|Network Working Group||R. Fielding, Editor|
|Internet Draft||Day Software|
|Obsoletes: 2616 (if approved)||One Laptop per Child|
|Intended status: Standards Track||J. Mogul|
|Expires: July 2008||HP|
|Y. Lafon, Editor|
|J. F. Reschke, Editor|
HTTP/1.1, part 5: Range Requests and Partial Responses
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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 (email@example.com). The current issues list is at <http://www.tools.ietf.org/wg/httpbis/trac/report/11> and related documents (including fancy diffs) can be found at <http://www.tools.ietf.org/wg/httpbis/>.
This draft incorporates those issue resolutions that were either collected in the original RFC2616 errata list (<http://purl.org/NET/http-errata>), or which were agreed upon on the mailing list between October 2006 and November 2007 (as published in "draft-lafon-rfc2616bis-03").
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 [RFC2119].
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."
HTTP/1.1 allows a client to request that only part (a range of) the response entity be included within the response. HTTP/1.1 uses range units in the Range (Section 5.4) and Content-Range (Section 5.2) header fields. An entity can be broken down into subranges according to various structural units.
range-unit = bytes-unit | other-range-unit bytes-unit = "bytes" other-range-unit = token
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 implementations MAY ignore ranges specified using other units.
HTTP/1.1 has been designed to allow implementations of applications that do not depend on knowledge of ranges.
The server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field (Section 5.4) indicating the desired range, and MAY have included an If-Range header field (Section 5.3) to make the request conditional.
The response MUST include the following header fields:
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 Section 4.
A cache that does not support the Range and Content-Range headers MUST NOT cache 206 (Partial Content) responses.
A server SHOULD return a response with this status code if a request included a Range request-header field (Section 5.4), and none of the range-specifier values in this field overlap the current extent of the selected resource, and the request did not include an If-Range request-header field. (For byte-ranges, this means that the first-byte-pos of all of the byte-range-spec values were greater than the current length of the selected resource.)
When this status code is returned for a byte-range request, the response SHOULD include a Content-Range entity-header field specifying the current length of the selected resource (see Section 5.2). This response MUST NOT use the multipart/byteranges content-type.
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.
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:
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 Accept-Ranges response-header field allows the server to indicate its acceptance of range requests for a resource:
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges acceptable-ranges = 1#range-unit | "none"
Origin servers that accept byte-range requests MAY send
but are not required to do so. Clients MAY generate byte-range requests without having received this header for the resource involved. Range units are defined in Section 2.
Servers that do not accept any kind of range request for a resource MAY send
to advise the client not to attempt a range request.
The Content-Range entity-header is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied. Range units are defined in Section 2.
Content-Range = "Content-Range" ":" content-range-spec content-range-spec = byte-content-range-spec byte-content-range-spec = bytes-unit SP byte-range-resp-spec "/" ( instance-length | "*" ) byte-range-resp-spec = (first-byte-pos "-" last-byte-pos) | "*" instance-length = 1*DIGIT
The header SHOULD indicate the total length of the full entity-body, unless this length is unknown or difficult to determine. The asterisk "*" character means that the instance-length is unknown at the time when the response was generated.
Unlike byte-ranges-specifier values (see Section 5.4.1), a byte-range-resp-spec MUST only specify one range, and MUST contain absolute byte positions for both the first and last byte of the range.
A byte-content-range-spec with a byte-range-resp-spec whose last-byte-pos value is less than its first-byte-pos value, or whose instance-length value is less than or equal to its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec MUST ignore it and any content transferred along with it.
A server sending a response with status code 416 (Requested range not satisfiable) SHOULD include a Content-Range field with a byte-range-resp-spec of "*". The instance-length specifies the current length of the selected resource. A response with status code 206 (Partial Content) MUST NOT include a Content-Range field with a byte-range-resp-spec of "*".
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to a request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial Content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif
When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart message. The multipart media type used for this purpose is "multipart/byteranges" as defined in Appendix A. See Appendix B.1 for a compatibility issue.
