Idea to check availability of external ftp server from a rem



I am a newbie in Nagios.
Now I have installed Nagios 2.8 and Nagios plugins 1.4.5 on RHEL5.
So far basic monitoring works fine.

Now, a bit challenge for me was that I was asked to monitor the availability of external ftp server on a remote host.
Basically, only the remote host can perform the ping and ftp to the ftp server.
Can anybody share the idea which plugin to use or how can it be done by any means like perl script?
Thanks in advance.