Reviewer: Valery Smyslov Review result: Ready I have reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last call comments. This is a short document that defines additional measurement units and subunits to be used in SenML (RFC8428). As such the primary purpose of the document is to request IANA to update "SenML Units" registry and to create a new one - a "secondary units" sub-registry. The document doesn't define any actual protocol. The Security Considerations section refers to RFC8428, since introduction of new measurement units is believed to not bring any new security implications. Nit (not related to security). In para: SenML packs MAY, but SHOULD NOT, use secondary units in place of SenML units, where the exception of the "SHOULD NOT" lies in the context of specific existing data models that are based on these secondary units. I think that uppercase "MAY" is redundant here, because using "SHOULD NOT" always assumes that there may be exceptions.