|HTTPbis Working Group||R. Fielding, Editor|
|Obsoletes: 2616 (if approved)||Y. Lafon, Editor|
|Intended status: Standards Track||W3C|
|Expires: July 23, 2013||J. Reschke, Editor|
|January 19, 2013|
Hypertext Transfer Protocol (HTTP/1.1): Range Requests
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 (email@example.com), which is archived at <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related documents (including fancy diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix E.3.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.
This Internet-Draft will expire on July 23, 2013.
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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. 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 [RFC2119].
Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [Part1].
This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 1.2 of [Part1]. Appendix C describes rules imported from other documents. Appendix D 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. Such a range unit can be used in the Range (Section 5.4) and Content-Range (Section 5.2) header fields to delineate the parts of a representation that are either requested or transferred, respectively.
range-unit = bytes-unit / other-range-unit bytes-unit = "bytes" other-range-unit = token
The only range unit defined by HTTP/1.1 is "bytes" (Section 5.4.1). Additional units can be defined as described in Section 2.1.
The HTTP Range Unit Registry defines the name space for the range unit names and refers to their corresponding specifications.
Registrations MUST include the following fields:
Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).
The registry itself is maintained at <http://www.iana.org/assignments/http-range-specifiers>.
The 206 (Partial Content) status code indicates that the server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field (Section 5.4) indicating the desired range, and MAY have included an If-Range header field (Section 5.3) to make the request conditional.
When a 206 response is generated, the sender MUST generate the following header fields:
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 described above. Otherwise, the sender MUST generate all of the same representation header fields that would have been sent in a 200 (OK) response to the same request.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 206 responses.
The 416 (Range Not Satisfiable) status code indicates that none of the ranges-specifier values in the request's Range header field (Section 5.4) overlap the current extent of the selected resource and the request did not include an If-Range header field (Section 5.3). (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 representation.)
When this status code is sent in response to a byte-range request, the sender SHOULD generate a Content-Range header field specifying the current length of the selected representation (see Section 5.2).
HTTP/1.1 416 Range Not Satisfiable Date: Mon, 20 Jan 2012 15:41:54 GMT Content-Range: bytes */47022 Content-Type: image/gif
Note: Clients cannot depend on servers to send a 416 (Range Not Satisfiable) response instead of a 200 (OK) response for an unsatisfiable Range header field, since not all servers implement this header field.
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 field, and a Content-Length header field showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial Content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif
When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart message. The multipart media type used for this purpose is "multipart/byteranges" as defined in Appendix A.
A server MAY combine requested ranges when those ranges are overlapping (see Section 7.1).
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 asks for multiple ranges in one request, the server SHOULD send them in the order that they appeared in the request.
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 [Part4]) or, if no entity-tag is provided, a Last-Modified value that is strong in the sense defined by Section 2.2.2 of [Part4].
When a client receives an incomplete 200 (OK) response or a 206 (Partial Content) response, and already has one or more partial responses for the same method and effective request URI that have the same strong validator as present in the new response, the recipient MAY combine some or all of those responses into a set of continuous ranges. A client MUST NOT combine responses that differ in the strong validator or that do not have a strong validator.
If the new response is an incomplete 200 (OK) response, then the header fields of that new response are used for any combined response and replace those of the matching stored responses.
If the new 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.
This section defines the syntax and semantics of HTTP/1.1 header fields related to range requests and partial responses.
The "Accept-Ranges" header field allows a resource to indicate its acceptance of range requests.
Accept-Ranges = acceptable-ranges acceptable-ranges = 1#range-unit / "none"
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 field for the resource involved. Range units are defined in Section 2.
Servers that do not accept any kind of range request for a resource MAY send
to advise the client not to attempt a range request.
The "Content-Range" header field is sent with a partial representation to specify where in the full representation the payload body is intended to be applied.
