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INTERNET-DRAFT                                              Ari Luotonen
Expires: August, 1996                Netscape Communications Corporation
                                                             John Franks
                                                 Northwestern University
<draft-ietf-http-range-retrieval-00.txt>               February 22, 1996


                 Byte Range Retrieval Extension to HTTP


STATUS OF THIS MEMO

   This document supersedes <draft-luotonen-http-url-byterage-02.txt>.

   A subgroup of the HTTP working group has developed this after several
   rounds of discussion. Some but not all parts of this proposal are
   currently implemented in commercial Web servers and browsers.

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).


TABLE OF CONTENTS

   1.   Overview ................................................. 2

   2.   Accept-Ranges HTTP response header ....................... 3

   3.   Byte range HTTP request .................................. 3
   3.1. The Range HTTP request header ............................ 3
   3.2. Conditional Range retrievals.............................. 5

   4.   Byte range HTTP response ................................. 7
   4.1. 206 Partial Content status code .......................... 7
   4.2. The Content-Range HTTP response header ................... 8



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   4.3. Multiple ranges as multipart MIME messages ............... 9
   4.4. Caching issues .......................................... 10

   5.   Future considerations ................................... 10
   5.1. Extending Accept-Ranges, Range and Content-Range headers  10
   5.2. Other possible ranges ................................... 11

   6.   References .............................................. 12

   7.   Authors' Addresses ...................................... 12
        Contributors ............................................ 12


1. OVERVIEW

   It is possible in Web clients to interrupt the connection before the
   data transfer has finished.  As a result the client may have partial
   documents or images loaded into its memory.  It would be useful to be
   able to request the server to return the missing portion of the
   document only, instead of retransferring the entire file, if the page
   is re-entered later.

   There are also a number of Web applications that would benefit from
   being able to request the server to give a byte range of a document.
   As an example an Adobe PDF viewer would need to be able to access
   individual pages by byte range; the table that defines those ranges
   is located at the end of the PDF file (this is the case in the new
   to-be-released PDF format).

   Setting this standard will promote interoperability between clients,
   servers and intermediate proxy servers, make (partial) caching
   effective, and save bandwidth.

   This specification defines only the byte ranges.  It shows other
   types of ranges as an example of how this specification could be
   extended, as proof of its generality.  Those examples should not be
   viewed as their definition.

   This specification is simple enough to be adopted quickly by the
   server authors/vendors, and be quickly and easily exploited on the
   client side.  The proposed solution will be backward compatible with
   existing proxy servers, and once this specification becomes official
   it will actually be possible to support this in a smart way in proxy
   servers.

   This specification can be applied to document types for which byte
   ranges make sense; there are types for which they don't, and this
   specification is not trying to enforce semantics for byte ranges for



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   them.  In practice most of the data in the Web is represented as a
   byte stream, and can be addressed with a byte range to retrieve a
   desired portion of it.  This is especially useful when there is a
   partial copy of the document, the transfer of which was interrupted
   by the user, but later resumed, in which case only the missing
   portion needs to be transferred.

   Byte range requests are typically generated by software, not written
   by humans.


2. ACCEPT-RANGES HTTP RESPONSE HEADER

   The server needs to let the client know that it can support byte
   ranges.  This is done through the Accept-Ranges HTTP header when a
   server is returning a document that supports byte ranges:

        Accept-Ranges: bytes

   The server will send this header only for documents for which it will
   be able to satisfy the byte range request, e.g. for PDF documents, or
   images, which can be partially reloaded if the user interrupts the
   page load, and image gets only partially cached.

   Because of the architecture of the byte range request and response,
   the client is not limited to attempting to use byte ranges only when
   this header is present.  The Request-Range header is simply ignored
   by a server that does not support it, and it will send the entire
   document as a response.


3. BYTE RANGE HTTP REQUEST

   Byte range request is made like any other HTTP request, with the
   addition of the Range: HTTP Request header.


3.1. The Range HTTP Request Header

   The client requests a byte range via the Range: HTTP header:

        Range: bytes=0-500,5000-


The Range: header is defined extensibly so that it can take a generic
parameter specifying the type of range.  The parameter name for byte
ranges is "bytes".  The syntax of this parameter is described below.




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   The name of the byte range parameter is bytes. It is passed to the
   server in the Range: HTTP request header, followed by an equal sign
   and the byte range specification.  (In an earlier version of this
   draft, it was passed to the server appended to the end of the path
   part of the URL, separated by a semicolon).


