What do MP4 metadata tags such as "gssd" or "gsst" mean?

I am extracting metadata from mp4 files and found some keys that I cannot identify: "gssd", "gsst", "gstd" and "gspu" are some of them.

I know of keys defined for iTunes (in AtomicParsley's website there is some info), but those won't appear in the .mp4 files I have analyzed with a hex editor. Instead I find these not-telling-me-anything keys. Their values are also not very representative: their type is text and they look like "0", "300953" or "B4A7D6381H开发者_高级运维H"

Has anyone dealt with these tags before? What do they mean?

This superuser question and this forum post suggest that those meta tags are added by Google for tracking purposes.

I did a little test of my own. I took a video I uploaded to vimeo and checked the metadata using MediaInfo. It did not contain any of the tags gsst, gstd, gssd, gshh in the general section or any other section.

Next I uploaded the downloaded Vimeo video to YouTube. I proceeded to download the video once again and checked the meta tags. Turns out the meta tags were present.

gsst : 0
gstd : 156990
gssd : B4A7DD6E1MM1359518628237025
gshh : r1---sn-p5qlsu7r.c.youtube.com

Using Chrome's Developer tools > Resources it looks like gshh points to the XHR videoplayback. It's the direct link in the page and seems to be embedded in the file. I couldn't determine what the others store.

Minor Update

I went back today and used Youtube's editing capabilities to add a filter to my video. I downloaded the video and some of the values changed.

gsst : 0
gstd : 156850
gssd : BADC235C1MM1359899059491718 
gshh : r1---sn-p5q7ynl7.c.youtube.com

The superuser question noted that some of these values change. In this instance, same video with an edit performed via Youtube, only gstd and gssd changed. I thought gssd might be a hash, but I found no evidence to support this idea.

For the curious, this is the video I used for the test.

• Vimeo: https://vimeo.com/15508855
• Youtube: http://www.youtube.com/watch?v=rxFqL9I66-U