source: draft-ietf-httpbis/00/draft-ietf-httpbis-p5-range-00.txt @ 187

Network Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2068, 2616                                          J. Gettys
(if approved)                                       One Laptop per Child
Intended status: Standards Track                                J. Mogul
Expires: June 22, 2008                                                HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                       December 20, 2007


         HTTP/1.1, part 5: Range Requests and Partial Responses
                     draft-ietf-httpbis-p5-range-00

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Copyright Notice

   Copyright (C) The IETF Trust (2007).



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Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 5 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 5 defines
   range-specific requests and the rules for constructing and combining
   responses to those requests.

Editorial Note (To be removed by RFC Editor)

   This version of the HTTP specification contains only minimal
   editorial changes from [RFC2616] (abstract, introductory paragraph,
   and authors' addresses).  All other changes are due to partitioning
   the original into seven mostly independent parts.  The intent is for
   readers of future drafts to able to use draft 00 as the basis for
   comparison when the WG makes later changes to the specification text.
   This draft will shortly be followed by draft 01 (containing the first
   round of changes that have already been agreed to on the mailing
   list).  There is no point in reviewing this draft other than to
   verify that the partitioning has been done correctly.  Roy T.
   Fielding, Yves Lafon, and Julian Reschke will be the editors after
   draft 00 is submitted.

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org).  The current issues list is
   at <http://www3.tools.ietf.org/wg/httpbis/trac/report/11> and related
   documents (including fancy diffs) can be found at
   <http://www3.tools.ietf.org/wg/httpbis/>.




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Range Units  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Status Code Definitions  . . . . . . . . . . . . . . . . . . .  4
     3.1.  206 Partial Content  . . . . . . . . . . . . . . . . . . .  4
     3.2.  416 Requested Range Not Satisfiable  . . . . . . . . . . .  5
   4.  Combining Byte Ranges  . . . . . . . . . . . . . . . . . . . .  5
   5.  Header Field Definitions . . . . . . . . . . . . . . . . . . .  6
     5.1.  Accept-Ranges  . . . . . . . . . . . . . . . . . . . . . .  6
     5.2.  Content-Range  . . . . . . . . . . . . . . . . . . . . . .  7
     5.3.  If-Range . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Range  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       5.4.1.  Byte Ranges  . . . . . . . . . . . . . . . . . . . . .  9
       5.4.2.  Range Retrieval Requests . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Internet Media Type multipart/byteranges  . . . . . . 13
   Appendix B.  Changes from RFC 2068 . . . . . . . . . . . . . . . . 14
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 18



























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1.  Introduction

   This document will define aspects of HTTP related to range requests,
   partial responses, and the multipart/byteranges media type.  Right
   now it only includes the extracted relevant sections of RFC 2616
   [RFC2616] without edit.


2.  Range Units

   HTTP/1.1 allows a client to request that only part (a range of) the
   response entity be included within the response.  HTTP/1.1 uses range
   units in the Range (Section 5.4) and Content-Range (Section 5.2)
   header fields.  An entity can be broken down into subranges according
   to various structural units.

      range-unit       = bytes-unit | other-range-unit
      bytes-unit       = "bytes"
      other-range-unit = token

   The only range unit defined by HTTP/1.1 is "bytes".  HTTP/1.1
   implementations MAY ignore ranges specified using other units.

   HTTP/1.1 has been designed to allow implementations of applications
   that do not depend on knowledge of ranges.


3.  Status Code Definitions

3.1.  206 Partial Content

   The server has fulfilled the partial GET request for the resource.
   The request MUST have included a Range header field (Section 5.4)
   indicating the desired range, and MAY have included an If-Range
   header field (Section 5.3) to make the request conditional.

   The response MUST include the following header fields:

   o  Either a Content-Range header field (Section 5.2) indicating the
      range included with this response, or a multipart/byteranges
      Content-Type including Content-Range fields for each part.  If a
      Content-Length header field is present in the response, its value
      MUST match the actual number of OCTETs transmitted in the message-
      body.

   o  Date





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   o  ETag and/or Content-Location, if the header would have been sent
      in a 200 response to the same request

   o  Expires, Cache-Control, and/or Vary, if the field-value might
      differ from that sent in any previous response for the same
      variant

   If the 206 response is the result of an If-Range request that used a
   strong cache validator (see Section 4 of [Part4]), the response
   SHOULD NOT include other entity-headers.  If the response is the
   result of an If-Range request that used a weak validator, the
   response MUST NOT include other entity-headers; this prevents
   inconsistencies between cached entity-bodies and updated headers.
   Otherwise, the response MUST include all of the entity-headers that
   would have been returned with a 200 (OK) response to the same
   request.

