"t0 is the time when the Echo option value was generated."
"If the server cannot verify that the Echo option value was generated by the server or the round-trip time is not below the threshold the request is not processed further, and an error message MAY be sent. The error message SHOULD include a new Echo option."
Clarifications based on Jim's review.
"t0 is the time when the Echo option value was generated."
"If the server cannot verify that the Echo option value was generated by the server or the round-trip time is not below the threshold the request is not processed further, and an error message MAY be sent. The error message SHOULD include a new Echo option."