This mechanism is needed because user context does not exist in modules and drivers when ioctl processing occurs. This prevents them from using the kernel itself on copyin and copyout functions
consider the following ioctl call:

ioctl (stream_fildes, user_command, &ioctl_struct);

where ioctl_struct is a structure containing the members:
int stringlen; /* string length */
char *string;
struct other_struct *other1;

To read (or write) the elements of ioctl_struct, a module would have to do a series of copyin( ) and copyout( ) calls using pointer information from a prior copyin( ) to transfer additional data. A non-STREAMS character driver could directly execute these copy functions because user context exists during all calls to the driver. However, in STREAMS, user context is only available to modules and drivers in their open( ) and close( ) routines.
The transparent mechanism enables modules and drivers to request that the Stream head do a copyin( ) or copyout( ) operation on their behalf to transfer ioctl data between their kernel space and various user space locations. The related data is sent in message pairs exchanged between the Stream head and the module. A pair of messages is required so that each transfer can be acknowledged.