The HyperText Transfer Protocol

1
The HyperText Transfer Protocol
2
History

• HTTP has been in use since 1990 (HTTP/0.9)
• HTTP/1.0 was defined in RFC 1945 (May 1996) and
  included metainformation
• HTTP/1.1 was defined in RFC 2068 (January 1997)
  and included caching, and persistent connections
• "Specifies an Internet standards track protocol
  for the Internet Community"
• HTTP is "a generic, stateless, object-oriented
  protocol"
• Good site http//www.freesoft.org/CIE/RFC/2068/in
  dex.htm

3
Overall Operation

• Client sends a request in the form of a URI
  (Uniform Resource Identifier) and protocol
  version, followed by optional modifiers
• Server responds with a status line,
  metainformation, and entity
• Default port is 80 (but others can be used)
• In HTTP/1.0, a single connection is used for each
  request
• HTTP/1.1 a connection may be used for multiple
  requests

4
HTTP Version

• HTTP version uses ltmajorgt.ltminorgt
• HTTP-Version"HTTP" "/" 1DIGIT "." 1DIGIT
• HTTP/2.4 is lower than HTTP/2.13
• To follow the specification, you MUST send a
  version number!

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URLs and URIs(Using Backus-Naur Form or BNF)

• Identify several things
• http_URL "http" "//" host "" port
  abs_path
• host ltA legal internet domaingt
• port DIGIT
• abs_path "/" rel_path
• rel_path path "" params "?" query

6
Date and Time Format

• Three formats
• Sun, 06 Nov 1994 084937 GMT RFC 822, updated
  by RFC 1123
• Sunday, 06-Nov-94 084937 GMT RFC 850,
  obsoleted by RFC 1036
• Sun Nov 6 084937 1994 ANSI C's asctime()
  format
• It, too, has a long BNF!

7
Content/Transfer Codings

• Content indicates an encoding transformation
  that has been, or can be applied to an entity
• gzip (GNU zip)
• compress (UNIX)
• deflate (zlib format)
• Transfer used to ensure "safe transport" through
  the network
• If server doesn't understand, returns 501
  (unimplemented)
• Rarely used

8
Media Types

• Used in the content-type and accept header fields
  for type negotiations
• media-type type "/" subtype ("" parameter)
• type token
• subtype token
• Example text/html or text/plain

9
Multipart Types

• Encapsulation of one or more entities within a
  single message body
• Must use CRLF to represent line breaks between
  body parts

10
Product Tokens

• Used to identify server and client by software
  name and version
• Examples
• User-Agent CERN-LineMode/2.15 libwww/2.17
• Server Apache/0.8.4

11
Language Tags

• Identifies the natural (spoken) language of the
  entity
• Examples en, en-US, en-cockney, x-pig-latin

12
HTTP Messages

• Messages consist of two types
• HTTP-message Request Response
• generic-message start-line
• message-header
• CRLF
• message-body

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Requests

• Requests an entity by a client from a server
• Request Request-line
• (general-header
• request-header
• entity-header)
• CRLF
• message-body

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Request-Line

• The actual request from the client
• Request-line Method SP Request-URI SP
  HTTP-version CRLF
• Method "OPTIONS" "GET" "HEAD" "POST"
  "PUT" "TRACE"
• Request-URI "" absolute-URI abs-path
• Examples
• GET /index.html HTTP/1.0
• GET http//kahuna.clayton.edu/index.html HTTP/1.1
• OPTIONS HTTP/1.0

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OPTIONS Example
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Request Header Fields

• Allow client to specify more information about
  itself
• request-header Accept Accept-Charset
• Accept-Encoding
• Accept-Language
• Authorization From Host
• If-Modified-Since If-Match
• If-None-Match If-Range
• If-Unmodified-Since
• Max-Forwards
• Proxy-Authorization Range
• Referer User-Agent

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Response

• After receiving a request, the server responds
• Response Status-Line
• (general-header
• response-header
• entity-header)
• CRLF
• message-body

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Status-Line and Reason-Phrase

