[NUT-devel] NUT?

[Ralph Giles]

On Mon, Nov 06, 2006 at 01:56:53AM +0100, Michael Niedermayer wrote:

> btw, ralph if you disagree here, iam curious about the cases where you think
> that a crc field on frames makes sense

The main thing, I think, is that most decoder implementations are not 
robust against garbage data (and in some cases cannot be, entirely) so 
a crc on the frame data helps catch corruption that could crash or 
mangle playback. It's not a security issue per se, since malicious code 
could of course set correct crcs, but it still improves robustness.

As a counterexample, RTP packets don't have an internal CRC, but since 
they're supposed to fit within an unfragmented UDP packet, they can 
piggyback on the transport's checksum for a similar level of integrity.

The other point is file verification. Since people collect and serve 
media files, it's convenient to have an embedded integrity check to
detect corruption. An embedded hash or separate checksum can do the same 
thing, but it's not automatic, and doesn't work with streaming. They do 
help with truncation detection though.

PNG images (and FLAC frames) have CRCs, JFIF jpeg and GIF do not, so 
perhaps you can make an argument that it's more important for lossless 
storage. But I've only heard the png developers talk in terms of 
integrity verification motivated as above.

 -r