ipf ttl question

[Dave+Seddon]

Greetings, 

I'm running ipf+ipnat and proftp.  I'm encountering a problem where the data 
connection is working fine, however because there's a large tranfer no data 
is tranferred on port 21, so the port 21 session dies (ttl expires). 

The transfer is running now. 

How can I change the ttl on the port 21 session, without dropping the 
session?
Or can I change the ruleset to allow everything without dropping the 
session? 

Regards,
Dave