Network Time Foundation NTPv4 is defined by RFC 5905 [RFC5905], and it and earlier versions of the NTP Protocol have supported symmetric private key MAC authentication. MACs pre-date the Extension Fields introduced in RFC 5905 [RFC5905], and as the number of Extension Fields grows there is an increasing chance of ambiguity when deciding if the "next" set of data is an Extension Field or a MAC. This proposal defines a new Extension Field which is used to signifiy that it is the last Extension Field in the packet. If present, any subsequent data SHOULD be considered to be a legacy MAC.