The application layer
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The Application Layer. Chapter 7. DNS – The Domain Name System: RFC 1034, 1035. The DNS Name Space : No rule against registering under two top-level domains but not many organizations except multinationals do it( For Eg. and

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The application layer

The Application Layer

Chapter 7

Dns the domain name system rfc 1034 1035

DNS – The Domain Name System: RFC 1034, 1035

  • The DNS Name Space : No rule against registering under two top-level domains but not many organizations except multinationals do it( For Eg. and

  • Resource Records: Every DNS keeps lot of information about its domain in what are called the resource records.

  • Name Servers

The dns name space

The DNS Name Space

A portion of the Internet domain name space.

The dns name space1

The DNS Name Space

Note that

  • Yale Univ of US is directly under generic domain “edu”

  • Keio Univ of Japan is under “” (ac mirrors edu in japan)

  • Vrije Univ of Netherland is directly under country domain “nl”.

Resource records

Resource records

Five Tuple

Domain_name TTL Class Type value

TTL: How much stable/reliable is this entry, for heavily used site this could be big and infrequently used site it could be small.

Class: Theoretically, for Internet related information it is IN, for other purposes other codes may be used, but in practice other codes are rarely seen.

Resource records1

Resource Records

The principal DNS resource records types.

Resource records 2

Resource Records (2)

A portion of a possible DNS database for

Name servers

Name Servers

Part of the DNS name space showing the division into zones.

Name servers 2

Name Servers (2)

How a resolver looks up a remote name in eight steps.



As resource records trace back through various name servers, they are entered into a cache there.

This information is not authoritative, since the changes made at before the use of the entry are not reflected in the cache entry.

Hence the TTL field. Sites which have held their IP addresses for long can have a large value(1 day or 86400secs) for this field and others can have smaller value depending upon how frequently they change.

Electronic mail

Electronic Mail

  • Architecture and Services

  • The User Agent

  • Message Formats

  • Message Transfer

  • Final Delivery

Electronic mail 2

Electronic Mail (2)

Some smileys. They will not be on the final exam :-).

Architecture and services

Architecture and Services

Architecture: Header and Body.

Basic functions

  • Composition (compose)

  • Transfer (send/recv)

  • Reporting (whether successfully delivered or not etc)

  • Displaying

  • Disposition (delete/save etc)

Advanced features

Advanced Features

  • Create and maintain address book

  • Forward etc

User agent

User Agent

  • User Agents deals with providing an interface to the user to perform various functions. For Eg. Outlook Express, Yahoo mail,, etc.

Reading e mail

Reading E-mail

An example display of the contents of a mailbox.

Message formats rfc 822

Message Formats – RFC 822

RFC 822 header fields related to message transport.

Message formats rfc 822 2

Message Formats – RFC 822 (2)

Some fields used in the RFC 822 message header.

Problems with rfc 822

Problems with RFC 822

  • Messages in languages without alphabet (e.g. Chinese and Japanese)

  • Messages in languages with accents (e.g. French and German)

  • Messages in non-Latin alphabets (e.g. Russian)

  • Messages containing audio and images.


    MIME – Multipurpose Internet Mail Extensions: RFC 2045-2049

The application layer


RFC 822 headers added by MIME.

The application layer


The MIME types and subtypes defined in RFC 2045.

Mime 4

MIME (4)

A multipart message containing enriched and audio alternatives.

Message transfer

Message Transfer

  • SMTP (Simple Mail Transfer Protocol)

  • POP3(Post Office Protocol Version 3) and IMAP (Internet Message Access Protocol)

The application layer


An SMTP client (running on the sender’s mail server) establishes a TCP connection at port 25 with SMTP server (running on receiver’s mail server).

SMTP server is continuously listening to port 25 as explained earlier.

If the server is ready to accept mail from the client it informs else try later.

Once the connection is established, client announces whom the mail is coming from and whom is it meant for.

If such a recipient exists at the server’s side it gives a go-ahead.

Client sends the message and the server acknowledges. Send more if required else connection is released.

Message transfer using smtp

Message Transfer using SMTP

Transferring a message from to

Problems in this approach

Problems in this approach

  • Receiver was expected to be online all the time.

  • Worked well when the Internet was small (ARPANET)

  • But not now when the IP address being a scarce resource, is allocated dynamically.

Final delivery

Final Delivery

(a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent. (b) Reading e-mail when the receiver has a dial-up connection to an ISP.

Problems with pop3

Problems with POP3

Once the receiver downloaded the message, it was erased from the mailbox at the ISP.

The only copy was at the user’s hard disk and if it crashes all the mails are gone.

The application layer


A comparison of POP3 and IMAP.

The world wide web

The World Wide Web

  • Architectural Overview

  • Static Web Documents

  • Dynamic Web Documents

  • HTTP – The HyperText Transfer Protocol

  • Performance Enhancements

  • The Wireless Web

Architectural overview

Architectural Overview

(a) A Web page (b) The page reached by clicking on Department of Animal Psychology.

The url


Three parts:

  • Name of the protocol (http: HyperText Transfer Protocol)

  • DNS name of the machine where the desired page is located (

  • The name of the file containing the page (products.html)

Architectural overview 2

Architectural Overview (2)

The parts of the Web model.

Client side

Client Side

  • The browser determines the URL (say

  • The browser asks its DNS for the IP address of the remote DNS

  • DNS replies with

  • The browser makes a TCP connection to port 80 on

  • It then sends a request asking for file /home/index.html.

  • server sends the file /home/index.html.

  • The TCP connection is released.

  • The browser displays the file as required (using a plug in or an application program like Adobe Reader or Word)

The client side

The Client Side

(a) A browser plug-in. (b) A helper application.

The server side

The Server Side

  • Accept a TCP connection from a client.

  • Get the name of the requested file.

  • Get the file (from the disk).

  • Return the file to the client.

  • Release the TCP connection.

The server side1

The Server Side

A multithreaded Web server with a front end and processing modules.

Modern servers do much more

Modern Servers do much more

  • Resolve the name of the web page requested.

  • Authenticate the client.

  • Perform access control on the client.

  • Perform access control on the Web page.

  • Check the cache.

  • Fetch the requested page from the disk.

  • Determine the MIME type to include in response.

  • Other miscellaneous.

  • Return the reply to the client.

  • Make an entry in the server log.

The server side 2

The Server Side (2)

A server farm.

The server side 3

The Server Side (3)

(a) Normal request-reply message sequence.

(b) Sequence when TCP handoff is used.

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