The currently implemented importer plugins invoke rsync directly against the remote site. The intent is to provide an additional importer->distributor->importer plugin chain that uses a remote Pulp server to generate the rsync delta files over the LAN, passes the delta file over the WAN, then applies the delta locally at the destination site.

1. This will initially only be supported for "snapshot" tree layouts, since the previous version of the tree is needed to determine the deltas and some marker is needed to indicate when a new version of the tree is ready for synchronisation.
2. The upstream importer invokes the rsync CLI as usual, but adds the "--only-write-batch" flag. The generated delta is then stored in Pulp as a content unit. Initially, a delta will be generated solely for the most recent version of the tree. Longer term, a config option to allow generation and storage of additional delta files to update from older versions of the tree may be added.
3. The upstream distributor then makes these content units available via HTTP.
4. The downstream importer queries the upstream distributor, reporting its latest local version. The upstream distributor then either replies with a link to the appropriate delta file (to be downloaded and applied), or details of the upstream repo for a direct rsync call (i.e. the client is sufficiently out of date that the required delta file has already been purged).