Question: What action should a receiver take when it encounters an unexpected Information Element with a known Type value but an undefined instance value in a GTP message?
Option 1: The receiver should reject the entire message and send an error response.
Option 2: The receiver should discard the IE, log an error, and process the rest of the message as if the IE was not present.
Option 3: The receiver should skip the IE and continue processing the message without logging any errors.
Option 4: The receiver should silently discard the IE and process the rest of the message according to the defined grammar.
Explanation: According to the specification, when a receiver encounters an unexpected Information Element with a known Type value but an undefined instance value in a GTP message, it should discard the IE, log an error, and continue processing the message. This ensures that the rest of the message is processed correctly while also logging any errors for potential debugging purposes.