RFC7233: Hypertext Transfer Protocol (HTTP/1.1): Range Requests

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].

本文档中的关键词 必须MUST、禁止MUST NOT、要求REQUIRED、必须SHALL、禁止SHALL NOT、应当SHOULD、不应当SHOULD NOT、推荐RECOMMENDED、可以MAY 和 可选OPTIONAL 的意义与【RFC2119】一致。

Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [RFC7230].

关于错误处理的一致性标准以及注意事项已在【RFC7230】章节 2.5 中定义了。

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with a list extension, defined in Section 7 of [RFC7230], that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates repetition). Appendix C describes rules imported from other documents. Appendix D shows the collected grammar with all list operators expanded to standard ABNF notation.

本规范使用了扩展巴科斯范式（ABNF）标记法【RFC5234】，另外，出于定义的紧凑性的考虑，本规范对 ABNF 规则进行了扩展（见【RFC7230】章节 7），允许使用一个 # 操作符（类似于 * 操作符，指代“重复”）来定义一种以逗号分隔的列表。附录 C 描述了从其他文档中引进的规则。附录 D 展示了所有已收集的包含列表扩展规则以及标准 ABNF 标记的语法。

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 [RFC7231]). The "bytes" range unit is defined for expressing subranges of the data's octet sequence.

因为表示形式数据是以字节octets序列的方式在有效载荷里传输的，因此，对于任何可通过 HTTP 进行传输的表示形式，字节范围byte range都是有意义的基础结构meaningful substructure。

译注："octet" 和 "byte" 都译作字节，其区别是，"octet" 是特指 8 位的字节，因此也被称为“八位组”。而 "byte" 可能是 8 位，也可能为其他位。

bytes-unit       = "bytes"

A byte-range request can specify a single range of bytes or a set of ranges within a single representation.

一个字节范围请求能够指定一个单一的字节范围，也可以指定同一个表示形式里的一个范围集合。

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.

在一个 byte-range-spec 中的 first-byte-pos 的值给出了在一个范围内第一个字节的偏移量byte-offset。last-byte-pos 的值给出了在一个范围内最后一个字节的偏移量，也就是说，是包括inclusive所指定的这个位置的字节的。字节偏移量从 0 开始。

Examples of byte-ranges-specifier values:

byte-ranges-specifier 值的一些示例：

• The first 500 bytes (byte offsets 0-499, inclusive):

  第一组 500 个字节（字节偏移量为 0-499，闭合区间inclusive）：

  bytes=0-499
  

• The second 500 bytes (byte offsets 500-999, inclusive):

  第二组 500 个字节（字节偏移量为 500-999，闭合区间）：

  bytes=500-999
  

A byte-range-spec is invalid if the last-byte-pos value is present and less than the first-byte-pos.

在 byte-range-spec 中，如果有出现 last-byte-pos 值，并且它小于 first-byte-pos，那么这个 byte-range-spec 是无效的。

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).

客户端可以在不知道已选表示形式大小的情况下限制所请求的字节数量。如果缺少 last-byte-pos 值，或者其值大于或等于表示形式数据的当前长度，那么，字节范围会被解释为这个表示形式的剩余部分（也就是说，服务器将这个 last-byte-pos 的值替换为已选定表示形式的长度减 1 的值）。

A client can request the last N bytes of the selected representation using a suffix-byte-range-spec.

客户端可以使用一个 suffix-byte-range-spec 来请求已选表示形式的最后 N 个字节。

suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT

译注：需要注意的是，suffix-length 的语义是“字节的长度”，而 first-byte-pos 和 last-byte-pos 的语义是“字节的序号（偏移量）”。

If the selected representation is shorter than the specified suffix-length, the entire representation is used.

