What method does Splunk use when moving warm buckets to cold?

Hello all, Hot and warm buckets reside on our fastest disks, but when we move to cold it is shipped off to a slower disk. There is a command to manually force the hot to warm transition but is there one for warm to cold? My real curiosity is what splunk actually uses to transfer the data to the cold disk? A certain protocol or port? Something like scp or is it a splunk built in utility similar to indexing? Thanks!