The Transfer VM is a BusyBox virtual appliance that can be used to expose virtual disk images (VDIs) over a number of protocols. The disk can be exposed over the network for both uploading and downloading disks.

The currently supported transport protocols are:

• HTTP
• HTTPS
• BITS (Microsoft's Background Intelligent Transfer Service)
• iSCSI
• iSCSI over SSL

A client makes a request to the XenServer host using a XAPI plugin 'transfer' which is installed along with the Transfer VM VPX image. Once the plugin has been called to expose a disk, a record can be returned by the client which contains a URL over which the Trnsfer VM can be contacted to either upload/download a disk image.