What is the correct way to submit only unchecked checkboxes and the correct way to submit both checked and unchecked

I would like to know the correct approach to submit unchecked checkboxes only(opposite of default behaviour)

Why?

If all checkboxes are checked by default, lets say 50 of them. It only really makes sense to submit changed values, in this case only the unchecked checkboxes need to be posted.

I would like to know the correct approach to submit a checkbox regardless of whether it is checked or not.

Why?

I would like a default value to be submitted it a checkbox was not checked.

Please no answers which are dependent on a specific server-side language see here