[bitfolk] IPv6 tunnel timeout

I've set up IPv6 tunnelling between my VPS and a linux box at home (both CentOS) and now have IPv6 working on my home network a treat. The problem is, though, that the tunnel appears to time out after a few minutes when traffic stops flowing and I start getting "network unreachable" errors when trying to connect out from my home network. If I ping6 something on the home network from the VPS it appears to wake the tunnel up again. Is there a way round this (other than having a cron job ping something every minute or so) - maybe some sort of timeout parameter when setting up a sit tunnel? Thanks in advance, Mike