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HTTP and HTML

Computer Science 01iIntroduction to the Internet

Neal Sample30 January 2001

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We will talk about...

• Domain Name System (DNS)• Client/Server Relationships• HTTP• Web Browsers• HTML

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IP Addresses for Humans

• Until now, each machine has been represented by an ugly number (xxx.xxx.xxx.xxx)

• The Domain Name Service (DNS) abstracts numeric IP addresses into something easier for humans to use

• “Internet computer system white pages”

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Domain Name System (DNS)

• No packet on the net uses a computer’s “name” for routing

• A distributed database that knows the IP address of every machine on the net

• Like routers, the DNS is highly decentralized

• Unlike Routers, the DNS is hierarchical

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How each machine is named

• Much like a street address• Left-most part is the actual name• Several hierarchical zones becoming more

and more generalgates 195, Stanford University, CA, USA

-vs-cse.stanford.edu

• No geographical restriction on host names like there is with IP addresses

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How each machine is named, II

• Every machine is part of a top-level domain .com = commercial .gov = government .edu = educational .net = networks .org = other organizations (often nonprofits) .mil = military installations Two-letter country codes for geographic sites:

• .au (Australia), .uk (United Kingdon), .ae (United Arab Emirates), .zw (Zimbabwe)

• And the winner is? Tuvalu!

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Name Servers

• Every bottom-level domain has at least one local name server - a very busy machine

• Local name servers know the IP address and name of every machine in their domain

• Forward requests for unknown names up the pyramid, then back down

• Local machine caches recently used IP addresses in other domains

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More on DNS

• Nearly every transaction on the net begins with a DNS lookup

• One IP address can have multiple names. Why is this nice?

• DNS supports reverse lookups, so you cannot hide the machine’s ownership

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Problems with DNS

• Any one can register any domain for a small fee (great in theory)

• This causes massive problems Who owns american.com? Who should own

it? What about america.com? (no “n”) Domain name resellers (greatdomains.com)

• Massive political battles about this. DNS’s design shows its age and needs to evolve.

• Uzi Nissan?

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More Top Level Domains Proposed

• .biz - registered companies• .store - net-shopping sites• .arts - art-related and creative sites• .web- things pertaining to the WWW• .rec - recreational subject matter• .info - informational sites• .name - personal sites• .sex - sex related sites

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Building Service

• Most services on the net are built using the Client/Server model

• Client contacts the server and they exchange information following some protocol

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Servers are:

• On all the time• Usually have permanent IP addresses• Generally connected to the net “24/7”• Good for:

centralization and synchronization large amounts of data that lots of people want

• What other reasons are there to centralize information on servers?

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Clients are:

• The PC you use• Good at real-time interaction

why are servers generally bad at real-time interaction?

• Usually less powerful than the server• The only technical difference is that

the server runs some programs that the client does not. Why?

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Almost Home!

• We have atoms and bits and pipes• We have packets with IP and TCP/IP• We have a way to find machines with

DNS• We have our clients and servers

… what do we need now, to finally browse the web?!

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Hypertext Transfer Protocol

• Another layer of abstractions, over TCP/IP

• Unifies and integrates other protocols

• Is extensible: plan for the future built in

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How HTTP works:

• Web browser makes a request for a page• Server sends back the page in ASCII• Web browser reads the page and

interprets codes for colors, fonts and formatting and displays the result

• Browser requests any additional files (such as images) then displays them also

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Web Browser

• Made possible by increased desktop power

• Know how to find pages referenced by another automatically -- big innovation

• Fluent in numerous data types(pictures, sounds, fonts)

• Can display multiple types at once• Wraps everything into one tidy,

convenient package

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Uniform Resource Locators (URLs)

• Addresses of information on the web

• Three parts: Protocol Host name (DNS name) Path

• A “bookmark” is a URL that your browser has saved

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An HTTP Dialog Part I:

• User types URL in browser window http://news.excite.com/odd/

• Browser does DNS lookup on host news.excite.com

• Browser contacts host via protocol http:

• Browser requests the file in the given path: /odd/

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An HTTP Dialog Part II:

• Server sends requested document back

• Browser reads it, requests any images or additional files needed

• Server sends those files• Browser displays the page to you

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Things to note:

• Most of the work happens at the browser requires a relatively smart machine server just pumps bits down the network

as fast as it can

• All viewing, scrolling, interaction, handled at the browser

• Browser usually caches pages for speed• “Back” button doesn’t require any

interaction with the server• Delete right part of the URL trick

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Hypertext Markup Language (HTML)

• The “lingua franca” of the web• Pages are written in HTML• HTML is written in ASCII• Defines the structure, not the appearance

of a document ie, HTML says “here is a title” not

“print this in 30 point Times-New-Roman” The browser determines the documents looks Why do it this way?

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More on HTML

• Images and sounds are separate files on the server and have their own URLs

• Images are relatively large images are fun and descriptive slow to transmit can turn off in most browsers

• Pages can be interactive• HTML is evolving rapidly

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Project: Say what?!

• Long live the babelfish! http://babelfish.altavista.com French, German, Italian, Portuguese, Spanish,

and English

• So you want to speak Bengali? http://rivendel.com/~ric/resources/

dictionary.html

• Plane Meals? http://24.2.81.10/fcgi-bin/anagrams.pl http://www.wordplays.com/