HTTP messages Entities and Encoding

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HTTP messagesEntities and Encoding

• Herng-Yow Chen

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Outline

• The format and behavior of HTTP message entities
  as HTTP containers
• How HTTP describes the size of entity bodies, and
  what HTTP requires in the way of sizing
• The entity headers used to describe the format,
  alphabet, and language of content, so clients can
  process it properly

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• Reversible content encoding transforms data
  format to take up less space or be more secure
• Transfer encoding modifies how HTTP ships data to
  enhance the communication of some kinds of data
• Chunked encoding chops data into multiple pieces
  to deliver content of unknown length safely

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• The assortment of tags, labels, times, and
  checksums help clients get the latest version of
  requested content
• Ranges are useful for continuing aborted
  downloads where they left off
• Delta encoding extensions allow client to request
  just those parts of a web page that actually have
  changed since a previously viewed revision

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• Checksums of entity bodies are used to detect
  changes in entity content as it passes through
  proxies

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Message is made up of header and body
HTTP/1.0 200 OK Server Netscape_Enterprise/3.6 Da
te Sun, 17 Sep 2000 000105 GMT Content_type
text/plain Content-length 18 Hi!Im a message!
Entity headers
Entity
Entity body
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HTTP 1.1 defines 10 entity headers

• Content-Type
• Content-Length
• Content-Language
• Content-Encoding
• Content-Location
• Content-Range

• Content-MD5
• Last-Modified
• Expires
• Allow
• ETag
• Cache-Control

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Entity Bodies
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Why content-length is important?

• Detecting Truncation
• Incorrect Content-Length problems?
• When connection is persistent, where one entity
  body ends and the next message begins.
• Chunked encoding is an alternate, sending the
  data in a series of chunks, each with a specified
  chunk size.
• When content-encoding is applied
• Content-length refers to the encoded body, not
  the length of the original, unencoded body.

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Entity Digest

• Content-MD5
• Is used to check message integrity
• Also can be used as a key into a hash table to
  quickly locate documents and reduce duplicate
  storage of content.

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Media type and Charset

• Content-type refers to original entity body type
  before encoding.
• Support optional parameters to further specify
  the content type.
• Character Encodings for Text Media
• Content-Type text/html charsetiso-8859-4

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Common media types
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Multipart Media Types

• MIME multipart email messages contain multiple
  messages stuck together and sent as a single,
  complex message.
• Each component is self-contained, with its own
  headers describing its contents the different
  components are concatenated together and
  delimited by a string.
• HTTP also supports multipart bodies however,
  only used in two cases fill-in form submission
  and range responses carrying pieces of a document.

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Multipart Form Submissions

• ltform actionhttp//xxx/cgi
  enctype"multipart/form-data
  methodPOSTgt ltPgt Your Name? ltINPUT
  typetext namesubmit-namegtltbrgt Your File
  to send? ltINPUT typefile namefilesgt
  ltbrgt ltINPUT typesubmit valuesendgt
  ltINPUT typeresetgtltformgt

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If the user enters John and selects the text
file hello.txt

• Content-Type multipart/form-data
  boundaryAaBo3x
• --AaBo3x
• Content-Disposition form-data
  namesubmit-name
• John
• --AaBo3x
• Content-Disposition form-data namefiles
  filenamehello.txt
• Content-Type text/plain
• contents of hello.txt
• --AaBo3x

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If selects the text file hello.txt and the
second image file image.gif

• Content-Type multipart/form-data
  boundaryAaBo3x
• --AaBo3x
• Content-Disposition form-data
  namesubmit-name
• John
• --AaBo3x
• Content-Disposition form-data namefiles
• Content-type multipart/mixed boundaryBbC04y
• --BbC04y
• Content-Disposition file filenamehello.txt
• Content-type text/plain
• contents of hello.txt
• --BbC04y
• Content-Disposition file filenameimage.gif
• Content-Type image/gif
• Content-Transfer-Encoding binary
• contents of image.gif
• --BbC04y
• --AaBo3x

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Multipart Range Response

• HTTP/1.0 206 Partial Content
• Server Microsoft-IIS/5.0
• Content-Location http//xxx/hello.txt
• Content-Type martipart/x-byteranges
  boundary--abcdefghikz--
• ----abcdefghikz
• Content-Type text/plain
• Content-Range bytes 0-174/1441
• . Part I content ---
• --abcdefghikz--
• Content-Type text/plain
• Content-Range bytes 1344-1441/1441
• . Part II content ---
• --abcdefghikz--

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Content-Encoding

• HTTP applications sometimes want to encode
  content before sending it, to help lesson the
  time it takes to transmit the data.
• Content-Type is the type of the original format,
  before encoding
• Content-Length is the length of the encoded length

