Hypertext Transfer Protocol (HTTP/1.1): Range RequestsAdobe Systems Incorporated345 Park AveSan JoseCA95110USAfielding@gbiv.comhttp://roy.gbiv.com/World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-AntipolisAM06902Franceylafon@w3.orghttp://www.raubacapeu.net/people/yves/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
distributed, collaborative, hypertext information systems. This document
defines range requests and the rules for constructing and combining
responses to those requests.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
Hypertext Transfer Protocol (HTTP) clients often encounter interrupted data
transfers as a result of canceled requests or dropped connections. When a
client 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. Likewise, devices with limited local
storage might benefit from being able to request only a subset of a larger
representation, such as a single page of a very large document, or the
dimensions of an embedded image.
This document defines HTTP/1.1 range requests, partial responses, and the
multipart/byteranges media type, obsoleting those parts previously defined
in . Range requests are an OPTIONAL feature
of HTTP, designed so that recipients not implementing this feature (or not
supporting it for the target resource) 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 caches that might not implement the feature.
Although the 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 .
Conformance criteria and considerations regarding error handling
are defined in Section 2.5 of .
This specification uses the Augmented Backus-Naur Form (ABNF) notation
of with the list rule extension defined in
Section 1.2 of . describes rules imported from
other documents. shows the collected ABNF
with the list rule expanded.
A representation can be partitioned into subranges according to various
structural units, depending on the structure inherent in the
representation's media type. This "range unit" is used
in the Accept-Ranges ()
response header field to advertise support for range requests, the
Range () request header field
to delineate the parts of a representation that are requested, and the
Content-Range ()
payload header field to describe which part of a representation is being
transferred.
Since representation data is transferred in payloads as a sequence of
octets, a byte range is a meaningful substructure for any representation
transferable over HTTP (Section 3 of ). We define the "bytes" range
unit for expressing subranges of the data's octet sequence.
A byte range operation MAY specify a single range of bytes, or a set
of ranges within a single representation.
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.
Examples of byte-ranges-specifier values:
The first 500 bytes (byte offsets 0-499, inclusive):
The second 500 bytes (byte offsets 500-999, inclusive):
A byte-range-spec is invalid if the
last-byte-pos value is present and less than the
first-byte-pos.
A client can limit the number of bytes requested without knowing the size
of the selected representation.
If the last-byte-pos value is absent, or if the value is
greater than or equal to the current length of the representation data, the
byte range is interpreted as the remainder of the representation (i.e., the
server replaces the value of last-byte-pos with a value that
is one less than the current length of the selected representation).
A client can request the last N bytes of the selected representation using
a suffix-byte-range-spec.
If the selected representation is shorter than the specified
suffix-length, the entire representation is used.
For example (assuming a representation of length 10000):
The final 500 bytes (byte offsets 9500-9999, inclusive):
Or:
The first and last bytes only (bytes 0 and 9999):
Other valid (but not canonical) specifications of the second 500
bytes (byte offsets 500-999, inclusive):
If a valid byte-range-set includes at least one
byte-range-spec with a first-byte-pos that is
less than the current length of the representation, 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.
In the byte range syntax, first-byte-pos,
last-byte-pos, and suffix-length are
expressed as decimal number of octets. Since there is no predefined limit
to the length of a payload, recipients ought to anticipate potentially
large decimal numerals and prevent parsing errors due to integer conversion
overflows.
Range units are intended to be extensible. New range units ought to be
registered with IANA, as defined in .
The "Accept-Ranges" header field allows a server to indicate that it
supports range requests for the target resource.
Origin servers that support byte-range requests MAY send
but are not required to do so. Clients MAY generate range
requests without having received this header field for the resource
involved. Range units are defined in .
Servers that do not support any kind of range request for the target
resource resource MAY send
to advise the client not to attempt a range request.
The "Range" header field on a GET request modifies the method semantics to
request transfer of only one or more subranges of the selected
representation data, rather than the entire selected representation data.
A server MAY ignore the Range header field. However, origin servers and
intermediate caches ought to support byte ranges when possible, since Range
supports efficient recovery from partially failed transfers and partial
retrieval of large representations. A server MUST ignore a Range header
field received with a request method other than GET.
An origin server MUST ignore a Range header field that contains a range
unit it does not understand. A proxy MAY either discard a Range header
field that contains a range unit it does not understand or pass it to the
next inbound server when forwarding the request.
A server that supports range requests ought to ignore or reject a
Range header field that consists of more than two
overlapping ranges, or a set of many small ranges that are not listed
in ascending order, since both are indications of either a broken client or
a deliberate denial of service attack ().
