A Closer Look at HTTP

1
A Closer Look at HTTP

• HyperText Transfer Protocol

2
HTTP A Closer Look

• World Wide Web Standards
• HTTP (HyperText Transfer Protocol) governs
  requests and responses between the browser and
  the webserver application program
• HTML (HyperText Markup Language) governs the
  structure of the HTML webpage

HTTP
Browser
Webserver Program
HTML
3
HTTP A Closer Look

• Sample HTTP Request Line
• All text, often only a single line
• Keywordabsolute pathHTTP/ version
• Keyword is GET for Retrievals
• GET must be capitalized
• Absolute path is URL without http//hostname

GET /report1/home.htm HTTP/1.1
Absolute Path
4
HTTP A Closer Look
New

• Sample HTTP Request Header
• Although the book only discusses the
  request-line, if only the absolute path is give,
  the request line is supposed to be followed by a
  header line naming the host.
• So if the host is puka.hawaii.edu, the
  request-line and header would be (where CRLF is
  carriage return followed by line feed)

GET /report1/home.htm HTTP/1.1CRLF Hostpuka.haw
aii.eduCRLF
5
HTTP A Closer Look

• Sample HTTP Response Message
• Header is multiple lines of text
• Each ends with CRLF

HTTP/1.1 200 OKCRLFDate Tuesday, 20-JAN-1999
183215 GMTCRLFServer name of server
softwareCRLFMIME-version 1.0CRLFContent-typ
e text/plainCRLFCRLFFile to be downloaded.
6
HTTP A Closer Look

• Sample HTTP Response Message
• 200 is a code for a successful retrieval,
  followed by the humanly readable code, OK

HTTP/1.1 200 OKCRLFDate Tuesday, 20-JAN-1999
183215 GMTCRLFServer name of server
softwareCRLFMIME-version 1.0CRLFContent-typ
e text/plainCRLFCRLFFile to be downloaded.
7
HTTP A Closer Look

• Sample HTTP Response Message
• There are other codes, indicating errors, such as
  404
• Followed by a humanly readable code, such as,
  host not found

HTTP/1.1 404 Host not FoundCRLFDate Tuesday,
20-JAN-1999 183215 GMTCRLFServer name of
server softwareCRLFMIME-version
1.0CRLFContent-type text/plainCRLFCRLFFi
le to be downloaded.
8
HTTP A Closer Look

• Sample HTTP Response Message
• MIME-version and Content-type tell the type of
  file being retrieved
• For HTML document, text/plain

HTTP/1.1 200 OKCRLFDate Tuesday, 20-JAN-1999
183215 GMTCRLFServer name of server
softwareCRLFMIME-version 1.0CRLFContent-typ
e text/plainCRLFCRLFFile to be downloaded.
9
HTTP A Closer Look

• HTML Pages Contain Text
• To be displayed
• HTML Pages Contain Tags
• Some tags say place a graphic here, place a
  JAVA applet in a box here, etc.
• For instance, the tag says
  Get file big.jpg and place it here in the HTML
  document
• Graphics, Java programs, etc. are separate files

10
HTTP A Closer Look

• Downloading a page may require several
  downloads
• One for the HTML document
• One each for the other files it calls for

HTML
App. java
1
File Downloads
Big. jpg
Webserver
User PC
2
3
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HTTP A Closer Look

• Each download requires a separate HTTP
  request-response cycle!
• So downloading complex webpages may require many
  HTTP request-response cycles

HTTP Request-Response Cycle 1
HTML
App. java
Big. jpg
Webserver
User PC
2
3
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HTTP A Closer Look

• How Many HTTP messages will be created to
  download the three files?
• 3 HTTP request-response cycles
• 2 HTTP messages per cycle (request and response)
• 6 HTTP messages total

HTML
App. java
HTTP Request-Response Cycle 1
Big. jpg
Webserver
User PC
2
3
13
HTTP A Closer Look

• HTTP is Unreliable
• There is no error detection and correction for
  transmission errors
• This helps makes HTTP simple, allowing browsers
  and webserver application programs to be simple
  and inexpensive
• As we will see later, TCP at the transport layer
  is reliable, offering error detection and
  correction
• TCP gives HTTP clean data, so there is no need
  for HTTP to do error checking

14
HTTP A Closer Look

• HTTP is Connectionless
• No connection (agreement to communicate) is made
  between the browser and webserver application
  before an HTTP message is sent
• Like sending a letter or an E-mail message
  (connectionless) as compared to taking on the
  telephone, where a conversation (connection) must
  be opened, managed, and closed

HTTPRequest
Browser
Webserver Application