Network map using nmap

$ nmap -O -sS -p 20-23,3300,80,443
...
Interesting ports on localhost (127.0.0.1):
PORT    STATE  SERVICE
20/tcp  closed ftp-data
21/tcp  closed ftp
22/tcp  closed ssh
23/tcp  closed telnet
80/tcp  open   http
443/tcp closed https
3300/tcp  open   unknown
Device type: general purpose
Running: Linux 2.6.X
OS details: Linux 2.6.17 - 2.6.25
Network Distance: 0 hops
...

I configured sshd to listen on port 3300, but nmap can’t resolve the type of service in the above sample. (It is found to be open but unknown service)