s. The ITU-T subtype (octets number 19-20 in the ESMC PDU) is defined by G.8264 as a fixed value of 0x00-01 for ESMC frames. There are two types of ESMC PDUs that are generated by a clock. The ESMC information PDUs carrying QL information are generated once per second. On the other hand, an ESMC event PDU (which has the event flag shown in Table 6-3 set to 0x1) is generated upon events that involve a change in QL (such as physical link failure or detection of a change in QL from an incoming ESMC PDU). This helps to ensure the latest QL information is passed to the peer EEC as quickly as pos- sible following a change in clock status. The QL information itself is carried in a QL TLV structure in an ESMC PDU. Table 6-4 shows the format of the QL TLV and how the SSM is encoded within the QL TLV as a 4-bit field. Table 6-4 QL TLV Format Octet Number Size/bits Field 8 bits Type: 0x01 16 bits Length: 00-04 bits 7:4 0x0 (unused) bits 3:0 SSM code Source: ITU-T G.8264, Table 11-4 As specified in Annex 57A and 57B of IEEE 802.3-2018 (specifying slow protocols) and restated by ITU-T G.8264 (describing ESMC), in no case should more than ten ESMC PDUs (information and/or event) be generated in any 1-second period. ESMC PDUs are based on slow protocols that are applied to single physical links. However, the case of multiple parallel links, such as Link Aggregation Group (LAG) between two NEs, brings up an interesting problem (LAG is specified in IEEE 802.1AX-2020-formerly IEEE 802.3ad). Each physical link of a LAG can exchange its own independent ESMC PDUs. So, when using a LAG, the QL value in ESMC PDUs sent from one NE to another on one physical link can get returned by the other NE on a different physical link that belongs to the same LAG. This condition has a potential for creating a critical condition for timing networks, referred to as a timing loop (see the section "Timing Loops" later in this chapter), where an NE is synchronized to the delayed version of its own clock that has returned to it from another connected NE.