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Content Encoding
Content-encoded content Content-Type
text/html Content-Length 5746 content-encoding
gzip
Original content Content-Type text/html Content-L
ength 17571
Original content Content-Type text/html Content-L
ength 17571
01110001 00110010
Gzip content decoder
Gzip content encoder
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Content-encoding tokens
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Accept-Encoding Headers
Request message
GET /logo.gif HTTP/1.1 Accept-encoding gzip
client
server
HTTP/1.1 200 OK Content-type image/gif Content-en
coding gzip
gzip
gunzip
Response message
00101101
00101101
The server compresses the image with gzip to
transport a smaller file over the thin Network
connection between itself and the client.This
saves network bandwidth And reduces the amount of
time that the client waits for the
transfer.Though,the Client will have to spend
time decompressing the image once the image is
served.
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Client can indicate preferred encodings by
attaching Q values

• Accept-Encoding compress, gzip
• Accept-Encoding
• Accept-Encoding
• Accept-Encoding compressq0.5, gzipq1.0
• Accept-Encoding gzipq1.0, identityq0.5
  q0

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Transfer Encoding

• Content-Encodings are to deal with the entity
  content to be encoded for less-space or security
  reason, tightly associated with the content
  format.
• In comparison, transfer encodings are applied for
  architectural reasons and are independent of the
  content format.

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Content encoding vs. transfer encoding
Content-encoded response
Normal header block
Normal entity (just encoded)
A content-encoded message just encodes the
entity Section of the message. With
Transfer-encoded Messages the encoding is a
function of the entire Message, changing the
structure of the message itself
Transfer-encoded response
Basic header
Encoded blocks
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Transfer-Encoding Headers

• TE
• Used in the request header to tell the server
  what extension transfer encoding are okay to use.
• Transfer-Encoding
• Used in the response header to tell the receiver
  (client) what encoding has been perform

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Example

• GET /1.html HTTP/1.1
• Host www.csie.ncnu.edu.tw
• User-Agent Mozilla/4.61
• TE trailers, chunked
• HTTP/1.1 200 ok
• Transfer-Encoding chunked
• Server Apache 3.0

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Chunked Encoding
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Chunked Encoding (continued)

• Chunking and Persistent connection
• Trailers in chunked messages
• Combining Content and Transfer Encoding

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Combining Content and Transfer Encodings
Content-type text/heml
Content encoding
9BF2578EA4 2670CD
9BF2578EA4 2670CD
Content-Type text/html content-encoding gzip
Transfer encoding (chunking)
426
426
Content-Type text/html content-encoding
gzip Transfer-encoding chunked
8EA
8EA
257
257
98B
98B
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Time-Varying Instance

• Web objects usually are not static.
• The same URL can, over time, point to different
  versions of an object.
• For example, the website of any media company
  like CNN, and BBC.

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Time-Varying Instances
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Validators and Freshness

• In the previous CNN example, the client got the
  initial resource V1 and can cache this copy, but
  for how long?
• Once the document has expired at the client, it
  must request a fresh copy from the server.
• Using a conditional request to tell the server
  which version it currently has, using a
  validator, and ask for a copy to be sent only if
  its current copy is no long valid.

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Cache-Control header directives
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Cache-Control header directives
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Conditional request types
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Range Request

• HTTP allows clients to actually request just part
  or a range of a document.
• Applications
• Request RoI (Region of Interest)
• Media Indexing and Access
• Streaming applications

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Range Requests
Request message
GET /bigfile.html HTTP/1.1
client
Response message
HTTP/1.1 200 OK Content-Type text/html Content-Le
ngth 65537 Accept-Ranges bytes
110100 111001 101001 110010
www.csie.ncnu.edu.tw
Range request message
GET /bigfile.html HTTP/1.1 Range bytes20224-
Range response message
HTTP/1.1 200 OK Content-Type text/html Range
bytes20224- Accept-Ranges bytes
The clients original request was Interrupted,but
a second request For the part of the message that
Was not received allows the Client to resume
form the point Of the interruption
www.csie.ncnu.edu.tw
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Delta Encoding

• An extension to the HTTP protocol that optimizes
  transfer by communicating changes instead of
  entire objects.
• RFC 3229 describe delta encoding.

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Delta Encoding
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Delta Encoding
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Delta-encoding headers

• Etag
• If-None-Match
• A-IM
• IM
• Delta-Base

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IANA registered types of instance manipulations
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For More Information

• http//www.ietf.org/rfc/rfc2616.txt
• Hypertext Transfer Protocol -- HTTP/1.1
• http//www.ietf.org/rfc/rfc3229.txt
• Delta encoding in HTTP
• http//www.ietf.org/rfc/rfc1521.txt
• MIME (Multipurpose Internet Mail Extensions) Part
  OneMechanisms for Specifying and Describing the
  Format of Internet Message Bodies
• http//www.ietf.org/rfc/rfc2045.txt
• Multipurpose Internet Mail Extensions(MIME) Part
  OneFormat of Internet Message Bodies
• http//www.ietf.org/rfc/rfc1864.txt
• The Content-MD5 Header Field
• http//www.ietf.org/rfc/rfc3230.txt
• Instance Digests in HTTP