onous Ethernet equipment clock (EEC), by using ESMC PDUs. The ESMC is defined by ITU-T G.8264 and is covered in more detail in Chapter 8. Even though an ESMC is sometimes called "SSM for Ethernet," there are some differences between the SSM and ESMC mechanisms. Although SSM messages are carried in a fixed location within an SDH/SONET frame, the same information is carried via ESMC PDUs in Ethernet because it does not have a fixed frame. The QL values are carried using a special Ethernet method known as a slow protocol, with its own Ethertype (88-09), similar to other overhead functions for Ethernet, such as OAM, Link Aggrega- tion Control Protocol (LACP), and SO on. Specifically, the ITU-T defines ESMC using one of the slow protocol mechanisms called an organization specific slow protocol (OSSP), originally spe- cified in IEEE 802.3ay. Note that the definition of OSSP is now included in Annex 57A and 57B of IEEE 802.3-2018. The ESMC packet is composed of the standard Ethernet header for an OSSP frame; the Organiza- tionally Unique Identifier (OUI) representing the ITU-T; an ESMC-specific header; a flag field; and a type, length, value (TLV) structure. ESMC packets, like SSM, carry a QL identifier that identifies the timing quality of the synchronization trail. QL values in the QL-TLV structures are the same QL values defined for the SDH/SONET SSM messages. Table 6-3 shows the ESMC PDU format. Table 6-3 ESMC PDU Format Octet Number Field 6 octets Destination address = 01-80-C2-00-00-02 6 octets Source address 13-14 2 octets Slow protocol Ethertype = 0x88-09 1 octet Slow protocol subtype = 0x0A 16-18 3 octets ITU-OUI = 0x00-19-A7 19-20 2 octets ITU-T subtype = 0x00-01 = ESMC 4 bits Version = 0x1 1 bit Event flag 3 bits Reserved 22-24 3 octets Reserved 25-1514 36-1490 octets Data and padding Last 4 4 octets Frame check sequence (FCS) Source: ITU-T G.8264, Table 11-3 The ITU-OUI and ITU-T subtype are important fields in the OSSP PDU for QL values because these fields differentiate the ESMC PDU from the other OSSP PDU