Suspected nmap listing - am I under attack?



Did an nmap scan on my Cisco router from the outside world. Found the following output:

Interesting ports on *.*.*.1:

Not shown: 1676 closed ports
PORT STATE SERVICE
23/tcp filtered telnet
135/tcp filtered msrpc
1524/tcp filtered ingreslock
27665/tcp filtered Trinoo_Master

There is a Cisco PIX behind the inside interface though I ran nmap from across the serial.
What does the state "filtered" signify?

How can I be sure if I have been used as an agent for launching DDoS attack?

Please advise!!

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