如果已选表示形式的长度小于 suffix-length 所指定的值，表示使用整个表示形式。

Additional examples, assuming a representation of length 10000:

再给出一些例子，假设一个表示形式的长度为 10000：

• The final 500 bytes (byte offsets 9500-9999, inclusive):

  最后 500 个字节（字节偏移量为 9500-9999，闭合区间）：

  bytes=-500
  

  Or:

  或者

  bytes=9500-
  

• The first and last bytes only (bytes 0 and 9999):

  第一个字节和最后一个字节（第 0 号字节和第 9999 号字节）：

  bytes=0-0,-1
  

• Other valid (but not canonical) specifications of the second 500 bytes (byte offsets 500-999, inclusive):

  其他有效的（但不规范的not canonical）对第二组 500 个字节的指定（字节偏移量为 500-999，闭合区间）：

  bytes=500-600,601-999
  bytes=500-700,601-999
  

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.

如果一个有效的 byte-range-set 包含至少一个 byte-range-spec 带有少于表示形式的当前长度的一个 first-byte-pos 值，或者包含至少一个 suffix-byte-range-spec 带有一个非零的 suffix-length，那么，这个 byte-range-set 是可以满足的satisfiable。否则，这个 byte-range-set 是不可满足的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 MUST anticipate potentially large decimal numerals and prevent parsing errors due to integer conversion overflows.

在字节范围句法中，first-byte-pos、last-byte-pos、以及 suffix-length 所表达的是八位组字节的十进制数decimal number of octets。由于没有预定义有效载荷的长度限制，因此，接收端 必须 能预料到特大数值big decimal numbers的可能，并且避免由于整型类型转换溢出所引起的解析错误。

Range units are intended to be extensible. New range units ought to be registered with IANA, as defined in Section 5.1.

范围单位是可扩展的。新的范围单位应该在 IANA 上进行登记，如章节 5.1 所述。

other-range-unit = token

The "Accept-Ranges" header field allows a server to indicate that it supports range requests for the target resource.

Accept-Range 头字段让服务器可以表明它支持对这个目标资源的范围请求。

Accept-Ranges     = acceptable-ranges
acceptable-ranges = 1#range-unit / "none"

An origin server that supports byte-range requests for a given target resource MAY send

支持对一个给定目标资源进行范围请求的源服务器 可以 发送：

Accept-Ranges: bytes

to indicate what range units are supported. A client MAY generate range requests without having received this header field for the resource involved. Range units are defined in Section 2.

来表明它支持什么范围单位。客户端 可以 在没有接收到这个头字段的情况下生成范围请求。范围单位定义在章节 2。

A server that does not support any kind of range request for the target resource MAY send

不支持对目标资源进行任何类型的范围请求的服务器，可以 发送：

Accept-Ranges: none

to advise the client not to attempt a range request.

来通告客户端不要试图发送范围请求。

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.

在一个 GET 请求中的 Range 头字段将请求方法的语义修改为去请求传输已选表示形式数据selected representation data的仅仅一个或多个子范围，而不是整个已选表示形式数据。

Range = byte-ranges-specifier / other-ranges-specifier
other-ranges-specifier = other-range-unit "=" other-range-set
other-range-set = 1*VCHAR

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.

服务器 可以 忽略 Range 头字段。但是，源服务器和中间人缓存服务器应该尽可能去支持字节范围byte ranges，因为 Range 支持从传输一部分数据后（因某种原因导致）传输失败了的场景中高效地恢复过来，还支持获取一个大的表示形式中的一部分。如果所接收到的请求方法不是 GET，那么，服务器 必须 忽略这个消息中的 Range 头字段。

An origin server MUST ignore a Range header field that contains a range unit it does not understand. A proxy MAY discard a Range header field that contains a range unit it does not understand.

如果 Range 头字段包含了源服务器所不能理解的范围单位range unit，那么，源服务器 必须 忽略这个头字段。如果 Range 头字段包含了代理所不能理解的范围单位，代理 可以 丢弃掉这个头字段。

A server that supports range requests MAY 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 (Section 6.1). 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.

如果 Range 头字段由不止两个的重叠范围组成，或者由许多不是以升序排列的小范围组成，那么，支持范围请求的服务器 可以 忽略或者拒绝这个头字段。因为之所以出现这两种组成方式，要不就是客户端的实现有问题，要不就是出现了刻意的拒绝服务攻击denial-of-service attack（章节 6.1）。如果划分为多个小范围进行处理和传输本质上比包含同样数据的单个大范围的效率要差的话，客户端 不应当 请求多个范围。

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.

