anguages, including MIB (for SNMP) and YANG. So, section 8 has expanded from the defini- tion of data used in the operation of the PTP protocol to an information model. 7.7.1 New Features in v2.1 The following is a list of the most important new features and interesting additions to v2.1: Improved accuracy: Accuracy is improved by making use of layer 1 frequency synchroniza- tion (Annex L) and asymmetry calibrations (16.7, , 16.8), and a new High Accuracy Delay Re- quest-Response Default PTP Profile in Annex I.5. Two new informative annexes on enhanced performance, namely M and N, were added, with Annex M including the use of a DDMTD- based phase offset detector (as used by White Rabbit). Special PTP ports: Special ports enable the inclusion of network links based on technologies that provide inherent timing support as opposed to the use of PTP timing messages, for ex- ample, IEEE 802.11 Wi-Fi and passive optical networking (PON) (see section 19.4.6). From the point of view of a PTP network, a special PTP port can only be connected to a compatible spe- cial port. Mixed multicast/unicast model of operation: The new edition includes new TLVs to provide stronger support for mixing multicast and unicast messages in a PTP session (see section 16.9, "Mixed multicast/unicast operation"). This optional feature was available but not well described in the 2008 version. External configuration: A new (optional) feature allows the external configuration of the port state under management control (see section 17.6). The management method to do this is not defined but could include a standard mechanism such as SNMP or YANG. Previously port state could only be determined resulting from the BMCA process. The (optional) section 17.7, "Re- duced state sets and use of the <foreignMasterList> feature," also allows the use of a restricted set of port states. Security enhancements: The 1588-2008 (optional) Annex K on security has been removed, and 1588-2019 introduces an (optional) section 16.14 on the PTP integrated security mechanism