source: draft-ietf-httpbis/latest/p5-range.xml @ 98

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE rfc [
  <!ENTITY MAY "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MAY</bcp14>">
  <!ENTITY MUST "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST</bcp14>">
  <!ENTITY MUST-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>MUST NOT</bcp14>">
  <!ENTITY OPTIONAL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>OPTIONAL</bcp14>">
  <!ENTITY RECOMMENDED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>RECOMMENDED</bcp14>">
  <!ENTITY REQUIRED "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>REQUIRED</bcp14>">
  <!ENTITY SHALL "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL</bcp14>">
  <!ENTITY SHALL-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHALL NOT</bcp14>">
  <!ENTITY SHOULD "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD</bcp14>">
  <!ENTITY SHOULD-NOT "<bcp14 xmlns='http://purl.org/net/xml2rfc/ext'>SHOULD NOT</bcp14>">
  <!ENTITY ID-VERSION "latest">
  <!ENTITY ID-MONTH "December">
  <!ENTITY ID-YEAR "2007">
  <!ENTITY messaging                  "<xref target='Part1' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
  <!ENTITY weak-and-strong-validators "<xref target='Part4' x:rel='#weak.and.strong.validators' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
]>
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc compact="yes"?>
<?rfc subcompact="no" ?>
<?rfc linkmailto="no" ?>
<?rfc editing="no" ?>
<?rfc-ext allow-markup-in-artwork="yes" ?>
<?rfc-ext include-references-in-index="yes" ?>
<rfc obsoletes="2068, 2616" category="std"
     ipr="full3978" docName="draft-ietf-httpbis-p5-range-&ID-VERSION;"
     xmlns:x='http://purl.org/net/xml2rfc/ext' xmlns:ed="http://greenbytes.de/2002/rfcedit">
<front>

  <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>

  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    <organization abbrev="Day Software">Day Software</organization>
    <address>
      <postal>
        <street>23 Corporate Plaza DR, Suite 280</street>
        <city>Newport Beach</city>
        <region>CA</region>
        <code>92660</code>
        <country>USA</country>
      </postal>
      <phone>+1-949-706-5300</phone>
      <facsimile>+1-949-706-5305</facsimile>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>

  <author initials="J." surname="Gettys" fullname="Jim Gettys">
    <organization>One Laptop per Child</organization>
    <address>
      <postal>
        <street>21 Oak Knoll Road</street>
        <city>Carlisle</city>
        <region>MA</region>
        <code>01741</code>
        <country>USA</country>
      </postal>
      <email>jg@laptop.org</email>
      <uri>http://www.laptop.org/</uri>
    </address>
  </author>
  
  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    <organization abbrev="HP">Hewlett-Packard Company</organization>
    <address>
      <postal>
        <street>HP Labs, Large Scale Systems Group</street>
        <street>1501 Page Mill Road, MS 1177</street>
        <city>Palo Alto</city>
        <region>CA</region>
        <code>94304</code>
        <country>USA</country>
      </postal>
      <email>JeffMogul@acm.org</email>
    </address>
  </author>

  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
        <country>USA</country>
      </postal>
      <email>henrikn@microsoft.com</email>
    </address>
  </author>

  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
  </author>
  
  <author initials="P." surname="Leach" fullname="Paul J. Leach">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
      </postal>
      <email>paulle@microsoft.com</email>
    </address>
  </author>
   
  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
        <street>The Stata Center, Building 32</street>
        <street>32 Vassar Street</street>
        <city>Cambridge</city>
        <region>MA</region>
        <code>02139</code>
        <country>USA</country>
      </postal>
      <email>timbl@w3.org</email>
      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
  </author>

  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    <organization abbrev="W3C">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>W3C / ERCIM</street>
        <street>2004, rte des Lucioles</street>
        <city>Sophia-Antipolis</city>
        <region>AM</region>
        <code>06902</code>
        <country>France</country>
      </postal>
      <email>ylafon@w3.org</email>
      <uri>http://www.raubacapeu.net/people/yves/</uri>
    </address>
  </author>

  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <phone>+49 251 2807760</phone>    
      <facsimile>+49 251 2807761</facsimile>    
      <email>julian.reschke@greenbytes.de</email>       
      <uri>http://greenbytes.de/tech/webdav/</uri>      
    </address>
  </author>

  <date month="&ID-MONTH;" year="&ID-YEAR;"/>

<abstract>
<t>
   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.
</t>
</abstract>