客户端在请求多个范围时，应当 将这些范围按升序排序（按它们通常被接收成为一个完整的表示形式的顺序），除非它明确需要更早地请求一个更后的部分。例如，用户代理正在处理一个非常大的表示形式，这个表示形式带有一个自身组成部分的内部目录可能需要先请求更后的部分的情况，特别是如果这个表示形式由多页组成，以页码倒序的方式保存，并且用户代理希望一次一页地传输。

The Range header field is evaluated after evaluating the precondition header fields defined in [RFC7232], and only if the result in absence of the Range header field would be a 200 (OK) response. In other words, Range is ignored when a conditional GET would result in a 304 (Not Modified) response.

在前提条件头字段precondition header fields（定义在【RFC7232】）求值以后，并且假如没有 Range 头字段的话这次请求的最终结果将会是一个 200 (OK) 响应，那么，Range 头字段才会被求值。换句话说，当一个条件 GET 请求的结果是一个 304 (Not Modified) 响应的时候，Range 会被忽略。

The If-Range header field (Section 3.2) can be used as a precondition to applying the Range header field.

If-Range 头字段（章节 3.2）能够被用作应用 Range 头字段的一个前提条件。

译注：也就是说，当 If-Range 的求值结果为真（即条件成立）的时候，才会对 Range 进行求值。

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 Section 2.1), 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 Section 4.

如果所有前提条件皆为真，且服务器支持对目标资源应用 Range 头字段，且所指定的范围皆为有效的和可满足的satisfiable（其定义见章节 2.1），那么，服务器 应当 发送一个 206 (Partial Content) 响应，带有一个有效载荷来包含一个表示形式的一个或多个部分，这些部分对应于客户端所请求的可满足的范围satisfiable ranges，其定义见章节 4。

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.

如果所有前提条件皆为真，且服务器支持对目标资源应用 Range 头字段，但所指定的范围无效或不可满足的，那么，服务器 应当 发送一个 416 (Range Not Satisfiable) 响应。

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 precondition fails because the representation has been modified, the client would then have to make a second request to obtain the entire current representation.

如果客户端已有一个表示形式的一部分副本partial copy，它希望获得整个表示形式的一份最新副本，那么，它可以使用在一个条件 GET 请求中带有 Range 头字段（使用 If-Unmodified-Since 和 If-Match 两者或两者之一）。但是，如果因表示形式已经被修改而导致前提条件不成立的话，客户端将不得不发起第二次请求来获得整个当前表示形式。

The "If-Range" header field allows a client to "short-circuit" the second request. Informally, its meaning is as follows: if the representation is unchanged, send me the part(s) that I am requesting in Range; otherwise, send me the entire representation.

If-Range 头字段让客户端可以“短路short-circuit”第二次请求。通俗地说，它的含义如下：如果表示形式未发生变化，把我在 Range 所请求的范围的那部分发送给我；否则，把整个表示形式发送给我。

译注：编程语言中的逻辑表达式 and 和 or 通常都带有“短路”功能。

If-Range = entity-tag / HTTP-date

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.

如果一个请求中不包含有 Range 头字段，那么，客户端 禁止 在这个请求中生成 If-Range 头字段。如果服务器所接收到的请求中不包含有 Range 头字段，那么，服务器 必须 忽略这个请求中的 If-Range 头字段。如果源服务器所接收到的请求所指的目标资源并不支持 Range 范围请求，那么，源服务器 必须 忽略这个请求中的 If-Range 头字段。

A client MUST NOT generate an If-Range header field containing an entity-tag that is marked as weak. A client MUST NOT generate an If-Range header field containing an HTTP-date unless the client has no entity-tag for the corresponding representation and the date is a strong validator in the sense defined by Section 2.2.2 of [RFC7232].