A client SHOULD NOT request multiple ranges that are inherently less
efficient to process and transfer than a single range that encompasses the
same data.
A client that is requesting multiple ranges SHOULD list those ranges in
ascending order (the order in which they would typically be received in a
complete representation) unless there is a specific need to request a later
part earlier. For example, a user agent processing a large representation
with an internal catalog of parts might need to request later parts first,
particularly if the representation consists of pages stored in reverse
order and the user agent wishes to transfer one page at a time.
The Range header field is evaluated after evaluating the preconditions of
and only if the result of their evaluation is
leading toward a 200 (OK) response. In other words, Range
is ignored when a conditional GET would result in a
304 (Not Modified) response.
The If-Range header field () can be used as
a precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range header
field for the target resource, and the specified range(s) are valid and
satisfiable (as defined in ), the
server SHOULD send a 206 (Partial Content) response with a
payload containing one or more partial representations that correspond to
the satisfiable ranges requested, as defined in
.
If all of the preconditions are true, the server supports the Range header
field for the target resource, and the specified range(s) are invalid or
unsatisfiable, the server SHOULD send a
416 (Range Not Satisfiable) response.
If a client has a partial copy of a representation and wishes
to have an up-to-date copy of the entire representation, it could use the
Range header field with a conditional GET (using
either or both of If-Unmodified-Since and
If-Match.) However, if the condition fails because the
representation has been modified, the client would then have to make a
second request to obtain the entire current representation.
The "If-Range" header field allows a client to "short-circuit" the second
request. Informally, its meaning is: if the representation is unchanged,
send me the part(s) that I am requesting in Range; otherwise, send me the
entire representation.
Clients MUST NOT use an entity-tag marked as weak in an If-Range
field value and MUST NOT use a Last-Modified date in an
If-Range field value unless it has no entity-tag for the representation and
the Last-Modified date it does have for the representation is strong
in the sense defined by Section 2.2.2 of .
A server that evaluates a conditional range request that is applicable
to one of its representations MUST evaluate the condition as false if
the entity-tag used as a validator is marked as weak or, when an HTTP-date
is used as the validator, if the date value is not strong in the sense
defined by Section 2.2.2 of . (A server can distinguish between a
valid HTTP-date and any form of entity-tag by examining the first
two characters.)
A client MUST NOT generate an If-Range header field in a request that
does not contain a Range header field.
A server MUST ignore an If-Range header field received in a request that
does not contain a Range header field.
An origin server MUST ignore an If-Range header field received in a
request for a target resource that does not support Range requests.
If the validator given in the If-Range header field matches the current
validator for the selected representation of the target resource, then
the server SHOULD process the Range header field as requested.
If the validator does not match, then the server MUST ignore the
Range header field.
The 206 (Partial Content) status code indicates that the
server is successfully fulfilling a range request for the target resource
by transferring one or more parts of the selected representation that
correspond to the satisfiable ranges found in the requests's
Range header field ().
If a single part is being transferred, the server generating the 206
response MUST generate a Content-Range header field,
describing what range of the selected representation is enclosed, and a
payload consisting of the range. For example:
If multiple parts are being transferred, the server generating the 206
response MUST generate a "multipart/byteranges" payload, as defined
in , and a
Content-Type header field containing the
multipart/byteranges media type and its required boundary parameter.
To avoid confusion with single part responses, a server MUST NOT generate
a Content-Range header field in the HTTP header block of a
multiple part response (this field will be sent in each part instead).
Within the header area of each body part in the multipart payload, the
server MUST generate a Content-Range header field
corresponding to the range being enclosed in that body part.
If the selected representation would have had a Content-Type
header field in a 200 (OK) response, the server SHOULD
generate that same Content-Type field in the header area of
each body part. For example:
When multiple ranges are requested, a server MAY coalesce any of the
ranges that overlap or that are separated by a gap that is smaller than the
overhead of sending multiple parts, regardless of the order in which the
corresponding byte-range-spec appeared in the received Range
header field. Since the typical overhead between parts of a
multipart/byteranges payload is around 80 bytes, depending on the selected
representation's media type and the chosen boundary parameter length, it
can be less efficient to transfer many small disjoint parts than it is to
transfer the entire selected representation.
A server MUST NOT generate a multipart response to a request for a single
range, since a client that does not request multiple parts might not
support multipart responses. However, a server MAY generate a
multipart/byteranges payload with only a single body part if multiple
ranges were requested and only one range was found to be satisfiable or
only one range remained after coalescing.
