ptional security mechanism. The new standard goes to great lengths to point out any differences and specifically addresses com- patibility. Section 19.4 of the new standard gives details of the areas where compatibility between former and newer implementations can arise, while Table 139 specifically cautions on the compat- ibility of options. However, there is a lot more material written into the new version than previously, with the page count dramatically increasing. As a short summary, the following main areas have been addressed by the new version, 1588-2019: High accuracy: Improved time sync performance with high accuracy techniques with an op- tional new profile to support it (which might likely replace the White Rabbit specification). Management: Performance monitoring and data information models. Methods and options for monitoring PTP implementations are covered in 16.11, 16.12, and Annex J and adoption of an information model for the PTP datasets (to allow remote management). Architecture update and clarifications: Methods for profile isolation; PTP redundancy; and restructuring of the standard to separate the media-dependent functions from the media-inde- pendent functions, allowing the creation of special PTP ports. One other architectural change is the adoption of the concept of the PTP Instance as an instantiation of the PTP protocol, operat- ing in a single device, within exactly one domain (separating the concept of a running PTP in- stance from the PTP domain). Security: A PTP integrated security mechanism to provide a security layer to PTP encom- passing message integrity and authentication. IEEE added a new Annex P to aid implementation of a secure timing distribution network. Of course, 1588-2019 includes the usual clarifications, small amendments, and typographical cor- rections. The following two sections cover the details of the more substantial changes between the versions and the new features added to PTPv2.1. 7.7.1 Changes from PTPv2 to PTPv2.1 The following major changes have been made b