客户端 禁止 生成这样一种 If-Range 头字段，其值是一个标记为弱验证类型的实体标签。客户端 禁止 生成这样一种 If-Range 头字段，其值是一个 HTTP-date，除非这个客户端的表示形式没有对应的实体标签，并且这个日期是一种【RFC7232】章节 2.2.2 所说的强验证类型。

A server that evaluates an If-Range precondition MUST use the strong comparison function when comparing entity-tags (Section 2.3.2 of [RFC7232]) and MUST evaluate the condition as false if an HTTP-date validator is provided that is not a strong validator in the sense defined by Section 2.2.2 of [RFC7232]. A valid entity-tag can be distinguished from a valid HTTP-date by examining the first two characters for a DQUOTE.

服务器对一个 If-Range 前提条件进行求值的时候，如果它的值是一个实体标签的话，那么，服务器 必须 使用强对比方法（【RFC7232】章节 2.3.2）；另外，如果它的值是一个 HTTP-date 验证器，但不是一种【RFC7232】章节 2.2.2 所说的强验证类型，那么，服务器 必须 将这个条件求值为假。一个有效的（强验证类型的）实体标签可以通过检测前两个字符是否有一个双引号 DQUOTE 来区分一个有效的 HTTP-date。

译注： 强实体标签第一个字符必定为双引号，如： "xyzzy"，弱实体标签的开头带有一个 W\，如：W\"zyzzy"。详细的 ABNF 见章【RFC7232】章节 2.3。

HTTP-date 并不是一个字符串，它没被双引号包裹，见【RFC7231】章节 7.1.1.1。

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, the server MUST ignore the Range header field. Note that this comparison by exact match, including when the validator is an HTTP-date, differs from the "earlier than or equal to" comparison used when evaluating an If-Unmodified-Since conditional.

如果 If-Range 头字段所给出的验证器匹配上了目标资源的已选表示形式的当前验证器，那么，服务器 应当 将 Range 头字段作为所要请求的范围来处理。如果验证器没有匹配，那么，服务器 必须 忽略 Range 头字段。需要注意的是，这种对比是通过精准匹配的，包括验证器是一个 HTTP-date 的情况，区别于对一个 If-Unmodified-Since 条件式进行求值时所使用的“早于或等于”对比方式。

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 request's Range header field (Section 3.1).

206 (Partial Content) 状态码表明：服务器通过传输已选表示形式的一个或多个部分，已经成功地实现fulfilling了客户端对目标资源的一个范围请求，这些部分的表示形式对应于请求消息的 Range 头字段（章节 3.1）里的可满足的范围。

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:

服务器在生成一个 206 响应的时候，如果即将传输的是表示形式的单个部分single part（区别于多个部分multiple part），那么，服务器 必须 生成一个 Content-Range 头字段来描述这个响应所封装的是已选表示形式的什么范围，以及生成这个范围所组成的一个有效载荷。例如：

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

... 26012 bytes of partial image data ...

If multiple parts are being transferred, the server generating the 206 response MUST generate a "multipart/byteranges" payload, as defined in Appendix A, 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 section of a multiple part response (this field will be sent in each part instead).

服务器在生成一个 206 响应的时候，如果即将传输的是表示形式的多个部分multiple parts（区别于单个部分single part），那么，服务器 必须 生成一个 "multipart/byteranges" 类型的有效载荷（其定义见附录 A），以及生成一个 Content-Type 头字段，这个头字段包含 multipart/byteranges 媒体类型以及该媒体类型所要求的 boundary 参数。为了避免与单个部分的响应single-part responses产生混淆，服务器 禁止 在多个部分multiple part的 HTTP 响应头里生成 Content-Range 头字段（而是改为将这个字段在每一个部分内部进行发送）。

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:

对于 multipart 类型的有效载荷，服务器 必须 在这种有效载荷中的每一个 body-part 的头部区域header area内都生成一个 Content-Range 头字段，每个头字段的值表示被置入到对应的 body-part 的表示形式数据的范围。假设这个已选表示形式在 200 (OK) 响应的时候会带有一个 Content-Type 头字段，那么，在 206 响应中，服务器 应当 在每个 body-part 的头部区域中生成同样的 Content-Type 字段。例如：

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--

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.