Content-Range = byte-content-range / other-content-range byte-content-range = bytes-unit SP ( byte-range-resp "/" unsatisfied-range ) byte-range-resp = byte-range "/" ( complete-length / "*" ) byte-range = first-byte-pos "-" last-byte-pos unsatisfied-range = "*/" complete-length complete-length = 1*DIGIT other-content-range = other-range-unit SP other-range-resp other-range-resp = *CHAR
Range units are defined in Section 2. A recipient of a 206 (Partial Content) response containing a Content-Range header field with a range unit that the recipient does not understand 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.
A Content-Range field value with 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, is invalid. The recipient of an invalid Content-Range MUST NOT attempt to recombine the received content with a stored representation.
A server generating a 206 (Partial Content) response to a byte range request MUST send, in each body-part of a multipart response or in the header block of a single part response, a Content-Range header field containing a byte-range-resp value that reflects the corresponding range being sent. The following example would apply when the complete length of the selected representation is known by the sender to be 1234 bytes:
Content-Range: bytes 42-1233/1234
or this second example would apply when the complete length is unknown:
Content-Range: bytes 42-1233/*
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:
Content-Range: bytes */1234
The complete-length in a 416 response indicates the current length of the selected representation, which will be known by the server generating the response because that is how it determined the range to be unsatisfiable.
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.
More examples of Content-Range values, assuming that the representation contains a total of 1234 bytes:
Content-Range: bytes 0-499/1234
Content-Range: bytes 500-999/1234
Content-Range: bytes 500-1233/1234
Content-Range: bytes 734-1233/1234
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.
If-Range = entity-tag / HTTP-date
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 [Part4].
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 [Part4]. (A server can distinguish between a valid HTTP-date and any form of entity-tag by examining the first two characters.)
The If-Range header field SHOULD only be sent by clients together with a Range header field. The If-Range header field MUST be ignored if it is received in a request that does not include a Range header field. The If-Range header field MUST be ignored by a server that does not support the sub-range operation.
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 send the specified sub-range of the representation using a 206 (Partial Content) response. If the validator does not match, then the server SHOULD send the entire representation using a 200 (OK) response.
Since all HTTP representations are transferred as sequences of bytes, the concept of a byte range is meaningful for any HTTP representation. (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 representation data (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 representation.
byte-ranges-specifier = bytes-unit "=" byte-range-set byte-range-set = 1#( byte-range-spec / suffix-byte-range-spec ) byte-range-spec = first-byte-pos "-" [ last-byte-pos ] first-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it MUST be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte-range-spec is syntactically invalid. The recipient of a byte-range-set that includes one or more syntactically invalid byte-range-spec values MUST ignore the header field that includes that byte-range-set.
If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the representation data, last-byte-pos is taken to be equal to one less than the current length of the representation in bytes.
By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the representation.
suffix-byte-range-spec = "-" suffix-length suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the representation data, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of a representation.) If the representation is shorter than the specified suffix-length, the entire representation 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 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. If the byte-range-set is unsatisfiable, the server SHOULD send a response with a 416 (Range Not Satisfiable) status code. Otherwise, the server SHOULD send a response with a 206 (Partial Content) status code containing the satisfiable ranges of the representation.
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 an HTTP payload, recipients SHOULD anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows.
Examples of byte-ranges-specifier values (assuming a representation of length 10000):
The "Range" header field on a GET request modifies the method semantics to request transfer of only one or more sub-ranges of the selected representation data in a successful response, rather than the entire representation data.
Range = byte-ranges-specifier / other-ranges-specifier other-ranges-specifier = other-range-unit "=" other-range-set other-range-set = 1*CHAR
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.
The Range header field is evaluated after evaluating the preconditions of [Part4] 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 (Section 5.3) 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, the specified range(s) are syntactically correct (as defined in Section 5.4.1), and at least one of the ranges has a non-empty intersection with the current selected representation extent, then the server MAY respond with a status code of 206 (Partial Content) and a payload containing one or more partial representations that correspond to those requested, as defined in Section 4.
The HTTP Status Code Registry located at <http://www.iana.org/assignments/http-status-codes> shall be updated with the registrations below:
The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be updated with the permanent registrations below (see [BCP90]):
|Header Field Name||Protocol||Status||Reference|
The change controller is: "IETF (firstname.lastname@example.org) - Internet Engineering Task Force".
