3.2.5.3.2 Sending the Soft-Sync Response PDU
The structure and fields of the Soft-Sync Response PDU are specified in section 2.2.5.2, and the PDU MUST be initialized according to this specification. For each dynamic virtual channel, the client DVC manager SHOULD begin to send data on the same multitransport tunnel that the server indicated will be used in the Soft-Sync Request PDU.
The client DVC manager MUST NOT send the Soft-Sync Response PDU until it has received the Soft-Sync Request PDU and sent a successful Initiate Multitransport Response PDU ([MS-RDPBCGR] section 2.2.15.2) for each multitransport tunnel that will be used to send dynamic virtual channel data. Each multitransport tunnel that the client manager will use to send dynamic virtual channel data MUST be specified in the TunnelsToSwitch field.
The Soft-Sync Response PDU MUST be sent over the DRDYNVC static virtual channel on the main RDP connection. Immediately after sending this PDU, for each dynamic virtual channel, the client DVC manager MUST consistently use either a multitransport tunnel or the DRDYNVC static virtual channel on the main RDP connection to send data.