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Hyperlinks and Protocols
Introduction to PCs and the Internet Spring 2009
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A Basic Understanding Hyperlinks
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All of the following slides assume that you have a connection to the Internet, and
that you are viewing a web page in the web browser Internet Explorer 7. You Internet
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You click on a hyperlink to http://www.google.com/intl/en/about.html
Internet
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Your web browser detects your click
Your web browser detects your click. It analyzes the URL that you clicked on: It must retrieve a webpage using the Hyper Text Transfer Protocol It must connect to a computer known as It must access the HTML file /intl/en/about.html You Internet
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Your web browser connects to www. google
Your web browser connects to using HTTP and sends a request for the HTML file /intl/en/about.html Web browser client connects to Google server via the Internet You Internet HTTP Request GET /intl/en/about.html
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The web server at www. google
The web server at receives your request and finds the HTML file /intl/en/about.html. It then sends a response to your web browser that contains the HTML file. You Internet /intl/en/about.html: <html> <head> <title>About Google</tit </head> <body> <p>Reach the right aud HTTP Request GET /intl/en/about.html
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Your web browser receives the response and renders the HTML as a web page for
your viewing pleasure. At this time the HTTP connection can be closed. You Internet /intl/en/about.html: <html> <head> <title>About Google</tit </head> <body> <p>Reach the right aud
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More Complicated Version
Hyperlinks
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All of the following slides assume that you have a connection to the Internet, and
that you are viewing a web page in the web browser Internet Explorer 7. You Internet
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You click on a hyperlink to http://www.google.com/intl/en/about.html
Internet
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Your web browser detects your click
Your web browser detects your click. It analyzes the URL that you clicked on: It must retrieve a webpage using the Hyper Text Transfer Protocol It must connect to a computer known as It must access the HTML file /intl/en/about.html You Internet
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Your web browser wants to send the request to www. google
Your web browser wants to send the request to but first it needs to determine the Internet Protocol (IP) address of the computer at On the Internet, names like are called domain names. Computers are actually assigned IP addresses made up of numbers such as A domain name is easier for humans to remember, but IP addresses are easier for computers to use. Thus domains names must first be translated to IP addresses. You Internet
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Your web browser wants to send the request to www. google
Your web browser wants to send the request to but first it needs to determine the Internet Protocol (IP) address of the computer at On the Internet, names like are called domain names. Computers are actually assigned IP addresses made up of numbers such as A domain name is easier for humans to remember, but IP addresses are easier for computers to use. Thus domains names must first be translated to IP addresses. You Internet
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Computers on the Internet called Domain Name System (DNS) servers hold the
translation from name to number. If your web browser does not yet know the translation, it must first send a DNS request to a DNS server that it does know. DNS Response To: From: The address for “ is DNS Server You Internet DNS Request To: From: What is the address for “
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Your web browser connects to www. google. com at the address 64. 233
using a TCP connection. It then sends an HTTP request for the HTML file: /intl/en/about.html Web browser client connects to Google server via the Internet You Internet HTTP Request GET /intl/en/about.html
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The web server at www. google
The web server at receives your request and finds the HTML file /intl/en/about.html. It then sends a response to your web browser that contains the HTML file. Keep in mind that the file is actually broken into smaller units called packets. Each packet is passed from router to router until it reaches its destination. Routers are machines that connect networks of computers. You Internet /intl/en/about.html: <html> <head> <title>About Google</tit </head> <body> <p>Reach the right aud HTTP Request GET /intl/en/about.html
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Your web browser receives the response and renders the HTML as a web page for
your viewing pleasure. At this time the HTTP connection can be closed. You Internet /intl/en/about.html: <html> <head> <title>About Google</tit </head> <body> <p>Reach the right aud
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Protocols A quick word about protocols
because the Internet and everything it contains is built on them Protocols
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What is a protocol? Protocol – An agreement about how to do something
This enables computers and software built by different people to be able to communicate in the same language Examples that we have seen: Hyper Text Transfer Protocol (HTTP) – Web Browser File Transfer Protocol (FTP) – File transfer Simple Mail Transfer Protocol (SMTP) – Internet Protocol (IP) – Packets across the Internet
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Human Protocols Sending a letter via the postal service Daniel Cole
5501 Sennott Sq. Pittsburgh, PA Contents! Jason Fuller 123 Somewhere St. Someplace, PA
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Computer Protocols Sending a packet via the network
To: on port 23 From: on port 6710 Payload: The contents of the message belong here.
