ccnet lec 15 dns http

8/6/2019 Ccnet Lec 15 DNS HTTP

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Lecture

15Data Communication & Networks

Application Layer

(DNS, HTTP)

Muhammad Yousaf


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Where we are in Protocol Stack:


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Application Layer:

Top most layer of protocol stackIn contrast to lower layer protocols, applicationlayer protocols are not implemented as kernelmodules of operating system

Rather they are implemented as OS utilities/Server

Either user itself or user-applications can interactwith these application layer protocols

Hence application layer provides the interfacebetween user/applications & network protocol stack


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Application Layer Protocols:

Most famous application layer protocols are:DNS (Domain Name System)

SNMP (Simple Network Management Protocol)

HTTP (Hyper Text Transfer Protocol)

FTP (File Transfer Protocol)

SMTP (Simple Mail Transfer Protocol)

MIME (Multipurpose Internet Mail Exchange)

When to use which protocol obviously dependsupon user requirements


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Domain Name System


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Domain Name System:

You might be unaware of DNSBut whenever you use Internet, you use DNS

Machines (computers, routers, etc) use numbers toidentify each other

e.g: IP= 172.16.0.153, MAC= 00-16-D3-03-0A-D4But humans are comfortable to use names toidentify others

I can remember your names instead of your roll-numbers

Humans like to remember names of computer/servers on Internet

e.g: www.mail.yahoo.com


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Domain Names:

FQDN is used for fully identifying a host on InternetFully Qualified Domain Names (FQDN)

It provides absolute path to access web pagesresiding on the target host

http://www.mail.yahoo.com/index.htmlhttp://www.groups.yahoo.com/index.html

Domain names must be registered with somedomain name registrar (Authority)

IANA delegated its responsibility to manage thedomain names to ICANN

Internet Corporation for Assigned Names & Numbers(ICANN)


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Domain Names: cont

http://www.mail.yahoo.com/index.html

Top Level Domain

Second Level: Company Name

Third Level: Division/Server Name

Accessible through Web Server

Protocol used to transfer data

File Name


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Top Level Domains:

Right most part of FQDN represents the top level domain(TLD)

TLDs are very few in number

Domain names reside under these TLDs

Some of them are:.com (for commercial business sites)

.net (for networking & telecomm sites)

.org (for non-profit organizations)

.edu (for educational institutes)

.gov (for government organizations)

.mil (for military sites only)

Besides these, there are country codes also


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Country Codes:

Special top level domains to identify geographicalregions i.e. countries

Represented as two letter codes

.au (Australia)

.ca (Canada)

.cn (China)

.in (India)

.jp (Japan)

.pk (Pakistan)

.uk (United Kingdom)


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Naming Hierarchy:

Hosts are arranged in hierarchical mannerRight-most part of FQDN represent Top LevelDomain & so on

org com edunet govmil pk fr

ietf ibmyahoo

mituiuc

eduinria

wwwcs

eecase

cupidonresearch


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DNS: cont

We need a mechanism that can translate namesinto IP addresses

DNS performs this translation

There should be some DNS server (also known as

Name Server) on the network

There is a hierarchy of Name Servers

DNS server runs on port-53


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Name Resolution:

User computers are DNS clientsClients query local DNS server with some domainname and asks for the IP address

Local DNS server checks its cache for

corresponding translation entry

If found then return the IP address

Else forward the query to top level DNS server

Upper level name server traces the lower levelname server


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Name Resolution: (example)

clientlocalname

server

.edu rootname

server

MITname

server

ECEname

server

www.ece.mit.edu

www.ece.mit.edu

mit.edu; 131.126.10.23

www.ece.mit.edu

ece.mit.edu; 131.126.150.1

www.ece.mit.edu

www.ece.mit.edu; 131.126.150.25

131.126.150.25


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Displaying DNS Cache:

C:\RIU>ipconfig/displaydns

Windows IP Configuration1.0.0.127.in-addr.arpa

----------------------------------------

Record Name . . . . . : 1.0.0.127.in-addr.arpa.

Record Type . . . . . : 12

Time To Live . . . . : 603476

Data Length . . . . . : 4Section . . . . . . . . . : Answer

PTR Record . . . . . : localhost

localhost

----------------------------------------

Record Name . . . . . : localhostRecord Type . . . . . : 1

Time To Live . . . . : 603476

Data Length . . . . . : 4

Section . . . . . . . : Answer

A (Host) Record . . . : 127.0.0.1


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Hyper Text Transfer Protocol


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Hyper Text Transfer Protocol:

World wide web is most successful application ofthe Internet

Consists of a large set of clients & servers; allspeaking the same language (HTTP)

HTTP clients = web browser (Internet Explorer etc.)

HTTP servers = web server

HTTP web server runs on port-80

It is the protocol that defines how web browserdownload information from web servers


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Evolution of HTTP:

First developed in 1990Later standardized in 1996 as RFC-1945

Current version is HTTP/1.1 (RFC-2616)

Dont confuse HTTP with HTML

HTML is the language for creating web pages

HTTP is the protocol that carry the HTML data fromone computer to other computer

HTTP can also carry text files, images, audio files,video files, pdf files, etc


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Working of HTTP:

HTTP is a stateless request/response protocolClients make requests to the server

Server then responds to the clients

Stateless means that each request/responsesession is independent from previous session


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HTTP request message:


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HTTP request line:

HTTP request message specifies the:operation to be performed

web page on which the operation should be performed

the HTTP version

e.g: GET http://www.case.edu.pk HTTP/1.1


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HTTP response message:


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HTTP response status line:

Response message begins with the status linespecifying the:

Version of HTTP

3-digit code indicating success or not

Text string giving the reasone.g: HTTP/1.1 202 Accepted


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HTTP example:


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Server Name resolution:

You can access web server using following type ofURLs (Universal Resource Locator)

http://207.200.83.29:80/index.html

http://www.netscape.com:80/index.html

http://www.netscape.com/index.html

Port-80 is default for web servers

Given the host name, IP address can be

determined using DNSNow having Port-number & IP address of server,client can easily establish the TCP connection withthe server


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Putting All Together:

See what web browser (client) is doing:Extracts server's hostname from the URL

Converts server's hostname into server's IP addressusing DNS

Extracts port number (if any) from the URLEstablishes a TCP connection with web server

Sends an HTTP request message to the server

Receives an HTTP response from the server

Connection is closed

Browser displays received web page


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Getting HTML page via HTTP:


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Last words:

This is not the END

This is not even the BEGINING OF END

This is just the END OF THE BEGINNING


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