HTTP/1.1, part 5: Range Requests and Partial ResponsesDay Software23 Corporate Plaza DR, Suite 280Newport BeachCA92660USA+1-949-706-5300+1-949-706-5305fielding@gbiv.comhttp://roy.gbiv.com/One Laptop per Child21 Oak Knoll RoadCarlisleMA01741USAjg@laptop.orghttp://www.laptop.org/Hewlett-Packard CompanyHP Labs, Large Scale Systems Group1501 Page Mill Road, MS 1177Palo AltoCA94304USAJeffMogul@acm.orgMicrosoft Corporation1 Microsoft WayRedmondWA98052USAhenrikn@microsoft.comAdobe Systems, Incorporated345 Park AveSan JoseCA95110USALMM@acm.orghttp://larry.masinter.net/Microsoft Corporation1 Microsoft WayRedmondWA98052paulle@microsoft.comWorld Wide Web ConsortiumMIT Computer Science and Artificial Intelligence LaboratoryThe Stata Center, Building 3232 Vassar StreetCambridgeMA02139USAtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-AntipolisAM06902Franceylafon@w3.orghttp://www.raubacapeu.net/people/yves/greenbytes GmbHHafenweg 16MuensterNW48155Germany+49 251 2807760+49 251 2807761julian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
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.
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
and related documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
HTTP clients often encounter interrupted data transfers as a result
of cancelled requests or dropped connections. When a cache has stored
a partial representation, it is desirable to request the remainder
of that representation in a subsequent request rather than transfer
the entire representation.
There are also a number of Web applications that benefit from being
able to request only a subset of a larger representation, such as a
single page of a very large document or only part of an image to be
rendered by a device with limited local storage.
This document defines HTTP/1.1 range requests,
partial responses, and the multipart/byteranges media type.
The protocol for range requests is an OPTIONAL feature of HTTP,
designed so resources or recipients that do not implement this feature
can respond as if it is a normal GET request without impacting
interoperability. Partial responses are indicated by a distinct status
code to not be mistaken for full responses by intermediate caches
that might not implement the feature.
Although the HTTP range request mechanism is designed to allow for
extensible range types, this specification only defines requests for
byte ranges.
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 .
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."
This specification uses the ABNF syntax defined in Section 1.2 of (which
extends the syntax defined in with a list rule).
shows the collected ABNF, with the list
rule expanded.
The following core rules are included by
reference, as defined in , Appendix B.1:
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space),
VCHAR (any visible USASCII character),
and WSP (whitespace).
The core rules below are defined in Section 1.2.2 of :
OWS =
]]>
The ABNF rules below are defined in other parts:
]]>
]]>
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 () and Content-Range ()
header fields. An entity can be broken down into subranges according
to various structural units.
HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges. The only range unit defined
by HTTP/1.1 is "bytes".
If a range unit is not understood in a request, a server MUST ignore
the whole Range header ().
If a range unit is not understood in a response, an intermediary
SHOULD pass the response to the client; a client MUST fail.
The server has fulfilled the partial GET request for the resource.
The request MUST have included a Range header field ()
indicating the desired range, and MAY have included an If-Range
header field () to make the request conditional.
The response MUST include the following header fields:
Either a Content-Range header field () 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.
Date
ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request
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, 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.
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 .
A cache that does not support the Range and Content-Range headers
MUST NOT cache 206 (Partial Content) responses. Furthermore,
if a response uses a range unit that is not understood by the cache,
then it MUST NOT be cached either.
A server SHOULD return a response with this status code if a request
included a Range request-header field (), and none of
the ranges-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 ).
This response MUST NOT use the multipart/byteranges content-type.
A response might transfer only a subrange of an entity-body, either
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.
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:
Both the incoming response and the cache entry have a cache
validator.The two cache validators match using the strong comparison
function (see Section 4 of ).
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.
This section defines the syntax and semantics of HTTP/1.1 header fields
related to range requests and partial responses.
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.
The response-header "Accept-Ranges" field allows the server to
indicate its acceptance of range requests for a resource:
Origin servers that accept byte-range requests MAY send
but are not required to do so. Clients MAY generate range
requests without having received this header for the resource
involved. Range units are defined in .
Servers that do not accept any kind of range request for a
resource MAY send
to advise the client not to attempt a range request.
The entity-header "Content-Range" 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 .
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 ), 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.
In the case of a byte range request:
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 as a decimal number. 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:
The first 500 bytes:
The second 500 bytes:
All except for the first 500 bytes:
The last 500 bytes:
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,
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 . See 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
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-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) ().
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.
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 request header "If-Range" 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 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.
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.)
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.
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 are decimal and 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.
