[NUT-devel] NUT?

[Ralph Giles]

On Sun, Nov 05, 2006 at 12:05:30PM -0500, Rich Felker wrote:

> Ogg's problem with streaming is that the only way it avoids massive
> overhead is by packing large numbers of frames into a single Ogg
> "page". The page cannot be transmitted until it's complete, adding an
> extra senseless stage of latency to the stream.

To be a bit more explicit, Ogg fragments and/or packs individual data 
packets (I guess those are "frames" in NUT terminology?) into data 
chunks called "pages". So Mr. Felker is correct that one must buffer at 
least one page (one page from each stream in the general multiplexed 
case).

Each Ogg page as a 27-byte header which includes a sync magic, 
timestamp, and crc. Additionally, the length of the packets (or 
fragments) in the page body are encoded in a series of bit counts, with 
counts less than 255 marking packet divisions. For a sequence of packets 
(frames) < 255 bytes each, where the length encoding requires one byte 
per packet (frame). The case mentioned above, where one puts one packet 
per page to minimize latency, that's an overhead of 28 bytes per packet.

NUT appears to use variable length coding (a la utf-8) and table lookup 
(and delta coding for timestamps?) for most headers and the docs claim a 
2 byte-per-packet overhead for a single-stream audio case, and that one 
can send the packets one-at-a-time, but I'm not clear how this 
interacts with the CRC field. Maybe that's assuming you write 
it for a group of frames at the end?

In practice we've never cared about the latency. For interactive 
applications Ogg is not appropriate, and RTP over UDP is the already
existing solution for low-latency work. For unicast delivery (what we 
mean when we say ogg was designed for streaming) the latency is not 
significant.

For the more typical case (4k pages, 10 packets/page) the Ogg overhead 
works out to 4.8 bytes per packet (frame) or 1.2% vs 0.5% for NUT. So 
NUT is better by about a factor of two there, but both are very low.
Ogg has bounded overhead as packets get bigger as well, but I 
don't understand NUT well enough to compare that case (say 17k video 
frames, where Ogg is 28+17k/255 bytes or 0.5%). Could you do a sketch 
there?

So NUT seems to be intended for TCP streaming and file storage, much 
like Ogg. It uses a more complicated framing scheme to save a fraction 
of a percent of bitrate. Technically I think the more significant 
difference is that metadata and codec-independent muxing is embedded 
within the format rather than layered on top.

IMO,
 -r