当服务器被请求多个范围multiple ranges，无论这些范围所对应的 byte-range-spec 按什么顺序出现在服务器接收到的 Range 头字段里，服务器都 可以 将这些范围的重叠部分合并起来，或者将以一个小间隙分隔的范围合并起来——如果这个间隙小于发送多个部分的开销overhead的话。因为一个 multipart/byteranges 类型的有效载荷的各个部分之间的一般开销是 80 字节bytes，根据已选表示形式的媒体类型以及所选择的 boundary 参数的长度，传输许多小的分离部分的效率可能会低于传输整个已选表示形式。

overhead，在计算机网络的帧结构中，除了有用数据以外，还有很多控制信息，这些控制信息用来保证通信的完成。这些控制信息被称作系统开销。 ——百度百科

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 generate a request that asks for multiple ranges.

当客户端请求一个单个范围single range的时候，服务器 禁止 生成一个多个部分的响应multipart response来回应，这是因为客户端之所以不请求多个部分，可能是由于它不支持多个部分的响应。但是，如果所请求的是多个范围，并且最后发现只有一个范围可被满足，或者经合并后只剩下一个范围，那么，服务器 可以 生成一个 multipart/byteranges 有效载荷，其仅带有单个 body-part。不能处理 multipart/byteranges 响应的客户端 禁止 生成一个请求asks for多个范围的请求消息。

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.

当服务器生成了一个多个部分的响应有效载荷multipart response payload的时候，服务器 应当 按照它所接收到的 Range 头字段里的 byte-range-spec 的出现顺序（排除掉它认为是不可满足的或合并成其他范围的那些范围）来发送这些部分。客户端在接收到一个多个部分的响应时，必须 检查出现在每一个 body-part 里的 Content-Range 头字段，为了确定哪一个范围是包含在那个主体部分里的。客户端不能完全相信可以接收到与它所请求相一致的范围，或者与它所请求相一致的顺序。

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.

服务器生成一个 206 响应的时候，服务器除了需要生成上述所要求的头字段以外，假如生成的是一个 200 (OK) 响应到同一个请求，如果这个 200 响应包含下列头字段，那么，服务器 必须 将这些头字段同样生成到 206 响应里，包括：Date、Cache-Control、ETag、Expires、Content-Location、以及 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.

如果一个请求带有一个 If-Range 头字段，那么，发送端在生成一个 206 来回应这个请求的时候，不应当 生成上述所要求的头字段以外的其他的表示形式头字段，因为客户端被认为是已经有一个之前的响应包含那些头字段了。否则，发送端 必须 像生成一个 200 (OK) 响应给同一个请求时所带有的所有表示形式头字段生成到这个 206 响应里。

A 206 response is cacheable by default; i.e., unless otherwise indicated by explicit cache controls (see Section 4.2.2 of [RFC7234]).

206 响应默认是可缓存的cacheable，也就是说，除非由显式带有的缓存控制cache controls（【RFC7234】章节 4.2.2）另有指定。

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.

如果 Content-Range 头字段发送在一种单个部分single part的 206 (Partial Content) 响应里，那么它表明一个范围，这个范围是指封装为消息有效载荷的已选表示形式的局部范围partial range。如果它发送在一种多个部分multipart的 206 响应里的每一个 body-part 中，那么它表示一个范围，这个范围是指封装在每个 body-part 中的表示形式的范围。如果它发送在 416 (Range Not Staisfiable) 响应中，它提供了已选表示形式的相关信息。

译注：下文会多次提及 body-part 这个概念，可在【RFC2046】章节 5 找到更详细的信息。

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

If a 206 (Partial Content) response contains a Content-Range header field with a range unit (Section 2) 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.

如果一个 206 (Partial Content) 响应包含有一个 Content-Range 头字段，这个头字段使用了一种接收端无法理解的范围单位range unit（章节 2），那么，接收端 禁止 试图去使用一个已存储的表示形式来重组recombine它。代理在接收到这种消息时 应当 将它转发给下游。

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.

