection established, POP3 progresses through
three phases: authorization, transaction and update.
▪ In authorization, the user agent sends a username and a password
to authenticate the user.
▪ In transaction, the user agent retrieves messages, mark messages
for deletion, remove deletion marks and obtain mail statistics.
▪ In update, after the quit command by client, ending the POP3
session; the mail server deletes marked messages.
▪ POP3 is designed to delete mail on the server as soon as the user
has downloaded it.

Unit: 2 - Application Layer 44 Darshan Institute of Engineering & Technology

 IMAP - Internet Mail Access Protocol
▪ To keeps all messages in one place: at server
▪ The recipient can then move and organize the message into a
new, user-created folder, read the message, delete the message,
move messages from one folder to another and so on.
▪ To allow users to search remote folders for messages matching
specific criteria.
▪ Also permit a user agent to obtain components of messages,
When low-bandwidth connection between the user agent and its
mail server.
▪ In this case, user not to download all of the messages in its
mailbox, particularly avoiding long messages like an audio or video
clip.
Unit: 2 - Application Layer 45 Darshan Institute of Engineering & Technology
 DNS - Domain Name System

Alphabetic name remember IP Address

by human
▪ It is an internet service that translates domain names into IP
addresses.
▪ It is application-layer protocol. DNS service must translate the
domain name into the corresponding IP address.
▪ In DNS system, If one DNS server doesn't know how to translate a
particular domain name, it asks another one, and so on, until the
correct IP address is returned.

Unit: 2 - Application Layer 46 Darshan Institute of Engineering & Technology

 DNS - Example

▪ DNS client wants to determine the IP address for the hostname

www.amazon.com
▪ The client first contacts one of the root servers, which returns IP addresses for
TLD servers - top-level domain .com.
▪ Then contacts TLD servers, which returns the IP address of an authoritative
server for www.amazon.com
▪ Finally, contacts one of the authoritative servers for www.amazon.com, which
returns the IP address for the hostname www.amazon.com.

Unit: 2 - Application Layer 47 Darshan Institute of Engineering & Technology

 DNS: A distributed - hierarchical database
▪ Root DNS Servers – Total 13

Unit: 2 - Application Layer 48 Darshan Institute of Engineering & Technology

 DNS – Cont…
▪ Top-level domain (TLD) servers:
✔ It is responsible for com, org, net, edu, aero, jobs, museums, and all top-
level country domains, e.g.: uk, fr, ca, jp
✔ Network Solutions maintains servers for .com TLD
✔ Education for .edu TLD
▪ Authoritative DNS servers:
✔ To organization’s own DNS servers, providing authoritative hostname to IP
mappings for organization’s named hosts.
✔ It can be maintained by organization or service provider.
▪ Local DNS name servers:
✔ It does not strictly belong to hierarchy
✔ when host makes DNS query, query is sent to its local DNS server.
▪ It acts as proxy, forwards query into hierarchy.

Unit: 2 - Application Layer 49 Darshan Institute of Engineering & Technology

 DNS - Example

Unit: 2 - Application Layer 50 Darshan Institute of Engineering & Technology

 DNS name resolution example
root DNS server
▪ Iterated query:
▪ A host at cis.poly.edu wants IP
2
address for gaia.cs.umass.edu 3
TLD DNS server
4

local DNS server

dns.poly.edu
7 6
1 8

authoritative DNS server

dns.cs.umass.edu
requesting host
cis.poly.edu

gaia.cs.umass.edu
Unit: 2 - Application Layer 51 Darshan Institute of Engineering & Technology
 DNS name resolution example
root DNS server
▪ Recursive query:
▪ A host at cis.poly.edu wants IP
2 3
address for gaia.cs.umass.edu 7
6
TLD DNS
server

local DNS server

dns.poly.edu 5 4

1 8

authoritative DNS server

dns.cs.umass.edu
requesting host
cis.poly.edu

gaia.cs.umass.edu
Unit: 2 - Application Layer 52 Darshan Institute of Engineering & Technology
 DNS – Cont...
▪ Distributed database design is more preferred over centralized design to
implement DNS in the Internet.
▪ A single point of failure: If the DNS server crashes then the entire
Internet will not stop.
▪ Traffic volume: With millions of device and users accessing its services
from whole globe at the same time.
▪ A Single DNS Server cannot handle huge DNS traffic but with distributed
system its distributed and reduce overload on server.
▪ Distant centralized database: A single DNS server cannot be “close to” all
the querying clients.
▪ If it is in New York City, then all queries from Australia must travel to the
other side of the globe, perhaps over slow and congested links cause
significant delays.
▪ Maintenance: To keep records for all Internet hosts. it would have to be
updated frequently to account for every new host.

Unit: 2 - Application Layer 53 Darshan Institute of Engineering & Technology

 Socket Programming
▪ Socket is interface between application and network.
✔ An application creates a socket.
✔ Two type of socket:
1. TCP Socket – Reliable Transmission
2. UDP Socket – Unreliable Transmission
▪ Once configured the application can pass data to the socket for
transmission and receive data from the socket (transmitted
through the network by some other host).
application application
proce socke proce controlled by
ss t ss app developer

transport transport
network network controlled
link
by OS
link Internet
physical physical

Unit: 2 - Application Layer 54 Darshan Institute of Engineering & Technology

 Type of Socket
▪ SOCK_STREAM ▪ SOCK_DGRAM
✔ E.g. TCP ✔ E.g. UDP
✔ Reliable delivery ✔ Unreliable delivery
✔ In-order guaranteed ✔ No order guarantees
✔ Connection-oriented ✔ Connection-less
✔ Bidirectional ✔ Unidirectional

App D1
App

3 2 3 2
1 D2
socket Dest. 1 socket
D3

Unit: 2 - Application Layer 55 Darshan Institute of Engineering & Technology

 Outline - Summary
▪ Principles of Computer Applications
✔ Browser, Web Server, Email, P2P Applications etc…
▪ Application Layer (TCP – UDP Services)
▪ Web (Web Pages – Objects like html, jpeg, mp3, etc…)
▪ HTTP (TCP connection, port-80, persistent & non-persistent conn.),
Request & Response Message format, Cookies, Web caches, FTP,
Port-21
▪ E-mail (User agent, Mail Server, SMTP port - 25), POP3, IMAP
▪ DNS (Domain names to IP Address), hierarchy structure
▪ Socket programming with TCP and UDP (TCP – Sock_Stream, UDP
– Sock_DGram)

Unit: 2 - Application Layer 56 Darshan Institute of Engineering & Technology

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