Since 2.8.0 and update to 4.x Smack file transfer feature has been degraded. Maybe due to bugs in Smack or maybe Spark needs better tuning to use newer Smack version. Now file transfers between different subnets fail and in some other cases where 2.7.0 worked fine. Until this bugs are fixed we can introduce an option to force In-band bystream (IBB) type transfer. Which is slower, but usually works in many cases.