对于字节范围byte ranges，发送端 应当 指明表示形式的完整长度complete length，除非完整长度是未知的或者是难以确定的，所述表示形式指的是提取出这个字节范围的那一个表示形式。使用一个星号字符（"*"）来代替 complete-length，表明：当头字段生成的时候，表示形式的长度还是未知的。

The following example illustrates when the complete length of the selected representation is known by the sender to be 1234 bytes:

以下示例说明了当发送端知道已选表示形式的完整长度是 1234 个字节的时候：

Content-Range: bytes 42-1233/1234

and this second example illustrates when the complete length is unknown:

第二个示例说明当完整长度是未知的时候：

Content-Range: bytes 42-1233/*

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.

如果 Content-Range 的值包含有带有 last-byte-pos 值的一个 byte-range-resp 符合下列情况之一，那么，这个 Content-Range 的值就是无效的：

• last-byte-pos 的值小于 first-byte-pos 的值。
• complete-length 的值小于或等于 last-byte-pos 的值。

接收端在接收到一个无效的 Content-Range 头字段的时候，禁止 试图使用一个已存储的表示形式去重组所接收到的内容。

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:

服务器在生成一个 416 (Range Not Satisfiable) 响应来回应给一个字节范围请求byte-range request的时候，应当 发送一个 Content-Range 头字段，它的值表示一个不可满足的范围unsatisfied-range，见以下示例：

Content-Range: bytes */1234

The complete-length in a 416 response indicates the current length of the selected representation.

416 响应里的 complete-length 表明已选表示形式的当前长度。

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.

Content-Range 使用在没有明确描述它的语义的那些状态码里是没有任何意义的。对于本规范，只有 206 (Partial Content) 和 416 (Range Not Satisfiable) 有描述过 Content-Range 的意义。

The following are examples of Content-Range values in which the selected representation contains a total of 1234 bytes:

以下是一个总大小为 1234 字节的已选表示形式中的 Content-Range 值的示例：

• The first 500 bytes:

  第一组 500 个字节：

  Content-Range: bytes 0-499/1234
  

• The second 500 bytes:

  第二组 500 个字节：

  Content-Range: bytes 500-999/1234
  

• All except for the first 500 bytes:

  所有字节，除了第一组 500 个字节以外：

  Content-Range: bytes 500-1233/1234
  

• The last 500 bytes:

  最后一组 500 个字节：

  Content-Range: bytes 734-1233/1234
  

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 (Section 2.1 of [RFC7232]).

如果连接被过早地关闭了，或者请求使用了一个或多个 Range 规格，那么，一个响应可能只对一个表示形式的一个子范围subrange进行传输。历经多次这样的传输以后，客户端可能已接收到同一个表示形式的许多范围。这些范围只有在它们都具有同一个强验证器strong validator（【RFC7232】章节 2.1）的时候才能够被安全地结合combined起来。

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.

在向一个目标资源发起多次 GET 请求后，客户端接收到多个部分响应partial responses，那么，客户端 可以 将这些响应结合为一个大的连续的范围，如果它们共享同一个强验证器的话。

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.

如果最近一次响应是一个不完整的 200 (OK) 响应，那么，这个响应的头字段会被用作任何结合后的响应，并替换掉与已存储的响应相匹配的那些头字段。

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.

如果最近一次响应是一个 206 (Partial Content) 响应，并且与之匹配的已存储的响应中至少有一个的状态码是 200 (OK)，那么，结合后的响应的头字段由最后一次 200 响应的那些头字段组成。如果与之匹配的所有已存储的响应都是 206 响应，那么，已存储的响应中，以具有最新的头字段的那个响应来作为结合后的响应的头字段的来源，除了以下情况的头字段以外：客户端 必须 使用提供自新响应的（除了 Content-Range 以外还有）其他的头字段来替换有已存储的响应中所有对应的头字段的实际值。

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 process 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 process 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.

结合后的响应消息体由新响应内的部分内容范围以及选定的响应的每个部分联合组成。如果这个联合是由表示形式的完整范围组成，那么，客户端 必须 像处理一个完整的 200 (OK) 响应一样来处理这个结合后的响应，包括使用一个 Content-Length 头字段来反映完整的长度。否则，客户端 必须 按以下的方式之一来处理连续的范围：

• 一个不完整的 200 (OK) 响应，如果结合后的响应是表示形式的开头部分
• 一个单个的 206 (Partial Content) 响应，包含一个 multipart/byteranges 主体部分
• 多个 206 (Partial Content) 响应，其中每个都是一个连续的范围并且都由一个 Content-Range 头字段来指明。

The 416 (Range Not Satisfiable) status code indicates that none of the ranges in the request's Range header field (Section 3.1) 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.