   A cache MUST NOT combine a 206 response with other previously cached
   content if the ETag or Last-Modified headers do not match exactly,
   see 4.

   A cache that does not support the Range and Content-Range headers
   MUST NOT cache 206 (Partial) responses.

3.2.  416 Requested Range Not Satisfiable

   A server SHOULD return a response with this status code if a request
   included a Range request-header field (Section 5.4), and none of the
   range-specifier values in this field overlap the current extent of
   the selected resource, and the request did not include an If-Range
   request-header field.  (For byte-ranges, this means that the first-
   byte-pos of all of the byte-range-spec values were greater than the
   current length of the selected resource.)

   When this status code is returned for a byte-range request, the
   response SHOULD include a Content-Range entity-header field
   specifying the current length of the selected resource (see
   Section 5.2).  This response MUST NOT use the multipart/byteranges
   content-type.


4.  Combining Byte Ranges

   A response might transfer only a subrange of the bytes of an entity-
   body, either because the request included one or more Range
   specifications, or because a connection was broken prematurely.
   After several such transfers, a cache might have received several
   ranges of the same entity-body.



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   If a cache has a stored non-empty set of subranges for an entity, and
   an incoming response transfers another subrange, the cache MAY
   combine the new subrange with the existing set if both the following
   conditions are met:

   o  Both the incoming response and the cache entry have a cache
      validator.

   o  The two cache validators match using the strong comparison
      function (see Section 4 of [Part4]).

   If either requirement is not met, the cache MUST use only the most
   recent partial response (based on the Date values transmitted with
   every response, and using the incoming response if these values are
   equal or missing), and MUST discard the other partial information.


5.  Header Field Definitions

   This section defines the syntax and semantics of all standard
   HTTP/1.1 header fields.  For entity-header fields, both sender and
   recipient refer to either the client or the server, depending on who
   sends and who receives the entity.

5.1.  Accept-Ranges

   The Accept-Ranges response-header field allows the server to indicate
   its acceptance of range requests for a resource:

          Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
          acceptable-ranges = 1#range-unit | "none"

   Origin servers that accept byte-range requests MAY send

          Accept-Ranges: bytes

   but are not required to do so.  Clients MAY generate byte-range
   requests without having received this header for the resource
   involved.  Range units are defined in Section 2.

   Servers that do not accept any kind of range request for a resource
   MAY send

          Accept-Ranges: none

   to advise the client not to attempt a range request.





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5.2.  Content-Range

   The Content-Range entity-header is sent with a partial entity-body to
   specify where in the full entity-body the partial body should be
   applied.  Range units are defined in Section 2.

       Content-Range = "Content-Range" ":" content-range-spec

       content-range-spec      = byte-content-range-spec
       byte-content-range-spec = bytes-unit SP
                                 byte-range-resp-spec "/"
                                 ( instance-length | "*" )

       byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                      | "*"
       instance-length           = 1*DIGIT

   The header SHOULD indicate the total length of the full entity-body,
   unless this length is unknown or difficult to determine.  The
   asterisk "*" character means that the instance-length is unknown at
   the time when the response was generated.

   Unlike byte-ranges-specifier values (see Section 5.4.1), a byte-
   range-resp-spec MUST only specify one range, and MUST contain
   absolute byte positions for both the first and last byte of the
   range.

   A byte-content-range-spec with a byte-range-resp-spec whose last-
   byte-pos value is less than its first-byte-pos value, or whose
   instance-length value is less than or equal to its last-byte-pos
   value, is invalid.  The recipient of an invalid byte-content-range-
   spec MUST ignore it and any content transferred along with it.