• Status-line HTTP-Version SP Status-Code SP
  Reason-Phrase CRLF
• Status-Code "100" Continue "101"
  Switching Protocols
• "200" OK
• "201" Created "202" Accepted
• "203" Non-Authoritative Information
  "204" No Content
• "205" Reset Content "206" Partial
  Content
• "300" Multiple Choices "301" Moved
  Permanently
• "302" Moved Temporarily "303" See
  Other "304" Not Modified
• "305" Use Proxy
• "400" Bad Request "401"
  Unauthorized
• "402" Payment Required "403"
  Forbidden "404" Not Found
• "405" Method Not Allowed "406" Not
  Acceptable
• "407" Proxy Authentication Required
  "408" Request Time-out
• "409" Conflict "410" Gone "411"
  Length Required
• "412" Precondition Failed "413"
  Request Entity Too Large
• "414" Request-URI Too Large "415"
  Unsupported Media Type

Successful
Redirection
Client Error
Server Error
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Response Header Fields

• Allows server to pass additional information to
  client
• Response-header Age Location
• Proxy-Authenticate
• Public Retry-After Server
• Vary Warning
• WWW-Authenticate

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Entity Header

• Identify optional information about the entity
• entity-header Allow Content-Base
• Content-Encoding
• Content-Language
• Content-Length
• Content-Location
• Content-MD5
• Content-Range
• Content-Type
• Expires
• Last-Modified

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Persistent Connections
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Request Methods

• OPTIONS requests options available for
  communications
• GET requests an entity
• HEAD requests only the header info for an
  entity
• POST used for sending info from the client as a
  separate entity
• PUT puts info in the URI instead of a separate
  entity
• DELETE delete an entity

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Persistent Connections

• Open and close fewer TCP connections
• Saves time
• Saves CPU
• Allows pipelining of requests (without waiting
  for each response)
• Network congestion reduced (caused by TCP opens)

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Authorization

• Websites are stored as 'realms'
• Upon receipt of unauthorized request, server
  responds with
• WWW-Authenticate Basic realm"My Stuff"
• Client sends userID and passwd
• Authorization Basic bobhi

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Content Negotiation

• Can be either Server or Client side
• Accept the types of media that will be accepted
  on client side. Can specify quality as well
• Accept audio/ q0.2 audio/basic
• If no accept, assumed to accept all media
• Accept-Charset specifies the character set
• Accept-Charset iso-8859-1
• Accept-Encoding allowable compressions
• Accept-Encoding compress, gzip

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Content Negotiation

• Accept Language restricts the set of languages
• Accept-Language da, en-gb q0.8
• Accept Ranges how an entity is requested. Can be
  either bytes or none
• Accept Ranges bytes
• Allow The kind of methods allowed
• Allow GET, HEAD, PUT
• Authorization allows client to send
  authorization info
• Authorization ltcredentialsgt

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Content Negotiation

• Connection specifies the state of the connection

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Content Negotiation

• Content-Encoding When present, indicates codings
  have been applied to entity
• Content-Encoding gzip
• Content-Language the language of the entity
• Content-Language en
• Content-Length size of message body in bytes
• Content-Length ltsize of filegt
• Content-Range can be used for partial entities
• Skip this

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Content Negotiation

• Content-Type indicates the type of media
• Content-Type text/html charsetISO-8859-1
• Date Date of message (described previously)
• Expires when the response should be considered
  stale (same format as Date)
• From can contain an email address of person
  responsible for entity
• From bob_at_bob.com

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Content Negotiation

• Host specifies the host of the entity and port
  number (remember, there can be gateways)
• Host kahuna.clayton.edu8080
• If-Modified-Since will not return if entity is
  older than date
• If-Modified-Since Sun, 9 Feb 2003 90000 GMT
• If-Unmodified-Since hmmm
• Last-Modified the time which the server believes
  the file was last modified
• Last-Modified Sun, 9 Feb 2003 90000 GMT

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Content Negotiations

• Public specifies methods supported by the server
• Public GET, OPTIONS, HEAD
• Referer allows client to specify the address of
  the resource from which the request URI was
  obtained
• Retry-After server can specify this with a 503
  (Service Unavailable) response
• Retry-After Sun, 9 Feb 2010 120000 GMT
• Retry-After 30

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Content Negotiations

• Server specifies software and sub-software
• Server Apache/1.3.9 mod_perl/1.21
• Transfer-Encoding shows what, if any, type of
  transformation was applied to safely deliver it
• Transfer-Encoding chunked
• Upgrade allows client to specify additional
  protocols it supports
• Upgrade HTTP/2.0, IRC/6.9, SHTTP/1.3
• Server must send 101 (Switching Protocols)
• User-Agent specifies the client's browser
• WWW-Authenticate Basic realm"My Stuff"