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My question is about popular YouTube downloaders like youtube-dl (a command line program) or VideoDownloadHelper (a Firefox-browser extension).

Comparing two cases:

  1. Watching a video on YouTube
  2. Download the video using a downloader (to be specific let's assume youtube-dl)

Is it possible to tell – for instance by inspecting the network traffic – that the video was downloaded and not "only watched" on YouTube?

Maybe one could compare network traffic using programs like Wireshark? I cannot do that myself, but maybe this will help somebody to answer the question.

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Unless it's some kind of special player+stream combo using anti-copying measures, when online videos are played they're also downloaded to your local machine, and you can copy them from your browser cache. – Karan Nov 10 '12 at 16:00
I have to give this browser cache some background research. Maybe this way I can find a way / software which when downloading does not generate any difference to the simple watching of a video. – humanityANDpeace Nov 10 '12 at 16:28
up vote 1 down vote accepted

Yes, it's possible to differentiate between these two use cases when looking at network traffic. The simple explanation is:

  • When you're downloading the raw video file with youtube-dl, you're loading a complete file at once.
  • When you're watching YouTube video through the browser, the Flash client downloads the video in chunks. The chunks fill up a buffer, and once that buffer is about to run out, the player fetches the next chunks.

Both can be done through HTTP these days. You can observe the client behavior when you load up a video. It is never completely downloaded at once: The buffer will be played out, then the next part will be loaded. This of course is visible in network traffic, as multiple requests are sent to YouTube for one resource over the course of time.

To cite Kuschnig et al. (see below):

A video segment is split into chunks of size lch, which are served by a standard HTTP server. The download of the video chunks is coordinated by the client. For that purpose, the client maintains nc HTTP-based request-response streams and schedules the downloads of the different chunks by using a separate queue for each stream

If you want more specifics about the YouTube streaming traffic, I could of course explain more. We currently conduct various simulated experiments regarding optimization of YouTube buffering and analysis of diverse video streaming scenarios.

Further reading:

  • Kuschnig, Robert, Ingo Kofler, and Hermann Hellwagner. "Evaluation of http-based request-response streams for internet video streaming." Proceedings of the second annual ACM conference on Multimedia systems. ACM, 2011 (PDF)

  • Stockhammer, Thomas. "Dynamic adaptive streaming over HTTP--: standards and design principles." Proceedings of the second annual ACM conference on Multimedia systems. ACM, 2011. (PDF)

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So is it not entirely possible for a downloader to mimic data requests in a manner similar to that of the Flash client? Along with the correct user agent string and what not, would it still be possible to differentiate? – Karan Nov 10 '12 at 22:03
Well, then of course it's splitting hairs between what's a proper video client or merely a downloader acting as such :) You're right of course: You could definitely mimic video player requests, and changing user agent strings would be another way to obscure traffic. I'm sure if you're clever enough you could fool any detection algorithm. – slhck Nov 10 '12 at 22:07
True. Referring to the original question, Google/the music industry is not so stupid as to be ignorant of the fact that content can and is downloaded (often with multiple connections to the server using download accelerators). Guess either they don't care as long as it's for personal use, or don't want to reduce their popularity and/or spark off an arms race by introducing some form of DRM, or whatever. In any case, I doubt there'd be much left if all copyrighted content not uploaded by the copyright owners themselves were to be removed from YouTube. :) – Karan Nov 10 '12 at 22:14

Yes it is different (in the special case of using youtube-dl) which can be seen by the fact that the traffic while watching on website uses a https:// transfer and the traffic generated by youtube-dl is using an unencrypted http://.

If somebody sniffes the packages he can tell that the file was not watched on youtube. At least not the ordinary way

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Can't youtube-dl be made to use an https connection? Does YouTube always use https? – Karan Nov 10 '12 at 16:36
I see no reason why it should be impossible to make youtube-dl use https connections. Still the handling of https is a little more tricky and as it seems not required to achieve the goal (to provide a mechanism to download the resource). In the current way it would still not achieve the side-goal of downloading the data an "mimic video watching way". This goal (even with using https-connections) would not be achieved since I doubt the elaborate behaviour of the browser is immitated. I think youtube-dl is more like small python app. – humanityANDpeace Nov 12 '12 at 8:57
why the downvoting? It answers the question by showing an example of a case where it is different. At least it thereby partially responds to the question. It took some work to use wireshark and investigate this. I feel unappreciated for this work. – humanityANDpeace Nov 12 '12 at 8:59
Although I don't know who downvoted, don't take it so seriously. It's just how the site works. – Karan Nov 12 '12 at 16:51
@ Karan :thanks for the consolation. Still I am confused to see a downvote on an "not-wrong" even partly helpful answer of mine. Instead of downvoting I would rather see better answers to be voted up. I am confused, since I though the site works the way that wrong answers are downvoted. – humanityANDpeace Nov 14 '12 at 9:24

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