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3.1.5.3 Soft-Sync

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Soft-Sync is an optional feature used to defer the use of the multitransport tunnels (reliable UDP and lossy UDP as described in [MS-RDPEMT] section 1.3) as a transport for dynamic virtual channels until after these tunnels have been successfully created.<10>

Soft-Sync negotiation

Figure 9: Soft-Sync negotiation

The server and client indicate support for Soft-Sync by setting the SOFTSYNC_TCP_TO_UDP flag in the Server Multitransport Channel Data ([MS-RDPBCGR] section 2.2.1.3.8) and Client Multitransport Channel Data ([MS-RDPBCGR] section 2.2.1.4.6). Soft-Sync MUST NOT be used unless it is supported by both the server and client.

If Soft-Sync is supported by the server and client, the DRDYNVC static virtual channel on the main RDP connection MUST initially be used by the server manager and client manager for reading and writing all dynamic virtual channel data. The server manager and client manager MUST NOT send or receive any dynamic virtual channel data on the multitransport tunnels until the Soft-Sync negotiation has completed.

The Initiate Multitransport Response PDU ([MS-RDPBCGR] section 2.2.15.2) MUST be sent to the server after a multitransport tunnel is created. Soft-Sync MUST NOT occur until a successful Initiate Multitransport Response PDU has been received by the server for all supported multitransport tunnels.

The server manager sends a Soft-Sync Request PDU (section 2.2.5.1) to the client manager and then starts to send dynamic virtual channel data on the multitransport tunnels. After receiving the Soft-Sync Request PDU, the client manager sends a Soft-Sync Response PDU (section 2.2.5.2) to the server manager and then starts sending and receiving dynamic virtual channel data on the multitransport tunnels. After receiving the Soft-Sync Response PDU, the server manager starts to receive dynamic virtual channel data on the multitransport tunnels.