What is DASH (Dynamic Adaptive Streaming over HTTP) and why did it broke my yt-dlp filter?

When you run yt-dlp -F on a YouTube video you will see lots of entries ending in mp4_dash, webm_dash or m4a_dash. I thought at first that this is a new format, so I started searching. As my current filter would complain: "No video formats found!" The short version DASH stands for Dynamic Adaptive Streaming over HTTP, also known as MPEG-DASH. It is an international standard (first described in ISO/IEC 23009-1) for streaming video and audio over plain HTTP. The same protocol your browser uses to load any webpage. The core idea: Don't send one big file. Cut the media into many small segments, offer them in several qualities, and let the player decide which quality it wants to fetch next. And reading this, I finally understood a bit more how some things related to streaming work (yeah, all that audio/video stuff is not my bright side). The problem DASH solves Before adaptive streaming there were basically two options to watch a video on the web: Download the whole file (progressive…