This is called "extensibility". In ASN.1 people were doing the same thing unofficially in the mid-80s, then explicit support was added in the form of "extensib

This is called "extensibility". In ASN.1 people were doing the same thing unofficially in the mid-80s, then explicit support was added in the form of "extensibility markers". There's nothing new in protobufs. EDIT: I should add that the non-TLV encoding rules for ASN.1 all support extensibility. Also, there are good engineering practices and security reasons why one should want explicit documentation of what structures/enums/choices/typeranges are expected to be extensible. Relying on decoders to ignore unexpected bits is not a sound practice.