<note title="Editorial Note (To be removed by RFC Editor)">
  <t>
    This version of the HTTP specification contains only minimal editorial
    changes from <xref target="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.
  </t>
  <t>
    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 <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/report/11"/>
    and related documents (including fancy diffs) can be found at
    <eref target="http://www3.tools.ietf.org/wg/httpbis/"/>.
  </t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>
   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
   <xref target="RFC2616">RFC 2616</xref> without edit.
</t>

<section title="Requirements" anchor="intro.requirements">
<t>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
   An implementation is not compliant if it fails to satisfy one or more
   of the &MUST; or &REQUIRED; level requirements for the protocols it
   implements. An implementation that satisfies all the &MUST; or &REQUIRED;
   level and all the &SHOULD; level requirements for its protocols is said
   to be "unconditionally compliant"; one that satisfies all the &MUST;
   level requirements but not all the &SHOULD; level requirements for its
   protocols is said to be "conditionally compliant."
</t>
</section>
</section>

<section title="Range Units" anchor="range.units">
<t>
   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 (<xref target="header.range"/>) and Content-Range (<xref target="header.content-range"/>)
   header fields. An entity can be broken down into subranges according
   to various structural units.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="range-unit"/><iref primary="true" item="Grammar" subitem="bytes-unit"/><iref primary="true" item="Grammar" subitem="other-range-unit"/>
   range-unit       = bytes-unit | other-range-unit
   bytes-unit       = "bytes"
   other-range-unit = token
</artwork></figure>
<t>
   The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
   implementations &MAY; ignore ranges specified using other units.
</t>
<t>
   HTTP/1.1 has been designed to allow implementations of applications
   that do not depend on knowledge of ranges.
</t>
</section>

<section title="Status Code Definitions">
<section title="206 Partial Content" anchor="status.206">
  <iref primary="true" item="206 Partial Content (status code)" x:for-anchor=""/>
  <iref primary="true" item="Status Codes" subitem="206 Partial Content" x:for-anchor=""/>
<t>
   The server has fulfilled the partial GET request for the resource.
   The request &MUST; have included a Range header field (<xref target="header.range"/>)
   indicating the desired range, and &MAY; have included an If-Range
   header field (<xref target="header.if-range"/>) to make the request conditional.
</t>
<t>
   The response &MUST; include the following header fields:
  <list style="symbols">
    <t>
        Either a Content-Range header field (<xref target="header.content-range"/>) 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.
    </t>
    <t>
        Date
    </t>
    <t>
        ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request
    </t>
    <t>
        Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant
    </t>
  </list>
</t>
<t>
   If the 206 response is the result of an If-Range request, the response
   &SHOULD-NOT; include other entity-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.
</t>
<t>
   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 <xref target="combining.byte.ranges" format="counter"/>.
</t>
<t>
   A cache that does not support the Range and Content-Range headers
   &MUST-NOT; cache 206 (Partial) responses.
</t>
</section>

<section title="416 Requested Range Not Satisfiable" anchor="status.416">
  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)" x:for-anchor=""/>
  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable" x:for-anchor=""/>
<t>
   A server &SHOULD; return a response with this status code if a request
   included a Range request-header field (<xref target="header.range"/>), 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.)
</t>
<t>
   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 <xref target="header.content-range"/>).
   This response &MUST-NOT; use the multipart/byteranges content-type.
</t>
</section>
</section>

<section title="Combining Byte Ranges" anchor="combining.byte.ranges">
<t>
   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.
</t>
<t>
   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:
  <list style="symbols">
    <t>Both the incoming response and the cache entry have a cache
        validator.</t>
    <t>The two cache validators match using the strong comparison
        function (see &weak-and-strong-validators;).</t>
  </list>
</t>
<t>
   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.
</t>
</section>

<section title="Header Field Definitions" anchor="header.fields">
<t>
   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.
</t>
<section title="Accept-Ranges" anchor="header.accept-ranges">
  <iref primary="true" item="Accept-Ranges header" x:for-anchor=""/>
  <iref primary="true" item="Headers" subitem="Accept-Ranges" x:for-anchor=""/>
<t>
      The Accept-Ranges response-header field allows the server to
      indicate its acceptance of range requests for a resource:
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Accept-Ranges"/><iref primary="true" item="Grammar" subitem="acceptable-ranges"/>
       Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
       acceptable-ranges = 1#range-unit | "none"
</artwork></figure>
<t>
      Origin servers that accept byte-range requests &MAY; send
</t>
<figure><artwork type="example">
       Accept-Ranges: bytes
</artwork></figure>
<t>
      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 <xref target="range.units"/>.
</t>
<t>
      Servers that do not accept any kind of range request for a
      resource &MAY; send
</t>
<figure><artwork type="example">
       Accept-Ranges: none
</artwork></figure>
<t>
      to advise the client not to attempt a range request.
</t>
</section>

