World Wide Web Hypertext model Use of hypertext in World Wide Web (WWW) WWW client-server model Use of TCP/IP protocols in WWW

Hypertext and Hypermedia  Hypermedia system allows interactive access to collections of documents  Document can hold:  Text (hypertext)  Graphics  Sound  Animations  Video  Documents linked together  Nondistributed - all documents stored locally (like CD-ROM)  Distributed - documents stored on remote servers

HyperPointers  Each document contains links (pointers) to other documents  Link represented by "active area" on screen  Graphic - button  Text - highlighted  Selecting link fetches referenced document for display  Links may become invalid  Link is simply a text name for a remote document  Remote document may be removed while name in link remains in place

Browser Interface  Interactive, "point-and-click" interface to hypermedia documents  Each document is displayed in screen  User can select and follow links - "point- and-click"  Application is called a browser (infinite time sink)

Each WWW document is called a page Initial page for individual or organization is called a home page Page can contain many different types of information; page must specify  Content  Type of content  Location  Links Rather than fixed WYSIWYG representation (e.g., Word), pages are formatted with a mark up language (like TeX)  Allows browser to reformat to fit display  Allows text-only browser to discard graphics Standard is HyperText Markup Language (HTML)

 HTML specifies  Major structure of document  Formatting instructions  Hypermedia links  Additional information about document contents  Two parts to document:  Head contains details about the document  Body contains information/content  Page is represented in ASCII text with embedded HTML tags formatting instructions  Tags have format  End of formatted section is

 Page identified by:  Protocol used to access page  Computer on which page is stored  TCP port to access page  Pathname of file on server  Specific syntax for Uniform Resource Locator (URL): protocol://computer_name:port/document_name  Protocol can be http, ftp, file, mailto  Computer name is DNS name  (Optional) port is TCP port  document_name is path on computer to page

 Browser is client, WWW server is server  Browser:  Makes TCP connection  Sends request for page  Reads page  Each different item - e.g., IMG - requires separate TCP connection  HyperText Transport Protocol (HTTP) specifies commands and client-server interaction Client/Server Model

Server Architecture Much like ftp server  Waits for incoming connection  Accepts command from connection  Writes page to connection Performance is hard issue

Browser Architecture  Browser has more components:  Display driver for painting screen  HTML interpreter for HTML-formatted documents  Other interpreters (e.g., Shockwave) for other items  HTTP client to fetch HTML documents from WWW server  Other clients for other protocols (e.g., ftp)  Controller to accept input from user  Must be multi-threaded

Caching  Downloading HTML documents from servers may be slow  Internet congested  Dialup connection  Server busy  Returning to previous HTML document requires reload from server  Local cache can be used to hold copies of visited pages  Also can implement organizational HTTP proxy that caches documents for multiple users

Security  Routers forward packets - from any source  Bad guys can send in packets from outside  How to avoid security breaches?

Security Policies Can't describe a network as secure in the abstract University may have different notion of security than military installation Must define a security policy Many possibilities to consider:  Data stored on servers  Messages traversing LANs  Internal or external access  Read/write versus read-only access

Encryption  Encryption - rewrite contents so that they cannot be read without key  Encrypting function - produces encrypted message  Decrypting function - extracts original message  Encryption key - parameter that controls encryption/decryption; sender and receiver share secret key  Sender produces: E = encrypt(K, M)  Sender transmits E on network  Receiver extracts: M = decrypt(K, E)

Public Key Previous scheme requires shared secret K If K is discovered, security is compromised Public key encryption uses two keys:  Private key - kept secret by user  Public key - published by user  To send to user, encrypt using public key, decrypt using private key

Digital Signatures Goal - guarantee that message must have originated with certain entity Authenticate Sender Idea - encrypt with private key, decrypt with public key Only owner of private key could have generated original message

Packet Filtering  Can configure packet forwarding devices - esp. routers - to drop certain packets  Consider example:  Suppose is test network and has controlling workstations  Install filter to allow packets only from to  Keeps potentially bad packets away from remainder of Internet s

Internet Firewall  Packet filter at edge of intranet can disallow unauthorized packets  Restricts external packets to just a few internal hosts  Proxies forward packets through firewall after authorization