on chunk body data.
                </p>
                <p>
                  The callback will be invoked one or more times to provide buffers
                  corresponding to the chunk body for the current chunk. The callback
                  receives the number of octets remaining in this chunk body including
                  the octets in the buffer provided.
                </p>
                <p>
                  The callback must return the number of octets actually consumed.
                 