14.36.1 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 invalid. The recipient of an invalid byte-range-spec must ignore it.

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.

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