World Wide Web

The World Wide Web

Formats and protocols

• What do those words mean?
  • A convention
  • Gives meaning to “bits on the wire”
  • Example:
• 3 critical protocols or formats
  • HTTP - Hyptertext Transfer ProtocolP
  • URI - Universal Resource Identifier
  • HTML - Hypertext Markup Language
• Actually there are a bunch more that are almost as important
  • CSS - Cascading Style Sheets
  • JS (or ECMA) - Javascript
  • More?
• All this is based on foundational Internet protocols
  • TCP/IP
  • DNS
• Dissecting the URL
  • Protocol
  • Host Name (see also: domain name, subdomain name)
  • Port (default for http is 80 and for telnet is 23, etc.)
  • Path
  • Parameters
• HTTP Protocol
  • Client makes a request (a packet is sent)
  • Server Responds to the request (a packet is sent back)
  • URL is on the “envelope” of the request
  • Stateless
• HTTP Requests contain:
  • Request “method”
  • URL
    • Host name
    • Path
    • Parameters
  • Request Headers
  • Methods:
    • GET, HEAD, POST, PUT, DELETE, a few others
• HTTP Responses contain
  • Status code
  • Response headers:
    • Content-type
    • Content-length
    • Lots of other stuff

Understanding this is critical!

Use Telnet to experiment with TCP/IP

• TCP/IP sends and receives ‘text’ protocols
• Telnet utility
  • command line indicates the target host and port
  • Then you can type in the messages
  • What happens if I don’t specify port 80?

telnet example.com 80
GET /index.html HTTP/1.1
Host: www.example.com

Use an app to visualize HTTP

• Mac only: Rested
• Other platforms: rest-client
• Mac version of rest-client

Review: What does TCP/IP do?

• Sends a ‘message’ from one computer to another (often message is text)
• Destination computer is designated by a host or domain name
• Destination within the computer is designated by a port number
• The format or Interpretation of the ‘message’ is determined by the port
• Usually port 80 messages are interpreted as HTTP

Review: What does HTTP do?

• Expects a “request” message which is answered by a “response”
• Request message
  • Defines a “method”
  • Defines the “path”
  • Supplies information about the format and lots of other stuff
• Response message
  • Defines a status code
  • Defines format of the response
  • Defines the response body

Review: How is the URL processed?

• URL: http://www.salas.com:80/toplevel?catalog=cosi goes:
  • http -> use http protocol
  • :80 -> over port 80
  • -> method is implictly GET
  • www.salas.com -> host www.salas.com
  • /toplevel -> the path
  • Params ->
    • catalog = “cosi”