This describes STUN UNKNOWN-ATTRIBUTES attribute. The UNKNOWN-ATTRIBUTES attribute is present only in an error response when the response code in the ERROR-CODE attribute is 420. The attribute contains a list of 16 bit values, each of which represents an attribute type that was not understood by the server. If the number of unknown attributes is an odd number, one of the attributes MUST be repeated in the list, so that the total length of the list is a multiple of 4 bytes.