   A server sending a response with status code 416 (Requested range not
   satisfiable) SHOULD include a Content-Range field with a byte-range-
   resp-spec of "*".  The instance-length specifies the current length
   of the selected resource.  A response with status code 206 (Partial
   Content) MUST NOT include a Content-Range field with a byte-range-
   resp-spec of "*".

   Examples of byte-content-range-spec values, assuming that the entity
   contains a total of 1234 bytes:

   o  The first 500 bytes:

      bytes 0-499/1234





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   o  The second 500 bytes:

      bytes 500-999/1234

   o  All except for the first 500 bytes:

      bytes 500-1233/1234

   o  The last 500 bytes:

      bytes 734-1233/1234

   When an HTTP message includes the content of a single range (for
   example, a response to a request for a single range, or to a request
   for a set of ranges that overlap without any holes), this content is
   transmitted with a Content-Range header, and a Content-Length header
   showing the number of bytes actually transferred.  For example,

       HTTP/1.1 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-Range: bytes 21010-47021/47022
       Content-Length: 26012
       Content-Type: image/gif

   When an HTTP message includes the content of multiple ranges (for
   example, a response to a request for multiple non-overlapping
   ranges), these are transmitted as a multipart message.  The multipart
   media type used for this purpose is "multipart/byteranges" as defined
   in Appendix A.  See Appendix B for a compatibility issue.

   A response to a request for a single range MUST NOT be sent using the
   multipart/byteranges media type.  A response to a request for
   multiple ranges, whose result is a single range, MAY be sent as a
   multipart/byteranges media type with one part.  A client that cannot
   decode a multipart/byteranges message MUST NOT ask for multiple byte-
   ranges in a single request.

   When a client requests multiple byte-ranges in one request, the
   server SHOULD return them in the order that they appeared in the
   request.

   If the server ignores a byte-range-spec because it is syntactically
   invalid, the server SHOULD treat the request as if the invalid Range
   header field did not exist.  (Normally, this means return a 200
   response containing the full entity).

   If the server receives a request (other than one including an If-



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   Range request-header field) with an unsatisfiable Range request-
   header field (that is, all of whose byte-range-spec values have a
   first-byte-pos value greater than the current length of the selected
   resource), it SHOULD return a response code of 416 (Requested range
   not satisfiable) (Section 3.2).

      Note: clients cannot depend on servers to send a 416 (Requested
      range not satisfiable) response instead of a 200 (OK) response for
      an unsatisfiable Range request-header, since not all servers
      implement this request-header.

5.3.  If-Range

   If a client has a partial copy of an entity in its cache, and wishes
   to have an up-to-date copy of the entire entity in its cache, it
   could use the Range request-header with a conditional GET (using
   either or both of If-Unmodified-Since and If-Match.)  However, if the
   condition fails because the entity has been modified, the client
   would then have to make a second request to obtain the entire current
   entity-body.

   The If-Range header allows a client to "short-circuit" the second
   request.  Informally, its meaning is `if the entity is unchanged,
   send me the part(s) that I am missing; otherwise, send me the entire
   new entity'.

        If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

   If the client has no entity tag for an entity, but does have a Last-
   Modified date, it MAY use that date in an If-Range header.  (The
   server can distinguish between a valid HTTP-date and any form of
   entity-tag by examining no more than two characters.)  The If-Range
   header SHOULD only be used together with a Range header, and MUST be
   ignored if the request does not include a Range header, or if the
   server does not support the sub-range operation.

   If the entity tag given in the If-Range header matches the current
   entity tag for the entity, then the server SHOULD provide the
   specified sub-range of the entity using a 206 (Partial content)
   response.  If the entity tag does not match, then the server SHOULD
   return the entire entity using a 200 (OK) response.

5.4.  Range

5.4.1.  Byte Ranges

   Since all HTTP entities are represented in HTTP messages as sequences
   of bytes, the concept of a byte range is meaningful for any HTTP



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   entity.  (However, not all clients and servers need to support byte-
   range operations.)

   Byte range specifications in HTTP apply to the sequence of bytes in
   the entity-body (not necessarily the same as the message-body).

