The initial data in the message, or NULL.
The length of the allocated data buffer.
If non-zero, indicates that datalen is fixed.
This function creates a new IP_Msg structure whose message is an IP_MsgBuffer with a data buffer that is datalen bytes in length.
If data is non-NULL, then datalen bytes are copied into the newly allocated data buffer in the IP_MsgBuffer component of the IP_Msg. If noresize is 0, then the IP library may enlarge the data buffer in the future to accomodate requests larger than datalen. If datalen is 0, then IP_MsgDefaultSize is used. The message is marked as dynamic.