The TS_UD_CS_MCS_MSGCHANNEL packet indicates support for the message channel which is used to transport the Initiate Multitransport Request PDU (section 2.2.15.1). This packet is an Extended Client Data Block and MUST NOT be sent to a server which does not advertise support for Extended Client Data Blocks by using the EXTENDED_CLIENT_DATA_SUPPORTED flag (0x00000001) as described in section 2.2.1.2.1.