416 (Range Not Satisfiable) 状态码表明：请求的 Range 头字段中没有一个范围与已选资源的当前范围有重合overlap，或者所请求的范围集由于范围无效或者请求了太多小的或重合的范围已被拒绝。

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 (Section 4.2).

对于字节范围byte ranges，与当前范围没有重合的意思是所有 byte-range-spec 的值里的 first-byte-pos 都大于已选表示形式的当前长度。当生成这个状态码来回应一个字节范围请求byte-range request的时候，发送端 应当 生成一个 Content-Range 头字段来指定已选表示形式的当前长度（章节 4.2）。

For example:

例如：

HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022

Note: Because servers are free to ignore Range, many implementations will simply respond with the entire selected representation in a 200 (OK) response. 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.

注意： 因为服务器可以自由忽略 Range，许多实现会简单地使用一个 200 (OK) 包含整个已选表示形式来响应这种请求。有部分原因是因为大多数客户端都准备使用一个 200 (OK) 来完成请求任务（即使不那么高效），以及有部分原因是因为客户端可能不会停止发送一个无效的部分请求partial request直到它们已经接收到一个完整的表示形式。所以，客户端不能完全相信一个 416 (Range Not Satisfiable) 响应，即使这种响应是最合适的。

The "HTTP Range Unit Registry" defines the namespace for the range unit names and refers to their corresponding specifications. The registry has been created and is now maintained at http://www.iana.org/assignments/http-parameters.

《HTTP 范围单位登记表》定义了范围单位的命名空间，并引用到它们对应的规范上。这份登记表已经创建，目前维护在 http://www.iana.org/assignments/http-parameters。

The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located at http://www.iana.org/assignments/http-status-codes has been updated to include the registrations below:

《超文本传输协议（HTTP）状态码登记表》位于 http://www.iana.org/assignments/http-status-codes，已经更新了以下登记项。

HTTP header fields are registered within the "Message Headers" registry maintained at http://www.iana.org/assignments/message-headers/.

HTTP 头字段被登记在《消息头部》登记表中，并维护在 http://www.iana.org/assignments/message-headers/。

This document defines the following HTTP header fields, so their associated registry entries have been updated according to the permanent registrations below (see [BCP90]):

本文档定义了以下 HTTP 头字段，因此它们相关联的登记项已经根据以下永久登记条目进行更新（见【BCP90】）：

The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".

主管以上条目的变更的是 "IETF (iesg@ietf.org) - Internet Engineering Task Force"。

IANA maintains the registry of Internet media types [BCP13] at http://www.iana.org/assignments/media-types.

IANA 将互联网媒体类型的登记表【BCP13】维护在 http://www.iana.org/assignments/media-types。

This document serves as the specification for the Internet media type "multipart/byteranges". The following has been registered with IANA.

本文档充当 "multipart/byteranges" 互联网媒体类型的规范。以下信息已经在 IANA 进行过登记。

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.

不加约束的多个部分的范围请求multiple range requests容易受到拒绝服务攻击denial-of-service attacks的影响，这是因为请求相同数据的许多重合范围的工作量与试图提供所请求的许多部分的数据所消耗的时间、内存、以及宽带相比显得太微不足道了。服务器应该忽略、合并、或者拒绝异常的范围请求，比如请求超过两个以上的重合范围，或者在在一个范围集里面请求了许多小范围，特别是当这些范围顺序混乱又没有合理的理由的情况。多个部分的范围请求multipart range requests不是为了支持随机访问random access而设计的。

[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.

[RFC7230]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, June 2014.

[RFC7231]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, June 2014.

[RFC7232]

Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests”, RFC 7232, June 2014.

[RFC7234]

Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Caching”, RFC 7234, June 2014.

[BCP13]

Freed, N., Klensin, J., and T. Hansen, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 6838, January 2013.

[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.