A client that cannot process a multipart/byteranges response MUST NOT ask
for multiple ranges in a single request.
When a multipart response payload is generated, the server SHOULD send
the parts in the same order that the corresponding byte-range-spec appeared
in the received Range header field, excluding those ranges
that were deemed unsatisfiable or that were coalesced into other ranges.
A client that receives a multipart response MUST inspect the
Content-Range header field present in each body part in
order to determine which range is contained in that body part; a client
cannot rely on receiving the same ranges that it requested, nor the same
order that it requested.
When a 206 response is generated, the server MUST generate the following
header fields, in addition to those required above, if the field would
have been sent in a 200 (OK) response to the same request:
Date, Cache-Control, ETag,
Expires, Content-Location, and
Vary.
If a 206 is generated in response to a request with an If-Range
header field, the sender SHOULD NOT generate other representation header
fields beyond those required above, because the client is understood to
already have a prior response containing those header fields.
Otherwise, the sender MUST generate all of the representation header
fields that would have been sent in a 200 (OK) response
to the same request.
A 206 response is cacheable unless otherwise indicated by
explicit cache controls (see Section 4.1.2 of ).
The "Content-Range" header field is sent in a single part
206 (Partial Content) response to indicate the partial range
of the selected representation enclosed as the message payload, sent in
each part of a multipart 206 response to indicate the range enclosed within
each body part, and sent in 416 (Range Not Satisfiable)
responses to provide information about the selected representation.
If a 206 (Partial Content) response contains a
Content-Range header field with a range unit
() that the recipient does not understand, the
recipient MUST NOT attempt to recombine it with a stored representation.
A proxy that receives such a message SHOULD forward it downstream.
For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the complete
length is unknown or difficult to determine. An asterisk character ("*") in
place of the complete-length indicates that the representation length was
unknown when the header field was generated.
The following example illustrates when the complete length of the selected
representation is known by the sender to be 1234 bytes:
and this second example illustrates when the complete length is unknown:
A Content-Range field value is invalid if it contains a
byte-range-resp that has a last-byte-pos
value less than its first-byte-pos value, or a
complete-length value less than or equal to its
last-byte-pos value. The recipient of an invalid
Content-Range MUST NOT attempt to recombine the received
content with a stored representation.
A server generating a 416 (Range Not Satisfiable) response
to a byte range request SHOULD send a Content-Range header field with an
unsatisfied-range value, as in the following example:
The complete-length in a 416 response indicates the current length of the
selected representation.
The "Content-Range" header field has no meaning for status codes that do
not explicitly describe its semantic. For this specification, only the
206 (Partial Content) and
416 (Range Not Satisfiable) status codes describe a meaning
for Content-Range.
The following are examples of Content-Range values in which the
selected representation 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:
A response might transfer only a subrange of a representation if the
connection closed prematurely or if the request used one or more Range
specifications. After several such transfers, a client might have
received several ranges of the same representation. These ranges can only
be safely combined if they all have in common the same strong validator,
where "strong validator" is defined to be either an entity-tag that is
not marked as weak (Section 2.3 of ) or, if no entity-tag is provided, a
Last-Modified value that is strong in the sense defined by
Section 2.2.2 of .
A client that has received multiple partial responses to GET requests on a
target resource MAY combine those responses into a larger continuous
range if they share the same strong validator.
If the most recent response is an incomplete 200 (OK)
response, then the header fields of that response are used for any
combined response and replace those of the matching stored responses.
If the most recent response is a 206 (Partial Content)
response and at least one of the matching stored responses is a
200 (OK), then the combined response header fields consist
of the most recent 200 response's header fields. If all of the matching
stored responses are 206 responses, then the stored response with the most
recent header fields is used as the source of header fields for the
combined response, except that the client MUST use other header fields
provided in the new response, aside from Content-Range, to
replace all instances of the corresponding header fields in the stored
response.
The combined response message body consists of the union of partial
content ranges in the new response and each of the selected responses.
If the union consists of the entire range of the representation, then the
client MUST record the combined response as if it were a complete
200 (OK) response, including a Content-Length
header field that reflects the complete length.
Otherwise, the client MUST record the set of continuous ranges as one of
the following:
an incomplete 200 (OK) response if the combined response is
a prefix of the representation,
a single 206 (Partial Content) response containing a
multipart/byteranges body, or
multiple 206 (Partial Content) responses, each with one
continuous range that is indicated by a Content-Range header
field.
The 416 (Range Not Satisfiable) status code indicates that
none of the ranges in the request's Range header field
() overlap the current extent of the selected
resource or that the set of ranges requested has been rejected due to
invalid ranges or an excessive request of small or overlapping ranges.