Note About CGI Applications

      As defined by the CGI/1.1 specification, the value of the Range:
      header will be passed to CGI scripts in the HTTP_RANGE environment
      variable.  The CGI applications can choose to support it if they
      so desire, and if it is possible.  If the CGI applications do not
      support it, or if the content they return changes from call to
      call, they simply ignore the presence of that header, and return
      the entire document.


   Each range consists of one or two non-negative integers, separated by
   a hyphen.  The first integer must always be less than or equal to the
   second one. One of these integers may be missing, but not both at the
   same time.  The hyphen is always there, so it is possible to tell
   which number is missing.

   If the first number is missing, it means to return the n last bytes
   of the document, where n is the second number. If n is equal to, or
   larger than, the size of the document minus one, then the entire file
   is returned.

   If the second number is missing, it means the end of document.  That
   is, all the bytes starting from byte n until the end of the document,
   where n is the first number.

   The first byte in a document is byte number 0.

   If the second number is larger than the size of the document minus
   one, it is taken to mean the size of the document minus one (that is,
   the end of the document).

   The range is inclusive; as an example, the range 500-1000 includes
   bytes from 500 to 1000, including 500 and 1000.

   There may be multiple ranges, separated by a comma. The order of the
   ranges is the preferred order in which the ranges should be returned.

   In the case that the second integer is smaller than the first one,
   that particular range is tagged as invalid, and ignored.  If it was
   the only requested byte range, the entire document is returned.



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   Otherwise the remaining valid ranges will be returned.

   The byte ranges refer to ranges in data as they are transferred over
   the network (and retrieved by the client). E.g. if in an imaginary
   system the server stores all lines terminated by CR LF, but turns
   them into a single LF before sending the data, then byte ranges refer
   to ranges inside this modified data (the one with single LF line
   separators). That is, the ranges refer to the data that the client
   would see.

   The byte ranges apply to the "raw" data, that is, the data encoded by
   Content-encoding; but not to the "armored" data, that is, the data
   encoded by content-transfer-encoding.


Examples of the Range: header with the bytes parameter

   The first 500 bytes:

        Range: bytes=0-499

   The second 500 bytes:

        Range: bytes=500-999

   All bytes except for the first 500 until the end of document:

        Range: bytes=500-

   The last 500 bytes of the document:

        Range: bytes=-500

   Two separate ranges:

        Range: bytes=50-99,200-249

   The first 100 bytes, 1000 bytes starting from the byte number 500,
   and the remainder of the document starting from byte number 4000
   (byte numbering starts from zero):

        Range: bytes=0-99,500-1499,4000-

   The first 100 bytes, 1000 bytes starting from the byte number 500,
   and the last 200 bytes of the document:

        Range: bytes=0-99,500-1499,-200




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3.2 Conditional Range Retrievals

   There are three cases of Range retrievals:

           A) Unconditional
           B) Insertion
           C) Tentative

A) Unconditional Range Retrieval

   An unconditional range retrieval always returns the selected range
   (if it exists).

   Servers that do not support Range: return entire resource.

B) Insertion Range Retrieval

   Insertion type range retrieval returns the selected range if the
   request-validator is valid, otherwise returns entire resource.

   Servers that do not support Range: return entire resource if the
   validator is valid, else return nothing + "306 Modified".

   Case (B) is for filling in a hole in a cached resource, perhaps after
   an interrupted retrieval, or perhaps after a previous Unconditional
   or Tentative Range: request.

   Note that if the server does not support Range:, it requires an extra
   round-trip to update a cached resource with a hole in it.

C) Tentative Range Retrieval

   A tentative range retrieval returns the selected range if the
   request-validator is NOT valid, otherwise returns nothing + "304 Not
   Modified"

   Servers that do not support Range: return entire resource if the
   validator is not valid, else return nothing + "304 Not Modified".

   Case (C) is a way of asking only for a specified range, but in a way
   that reloads it only if the client's cache is stale.  For example,
   you may have a large GIF image in your cache, and you may want to
   know if its GIF header has changed (so that you can do an early
   rendering of the enclosing HTML file), but you do not want to
   retrieve the whole image right away.

