Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 1

7. The Application Layer and Services

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 2

Outline

� Domain Name Service (DNS)� Electronic Mail (Email)

• Multipurpose Internet Mail Extension (MIME)• Simple Mail Transfer Protocol (SMTP) • Post Office Protocol (POP3)

� Telnet and Rlogin� World Wide Web (WWW)

• HyperText Markup Language (HTML)• HyperText Transfer Protocol (HTTP)• Dynamic Web pages (CGI)• Active Web Pages (Java)

� VoIP

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 3

(RFC 1034 & 1035)

Domain Name Systems (DNS)

� Binary machine address is not very convenient

� DNS is a hierarchical domain based naming scheme and a distributed database system for implementing this naming scheme

� Used for mapping host names to IP address (RFC 1034 and 1035)

� Program calls resolver to contact DNS server to lookup the name and return the IP address

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 4

Resource records and an example

IN: Internet Information

Example: host –a –v surrey.ac.uk

Domain_name, Time-to-Live, Class, Type, Value

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 5

Name Servers and Resolver looks up

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 6

Electronic mail (E-mail)(RFC 821: transmission, 822: Message format)

� Two subsystems: the user agent allows people to read and send email; and the message transfer agentmoves the message from the source to the destination.

� Support five basic functions:• Composition• Transfer• Reporting• Display• Disposition

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 7

Reading Email

K: Keep, A: Answer, F: Forwarded

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 8

Message header format (RFC822)

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 9

Multipurpose Internet Mail Extension (MIME - RFC 1341, 2045-2049)

� To add structure to the message body and encoding rules for non-ASCII messages

� Using the existing mail programs and protocols

� Five new message header� The MIME types and subtypes

defined in RFC 2045

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 10

An example of multimedia email

Same message in different format separated by the boundary

The text message

The text format of the song.

get the sound file named using FTP at the site under the directory

The audio format of the song using base64 for transfer encoding.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 11

Simple Message Transfer Protocol (SMTP)

Messages from the server xyz.com

Messages from the client abc.com

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 12

Email 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.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 13

Post Office Protocol (POP3) (RFC 1225)

11

n It has commands for users to login, fetch message, delete message, logout.

n It is used to fetch email from the remote mailbox and store it on the user’s local machine to be read later.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 14

POP3

Using POP3 to fetch three messages.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 15

IMAP

A comparison of POP3 and IMAP.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 16

Telnet

� telnet:• client-server• network virtual terminal (23/TCP)

� Provides:• session with server• ASCII presentation

� Allows process-to-process communication

12

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 17

The World Wide Web (WWW)

� Began in 1989 at CERN - the European Centre for Nuclear Research

� Linked document with first graphical interface in 1993 (Mosaic by Marc Andreessen)

� In 1994, CERN and MIT set up WWW consortium to develop standard protocols and encourage interoperability

� Client side: browser, pages, hypertext, hyperlink, hypermedia

� More info: www.w3.org

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 18

The client and server side

� The browser determines the Uniform Resource Locator (URL)

� The browser asks DNS for the IP address of the URL

� DNS reply with the IP address� The browser makes a TCP connection

to port 80 of the IP address� The browser sends a GET the page

pointed by the URL command� The server sends the page in a file� The TCP connection is released� The browser displays all the text in

the file� The browser fetches and displays all

images in the page

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 19

An example: telnet the WWW server

� The status information is displayed in the bottom of the screen

� If the performance is poor, it may be due to• DNS not responding• server not responding• network congestion during

transmission� Each in-line image (icon,

drawing, photo, etc.) on a page is fetched in a separate new connection.

Type in command

Terminal response

Server response

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 20

Proxy server

� It is a kind of gateway that speaks HTTP to the browser but FTP, Gopher, or some other protocols to the server.

� The translate HTTP request into FTP request, for example.

� It can be in the same machine as the browser or independent machine

� Caching is an important function

� Put a proxy server inside firewall to give users access to web but not full access to Internet

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 21

HyperText Transfer Protocol (HTTP)

� It is the standard Web transfer protocol

� It is constantly evolving.� There are two fairly distinct items:

• The set of request from browsers to servers

• The set of responses going back the other way

� Simple requests: just GET named page without the version; response is just a raw page

� Full requests: with version, MIME, encoding, ...

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 22

Uniform Resource Locators (URL)

� The URL gives a worldwide name, location and how to access to it

� The URL has three parts:• The protocol• DNS name of the machine

on which the page is located• Local name uniquely

indicate the specific page� HTTP is the Web’s native

language, the one spoken by HTTP server

� It is also possible to use FTP to access web pages.

n What is the page called?n Where is the page located?n How can the page be accessed?

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 23

HyperText Markup Language (HTML)

� HTML is an application of ISO standard 8879, SGML (Standard Generalised Mark-up Language), but specialised to hypertext and adapted to the Web

� It is a mark-up language for describing how documents are to be formatted

� Proprietary WYSIWYG word processors cannot be used on wobbecause their internal markuplanguages (if any) are not standardised across vendors, machines and operating systems.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 24

Some common HTML tags

� The embedding the mark-up commands within each HTML makes it possible for any browser to read and reformat any web page

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 25

Forms

(a) The HTML for an order form.(b) The formatted page. (b)

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 26

Forms

A possible response from the browser to the server with information filled in by the user.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 27

Common Gateway Interface (CGI) for Dynamic Web documents

� Specifies input/output to/from a program that is a hyperlink:• standard input format for arguments to program• output format from program is HTML• allows dynamic WWW pages

� Server-side processing:• CGI program invoked each time link is used• extra processing overhead on server

18

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 28

Dynamic Web Documents

Steps in processing the information from an HTML form.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 29

Java for Active Web documents

� Active content:• small program - applet• downloads and executes

on client-side• requires virtual machine

in client� Java:

• full programming language/platform

• includes security features

request to server for applet

applet downloads to clientand executes locally

19

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 30

Voice over IP

The H323 architectural model for Internet telephony.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 31

Voice over IP (2)

The H323 protocol stack.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 32

Voice over IP (3)

Logical channels between the caller and callee during a call.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 33

SIP – The Session Initiation Protocol

The SIP methods defined in the core specification.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 34

SIP (2)

Use a proxy and redirection servers with SIP.

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 35

Comparison of H.323 and SIP

Computer and Data Networks,7. The Application Layer and Services ©Dr.Z.Sun 36

Summary

� Domain Name Service (DNS)� Electronic Mail (Email)� Telnet and Rlogin� World Wide Web (WWW) � VoIP