He failed to see how adding additional messages that are ignored by older peers amounts to a lack of backward compatibility. Matt Corallo, however, argued that an explicit part of their version handshake and protocol negotiation is exchanging otherwise-ignored messages to set up optional features. Peers that do not support this ignore such messages, just as if they indicated they wouldn't support it. This is not the same as BIP152, as control messages apart from BIP151 are not sent until after the version is negotiated. BIP151 is different as it desires to negotiate encryption before any other communications, including version.