© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 1 Traditional Internet Applications Asst. Prof. Chaiporn Jaikaeo, Ph.D. Computer Engineering Department Kasetsart University, Bangkok, Thailand Adapted from the notes by Lami Kaya, and notes by J.F Kurose and K.W.

2 Application-Layer Protocols Specs for network applications The syntax and semantics of messages Actions to be taken if an error arises How the two sides know when to terminate communication There are two broad types of application-layer protocols that depend on the intended use: Private communication—proprietary, limited scope Standardized service—standard, Internet-wide: Web server, FTP, etc. Standards for the Internet are known as Request for Comments (RFCs)

3 Representation And Transfer Application-layer protocols specify two aspects of interaction: Representation Transfer

World Wide Web

5 Web Protocols The World Wide Web (WWW) is one of the most widely used services in the Internet

6 Document Representation With HTML HyperText Markup Language (HTML) Specifies the syntax of a web page Uses a textual representation Describes pages that contain multimedia Provides markup specifications instead of formatting Permits a hyperlink to be embedded in an arbitrary object

7 Document Representation With HTML Whitespaces do not matter Document Title Hello Document Title Hello

8 Uniform Resource Locators URLs identify where resources can be found and how they can be obtained The general form of a URL is: Example:

9 Uniform Resource Locators In a typical URL, a user can omit many of the parts E.g., Which omits the protocol (http is assumed) the port (80 is assumed) the document name (index.html is assumed) and parameters (none are assumed)

10 Hypertext Transfer Protocol HyperText Transfer Protocol (HTTP) Primary transfer protocol between web server and browser (client) A browser is a client that extracts a server name from a URL and contacts the server Most URLs contain an explicit protocol reference of or omit the protocol altogether (HTTP is assumed)

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.11 HTTP Once it establishes a connection a browser sends an HTTP request to the server

12 HTTP GET Requests/Responses The browser (client) sends a GET request to request a resource (e.g., HTML doc, images) The server responds by sending a header, a blank line, and the requested document GET /index.html HTTP/1.1 CRLF ( \r\n ) Sample GET request HTTP/ OK Server: Apache/ Content-Length: 221 Content-Type: text/html : Sample response

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.13 Response Details The first line of a response header contains a status code Additional lines of the header give further information, such as its length when it was last modified and the content type

14 Web Browser Architecture A browser must understand HTTP (at least) A browser also provides support for other protocols Knows how to interact with a server and how to interpret responses E.g., a browser must know how to access the FTP (File Transfer Protocol) service Knows what external program to launch for particular protocol code

File Transfer

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.16 File Transfer Protocol (FTP) FTP provides a powerful mechanism for the exchange of data between computers File transfer across the Internet is complicated because computers are heterogeneous, since each computer system file representations type information naming file access mechanisms

17 File Transfer Protocol (FTP) On some OS, each line in a text file is terminated by a single LINEFEED (LF) character On some OS may require both CARRIAGE RETURN (CR) and LINEFEED (LF) Some OS use slash (/) as a separator in file names, and others use a backslash (\)

18 FTP Communication Paradigm Based on client-server architecture Employs two types of connections Control connection – used for sending control commands Data connections – used for transferring file contents or directory listings

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.19 Typical FTP Connections

Electronic Mail

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.21 Electronic Mail One of the most widely used Internet applications Because it was conceived before personal computers and hand-held PDAs were available, software is divided into two parts interface application A mechanism for a user to compose and edit outgoing messages as well as read and process incoming transfer program acts as a client to send a message to the mail server on the destination computer; the mail server accepts incoming messages and deposits each in the appropriate user's mailbox

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.22 Electronic Mail The specifications used for Internet can be divided into three broad categories

23 Simple Mail Transfer Protocol (SMTP) Standard protocol that a mail transfer program uses SMTP has a restriction to send only textual content MIME standard that allows to include attachments such as graphic images or binary files SMTP can send a single message to multiple recipients

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.24 Example SMTP Session

25 ISPs, Mail Servers, And Mail Access Most users do not know how to configure and manage an server ISPs began offering services An ISP runs an server and provides a mailbox for each user access follows one of two forms: A special-purpose interface application A web browser that accesses an web page

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.26 ISPs, Mail Servers, And Mail Access

27 Web Browser as Mail Access ISP provides a special web page that displays messages from a user's mailbox Advantages ability to read from any computer a user does not need to run a special mail interface application

28 Mail Access Protocols (POP, IMAP) Provide access to a user’s mailbox Permit a user to view headers, download, delete, or send messages Client runs on user’s personal computer Server runs on a computer that stores user’s mailbox

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.29 Representation Standards Two important representation standards exist: RFC (Request For Comments) 2822 Internet Message Format Multi-purpose Internet Mail Extensions (MIME)

RFC 2822 Mail Message Format Takes its name from the IETF standards document RFC 2822 A mail message is represented as a text file and consists of a header section a blank line and a body Header lines each have the form: Keyword: information where the set of keywords is defined to include From:, To:, Subject:, Cc:

31 Multi-purpose Internet Mail Extensions (MIME) The MIME standard extends the functionality of to allow the transfer of non-text data in a message MIME specifies how a binary file can be encoded into printable characters, included in a message, and decoded by the receiver The Base64 encoding standard is most popular, but MIME does not restrict encoding to a specific form

Domain Name System

33 Domain Name System (DNS) DNS provides a service that maps human- readable symbolic names to computer addresses Just like a phonebook of the Internet Browsers, mail software, and most other Internet applications use the DNS

34 Domain Name System (DNS) Each name consists of a sequence of alpha-numeric segments separated by periods For example, a computer in the Computer Engineering Department at Kasetsart University has the domain name: garnet.cpe.ku.ac.th A computer at Cisco, Incorporated has the domain name: anakin.cisco.com Domain names are hierarchical, with the most significant part of the name on the right

35 Top-Level Domains (TLDs) The most significant segment of each domain name is called a top-level domain (TLD) Controlled by the Internet Corporation for Assigned Names and Numbers (ICANN) ICANN designates one or more domain registrars to administer a given top-level domain and approve specific names

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.36 Examples of TLDs

Examples of Country Code TLDs ccTLDCountry at Austria be Belgium ca Canada ch Switzerland cn Mainland China fm Federated States of Micronesia jp Japan th Thailand to Tonga tv Tuvalu

38 DNS Hierarchy And Server Model Each organization is free to choose the details of its servers A small organization that only has a few computers can contract with an ISP to run a DNS server. An organization that runs its own server can choose to place all names for the organization in a single physical server, or it can choose to divide its names among multiple servers

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.39 Possible Server Setups

40 Name Resolution Software (or library) to perform name-address translation is known as a name resolver The resolver becomes a client of a DNS server DNS server returns an answer to the caller Each resolver is configured with the address of one or more local domain name servers The resolver forms a DNS request message sends the message to the local server waits for the server to send a DNS reply message for the answer

requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8.edu DNS server Example Host at cis.poly.edu wants IP address for gaia.cs.umass.edu There are 13 root DNS servers around the world

DNS Caching Once (any) name server learns mapping, it caches mapping Cache entries timeout (disappear) after some time TLD servers typically cached in local name servers Thus root name servers not often visited

nslookup Command Tool for querying DNS Available for many operating systems Including Windows and Linux $ nslookup > Server: Address: #53 Non-authoritative answer: Name: Address: >

Summary Common Internet applications and protocols World Wide Web Electronic Mail File Transfer Domain Name System