Examples




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   The three cases are implemented in HTTP/1.1 using some combination of
   these three headers
           Range:
           If-invalid:
           If-valid:
   or
           Range:
           If-Modified-Since:
           Unless-Modified-Since:

   See (*) below for more about using *-Modified-Since.

   Case A (Unconditional) is done using

           GET /image.gif HTTP/1.0
           Range: bytes=0-128

   Case B (Insertion) is done using

           GET /image.gif HTTP/1.0
           Range: bytes=0-128
           If-valid: "xxxyxxyx"
   or
           GET /image.gif HTTP/1.0
           Range: bytes=0-128
           Unless-modified-since: Wed, 15 Nov 1995 06:25:15 GMT

   Case C (Tentative) is done using

           GET /image.gif HTTP/1.0
           Range: bytes=0-128
           If-invalid: "xxxyxxyx"
   or
           GET /image.gif HTTP/1.0
           Range: bytes=0-128
           If-modified-since: Wed, 15 Nov 1995 06:25:15 GMT


   (*) We can get rid of "Unless-modified-since:" if we adopt the rule
   that the "If-valid:" header either takes a quoted string (in which
   case it is an opaque validator) or a string that does not include
   quotes (in which case it must be an HTTP-date value).  This saves a
   few header bytes.


4. BYTE RANGE HTTP RESPONSE

4.1. 206 Partial Content Status Code



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   The byte range response uses the 206 Partial Content HTTP response
   status.  Servers and CGI applications not supporting byte ranges will
   simply ignore the Range: header in the request, and return the entire
   document in a 200 response.

   Existing proxy servers only cache 200 Ok responses.  This way
   intermediate proxy servers will not mistakenly cache a partial
   document as if it was the entire document.

   If the request includes multiple ranges, the response is sent back as
   a multipart MIME message, with content-type multipart/x-byteranges.
   A server may, but is not required to, send also a single byte range
   as a multipart message.

   If there are overlapping ranges the behavior for each range doesn't
   change. That is, a range will not be truncated, merged, or left out,
   just because there is an overlap.

   If a request that includes a Range: header also includes an Unless-
   Modified-Since header and the resource was modified since the
   specified time, or also includes an If-Valid header and the validator
   does not match,

      if the server supports Range:, it will send a normal 200 OK
      response, and transfer the entire resource instead.

      if the server does not support Range:, it will send 306 Modified
      response, and will not transfer any part of the resource.

      if the server supports neither Range: nor the validation header
      (Unless-Modified-Since or If-Valid) then it will send a normal 200
      OK response, and transfer the entire resource.

   If a request that includes a Range: header also includes an If-
   Modified-Since header and the resource was not modified since the
   specified time, or also includes an If-Invalid header and the
   validator matches, the server will send a 304 Not Modified response,
   and will not transfer any part of the resource.


4.2 The Content-Range HTTP Response Header

   The Content-Range HTTP response header is sent back to provide
   verification and information about the range and total size of the
   document.  This header can be used by the client to determine which
   one of the requested ranges is in question.  Syntax:

        Content-Range: bytes X-Y/Z



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   where:

      X      is the number of the first byte returned (the first byte is
             byte number zero).

      Y      is the number of the last byte returned (in case of the end of
             the document this is one smaller than the size of the document
             in bytes).

      Z      is the total size of the document in bytes.


Examples of the Content-Range: HTTP Response Header

   The first 500 bytes of a 1234 byte document:

        Content-Range: bytes 0-499/1234

   The second 500 bytes of the same document:

        Content-Range: bytes 500-999/1234

   All bytes until the end of document, except for the first 500 bytes:

        Content-Range: bytes 500-1233/1234

   The last 500 bytes of the same document:

        Content-Range: bytes 734-1233/1234


Example of a response:

   HTTP/1.0 206 Partial content
   Server: Netscape-Communications/2.0
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-range: 21010-47021/47022
   Content-length: 26012
   Content-type: image/gif



4.3. Multiple Ranges as Multipart MIME Messages

   Multipart MIME is defined in [RFC-1521].  With byteranges, the
   multipart MIME message uses content-type multipart/x-byteranges, with
   a boundary parameter.