   A byte range operation MAY specify a single range of bytes, or a set
   of ranges within a single entity.

       ranges-specifier = byte-ranges-specifier
       byte-ranges-specifier = bytes-unit "=" byte-range-set
       byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
       byte-range-spec = first-byte-pos "-" [last-byte-pos]
       first-byte-pos  = 1*DIGIT
       last-byte-pos   = 1*DIGIT

   The first-byte-pos value in a byte-range-spec gives the byte-offset
   of the first byte in a range.  The last-byte-pos value gives the
   byte-offset of the last byte in the range; that is, the byte
   positions specified are inclusive.  Byte offsets start at zero.

   If the last-byte-pos value is present, it MUST be greater than or
   equal to the first-byte-pos in that byte-range-spec, or the byte-
   range-spec is syntactically invalid.  The recipient of a byte-range-
   set that includes one or more syntactically invalid byte-range-spec
   values MUST ignore the header field that includes that byte-range-
   set.

   If the last-byte-pos value is absent, or if the value is greater than
   or equal to the current length of the entity-body, last-byte-pos is
   taken to be equal to one less than the current length of the entity-
   body in bytes.

   By its choice of last-byte-pos, a client can limit the number of
   bytes retrieved without knowing the size of the entity.

       suffix-byte-range-spec = "-" suffix-length
       suffix-length = 1*DIGIT

   A suffix-byte-range-spec is used to specify the suffix of the entity-
   body, of a length given by the suffix-length value.  (That is, this
   form specifies the last N bytes of an entity-body.)  If the entity is
   shorter than the specified suffix-length, the entire entity-body is
   used.

   If a syntactically valid byte-range-set includes at least one byte-
   range-spec whose first-byte-pos is less than the current length of
   the entity-body, or at least one suffix-byte-range-spec with a non-



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   zero suffix-length, then the byte-range-set is satisfiable.
   Otherwise, the byte-range-set is unsatisfiable.  If the byte-range-
   set is unsatisfiable, the server SHOULD return a response with a
   status of 416 (Requested range not satisfiable).  Otherwise, the
   server SHOULD return a response with a status of 206 (Partial
   Content) containing the satisfiable ranges of the entity-body.

   Examples of byte-ranges-specifier values (assuming an entity-body of
   length 10000):

   o  The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499

   o  The second 500 bytes (byte offsets 500-999, inclusive): bytes=500-
      999

   o  The final 500 bytes (byte offsets 9500-9999, inclusive): bytes=-
      500

   o  Or bytes=9500-

   o  The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1

   o  Several legal but not canonical specifications of the second 500
      bytes (byte offsets 500-999, inclusive):
      bytes=500-600,601-999
      bytes=500-700,601-999

5.4.2.  Range Retrieval Requests

   HTTP retrieval requests using conditional or unconditional GET
   methods MAY request one or more sub-ranges of the entity, instead of
   the entire entity, using the Range request header, which applies to
   the entity returned as the result of the request:

      Range = "Range" ":" ranges-specifier

   A server MAY ignore the Range header.  However, HTTP/1.1 origin
   servers and intermediate caches ought to support byte ranges when
   possible, since Range supports efficient recovery from partially
   failed transfers, and supports efficient partial retrieval of large
   entities.

   If the server supports the Range header and the specified range or
   ranges are appropriate for the entity:

   o  The presence of a Range header in an unconditional GET modifies
      what is returned if the GET is otherwise successful.  In other
      words, the response carries a status code of 206 (Partial Content)



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      instead of 200 (OK).

   o  The presence of a Range header in a conditional GET (a request
      using one or both of If-Modified-Since and If-None-Match, or one
      or both of If-Unmodified-Since and If-Match) modifies what is
      returned if the GET is otherwise successful and the condition is
      true.  It does not affect the 304 (Not Modified) response returned
      if the conditional is false.

   In some cases, it might be more appropriate to use the If-Range
   header (see Section 5.3) in addition to the Range header.

   If a proxy that supports ranges receives a Range request, forwards
   the request to an inbound server, and receives an entire entity in
   reply, it SHOULD only return the requested range to its client.  It
   SHOULD store the entire received response in its cache if that is
   consistent with its cache allocation policies.


6.  IANA Considerations

   TBD.


7.  Security Considerations

   No additional security considerations have been identified beyond
   those applicable to HTTP in general [Part1].


