At what point NMAP decides if host is UP ?

Hi,
I use TCP SYN scan to scan remote hosts and also use OS fingerprinting.
I use command something like that -
 
nmap -sS -O -p <port list> <ip address>
I also depend upon nmap response to find if host is UP or DOWN.
 
So I am just qurious to know how NMAP will decide if host is UP if command listed above is used.
Does it use ICMP at all in this case ?
 
If NMAP uses SYN packets to find if host is UP, then it uses any default port or uses list of ports specified in the command.
 
Thanks
 

                
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Received on Aug 05 2004