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URL - UMIACS

Transcript URL - UMIACS

Structured Documents
Week 3
LBSC 690
Information Technology
Outline
• Muddiest points
• Building the Web
• Building a better Web
Muddiest Points
• Encryption
• Packet vs. circuit switching
• The TCP/IP “protocol stack”
Encryption
• Secret-key systems (e.g., DES)
– Use the same key to encrypt and decrypt
• Public-key systems (e.g., PGP, PKI)
– Public key: open, for encryption
– Private key: secret, for decryption
• Digital signatures
– Encrypt with private key, decrypt with public key
Encryption Issues
• Key length
– 128 bits balances speed and protection today
• Trust infrastructure
– How do you prevent “bait and switch”?
– Who certifies a digital signature is valid?
Encrypted Applications
• Secure Shell (SSH)
– Replaces Telnet
• Secure FTP (SFTP)/Secure Copy (SCP)
– Replaces FTP
• Secure HTTP (HTTPS)
– Used for financial and other private data
• Wired Equivalent Protocol (WEP)
– Used on wireless networks
Packet vs. Circuit Networks
• Telephone system (“circuit-switched”)
– Fixed connection between caller and called
– High network load results in busy signals
• Internet (“packet-switched”)
– Each transmission is routed separately
– High network load results in long delays
Packet Switching
• Break long messages into short “packets”
– Keeps one user from hogging a line
• Route each packet separately
– Number them for easy reconstruction
• Request retransmission for lost packets
– Unless the first packet is lost!
The TCP/IP “Protocol Stack”
• Link layer moves bits
– Ethernet, cable modem, DSL
• Network layer moves packets
– IP
• Transport layer provides services to applications
– UDP, TCP
• Application layer uses those services
– DNS, FTP, SSH, …
TCP/IP layer architecture
Application
Application
Virtual network service
Transport
Transport
Virtual link for end to end packets
Network
Link
Network
Link
Link for bits
Virtual link for packets
Link
Link
Link for bits
Network
Network
Link
Link
Link for bits
The World-Wide Web
My
Browser
Local copy of
Page requested
Page Requested
Proxy Server
Fetch Page
Send Request
Internet
Remote
Sever
Web Standards
• HTML
– How to write and interpret the information
• URL
– Where to find it
• HTTP
– How to get it
Uniform Resource Locator (URL)
• Uniquely identify web pages on the WWW
– Domain name
– Directory path
– File name
Domain name
File name
URL: http://www.clis.umd.edu/courses/schedules/fall2003.html
Directory path
HyperText Markup Language (HTML)
• Simple document structure language for Web
• Advantages
– Adapts easily to different display capabilities
– Widely available display software (browsers)
• Disadvantages
– Does not directly control layout
Hands On:
Learning HTML From Examples
• Use Internet Explorer to find a page you like
– http://www.umiacs.umd.edu/~daqingd/simplepage.html
– http://www.glue.umd.edu/~oard
• On the “View” menu select “Source”
– Opens a notepad window with the source
• Compare HTML source with the Web page
– Observe how each effect is achieved
Hands On: “Adopt” a Web Page
• Modify the HTML source using notepad
– For example, change the page to yours
• Save the HTML source on your “M:” drive
– In the “File” menu, select “Save As”
– Select “All Files” and name it “test.html”
• FTP it to your ~/pub directory on WAM
– ftp wam.umd.edu
– cd ../pub/
– put test.html
• View it
– http://www.wam.umd.edu/~(yourlogin)/test.html
HTML Document Structure
• “Tags” mark structure
– <html>a document</html>
– <ol>an ordered list</ol>
– <i>something in italics</i>
• Tag name in angle brackets <>
– Not case sensitive
• Open/Close pairs
– Close tag may be optional (if unambiguous)
Logical Structure Tags
• Head
– Title
• Body
–
–
–
–
–
–
Headers: <h1> <h2> <h3> <h4> <h5>
Lists: <ol>, <ul> (can be nested)
Paragraphs:<p>
Definitions: <dt><dd>
Tables: <table> <tr> <td> </td> </tr> </table>
Role: <cite>, <address>, <strong>, …
Rendering
• Different devices have different capabilities
– Desktop
– PDA
• Rendering maps logical tags to physical layout
– Controls line wrap, size, font…
• Place the title in the page border
• Render <h1> as 24pt Times
• Render <strong> as bold
• Somewhat browser-dependent
– Internet Explorer and Netscape make different choices
Physical Structure Tags
• Font
– Typeface: <font face=“Arial”></font>
– Size: <font size=“+1”></font>
– Color: <font color=“990000”></font>
• http://www.barasch.com/excel/colorfonts.htm
• Emphasis
– Bold: <b></b>
– Italics: <i></i>
Hypertext “Anchors”
• Links make the Web a web!
