There is ZERO caching of YT content. Every single vidoe/audio source link your client receives is individually cryptographically signed for you (IP) and for this particular time (timestamp, expires in couple hours). Refresh the page and you receive brand new list of different links to same files.

No. The cached files will remain intact, since the "video chunks" playlist will add new items, but not change or remove the existing ones like that:

    video_part0076.ts
    video_part0077.ts
    video_part0078.ts
    ad_part0001.ts
    ad_part0002.ts
    video_part0079.ts
    video_part0080.ts

Wouldn't this be trivial to defeat by pulling the playlist multiple times (maybe via a proxy to get a different IP), and only showing the chunks which did not change?

I guess they could prevent that if they randomize the part names (with some m:1 mapping on the server side). As long as you have control of your CDN, this would even not mess with caching.

> Wouldn't this be trivial to defeat by pulling the playlist multiple times (maybe via a proxy to get a different IP), and only showing the chunks which did not change?

Since the YouTube marks ads, I guess pulling the playlist twice won't really be needed. YouTube's player should somehow know when the ad starts and when it ends after all.

Good point. One could probably patch the javascript on the fly to go from unskippable ad to auto-skip....

Regarding autoskip, I have an interesting experience with it in the past.

My friend asked me to write a userscript that simulates pressing the "Skip ad" button, so he won't stain his mouse with grease while eating.

Checking if there is a "Skip ad" button every quarter of a second and invoking its click() method eliminated the video ads completely. As it turns out, the button existed alongside the 5 second countdown, just wasn't visible.

I don't know if it still works the same way today, but it felt good to accidentally make an effective adblock with five lines of code.

Not really! The main live video technologies work by having a playlist file which is a list of URLs that updates frequently and a bunch of short video files. So the caching of the short video files would be unaffected.