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I wonder whether there would be any problems if Tor grew to serve a large portion (e.g. 5 or 30 %) of internet (web) usage?

As I understand it Tor is helped by new nodes at the moment – but is there any limit when its usability or safety would decrease?

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    Scalability asside (which is very good), the "safety" factor is illustrated very well by this blog post that Schneier posted thismorning (the gist is: The more people who use Tor, the safer it becomes).
    – user5
    Dec 18, 2013 at 18:46
  • An underlying reason for this question is whether Tor could solve the issues of a free and safe internet that among others Schneier discuss. Or is something other (another system/protocol) than Tor needed for common users to have access to anonymous and free internet access?
    – trmdttr
    Dec 18, 2013 at 19:08

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The whole Tor network advertises 6 GiB/s according to the metrics page. DE-CIX statistics say they have an average bandwidth of 200 GiB/s. So if you assume that web traffic is roughly 20% of all traffic, we talk about 40 GiB/s. Numbers between 5 and 30% are 2 to 12 GiB/s of traffic in the DE-CIX case.

So if those estimation is correct, Tor should be able to handle (DE-CIX) web traffic right now. It would need some fast relays, but not enormous amounts of relays. I assume the numbers don't grow that much on a worldwide base. So with a fair number of fast relays web traffic should work.

However when when talk about general Internet traffic (BitTorrent alone is estimated to account for >40% of internet traffic), you will needs ten of thousands, if not millions of relays. A client will have to download all those information at the beginning. I would estimate that this file can easily grow to some hundreds of MB. Furthermore, as Peter wrote, every relay talks to each other which simply doesn't work with so many relays.

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Caveat: not a complete answer, but still a start:

There probably is a limit, given Tor's current design. For instance, right now it's assumed that every relay can and does talk directly to every other relay. Once you have many tens of thousands of relays that just won't work anymore.

For now, Tor has kept ahead of the curve. However, this requires constant updates, discussion, and reflection. There is a discussion from a couple years back in the tor specification repository.

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  • Just as the two answers below, you seem to suggest that the answer is no. But if Tor was developed to solve the problems, it might be possible.
    – trmdttr
    Dec 20, 2013 at 17:18
  • Is it (still?) true in practice that each relay does talk to every single other relay? What do they talk "about"?
    – agentofuser
    Apr 4, 2014 at 17:35
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Another bottleneck is the distribution of the network consensus. To maintain anonymity, every client must have the full consensus for relay selection. As more relays and users join the network, the distribution gets more and more expensive. After all, every relay adds another entry to the consensus and every user will periodically want to download the consensus.

This paper gives a good overview of the problem: http://freehaven.net/anonbib/#ccs09-torsk

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