BIP151 protocol incompatibility

In a discussion about the protocol negotiation and version handshake, Matt Corallo explains that part of this process involves the exchange of otherwise-ignored messages to set up optional features. However, these messages are only exchanged if the peer is at the protocol level that allows it. Peers that do not support these messages ignore them, just as if they had indicated they wouldn't support it. This is not considered backwards incompatible because old nodes can still communicate effectively. The discussion also touches on the fact that BIP151 negotiates encryption before any other communications, including version.