Question: What triggers a GTP entity to send a Supported Extension Headers Notification message to its peer?
Option 1: When a GTP entity is upgraded to support new extension headers and wants to advertise its updated capabilities.
Option 2: When a GTP entity encounters an unknown extension header but has the capability to interpret it after a software update.
Option 3: When a GTP entity needs to interpret a mandatory Extension Header but lacks the necessary upgrade to support that specific header.
Option 4: During initial setup of GTP tunnels as a standard procedure to declare all potential extensions.
Explanation: This is correct because, according to the context, the message is sent specifically in response to a situation where a GTP entity is required to interpret a mandatory Extension Header but does not yet have the upgrade to support it. This notification informs the peer about its limitations, prompting potential retries with all headers to verify if an upgrade has occurred since the last communication.