Bitcoin Q&A: Lightning's Security Model

Are Lightning Network transactions still bitcoin transactions? Do you have to be online? What happens when a party does not commit to an action / send a reimbursement in LN? How will the Lightning Network handle DDoS attacks? How does node banning work?
These questions are from the MOOC 9.5 session which took place on March 16th, 2018. Andreas is a teaching fellow with the University of Nicosia. The first course in their Master of Science in Digital Currency degree, DFIN-511: Introduction to Digital Currencies, is offered for free as an open enrollment MOOC course to anyone interested in learning about the fundamental principles.