 send this acknowledge cancel TLV message and cease using the service. UNICAST NEGOTIATION ENABLE: This message TLV is used to enable or disable the unicast negotiation mechanism. A grant port receiving this TLV message will cancel all out- standing grants and disable the mechanism. PTP ports in the following states are not allowed to participate in the negotiation process: initializing, faulty, or disabled. According to Annex A.9.4.2 of IEEE 1588-2008, for unicast implementations, the recommended and allowed values for the message intervals of the request TLV are outlined in Table 7-6. Table 7-6 Recommended Message Interval Rates for Unicast Negotiation Message Default Message Interval Minimum Message Interval Maximum Message Interval Announce 1 (1 per 2 sec) -3 (8 per sec) 3 (1 per 8 sec) -4 (16 per 1 sec) -7 (128 per sec) 1 (1 per 2 sec) Delay_Resp -4 (16 per 1 sec) -7 (128 per sec) 6 (1 per 64 sec) According to the same recommendation, the duration field should have a configurable range of 1000 seconds (default is 300 seconds). Figure 7-6 illustrates how the negotiation process between two PTP ports would work. Request for Announce Grant for Announce Announce Request Grant Announce grant Announce duration message Slave Request for Delay_Resp Grant interval Grant Delay Announce Request for Announce Grant for Announce Signaling message Delay Req Delay Resp Figure 7-6 Example Negotiation Process The slave begins by asking for a service of Announce messages to determine the quality of the up- stream clock. The grant port responds with the acknowledgement of the grant and commences the transmission of (unicast) Announce messages. The BMCA on the slave determines (via the clock quality and priority) that this grant port is an acceptable master clock and then requests a flow of Sync event messages. The grant port acknowledges the grant of Sync service and immediately starts sending the Sync messages at the requested rate (for a two-step clock, each Sync message will be followed by a Follow_Up message). If this is a 