+ /** Specify the size of the data to send. Not to be mixed with @ref Activity::set_remaining()
+ *
+ * That's the size of the data to actually copy in the simulator (ie, the data passed with Activity::set_src_data()).
+ * That's completely unrelated from the simulated size (given with @ref Activity::set_remaining()):
+ * you can send a short buffer in your simulator, that represents a very large message
+ * in the simulated world, or the opposite.
+ */