8.  Acknowledgments

   Most of the specification of ranges is based on work originally done
   by Ari Luotonen and John Franks, with additional input from Steve
   Zilles.

   Based on an XML translation of RFC 2616 by Julian Reschke.


9.  References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 1: URIs, Connections, and Message Parsing",
              draft-ietf-httpbis-p1-messaging-00 (work in progress),
              December 2007.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,



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              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 4: Conditional Requests",
              draft-ietf-httpbis-p4-conditional-00 (work in progress),
              December 2007.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.


Appendix A.  Internet Media Type multipart/byteranges

   When an HTTP 206 (Partial Content) response message includes the
   content of multiple ranges (a response to a request for multiple non-
   overlapping ranges), these are transmitted as a multipart message-
   body.  The media type for this purpose is called "multipart/
   byteranges".

   The multipart/byteranges media type includes two or more parts, each
   with its own Content-Type and Content-Range fields.  The required
   boundary parameter specifies the boundary string used to separate
   each body-part.

   Media Type name:  multipart

   Media subtype name:  byteranges

   Required parameters:  boundary

   Optional parameters:  none

   Encoding considerations:  only "7bit", "8bit", or "binary" are
      permitted

   Security considerations:  none












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   For example:

      HTTP/1.1 206 Partial Content
      Date: Wed, 15 Nov 1995 06:25:24 GMT
      Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
      Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

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

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

      ...the second range
      --THIS_STRING_SEPARATES--

   Notes:

   1.  Additional CRLFs may precede the first boundary string in the
       entity.

   2.  Although RFC 2046 [RFC2046] permits the boundary string to be
       quoted, some existing implementations handle a quoted boundary
       string incorrectly.

   3.  A number of browsers and servers were coded to an early draft of
       the byteranges specification to use a media type of multipart/
       x-byteranges, which is almost, but not quite compatible with the
       version documented in HTTP/1.1.


Appendix B.  Changes from RFC 2068

   There are situations where a server (especially a proxy) does not
   know the full length of a response but is capable of serving a
   byterange request.  We therefore need a mechanism to allow byteranges
   with a content-range not indicating the full length of the message.
   (Section 5.2)

   Range request responses would become very verbose if all meta-data
   were always returned; by allowing the server to only send needed
   headers in a 206 response, this problem can be avoided.

   Fix problem with unsatisfiable range requests; there are two cases:
   syntactic problems, and range doesn't exist in the document.  The 416



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   status code was needed to resolve this ambiguity needed to indicate
   an error for a byte range request that falls outside of the actual
   contents of a document.  (Section 3.2, 5.2)


Index

   2
      206 Partial Content (status code)  4

   4
      416 Requested Range Not Satisfiable (status code)  5

   A
      Accept-Ranges header  6

   C
      Content-Range header  7

   G
      Grammar
         Accept-Ranges  6
         acceptable-ranges  6
         byte-content-range-spec  7
         byte-range-resp-spec  7
         byte-range-set  10
         byte-range-spec  10
         byte-ranges-specifier  10
         bytes-unit  4
         Content-Range  7
         content-range-spec  7
         first-byte-pos  10
         If-Range  9
         instance-length  7
         last-byte-pos  10
         other-range-unit  4
         Range  11
         range-unit  4
         ranges-specifier  10
         suffix-byte-range-spec  10
         suffix-length  10

   H
      Headers
         Accept-Ranges  6
         Content-Range  7
         If-Range  9
         Range  9



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   I
      If-Range header  9

   M
      Media Type
         multipart/byteranges  13
         multipart/x-byteranges  14
      multipart/byteranges Media Type  13
      multipart/x-byteranges Media Type  14

   R
      Range header  9

   S
      Status Codes
         206 Partial Content  4
         416 Requested Range Not Satisfiable  5


Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660
   USA

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305
   Email: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Jim Gettys
   One Laptop per Child
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   Email: jg@laptop.org
   URI:   http://www.laptop.org/










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   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   Email: JeffMogul@acm.org


   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   Email: henrikn@microsoft.com


   Larry Masinter
   Adobe Systems, Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   Email: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   Email: paulle@microsoft.com


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   Email: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/




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Full Copyright Statement

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