 expressed as the log (base 2) of the interval between messages, SO that, for example, -4 is 16 packets per second, while +1 is one packet every 2 seconds. The mechanism behind the use of logarithm for inter-message period is explained in the section "Message Rates" earlier in the chapter. The grant port that receives this request TLV shall respond with a signaling message containing a GRANT_UNICAST_TRANSMISSION TLV. This transmitted grant TLV signals to the request- ing slave whether it grants or denies the request. These are the possible operations available for the negotiation process: REQUEST UNICAST TRANSMISSION: The slave uses this TLV to request a flow of one of the timing messages (for example, Announce messages to see how good the configured master is). This message TLV contains fields that specify the log of the desired message interval as well as the duration of the grant-the default being 300 seconds. Before the grant expires, the slave must issue another request if it wants to continue receiving the service. GRANT UNICAST TRANSMISSION: Any grant port that receives a request TLV responds with a grant TLV confirming the grant or denying it. The TLV contains fields that specify the message type, the log of the granted message interval, and the duration of the grant. If the grant is denied, the duration will be zero seconds. CANCEL UNICAST TRANSMISSION: A slave that has requested a grant may inform the grant port that it no longer needs the service it was granted. It does this by transmitting this can- cel TLV message with the name of the message type that it wishes to cancel. A grant port providing the service may also decide to inform the grant port that it can no longer provide the message flow and send the same TLV to the slave port. ACKNOWLEDGE CANCEL UNICAST TRANSMISSION: The grant port receiving the cancel TLV responds with this acknowledge cancel TLV and then immediately stops transmit- ting the messages the slave wants to cancel. Similarly, any slave port that receives the cancel TLV should also