A suffix-byte-range-spec is used to specify the suffix of the
entity-body, of a length given by the decimal 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-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):
The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500Or bytes=9500-The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1Several 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
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:
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:
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).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 ) 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.
The Message Header Registry located at should be updated
with the permanent registrations below (see ):
Header Field NameProtocolStatusReferenceAccept-RangeshttpstandardContent-RangehttpstandardIf-RangehttpstandardRangehttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
No additional security considerations have been identified beyond
those applicable to HTTP in general .
Most of the specification of ranges is based on work originally done
by Ari Luotonen and John Franks, with additional input from Steve
Zilles, Daniel W. Connolly, Roy T. Fielding, Jim Gettys, Martin Hamilton,
Koen Holtman, Shel Kaplan, Paul Leach, Alex Lopez-Ortiz, Larry Masinter,
Jeff Mogul, Lou Montulli, David W. Morris, Luigi Rizzo, and Bill Weihl.
HTTP/1.1, part 1: URIs, Connections, and Message ParsingDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 3: Message Payload and Content NegotiationDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 4: Conditional RequestsDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 6: CachingDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deMultipurpose Internet Mail Extensions (MIME) Part Two: Media TypesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorking675 Spruce Dr.SunnyvaleCA94086US+1.408.246.8253dcrocker@bbiw.netTHUS plc.1/2 Berkeley Square99 Berkely StreetGlasgowG3 7HRUKpaul.overell@thus.netHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgMedia Type Specifications and Registration ProceduresSun Microsystemsned.freed@mrochek.comklensin+ietf@jck.com
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 (, Section 5.1). The media type for this purpose is called
"multipart/byteranges". The following is to be registered with IANA .
The multipart/byteranges media type includes one 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.
multipart
byteranges
boundary
none
only "7bit", "8bit", or "binary" are permitted
none
none
This specification (see ).
nonenonenone
See Authors Section.
COMMON
none
IESG
For example:
Notes:
Additional CRLFs may precede the first boundary string in the
entity.Although permits the boundary string to be
quoted, some existing implementations handle a quoted boundary
string incorrectly.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.
Transfer-coding and message lengths all interact in ways that
required fixing exactly when chunked encoding is used (to allow for
transfer encoding that may not be self delimiting); it was important
to straighten out exactly how message lengths are computed.
(,
see also , and )
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.
()
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.
(Section
and )
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 , )
Clarify that it is not ok to use a weak cache validator in a 206 response.
()
Clarify that multipart/byteranges can consist of a single part.
()
If-Range = "If-Range:" OWS If-Range-v
If-Range-v = entity-tag / HTTP-date
OWS =
Range = "Range:" OWS Range-v
Range-v = byte-ranges-specifier / other-ranges-specifier
acceptable-ranges = ( *( "," OWS ) range-unit *( OWS "," [ OWS
range-unit ] ) ) / "none"
byte-content-range-spec = bytes-unit SP byte-range-resp-spec "/" (
instance-length / "*" )
byte-range-resp-spec = ( first-byte-pos "-" last-byte-pos ) / "*"
byte-range-set = ( *( "," OWS ) byte-range-spec ) / (
suffix-byte-range-spec *( OWS "," [ ( OWS byte-range-spec ) /
suffix-byte-range-spec ] ) )
byte-range-spec = first-byte-pos "-" [ last-byte-pos ]
byte-ranges-specifier = bytes-unit "=" byte-range-set
bytes-unit = "bytes"
content-range-spec = byte-content-range-spec /
other-content-range-spec
entity-tag =
first-byte-pos = 1*DIGIT
instance-length = 1*DIGIT
last-byte-pos = 1*DIGIT
other-content-range-spec = other-range-unit SP other-range-resp-spec
other-range-resp-spec = *CHAR
other-range-unit = token
other-ranges-specifier = 1*CHAR
range-unit = bytes-unit / other-range-unit
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
token =
]]>ABNF diagnostics:
Extracted relevant partitions from .
Closed issues:
:
"Cache validators in 206 responses"
()
:
"Normative and Informative references"
:
"Normative up-to-date references"
Closed issues:
:
"Updating to RFC4288"
Ongoing work on ABNF conversion ():
Add explicit references to BNF syntax and rules imported from other parts of the specification.
Ongoing work on IANA Message Header Registration ():
Reference RFC 3984, and update header registrations for headers defined
in this document.
Closed issues:
:
"multipart/byteranges minimum number of parts"
Ongoing work on ABNF conversion ():
Use "/" instead of "|" for alternatives.
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
Rewrite ABNFs to spell out whitespace rules, factor out
header value format definitions.
Closed issues:
:
"State base for *-byte-pos and suffix-length"
Ongoing work on Custom Ranges ():
Remove bias in favor of byte ranges; allow custom ranges in ABNF.
Final work on ABNF conversion ():
Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.