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):