The registration procedure for HTTP Range Specifiers is defined by Section 2.1 of this document.
The HTTP Range Specifier Registry shall be created at <http://www.iana.org/assignments/http-range-specifiers> and be populated with the registrations below:
|Range Specifier Name||Description||Reference|
|bytes||a range of octets||Section 2|
|none||reserved as keyword, indicating no ranges are supported||Section 5.1|
The change controller is: "IETF (email@example.com) - 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 [Part1] and semantics [Part2].
Range requests containing overlapping ranges can lead to a situation where the server is sending far more data than the size of the complete resource representation.
See Section 9 of [Part1].
|[Part1]||Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, Internet-Draft draft-ietf-httpbis-p1-messaging-latest (work in progress), January 2013.|
|[Part2]||Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, Internet-Draft draft-ietf-httpbis-p2-semantics-latest (work in progress), January 2013.|
|[Part4]||Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-latest (work in progress), January 2013.|
|[Part6]||Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-latest (work in progress), January 2013.|
|[RFC2046]||Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996.|
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC5234]||Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.|
|[BCP13]||Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.|
|[BCP90]||Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.|
|[RFC2616]||Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.|
|[RFC5226]||Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.|
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 ([RFC2046], Section 5.1). The media type for this purpose is called "multipart/byteranges". The following is to be registered with IANA [BCP13].
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.
Note: Despite the name "multipart/byteranges" is not limited to the byte ranges only.
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-Length: 1741 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--
Another example, using the "exampleunit" range unit:
HTTP/1.1 206 Partial Content Date: Tue, 14 Nov 1995 06:25:24 GMT Last-Modified: Tue, 14 July 04:58:08 GMT Content-Length: 2331785 Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES --THIS_STRING_SEPARATES Content-Type: video/example Content-Range: exampleunit 1.2-4.3/25 ...the first range... --THIS_STRING_SEPARATES Content-Type: video/example Content-Range: exampleunit 11.2-14.3/25 ...the second range --THIS_STRING_SEPARATES--
A weak validator cannot be used in a 206 response. (Section 3.1)
The Content-Range header field only has meaning when the status code explicitly defines its use. (Section 5.2)
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. (Appendix A)
This specification introduces a Range Specifier Registry. (Section 2.1)
The following core rules are included by reference, as defined in Appendix B.1 of [RFC5234]: 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 [Part1]:
OWS = <OWS, defined in [Part1], Section 3.2.3> token = <token, defined in [Part1], Section 3.2.6>
The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in [Part2], Section 18.104.22.168> entity-tag = <entity-tag, defined in [Part4], Section 2.3>
Accept-Ranges = acceptable-ranges Content-Range = byte-content-range / other-content-range HTTP-date = <HTTP-date, defined in [Part2], Section 22.214.171.124> If-Range = entity-tag / HTTP-date OWS = <OWS, defined in [Part1], Section 3.2.3> Range = byte-ranges-specifier / other-ranges-specifier acceptable-ranges = ( *( "," OWS ) range-unit *( OWS "," [ OWS range-unit ] ) ) / "none" byte-content-range = bytes-unit SP ( byte-range-resp "/" unsatisfied-range ) byte-range = first-byte-pos "-" last-byte-pos byte-range-resp = byte-range "/" ( complete-length / "*" ) 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" complete-length = 1*DIGIT entity-tag = <entity-tag, defined in [Part4], Section 2.3> first-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT other-content-range = other-range-unit SP other-range-resp other-range-resp = *CHAR other-range-set = 1*CHAR other-range-unit = token other-ranges-specifier = other-range-unit "=" other-range-set range-unit = bytes-unit / other-range-unit suffix-byte-range-spec = "-" suffix-length suffix-length = 1*DIGIT token = <token, defined in [Part1], Section 3.2.6> unsatisfied-range = "*/" complete-length
Changes up to the first Working Group Last Call draft are summarized in <http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-19#appendix-D>.