A response to a request for a single range MUST NOT be sent using the multipart/byteranges media type. A response to a request for multiple ranges, whose result is a single range, MAY be sent as a multipart/byteranges media type with one part. A client that cannot decode a multipart/byteranges message MUST NOT ask for multiple byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request.
If the server ignores a byte-range-spec because it is syntactically invalid, the server SHOULD treat the request as if the invalid Range header field did not exist. (Normally, this means return a 200 response containing the full entity).
If the server receives a request (other than one including an If-Range request-header field) with an unsatisfiable Range request-header field (that is, all of whose byte-range-spec values have a first-byte-pos value greater than the current length of the selected resource), it SHOULD return a response code of 416 (Requested range not satisfiable) (Section 3.2).
If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request-header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity-body.
The If-Range header allows a client to "short-circuit" the second request. Informally, its meaning is `if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity'.
If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
If the client has no entity tag for an entity, but does have a Last-Modified date, it MAY use that date in an If-Range header. (The server can distinguish between a valid HTTP-date and any form of entity-tag by examining no more than two characters.) The If-Range header SHOULD only be used together with a Range header, and MUST be ignored if the request does not include a Range header, or if the server does not support the sub-range operation.
If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server SHOULD provide the specified sub-range of the entity using a 206 (Partial Content) response. If the entity tag does not match, then the server SHOULD return the entire entity using a 200 (OK) response.
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.
ranges-specifier = byte-ranges-specifier byte-ranges-specifier = bytes-unit "=" byte-range-set byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) byte-range-spec = first-byte-pos "-" [last-byte-pos] first-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it MUST be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte-range-spec is syntactically invalid. The recipient of a byte-range-set that includes one or more syntactically invalid byte-range-spec values MUST ignore the header field that includes that byte-range-set.
If the last-byte-pos value is absent, or if the value is greater than or equal to the current length of the entity-body, last-byte-pos is taken to be equal to one less than the current length of the entity-body in bytes.
By its choice of last-byte-pos, a client can limit the number of bytes retrieved without knowing the size of the entity.
suffix-byte-range-spec = "-" suffix-length suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the entity-body, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity-body.) If the entity is shorter than the specified suffix-length, the entire entity-body is used.
If a syntactically valid byte-range-set includes at least one byte-range-spec whose first-byte-pos is less than the current length of the entity-body, or at least one suffix-byte-range-spec with a non-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):
HTTP retrieval requests using conditional or unconditional GET methods MAY request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request:
Range = "Range" ":" ranges-specifier
A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches ought to support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities.
If the server supports the Range header and the specified range or ranges are appropriate for the entity:
In some cases, it might be more appropriate to use the If-Range header (see Section 5.3) in addition to the Range header.
If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client. It SHOULD store the entire received response in its cache if that is consistent with its cache allocation policies.
No additional security considerations have been identified beyond those applicable to HTTP in general [Part1].
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.
|[Part1]||Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. F. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-latest (work in progress), January 2008.|
|[Part3]||Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. F. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-latest (work in progress), January 2008.|
|[Part4]||Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. F. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-latest (work in progress), January 2008.|
|[Part6]||Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. F. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-latest (work in progress), January 2008.|
|[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.|
|[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.|
When an HTTP 206 (Partial Content) response message includes the content of multiple ranges (a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart message-body. The media type for this purpose is called "multipart/byteranges".
The multipart/byteranges media type includes two or more parts, each with its own Content-Type and Content-Range fields. The required boundary parameter specifies the boundary string used to separate each body-part.
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--
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. (Section 5.2, see also [Part1], [Part3] and [Part6])
There are situations where a server (especially a proxy) does not know the full length of a response but is capable of serving a byterange request. We therefore need a mechanism to allow byteranges with a content-range not indicating the full length of the message. (Section 5.2)
Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send needed headers in a 206 response, this problem can be avoided. (Section 3.1 and 5.3)
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 3.2, 5.2)
Clarify that it is not ok to use a weak cache validator in a 206 response. (Section 3.1)
Extracted relevant partitions from [RFC2616].
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