In the sync settings page(workarea > settings > configuration > synchronization > settings) there is a drop down which is typically set to proxy. Also in the drop down are direct and auto-detect.
In some cases one type may work where the other does not. For instance, if you see that sync is stuck at 50% you might have an issue with the remote EWS which can be bypassed by switching to the direct type(see below for description).
Direct - The source Ektron Windows Service(EWS) will make changes directly to the remote database without interacting with the destination EWS. Instead of batching up changes in the database folder, changes are stored and executed from memory. This only works if the source EWS can communicate to the destination SQL server over 1433 and if the source EWS has permission to change or create files on the remote site and assetlibrary.
Proxy - The source EWS batches changes and sends them to the destination EWS for evaluation, then applies the prod EWS applies the changes to the prod database. After applying the changes the destination EWS notifies the source EWS.
Auto-detect - It will first try the direct connection and if that fails it will use the proxy.