HTTP

1
HTTP

• Hypertext Transfer Protocol

2
HTTP messages

• HTTP is the language that web clients and web
  servers use to talk to each other
• HTTP is largely under the hood, but a basic
  understanding can be helpful
• Each message, whether a request or a response,
  has three parts
• The request or the response line
• A header section
• The body of the message

3
What the client does, part I

• The client sends a message to the server at a
  particular port (80 is the default)
• The first part of the message is the request
  line, containing
• A method (HTTP command) such as GET or POST
• A document address, and
• An HTTP version number
• Example
• GET /index.html HTTP/1.0

4
Other methods

• Other methods beside GET and POST are
• HEAD Like GET, but ask that only a header be
  returned
• PUT Request to store the entity-body at the URI
• DELETE Request removal of data at the URI
• LINK Request header information be associated
  with a document on the server
• UNLINK Request to undo a LINK request
• OPTIONS Request information about communications
  options on the server
• TRACE Request that the entity-body be returned
  as received (used for debugging)

5
What the client does, part II

• The second part of a request is optional header
  information, such as
• What the client software is
• What formats it can accept
• All information is in the form Name Value
• Example
• User-Agent Mozilla/2.02Gold (WinNT I)Accept
  image/gif, image/jpeg, /
• A blank line ends the header

6
Client request headers

• Accept type/subtype, type/subtype, ...
• Specifies media types that the client prefers to
  accept
• Accept-Language en, fr, de
• Preferred language (For example English, French,
  German)
• User-Agent string
• The browser or other client program sending the
  request
• From dave_at_acm.org
• Email address of user of client program
• Cookie namevalue
• Information about a cookie for that URL
• Multiple cookies can be separated by commas

7
What the client does, part III

• The third part of a request (after the blank
  line) is the entity-body, which contains optional
  data
• The entity-body part is used mostly by POST
  requests
• The entity-body part is always empty for a GET
  request

8
What the server does, part I

• The server response is also in three parts
• The first part is the status line, which tells
• The HTTP version
• A status code
• A short description of what the status code means
• Example HTTP/1.1 404 Not Found
• Status codes are in groups
• 100-199 Informational
• 200-299 The request was successful
• 300-399 The request was redirected
• 400-499 The request failed
• 500-599 A server error occurred

9
Common status codes

• 200 OK
• Everything worked, heres the data
• 301 Moved Permanently
• URI was moved, but heres the new address for
  your records
• 302 Moved temporarily
• URL temporarily out of service, keep the old one
  but use this one for now
• 400 Bad Request
• There is a syntax error in your request
• 403 Forbidden
• You cant do this, and we wont tell you why
• 404 Not Found
• No such document
• 408 Request Time-out, 504 Gateway Time-out
• Request took too long to fulfill for some reason

10
What the server does, part II

• The second part of the response is header
  information, ended by a blank line
• Example
• Content-Length 2532Connection CloseServer
  GWS/2.0Date Sun, 01 Dec 2002 212450
  GMTContent-Type text/htmlCache-control
  privateSet-Cookie PREFID05302a93093ec661TM10
  38777890LM1038777890SyNWNjraftUz299RH
  expiresSun, 17-Jan-2038 191407 GMT path/
  domain.google.com

11
Viewing the response

• There is a header viewer at http//www.delorie.com
  /web/headers.html(with nasty jittery
  advertisements)
• Example 2.3 (GetResponses) in the Gittleman book
  does the same thing
• Heres an example (from GetResponses)
• java GetResponses http//www.cis.upenn.edu/matu
  szek/cit597-
  
  2003/index.htmlStatus line HTTP/1.1
  200 OKResponse headers Date Wed, 10
  Sep 2003 002653 GMT Server
  Apache/1.3.26 (Unix) PHP/4.2.2 mod_perl/1.27
  
  mod_ssl/2.8.10 OpenSSL/0.9.6e
  Last-Modified Tue, 09 Sep 2003 192450 GMT
  ETag "1c1ad5-1654-3f5e2902
  Accept-Ranges bytes Content-Length
  5716 Keep-Alive timeout15, max100
  Connection Keep-Alive Content-Type
  text/html

12
The GetResponses program, I

• Heres just the skeleton of the program that
  provided the output on the last slide
• import java.net.import java.io.public
  class GetResponses public static void
  main(String args) try
  ...interesting code goes here...
  catch(Exception e)
  e.printStackTrace()

13
The GetResponses program, II

• Heres the interesting part of the code
• URL url new URL(args0)URLConnection c
  url.openConnection()System.out.println("Status
  line ")System.out.println('\t'
  c.getHeaderField(0))System.out.println("Response
  headers")String value ""int n 1 while
  (true) value c.getHeaderField(n) if
  (value null) break System.out.println('\t'
  c.getHeaderFieldKey(n)
  " " value)

14
Server response headers

• Server NCSA/1.3
• Name and version of the server
• Content-Type type/subtype
• Should be of a type and subtype specified by the
  clients Accept header
• Set-Cookie namevalue options
• Requests the client to store a cookie with the
  given name and value

15
What the server does, part III

• The third part of a server response is the entity
  body
• This is often an HTML page
• But it can also be a jpeg, a gif, plain text,
  etc.--anything the browser (or other client) is
  prepared to accept

16
The ltmeta http-equivgt tag

• The ltmeta http-equivstring contentstringgt tag
  may occur in the ltheadgt of an HTML document
• http-equiv and content typically have the same
  kinds of values as in the HTTP header
• This tag asks the client to pretend that the
  information actually occurred in the header
• The information is not really in the header
• This tag is available because you have little
  direct control over what is in the header (unless
  you write your own server)
• As usual, not all browsers handle this
  information the same way
• Example ltmeta http-equiv"Set-Cookie"
  content"valuenexpiresdate pathurl"gt

17
Summary

• HTTP is a fairly straightforward protocol with a
  lot of possible kinds of predefined header
  information
• More kinds can be added, so long as client and
  server agree
• A request from the client consists of three
  parts
• A header line
• A block of header information, ending with a
  blank line
• The (optional) entity body, containing data
• A response from the server consists of the same
  three parts
• HTTP headers are under the hood information,
  not normally displayed to the user
• As with most of the things covered in CIT597,
• We have covered only the fundamentals
• Much more detail can be found on the Web

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The End