Diretta manages the data flow over the Ethernet link between the Diretta host and the Diretta target. With a modern USB DAC supporting UAC-2 and async USB, it does absolutely nothing to change the data transfer pattern over the USB link to the DAC. With or without Diretta, the data transfer timing over Ethernet is only very loosely associated with the data transfer timing of that same data over the USB bus and that in turn is only very loosely associated with the timing of the delivery of samples to the digital to analogue conversion stage in a USB connected DAC.
I have already explained how the async USB audio data transfer works in my earlier post. I won’t do it again.
That USB protocol allows very little room for changing the USB data pattern (and what there is is likely determined by the USB protocol stack on the USB host end and the driver/abstraction layer that provides access to applications - e.g. ALSA. It is certainly not something that Diretta influences.
Connecting a DAC via USB to a Diretta target rather than a RAAT target (or any other streaming protocol) does not affect the nature and delivery pattern of data to the DAC and, as has been said multiple times before, the data delivery over USB has no relation to the timing of the clocking of the data onto the actual digital to analogue conversion stage within the DAC (which is the only point at which data delivery timing matters) other than the simple fact that the average data transport rate over USB must match the average data consumption rate by the analogue to digital converter.
You can argue that Diretta has an influence on processor activity patterns, power consumption patterns, network activity patterns etc in the Diretta target device and you can argue that it isolates the streamer (the Diretta target) from broadcast/multicast traffic on the network and the limited amount of extra processing that this involves (again - in the Diretta target device). However, you then have to explain how any of that can affect the performance of the DAC which is separate to the Diretta system and has already been designed to isolate the analogue performance from external influences (as is the case with any half-decent DAC).
Indeed. There is no technical reason why Diretta could not support both IPv4 and IPv6. I suspect that IPv6 is specified because it has mechanisms to automatically assign ip addresses without a DHCP server (which, following the Diretta logic, would be undesirable) and so it eliminates a network configuration stage - IPv4 would require both host and target to be assigned static ip addresses in a separate subnet to the main network by the user thus making system setup potentially more complicated.
In fact, IPv4 packets have a smaller header and so, following the Diretta logic, it would seem to me that it would be more desirable to use IPv4 because the amount of processing required to decode the packet header would be reduced.