Http Range

Http Range规范定义：http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
14.35 Range 14.35.1 Byte Ranges

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):

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

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

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

      - Or bytes=9500-

      - The first and last bytes only (bytes 0 and 9999):  bytes=0-0,-1

      - Several legal but not canonical specifications of the second 500
        bytes (byte offsets 500-999, inclusive):
        bytes=500-600,601-999
        bytes=500-700,601-999