• Internal anchors: somewhere on the same page
– <a href=“#students”> Students</a>
• Links to: <a name=“students”>Student Information</a>
• External anchors: to another page
– <a href=“http://www.clis.umd.edu”>CLIS</a>
– <a href=“http://www.clis.umd.edu#students”>CLIS students</a>
Images
• <img src=“URL”> or <img src=“path/file”>
–
–
–
–
–
<img src=“http://www.clis.umd.edu/IMAGES/head.gif”>
SRC: can be url or path/file
ALT: a text string
ALIGN: position of the image
WIDTH and HEIGHT: size of the image
• Can use as anchor:
–
<a href=URL><img src=URL2></a>
• Example:
– http://www.umiacs.umd.edu/~daqingd/Image-Alignment.html
Tables
<table align=“center”>
<caption align=“right”>The caption</caption>
< tr align=“LEFT”>
<th> Header1 </th>
<th> Header2</th>
</tr>
<tr><td>first row, first item </td>
<td>first row, second item</td></tr>
< tr><td>second row, first item</td>
<td>second row, second item</td></tr>
</table>
Example: http://www.umiacs.umd.edu/~daqingd/Simple-Table.html
Frames
• Divide browser pages into separate sections
– Useful when you want to scroll separately
• Each section can display an HTML page
• Example 1: menu frame on the left side of a page
<frameset cols=“10%,90%" >
<frame src=“template.html">
<frame src=“images.html">
</frameset>
• Example 2:
– http://www.scms.rgu.ac.uk/research/ir/members.html
Designing Web Pages
• Key design issues:
– Content: What do you want to publish?
– Style: How do you want to present it?
– Syntax: How can you achieve that presentation?
• Sources of information
– Online tutorials (Yahoo points to lots of these)
– Technical materials (e.g., the HTML 4.0 spec)
Some Style Guidelines
• Design for generic browsers
– And test on every version you wish to support ?
• Provide appropriate “access points”
– User needs and navigation strategies differ
• Design useful navigational aids
– A Web search may lead to the middle of a site
• Include some indication of currency
– Date of last update, “new” icons, etc.
• Indicate who is responsible for the content
– Helps readers assess authority
Accessibility Guidelines
• Design for device independence
• Maintain backward compatibility
– Provide alternative pages if necessary
• Provide alternatives for aural and visual content
– Alt tags for images, transcripts for audio
• Make is easy for assistive devices to work
–
–
–
–
Combine structural markup and style sheets
Give a title to each frame
Use HTML tables only for tabular data
Use markup to indicate language switching
HTML Editors
• Goal is to create Web pages, not learn HTML!
• Several are available
– In Explorer, “File” - “Edit with Front Page”
– In Netscape, “File” – “Edit Page” for Composer
• You may still need to edit the HTML file
– Some editors use browser-specific features
– Some HTML features may be missing entirely
– File names may be butchered by FTP
• Tend to use physical layout tags extensively
– Detailed control can make hand-editing difficult
HTML Validators
• Syntax checking: cross-browser compatibility
– http://validator.w3.org
• Style checking: improved accessibility
– http://bobby.watchfire.com
What’s Wrong with the Web?
• HTML
– Confounds structure and appearance (XML)
• HTTP
– Can’t recognize related transactions (Cookies)
• URL
– Links breaks when you move a file (PURL)
Discussion Point:
Describing the Structure of Text
• Entities
– Span
– Type/Attributes
• Relationships
– Part-whole
– Is-a
What’s a Document?