<section title="Content-Range" anchor="header.content-range">
  <iref primary="true" item="Content-Range header" x:for-anchor=""/>
  <iref primary="true" item="Headers" subitem="Content-Range" x:for-anchor=""/>
<t>
   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 <xref target="range.units"/>.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Content-Range"/><iref primary="true" item="Grammar" subitem="content-range-spec"/><iref primary="true" item="Grammar" subitem="byte-content-range-spec"/><iref primary="true" item="Grammar" subitem="byte-range-resp-spec"/><iref primary="true" item="Grammar" subitem="instance-length"/>
    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
</artwork></figure>
<t>
   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.
</t>
<t>
   Unlike byte-ranges-specifier values (see <xref target="byte.ranges"/>), 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.
</t>
<t>
   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.
</t>
<t>
   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 "*".
</t>
<t>
   Examples of byte-content-range-spec values, assuming that the entity
   contains a total of 1234 bytes:
   <list style="symbols">
      <t>
        The first 500 bytes:
<figure><artwork type="text/plain">
   bytes 0-499/1234
</artwork></figure>
      </t>    
      <t>
        The second 500 bytes:
<figure><artwork type="text/plain">
   bytes 500-999/1234
</artwork></figure>
      </t>    
      <t>
        All except for the first 500 bytes:
<figure><artwork type="text/plain">
   bytes 500-1233/1234
</artwork></figure>
      </t>    
      <t>
        The last 500 bytes:
<figure><artwork type="text/plain">
   bytes 734-1233/1234
</artwork></figure>
      </t>    
   </list>
</t>
<t>
   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,
</t>
<figure><artwork type="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
</artwork></figure>
<t>
   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 <xref target="internet.media.type.multipart.byteranges"/>. See <xref target="changes.from.rfc.2068"/> for a compatibility issue.
</t>
<t>
   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.
</t>
<t>
   When a client requests multiple byte-ranges in one request, the
   server &SHOULD; return them in the order that they appeared in the
   request.
</t>
<t>
   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).
</t>
<t>
   If the server receives a request (other than one including an If-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) (<xref target="status.416"/>).
  <list><t>
      <x:h>Note:</x:h> 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.
  </t></list>
</t>
</section>

<section title="If-Range" anchor="header.if-range">
  <iref primary="true" item="If-Range header" x:for-anchor=""/>
  <iref primary="true" item="Headers" subitem="If-Range" x:for-anchor=""/>
<t>
   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.
</t>
<t>
   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'.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="If-Range"/>
     If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
</artwork></figure>
<t>
   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.
</t>
<t>
   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.
</t>
</section>

<section title="Range" anchor="header.range">
  <iref primary="true" item="Range header" x:for-anchor=""/>
  <iref primary="true" item="Headers" subitem="Range" x:for-anchor=""/>

<section title="Byte Ranges" anchor="byte.ranges">
<t>
   Since all HTTP entities are represented in HTTP messages as sequences
   of bytes, the concept of a byte range is meaningful for any HTTP
   entity. (However, not all clients and servers need to support byte-range
   operations.)
</t>
<t>
   Byte range specifications in HTTP apply to the sequence of bytes in
   the entity-body (not necessarily the same as the message-body).
</t>
<t>
   A byte range operation &MAY; specify a single range of bytes, or a set
   of ranges within a single entity.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="ranges-specifier"/><iref primary="true" item="Grammar" subitem="byte-ranges-specifier"/><iref primary="true" item="Grammar" subitem="byte-range-set"/><iref primary="true" item="Grammar" subitem="byte-range-spec"/><iref primary="true" item="Grammar" subitem="first-byte-pos"/><iref primary="true" item="Grammar" subitem="last-byte-pos"/>
    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
</artwork></figure>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   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.
</t>
<t>
   By its choice of last-byte-pos, a client can limit the number of
   bytes retrieved without knowing the size of the entity.
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="suffix-byte-range-spec"/><iref primary="true" item="Grammar" subitem="suffix-length"/>
    suffix-byte-range-spec = "-" suffix-length
    suffix-length = 1*DIGIT
</artwork></figure>
<t>
   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.
</t>
<t>
   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-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.
</t>
<t>
   Examples of byte-ranges-specifier values (assuming an entity-body of
   length 10000):
  <list style="symbols">
     <t>The first 500 bytes (byte offsets 0-499, inclusive):  bytes=0-499</t>

     <t>The second 500 bytes (byte offsets 500-999, inclusive):
        bytes=500-999</t>

