3.3.5.15.1 Sending the Initiate Multitransport Request PDU

The structure and fields of the Initiate Multitransport Request PDU are described in section 2.2.15.1 and the PDU MUST be initialized according to this specification. The embedded initiator field of the mcsSDin field MUST be set to the MCS server channel ID held in the Server Channel ID store (section 3.3.1.5), while the embedded channelId field MUST be set to the MCS message channel ID held in the Message Channel ID store (section 3.3.1.4). Furthermore, the embedded flags field of the securityHeader MUST contain the SEC_TRANSPORT_REQ (0x0002) flag (section 2.2.8.1.1.2.1).

A single Initiate Multitransport Request PDU MUST be sent to the client for each type of sideband channel being requested. A sideband channel utilizes either reliable UDP or lossy UDP as a transport protocol ([MS-RDPEMT] section 1.3) and hence only a maximum of two Initiate Multitransport Request PDUs can be sent to the client.

The server MUST save the request ID (specified in the requestId field), requested protocol (specified in the requestedProtocol field) and the security cookie (specified in the securityCookie field) in the Multitransport Request Data store (section 3.3.1.14) so that the sideband initiation request can be correctly correlated with the Tunnel Create Request PDU ([MS-RDPEMT] section 2.2.2.1) or Initiate Multitransport Response PDU (section 2.2.15.2).

The Initiate Multitransport Request PDU is only used to bootstrap the creation of a sideband channel. More information on the creation, setup sequence, and operation of sideband channels is available in [MS-RDPEMT] section 1.3.1.