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   Example:

       Content-type: multipart/x-byteranges; boundary=THIS_STRING_SEPARATES

       --THIS_STRING_SEPARATES
       Content-type: application/x-pdf
       Content-range: bytes 500-999/8000

       ...the first range...
       --THIS_STRING_SEPARATES
       Content-type: application/x-pdf
       Content-range: bytes 7000-7999/8000

       ...the second range...
       --THIS_STRING_SEPARATES--


4.4. Caching Issues

   The server must give Last-modified headers for each range request
   whenever possible, and the client side must take care of having all
   the fragments in sync. Conditional GET (the GET request with the If-
   modified-since header) works as expected with byte ranges.  That is,
   the requested range is returned if the document has been modified
   since the given date.  Otherwise, a 304 Not Modified response is
   sent.

   Ranges can be cached, and if the Last-modified header matches they
   can be combined.  If a received Last-modified date at any time
   differs from the ones in the cache, all the cached ranges will be
   discarded.

   The client side should monitor the Last-modified header value
   returned by the server, and make sure that all of its individual
   fragments are in sync. If there are older ones they should be
   immediately discarded and re-retrieved.


5. FUTURE CONSIDERATIONS

5.1. Extending Accept-Ranges, Range and Content-Range headers

   If at some point there will be additional parameters for the Range:
   header, they should be separated by the semicolon character.

   Example:

        Range: param1=bar; param2=xyzzy



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   This specification does not define semantics for cases with multiple
   Range: parameters. Future specifications should define semantics for
   these. Until then, Range: headers with parameters that cannot be
   understood should be ignored.


5.2. Other Possible Ranges

   There are other kinds of ranges that can be addressed in a similar
   fashion; this document does not define them, but both the Range: HTTP
   request header and the Content-Range: HTTP header are defined so that
   it is possible to extend them.

   As an example, there might be a "lines" parameter, with the same kind
   of range specification, and the Content-Range: header would then
   specify the numbers in lines. Example:

        GET /dir/foo HTTP/1.0
        Range: lines=20-30

   The response from a 123 line document would be:

        HTTP/1.0 206 Partial Content
        Content-Range: lines 20-30/123
        Last-Modified: ...
        Content-Length: 773
        Content-Type: text/plain

   This could be useful for such things as structured text files like
   address lists or digests of mail and news, but isn't meaningful to
   such document types as GIF or PDF.

   Other examples might be document format specific ranges, such as
   chapters:

        GET /dir/foo HTTP/1.0
        Range: chapters=6-9

        206 Partial Content
        Content-Range: chapters 6-9/12
        Last-Modified: ...
        Content-Length: 36023
        Content-Type: application/x-book-type








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6. References

   [RFC-1521] N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail
              Extensions), Part One: Mechanisms for Specifying and
              Describing the Format of Internet Message Bodies",
              RFC 1521, Bellcore, Innosoft, September 1993

   [HTTP]     T. Berners-Lee, R. Fielding, H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0",
              draft-ietf-http-v10-spec-04.html, October 14, 1995.

   [CGI]      R. McCool et al, "Common Gateway Interface -- CGI/1.1",
              http://hoohoo.ncsa.uiuc.edu/cgi/, NCSA, 1994.


7. Authors' Addresses:

   Ari Luotonen                                       <ari@netscape.com>
   Netscape Communications Corporation
   501 E. Middlefield Road
   Mountain View, CA 94043
   USA

   John Franks                                       <john@math.nwu.edu>
   Department of Mathematics
   Northwestern University
   Evanston, IL 60208-2730

   The subgroup of the HTTP working group that contributed in producing
   this draft (in alphabetical order; apologies for any omissions):

   Daniel W. Connolly connolly@beach.w3.org
   Roy T. Fielding    fielding@avron.ics.uci.edu
   John Franks        john@math.nwu.edu
   Jim Gettys         jg@w3.org
   Martin Hamilton    martin@mrrl.lut.ac.uk
   Koen Holtman       koen@win.tue.nl
   Shel Kaplan        sjk@amazon.com
   Paul Leach         paulle@microsoft.com
   Alex Lopez-Ortiz   alopez-o@barrow.uwaterloo.ca
   Ari Luotonen       luotonen@netscape.com
   Larry Masinter     masinter@parc.xerox.com
   Jeff Mogul         mogul@pa.dec.com
   Lou Montulli       montulli@mozilla.com
   David W. Morris    dwm@shell.portal.com
   Luigi Rizzo        luigi@labinfo.iet.unipi.it
   Bill Weihl         weihl@pa.dec.com
   Steve Zilles       szilles@mv.us.adobe.com



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Luotonen, Franks                                               [Page 13]


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