     <t>The final 500 bytes (byte offsets 9500-9999, inclusive):
        bytes=-500</t>

     <t>Or bytes=9500-</t>

     <t>The first and last bytes only (bytes 0 and 9999):  bytes=0-0,-1</t>

     <t>Several legal but not canonical specifications of the second 500
        bytes (byte offsets 500-999, inclusive):
        <vspace/>
         bytes=500-600,601-999<vspace/>
         bytes=500-700,601-999</t>
  </list>
</t>
</section>

<section title="Range Retrieval Requests" anchor="range.retrieval.requests">
<t>
   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:
</t>
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Range"/>
   Range = "Range" ":" ranges-specifier
</artwork></figure>
<t>
   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.
</t>
<t>
   If the server supports the Range header and the specified range or
   ranges are appropriate for the entity:
  <list style="symbols">
     <t>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) instead of 200 (OK).</t>

     <t>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.</t>
  </list>
</t>
<t>
   In some cases, it might be more appropriate to use the If-Range
   header (see <xref target="header.if-range"/>) in addition to the Range header.
</t>
<t>
   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.
</t>
</section>
</section>
</section>

<section title="IANA Considerations" anchor="IANA.considerations">
<t>
   TBD.
</t>
</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
   No additional security considerations have been identified beyond
   those applicable to HTTP in general &messaging;.
</t>
</section>

<section title="Acknowledgments" anchor="ack">
<t>
   Most of the specification of ranges is based on work originally done
   by Ari Luotonen and John Franks, with additional input from Steve
   Zilles.
</t>
</section>
</middle>
<back>
<references>

<reference anchor="Part1">
   <front>
      <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
         <organization abbrev="Day Software">Day Software</organization>
         <address><email>fielding@gbiv.com</email></address>
      </author>
      <author initials="J." surname="Gettys" fullname="Jim Gettys">
         <organization>One Laptop per Child</organization>
         <address><email>jg@laptop.org</email></address>
      </author>
      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
         <organization abbrev="HP">Hewlett-Packard Company</organization>
         <address><email>JeffMogul@acm.org</email></address>
      </author>
      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
         <organization abbrev="Microsoft">Microsoft Corporation</organization>
         <address><email>henrikn@microsoft.com</email></address>
      </author>
      <author initials="L." surname="Masinter" fullname="Larry Masinter">
         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
         <address><email>LMM@acm.org</email></address>
      </author>
      <author initials="P." surname="Leach" fullname="Paul J. Leach">
         <organization abbrev="Microsoft">Microsoft Corporation</organization>
         <address><email>paulle@microsoft.com</email></address>
      </author>
      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
         <address><email>timbl@w3.org</email></address>
      </author>
      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
         <organization abbrev="W3C">World Wide Web Consortium</organization>
         <address><email>ylafon@w3.org</email></address>
      </author>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
         <organization abbrev="greenbytes">greenbytes GmbH</organization>
         <address><email>julian.reschke@greenbytes.de</email></address>
      </author>
      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
   </front>
   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;"/>
   <x:source href="p1-messaging.xml" basename="p1-messaging"/>
</reference>

<reference anchor="Part4">
   <front>
      <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
      <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
         <organization abbrev="Day Software">Day Software</organization>
         <address><email>fielding@gbiv.com</email></address>
      </author>
      <author initials="J." surname="Gettys" fullname="Jim Gettys">
         <organization>One Laptop per Child</organization>
         <address><email>jg@laptop.org</email></address>
      </author>
      <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
         <organization abbrev="HP">Hewlett-Packard Company</organization>
         <address><email>JeffMogul@acm.org</email></address>
      </author>
      <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
         <organization abbrev="Microsoft">Microsoft Corporation</organization>
         <address><email>henrikn@microsoft.com</email></address>
      </author>
      <author initials="L." surname="Masinter" fullname="Larry Masinter">
         <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
         <address><email>LMM@acm.org</email></address>
      </author>
      <author initials="P." surname="Leach" fullname="Paul J. Leach">
         <organization abbrev="Microsoft">Microsoft Corporation</organization>
         <address><email>paulle@microsoft.com</email></address>
      </author>
      <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
         <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
         <address><email>timbl@w3.org</email></address>
      </author>
      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
         <organization abbrev="W3C">World Wide Web Consortium</organization>
         <address><email>ylafon@w3.org</email></address>
      </author>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
         <organization abbrev="greenbytes">greenbytes GmbH</organization>
         <address><email>julian.reschke@greenbytes.de</email></address>
      </author>
      <date month="&ID-MONTH;" year="&ID-YEAR;"/>
   </front>
   <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;"/>
   <x:source href="p4-conditional.xml" basename="p4-conditional"/>
</reference>

