Software backup solutions use different protocols called transport modes to retrieve VM data from the storage. The transport mode to use for backups depends on the design of the network and the storage architecture.

There are four main transport modes that are supported for handling data:

• Network Block Device (NBD): The ESXi host reads data from the storage and sends it to the application, across the network, using the NBD protocol. This mode can be used in any infrastructure configuration and is the simplest method to implement.
• Network Block Device Secure Socket Layer (NBDSSL): This is the same as NBD but uses SSL to encrypt the data passed over the TCP/IP connection.
• SCSI HotAdd: This is a LAN free data transfer mode where the .vmdk files of a VM are attached to the backup application. Data won't go through the network but is read and written directly from/to the datastore. In a lot of environments, this is the preferred mode.
• Direct SAN: In this, data is read directly from the SAN or iSCSI LUN; this provides the fastest data transfer speed. Direct SAN transport mode is recommended if VM's disks are stored on shared SAN LUNs connected to ESXi host over FC, FCoE, and iSCSI.

Not all transport modes can be used in all cases: for example with Virtual Volumes, actually, Direct SAN mode is not supported. 

Also, some specific backup products can implement other specific transport modes (for example Direct NFS with Veeam Backup & Replication.