IEC 61158-3-21:2019 defines in an abstract way the externally visible service provided by the Type 21 fieldbus data-link layer in terms of the primitive actions and events of the service; the parameters associated with each primitive action and event, and the form which they take and the interrelationship between these actions and events, and their valid sequences. It cancels and replaces the first edition published in 2010 and constitutes a technical revision. The main changes are listed below: lt;br /gt;•Added Network Control Message Type; lt;br /gt;•Miscellaneous editorial corrections.