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Visual Comparison To: 192.168.1.1 on port 23
From: on port 6710 Payload: The contents of the message belong here. Contents!!! Daniel Cole 5501 Sennott Sq. Pittsburgh, PA Contents! Jason Fuller 123 Somewhere St. Someplace, PA
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Textual Comparison The address of the letter is the destination of the packet The return address of the letter is the source of the packet The content of the envelope is the payload of the packet In both protocols, if you disobey the rules your message will not be received
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Protocols can be layered on top of other protocols to enable new ways of communicating
Protocol Layers
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Example: Writing a letter
A common protocol that is followed when writing a letter is: [Date] Dear [Recipient], [Body of message goes here] [Closing] [Sender signature] [Sender name]
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Example: Writing a letter
A common protocol that is followed when writing a letter is: 04/01/2009 Dear Jason, Hey there! What’s up? I’m good. Thanks. Sincerely, Daniel Cole
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Example: Writing a Letter
After following the protocol for writing a letter, I can use the protocol for sending a letter via the postal service: Daniel Cole 5501 Sennott Sq. Pittsburgh, PA 04/01/2009 Dear Jason, Hey there! What’s up? I’m good. Thanks. Sincerely, Daniel Cole Jason Fuller 123 Somewhere St. Someplace, PA
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Example: Writing a letter
Thus, I have layered two protocols on top of each other. The lower protocol provides a service used by the higher protocols. Letter Writing Protocol Sending a Letter via USPS Protocol
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Protocol Layering: The Internet is build this way
Internet Protocol (IP) provides a way to deliver packets to a destination SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol
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Protocol Layering: The Internet is build this way
TCP uses packets to maintain connections across a network, and thus is layered above IP SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol
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Protocol Layering: The Internet is build this way
UDP just sends or receives raw packets with a best-effort approach, also layered above IP SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol
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Protocol Layering: The Internet is build this way
SSH, FTP, HTTP, SMTP and many more applications use TCP connections to communicate data back and forth SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol
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Protocol Layering: The Internet is build this way
DNS, VoIP, and many more applications use UDP packets to communicate data SSH, FTP, HTTP, SMTP DNS, VoIP TCP UDP Internet Protocol
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The Internet is built this way
The Internet Protocol is near the bottom layer It provides the rules for forming packets and passing them to their destination Above that are the TCP / UPD protocols Above that are the HTTP/FTP/SSH/SMTP protocols
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Protocol Layers Thus we have the following protocol layers:
Reversing Messages Protocol Letter Writing Protocol Postal Service Protocol
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Computer Communication Protocol Layers
Here are some common protocols found on the Internet and how they are layered: HTTP, FTP, IMAP, DNS Protocols Application Layer TCP and UDP Protocols Transport Layer Internet Protocol Network Layer [Hardware Stuff] Data link and Physical Layers
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Final Notes
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Summary of Internet/Networking
The Internet WWW + HTML DNS Clients/Servers and UNIXS SSH and telnet clients FTP client Networking Networks, Routers, and Packets Connections Hostnames, IP addresses, ports Protocols
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Remaining Topics PC Maintenance/Health PC Safety
Disk cleanup, Scan disk, Defragging, Safe-Mode, Browser cleanup, Backup PC Safety Protection against/Removal of Malware: Viruses, Worms, Spyware, Adware Identity theft: Phishing schemes, key loggers, trojan horses University resources for protection
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