Comprehensive and Detailed In-Depth Explanation:

In the Label Distribution Protocol (LDP) session establishment process, the Initialization message is sent by the initiator to propose parameters for the session. If the responder finds that the proposed parameters are not acceptable, it responds with an Error Notification Message. This message indicates that the session could not be established due to incompatible or incorrect parameters.

The Error Notification Message typically includes details about why the session could not be established, such as parameter mismatch or unsupported features. If the parameters are acceptable, the responder would instead reply with a Keepalive Message to maintain the session.