• Content
• Structure
• Appearance
• Behavior
History of Structured Documents
• Early standards were “typesetting languages”
– NROFF, TeX, LaTeX, SGML
• HTML was developed for the Web
– Too specialized for other uses
• Specialized standards met other needs
– Change tracking in Word, annotating manuscripts, …
• XML seeks to unify these threads
– One standard format for printing, viewing, processing
Goals of XML
• Meta language
– A toolkit for design markup languages
• Unambiguous markup
– Clear span of tags
• Separate markup from presentation
– Style info => stylesheet, so easy to change
• Be simple
A Family of Standards
• Definition: DTD
– Names known types of entities with “labels”
– Defines part-whole and is-a relationships
• Markup: XML
– “Tags” regions of text with labels
• Markup: XLink
– Defines “hypertext” (and other) link relationships
• Presentation: XSL
– Specifies how each type of entity should be “rendered”
XML Example
• View “The Song of the Wandering Aengus”
– http://glue.umd.edu/~rba/COURSES/TECHNOLOGY/XML/DTD/
• Built from three files
– yeats01.xml
– poem01.dtd
– poem01.xsl
An XML Example
<?xml version="1.0"?>
<!DOCTYPE POEM SYSTEM "poem01.dtd">
<?xml-stylesheet type="text/xsl" href="poem01.xsl"?>
<POEM>
<TITLE>The Song of Wandering Aengus</TITLE>
<AUTHOR> <FIRSTNAME>W.B.</FIRSTNAME>
<LASTNAME>Yeats</LASTNAME>
</AUTHOR>
<STANZA>
<LINE>I went on to the hazel wood,</LINE>
<LINEIN>Because a fire was in my head,</LINEIN>
<LINE>And cut and peeled a hazel wand,</LINE>
</STANZA>
</POEM>
Document Type Definition (DTD)
<!ELEMENT poem ( (title, author, stanza)* )>
<!ELEMENT title (#PCDATA) >
<!ELEMENT author (firstname, lastname) >
<!ELEMENT firstname (#PCDATA) >
<!ELEMENT lastname (#PCDATA) >
<!ELEMENT stanza (line+ | linein+) >
<!ELEMENT line (#PCDATA) >
<!ELEMENT linein (#PCDATA) >
#PCDATA
a,b
a|b
a*
a+
span of text
a followed by b
either a or b
0 or more a’s
1 or more a’s
Specifying Appearance: XSL
<xsl:template match="POEM">
<xsl:template match="TITLE">
<HTML>
<H1>
<BODY BGCOLOR="#FFFFCC">
<FONT COLOR="Green">
<xsl:apply-templates/>
<xsl:value-of/>
</BODY>
</FONT>
</HTML>
</H1>
</xsl:template>
</xsl:template>
An XLink Example
……
<poem xmlns:xlink="http://www.w3.org/1999/xlink">
<author xlink:href="yeatsRDFS3.xml“
xlink:type="simple">W. B. Yeats</author>
<poems>
<poem1
xlink:href="http://www.geocities.com/Athens/5379/yeats_index.html"
xlink:type="simple">The Rose</poem1>
<poem2
xlink:href="http://www.geocities.com/Athens/5379/yeats_index.html"
xlink:type="simple">The Tower</poem2>
</poems>
</poem>
……….
Some XML Applications
• Text Encoding Initiative
– For adding annotation to historical manuscripts
– http://www.tei-c.org/
• Encoded Archival Description
– To enhance automated processing of finding aids
– http://www.loc.gov/ead/
• Metadata Encoding and Transmission Standard
– Bundles descriptive and administrative metadata
– http://www.loc.gov/standards/mets/
What’s Wrong with the Web?
• HTML
– Confounds structure and appearance (XML)
• HTTP
– Can’t recognize related transactions (Cookies)
• URL
– Links breaks when you move a file (PURL)
Cookies
• Servers know users by IP address and port
– Because that’s where they send the Web pages
• Cookies preserve “state”
– Server sends data to the browser
– Browser later responds with the same data
• A unique code (server-side state)
• Information about the user (client-side state)
Uniform Resource Names (URN)
• Persistent URLs (www.purl.org)
– http://purl.oclc.org/OCLC/PURL/FAQ/
My
Browser
PURL
URL
URL
PURL Sever
Page
Resource Sever
Summary
• Learning to build simple Web pages is easy
– Which is good news for the homework!
• All documents are structured documents
• XML is a flexible markup language toolkits
• The key is to understand its capabilities
– XML editors can hide much of the complexity
Before You Go!
• On a sheet of paper (no names), answer the
following question:
What was the muddiest point in today’s
class?