When TCP Idle Timeout counter starts

Hi, Just working on some migration from Cisco CSS. There is setting in config "flow permanent" that is described in manual like that: To define a set of TCP or UDP ports that will have permanent connections and will not be reclaimed by the CSS when the flows are inactive, use the flow permanent command. By default, the CSS may reclaim TCP/UDP flows that have not received an ACK or content request after approximately 15 seconds. Use the no form of this command to disable a permanent connection by setting its port number to 0. Most important from above is sentence: "By default, the CSS may reclaim TCP/UDP flows that have not received an ACK or content request after approximately 15 seconds" Maybe I am wrong but "have not received an ACK" in above sentence sounds for me as situation when client sending SYN was replied with SYN+ACK but never send ACK - typical not finished 3WHS - but maybe I am wrong as I am not Cisco expert. That's why I am trying to figure out what is responsible for timeout in such situation on F5. Idle Timeout should handle without issue situation when 3WHS was finished (so we have established TCP connection in Connection Table) but at some point no more packets is arriving via this connection. I assume that counter is starting each time packet is received and is reset when next packet arrives. Question is if the same counter is used for unfinished 3WHS situation or some other, or some RFC defined default is used? Piotr