For byte ranges, failing to overlap the current extent means that the
first-byte-pos of all of the byte-range-spec
values were greater than the current length of the selected representation.
When this status code is generated in response to a byte range request, the
sender SHOULD generate a Content-Range header field
specifying the current length of the selected representation
().
Note: Because servers are free to ignore Range, many
implementations will simply respond with 200 (OK) if the
requested ranges are invalid or not satisfiable. That is partly because
most clients are prepared to receive a 200 (OK) to
complete the task (albeit less efficiently) and partly because clients
might not stop making an invalid partial request until they have received
a complete representation. Thus, clients cannot depend on receiving a
416 (Range Not Satisfiable) response even when it is most
appropriate.
The HTTP Range Unit Registry defines the name space for the range
unit names and refers to their corresponding specifications.
The registry is maintained at
.
Registration of an HTTP Range Unit MUST include the following fields:
NameDescriptionPointer to specification text
Values to be added to this name space require IETF Review
(see , Section 4.1).
The initial HTTP Range Unit Registry shall contain the registrations
below:
Range Unit NameDescriptionReferencebytesa range of octetsnonereserved as keyword, indicating no ranges are supported
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
The HTTP Status Code Registry located at
shall be updated with the registrations below:
ValueDescriptionReference206Partial Content416Range Not Satisfiable
The Message Header Field Registry located at shall 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".
This section is meant to inform developers, information providers, and
users of known security concerns specific to the HTTP/1.1 range
request mechanisms. More general security considerations are addressed
in HTTP messaging and semantics .
Unconstrained multiple range requests are susceptible to denial of service
attacks because the effort required to request many overlapping ranges of
the same data is tiny compared to the time, memory, and bandwidth consumed
by attempting to serve the requested data in many parts.
Servers ought to ignore, coalesce, or reject egregious range requests, such
as requests for more than two overlapping ranges or for many small ranges
in a single set, particularly when the ranges are requested out of order
for no apparent reason. Multipart range requests are not designed to
support random access.
See Section 9 of .
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporatedfielding@gbiv.comAkamaimnot@mnot.netgreenbytes 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 InternetWorkingdcrocker@bbiw.netTHUS plc.paul.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 ProceduresOraclened+ietf@mrochek.comjohn+ietf@jck.comAT&T Laboratoriestony+mtsuffix@maillennium.att.comGuidelines for Writing an IANA Considerations Section in RFCsIBMnarten@us.ibm.comGoogleHarald@Alvestrand.no
When a 206 (Partial Content) response message includes the
content of multiple ranges, they are transmitted as body parts in a
multipart message body (, Section 5.1)
with the media type of "multipart/byteranges". The following definition is
to be registered with IANA .
The multipart/byteranges media type includes one or more body 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 ).
HTTP components supporting multiple ranges in a single request.
nonenonenone
See Authors Section.
COMMON
none
IESG
Implementation Notes:
Additional CRLFs might precede the first boundary string in the body.Although permits the boundary string to be
quoted, some existing implementations handle a quoted boundary
string incorrectly.A number of clients and servers were coded to an early draft
of the byteranges specification that used a media type of
multipart/x-byteranges,
which is almost (but not quite) compatible with this type.
Despite the name, the "multipart/byteranges" media type is not limited to
byte ranges. The following example uses an "exampleunit" range unit:
A weak validator cannot be used in a 206 response.
()
The Content-Range header field only has meaning when the status code
explicitly defines its use.
()
Servers are given more leeway in how they respond to a range request,
in order to mitigate abuse by malicious (or just greedy) clients.
multipart/byteranges can consist of a single part.
()
This specification introduces a Range Unit Registry.
()
The following core rules are included by
reference, as defined in Appendix B.1 of :
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), and
VCHAR (any visible US-ASCII character).
Note that all rules derived from token are to
be compared case-insensitively, like range-unit and
acceptable-ranges.
The rules below are defined in :
The rules below are defined in other parts:
Changes up to the first Working Group Last Call draft are summarized
in .
Closed issues:
:
"ABNF list expansion code problem"
:
"ABNF requirements for recipients"
:
"reserve 'none' as byte range unit"
:
"note introduction of new IANA registries as normative changes"
:
"range units vs leading zeroes vs size"
Conformance criteria and considerations regarding error handling are
now defined in Part 1.
Closed issues:
:
"Security consideration: range flooding"
:
"Allowing heuristic caching for new status codes"
:
"Add limitations to Range to reduce its use as a denial-of-service tool"