FTP Data Port must always be N+1?

Hi,

I have a question regarding FTP protocol - data port.
I have been referred to a website URL that states, for active or passive mode, the client's control port if is N, the data port is N+1.
I would like to know if N+1 for data port is a must have, or should have concept. Currently, in my program, the data port that I use is any random number > 1024. The rfc 959 does not state that data port has to be N+1.

Please kindly advise or if possible, refer me to other info.

Thanks,
Claudia