<reference anchor="RFC2616">
   <front>
      <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
      <author initials="R." surname="Fielding" fullname="R. Fielding">
         <organization>University of California, Irvine</organization>
         <address><email>fielding@ics.uci.edu</email></address>
      </author>
      <author initials="J." surname="Gettys" fullname="J. Gettys">
         <organization>W3C</organization>
         <address><email>jg@w3.org</email></address>
      </author>
      <author initials="J." surname="Mogul" fullname="J. Mogul">
         <organization>Compaq Computer Corporation</organization>
         <address><email>mogul@wrl.dec.com</email></address>
      </author>
      <author initials="H." surname="Frystyk" fullname="H. Frystyk">
         <organization>MIT Laboratory for Computer Science</organization>
         <address><email>frystyk@w3.org</email></address>
      </author>
      <author initials="L." surname="Masinter" fullname="L. Masinter">
         <organization>Xerox Corporation</organization>
         <address><email>masinter@parc.xerox.com</email></address>
      </author>
      <author initials="P." surname="Leach" fullname="P. Leach">
         <organization>Microsoft Corporation</organization>
         <address><email>paulle@microsoft.com</email></address>
      </author>
      <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
         <organization>W3C</organization>
         <address><email>timbl@w3.org</email></address>
      </author>
      <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
         <organization abbrev="W3C">World Wide Web Consortium</organization>
         <address><email>ylafon@w3.org</email></address>
      </author>
      <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
         <organization abbrev="greenbytes">greenbytes GmbH</organization>
         <address><email>julian.reschke@greenbytes.de</email></address>
      </author>
      <date month="June" year="1999"/>
   </front>
   <seriesInfo name="RFC" value="2616"/>
</reference>

<reference anchor="RFC2046">
  <front>
    <title abbrev="Media Types">Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types</title>
    <author initials="N." surname="Freed" fullname="Ned Freed">
      <organization>Innosoft International, Inc.</organization>
      <address><email>ned@innosoft.com</email></address>
    </author>
    <author initials="N." surname="Borenstein" fullname="Nathaniel S. Borenstein">
      <organization>First Virtual Holdings</organization>
      <address><email>nsb@nsb.fv.com</email></address>
    </author>
    <date month="November" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="2046"/>
</reference>

<reference anchor="RFC2119">
  <front>
    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    <author initials="S." surname="Bradner" fullname="Scott Bradner">
      <organization>Harvard University</organization>
      <address><email>sob@harvard.edu</email></address>
    </author>
    <date month="March" year="1997"/>
  </front>
  <seriesInfo name="BCP" value="14"/>
  <seriesInfo name="RFC" value="2119"/>
</reference>

</references>

<section title="Internet Media Type multipart/byteranges" anchor="internet.media.type.multipart.byteranges">
<iref item="Media Type" subitem="multipart/byteranges" primary="true"/>
<iref item="multipart/byteranges Media Type" primary="true"/>
<t>
   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".
</t><t>
   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.
</t>
<t>
  <list style="hanging" x:indent="12em">
    <t hangText="Media Type name:">
      multipart
    </t>
    <t hangText="Media subtype name:">
      byteranges
    </t>
    <t hangText="Required parameters:">
      boundary
    </t>
    <t hangText="Optional parameters:">
      none
    </t>
    <t hangText="Encoding considerations:">
      only "7bit", "8bit", or "binary" are permitted
    </t>
    <t hangText="Security considerations:">
      none
    </t>
  </list>
</t>
<figure><preamble>
   For example:
</preamble><artwork type="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--
</artwork></figure>
<t>
      Notes:
  <list style="numbers">
      <t>Additional CRLFs may precede the first boundary string in the
         entity.</t>

      <t>Although <xref target="RFC2046"/> permits the boundary string to be
         quoted, some existing implementations handle a quoted boundary
         string incorrectly.</t>

      <t>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<iref item="multipart/x-byteranges Media Type"/><iref item="Media Type" subitem="multipart/x-byteranges"/>, which is almost, but not quite
         compatible with the version documented in HTTP/1.1.</t>
  </list>
</t>
</section>

<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
<t>
   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.
   (<xref target="header.content-range"/>)
</t>
<t>
   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.
</t>
<t>
   Fix problem with unsatisfiable range requests; there are two cases:
   syntactic problems, and range doesn't exist in the document. The 416
   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 <xref target="status.416" format="counter"/>, <xref target="header.content-range" format="counter"/>)
</t>
</section>

</back>
</rfc>