public void
setActive(int len) {
	if (len > byteBuffer.capacity() - byteBuffer.position()) {
		throw new IllegalArgumentException("cannot set active " +
						   "region past end of input");
	}
	byteBuffer.limit(byteBuffer.position() + len);
}
Marks the following bytes in the stream as active.
@param len The number of bytes in the active region.
@throws IllegalArgumentException The number of bytes in the active region
is longer than the remainder of the input.
src/main/java/core/org/xbill/DNS/DNSInput.java