BIP151 protocol incompatibility

The issue cited was that the protocol violates previous patterns of behavior by allowing the new control message to be sent before the version handshake. This could cause both irrelevant and broken handshakes for all existing protocol versions. Potential DOS attacks were also mentioned, where a node cannot determine whether an unknown message is an attack or a correct behavior. However, the implementation properly validates the protocol and supports a configurable protocol range, making it helpful for testing against each protocol level one wishes to support. Despite this, there are already nodes breaking connections based on the BIP, so it may need further revision.